CdTe, CdTe/CdS Core/Shell, and CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Study. A dissertation presented to. the faculty of

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1 CdTe, CdTe/CdS Core/Shell, and CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Study A dissertation presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Doctor of Philosophy Yueran Yan March Yueran Yan. All Rights Reserved.

2 2 This dissertation titled CdTe, CdTe/CdS Core/Shell, and CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Study by YUERAN YAN has been approved for the Department of Chemistry and Biochemistry and the College of Arts and Sciences by Paul G. Van Patten Associate Professor of Chemistry and Biochemistry Howard D. Dewald Interim Dean, College of Arts and Sciences

3 3 Abstract YAN YUERAN, Ph.D., March 2012, Chemistry CdTe, CdTe/CdS Core/Shell, and CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Study Director of Dissertation: Paul G. Van Patten CdTe, CdTe/CdS core/shell, and CdTe/CdS/ZnS core/shell/shell quantum dots (QDs) are potential candidates for bio-imaging and solar cell applications because of some special physical properties in these nano materials. For example, the band gap energy of the bulk CdTe is about 1.5 ev, so that principally they can emit 790 nm light, which is in the near-infrared range (also called biological window). Moreover, theoretically hot exciton generated by QDs is possible to be caught since the exciton relaxation process in QDs is slower than in bulk materials due to the large intraband energy gap in QDs. In this dissertation, we have synthesized the CdTe and CdTe/CdS core/shell QDs, characterized their structure, and analyzed their photophysical properties. We used organometallic methods to synthesize the CdTe QDs in a noncoordinating solvent. To avoid being quenched by air, ligands, solvent, or other compounds, CdS shell was successfully deposited on the CdTe QDs by different methods, including the slow injection method, the successive ion layer adsorption and reaction (SILAR) method, and thermal-cycling coupled single precursor method (TC-SP). Our final product, quasi-type- II CdTe/CdS core/shell QDs were able to emit at 770 nm with a fluorescence quantum yield as high as 70%. We also tried to deposit a second shell ZnS on CdTe/CdS core/shell QDs since some compounds can quench CdTe/CdS core/shell

4 4 QDs. Even though different methods were used to deposit ZnS shell on the CdTe/CdS core/shell QDs, CdTe/CdS/ZnS core/shell/shell QDs still can be quenched. Furthermore, the CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs were transferred into aqueous phase, phosphate buffered saline or deionized water, by switching the hydrophilic ligands (thiol or PEG ligands). The thioglycolic acid (TGA)- capped CdTe/CdS core/shell QDs can be kept in aqueous phase with high fluorescence quantum yield (60% - 70%) for more than two months. However, some other compounds in organic or aqueous phase can quench CdTe/CdS QDs. Additionally, the stability of the different ligands capped CdTe/CdS QDs was tested by dialysis measurement, the hydrodynamic diameters of CdTe and CdTe/CdS core/shell QDs were measured by dynamic light scattering, and dissolving issue was found when CdTe and CdTe/CdS core/shell QDs were diluted in CHCl 3. We have characterized the CdTe core and the CdTe/CdS core/shell QDs by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and ICP-OES measurements. We have found that the CdTe core was of a zincblende structure, and the shell was a wurtzite structure. And the CdTe/CdS QDs were core/shell QDs instead of alloying QDs. We have also analyzed the photophysical properties of CdTe and CdTe/CdS core/shell QDs. Time-resolved photoluminescence (PL) measurements showed the emission decay lifetimes in the tens of nanoseconds. Additionally, ultrafast charge carrier relaxation dynamics of the CdTe core and CdTe/CdS core/shell QDs were studied by the femtosecond transient absorption (TA) spectroscopy. The transient absorption spectra of

5 5 CdTe and CdTe/CdS core/shell QDs showed multiple bleaches, which have been assigned to the 1S 3/2 (h)-1s(e), 2S 3/2 (h)-1s(e), and 1P 3/2 (h)-1p(e) transitions. The spectral shifts of these bleaches after shell deposition have been analyzed in the context of a quasi-type-ii carrier distribution in the core/shell samples, and interestingly the red shift was only contributed from the conduction band energy levels shifting to lower energy. In addition, the ultrafast evolution of these bleach features has been examined to extract electron cooling rates in these samples. A fast decay component in the 1S 3/2 (h)-1s(e) transition of the small CdTe QDs was discovered due to the hole being trapped by the defects on the surface of QD. Further, we have studied the PL quenching process of the air exposed CdTe QDs via the PL decay and transient absorption measurements. Oxygen was shown to cause strong PL quenching of the CdTe QDs. There was no significant difference of the PL decay lifetimes between the CdTe QDs under argon and air, but a fast decay lifetime of 2.6 ps was observed in transient absorption data, indicating that the quenching process happened in a very short time scale (~ 2.6 ps). Approved: Paul G. Van Patten Associate Professor of Chemistry and Biochemistry

6 6 Acknowledgments The first person I would like to acknowledge is my advisor Dr. P. Gregory Van Patten. Through his guidance I become a qualified Ph.D. of physical chemistry and know how to investigate the unknown world using scientific method. I thank his encouragement, support, and help in every aspect of my academic progress in the past four and half years. I thank my wife, Juan Ding, for her love and support for me all these years. Especially, after she graduated she still stayed with me waiting for me to finish my Ph.D. study. I appreciate it that she gave me a fantastic daughter, who brought me more happiness in my life. I thank my parents, Wenbin Yan and Jianhua Chen, for their love and understanding. In my whole life, they always tolerate my shortcomings and mistakes, and encourage me to realize my dream. Also I thank them for coming to help us take care of our daughter. They are my heroes. I thank my daughter, Sabrina Z. Yan, who makes my life more meaningful. Besides my advisor, the other two very important people, who taught me how to become a scientist, are Dr. Hugh Richardson and Dr. Jeffrey Rack. I never forget that they spent a lot of time to improve my oral presentation for ACS meetings, and gave me suggestions about my research. Dr. Hugh Richardson always tried his best to help me solve the problems. Dr. Jeffrey Rack was very generous to let me use his instruments. I thank my committee member, Dr. Eric Stinaff, for the time he spent on my proposal and dissertation.

7 7 I thank Dr. Martin Kordesch for letting me use his TEM instrument. I thank Dr. Frazier Nyasulu for his help and inviting me to join the Thanksgiving party with his family in my first year. I thank my roommate, Yuhuan Jin, for his understanding and friendliness. I thank my lab mates, Lei Wang and Chris McCleese, for their friendship. I thank the National Science Foundation (NSF) and the Condensed Matter and Surface Science (CMSS) programs at Ohio University for funding my research and research assistantship. I thank Ohio University for providing me with such a good environment to do research, and for funding my scholarship. Finally, I thank all the American people who helped me during the time in the U.S. Because of these people, I feel U.S. is like my second home town. Yueran Yan Nov. 27 th, 2011 Athens, OH

8 8 Table of Contents Page Abstract... 3 Acknowledgments... 6 List of Tables List of Figures Chapter 1 Introduction Quantum Dots Basics Electronic Structure in Semiconductor Quantum Dots Synthesis Methods of QDs Structural Characterization of QDs Photoluminescence Emission Spectroscopy Small-Angle X-Ray Scattering (SAXS) Transmission Electron Microscopy (TEM) Powder X-Ray Diffraction (XRD) ICP-OES Dynamic Light Scattering Dialysis Ligand Effects on QDs QDs as Photovoltaic Material in Solar Cell Application Research Motivation... 60

9 9 Chapter 2 Synthesis of CdTe Core, CdTe/CdS Core/Shell and CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Chemicals in Synthesis General Introduction of Synthesis of QDs Synthesis of CdTe Core Quantum Dots Synthesis of CdTe Core QDs by One Injection Synthesis of CdTe Quantum Dots by Two Injections Synthesis of CdTe Quantum Dots by Multi-Injection Synthesis of CdTe/CdS Core/Shell Quantum Dots by Slow Injection Shell Growth Method Deposition of CdS Shell by Slow Injection of CdS Precursors Characterization of CdTe/CdS Core/Shell Quantum Dots Synthesized by Slow Injection Shell Growth Method Nucleation of CdS Quantum Dots during Slow Injection Characterization of CdTe/CdS Core/Shell Quantum Dots with 1.55 ev (800 nm) Emission Syntheized by Slow Injection Shell Growth Method Synthesis of CdTe/CdS Core/Shell Quantum Dots by SILAR Shell Growth Method Purification of CdTe Cores Deposition of CdS Shell by SILAR Shell Growth Method Characterization of CdTe and CdTe/CdS Core/Shell Quantum Dots Synthesized by SILAR Method... 93

10 CdS Shell Growth by SILAR Method with CdOA, S Precursors Calculations for Precursor Injections in the SILAR-based Deposition of CdS Shell onto CdTe QD Cores Synthesis of CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots ZnS Shell Growth by Slow Injection with Zinc Oleate (ZnOA) and S Precursors ZnS Shell Growth by Slow Injection with Zinc Stearate and Thioacetamide Precursors ZnS Shell Growth by SILAR Method ZnS Shell Growth by Thermal-Cycling Coupled Single Precursor Method Characterization of CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Synthesized by Thermal-Cycling Coupled Single Precursor ZnS Shell Growth Method Calculation of the ZnS Shell on CdTe/CdS Thickness Chapter 3 Quantum Dots Water Transfer Chemicals Synthesis of PEG Ligands Synthesis of PEG1500 Ligands Synthesis of PEG2500 Ligands PEG Ligands Characterization by 1 H NMR CdTe/CdS Core/Shell QDs Water Transfer Organic-Ligand-Capped CdTe/CdS Core/Shell QDs Precipitation

11 CdTe/CdS Core/Shell QDs Water Transfer by Two Phase Extraction CdTe/CdS Core/Shell QDs Water Transfer by One Layer Transfer UV-visible Spectra and Photoluminescence Spectra of CdTe/CdS Core/Shell Quantum Dots in Aqueous Phase TGA-Capped CdTe/CdS Core/Shell QDs Precipitation CdTe/CdS/ZnS Core/Shell/Shell QDs Water Transfer CdTe/CdS/ZnS Core/Shell/Shell QDs Water Transfer with TGA Ligand CdTe/CdS/ZnS Core/Shell/Shell QDs Water Transfer with PEG Ligands Chapter 4 Ligand Effects on CdTe/CdS Core/Shell Quantum Dots Introduction Experimental Results and Discussion Ligand Effects on PL QY Stability of CdTe/CdS Core/Shell Quantum Dots in Organic or Aqueous Phase Ligand Effects on Hydrodynamic Diameter Spontaneous Disintegration of CdTe and CdTe/CdS Core/Shell Quantum Dots Conclusions Chapter 5 QD Electronic Structure and Dynamics Charge Carrier Relaxation Pathways Types of Core/Shell QDs

12 Experimental Probes of Electronic Structure and Dynamics Photoluminescence Decay Transient Absorption Spectroscopy Ultrafast Charge Carrier Dynamics of CdSe Quantum Dots by Femtosecond Transient Absorption Spectroscopy Laser Power Dependent Bleach Intensity Chapter 6 Ultrafast Exciton Dynamics in CdTe Quantum Dots and Core/Shell CdTe/CdS Quantum Dots Introduction Electronic States in Colloidal CdTe Quantum Dots Measurements Results Basic Characterization Photophysics Discussion Assignments of TA Spectral Features Charge Transfer Bleach Intraband Relaxation and Ultrafast Hole Trapping Effects of Air, Ligand Exchange, and Aqueous Solvent on CdTe/CdS NCs Conclusions Chapter 7 Spectroscopic Investigation of Oxygen Sensitivity in CdTe and CdTe/CdS Quantum Dots

13 Introduction Experimental Results and Discussion Conclusion Chapter 8 Emission Wavelength Shift of CdTe/CdS Core/Shell QDs by Temperature. 247 Chapter 9 Conclusions References Appendix A: Details of Synthesis of CdTe Core, CdTe/CdS Core/Shell and CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Appendix B: Details of Quantum Dots Water Transfer Appendix C: List of Publications Appendix D: Copyright Permission

14 14 List of Tables Table 1. Lattice constants and densities of CdS in different crystal structures Table 2. Lattice constants and densities of ZnS in different crystal structures Table 3. Solubility of the organic capped CdTe and CdTe/CdS core/shell QDs in solvents Table 4. Dialysis of CdTe/CdS core/shell QDs with different ligands Table 5. Disintegration of CdTe and CdTe/CdS core/shell QDs under the different conditions Table 6. Different parameters values in wavefunctions calculation Table 7. Lifetime of CdTe and CdTe/CdS QDs via transient absorption measurement.204 Table 8. Emission decay lifetime of CdTe and CdTe/CdS core/shell QDs Table 9. Multiexponential fit parameters (amplitudes and lifetimes) for argon-protected and air-exposed CdTe QDs Table 10. Emission wavelengths of CdTe/CdS core/shell QDs at different temperature by various methods Table 11. Synthesis of CdTe QDs by one injection. TDPA was applied as the ligands for the cadmium precursors. Exp.: Number of the different experiments of synthesis of CdTe QDs. ODE 1 : The solvent for the cadmium precursors. ODE 2 : The solvent for the tellurium precursor Table 12. Synthesis of CdTe QDs by one injection. Oleic acid was applied as the ligands for the cadmium precursors

15 15 Table 13. Synthesis of CdTe QDs by one injection. HDA was applied as the ligands for the cadmium precursors. And TOP was used as the ligands for the tellurium precursors Table 14. Synthesis of CdTe QDs by two injections Table 15. Synthesis of the CdS shell growing on the CdTe core by slow injection Table 16. Synthesis of the CdS shell growth on the CdTe core via SILAR method. The precursors used in the reaction were CdOA and S Table 17. Synthesis of the CdS shell growth on the CdTe core via SILAR method. The precursors used in the reaction were dimethylcadmium and TOPS Table 18. Synthesis of the ZnS shell growth on the CdTe/CdS core/shell QDs by slow injection method. The precursors used in the reaction were ZnOA and S Table 19. Synthesis of the ZnS shell growth on the CdTe/CdS core/shell QDs by slow injection method. The ligands used in the reaction were zinc stearate and thioacetamide Table 20. Synthesis of the ZnS shell growth on the CdTe/CdS core/shell QDs via SILAR method. The precursors used in the reaction were ZnOA and S Table 21. Synthesis of the ZnS shell growth on the CdTe/CdS core/shell QDs via TC-SP method. The precursor used in the reaction was Zn(DDTC) Table 22. CdTe QDs precipitation by different solvents Table 23. Precipitation Experiments of organic ligands capped CdTe/CdS core/shell QDs by different solvents Table 24. Extraction of CdTe QDs

16 16 Table 25. Water transfer of CdTe/CdS core/shell QDs via two layers transfer method.323 Table 26. Water transfer of CdTe/CdS core/shell QDs via one layer transfer method with different hydrophilic ligands Table 27. Water transfer of CdTe/CdS core/shell QDs with MPA and TMAH Table 28. Water transfer of CdTe/CdS core/shell QDs with PEG ligands Table 29. Water transfer of CdTe/CdS core/shell QDs with CTAB Table 30. Water transfer of CdTe/CdS core/shell QDs with MHA ligands Table 31. Hydrodynamic diameter of the CdTe/CdS core/shell QDs capped by different ligands Table 32. Water transfer of CdTe/CdS/ZnS core/shell/shell QDs with TGA Table 33. Water transfer of CdTe/CdS/ZnS core/shell/shell QDs with PEG

17 17 List of Figures Figure 1-1. (a) Band diagram of a direct-gap semiconductor. (b) Optical transitions in a finite-size semiconductor QD Figure 1-2. (a) Band diagram of the bulk semiconductor material, E g0 is the fixed band gap of the material. (b) Energy levels of semiconductor nanoparticles. There are three energy states shown in each of conduction band (1S e, 1P e, 1D e ) and valence band (1S h, 1P h, 1D h ). The energy gap between the conduction band and the valence band in the nanoparticle equals the sum of the band gap of the bulk semiconductor material and the confinement energy of the electron-hole pair Figure 1-3. (a) The valence band structure of diamondlike semiconductors. Because of spin-orbital coupling ( so ), the valence band is split into two bands at k = 0, J = 3/2 and J = 1/2. And away from k = 0, J = 3/2 band is split into J m = ±3/2 heavy hole (A or hh) and J m = ±1/2 light hole (B or lh). C is the split-off band (J = 1/2). (b) The valence band structure of semiconductors with wurtzite structure. The A and B bands are split by the hexagonal crystal field ( cf ) Figure 1-4. Normalized UV-visible and PL emission spectra of CdTe QDs. Black solid line: UV-visible spectrum and red dot line: emission spectrum. A-D represent the different transitions of CdTe QDs. PL excitation: 400 nm Figure 1-5. Scheme of the SAXS experiment Figure 1-6. Layout of some components in a basic TEM... 45

18 18 Figure 1-7. Scheme of Bragg s law. x-rays from the source are represented by 1 and 2 while the 1 and 2 are the diffraction by the lattice planes L1 and L2. D-spacing between two planes is d Figure 1-8. Scheme of the X-ray diffraction experiment Figure 1-9. Scheme of the ICP-OES experiment Figure Scheme of the dynamic light scattering experiment. The black circles in the aqueous solution are water molecules and the red circles are the particles in the solution Figure Scheme of the dialysis tubing diffusion process. The columns are full of water or aqueous solvent. The yellow bars are clips to seal two mouths of the dialysis tubings (blue squares), which contain the solution with two sizes of molecules, the red and the black circle. The solvent is the same as the one out of the dialysis tubing. (a) The beginning of the dialysis experiment. (b) The end of the experiment Figure 2-1. PL emission spectra of the CdTe QDs at different reaction times (1, 12, 30 and 85 minutes) after the first precursor injection. FWHM is full width at half maximum. The second precursor injection started at 30 minutes Figure 2-2. PL spectra of CdTe QDs. Time (30, 65, 90, and 120 minutes) are the reaction time from the first injection, and the second injection started after 30 minutes. Black triangle line is the PL spectrum of the CdTe QDs at 30 minutes before the second injection. The rest three different lines are the PL spectra of the

19 19 CdTe QDs during the second injection. The emission wavelengths of these samples at 30, 65, 90, and 120 minutes are 658, 730, 768, and 776 nm, respectively Figure 2-3. Normalized PL (dot lines) and absorption (solid lines) spectra of CdTe QDs by multi-injection method. The numbers on the side of these PL spectra represent the injection times Figure 2-4. Normalized absorption and PL emission spectra of four different size CdTe/CdS core/shell QDs. Blue solid line: UV-visible spectra of CdTe/CdS core/shell QDs; red solid line: PL emission spectra of CdTe/CdS core/shell QDs. The numbers on the top of the spectra are the diameter (D) and emission wavelength (λ) of the core for each core/shell QDs Figure 2-5. Normalized PL emission spectra of CdTe core and CdTe/CdS core/shell QDs. Black solid line: CdTe core QDs; red solid line: CdTe/CdS core/shell QDs. Size (diameter) of CdTe QDs is 3.5 nm; size (diameter) of CdTe/CdS core/shell QDs is 4.5 nm. Shift between emission photon energy of CdTe core and CdTe/CdS core/shell QDs is 296 mev Figure 2-6. Normalized PL emission spectra of CdTe core and CdTe/CdS core/shell QDs. Black solid line: CdTe core QDs; red solid line: CdTe/CdS core/shell QDs. Size (diameter) of CdTe QDs is 9 nm; size (diameter) of CdTe/CdS core/shell QDs is 10 nm. Shift between emission photon energy of CdTe core and CdTe/CdS core/shell QDs is 52 mev Figure 2-7. Normalized PL spectra of CdTe/CdS QDs by different reaction time. The arrows show the position of the small peak, which is due to CdS QDs

20 20 Figure 2-8. TEM image of CdTe/CdS and CdS QDs. Big particles are CdTe/CdS QDs; small particles are CdS QDs Figure 2-9. TEM image of CdTe/CdS QDs Figure Normalized UV-visible absorption and PL emission spectra of CdTe/CdS QDs emitting at 1.55 ev (800 nm). Black solid line: UV-visible spectrum; red solid line: emission spectrum Figure TEM image of CdTe/CdS QDs emitting at 1.55 ev (800 nm) Figure Normalized UV-visible absorption and PL emission spectra of CdTe and CdTe/CdS core/shell QDs by SILAR method. Solid lines are UV-visible spectra and dot lines are PL emission spectra. Black spectra: CdTe QDs; other spectra are CdTe/CdS core/shell QDs with 1 to 5 monolayer shell from bottom to top Figure Normalized UV-visible absorption spectra of CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs. Black line: CdTe/CdS QDs; red line: CdTe/CdS/ZnS nanocrytals with 1 ML ZnS; green line: CdTe/CdS/ZnS QDs with 2 ML ZnS. Inset: UV-visible absorption spectra of CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs in larger energy scale Figure PL emission spectra of CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs. Black line: CdTe/CdS QDs; red line: CdTe/CdS/ZnS QDs with 1 ML ZnS; green line: CdTe/CdS/ZnS QDs with 2 ML ZnS Figure TEM image of CdTe/CdS/ZnS core/shell/shell QDs Figure 3-1. Reaction scheme to produce trithiol-peg ligands

21 21 Figure H NMR spectra, (a) 1 H NMR spectrum of PEG acrylate, (b) 1 H NMR spectrum of tetrathiol, (c) 1 H NMR spectrum of PEG ligands. The solvent is deuterated chloroform. Insets are the molecular structures Figure H NMR spectra, (a) 1 H NMR spectrum of 6 months aged PEG ligands in deuterated chloroform, (b) 1 H NMR spectrum of 6 months aged PEG ligands in D 2 O. Insets are the molecular structures Figure 3-4. UV-visible absorption spectra of the CdTe/CdS core/shell QDs in CHCl 3 (black solid line) and PBS (red solid line) Figure 3-5. Normalized PL emission spectra of CdTe/CdS core/shell QDs in CHCl 3 (black solid line) and PBS (red solid line) Figure 4-1. PL quantum yield of the CdTe/CdS core/shell QDs with three kinds of the ligands. The left bar represents OA- and TBP-capped CdTe/CdS core/shell QDs in CHCl 3, the middle one represents TGA-capped CdTe/CdS core/shell QDs in PBS, and the right one represents PEG-ligand-capped CdTe/CdS core/shell QDs in PBS Figure 4-2. PL emission intensities of the CdTe/CdS core/shell QDs mixed with different quenchers in CHCl 3. The left bar represents pure CdTe/CdS core/shell QDs solution without any quenchers. Other bars with the error represent the emission intensities of the CdTe/CdS core/shell QDs and quencher mixture Figure 4-3. PL quantum yield of the CdTe/CdS QDs with different surface ligands in various solvents versus time in days. Black square: TDPA-, HDA-, TOP-capped CdTe/CdS core/shell QDs in CHCl 3, red circle: TGA-capped CdTe/CdS core/shell

22 22 QDs in ph = 11.5 buffer; green triangle: TDPA-, HDA-, TOP-capped CdTe/CdS core/shell QDs in hexane; blue triangle: PEG-ligand-capped CdTe/CdS core/shell QDs in PBS Figure 4-4. PL quantum yield of the PEG-ligand-capped CdTe/CdS core/shell QDs versus exposure time under a xenon arc lamp (64 W) Figure 4-5. UV-visible absorption spectra of CdTe/CdS core/shell QDs in CHCl 3. Black line: absorption spectrum of the fresh CdTe/CdS core/shell QDs solution without extra TBP; red line: absorption spectrum of the CdTe/CdS core/shell QDs solution with extra TBP after one day Figure 4-6. UV-visible absorption spectra of the CdTe/CdS core/shell QDs under different conditions. Black line: absorption spectrum of the fresh washed QDs; red line: absorption spectrum of the washed QDs in CHCl 3 after 6 days under the light; green line: absorption spectrum of the washed QDs in toluene under the light after 8 days; blue line: absorption spectrum of the washed QDs in CHCl 3 in the dark after 8 days; cyan line: absorption spectrum of the washed QDs in CHCl3 after 8 days under light Figure 5-1. Scheme of the Auger type e-h energy transfer. (a) The electron (black circle) is at the 1P state of the conduction band (CB), while the hole (white circle) is at the 1S state of the valence band (VB). The down solid arrow shows the relaxation direction of the electron, and the dash arrow presents the Auger type e-h energy transfer from the electron to the hole. (b) Energy state diagram of the exciton after the Auger type e-h energy transfer

23 23 Figure 5-2. Scheme of the hot electron intra-band relaxation process affected by the suface Cd ion bond ligands in the CdSe QDs. The energy state diagram on the right side belongs to the ligand-surface Cd orbital (adapted from Guyot-Sionnest et al.) Figure 5-3. Scheme of the relaxation pathways of the hot electron (upper) and hole (lower) Figure 5-4. Energy state diagram of CdSe QDs shows surface trapping of conduction band electrons and valence band holes Figure 5-5. Band structure diagrams of core/shell QDs. (a) Band structure of type I core/shell QDs, black circle: electron, white circle: hole. (b) and (c) Band structure of type II core/shell QDs Figure 5-6. Band energy diagram of CdTe/CdS core/shell QDs. The CdTe and CdS materials have different dielectric constant, ε 1 and ε 2, respectively. The dielectric constant of the surrounding medium is ε 3. R is the radius of the CdTe core and the H is the height of the CdS shell. The vertical dash lines show the edges of the core and the shell. The top band is the conduction band (CB), and the bottom one is the valence band (VB). In the conduction band, the black circle represents the electron, e and U 0 is the conduction band offset energy between the core and the shell. The white circle in the valence band represents the hole, and U h 0 is the valence band offset energy between the core and shell. E g, E g,c, and E g,sh are the indirect band gap of the CdTe/CdS core/shell QDs, the band gap of the CdTe core, and the band gap of the CdS shell, respectively. The energy of the bulk conduction band edge of the shell

24 24 is set as zero (E e = 0) and the up arrow shows the direction of the electron energy increasing, while the energy of the bulk valence band edge of the core is set as zero (E h = 0) and the down arrow shows the direction of the hole energy increasing. (Adapted from Piryatinski, A.; Ivanov, S. A.; Tretiak, S.; Klimov, V. I., Effect of quantum and dielectric confinement on the exciton-exciton interaction energy in type II core/shell semiconductor nanocrystals. Nano Letters 2007, 7, (1), ) Figure 5-7. (a) Wavefunction of CdTe/CdS core/shell QDs. (b) Radial probability density of CdTe/CdS core/shell QDs. The dash line represents the edge of the core and shell at r = 2 nm Figure 5-8. Scheme of the time-resolved photoluminescence decay experiment Figure 5-9. UV-visible absorption spectrum of CdSe QDs, the different features in the spectrum were assigned to 1S(e)-1S 3/2 (h), 1S(e)-2S 3/2 (h), and 1P(e)-1P 3/2 (h) transition. (The UV-visible absorption data was collected by Chris McCleese.) Figure Energy level diagram of CdSe QDs (right two) and bulk semiconductor (left). The red arrow represents 1S (e) 1S 3/2 (h), the blue arrow represents 1S (e) 2S 3/2 (h), and the green arrow represents 1P (e) 1 P 3/2 (h). E g0 and E gqd (R) represent the band gap energy of the bulk semiconductor and QD, respectively Figure (a) TA spectra of CdSe QDs recorded at 0.5, 1, and 2 ps after excitation (without chirp correction) in comparison to the UV-visible absorption spectrum. (b) Normalized TA dynamics at the positions of the B 1 (solid line), and A 1 (dashed line)

25 25 features. (These TA data were from Dr. Van Patten, and the CdSe QDs were synthesized by Chris McCleese.) Figure Plot of B1 bleach intensity in CdTe/CdS core/shell QDs versus pump fluence. Pump laser wavelength was 400 nm. Inset: data points are within 50 µw laser power Figure 6-1. (a) Size-dependent energy levels of seven CdTe NCs, shown in different colors. Straight lines overlie the data to illustrate the correspondence between different samples. (b) Dark line: size-dependent energy levels replotted with the excitation energy of the states in relation to the first dark excited state versus the PL energy. Red line: effective mass calculations published by Efros et al. 207 (Reproduced from Zhong, H.; Nagy, M.; Jones, M.; Scholes, G. D., Electronic States and Exciton Fine Structure in Colloidal CdTe Nanocrystals. Journal of Physical Chemistry C 2009, 113, (24), Used with permission) Figure 6-2. Nomalized UV-visible absorption and PL emission spectra of (a) small CdTe core QDs, (b) small CdTe/CdS core/shell QDs, (c) large CdTe core QDs, and (d) large CdTe/CdS core/shell QDs. Solid lines are the UV-visible absorption spectra; dot lines are the PL emission spectra. PL quantum yield (Φ PL ) is also reported for each sample Figure 6-3. TEM images of (a) CdTe core and (b) CdTe/CdS core/shell QDs Figure 6-4. Measured powder diffraction patterns from CdTe core (bottom) and CdTe/CdS core/shell (top) QDs. Vertical bars at top and bottom of the figure show

26 26 the patterns for zincblende CdTe (blue dotted lines, PDF ) and wurtzite CdS (red solid lines, PDF ), respectively Figure 6-5. Photoluminescence emission decays of CdTe cores (black solid lines) and CdTe/CdS core/shell QDs (red squares). The left panel depicts decay traces from small core and core/shell QDs and the right panel depicts decay traces from large core and core/shell NCs Figure 6-6. Transient absorption spectra at three different pump-probe delays in (a) small CdTe core, (b) small CdTe/CdS core/shell, (c) large CdTe core, and (d) large CdTe/CdS core/shell QDs. Pump-probe delays were 0.2 ps (red squares), 0.5 ps (green triangles), and 2.0 ps (blue circles) Figure 6-7. Transient absorption kinetics at the B1 bleach (1S 3/2 (h)-1s(e)) position of four CdTe and CdTe/CdS QD samples. Inset shows the extended decay, out to 3 ns delay. Black, red, blue, and green curves represent small core, large core, large core/shell and small core/shell, respectively Figure 6-8. Early TA dynamics of the three principal bleaches in small CdTe cores Figure 6-9. Early TA dynamics of the three principal bleaches in large CdTe cores Figure (A) Transient absorption spectra of large core/shell QDs in PBS at three different pump-probe delays. (B) Kinetic traces at the position of the B1 bleach for large core/shell QDs in chloroform (black curve, triangles) and in PBS (red curve, circles) Figure Energy level diagram showing the lowest two levels for conduction band electrons and lowest three levels for valence band holes in CdTe (left) and

27 27 CdTe/CdS core/shell QDs (right). Dashed lines represent the carrier potential energies of the conduction and valence band due to the CdTe and CdS lattices. When the CdS shell is added, the confinement energy of conduction band electrons decreases as these carriers delocalize throughout the core and shell. Valence band holes remain confined to the core due to the large valence band offset between the two materials. Vertical arrows represent the transitions, B1 (blue), B2 (red), and B3 (green) that are discussed in the transient absorption spectra Figure Energy spacing, E, between 1S 3/2 and 2S 3/2 valence band hole levels plotted versus band gap (measured as 1S 3/2 (h) 1S(e) energy) for CdTe cores (blue circles) and CdTe/CdS core/shell QDs (magenta stars). The black line shows the relationship between E and lowest-energy PL excitation peak determined experimentally by Zhong et al for CdTe core NCs Figure 7-1. (a) UV-Vis spectra of the CdTe QDs under argon (solid black curve) and after brief air exposure (red dashed curve). (b) PL emission spectra of the CdTe QDs under argon (solid black curve) and after brief air exposure (red dashed curve). PL emission continued to decrease futher with continued exposure to air, until the quenching reached 100% Figure 7-2. Normalized time-resolved emission decay of the CdTe QDs under argon and the air Figure 7-3. TA kinetic traces of (a) 4-nm CdTe QDs and (b) 3-nm CdTe QDs under argon (black triangles) and after exposure to air (red circles). The inset shows the same data over a longer pump probe delay time (up to 150 ps)

28 28 Figure 7-4. TA kinetic traces of CdTe/CdS core/shell QDs under argon (black triangles) and after exposure to air (red circles). The inset shows the same data over a longer pump-probe delay time (up to 3 ns). These core/shell QDs were prepared from 4 nm CdTe cores similar to those represented in Figure 7-3(a) Figure 7-5. Comparisons of TA kinetic traces from CdTe cores (black triangles) and CdTe/CdS core/shell QDs (red circles) under (a) air and (b) argon Figure 8-1. Normalized PL emission spectra of the CdTe/CdS core QDs at different temperatures

29 29 Chapter 1 Introduction 1.1 Quantum Dots Basics In bulk semiconductor materials, the band gap that separates the conduction band from the valence band is a fixed parameter. That means a certain kind of material only has one band gap value in macroscopic size. However this situation is changed when the material size is below a certain critical value, which for typical semiconductor particles is smaller than about 10 nm. The band gap energy of semiconductor nanoparticles changes with nanoparticle size. For example, larger nanoparticles have narrower band gap than smaller ones. A nanoparticle, that the exciton is confined in in all three spatial dimensions, is referred to as quantum dot (QD). 1 In the past two decades, luminescent semiconductor quantum dots (QDs) have attracted attention due to their excellent properties such as narrow distribution of the emission spectra, size-tunable emission wavelength, better photo-stability and stronger fluorescence than other traditional fluorescent dyes. 2, 3 Accordingly, QDs have been used in a variety of applications including bio-labeling, 3, 4 solar cells, 5, 6 light-emitting diodes (LED), 7-9 and lasers. 10 Compared with their bulk counterparts, QDs have size- dependent electrical, magnetic, optical and chemical properties. The continuous fluorescent emission from QDs is also a very important size-dependent characteristic of QDs 11. QDs with different sizes of the same material can supply emission light with different colors. Once the size of QDs decreases, the energy gap of it increases and the emission wavelength shifts to blue.

30 Electronic Structure in Semiconductor Quantum Dots Researchers did many studies on the electronic structure in these nanoparticles to fully understand all the special properties of the QDs. Within these studies the most common tools for the experimentalist are optical spectroscopies, such as UV-visible absorption, photoluminescence emission spectroscopy, and transient absorption spectroscopy. Once bulk material absorbs a photon, an electron is excited directly from the valence band into the conduction band. Thus an electron-hole pair is created, as presented in Figure 1-1(a). In bulk materials the energy levels in the bands are continua. Nevertheless, when the size of the semiconductor structure is less than the exciton Bohr radius, the charge carriers are confined in the nanoparticle by the boundaries of the material. Figure 1-1(b) shows the optical transitions in a semiconductor QD under the quantum size effect. In the nanoscale, the optical behavior of a QD is like a single atom since the bands are quantized. Thus the absorption of a single QD becomes a series of the discrete electronic transitions instead of a continuum spectrum, and normally the band gap of QDs is higher than that of the bulk semiconductor.

31 31 (a) (b) Figure 1-1. (a) Band diagram of a direct-gap semiconductor. (b) Optical transitions in a finite-size semiconductor QD. As stated above, the quantum size effect occurs when the size of the QD is smaller than the exciton Bohr radius. Generally the Bohr radius of a particle determined by a B m = ε a * 0, (1.1) m where a B is the Bohr radius of the particle, ε is the dielectric constant of the material, m is the rest mass of the electron, * m is the mass of the particle, and a 0 is the Bohr

32 radius of the hydrogen atom. 12 There are three different Bohr radii for electron ( a e ), hole ( a h ) and electron-hole pair or exciton ( a exc ). Usually if the nanoparticle radius, a < a e, a h and a exc, then both electron and hole are strongly confined by the boundary of the nanoparticle. This situation is called strong confinement regime. If a e, a h < a < a exc, the only confined part is the center-of-mass motion of the exciton. It is referred to as the weak confinement regime. In the end, if a h < a < a e, a exc, the electron is strongly confined and the hole is not. This phenomenon is called intermediate confinement regime. To understand size-dependent electronic properties quantitatively, a quantum mechanical calculation needs to be introduced. It starts from a simple model: the particle in a sphere model. The boundary condition is the particle is only allowed to exist in the sphere. The potential energy out of the sphere is very high (assumed infinite) as shown in equation (1.2) 32 0 r < a V( r) =, (1.2) r > a where V (r) is the potential energy of the particle, a is the radius of the particle, and r is the distance between the particle and the center of the sphere. The Schrödinger equation is H ψ = Eψ, (1.3) where H is the Hamiltonian operator, ψ is the eigenfunction, and E is the eigenvalue. Since the potential energy is zero inside the sphere, the equation (1.3) only describes the motion of this particle in the sphere. Then the Hamiltonian operator is

33 33 H = + T V 2 = h 2m 2, (1.4) where T is the kinetic energy operator, h is h, h is the Planck constant, m is the 2π mass of the particle, and functions 2 is the Laplace operator. Therefore the Schrödinger equation becomes 2 h 2 ψ = Eψ. (1.5) 2m By using polar coordinates, the Schrödinger equation can be solved yielding wave m jl ( kn, lr) Yl ( θ, φ) Φ n, l, m ( r, θ, φ) = C, (1.6) r where C is a normalization constant, j ( k r l n, l ) is the l th-order spherical Bessel function, m and Y ( θ, φ) is a spherical harmonic. Within this equation, l k n, l α n, l =, (1.7) a where α n, l is the n th zero of j l. Hence the energy of the particle is given by E n, l h k, l h α n, l = n =. (1.8) 2 2m 2m a 0 0 In these equations, n is the principal quantum number ( n = 1, 2, 3, ), l is the quantum number of the angular momentum ( l = 0, 1, 2,, n -1, or represented by S, P, D, ), and the m 0 is the mass of the particle. From the equation (1.8), it shows the energy of the particle is proportional to 1/ 2 a, which explains the size dependent

34 properties of QD quantitatively. Then the energy equations of the electron and the hole in QD are given as 34 E e n, l 2 h k =, (1.9) 2m 2 n, l e eff E h n, l 2 h k =, (1.10) 2m 2 n, l h eff where e m eff and h m eff are the effective masses of the electron and hole, respectively. These atomic-like nanoparticles have similar properties as atoms in that the energy of the nanoparticles is discrete. Different from the bulk semiconductor material, the energy gap of QD between the conduction band and the valence band is not fixed, and it is the sum of the band gap of the bulk semiconductor and the confinement energy of the electron and the hole (Figure 1-2), which is represented by E gqd = E g0 + E n,l (E n,l = E, + e n l E, ). h n l The different energy states in the conduction band and the valence band are represented by 1S, 1P, and 1D following the energy level increasing. By equation (1.8), the energy gap between two adjacent energy states ( n 1, l 1 and n 2, l 2 ) in one band is given by E h = 2m n1, l1 n 2, l 2 ( α 2 n2, l 2 α n1, l1 eff a ), (1.11) 2 that shows the energy difference between intra-band states also depends on 1/ a. On the other hand, the effective masses of the electron and hole also affect the energy difference between intra-band states. For instance, in the CdTe QDs, the electron effective mass is e smaller than that of the hole ( m eff < m h eff ) so that the energy of the electron is larger than e that of the hole ( E, > E, ), with the same principal and angular momentum quantum n l h l n

35 numbers. Therefore the intra-band energy spacings in the conduction band are wider than those of the valence band. 35 (a) (b) Figure 1-2. (a) Band diagram of the bulk semiconductor material, E g0 is the fixed band gap of the material. (b) Energy levels of semiconductor nanoparticles. There are three energy states shown in each of conduction band (1S e, 1P e, 1D e ) and valence band (1S h, 1P h, 1D h ). The energy gap between the conduction band and the valence band in the nanoparticle equals the sum of the band gap of the bulk semiconductor material and the confinement energy of the electron-hole pair.

36 36 In Figure 1-1, the conduction band and the valence band of the semiconductor are approximated by using two simple bands. However the real band structure of II VI, III-V QDs is more complicated. For example, in II VI QDs, such as CdSe, the conduction band is from the linear combination of 5s orbitals of Cd (twofold degenerate), while the valence band arises from the linear combination of 4p orbitals of Se (sixfold degenerate). This sixfold degeneracy causes the substructure in the valence band. In the conduction band, the total angular momentum quantum number is J = 1/2 (J = l + s), where the l and the s are the orbital and spin contribution to the angular momentum. And in the valence band, the total angular momentum quantum number is J = 3/2 (J = l + s) or J =1/2 (J = l + s - 1). Furthermore, in the valence band of the diamondlike semiconductors, the band is split into p 3/2 and p 1/2 sub-bands because of strong spin-orbit coupling ( so ), as shown in Figure 1-3 (a). Away from k = 0, the p 3/2 band is further split into two bands, J = ±3/2 and J = ±1/2, where J is the projection of J along the z-direction. Also J = ±3/2, J = ±1/2, J = 1/2 bands are referred to the heavy hole (A or hh), the light hole (B or lh), and the splitoff band (C or so), respectively. In the wurtzite structure (hexagonal) semiconductors, because the diamond structure is more symmetric than the wurtzite structure along the c axis, the A, B bands are split by the crystal field ( cf ) at k = 0.

37 37 (a) (b) Figure 1-3. (a) The valence band structure of diamondlike semiconductors. Because of spin-orbital coupling ( so ), the valence band is split into two bands at k = 0, J = 3/2 and J = 1/2. And away from k = 0, J = 3/2 band is split into J m = ±3/2 heavy hole (A or hh) and J m = ±1/2 light hole (B or lh). C is the split-off band (J = 1/2). (b) The valence band structure of semiconductors with wurtzite structure. The A and B bands are split by the hexagonal crystal field ( cf ). Figure 1-4 shows the normalized UV-visible absorption (black solid line) and PL emission (red dot line) spectra of the CdTe QDs in CHCl 3, with predominant size (diameter) 3.5 nm, which had 2.15 ev band gap energy (577 nm). The sharp peak of absorption spectrum, A, at 2.15 ev is the 1S 3/2 (h) 1S(e) energy state, which is the band edge energy of the CdTe QDs. The other absorption features, B-D, are the higher energy states of the CdTe QDs. For the PL measurement, the sample was excited at 400 nm, and the emission peak at 2.09 ev (592 nm) had a small red shift (0.06 ev) from the

38 38 absorption peak A because of Stokes shift. Moreover, the FWHM of the emission of the CdTe QDs was 0.13 ev, which depends on the size distribution of the CdTe QDs. FWHM of the emission energy is from 2.02 ev (614 nm) to 2.15 ev (577 nm). The FWHM of the emission can be used to determine the size distribution of QDs. For example, when the FWHM of the emission wavelengths ( nm) was used to calculated the size of the CdTe QDs by the empirical fitting functions reported by Yu et al., 13 the diameter range of the CdTe QDs ( nm) can be estimated. It needs to be noticed that the empirical fitting functions are built on the absorption of CdTe QDs and the emission spectrum of CdTe QDs has a Stokes shift from the absorption spectrum so that the size deviation (0.31 nm) estimated by FWHM of the emission spectrum is less accurate than the estimation by the absorption spectrum. However, in the absorption spectrum of QDs, it is very difficult to ensure the FWHM of peak A (band gap) since the higher energy side is affected by other energy state. In this case the size deviation of the CdTe QDs is 8.6% estimated by FWHM of the emission spectrum (in Figure 1-4).

39 39 Figure 1-4. Normalized UV-visible and PL emission spectra of CdTe QDs. Black solid line: UV-visible spectrum and red dot line: emission spectrum. A-D represent the different transitions of CdTe QDs. PL excitation: 400 nm. 1.3 Synthesis Methods of QDs There are two distinct routes to synthesize QDs: one is the physical route and the other one is chemical route. In the physical route, QDs are synthesized by the lithographic 14, 15 or the molecular beam techniques. In the chemical route, QDs are

40 40 made by colloidal chemistry in a solvent medium. Compared to the physical approach, it is easy to synthesize the uniform QDs and control the particle size by the chemical method. In this work, we chose chemical approach (colloidal synthesis route), a method to synthesize semiconductor QDs by precursors dissolved in solution, to produce QDs. This approach is similar to traditional chemical reactions. The nucleation occurs when the different precursors (such as Cd, Se precursors) are mixed in the flask at a sufficiently high temperature. The QDs start growing and in the meantime the concentration of the precursors decreases. When the concentration of the precursors is lower than the critical concentration of QDs nucleation, the particles are still growing but there are no new nuclei formed. If the monomers are further depleted, Ostwald ripening occurs, in which case smaller particles dissociate, releasing monomers that integrate into the bigger particles. Eventually, big QDs form and there are fewer particles in the solution. Colloidal approach can be used to produce many different kinds of QDs, such as CdSe, 20, 21 CdS, 22 CdTe 23 and InAs Structural Characterization of QDs Photoluminescence Emission Spectroscopy PL emission spectroscopy is a complementary method to the UV-visible absorption spectroscopy. It measures the PL emission when a QD relaxes from the excited state to the ground state. A sample is excited by light with a certain wavelength, and the emitted photons are collected by the detector. Typically, QDs have wide absorption spectrum in wavelength but narrow emission spectrum.

41 41 The QD sample was diluted with solvent and placed in a cuvette. PL spectra were obtained using a model C-60 spectrofluorometer from Photon Technology International (PTI). Light from a 75 W xenon arc lamp was sent through a monochromator to produce the excitation beam. Luminescence from the sample was collected and focused through a double grating monochromator and then detected with a photomultiplier tube (PMT) (Hamamatsu R928) in photon counting mode. Emission spectra were corrected for the wavelength-dependent instrument response Small-Angle X-Ray Scattering (SAXS) Small-angle x-ray scattering is a scattering of x-ray technique used to investigate an inhomogeneous sample in the nm scale, and finally is recorded at very low angles ( degrees). SAXS data gives the information about the shape and size of the particles or macromolecules, which are between 5 and 25 nm size range. Therefore this method is often applied to characterize materials such as proteins or QDs. For the QDs, the averaged particle sizes, shapes, distribution, and surface-to-volume ratio can be determined by SAXS. 25 The scheme of the SAXS experiment is shown in Figure 1-5 that contains x-ray source, slit, block collimation system, sample, primary beam stop and scattering pattern. Most x-ray sources generate divergent beams that cause difficulty in the scattering measurements. Hence the block collimation system is employed to focus the x-rays in SAXS instrument. The slit merely controls the intensity of the x-rays but it does not focus the beam. These collimation systems roughly can be divided into two groups, pointcollimation and line-collimation systems. Only the x-rays that have the right direction can

42 42 travel through the collimation system otherwise the x-rays are blocked. The focused x-ray beam hit the sample to produce the scattering light. In order to detect the scattering pattern, the primary x-ray beam must be blocked by a stop, because compared with the primary x-ray beam, the intensity of the scattering light is very weak. Finally a scattering pattern is obtained by the detector. These SAXS patterns can be described as a scattered intensity function (I(q)) of the magnitude of the scattering vector q = 4πsin(θ)/λ. The half angle between the incident x-ray beam and the measuring position is θ (in Figure 1-5), λ is the x-ray s wavelength. In the two phase sample, for example, the CdTe QDs in toluene, the resolution of the measurement depends on the difference in the average electron density ( ρ) between the CdTe QDs and toluene, and the scattering amplitude and scattering intensity are proportional to ρ and ρ 2, respectively. If ρ = 0, there is nothing that can be seen by the x-rays, because the solution is a homogeneous electron density continuum, and the x- ray scattering in any direction is disappears. Through different calculations, the size and the shape of the particles can be obtained by the SAXS scattered intensity function.

43 43 Figure 1-5. Scheme of the SAXS experiment Transmission Electron Microscopy (TEM) Transmission electron microscopy uses a beam of electrons transmitted through an ultrathin specimen, such as QDs deposited on a carbon film grid. When the electrons are transmitted through the specimen, the interaction between the sample and the electrons forms an image. TEM has very high resolution, which is more than 1,000 times higher than light microscopes, so TEM is employed in various applications, such as material science, biology, nanotechnology and semiconductor researches. For QDs, TEM is a powerful tool to explore the shape and size of QDs. Some components in a basic TEM are shown below in Figure 1-6. The electron source may be a tungsten filament or a lanthanum hexaboride (LaB 6 ) source. By connecting a high voltage source (typically kv), the electron source can generate electrons. This electron stream is focused into a small, coherent beam by the first and second condenser lenses. The spot size of the electron beam usually is adjusted by the

44 44 first condenser lens. Then the beam is constrained by the condenser aperture by filtering some high angle electrons that are far from the optic axis. After the beam strikes the sample, some electrons are transmitted. These transmitted electrons are focused by the objective lens to make an image. The objective aperture improves the contrast by excluding high angle diffracted electrons. The image then is enlarged by passing the column through the intermediate and projector lenses. Finally, the electron beam strikes a phosphor screen to generate light, and the image can be seen on the screen. In an image the dark areas are the places that fewer electrons are transmitted, while the light areas are the places that more electrons are transmitted. The thickness of the sample therefore can be estimated by the darkness of the TEM image.

45 Figure 1-6. Layout of some components in a basic TEM. 45

46 Powder X-Ray Diffraction (XRD) X-ray diffraction measurement is applied to unravel the ordering of atoms in a crystal. A beam of x-rays is driven to strike a crystal and the diffracted x-rays are collected by the detector by different directions. A crystal sample is gradually rotated while being hit by x-rays, and the detector records the intensity of the signal by the different angles to produce a diffraction pattern. Each crystal structure can be distinguished by its certain diffraction pattern. Hence this method has been widely use in determining the crystal structure of QDs. However the resolution of XRD for QDs is poor because the size of the QDs is too small. Once the x-rays travel through the lattice planes, some x-rays are reflected by the plane, and the angle of reflection equals the angle of incidence (Figure 1-7). The rest of the x-rays transmit, and finally are reflected by a series lattice planes. Figure 1-7 shows the situation of two x-ray beams traveling through two lattice planes. These two beams 1 and 2 are reflected by the planes L1 and L2, respectively. The path of beam 2 is longer than beam 1 because of the d-spacing. If the extra distance xyz equals to a whole number of x-rays wavelengths, then the waves in beams 1 and 2 interfere constructively at the detector. It can be seen that xy = yz = dsinθ, and xyz = 2dsinθ. Since xyz = nλ,

47 47 we can show that 2dsinθ = nλ. (1.18) Equation (1.18) is Bragg s law. Using this equation, the diffraction angles for the different lattice planes under a certain x-ray source are obtained. Figure 1-7. Scheme of Bragg s law. x-rays from the source are represented by 1 and 2 while the 1 and 2 are the diffraction by the lattice planes L1 and L2. D-spacing between two planes is d. Figure 1-8 illustrates the basic x-ray diffraction experiment, which has x-rays source, sample, and detector. x-rays source depending on the different target materials (Cu, Cr, Co et al.) generates the x-rays in various wavelengths. Usually the sample is a single crystal, a solid piece, or a powder. The detector is a radiation counter or photographic film. During the experiment x-rays source, sample and detector move around within an angle range to collect the diffraction signal from the sample.

48 48 Figure 1-8. Scheme of the X-ray diffraction experiment ICP-OES Inductively coupled plasma optical emission spectrometry (ICP-OES) is a method to analyze the concentration of the elements in a sample. The excited atoms or ions are produced by the inductively coupled plasma, and then the specific elements are determined by analyzing the electromagnetic radiation emission. The intensity of the emission is proportional to the concentration of the element, and the exact concentration is obtained by comparison to a set of standard solutions. The amounts of the elements of QDs can be determined through the ICP-OES method.

49 49 Figure 1-9 illustrates the ICP-OES experiment, which is composed with two parts, ICP and the optical spectroscopy. In the ICP part, the plasma is created by argon gas ionized in an intense electromagnetic field that is generated in a coil controlled by a radio frequency generator. A sample (aqueous or organic) is delivered into a nebulizer by a peristaltic pump, and is pushed up directly to the plasma flame. The sample is crashed into charged ions by the electrons and charged ions in the plasma. The radiation at the characteristic wavelengths of the elements is given by the atoms losing electrons and recombining in the plasma. The elements can be distinguished by the emission wavelengths and the concentration of each element is determined by the emission intensity. The emission light from the plasma is transferred to the spectrometer and separated into different wavelengths. The light falls on a charge coupled device (CCD), which is a kind of semiconductor photodetector. Hence the light signal is transformed to an electronic signal, and then the signal is processed by a computer, which shows either the emission spectra of the elements or the exact concentrations of the elements on the screen.

50 50 Figure 1-9. Scheme of the ICP-OES experiment. For elemental analysis, QDs were removed from the glovebox and purified in air by precipitating with CHCl 3 and MeOH (volume ratio = 1:1). The sample was centrifuged, the clear supernatant was discarded, and then the QDs were re-dissolved in CHCl 3. This process was repeated twice more, but after the final precipitation, the QDs were dissolved in 5% HNO 3. Atomic spectroscopy calibration standards (Cd, Te, and S) were diluted in deionized water at several concentrations (2ppm, 4ppm, 5ppm, 8ppm, and 15ppm for Cd and Te; 0.2ppm, 0.4ppm, 0.8ppm, 1ppm, and 2ppm for S) to produce a calibration curve. The amount of Cd, Te and S elements were measured using a CCDbased, inductively-coupled-plasma optical emission spectrometer (Varian Vista-MPX).

51 Dynamic Light Scattering Dynamic light scattering is a technique that is applied to measure the hydrodynamic diameter of the small particles in solution. 26 Even though TEM measurement can also be used to investigate the size of QDs, it is only for the nanoparticles excluding the surface environment such as ligands and solvent. The hydrodynamic diameter is important for the biology application, because the total size of QD does not only depend on the diameter of particles but also ligands or solvent. For example, the diameter of the CdSe QDs is only 5 nm; however, the hydrodynamic diameter of the CdSe QDs can be 10 or 100 nm, which depends on the ligands capped on the surface of them and on the degree of aggregation. Since dynamic light scattering provides measurements in solution, it can also give information about aggregation, which can not usually be determined from TEM images. Therefore the study of the hydrodynamic diameter of the nanoparticles is crucial. Figure 1-10 illustrates the scheme of the dynamic light scattering experiment for different sizes particles. Once the laser hits the particles, the light scatters in different direction and coherently. Hence a time-resolved scattering intensity is obtained (Figure 1-10), and the small particles fluctuate fast while the large particles have a slow fluctuation. The different fluctuation between the small and the large particles is due to the distance between the particles changed by Brownian motion. The large particles move slower than the small ones so that the fluctuation frequency of the large ones is lower than the small ones.

52 52 One way that solves the dynamic information of particles movement in aqueous solution by Brownian motion is called dynamic light scattering (quasi-elastic laser light scattering). The dynamic information is from the autocorrelation of the intensity trace recorded in the experiment. And the second order autocorrelation function is given by g 2 I( t) I( t + τ ) ( q; τ ) = (1.19) 2 I( t) where the I is the intensity, t is the time, and g 2 ( q; τ ) is the second order autocorrelation function with the parameters q, a particular wave vector, and τ, the delay time. From the equation (1.19), in the short time scale, the correlation is very high because the particles have not moved much so that g 2 ( q; τ ) almost equals to 1. However, as time increases, the correlation becomes lower and lower, and finally there is no correlation between the scattered light intensity of the beginning and the end. The first order autocorrelation function, g 1 ( q; τ ), can be obtained by 2 1 g ( q; τ ) 1 g ( q; τ ) = (1.20) β where β is a correction factor of the geometry and alignment of the laser beam. If 1 g ( q; τ ) is fit by the single exponential function, the first order autocorrelation function is shown as 1 g ( q; τ ) = exp( Γτ ) where Γ is the decay rate. Then Γ = q 2 D t

53 where D t is the translational diffusion coefficient, and the wave vector q is shown in equation (1.21). 53 4πn 0 θ q = sin( ) (1.21) λ 2 where n 0 is the refractive index, λ is the wavelength of the incident laser, and θ is the angle of the detector located with the respect to the incident beam. The hydrodynamic diameter of the particles are calculated by the translational diffusion coefficient D t following the Stokes-Einstein equation (1.22), D t k BT = (1.22) 6πηr where k B is Boltzmann s constant, T is the absolute temperature, η is viscosity, and r is the radius of the spherical particle.

54 54 Figure Scheme of the dynamic light scattering experiment. The black circles in the aqueous solution are water molecules and the red circles are the particles in the solution. In this measurement, a tiny volume of the solution (200 µl) is sufficient. The solution was taken by 1 ml syringe and filtered by the syringe filter (National Scientific, pore size = 200 nm). The organic solutions were filtered by Teflon filters while the aqueous solutions were filtered by cellulose acetate filters. The solution was injected into a cuvette, and placed in the holder. The instrument used for the light scattering measurement was the MicroSampler from Protein Solution Inc. The upper limit of the temperature is 60 C and the lower limit is 0 C. The final size distribution was calculated by the built-in software.

55 Dialysis Dialysis is a process that separates different sizes of molecules in solution by diffusion rate through a semipermeable membrane, such as dialysis tubing. 27 It is a common technique that operates on the same principle as medical dialysis. Typically if there are several molecules, which need to be separated, in a solution, dialysis is a timeconsuming but easy and inexpensive method. The solution is placed into a semipermeable dialysis bag, which can be purchased with specified pore sizes. The sealed dialysis bag is placed in a container filled with buffer or pure water. Because the concentration inside the dialysis bag is higher than outside, small molecules (salt: NaCl) move through the membrane to equalize the concentration; however, the large molecules (proteins), which are larger than the pore size, are blocked by the membrane. Usually dialysis is used to remove salts and impurities. Figure 1-11 shows the dialysis experiment process. Initially, all the molecules are in the tubing, and after a couple hours the smaller molecules move out of the tubing through the membrane but the larger molecules are blocked by the membrane and stay in the tubing. Therefore the small molecules can be diluted by thousands of times to be removed.

56 56 (a) (b) Figure Scheme of the dialysis tubing diffusion process. The columns are full of water or aqueous solvent. The yellow bars are clips to seal two mouths of the dialysis tubings (blue squares), which contain the solution with two sizes of molecules, the red and the black circle. The solvent is the same as the one out of the dialysis tubing. (a) The beginning of the dialysis experiment. (b) The end of the experiment. Dialysis tubing (molecular-weight cutoff (MWCO) 3,500 Daltons) was purchased from Fisher Scientific and always kept in a dry container. A tubing was cut as enough for the volume of the solution, which was used in the experiment. Two inches were left on the each end of the tubing for handling. One end of the cut tubing was sealed by a dialysis clamp, and placed in the DI water for minutes. The other end was opened by gently rubbing with the fingers. A small amount of buffer or DI water was added into

57 57 the tubing to check leaking. And then the tubing was rinsed by buffer or DI water. The solution was moved into the tubing, and air bubble was removed. Eventually, the open end was clamped and the tubing was placed into a large beaker, which had at least 10 times as the volume of the solution for dialysis. The beaker was filled with buffer and stirred by a stir bar on a hot plate. The buffer was changed 2-4 times during the experiment. The buffer can be used for 4 hours for the first time, and from the second time the process lasted over night. 1.5 Ligand Effects on QDs The ligands on QDs are crucial since the solubility of QDs relies on surface conditions. Ligands are used to improve the solubility of QDs in a certain medium. The strength of the interaction between ligands and the surface atoms of the QDs determines the stability of the nanoparticle-ligand complexes. Studies of the stability of the nanoparticle-ligand complexes has become very popular recently because applications of QDs are limited by their stability. Many researchers concentrate on the development of the ligands and different passivation strategies Typical ligands include Lewis bases such as thiols, amines, phosphonates, and carboxylates. 13, These ligands work only under certain ph. If ph decreases, the ligands may be protonated and separated from the QDs. This can be a big issue in the case of human stomach, which has a ph of 2. Currently, the new ligands, like polymers (polyethylene glycol (PEG)), attract researchers. These ligands have long chains, which have one hydrophilic side and the other side having functional group to attach QDs, and reduce the toxicity of QDs. 36

58 58 Also the toxicity of QDs is significantly affected by ligands. For example, many common ligands for QDs, such as the hydrophobic ligands (trioctylphosphine oxide (TOPO) and tributylphosphine (TBP)), 37 and the hydrophilic ligands (TGA, 11- mercaptoundecanoic acid (MUA), MPA, etc.), have been proven very toxic to biological systems. Therefore polymers (polyethylene glycol (PEG), poly(ethyleneimine) (PEI), and polyethylene oxide (PEO), etc.) become a better option for biocompatible QDs. 38 Although PEI as a common water-soluble polymer still makes QDs toxic; if PEG is added, the cytotoxicity will be reduced. Some research has shown that depending on the ratio of PEG / PEI, the viability of cells can be better. 44 Besides the coatings on QDs, size and concentration can also be the factors to influence the cytotoxicity. Some research groups found that toxicity of QDs is size-dependent with smaller particle sizes being more toxic because the smaller particles have the higher surface-to-volume ratio , 39, 45 Several studies show that higher concentration of QDs will cause higher cell death. The interaction between QDs and DNA or proteins is a factor of their toxicity. For instance, biotin coated CdSe/ZnS QDs were found to be able to nick DNA in a cell-free in vitro assay. 46 PEG seems to alleviate the toxicity in that it does not trigger immune response, and fewer than 50 genes (~0.2% of all the genes tested) changed expression significantly by PEG- QDs treatment, which was better than carbon nanotubes that induced significant changes in 216 genes. 42 Additionally, the ligand (such as PEG) can prevent the QDs made of these toxic elements (such as cadmium) from releasing the toxic metal ions to the environment. 47

59 QDs as Photovoltaic Material in Solar Cell Application As is well known, many bulk semiconductors, such as silicon, gallium nitride, 51 copper indium gallium selenide, 52 and CdTe, 53, 54 have been used as photovoltaic materials. Now QDs, as a kind of photovoltaic material, attract more attention since they may have the potential to increase the energy conversion efficiency beyond the traditional solar cells. Theoretically, QDs should have the longer lifetime for the exciton intraband relaxation, and mulitexcitons could be produced in QDs, so that a higher efficiency would be obtained by using QDs as photovoltaic material. In a solar cell, light is absorbed by the QDs producing an electron-hole pair. Once the charges are separated and flow, an electric current is generated. Basically these QDs are applied into solar cells as three types: first, metal junction solar cells; second, polymer hybrid solar cells; third, QD sensitized solar cells (QDSSC) In the metal junction solar cells, the laser excites the semiconductor and then the charges are separated at the interface of the metal and semiconductor. When QDs and the metal particles are in contact, the excited electrons are distributed between QDs and the metal particles Therefore the Fermi level of the QDs-metal nanocomposite moves closer to the conduction band of QDs because the Fermi level is increased by the accumulation of the electrons. For example, the CdS or CdSe QDs link the gold substrate by using thiol ligands and change the photophysical and charge transfer properties Also, ZnO-Au nanoparticles conjugated system was reported by Subramanian et. al. 78 Hence QDs-metal junction photovoltaic devices are an important potential role for the next generation solar cell.

60 60 The polymer-qds hybrid solar cells utilize the blend of the conducting polymer and QDs (e.g., CdSe QDs) to separate and transfer the charges because of the remarkable photoconversion efficiency and the solution processability. 83 For instance, Alivisatos and co-workers have studied the blend of regioregular poly(3-hexylthiophene) and the CdSe 62, 84 nanorods to create charge transfer junctions with high interfacial area. The photoconversion efficiency could be improved by this polymer-qds hybrid technology in the future. In the QDSSC solar cell, a small band gap QD is used as a dye to absorb light and then inject the excited electrons into another semiconductor, which has a large band gap, and the hole is removed by a redox couple. The electron and the hole can be separated in two different nanoparticles by this method so that the electrode has enough time to capture the charge. CdS (2.42 ev) 85 and CdSe (1.73 ev) 85 QDs have been applied to absorb the photons and inject the excited electrons into some wider band gap films or nanoparticles, such as TiO 2 (3.03 ev), SnO 2 (3.6 ev), and ZnO (3.3 ev). 70, A study of the CdSe QDs linked TiO 2 / N film showed higher current and power conversion of the film than without the CdSe QDs. 96 Other short band gap QDs, such as InP, InAs, PbS, and PbSe give more choices for light energy conversion in the visible and nearinfrared region Research Motivation QDs have many potential applications since they have the special properties in the nano-scale comparing to the bulk counterparts. However, before the certain QDs used in some specific applications, more properties need to be clarified. For example, in order to

61 61 applying the CdTe or CdTe/CdS QDs into the bio-imaging application, besides the optical properties, the ligand effects are also a key for the stability, size and toxicity of the QDs in the biological environment. Hence the investigation of the ligands effect on the QDs can give the researchers ideas about the different effects on the various ligands conjugated QDs so that help them choose the right ligands for the specific application. Furthermore, as introduced above, the status of solar cells with the QDs harvesting light energy is still on the beginning stage, and some problems need to be solved before the commercialization. First of all the photovoltaic material QDs must have the relative long charge carrier relaxation time so that the electron has the enough time to transfer into the acceptor. Therefore the charge carrier dynamics of QDs need to be clarified but many QDs (such as CdTe, CdTe/CdS, PbS, CuInS et. al.) are still not totally understood very well. In my research work, I did a lot of work to investigate the ultrafast charge carrier dynamics of CdTe and CdTe/CdS core/shell QDs under different environments. Compared with CdSe QDs, CdTe QDs have similar properties but smaller band gap, which can absorb the wider range light than the CdSe QDs. After the CdS shell is added to the CdTe core, the cooling time of the hot exciton in the CdTe/CdS core/shell QDs increases because of the less defects, weaker ligands vibration effect and spatial separation of electron and hole. Second, the energy level alignment between QDs and acceptors is also very important because it significantly affects the conversion efficiency. Many studies have been performed on these charge transfer processes. For example, Kamat and co-workers studied the charge transfer between TiO 2 and CdX QDs (X: Te or Se), 102 and the charge transfer between CdSe QDs and Ru-polypyridine complexes by

62 62 Sykora et. al. 103 Third, the costs of these materials are very expensive comparing with the commercial Si product because the abundances of some elements (Se, Te, In etc.) are very small. 104 Hence if people can use the elements of more rich-abundance (Cu, Pb, S et al.) to produce QDs, the cost of these materials will be reduced.

63 63 Chapter 2 Synthesis of CdTe Core, CdTe/CdS Core/Shell and CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots 2.1 Chemicals in Synthesis All the chemicals used in synthesizing the CdTe core, the CdTe/CdS core/shell, and the CdTe/CdS/ZnS QDs are listed below: n-tetradecylphosphonic acid (TDPA) was purchased from PCI Synthesis. Oleic acid (OA; 99%), zinc acetate dehydrate (99.5%), cadmium acetate dehydrate (98 %), sodium diethyldithiocarbamate trihydrate (98%), and hexadecylamine (HDA; 90%) were purchased from Alfa Aesar. Thioacetamide (99%) was purchased from Acros Organics. Cadmium oxide (CdO; 99 %) was purchased from Fluka. Sulfur (99.999%), trioctylphosphine (TOP; 97%), and dimethylcadmium (97%, 10 wt% in hexane) were obtained from Strem Chemicals. 1-Octadecene (ODE; tech. 90%), tributylphosphine (TBP; 97%), tellurium (200 mesh, 99.8%), oleylamine (70%), zinc stearate (technical grade) and zinc oxide (99%) were purchased from Aldrich. Chloroform (CHCl 3 ) was purchased from VWR. Methanol (MeOH) was obtained from J.T. Baker. All reagents were used as received. 2.2 General Introduction of Synthesis of QDs Synthesis of CdTe and CdTe/CdS core/shell QDs in this dissertation is all in hot, noncoordinating solvent, ODE. Initial synthesis procedure of the CdTe QDs followed the work from Yu et al. 13 by injecting Te precursor (TBPTe in ODE) into hot Cd precursor (CdTDPA in ODE) at 300 C. High temperature can supply more nucleation energy, so compared to the synthesis in aqueous phase, the higher temperature of the synthesis in

64 64 ODE leads to fast CdTe QDs nucleation and growth. For example, usually the synthesis time in ODE is only 30 minutes; this is much shorter than the synthesis time in aqueous phase under normal atmosphere, which takes hours or even more than one day. 105 Generally, time is a key to QD growth; the longer synthesis time yields larger particles. In a synthesis process, an ideal condition is nucleation happens quickly to produce QDs with the uniform size once the precursors mix together, and then QDs grow evenly by the time with narrow particle size distribution. However, although the size of QDs is uniform in the beginning, the growth rates of the different size QDs are not the same. Especially, when the concentration of the precursors is too low, Ostwald ripening, 106, 107 that small particles start to shrink and finally separate into monomers to help the large particles grow, occurs. Therefore, small QDs become smaller while the large QDs grow larger. As a result, size distribution of QDs becomes broader and broader. Additional precursors are useful to avoid Ostwald ripening happening. 108 Hence a slow injection method, which is injecting precursors by a syringe pump at a certain injection rate, was applied to the CdTe QDs growth in order to produce larger particles and still keep the small size distribution. 2.3 Synthesis of CdTe Core Quantum Dots Synthesis of CdTe Core QDs by One Injection Synthesis of TDPA- and TBP-capped CdTe core QDs followed the procedure previously reported by Yu et al. 13 Standard Schlenk-line or inert glovebox procedures were used to exclude air and water from the reactions. In a typical synthesis, CdO ( g, 0.20 mmol) and TDPA (0.114 g, 0.41 mmol) were added to 7.8 g ODE, and the mixture was heated to 300 C with vigorous

65 65 stirring under a flowing argon atmosphere. At this temperature, the solution was clear and colorless. A solution of Te ( g, 0.1 mmol) dissolved in g of TBP and g of ODE was prepared in an argon-filled glovebox and loaded into an airtight syringe. The syringe was removed from the glovebox, and the Te mixture was injected very rapidly into the Cd-containing mixture. The temperature rapidly decreased after injection, and was held at 260 C for growth of the QDs. The size of the QD cores was controlled by reaction time. Variations on the synthesis of the CdTe QDs by the one injection method are described in Table 11 (Appendix A) and the chemicals used in the reaction are described in the Section 2.1. Table 11 (Appendix A) lists different experiments done to produce the large CdTe QDs and to improve the quality of the CdTe QDs by one injection method. In this procedure, TDPA was used as the ligand for the Cd precursor preparation. Through the series of experiments, 280 C has been proven the best reaction temperature for the CdTe QDs growth since the large CdTe QDs can be obtained and the emission distribution of them was narrow (FWHM = 37 nm) at this temperature. If the temperature is lower than 280 C, the CdTe QDs grow slowly and the emission distribution is broad. In contrast, if the temperature is higher than 280 C, a small shoulder can be observed due to the nucleation of new CdTe QDs. Also the Cd/Te molar ratio plays a crucial role in the growth of the CdTe QDs. During these experiments, it was shown that more Te precursor can accelerate the CdTe QDs growth, which means the CdTe QDs grow larger and faster. The effect of the elements ratio in the CdSe QDs growing process was reported by Qu et al. 109 Furthermore, fast stirring and swift injection

66 66 at high temperature can avoid broad emission distribution or multiple emission peaks. However it is difficult to obtain larger CdTe QDs, which can emit longer than 700 nm light, by this procedure. In an effort to grow larger CdTe QDs, oleic acid was substituted for TDPA as a ligand for cadmium precursor. The procedure was similar with that described above. Table 12 (Appendix A) shows variations on the synthesis of the CdTe QDs with OA under different conditions. The growth rate of the CdTe QDs was faster by using OA, and they reacted only for 5 minutes to reach the emission wavelength longer than 700 nm. Additionally, the injection and reaction temperature in these experiments can be set at 200 C, which was much lower than the reaction using TDPA as a ligand for cadmium precursor. Nevertheless, the fast growth rate made the quality of the CdTe QDs synthesized by OA ligand relatively worse than those using TDPA ligand because fast growth makes the atoms in the crystals not arrange very well. The emission distribution of the CdTe QDs using OA was broad and the PL emission intensity was very weak since there are more defects in the OA-capped CdTe QDs. Table 13 (Appendix A) shows variations on the synthesis of the CdTe QDs by one injection by using HDA as a ligand for cadmium precursor and TOP as a ligand for tellurium precursor. Compared with the CdTe QDs synthesized with TDPA and OA ligand, the QDs with HDA ligand grew extremely slowly, because after cooking for 35 minutes the PL wavelength was merely 592 nm. In another experiment, TDPA, HDA, and TOP were all combined in one procedure. The synthesis of TDPA-, HDA-, and TOP-capped CdTe core QDs follows the

67 67 method reported by Smith et al. 4 The Te precursors were prepared by dissolving Te powder ( g, 0.1 mmol) in TOP (1 ml) and ODE ( g, 5 ml) in a glovebox. Then the Te precursors were swiftly injected into a hot mixture of CdO ( g, 0.2 mmol), TDPA ( g, 0.44 mmol), HDA ( g, 5.7 mmol) and ODE (10 ml) at 300 C. The size of CdTe QDs was controlled by the reaction time, and the growth temperature was set to 265 C. When the target size was reached, the heating was stopped and the CdTe QDs were allowed to cool down to room temperature. The CdTe QDs with HDA have bright PL and are stable in the air for hours. Hence, the extraction of these CdTe QDs can be done in the air and the solvents (such as CHCl 3 and MeOH) do not need to be degassed. The growth rate of the CdTe QDs with HDA is slower than that only with TDPA. The reason could be the bond between QDs and HDA is hard to break so that the deposition of the atoms on the QDs becomes more difficult Synthesis of CdTe Quantum Dots by Two Injections A second precursor injection was used in the CdTe QD growth in order to produce larger QDs and to narrow the size distribution of the QDs. For this precursor solution, CdO ( g, mmol) and TDPA (0.038 g, 0.14 mmol) were added to 2.65 g ODE, and this mixture was heated to 260 C to make the CdO dissolve in ODE. When the solution was clear, it was allowed to cool to room temperature. A solution of Te ( g, 0.1 mmol) dissolved in g of TBP and g of ODE was added into the as-prepared Cd solution. This mixture of Cd and Te precursors was injected into the CdTe QD solution by syringe pump (NE-300, New Era Pump Systems, Inc.) at a rate of 0.05 ml/min. After adding the entire precursor solution into the CdTe QDs solution, the

68 68 reaction was continued at 260 C for an additional 15 minutes in order to allow the Cd and Te monomers to fully react. The solution was cooled to room temperature and kept in a vial under argon. The whole second precursor injection was performed under flowing argon atmosphere. Table 14 (Appendix A) lists the different experiments for the synthesis of the CdTe QDs by two injections. Each experiment has two injection processes, injection one and two, which were marked by injection order 1 and 2, respectively. Usually within 30 minutes of the first injection, the concentration of the precursors in the reaction becomes too low to support size focusing. Meanwhile, Ostwald ripening affects the size distribution of the CdTe QDs and makes it broad. 106, 107 Hence it is necessary to add more precursors to the reaction to increase the size of the CdTe QDs and narrow the emission distribution. In the two-injection experiments, the second injection improves the emission distribution and increases the emission wavelength of the CdTe QDs. Figure 2-1 illustrates the normalized PL emission spectra of the CdTe QDs versus reaction time. The aliquots were taken from the reaction mixture via syringe and dissolved in CHCl 3 for the PL emission measurement. Compared with the spectrum of the CdTe QDs 1 minute after the first precursor injection, the spectrum of the CdTe QDs after 12 minutes had a red shift, which was 0.08 ev, and the FWHM of the CdTe QDs grown for 12 minutes (0.13 ev) was larger than that of the CdTe QDs grown for only 1 minute (0.10 ev). Also by the calculation (described in Section 1.2), the relative standard deviation in diameter of the CdTe QD grown for 12 minutes was 12.1% which is larger than that of the QDs grown for 1 minute (6.9%). Obviously, the size of the CdTe QDs

69 69 increases by the reaction time; furthermore, the size distribution of the CdTe QDs became broader after 12 minutes growth because Ostwald ripening happened when the concentration of the Cd and Te precursors decreased. 106, 107 The PL spectrum of the CdTe QDs after 30 minutes (green solid line) shows there is no change of the peak position and the FWHM after 12 minutes since the both PL peaks of the CdTe QDs after 12 minutes and 30 minutes were at the same position (2.02 ev). This means the CdTe QDs stopped growing as the concentration of the precursors fell below a certain value. For this reason, the supplement of the extra precursors is necessary for further CdTe QDs growth. After the second precursors injection, at 85 minutes the PL peak had red-shifted further from 2.02 ev to 1.87 ev. Also, the FWHM of the blue PL spectrum of the CdTe QDs (0.09 ev) was narrower than that of the green one (0.13 ev). However, the calculated size deviation of the CdTe QDs grown for 85 minutes is 18.2% which was larger than that of the CdTe QDs grown for 12 minutes (12.1%). Evidently, the additional precursors helped the CdTe QDs grow while narrowing the emission distribution since FWHM of QDs at 85 minutes (0.09 ev) is smaller than that at 30 minutes (0.13 ev). However, the size distribution of the CdTe QDs was still increasing, because the calculated size deviation of the CdTe QDs grown for 85 minutes is larger than that for 12 minutes.

70 70 Figure 2-1. PL emission spectra of the CdTe QDs at different reaction times (1, 12, 30 and 85 minutes) after the first precursor injection. FWHM is full width at half maximum. The second precursor injection started at 30 minutes. As mentioned above, Te-rich conditions make the CdTe QDs grow faster. Here, a Cd:Te ratio of 2:3 (mol/mol) was used in the second injection to increase the growth rate. Although more Te can further increase the growth rate, the PL quantum yield of the CdTe decreases. Moreover, the reaction temperature was set at 280 C since the CdTe QDs can

71 71 grow fast and large at this temperature. Slow injection is a very simple method to add precursors into the reaction. We found that 50 µl/min (injected by syringe pump) was the best injection rate for the second injection. Lower injection rate prolonged the reaction time and higher injection rate did not increase the growth rate of the CdTe QDs. Using the optimized procedure, we were able to synthesize large CdTe QDs, which emitted near 770 nm by the two-injection method. Attempts to achieve even longer emission wavelengths through further growth caused the QDs to precipitate and lose their PL emission intensity. Figure 2-2 shows the PL spectra of CdTe QDs at 30, 65, 90, and 120 minutes reaction time. The amount of aliquots taken from the solution was 0.2 ml, so that the concentration of each CdTe QDs sample was similar. Also, the excitation wavelength for these samples was 400 nm so that the relative PL quantum yield can be determined by the PL intensity of these samples. In Figure 2-2, the relative PL quantum yield of the CdTe QDs at 65 or 90 minutes was still high, and the emission wavelength was at 768 nm. However, at 120 minutes, when the emission wavelength reached 776 nm, the PL was almost gone. Consequently, the longest emission wavelength of CdTe QDs, obtained by the procedure described above, is about 770 nm. Furthermore, the size distributions of these samples were estimated by the method decribed in Section 1.2, and the relative size deviations of the CdTe QDs at 30, 65, 90, and 120 minutes are 29.9%, 63.4%, 39.4%, and 30.7%, respectively. The size distribution of the CdTe QDs became broader during the second injection in the early time and then

72 when they were growing close to the limitation (band gap of the bulk CdTe material), the size distribution was getting narrow again. 72 Figure 2-2. PL spectra of CdTe QDs. Time (30, 65, 90, and 120 minutes) are the reaction time from the first injection, and the second injection started after 30 minutes. Black triangle line is the PL spectrum of the CdTe QDs at 30 minutes before the second injection. The rest three different lines are the PL spectra of the CdTe QDs during the second injection. The emission wavelengths of these samples at 30, 65, 90, and 120 minutes are 658, 730, 768, and 776 nm, respectively.

73 Synthesis of CdTe Quantum Dots by Multi-Injection 110 In this procedure a multi-injection method was used to synthesize the CdTe QDs after the work reported by Yu et al. 110 The preparation of the three stock solutions is described below. TBPTe solution: 1.02 g Te powder was dissolved in g TBP, and the concentration was 0.40 mmol/g. Cd solution 1: CdO (0.064 g, 1.00 mmol), TDPA (0.285 g, 2.05 mmol) and g ODE were mixed, and then heated up to 300 C to form a colorless solution, in which the Cd concentration was mmol/g. Upon cooling to room temperature, the colorless solution became a gel-like solid. Cd solution 2: CdO (0.096 g, 3.00 mmol), TDPA ( g, 6.15 mmol) and g ODE were mixed, and then heated up to 300 C to form a colorless solution, which the Cd concentration was mmol/g. Upon cooling to room temperature, the colorless solution became gel-like solid. Te solution, Cd solution 1 and Cd solution 2 were kept at 100 C before injection. 1 st injection: CdO ( g, 0.10 mmol) and TDPA (0.057 g, mmol) were mixed with ODE (total weight was 4.00 g) and heated up to 300 C under argon. Then a Te solution (0.50 g TBPTe solution (0.196 mmol Te) mixed with 1.50 g ODE) was swiftly injected into the hot solution. Meanwhile time (t 0 ) was recorded after the first injection and the reaction temperature was set at 250 C to grow the CdTe QDs.

74 74 2 nd injection: At 4.5 minutes a mixture of g TBPTe solution (0.05 mmol Te), 0.20 g Cd solution 1 (0.05 mmol Cd) and g ODE was slowly injected into the reaction flask drop-wise from minutes. 3 rd injection: At 8 minutes a mixture of 0.25 g TBPTe solution (0.10 mmol Te), 0.40 g Cd solution 1 (0.10 mmol Cd) and 0.45 g ODE was slowly injected into the reaction flask drop-wise from 8 10 minutes. 4 th injection: At 15 minutes a mixture of 0.75 g TBPTe solution (0.20 mmol Te), g Cd solution 2 (0.30 mmol Cd and) and 0.30 g ODE was slowly injected into the reaction flask drop-wise in minutes. 5 th injection: At 22 minutes a mixture of 0.25 g TBPTe solution (0.10 mmol Te), 0.40 g Cd solution 1 (0.10 mmol Cd) and 0.45 g ODE was slowly injected into the reaction flask drop-wise in minutes. 6 th injection: At 36 minutes a mixture of 1.00 g TBPTe solution (0.30 mmol Te), 0.70 g Cd solution 1 (0.40 mmol Cd) was slowly injected into the reaction flask dropwise in minutes. Consequently, after the first injection, the PL peak was at 607 nm; after the second injection, the PL peak was at 622 nm; after the third injection, the PL peak was at 646 nm; after the fourth injection, the PL peak was at 668 nm; after the fifth injection, the PL peak was at 724 nm; after the sixth injection, PL was gone. This multi-injection method is not easy to operate, because all the precursors (six injections) are prepared in the glove box and the Cd precursor mixture forms a gel after cooling to the room temperature. Weighing this Cd precursor mixture is also difficult

75 75 since it is very sticky. Furthermore, the growth of the CdTe QDs is hard to control by this multi-injection method, because once injecting the sixth precursor, the QDs usually precipitate. Figure 2-3 illustrates the normalized PL and absorption spectra of CdTe QDs after each injection. The wavelengths of these spectra were increased by the 6 injections. Nevertheless, after the 6 th injection, the PL of the CdTe QDs was totally gone. Although the QDs were still growing because the absorption of the CdTe QDs had a red shift, the emission spectrum of QDs after 6 injections was only a straight line.

76 76 Figure 2-3. Normalized PL (dot lines) and absorption (solid lines) spectra of CdTe QDs by multi-injection method. The numbers on the side of these PL spectra represent the injection times. 2.4 Synthesis of CdTe/CdS Core/Shell Quantum Dots by Slow Injection Shell Growth Method This slow injection method also can be used in the deposition of CdS shell on the CdTe core. The operation of slow injection method to add precursors is very simple, and

77 77 the calculation of the amount of precursors is not necessary. The injection rate and time are controlled by the syringe pump, and the reaction temperature is constant. Nevertheless, the thickness of shell deposited on core cannot be controlled by this slow injection method Deposition of CdS Shell by Slow Injection of CdS Precursors In slow injection method, CdS shell was deposited onto the CdTe cores by gradually injecting a solution containing Cd and S precursors into the crude CdTe mixture using a syringe pump. 111 The shell precursor solution was prepared as follows. A mixture of CdO ( g, 0.33 mmol), OA (0.8 ml, 2.5 mmol), and ODE (3.6 ml) was heated up to 260 C under argon to produce a 0.08 M solution of cadmium oleate. In a separate flask, S ( g, 0.48 mmol) was dissolved in ODE (4 ml) by heating to 200 C under argon. These two precursor solutions were allowed to cool to room temperature and then mixed together. A portion of the crude reaction mixture containing CdTe cores (5 ml, containing approximately 50 nmol cores, as determined by UV-visible absorption measurements). was diluted with degassed ODE (3 ml) and extra TBP (0.5 ml) and then heated to 250 C under argon. The CdS precursors were then loaded into a syringe and injected into the CdTe mixture at a rate of 0.1 ml/min. After injection of the entire CdS precursor mixture, the heat was removed from the system. The crude product solutions of the core and core/shell QDs were kept under argon and stored in an argon-filled glovebox to prevent degradation by air, which is known to occur with the CdTe cores. Table 15 (Appendix A) shows the trials of the CdS shell growth. The ideal injection rate was set at 100 µl/min, because lower injection rate caused the shell to grow

78 78 slowly, while higher injection rate led the nucleation of CdS QDs. Also, S-rich shell precursor solution made CdS shell grow faster, and through the series experiments the perfect concentration of the Cd and S monomers were 0.08 M and 0.12 M, respectively. CdTe cores (extracted by CHCl 3 and MeOH) formed a black precipitate once the temperature was over 160 C since many ligands are removed away from the cores. Even HDA ligand was added into the flask; the washed CdTe cores were still not stable at high temperature. Therefore CdTe cores were not purified prior to the shell growth. Unpurified CdTe cores could be heated up to 250 C, which has been proved as the ideal reaction temperature by series experiments. When the temperature was lower than 250 C, CdS shell grew slowly; nevertheless, if the temperature was higher than 250 C, the nucleation of CdS QDs happened. Additionally the quality of CdTe/CdS core/shell QDs with TBP ligand was better than that with HDA ligand. For CdTe/CdS core/shell QDs with TBP ligand had higher PL quantum yield, narrower emission distribution, easier wash process, and almost the same properties after transfer to the aqueous phase Characterization of CdTe/CdS Core/Shell Quantum Dots Synthesized by Slow Injection Shell Growth Method Figure 2-4 shows the normalized absorption and PL emission spectra of CdTe/CdS core/shell QDs with different size CdTe core. For instance, the CdTe/CdS core/shell QDs with 1.84 ev emission grew on the smallest CdTe core (D = 3.4 nm, λ = 562 nm) while the CdTe/CdS core/shell QDs with 1.61 ev emission have the largest CdTe core (D = 7.4 nm, λ = 746 nm). Compared with CdTe core, CdTe/CdS core/shell QDs have a red shift in spectra because these CdTe/CdS core/shell QDs are type II QDs

79 79 (described in Section 5.2) and the band gap energy of core/shell QDs is smaller than that of core. Usually, it is easy to produce long emission wavelength core/shell QDs by large cores; however, the thickness of CdS shell also increases the emission wavelength of core/shell QDs. For example, the emission wavelength of the CdTe/CdS core/shell QDs with 1.77 ev emission (700 nm) with the larger core (D = 4.0 nm, λ = 646 nm) was smaller than that of the core/shell QDs with 1.72 ev emission (720 nm) with smaller core (D = 3.5 nm, λ = 592 nm) (in Figure 2-4). The reason was the amount of the CdS precursors (Cd: mmol, S: 0.46 mmol) injected in the larger core (D = 4.0 nm, λ = 646 nm) was less than that (Cd: mmol, S: 0.61 mmol) injected in the smaller core (D = 3.5 nm, λ = 592 nm). Therefore, the CdS shell on the core/shell QDs with 1.72 ev emission (720 nm) was thicker than that on the core/shell QDs with 1.77 ev emission (700 nm). These CdTe/CdS core/shell QDs can cover from 1.84 ev (674 nm) to 1.61 ev (770 nm), which is close or in the NIR range.

80 80 Figure 2-4. Normalized absorption and PL emission spectra of four different size CdTe/CdS core/shell QDs. Blue solid line: UV-visible spectra of CdTe/CdS core/shell QDs; red solid line: PL emission spectra of CdTe/CdS core/shell QDs. The numbers on the top of the spectra are the diameter (D) and emission wavelength (λ) of the core for each core/shell QDs.

81 81 Figure 2-5 and Figure 2-6 illustrate the normalized PL intensity of CdTe core and CdTe/CdS core/shell QDs. In Figure 2-5, the red spectrum was the CdTe/CdS core/shell QDs (4.5 nm), which were grown on the 3.5 nm CdTe QDs (the black spectrum). After the CdS shell addition, the emission peak of the CdTe/CdS core/shell QDs had a substantial red shift (296 mev) from the CdTe QDs, 4, 111, 112 which depends on a type II band alignment (described in Section 5.2) in CdTe and CdS. In Figure 2-6, the red spectrum was the CdTe/CdS core/shell QDs (10 nm), which were grown on the 9 nm CdTe QDs (the black spectrum). A minor red shift (52 mev) between the emission peaks of the CdTe core and the CdTe/CdS core/shell QDs was observed. The amount of the red shift should depend on the size of the core, the CdTe QDs if the thickness of the shell is assumed to be the same (0.5 nm). The smaller CdTe QDs had the larger red shift from the CdTe/CdS core/shell QDs since the CdS shell can affect energy states change of the smaller CdTe QDs in the conduction band more than that of the larger ones.

82 mev Figure 2-5. Normalized PL emission spectra of CdTe core and CdTe/CdS core/shell QDs. Black solid line: CdTe core QDs; red solid line: CdTe/CdS core/shell QDs. Size (diameter) of CdTe QDs is 3.5 nm; size (diameter) of CdTe/CdS core/shell QDs is 4.5 nm. Shift between emission photon energy of CdTe core and CdTe/CdS core/shell QDs is 296 mev.

83 83 52 mev Figure 2-6. Normalized PL emission spectra of CdTe core and CdTe/CdS core/shell QDs. Black solid line: CdTe core QDs; red solid line: CdTe/CdS core/shell QDs. Size (diameter) of CdTe QDs is 9 nm; size (diameter) of CdTe/CdS core/shell QDs is 10 nm. Shift between emission photon energy of CdTe core and CdTe/CdS core/shell QDs is 52 mev.

84 Nucleation of CdS Quantum Dots during Slow Injection The concentration of Cd and S precursors in the reaction is crucial for the CdS shell growth since the low concentration makes the shell grow slowly while the high concentration causes the nucleation of CdS QDs. Figure 2-7 shows the normalized PL spectra of CdTe/CdS core/shell QDs by the different reaction time (40, 55, 70 and 85 minutes) after starting the second injection. The down arrows represent the position of the emission from CdS QDs. The emission peak moved from 1.61 ev to 1.60 ev, and in the meantime the small peak, which was at around 1.84 ev, appeared after 70 minutes. After 85 minutes this peak became higher as showed in blue PL emission spectrum. Although the band gap energy of bulk CdS is about 2.42 ev, 85 which is much higher than the emission energy of CdS QDs observed, besides the band edge emission the deep-trap can also affect the emission energy and make this energy lower than the band gap energy. 113, 114 Moreover in Figure 2-8 the TEM image of the CdTe/CdS core/shell QDs illustrates that there are many small particles (CdS QDs) around the CdTe/CdS core/shell QDs (large particles). Obviously the size of these CdS QDs was much smaller than the size of the CdTe/CdS core/shell QDs. Therefore the ratio of surface area to volume of the CdS QDs was significant, and the surface defects of the CdS QDs led to the emission red shift. In order to avoid the nucleation of the CdS QDs, the concentration of Cd and S precursors needs to be controlled. In Figure 2-7 the PL emission spectra before 70 minutes had no small emission peak at 1.84 ev but the peak started appearing after 70 minutes. That means the concentration of the precursors reached a critical point between 55 minutes and 70 minutes. In this way, the second injection was paused after 60 minutes

85 85 for 15 minutes to expend the precursors in the solution. When the concentration of the precursors decreased, the injection was resumed to grow the thicker CdS shell. Consequently, after 85 minutes precursor injection, there is no emission peak at 1.84 ev in the PL emission spectra and no small particles observed in the TEM images (shown in Figure 2-9). Figure 2-7. Normalized PL spectra of CdTe/CdS QDs by different reaction time. The arrows show the position of the small peak, which is due to CdS QDs.

86 86 Figure 2-8. TEM image of CdTe/CdS and CdS QDs. Big particles are CdTe/CdS QDs; small particles are CdS QDs.

87 87 Figure 2-9. TEM image of CdTe/CdS QDs Characterization of CdTe/CdS Core/Shell Quantum Dots with 1.55 ev (800 nm) Emission Syntheized by Slow Injection Shell Growth Method Through the slow injection method, the CdTe/CdS core/shell QDs with 1.55 ev (800 nm) emission, which is close to the bulk band gap energy of CdTe, were produced if more Cd and S precursors were added into the reaction. Figure 2-10 shows the normalized UV-visible absorption and PL emission spectra of the CdTe/CdS core/shell

88 88 QDs with 1.55 ev emission. From the PL emission spectrum (red solid line), these CdTe/CdS core/shell QDs emitted at 1.55 ev. The black solid line was UV-visible absorption spectrum, and the absorption peak at the band edge was very smooth, which was due to the overlap of the energy states in the large CdTe/CdS core/shell QDs. However, these CdTe/CdS core/shell QDs with 1.55 ev emission were different from the core/shell QDs with the higher band gap energy such as 1.61 ev (770 nm). Using the phase transfer method (described in Section 3.3.3) to transfer the CdTe/CdS core/shell QDs of 1.55 ev emission to the aqueous phase, the PL of these QDs was strongly quenched. On the contrary, the CdTe/CdS core/shell QDs of 1.61 ev could be transferred into the aqueous phase with high PL QY.

89 89 Figure Normalized UV-visible absorption and PL emission spectra of CdTe/CdS QDs emitting at 1.55 ev (800 nm). Black solid line: UV-visible spectrum; red solid line: emission spectrum. Figure 2-11 illustrates the TEM image of the CdTe/CdS core/shell QDs of 1.55 ev. Apparently, the shapes of the particles in Figure 2-11 (1.55 ev) and in Figure 2-9 (1.61 ev) were different. Compared with the CdTe/CdS core/shell QDs of 1.61 ev, there were several small arms, which could be the CdS, around the particles of 1.55 ev.

90 90 Because the lattice mismatch between CdTe and CdS is 11.5 %, 115 the CdS shell grew small arms like tetrapods if the shell was too thick. Therefore, the CdS shell did not protect the CdTe core very well any more. Figure TEM image of CdTe/CdS QDs emitting at 1.55 ev (800 nm). 2.5 Synthesis of CdTe/CdS Core/Shell Quantum Dots by SILAR Shell Growth Method The successive ion layer adsorption and reaction (SILAR) method, which was invented by Li et al. 116 and modified by Smith et al., 4 was used to grow a CdS shell on the CdTe QDs. By the SILAR method, a shell of desired thickness can be deposited on core

91 91 QDs by adding a defined amount of precursors. The size measurement of core and the calculation of the amount of precursors for each layer need to be done before the reaction of shell deposition. Precursors (either cation or anion) are prepared separately and the injection of the cation and anion precursors is alternated. The SILAR method can maintain the size distribution of QDs during the shell growth because nucleation of shell materials is avoided, and the thickness of shell layers is uniform due to homogeneous monolayer growing on all cores. The operation of the SILAR method is more complicated than that of the slow injection method, because besides the core size measurement and the calculation of amount of precursors, the reaction temperature needs to be changed during the reaction, for instance, in the method reported by Li et al., 116 the temperature for the precursors injection is lower and the reaction temperature is higher since the low temperature can prevent nucleation happening, and in the method reported by Smith et. al., 4 the reaction temperature increases by the more shell layers deposited on core Purification of CdTe Cores Before CdS shell was deposited on CdTe core by SILAR method, the CdTe cores had to be washed. There are two methods to purify the CdTe QDs: one is extraction, and the other one is precipitation. For extraction, the amount of CHCl 3 is important for the extraction of the CdTe QDs. Too much CHCl 3 can make the solution mix together without separation. Table 24 (Appendix B) shows the experiments of the extraction of CdTe QDs and they all worked. Precipitation is another important approach to purify QDs.

92 92 Table 22 (Appendix B) shows the trials to precipitate CdTe QDs by different solvents. The best way to precipitate CdTe QDs (following the procedure from Yu. et al.) 13 is adding 2 ml MeOH, 2 ml hexane, and 2 ml acetone to 0.5 ml CdTe QDs crude solution, and almost all CdTe QDs can be precipitated. If only using one of these solvents, the solvents cannot mix together or CdTe QDs do not precipitate. Furthermore, the ratio of these three solvents is crucial. For example, if there is more acetone in the mixture, even the extra ligands (such as TDPA) precipitate; in contrast if there is less acetone, the CdTe QDs cannot precipitate totally. If following the method from Yu et. al., 13 only acetone is used to precipitate the CdTe QDs. By this method a lot of acetone needs to be used for precipitating these CdTe QDs (approximately ratio of the crude QDs to acetone is 1:20) Deposition of CdS Shell by SILAR Shell Growth Method In this procedure, the CdS shell was grown by SILAR method with dimethylcadmium and TOPS precursors, which was following the work of Smith et al. 4 At the certain temperature the first injection of precursor was 0.1 M dimethylcadmium dissolved in TOP. After 10 minutes another precursor (0.1 M sulfur in TOP) was injected into QDs solution. The amount of precursor of each injection was the amount required for 0.25 monolayer shell. The first layer of CdS shell was reacted at 150 C for 4 hours for each half layer. The growth temperature of the rest 4 CdS layers were set at 225 C, and each half layer grew for 30 minutes. Aliquots were taken after each CdS layer added on the CdTe QDs. The calculation of the amount of the precursors for each layer is explained in Section

93 93 Results from various trials are listed in Table 17 (Appendix A). The CdTe QDs used as the core materials were synthesized with HDA ligands, which were more stable in the air than the CdTe QDs with the TDPA ligand. These CdTe QDs were extracted before adding the CdS shell on them. Each time the amount of the precursors solution added into the flask was only enough for half CdS layers. The first CdS layer took 8 hours to grow on the CdTe QDs at 150 C. After 5 injections, the CdTe/CdS core/shell QDs solution became a little cloudy. The temperature in this SILAR method is kept the same during the precursor addition and the shell growth. After CdS shell was added, the PL wavelength of the CdTe/CdS core/shell QDs had a significant red shift, for example, the CdTe/CdS core/shell QDs had 763 nm PL wavelength with five layers CdS shell. Also these core/shell QDs were very stable since the PL QY of them was still high after being transferred into the water by TGA ligand. Consequently, the CdTe/CdS core/shell QDs synthesized by this SILAR method are very close to the ones synthesized by slow injection method. However, by this SILAR method, it takes 14 hours for the CdS shell growth that is too long to compare with 3 hours (at most) by the slow injection method Characterization of CdTe and CdTe/CdS Core/Shell Quantum Dots Synthesized by SILAR Method Figure 2-12 presents the normalized UV-visible absorption (solid line) and PL emission (dot line) spectra of CdTe and CdTe/CdS core/shell QDs grown by SILAR method. The black spectra were the spectra of CdTe QDs, which had 2.10 ev (590 nm) band gap energy. The remaining lines, in five colors, present the spectra of the CdTe/CdS core/shell QDs with the diverse CdS shell layer. After the first CdS shell layer was deposited on the CdTe core, there was a substantial red shift in the UV-visible absorption (0.12 ev) and PL emission (0.14 ev) spectra from the CdTe core to the CdTe/CdS

94 94 core/shell QDs. Also there was a red shift after each CdS shell layer was added; nevertheless, the red shift of the spectra between the QDs, which have one CdS shell layer difference, was much less than that between the core and the core/shell QDs. Figure Normalized UV-visible absorption and PL emission spectra of CdTe and CdTe/CdS core/shell QDs by SILAR method. Solid lines are UV-visible spectra and dot lines are PL emission spectra. Black spectra: CdTe QDs; other spectra are CdTe/CdS core/shell QDs with 1 to 5 monolayer shell from bottom to top.

95 CdS Shell Growth by SILAR Method with CdOA, S Precursors I also tried SILAR method to add the CdS shell on the CdTe QDs with CdOA and S precursors, following work by Li et al. about synthesis of the CdSe/CdS QDs. 116 The trials are listed in Table 16 (Appendix A). The injection temperature was low (less than 200 C) and the reaction temperature was high (250 C). It took 25 minutes for each layer growth. By this method the results were not very good; the CdS shell grew slowly since there was no large PL emission shift observed after the Cd and S precursors were added. For example, in the first experiment, the PL emission of the CdTe QDs was at 669 nm, after adding the first and second CdS layer the wavelength only moved to 674 nm and 678 nm, respectively. However, there is more thatn 40 nm red shift in emission wavelength if CdS shell is deposited by slow injection method. Also the operation of the synthesis is more complicated than the slow injection method Calculations for Precursor Injections in the SILAR-based Deposition of CdS Shell onto CdTe QD Cores Size of the CdTe QDs The size of the CdTe QDs was calculated from the absorption spectrum according to the empirical relations determined by Yu et al. 13 The absorption peak wavelength (λ) of the CdTe cores was 620 nm. The diameter (3.86 nm) can be obtained by equation (2.1) reported by Yu et al D = ( ) λ ( ) λ + (1.0064) λ (194.84) (2.1)

96 where D (nm) is the diameter of the CdTe QDs and λ (nm) is the absorption wavelength of the CdTe QDs. Thus, the volume (V = m 3 ) of one CdTe QD was obtained 96 by 4 3 ( D / 2) 3 π. Lattice Constants of CdS 117, 118 are listed in Table 1. The average thickness (d) of one monolayer of CdS was calculated by 3 a for zincblende, and by c ½ for wurtzite. 3 In this work, we used d = 0.34 nm, which is the average thickness calculated by zincblende and wurtzite structures. Table 1. Lattice constants and densities of CdS in different crystal structures. Structure Lattice Constants (a) Lattice Constants (c) Density Zincblende nm g/m 3 Wurtzite nm nm g/m 3 QD size. Also, Yu et al. reported an equation to relate the extinction coefficient to CdTe = 10043( D) 2.12 ε (2.2)

97 97 where ε (cm -1 M -1 ) is the molar extinction coefficient of the CdTe QDs and D (nm) is the diameter of the CdTe QDs. Hence, in this case the molar extinction coefficient of the CdTe QDs is cm -1 M -1. Then the concentration of the CdTe QDs solution was calculated by the Beer-Lambert Law. A = εcl (2.3) In equation (2.3), A represents the absorbance of the CdTe QDs at the peak position of the lowest energy state. The molar concentration of the CdTe QDs sample is represented as C (mol/l), and l represents the path length (cm) of the cuvette used for measuring the absorption spectrum of the CdTe QDs (1 cm). In this case, the absorbance is so that the concentration of CdTe QDs ( M) can be obtained by equation (2.3). Usually, 0.2 ml crude QDs solution is added into 3 ml CHCl 3 for the absorption measurement. Therefore, the concentration of the crude QDs solution is M. The number (N) of CdTe QDs contained in 10 ml (1 batch) was calculated, which was The volume (V 1 = m 3 ) of the first CdS shell layer was determined from π ( Rcore / shell Rcore ), where R core is the radius of core and R core / shell is R core plus 3 the thickness of shell. The densities (ρ) of CdS of the different crystal structures are listed in Table 1. In our calculation, we used the average density value, g/m 3, as CdS density. Then, the mass (m shell = g) of the 1st monolayer (ML) CdS shell in

98 98 one core/shell QD was calculated by V 1 ρ. Thus, the amount of CdS (n 2 = mol) in one QD is m shell /MW. The amount (M 1 = mmol) of the Cd and S precursors for the first CdS shell layer: N n 2. The volume of the precursor added for the first CdS layer is 0.22 ml if the concentrations of Cd and S precursor solutions are 0.1 M. The amount of each successive CdS layer can be calculated by the same method described above. 2.6 Synthesis of CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Cd releasing from CdTe/CdS core/shell QDs can cause health and environment problems, and a ZnS shell was tried to be deposited on CdTe/CdS core/shell QDs. Also CdTe/CdS/ZnS core/shell/shell QDs are type I/type II QDs 119 (described in Section 5.2) so that the exciton can be confined in QDs by ZnS shell to increase PL QY. Several shell deposition methods were applied and introduced below ZnS Shell Growth by Slow Injection with Zinc Oleate (ZnOA) and S Precursors The slow injection method was also applied to ZnS shell growth. Table 18 (Appendix A) shows the experiment of synthesis of the ZnS shell on the CdTe/CdS core/shell QDs by the slow injection approach. A mixture of ZnO ( g, 0.32 mmol), OA (0.8 ml, 2.5 mmol), and ODE (3.6 ml) was heated up to 260 C under argon to produce a 0.08 M solution of zinc oleate. In a separate flask, sulfur ( g, 0.48 mmol) was dissolved in ODE (4 ml) by heating to 200 C under argon. These two precursor solutions were allowed to cool to room temperature and then mixed together. A portion of the crude reaction mixture containing CdTe/CdS core/shell QDs (containing approximately 50 nmol QDs, synthesized by slow injection shell deposition method) was

99 99 diluted with degassed ODE (3 ml) and extra TBP (0.5 ml) and then heated to 250 C under argon. The ZnS precursors were then loaded into a syringe and injected into the CdTe/CdS mixture at a rate of 0.1 ml/min. After injecting for 60 minutes, the syringe pump was paused for 15 minutes, and then restarted injecting for 10 minutes. The temperature was still kept at 250 C when the injection was stopped. After 15 minutes, the heat was removed from the system. The crude product solutions of the CdTe/CdS/ZnS core/shell/shell QDs were kept under argon. The emission wavelength of the CdTe/CdS/ZnS core/shell/shell QDs was shifted beyond 800 nm. However, the thiol ligands were more difficult to bond to the QDs since the zinc atoms were on the surface of QDs and these core/shell/shell QDs precipitated in ODE. Hence the ZnS shell cannot perfectly cover the surface of the CdTe/CdS core/shell QDs by this approach ZnS Shell Growth by Slow Injection with Zinc Stearate and Thioacetamide Precursors The experiment listed in Table 19 (Appendix A) is the ZnS shell growth by the slow injection with zinc stearate and thioacetamide. Zinc stearate (0.1 mmol) and thioacetamide (0.1 mmol) were dissolved in ODE (5 ml) by heating to 110 C under argon and then the precursors solution was cooled down to room temperature. A portion of the crude reaction mixture containing CdTe/CdS core/shell QDs (containing approximately 50 nmol QDs, synthesized by slow injection shell deposition method) was mixed with extra TBP (0.25 ml) and then heated to 70 C under argon. The ZnS precursors were then loaded into a syringe. First time (t 0 ), 1 ml ZnS precursors were

100 100 injected into the QDs solution and then 1 ml ZnS precursors were injected for each 30 minutes until the entire precursors were injected into the QDs solution. The reaction was kept at 70 C under argon over night. The next day morning, the heat was removed from the system. The crude product solutions of the CdTe/CdS/ZnS core/shell/shell QDs were kept under argon. By this approach, the emission wavelength did not shift too much after the Zn and S precursors being added into the flask. If the temperature is too high, the ZnS QDs will be produced. Furthermore the zinc stearate and thioacetamide did not dissolve in ODE very well so that once the temperature cooling down, the Zn precursors precipitated. To avoid the precipitate, heating tape can be applied to wrap and warm the syringe during the injection ZnS Shell Growth by SILAR Method SILAR method was used for ZnS shell growth with ZnOA and S precursors. Table 20 (Appendix A) shows the trials of the synthesis of ZnS shell on CdTe/CdS core/shell QDs. These experiments followed work by Li et al. 116 Totally two layers ZnS shell were added on CdTe/CdS core/shell QDs. ZnS shell did not grow very well since the quenching issue still existed. Although two layers ZnS shell precursors were added into the flask, not all precursors were deposited on CdTe/CdS core/shell QDs as the shell. On the other hand, the lattice mismatches among CdTe, CdS, and ZnS materials can break the shell to make tetrapod shape instead of spherical shape so that the quenchers (such as O 2 ) are able to touch and quench the CdTe/CdS/ZnS core/shell/shell QDs.

101 ZnS Shell Growth by Thermal-Cycling Coupled Single Precursor Method The thermal-cycling coupled single precursor method (TC-SP), which was developed by Chen et al., 120 was tried to deposit ZnS shell on CdTe/CdS core/shell QDs. This method was combined with the thermal-cycling and single precursor methods. The thermal-cycling method is a kind of approach that addition/adsorption of precursors is at a low temperature while epitaxial growth is at a high temperature. Through TC-SP method, a shell can be deposited on a core following layer by layer like SILAR method (Section 2.6.3). However, TC-SP method is simpler than the SILAR method since both cation and anion precursors are in a same compound. Therefore the injection process of TC-SP method is easier than that of SILAR method, which needs to inject cation and anion precursors into the core solution alternately during the reaction. Zinc diethyldithiocarbamate (Zn(DDTC) 2 ) was used as the sole ZnS shell precursor. 120 The CdTe/CdS QDs were prepared by the slow injection described in Section 2.4. Before the ZnS shell addition, the CdTe/CdS QDs (~10 ml) were washed by CHCl 3 (2.5 ml) and MeOH (5 ml) twice and moved into a flask. The washed CdTe/CdS QDs were vacuumed and refilled with argon. Zn(DDTC) 2 (0.08 g) was dissolved in oleylamine (2.21 ml, 1.83 g) and the flask was flushed thoroughly with argon. The first ZnS shell precursors (0.731 ml) were injected into the CdTe/CdS QDs solution at 50 C and the temperature was increased to 180 C sharply for 20 minutes. Then the second ZnS shell precursors (0.810 ml) were injected when the temperature was decreased to 120 C. After injecting the temperature was raised to 180 C again for 20 minutes. Aliquots were taken after each ZnS layer deposited and dissolved in CHCl 3 for the UV-

102 visible and PL emission measurements. The CdTe/CdS/ZnS QDs were stored under argon. The amount of ZnS shell precursors was calculated from the model described in 102 Section 2.6.6, and the calculation was based on Zn (S was excess) since one Zn(DDTC) 2 compound contains 4 S atoms and 1 Zn atom. Table 21 (Appendix A) shows the experiments of the ZnS shell growth by TC-SP approach with Zn(DDTC) 2 precursor. The precursor compound, Zn(DDTC) 2, can be dissolved in oleylamine very well but not easy to be dissolved in CHCl 3. Because oleylamine can quench the CdTe/CdS core/shell QDs, it is better if use CHCl 3 as solvent for the precursors. Moreover the emission wavelength of CdTe/CdS core/shell was the same as CdTe/CdS/ZnS core/shell/shell QDs. Different from the CdTe/CdS/ZnS core/shell/shell QDs synthesized by slow injection and SILAR methods, by TC-SP method the emission wavelength of the core/shell/shell QDs did not change at all. This could be the shape of the QDs made by TC-SP was still spherical so that the energy states of the CdTe/CdS core/shell part in the core/shell/shell QDs are kept the same. The operation of TC-SP method is simpler than SILAR method, because two elements come from the same compound Characterization of CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Synthesized by Thermal-Cycling Coupled Single Precursor ZnS Shell Growth Method Figure 2-13 presents the normalized UV-visible absorption spectra of the CdTe/CdS core/shell (black solid line) and CdTe/CdS/ZnS core/shell/shell (red and green solid lines) QDs synthesized by TC-SP method. The concentrations of the samples for absorption measurement were different. Once the ZnS shell was added on the CdTe/CdS

103 103 core/shell QDs, the lowest absorption peaks of core/shell and core/shell/shell QDs were at almost the same position. The inset shows the UV-visible absorption spectra of the CdTe/CdS/ZnS core/shell/shell QDs in the wider energy range. An arrow marked the absorption peak (about 4.4 ev), and the absorbance at 4.4 ev was increased by the ZnS shell added on the CdTe/CdS core/shell QDs. From the absorption spectra, there was no peak at 4.4 ev in the absorption spectrum of the CdTe/CdS core/shell QDs; in contrast a peak was observed after one monolayer being added and the peak rose higher once the second ZnS shell layer was deposited.

104 104 Figure Normalized UV-visible absorption spectra of CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs. Black line: CdTe/CdS QDs; red line: CdTe/CdS/ZnS nanocrytals with 1 ML ZnS; green line: CdTe/CdS/ZnS QDs with 2 ML ZnS. Inset: UV-visible absorption spectra of CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs in larger energy scale. Figure 2-14 illustrates the normalized PL spectra of the CdTe/CdS core/shell QDs and the CdTe/CdS/ZnS core/shell/shell QDs synthesized by TC-SP method. After ZnS shell was added on the CdTe/CdS core/shell QDs, the PL spectra did not change

105 105 significantly even though the emission distribution became broad by the thickness of the ZnS shell increased. The PL QY of these core/shell/shell QDs was about 61%. The PL peaks of the CdTe/CdS/ZnS core/shell/shell QDs were almost at the same position as the CdTe/CdS QDs. Figure PL emission spectra of CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs. Black line: CdTe/CdS QDs; red line: CdTe/CdS/ZnS QDs with 1 ML ZnS; green line: CdTe/CdS/ZnS QDs with 2 ML ZnS.

106 106 Figure 2-15 presents the TEM image of the CdTe/CdS/ZnS core/shell/shell QDs synthesized by TC-SP method. The shape of the CdTe/CdS/ZnS core/shell/shell QDs was anomalous, such as triangle, square and spherical. The image shows the ZnS shell was not perfectly deposited on the CdTe/CdS core/shell QDs. Like the large lattice mismatch between CdTe and CdS (11.5 %), the lattice mismatch between CdS and ZnS is 7.5 %.121 Particularly, the shape of the CdTe/CdS core/shell QDs was not uniform so that the ZnS shell was not deposited on the CdTe/CdS core/shell QDs completely. Figure TEM image of CdTe/CdS/ZnS core/shell/shell QDs.

107 107 Although some measurements were done to characterize the CdTe/CdS/ZnS core/shell/shell QDs, it is still not very clear whether the ZnS shell was deposited on QDs evenly, or alloying QDs were produced instead of core/shell/shell QDs. The red shift was observed in the CdTe/CdS/ZnS core/shell/shell QDs synthesized by slow injection method and the SILAR method but was not observed in the core/shell/shell QDs synthesized by TC-SP method. This red shift also could be from the CdS shell growing by extra CdS precursors. Furthermore the goal to develop these CdTe/CdS/ZnS core/shell/shell QDs was to solve the quenching issue for CdTe/CdS core/shell QDs. However, QDs were still quenched once they were transferred into aqueous phase by PEG ligands even though ZnS shell was added on QDs (described in Section 3.4.2). If this project needs to be continued in the future, more characterization of CdTe/CdS/ZnS core/shell/shell QDs needs to be done Calculation of the ZnS Shell on CdTe/CdS Thickness An example of the calculation of the amount of the ZnS shell precursors for the injections is shown below: The diameter (D) of the CdTe/CdS core/shell QDs was 6 nm as determined from TEM. The volume (V = m 3 ) of one CdTe/CdS QD was obtained by 4 3 ( D / 2) 3 π.

108 108 Lattice constants of ZnS 117, 118 are listed in Table 2. The average thickness (d) of one monolayer of ZnS was calculated by 3 a for zincblende, and c ½ for wurtzite. In 3 this work, we used d = 0.31 nm, which is the average thickness calculated by zincblende and wurtzite structures. Table 2. Lattice constants and densities of ZnS in different crystal structures. Structure Lattice Constants a Lattice Constants c Density Zincblende nm g/m 3 Wurtzite nm nm g/m 3 The number of the CdTe QDs can be calculated by the method described in the section of Calculation of the CdS Shell on CdTe QDs. And in this work, the number (N) of the CdTe/CdS QDs was the same as the CdTe QDs, which was The volume (V 1 = m 3 ) of the first ZnS shell layer can be determined from π ( Rcore / shell R ) / shell core, where R / shell core shell 3 / is the radius of CdTe/CdS core/shell QD and R core/ shell is / shell core shell R / plus the thickness of ZnS shell. The densities (ρ) of ZnS are listed in Table 2. We used the average density value, g/m 3, as ZnS density

109 109 in calculation. Then, the mass (m shell = g) of the 1st ML ZnS shell in one core/shell QD was calculated by V 1 ρ. The amount (n 2 = mol) of ZnS in one QD was determined by m shell /MW. The amount (M 1 = mmol) of the Zn and S precursors for the first ZnS shell layer was calculated by N n 2. The volume of the precursor is added for the first CdS layer is 0.96 ml if the concentrations of Zn and S precursor solutions are 0.1 M. The amount of the rest ZnS layers can be calculated by the same method described above.

110 110 Chapter 3 Quantum Dots Water Transfer All QDs used in this dissertation were synthesized in organic phase (ODE). In order to apply these QDs in biological application, such as bio-imaging, they need to be transferred into aqueous phase. In this chapter, different kinds of the methods to transfer CdTe/CdS core/shell QDs and CdTe/CdS/ZnS core/shell/shell QDs are introduced below. 3.1 Chemicals Tetramethylammonium hydroxide pentahydrate (TMAH; 97%), 3- mercaptopropionic acid (MPA; 99%), 11-mercaptoundecanoic acid (MUA; 95%), 16- mercaptohexadecanoic acid (MHA; 90%), pentaerythritol tetrakis(3-mercaptopropionate) (tetrathiol, 97%), Hexadecyltrimethylammonium bromide (CTAB, 99%) N,Ndiisopropylethylamine (DIPEA, 99%), and thioglycolic acid (TGA, 98%) were purchased from Aldrich. Cysteine (99%) was purchased from Acros Organics. Dulbecco s phosphate buffered saline (PBS) (without calcium & magnesium salts, sterile filtered) was purchased from VWR. Atomic spectroscopy standards, such as Cd (2% HNO 3, 1000 mg/l), Te (5% HNO 3, 1000 mg/l), and S (H 2 O, 1000 mg/l), were obtained from PerkinElmer Pure. Polyethylene glycol monoacrylate (PEG acrylate, MW = 1000 and 2000) was purchased from Monomer-Polymer and Dajac Labs. All reagents were used as received. 3.2 Synthesis of PEG Ligands Synthesis of PEG1500 Ligands PEG ligands are hydrophilic and biocompatible ligands for QDs in biological application. The synthesis of tridentate thiolated PEG ligands was followed the method

111 111 developed by Thiry et al. 122 Figure 3-1 shows the reaction of PEG ligands. PEG acrylate (MW: 1000, 5 g, 5 mmol) and tetrathiol ( ml, 5 mmol) were placed in a flask. And the mixture was vacuumed and then refilled by argon. Degassed CHCl 3 was injected into the flask with vigorous stirring. The solution was cloudy before the catalyst being added. After DIPEA (2.6 ml, 15 mmol) was injected into the flask as the catalyst, the solution became clear immediately. The reaction was refluxed for 4 hours and the solution became pale yellow. The solution was cooled down by dry ice and cold, anhydrate ether was added dropwise into the flask to precipitate the PEG1500 ligands. The precipitate was filtered by a Buchner funnel and washed by cold dehydrate ether 3 times. The final product, PEG1500 ligands, was like gel or wax, and stored in a vial. Figure 3-1. Reaction scheme to produce trithiol-peg ligands.

112 Synthesis of PEG2500 Ligands The procedure of the synthesis of the PEG2500 ligands was similar with that of the PEG1500 ligands. PEG acrylate (MW: 2000, 10 g, 5 mmol) was used for the reaction and the final product was white solid PEG Ligands Characterization by 1 H NMR 1 H NMR method was used to characterize the PEG ligands. Figure 3-2(a) illustrates the 1 H NMR spectrum of the PEG acrylate, and the main peaks were assigned to the hydrogens in the molecule (inset). The peaks in the left side which are highlighted by the light yellow square are from the hydrogens on the sides of the double bond (marked by yellow arrow), because after tetrathiol (Figure 3-2(b)) bonds with PEG acrylate, this double bond is disappeared. The PEG ligands NMR spectrum (Figure 3-2(c)) shows the peaks in the light yellow square that contributed by the hydrogens on the sides of the double bond had gone. This means tetrathiol totally reacted with PEG acrylate.

113 113 Figure H NMR spectra, (a) 1 H NMR spectrum of PEG acrylate, (b) 1 H NMR spectrum of tetrathiol, (c) 1 H NMR spectrum of PEG ligands. The solvent is deuterated chloroform. Insets are the molecular structures.

114 114 However, after about 6 months, the PEG ligands became more difficult to bond on the CdTe/CdS core/shell QDs. One possibility was that the sulfur-sulfur bond existed. And in Figure 3-2(c), peak A is contributed to the hydrogen from thiol functional group, which was at around 1.7 ppm. Nevertheless in the NMR spectrum of the aged PEG ligands (Figure 3-3(a)), there was no peak at 1.7 ppm. That suggests the hydrogens of the thiol functional group were removed, and sulfur-sulfur bond could be produced. We also noticed a large peak at 2.2 ppm in the NMR spectrum of the aged PEG ligands (Figure 3-3(a)), which was not observed in the fresh sample. Then I changed the solvent using the D 2 O instead of deuterated chloroform. Figure 3-3(b) shows the peak at 2.2 ppm in the NMR spectrum of the aged ligands in the D 2 O was much smaller than that in the deuterated chloroform. On the other hand, the peak at around 4.8 ppm, which was contributed to the hydrogens in free H 2 O, became much higher in the NMR spectrum of the old ligands in the D 2 O than that in the deuterated chloroform. That means the peak at 2.2 ppm was from the hydrogens in H 2 O that combined with the PEG ligands, because D 2 O can switch hydrogen with deuterium, that caused the peak at 2.2 ppm decreased but 4.8 ppm peak increased. Therefore in the old PEG ligands, not only the sulfur-sulfur bond existed, but also the PEG ligands can adsorb the H 2 O from the air. In the future, the PEG ligands should be stored under argon to avoid exposing to the air.

115 115 Figure H NMR spectra, (a) 1 H NMR spectrum of 6 months aged PEG ligands in deuterated chloroform, (b) 1 H NMR spectrum of 6 months aged PEG ligands in D 2 O. Insets are the molecular structures. 3.3 CdTe/CdS Core/Shell QDs Water Transfer In this section, the precipitation procedure can be used for CdTe/CdS core/shell QDs synthesized by slow injection and SILAR method. The core/shell QDs used in water transfer part were synthesized by slow injection method.

116 Organic-Ligand-Capped CdTe/CdS Core/Shell QDs Precipitation In order to switch with the hydrophilic ligands, extra hydrophobic ligands have to be washed away. Table 23 (Appendix B) shows the experiments of the hydrophobiccapped CdTe/CdS core/shell QDs precipitation by different solvents. Because to the size of the CdTe/CdS core/shell QDs are larger than the CdTe QDs, and they exhibit different surface condition, precipitation of the core/shell QDs is easier than that of the CdTe QDs. In the beginning, I tried the same procedure as the CdTe QDs precipitation, and it still worked very well. However, following procedures developed in the Weller Group, it was possible to precipitate them using only by CHCl 3 and MeOH. By this procedure I only need two solvents and the amounts of these solvents are less than those in the procedure with MeOH, hexane, and acetone CdTe/CdS Core/Shell QDs Water Transfer by Two Phase Extraction In this water transfer procedure, the washed CdTe/CdS core/shell QDs were transferred from the organic phase to the aqueous phase by TGA ligand. Initially the CdTe/CdS core/shell QDs (~4 nmol) were in the CHCl 3 (0.8 ml) with organic ligands, and then TGA (40 µl) was added into the solution to switch the ligands on the surface of the CdTe/CdS core/shell QDs. When TGA was on the surface, the solution became cloudy and basic solution (1 ml 1M NaOH) was added into the solution. Then the aqueous phase was on the top while CHCl 3 was on the bottom. After about 2 hours, the TGA-capped CdTe/CdS core/shell QDs were moving to the aqueous phase. However the ph value affects the transfer strongly. The CdTe/CdS core/shell QDs were able to transfer to aqueous phase only in basic buffer. As showed in Table 25 (Appendix B), in

117 117 acidic buffer or aqueous solution of ph < 7, the TGA-capped CdTe/CdS core/shell QDs cannot be transferred into the aqueous phase CdTe/CdS Core/Shell QDs Water Transfer by One Layer Transfer In this procedure, the crude CdTe/CdS core/shell QDs were precipitated by CHCl 3 and MeOH first, and then washed by MeOH one time. After most precursors, solvent and ligands were washed away, the CdTe/CdS core/shell QDs (~4 nmol) were dissolved in CHCl 3 (2 ml). Once TGA (600 µl) was added into the solution, it became cloudy immediately. To help the TGA ligand switching with organic ligands, the solution was placed in the ultrasonic bath for 1-2 hours. After that, the solution was centrifuged, and the precipitate was washed by CHCl 3 again. The dried CdTe/CdS core/shell QDs were able to dissolve in PBS or basic buffers. In Table 26 (Appendix B), different hydrophilic ligands and solvents are listed. The short chain thiol ligands (TGA, MPA) help the CdTe/CdS core/shell QDs transfer into aqueous phase very well, but the long chain thiol-ligand (MUA)-capped core/shell QDs does not have a good solubility in the aqueous solution. Dihydrolipoic acid (DHLA)-capped core/shell QDs also can be dissolved in the aqueous phase very well, but the ligands can quench the QDs by an unknown mechanism. Moreover, the dodecanethiol and 2-mercaptoethylamine (MA) do not help the core/shell QDs transfer to the aqueous phase. In order to transfer the CdTe/CdS core/shell QDs to PBS, TGA ligand needs to be bonded on the surface. In the early experiments (Table 26 (Appendix B)), only 150 µl TGA was added for the ligands switching so that the core/shell QDs can merely dissolve

118 118 in strongly basic solution (ph=11.3). If more TGA (600 µl) was added, the core/shell QDs can be transferred into PBS, because there is an equilibrium between TGA and the organic ligands; more TGA molecules make more TGA ligand substitute the organic ligands. The CdTe/CdS core/shell QDs can also be transferred into the aqueous phase with MPA and TMAH. TMAH, an organic base, was introduced into the water transfer procedure to improve the transfer process. 123 The washed CdTe/CdS core/shell QDs (0.1 ml crude QDs solution), MPA (0.05 ml), TMAH (0.1 g) and CHCl 3 (1 ml) were mixed together, and then after 2 days, a small brown liquid ball, which was the CdTe/CdS core/shell QDs in water, was observed on the top of solution. The water was from the byproduct of the reaction between MPA and TMAH and also water of hydration from the TMAH. Table 27 (Appendix B) shows the experiments of the water transfer of the CdTe/CdS core/shell QDs with MPA and TMAH. Nevertheless, this aqueous transferring by MPA and TMAH is not an efficient method. It takes much solvent and time to transfer tiny amount of the CdTe/CdS core/shell QDs. Additionally, the concentration of the core/shell QDs in aqueous phase is relatively low.

119 119 Table 28Table 28 (Appendix B) shows the water transfer experiments using PEG ligands (1500) and (2500). There are three thiol functional groups on each PEG ligand so that the bond between the ligand and QD is stronger than that ligand with one or two thiol functional groups. The saturated solution (adding PEG ligands until they do not dissolve in CHCl 3 any more) of the PEG ligands in CHCl 3 (1 ml) was mixed with washed QDs (2 nmol). After about 10 minutes, excess hexane was added to precipitate CdTe/CdS QDs. Then the mixture was centrifuged. The precipitate was dissolved in PBS or DI water but the solubility of these PEG capped QDs is not very good in the aqueous phase. Moreover CdTe/CdS core/shell QDs are quenched after being transferred into the aqueous phase. CTAB capped CdSe/ZnS Core/Shell QDs micelles can be transferred into water (reported by Li et al.). 124 I tried to cap CTAB on the CdTe/CdS core/shell QDs but it did not work very well. The solution of the washed CdTe/CdS core/shell QDs (2 nmol) in CHCl 3 (1 ml) was added dropwise into the CTAB aqueous solution (20 ml, 0.1 mm) in a beaker. This mixture was heated up to 50 C to evaporate the CHCl 3. CTAB capped the CdTe/CdS core/shell QDs and they lightly dissolve in the aqueous phase. Table 29 (Appendix B) shows the experiments of water transfer of the CTAB capped CdTe/CdS core/shell QDs micelles. Table 30 (Appendix B) shows the experiments in which 16- mercaptohexadecanoic acid (MHA) was used as ligand to transfer CdTe/CdS core/shell QDs to the aqueous phase. The dried, washed CdTe/CdS core/shell QDs (2 nmol) and MHA (0.2 g) were mixed together in CHCl 3 (1 ml), and heated up to 70 C to melt

120 120 MHA. After one night, DI water was added to dissolve the CdTe/CdS core/shell QDs. However, the CdTe/CdS core/shell QDs did not dissolve in water by this method. Finally, I tried using cysteine as hydrophilic ligand to transfer CdTe/CdS core/shell QDs to the aqueous phase. The procedure of CdTe/CdS core/shell QDs water transfer with cysteine ligand is described below. Cysteine (0.4 g) was dissolved in 2 ml DI water, and the washed CdTe/CdS core/shell QDs (4 nmol) were dissolved in 0.5 ml CHCl 3. The mixture was sonicated for 1 hour, and then it turned cloudy. It became a little clear after being heated by water bath at 80 C. The disadvantage of this cysteine ligand is that it does not dissolve in non-polar solvents (such as CHCl 3 ), hence other hydrophilic ligands (such as TGA) have to be added on the QDs first, and then switching the cysteine ligand can be exchanged in the aqueous phase. After trying different methods to transfer the CdTe/CdS core/shell QDs to aqueous phase, the best procedure is shown below. As-prepared CdTe/CdS QDs (0.5 ml) were precipitated one time using 1 ml CHCl 3 and 1 ml MeOH. Approximately 5 nmol (in 0.5 ml crude solution) of CdTe/CdS QDs purified thusly were dissolved in 2 ml CHCl 3 and then mixed with a large excess (600 µl) of TGA. The QDs began to precipitate immediately upon addition of the TGA. The mixture was placed in a bath sonicator for 1 hour after which the QDs were centrifuged. The top liquid layer was removed, and the CdTe/CdS QDs were washed once with CHCl 3 to remove excess TGA. After drying, these QDs dissolved easily in PBS.

121 UV-visible Spectra and Photoluminescence Spectra of CdTe/CdS Core/Shell Quantum Dots in Aqueous Phase After CdTe/CdS core/shell QDs were transferred into aqueous phase, the absorption and emission spectra of them in aqueous phase were similar with those in organic phase. Figure 3-4 shows the normalized UV-visible absorption spectra of the CdTe/CdS core/shell QDs in CHCl 3 (black solid line) and PBS (red solid line). Compared with the absorption spectra of the CdTe/CdS core/shell QDs in CHCl 3, the absorption spectra of the CdTe/CdS core/shell QDs in PBS had no substantial change. The absorption peak of the CdTe/CdS core/shell QDs in PBS was at the same position as the CdTe/CdS core/shell QDs in CHCl 3. Consequently, after being transferred into PBS, the ligand, TGA, and the solvent, PBS, did not change the absorption of the CdTe/CdS core/shell QDs.

122 122 Figure 3-4. UV-visible absorption spectra of the CdTe/CdS core/shell QDs in CHCl 3 (black solid line) and PBS (red solid line). Figure 3-5 illustrates the normalized PL emission spectra of the CdTe/CdS core/shell QDs in CHCl 3 (black solid line) and PBS (red solid line). The PL emission spectra of the CdTe/CdS core/shell QDs in CHCl 3 and PBS are very similar. Ever though, the peak distribution of the CdTe/CdS core/shell QDs in PBS was broader than that in CHCl 3, the peak position of them were the same. Furthermore, the broader peak distribution of the CdTe/CdS core/shell QDs in PBS could be due to the effect of the ligand (TGA) and the solvent (PBS). As a result, these CdTe/CdS core/shell QDs can

123 maintain the high quality when they are transferred into the aqueous phase by switching ligands with TGA. 123 Figure 3-5. Normalized PL emission spectra of CdTe/CdS core/shell QDs in CHCl 3 (black solid line) and PBS (red solid line) TGA-Capped CdTe/CdS Core/Shell QDs Precipitation Ethanol is a very efficient solvent to precipitate TGA capped CdTe/CdS core/shell QDs in aqueous solution. To the TGA-capped CdTe/CdS core/shell QDs in PBS, ethanol

124 124 was added until the solution became cloudy. Then the solution was centrifuged, and the precipitate (the CdTe/CdS core/shell QDs) could be dissolved easily in DI water. 3.4 CdTe/CdS/ZnS Core/Shell/Shell QDs Water Transfer CdTe/CdS/ZnS Core/Shell/Shell QDs Water Transfer with TGA Ligand The procedure of the water transfer of the CdTe/CdS/ZnS core/shell/shell QDs is similar with that of the CdTe/CdS core/shell QDs. Table 32 (Appendix B) shows the experiment of this water transfer. Even though the CdTe/CdS/ZnS core/shell/shell QDs were successfully transferred into the aqueous phase by the TGA ligand, the PL QY is only 12 % CdTe/CdS/ZnS Core/Shell/Shell QDs Water Transfer with PEG Ligands Compared with procedure of the water transfer of the CdTe/CdS core/shell QDs with PEG ligands, TMAH needed to be added into the solution to help thiol functional group bond on the surface of the CdTe/CdS/ZnS core/shell/shell QDs. Table 33 (Appendix B) shows this experiment. After the CdTe/CdS/ZnS core/shell/shell QDs were capped by the PEG ligands, the QDs were totally quenched.

125 125 Chapter 4 Ligand Effects on CdTe/CdS Core/Shell Quantum Dots 4.1 Introduction Semiconductor QDs have attracted many researchers as a candidate for the biological applications to replace the traditional dyes. 2, 4, They have some special optical properties, such as wide excitation range, narrow PL emission spectra, tunable emission position adjusted by the size of particles, and high PL QY. 109 However, there are still some obstacles on the way to the final application for biology including the stability, toxicity, and biocompatible of the QDs. In order to solve these problems, the surface passivation of the QDs is crucial. 3, 129, 130 Other researchers have studied many ligands conjugated to QDs, such as thiolates, amphiphilic polymers, 131, , 134 and proteins. Here, we attached the different kinds of ligands (TDPA, TBP, OA, thiol, or PEG ligands) on the CdTe and CdTe/CdS core/shell QDs, and examined the optical and physical properties of them. The experiments were carried out under the different conditions (temperature, brightness, time, etc.) in organic or aqueous phases. 4.2 Experimental Quantum dots preparation. The CdTe QDs were synthesized by the hot injection in ODE, and the CdS shell was added by the slow injection method described in Section 2.4. The synthesis of the CdTe/CdS core/shell QDs by SILAR method was introduced in Section 2.5. The whole process was under argon flow and after synthesis

126 126 the crude CdTe/CdS core/shell QDs were stored in a sealed vial protected by argon in the glovebox. Surface modification by hydrophilic ligands. The aqueous transfer procedure of the CdTe/CdS core/shell QDs with TGA ligand has been introduced in Section 3.3. As-prepared CdTe/CdS QDs (0.5 ml) were precipitated by 1 ml CHCl 3 and 1 ml MeOH one time. Approximately 5 nmol of the purified CdTe/CdS QDs were dissolved in 2 ml CHCl 3 and then mixed with a large excess (600 µl) of TGA. The QDs began to precipitate immediately upon addition of the TGA. The mixture was placed in a bath sonicator for 1 hour after which the QDs were centrifuged. The top liquid layer was removed, and the CdTe/CdS QDs were washed once with CHCl 3 to remove excess TGA. After drying, these QDs dissolved easily in PBS. The other aqueous transfer procedure of the CdTe/CdS core/shell QDs with PEG ligands has been introduced in Section There are three thiol functional groups on each PEG ligand so that the bond between the ligand and QD is stronger than ligands with one or two thiol functional groups. The saturated solution (adding PEG ligands until they do not dissolve in CHCl 3 any more) of the PEG ligands in CHCl 3 was mixed with washed QDs. After about 10 minutes, hexane was added to precipitate the QDs. And then the mixture was centrifuged. The precipitate was dissolved in PBS or DI water. Dynamic Light Scattering. The details about the dynamic light scattering were described in Section Dialysis. The dialysis protocol was introduced in Section

127 127 Quenching Tests. All the quenchers were prepared in CHCl 3 as 4 ml solution (1 mm) while the washed CdTe/CdS core/shell QDs solution was in the same concentration (0.5 µmol/l). The UV-visible absorption and PL emission measurements were examined after the CdTe/CdS core/shell QDs mixed with the quenchers. The quenchers in the experiment include PEG polymer, ethyl acetate, diethylamine, DIPEA, MeOH, ethyl ether, PEG ligands, oleylamine, and tetrathiol. 4.3 Results and Discussion Ligand Effects on PL QY Although the PEG ligands are biocompatible molecules, they can quench CdTe/CdS core/shell QDs. Figure 4-1 shows the PL QY of three different ligands capped CdTe/CdS core/shell QDs in CHCl 3 and PBS. These CdTe/CdS core/shell QDs were synthesized by slow injection method. Obviously, the PL QY of the PEG-ligand-capped QDs (10%) dramatically decreased compared to those of the organic-ligand-capped QDs in CHCl 3 (70%) and the TGA capped QDs in PBS (70%). Although the PEG ligands are biocompatible molecules, some functional groups on them quench the CdTe/CdS core/shell QDs.

128 128 Figure 4-1. PL quantum yield of the CdTe/CdS core/shell QDs with three kinds of the ligands. The left bar represents OA- and TBP-capped CdTe/CdS core/shell QDs in CHCl 3, the middle one represents TGA-capped CdTe/CdS core/shell QDs in PBS, and the right one represents PEG-ligand-capped CdTe/CdS core/shell QDs in PBS. In order to find which kinds of specific functional groups cause this quenching to the CdTe/CdS core/shell QDs, a series of the samples of the different compounds mixed with the CdTe/CdS core/shell QDs were measured by the PL emission spectroscopy. Figure 4-2 illustrates the PL emission intensities of the CdTe/CdS core/shell QDs, which

129 129 were synthesized by slow injection method (Φ PL ), mixed with different quenchers in CHCl 3. The first bar from the left is the PL emission intensity of the CdTe/CdS core/shell QDs as a control, which had the highest intensity compared to the rest samples. Apparently these quenchers had the various effects on the CdTe/CdS QDs. The PEG acrylate had the weakest quenching effect, while the tetrathiol compound had the strongest effect. Also the amines performed as QDs quenchers such as primary amine (oleylamine), secondary amine (diethylamine), and tertiary amine (DIPEA). Additionally some other compounds, which have alkoxy functional group, like ethyl ether can quench CdTe/CdS core/shell QDs. Interestingly, besides CHCl 3, we tested several kinds of common solvents, such as hexane, toluene, THF and ODE; however all the PL emission intensities of the CdTe/CdS core/shell QDs in these solvents were lower than that in CHCl 3. Unfortunately, the processes that causes the quenching happening are still not clear. The quenching process could be from the electron or the hole donator, which depends on the quenchers. Further studies are necessary to investigate the different quenching mechanisms in the future.

130 130 Figure 4-2. PL emission intensities of the CdTe/CdS core/shell QDs mixed with different quenchers in CHCl 3. The left bar represents pure CdTe/CdS core/shell QDs solution without any quenchers. Other bars with the error represent the emission intensities of the CdTe/CdS core/shell QDs and quencher mixture. Usually the way to avoid the quenching is the perfect shell depositing. Therefore SILAR method, which was wished to deposit perfect CdS shell on CdTe core, was used to synthesize the CdTe/CdS core/shell QDs. Figure 4-3 shows the PL QY of the different ligand-capped CdTe/CdS core/shell QDs (SILAR) in various solvents in days. During the experiment, these samples were exposed to the air and kept under light. The black squares represent the TDPA-, HDA-, TOP-capped QDs in CHCl 3, the red circles

131 131 represent the TGA capped QDs in ph = 11.5 buffer, the green triangles represent the TDPA-, HDA-, TOP-ligand-capped QDs in hexane, and the blue triangles represent the PEG-ligand-capped QDs in PBS. The initial PL QY of the CdTe/CdS core/shell QDs in hexane was higher than that in CHCl 3, which was contrary to the CdTe/CdS core/shell QDs synthesized by slow injection. The ligands used in two synthesis methods were different, for example, TDPA, OA, and TBP were employed in the slow injection, while TDPA, HDA, and TOP were used in the SILAR method. Hence the CdTe/CdS core/shell QDs have the different behaviors for the PL QY in the same solvent. The PL QYs of TDPA-, HDA-, TOP-capped CdTe/CdS core/shell QDs in CHCl 3 and hexane decreased with time. After 12 days, the PL QY of the QDs in CHCl 3 had decreased 54%, from 13.7% to 6.25 %, and the PL QY of the QDs in hexane had decreased 84%, from 64.3% to 10.4 %. On the other hand, the PL QYs of the TGA and PEG-ligand-capped CdTe/CdS core/shell QDs gradually increased after 12 days. To find out the reason that causes the PL QY increasing of the thiol-ligand-capped CdTe/CdS core/shell QDs, I tried bubbling more air in the QDs solution, raising the temperature of the QDs solution, and exposing the QDs solution under the xenon arc lamp. The results showed that air and temperature increasing did not change the PL QY of the QDs, but if the QDs were exposed under the light, the PL QY increased with time.

132 132 Figure 4-3. PL quantum yield of the CdTe/CdS QDs with different surface ligands in various solvents versus time in days. Black square: TDPA-, HDA-, TOP-capped CdTe/CdS core/shell QDs in CHCl 3, red circle: TGA-capped CdTe/CdS core/shell QDs in ph = 11.5 buffer; green triangle: TDPA-, HDA-, TOP-capped CdTe/CdS core/shell QDs in hexane; blue triangle: PEG-ligand-capped CdTe/CdS core/shell QDs in PBS. Figure 4-4 presents the PL QY of the PEG-ligand-capped CdTe/CdS QDs in PBS at different times under the xenon arc lamp, which was 64 W as the output power. Initially the PL QY of the CdTe/CdS QDs was around 2%, and after 30 minutes the PL QY increased to 5%. The PL QY reached the highest value (~ 7%) after 90 minutes, and then it stayed between 6-7% for 60 minutes. After 150 minutes the PL QY started

133 133 decreasing again. The exact mechanism of the PL QY effect by the light is not very clear. That must have some photochemical reactions happen between the QDs and the thiol functional group in PEG ligands that can lead the PL QY to rise. Nevertheless other quenching process made the PL QY decreased again. Thus more studies need to be done for this phenomenon in the future. Figure 4-4. PL quantum yield of the PEG-ligand-capped CdTe/CdS core/shell QDs versus exposure time under a xenon arc lamp (64 W).

134 Stability of CdTe/CdS Core/Shell Quantum Dots in Organic or Aqueous Phase The stability of QDs in solvent depends on the ligands properties, and the bond strength between QDs and ligands. Table 3 shows the CdTe and CdTe/CdS core/shell QDs can dissolve in some solvents, such as CHCl 3, CH 2 Cl 2, toluene, hexane, and THF. However these QDs do not dissolve in acetonitrile or water. If the CdTe/CdS core/shell QDs need to be dissolved in water, the surface ligands have to be switched by other hydrophilic molecules, such as TGA, MPA, etc. The dialysis experiments followed the standard procedure of the dialysis experiment (described in Section 1.4.7). In the experiment, the TGA-capped CdTe/CdS core/shell QDs precipitated within 3 hours and PEG-ligand-capped CdTe/CdS core/shell QDs were stable in aqueous phase after 24 hours. According to the results in Table 4, PEG ligands with three thiol functional groups had stronger bond to the CdTe/CdS core/shell QDs. Table 3. Solubility of the organic capped CdTe and CdTe/CdS core/shell QDs in solvents. CdTe CdTe/CdS CHCl 3 Y Y CH 2 Cl 2 Y Y Toluene Y Y Hexane Y Y Acetonitrile N N Water N N THF Y

135 135 Table 4. Dialysis of CdTe/CdS core/shell QDs with different ligands. Exp. QDs Buffer Change Times Time Results Comments 1 In the first buffer, after 3 TGA hours the TGA capped capped PBS 0 3 hrs CdTe/CdS QDs started CdTe/CdS precipitate After the first and 2 more second water change, PEG than PEG aggregated, then capped Water 3 24 used sonic, no CdTe/CdS hrs precipitate observed in the end 3 PEG, TGA capped CdTe/CdS Water 3 more than 24 hrs No precipitate observed in the end, no PL neither Added some TGA into PEG, TGA capped CdTe/CdS QDs after dialysis, in the beginning there was no PL, after 3 hours PL came back (weak), a little precipitate, after sonicating the solution is clear.

136 136 The TGA-capped CdTe/CdS core/shell QDs precipitated very quickly (3 hours) in the dialysis experiment, which means the bond between the QDs and the TGA ligand is not strong so that the TGA ligand left the QDs when the concentration of the TGA decreased. However, the dialysis experiment shows the PEG and PEG-TGA-capped CdTe/CdS core/shell QDs (prepared by adding PEG2500 ligands into TGA-capped CdTe/CdS core/shell QDs solution in PBS) can be stable in solution even after 24 hours, because there are three thiol groups on one PEG molecule; hence the bond between the QDs and the ligands is much stronger than only one thiol group on the molecule Ligand Effects on Hydrodynamic Diameter Hydrodynamic diameter of the QDs is the effective diameter including not only the size of QDs but also the length of the ligands and some solvent molecules attached on the surface. Therefore the information of the hydrodynamic diameter of the QDs is very important for the biological applications. Table 31 (Appendix B) lists the measurements of the hydrodynamic diameters of the CdTe/CdS core/shell QDs under the different conditions. The QDs used in the experiments were synthesized by slow injection method. From the Table 31 (Appendix B), it shows at 20 C the TDPA-, TBP-, OA-capped CdTe/CdS core/shell QDs had the smallest hydrodynamic diameter (11.6 nm) in CHCl 3, while the PEG2500-ligand-capped QDs (the length of the PEG2500 ligands is about 15 nm as calculated from the bond length of C-C or C-O) had the largest hydrodynamic diameter (45-53 nm) in PBS. Therefore, there is no aggregation for PEG2500-ligandcapped QDs in PBS since the size of them is around 40 nm which equals the length of

137 137 two PEG2500 ligands (~15 nm) plus the diameter of CdTe/CdS core/shell QDs (~10 nm). Also the hydrodynamic diameter of the PEG2500 ligands in CHCl 3 (18.5 nm) was smaller than that in PBS because of the hydrophilic groups on the PEG chain. Furthermore the hydrodynamic diameter of the TGA or PEG-ligands-capped QDs increased when the temperature of the solution rose. And this process is reversible, because the ligands, especially PEG ligands, will stretch out at high temperature and shrink at low temperature. We also observed the aggregation happening in the thiolligand (TGA or PEG ligands)-capped CdTe/CdS core/shell QDs. The hydrodynamic diameter of the QDs increased by the days, for instance, the hydrodynamic diameter of the TGA-capped QDs in PBS increased from 13 nm to 38 nm after 8 days. However, the bonds between the QDs were not chemical bonds since the QDs can be separated again by sonicating. Interestingly, the hydrodynamic diameter (23 nm) of the PEG-TGAcapped CdTe/CdS core/shell QDs was between the hydrodynamic diameters of the QDs with only one of these two ligands. The reason for this situation is not clear. It could be the interaction between the TGA and PEG ligands that causes the PEG chain to be shorter Spontaneous Disintegration of CdTe and CdTe/CdS Core/Shell Quantum Dots During the measurements, we observed that under some conditions CdTe and CdTe/CdS core/shell QDs disintegrated after they were diluted by CHCl 3. Especially, when extra TBP (1-2 drops) was added into QDs solution, the disintegration process was promoted and the solution became colorless in the end. Figure 4-5 illustrates the UVvisible absorption spectra of the CdTe/CdS core/shell QDs in CHCl 3 without washing. The black spectrum represents the absorption of the fresh CdTe/CdS core/shell QDs in

138 138 CHCl 3 and the red one is the absorption of the CdTe/CdS core/shell QDs with extra TBP in CHCl 3 kept in test tube for one day. Obviously the red absorption of the CdTe/CdS core/shell QDs was a line that was around baseline. That means the QDs solution was colorless and they disintegrated in CHCl 3. The reason causes this issue is still not clear, and atoms that form QDs could combine with ligands and become monomers again. Although the QDs were protected by the CdS shells, TBP still can help the CdTe/CdS core/shell QDs disintegrate in CHCl 3 through some processes. Figure 4-5. UV-visible absorption spectra of CdTe/CdS core/shell QDs in CHCl 3. Black line: absorption spectrum of the fresh CdTe/CdS core/shell QDs solution without extra TBP; red line: absorption spectrum of the CdTe/CdS core/shell QDs solution with extra TBP after one day.

139 139 Furthermore, more measurements were done for the washed CdTe/CdS core/shell QDs to explore other factors that affect the disintegration process and at this time no extra TBP was added into solution. Figure 4-6 shows the absorption spectra of the washed CdTe/CdS core/shell QDs under the different conditions. The absorption spectra of the QDs in CHCl 3 under the light had the largest blue-shift of all spectra. On the contrary, the absorption spectra of the QDs in CHCl 3 in the dark had a small blue-shift. Therefore the light had a significant effect on the disintegration issue. The light could excite the electron to the conduction band to have the QDs be oxidized, which could cause the disintegration issue. Also solvents can affect the disintegration issue, where the QDs in toluene only had tiny blue-shift under the light after 8 days. Another experiment, which was a mixture of the washed CdTe QDs with TBP stored under argon in the glovebox, proved the air was another reason for the disintegration issue since the sample without air partially disintegrated in the CHCl 3.

140 140 Figure 4-6. UV-visible absorption spectra of the CdTe/CdS core/shell QDs under different conditions. Black line: absorption spectrum of the fresh washed QDs; red line: absorption spectrum of the washed QDs in CHCl 3 after 6 days under the light; green line: absorption spectrum of the washed QDs in toluene under the light after 8 days; blue line: absorption spectrum of the washed QDs in CHCl 3 in the dark after 8 days; cyan line: absorption spectrum of the washed QDs in CHCl3 after 8 days under light. Table 5 lists the results of the disintegration issue for CdTe and CdTe/CdS core/shell QDs. The diameters of the small CdTe QDs, large CdTe QDs, small CdTe/CdS core/shell QDs, and large CdTe/CdS core/shell QDs were 3.1 nm, 4.2 nm, 3.7 nm, and

141 nm, respectively. These QDs can disintegrate in CHCl 3 with extra TBP, but the large CdTe/CdS core/shell QDs can stay in CHCl 3 provided no extra TBP is added. TBP can help the QDs disintegrate in CHCl 3 faster. Even the washed QDs with extra TBP were easy to disintegrate in CHCl 3. However, the washed CdTe/CdS core/shell QDs without extra TBP did not disintegrate in CHCl 3. Without washing, there were still some excess ligands (TBP, TDPA et al.) in the solution that can lead the QDs to disintegrate in CHCl 3. Through the washing, most excess ligands were removed so that the CdS shell was able to provide a protection for the CdTe core in the CHCl 3 with low concentration of TBP. Additionally the large CdTe/CdS core/shell QDs were more difficult to disintegrate in CHCl 3 because of the smaller strain force on the larger surface area or the thicker CdS shell on the surface to protect the CdTe QDs. Table 5. Disintegration of CdTe and CdTe/CdS core/shell QDs under the different conditions. Dilute with CHCl 3 Diluted and added extra TBP Washed no TBP Washed and extra TBP CdTe (large) Disintegrate Disintegrate N/A Disintegrate CdTe/CdS (large) Not Disintegrate Not Disintegrate CdTe (small) Disintegrate Disintegrate Disintegrate Disintegrate CdTe/CdS (small) Disintegrate Disintegrate Not Disintegrate 4.4 Conclusions We reported the effect of some ligands on the CdTe and CdTe/CdS core/shell QDs under the different conditions. Significant quenching happened in the CdTe/CdS

142 142 core/shell QDs capped with PEG ligands in PBS. Besides the PEG ligands, other organic compounds and common solvents have also been proved as the quenchers to the CdTe/CdS core/shell QDs. The SILAR method was applied to synthesize the CdTe/CdS core/shell QDs in order to improve the CdS shell on the CdTe QDs. Unfortunately, the quenching problem still existed. The PEG ligands still quenched the CdTe/CdS core/shell QDs. However it was shown that white light can increase the PL QY of the PEG-ligandcapped CdTe/CdS core/shell QDs. The PL QY of the CdTe/CdS core/shell QDs was changed by the different ligands on the surface. In addition to the effect on the PL QY, the ligands also can determine the hydrodynamic diameter of the CdTe/CdS core/shell QDs by the length of the ligands. For instance, the PEG-ligand-capped CdTe/CdS core/shell QDs had the largest hydrodynamic diameter since the PEG chain (~15 nm) was much longer than the small molecules such as TGA or some organic ligands. Moreover the length of the ligands depends on the solvents and temperature. The bond between the CdTe/CdS core/shell QDs for the aggregation has been proved as the physical bond instead of the chemical bond by sonication. Finally, TBP can make the CdTe and CdTe/CdS core/shell QDs spontaneously disintegrate in CHCl 3. This disintegration issue is affected by many factors such as light, solvents, the CdS shell, air, or the size of the QDs. Nevertheless, some mechanisms of the ligands effect on the CdTe and CdTe/CdS core/shell QDs are still not clear. More studies need to be carried out in the future.

143 143 Chapter 5 QD Electronic Structure and Dynamics 5.1 Charge Carrier Relaxation Pathways Charge carrier relaxation study supplies the important information of the physical properties of QDs to apply in solar cell application. In an excited QD, the electron is promoted from the valence band into the conduction band, leaving a hole in the valence band. Thus, an electron-hole pair (or exciton) is produced, and the energy of the exciton depends on the energy of the excitation. Before an exciton relaxes to its ground state, this exciton is called a hot exciton due to its excess kinetic energy. Once a hot exciton is produced, if there is no more energy being introduced, this hot exciton cools from the high energy level to the exciton ground state (1S e and 1S h ). Finally, the exciton recombines through a radiative (emission) or nonradiative (energy transfer, surface defects trapping, Auger etc.) process. Hot exciton cooling (charge carrier relaxation dynamics) is a substantial issue in QD science. Researchers are attracted by the relaxation time scale and pathways of QDs, because these two aspects impact the design principles for the QD based optoelectronic devices and QD solar cell. 6, 66, 88 For example, the slow relaxation process is 137, 138 required by the interband laser while the fast one is required by the intraband laser. Also, in the solar cell application, QDs as a photovoltaic material, the long life time of the hot exciton cooling process is desired. After an exciton is produced, intraband charge carrier relaxation happens first, so the electron and the hole relax from the high energy level to the ground state. In bulk II VI semiconductors, charge carriers relax fast by the Fröhlich interaction with longitudinal

144 144 optical (LO) phonons However, in QDs, because of the restrictions forced by the quantized energy states, the energy level spacings of the QDs are much larger than those of the bulk semiconductors, so that the charge carrier relaxation, especially the electron relaxation, via the interactions with phonons is inhibited. This phenomenon is called a phonon bottleneck. 142, 143 Nevertheless, in the ultrafast transient absorption (TA) experiment of the CdSe QDs, the electron relaxation is very fast. For example, the electron relaxation time of CdSe QDs with 4.1 nm radius from 1P state to 1S state is about only 500 fs, 135 although the energy gap between 1S state and 1P state is around eight LO phonon energies. Moreover, this intra-band relaxation becomes faster with the size of the CdSe QDs decreasing, which has an opposite trend for the energy loss by LO phonon. This means the electron must have other, nonphonon energy-loss mechanism to relax from high energy level to the ground state. Some work shows Auger interactions with carriers outside the QDs, 144 the coupling to defects, 145 or Auger type e-h energy transfer 146 can produce this fast relaxation dynamics. Nevertheless, Auger interactions with carriers outside the QDs and the coupling to defects are not intrinsic to QDs. Therefore the energy relaxation inside QDs is due to the Auger type e-h energy transfer, in which an electron relaxes from high energy level to low one by transferring the energy to a hole. This process allows the electron to transfer its excess energy to the hole, so that leads to a significantly fast relaxation dynamics. In CdSe, the effective mass of the electron is smaller than that of the hole ( e m eff < m h eff ); therefore, the energy gap between adjacent energy levels in the

145 145 conduction band is wider than that in the valence band, as shown in equations (1.9) and (1.10). Therefore the relaxation process of the hole via phonon coupling is much faster than that of the electron. Figure 5-1 illustrates the process of the Auger type e-h energy transfer in QDs. In Figure 5-1(a), the hole has already relaxed to the ground state (1S h ) by phonon, while the electron is still at 1P e state since the energy spacing in the conduction band is much wider than that in the valence band. Then the electron can transfer the energy to the hole by Auger. Hence the hole absorbs the energy from the hot electron and jumps to a higher energy level. Through this process, the electron relaxes to the ground state by transferring the energy to the hole by Auger, as shown in Figure 5-1(b).

146 146 (a) (b) Figure 5-1. Scheme of the Auger type e-h energy transfer. (a) The electron (black circle) is at the 1P state of the conduction band (CB), while the hole (white circle) is at the 1S state of the valence band (VB). The down solid arrow shows the relaxation direction of the electron, and the dash arrow presents the Auger type e-h energy transfer from the electron to the hole. (b) Energy state diagram of the exciton after the Auger type e-h energy transfer. Additionally, Guyot-Sionnest et al , 148 and Kambhampati and co-workers showed that surface ligands also can strongly affect the cooling process of the hot exciton. For example, TDPA (tetradecylphosphonic acid)-capped CdSe QDs have a short cooling lifetime (3.8 ps) from 1P e -1S e, while the same size CdSe QDs capped with n-

147 147 dodecanethiol (DDT) have a much longer (27 ps) 1P e -1S e cooling lifetime. 138 Figure 5-2 shows the ligands effect on the hot electron cooling process via the surface Cd 2+, where the electron in the 1P e state is transferred to the antibonding orbital of surface Cd 2+. Because mostly the Cd atoms on the surface have the covalent bond with the ligands, this causes the filled bonding orbital below the 1S e state and the empty antibonding orbital close to 1P e state. Therefore, the electron can be transferred from 1P e state to 1S e state by this ligands effect, and the energy loss leads to ligand vibration. Moreover, the hot hole cooling process can also be affected by this ligands vibration with the similar mechanism as the hot electron. Figure 5-2. Scheme of the hot electron intra-band relaxation process affected by the suface Cd ion bond ligands in the CdSe QDs. The energy state diagram on the right side belongs to the ligand-surface Cd orbital (adapted from Guyot-Sionnest et al.). 138

148 148 In summary, there are multiple channels for the hot electron and hole to relax to the band edge. Figure 5-3 shows there are three pathways for the hot electron relaxation and two pathways for the hole cooling. LO phonon and ligands vibration pathways are found in the both hot electron and hole relaxation processes, and the Auger type e-h energy transfer is a very important pathway for the hot electron to relax to the ground state (1S e ) via transferring the energy to the hole. Therefore, the total relaxation rate for the electron is k(r )electron = k(r) Auger + k(r) e ligands + k(r) e phonon, (5.1) and for the hole, k(r) hole = k(r) h ligands + k(r) h phonon, (5.2) where k is the relaxation rate for the different pathways as a function with radius of the QDs (R). For the electron, k(r) Auge r > k(r) e ligands > k(r) e phonon, and for the hole k(r) h ligands > k(r) h phonon. 148 k(r) e ligands and k(r) h ligands are not the same, and the relaxation rates affected by ligands are determined by the interaction between electron/hole and ligands. Also, the relaxation rates affected by phonon are controlled by the QDs material; in our CdTe and CdTe/CdS core/shell QDs, k(r) e phonon < k(r) h phonon.

149 149 Figure 5-3. Scheme of the relaxation pathways of the hot electron (upper) and hole (lower).

150 150 During the relaxation process, sometimes the electron or the hole can be trapped at the surface, and that affects the photoluminescence (PL) spectra, the PL quantum yield (QY), and the charge carrier relaxation dynamics. Usually defects are on the surface and even inside. 28 These defects can trap the electron or the hole. Besides the defects, unsaturated dangling atomic orbitals also play a role of the surface trap. These orbitals can attract electrons and holes. Figure 5-4 shows the electron or the hole in a CdSe QD can be transferred to the trap state. Figure 5-4. Energy state diagram of CdSe QDs shows surface trapping of conduction band electrons and valence band holes.

151 Types of Core/Shell QDs Typically there are two types (type I and type II) for the core/shell QDs. Figure 5-5(a) shows the band structures of type I QDs, in which the shell band gap (E g,sh ) is larger than the core band gap (E g,c ), so that both the conduction band energy level and the valence band energy level of the shell material are higher than those of the core material. Hence the electron and the hole can be confined in the core. Compared to core QDs, the type I core/shell QDs have to higher quantum yield and better stability. The other type of core/shell QDs (type II), as shown in Figure 5-5(b), (c) has the various offsets in the conduction band and the valence band. In the type II core/shell QDs system, only one band (either the conduction band or the valence band) energy level of the shell material is higher than that of the core material and the other band energy level of the shell material is lower than that of the core material. Therefore, either the electron or the hole can be confined in the core while the other one moves into the shell and is localized there. Thus in the type II core/shell QDs, the electron and the hole are separated in the core and the shell so that the spatially indirect band gap is smaller than either the core band gap or the shell band gap (Figure 5-5(b), (c)). This is why the type II core/shell QDs have a significant red-shift emission to the core QDs. For example, quasi-type II CdTe/CdS core/shell QDs used in my research work are able to provide more red-shifted 111, PL emission than the CdTe QDs.

152 152 (a) (b) (c) Figure 5-5. Band structure diagrams of core/shell QDs. (a) Band structure of type I core/shell QDs, black circle: electron, white circle: hole. (b) and (c) Band structure of type II core/shell QDs. The conduction band of the CdTe/CdS core/shell QDs is different from that of typical type II core/shell QDs. The electron energy level in the conduction band is delocalized in the core and the shell because the conduction band energies of the core and the shell are very close. For this reason, CdTe/CdS core/shell QDs are called quasi-type II. Piryatinski et al. introduced a mathematical method to calculate the energy states in the type II core/shell QDs. 152 We used this method to calculate the energy states in our CdTe/CdS core/shell QDs. Therefore an electronic structure model for the CdTe/CdS core/shell QDs was built, as shown in Figure 5-6.

153 153 Figure 5-6. Band energy diagram of CdTe/CdS core/shell QDs. The CdTe and CdS materials have different dielectric constant, ε 1 and ε 2, respectively. The dielectric constant of the surrounding medium is ε 3. R is the radius of the CdTe core and the H is the height of the CdS shell. The vertical dash lines show the edges of the core and the shell. The top

154 154 band is the conduction band (CB), and the bottom one is the valence band (VB). In the e conduction band, the black circle represents the electron, and U 0 is the conduction band offset energy between the core and the shell. The white circle in the valence band represents the hole, and U h 0 is the valence band offset energy between the core and shell. E g, E g,c, and E g,sh are the indirect band gap of the CdTe/CdS core/shell QDs, the band gap of the CdTe core, and the band gap of the CdS shell, respectively. The energy of the bulk conduction band edge of the shell is set as zero (E e = 0) and the up arrow shows the direction of the electron energy increasing, while the energy of the bulk valence band edge of the core is set as zero (E h = 0) and the down arrow shows the direction of the hole energy increasing. (Adapted from Piryatinski, A.; Ivanov, S. A.; Tretiak, S.; Klimov, V. I., Effect of quantum and dielectric confinement on the exciton-exciton interaction energy in type II core/shell semiconductor nanocrystals. Nano Letters 2007, 7, (1), ) The boundary conditions of the radial wavefunction for the CdTe/CdS core/shell QDs are finite at the center of the core and zero outside the shell. Another boundary condition is the effective-mass-weighted wavefunctions and their derivatives are continuous at the core-shell interface. 153 In this calculation, only the lowest energy, the zero angular momentum (1S) states, in the conduction band and the valence band were considered. Then the wavefunctions that satisfied these boundary conditions are a a a sin( k r) R ( r) = N 0 r < R (5.3) a r sin( k R) a a a sin( q ( R + H r)) R ( r) = N R r < R + H (5.4) a r sin( q H )

155 where a is e or h for the electron and hole, respectively, r is the distance from the center a of the core/shell QD to the particle, R (r) is the wavefunction, 155 a N is the normalization a constant derived from setting the integral of R (r) over the volume of the core/shell QD to unity, R and H are the radius of the core and the thickness of the shell, respectively. Specifically in the CdTe/CdS core/shell QDs system, the electron wavevector components are e k = e e ( 2m ( E U h and c e 2 1/ 2 0 ) / ) e q = e e ( 2m ( E / h s 2 2 ) 1/, while the hole wavevector components are h k = ( 2 h h h 2 2 m ( / ) 1/ c E and h q = h h ( 2m ( E U h, s h 2 1/ 2 0 ) / ) where U 0 is the offset energy of the conduction band or the valence band, and e m c, e m s, h m c, h m s are the effective masses of the electron (e) and the hole (h) in the core (c) and shell (s). By solving the equation (5.5), the 1S eigenenergy a a a a a a [ 1 k R cot( k R)] m / m = 1+ q R cot( q H ), (5.5) s c a E is achieved as the lowest root. Coulombic interactions between the carriers were neglected in this treatment. We calculated two different sizes CdTe/CdS core/shell QDs, one is the small core/shell QDs, which have 1.6 nm radius for the core and 0.33 nm height for the shell, and the other one is the large core/shell QDs that the core has 2 nm radius and the shell has 0.42 nm height. The offset energies of the conduction band (U 0 e ) and the valence band (U 0 h ) were taken as 0.35 ev and 1.31 ev (listed in Table 6), respectively, which were calculated from the photoelectric threshold energies. 154 However, other different offset energy values were reported by other researchers, for instance, in Wang et al. s paper U 0 e = 0.04 ev, U 0 h = 0.99 ev, 155 in Schöps et al. s paper U 0 e = 0.83 ev, U 0 h = 1.8

156 ev, 149 in Ghosh and his co-workers s paper, U 0 e = 0.2 ev, U 0 h = 1.1 ev. 156 and in Chang et al. s paper, U 0 e = 0.1 ev, U 0 h = 0.99 ev. 111 Apparently there is large of uncertainty in these numbers, and the values we used are from the bulk material experimental data, which is within the range of these various values. Even though these values cannot be extremely precise, it is still valid for the calculation, because the values we used are between the highest and lowest values reported by those groups and the small difference of these offset energy values does not strongly affect the results of delocalization in the conduction band and strong confinement in the valence band. The effective masses are m = 0.1 m 0, e c e m s = 0.19 m 0, h m c = 0.4 m 0, and h m s = 0.8 m 0, where m ( kg) is the mass of the electron (listed in Table 6). 157 The dielectric constants ε 1 (core) and ε 2 (shell) are and 8.9 (listed in Table 6), 159 respectively. Now the a k and a q can be represented by the expressions of a E, and the equation (5.5) has only one unknown a E. Therefore solving this equation (5.5) by MATLAB software, we can gain the 1S eigenenergy of the electron and the hole states in the small and large CdTe/CdS core/shell QDs that eigenenergy in small QDs), e E sm = ev (the electron 1S h E sm = ev (the hole 1S eigenenergy in small QDs), = ev (the electron 1S eigenenergy in large QDs), and eigenenergy in large QDs). e E lg h E lg = ev (the hole 1S The wavefunctions for the CdTe/CdS core/shell QDs can be obtained by putting the eigenenergy into equations (5.3) and (5.4). In order to find out the difference of the wavefunction between the CdTe core and the CdTe/CdS core/shell QDs, wavefunctions

157 overlap of electron and hole wavefunctions in core and core/shell QDs R R+ H 157 e 2 h e 2 h ( R ( r ) r R ( r) dr + R ( r) r R ( r) dr ) was calculated by MATLAB software. The 0 R results are that both wavefunction overlaps of the small and large core QDs are 1 (100% overlap), but the wavefunction overlaps in the small and large core/shell QDs are and 0.975, respectively. Through the eigenenergies, e E sm, h E sm, e E lg, and h E lg, the band edge energies of the small and large core/shell QDs can be predicted as ev for the small core/shell QDs and ev for the large core/shell QDs. Compared to the experimental band edge energies of the small core/sell QDs (1.968 ev) and the large ones (1.771 ev), the differences in the small core/shell QDs and the large core/shell QDs are ev and ev, respectively. These errors are not only from the uncertainty of offset energies of conduction and valence bands but also from the neglect of Coulombic interactions between the carriers in calculation. Apparently, the hole 1S eigenenergy is much lower h than the valence band offset energy ( E, E lg < U h 0 ), so that the hole is localized in the h sm core. If the errors of the band edge energy are assumed only from the conduction band offset energy, the maximum conduction band offset energies of the small and large core/shell QDs is ev (U 0 e ev) and ev (U 0 e ev), respectively, which are still smaller than the electron 1S eigenenergy of the small and large core/shell e QDs ( E, E lg > U e 0 ). Also, the wavefunction overlaps of the core/shell QDs show the e sm wavefunctions of the electron and hole are different in the conduction and valence bands. If the hole wavefunction is localized in the core, the electron wavefunction should be

158 158 delocalized in the core and shell. Therefore, the electron is delocalized in the core and shell, because the electron 1S eigenenergy of the core/shell QDs is larger than the conduction band offset energy and the wavefunctions overlap of the core/shell QDs is not 100%. Figure 5-7(a) shows the wavefunctions versus the radius of CdTe/CdS core/shell QDs by the calculation. In calculation the radius of the CdTe core used is 2 nm and the thickness of the CdS shell is 1 nm. In Figure 5-7(a), the wavefunction of the conduction band exists almost from 0 to 3 nm radius, but the wavefunction of the valence band drops dramatically when it crosses the edge of the core and shell (dash line at 2 nm). Moreover, Figure 5-7(b) presents radial probability density versus radius of CdTe/CdS core/shell QDs. The radial probability density distribution of the conduction band covering both core and shell is broader than that of the valence band, which is mostly in the core. Besides these calculations, the experimental data also shows that the transition energy decreasing only came from the electron energy states decreasing after the CdS shell was deposited on the CdTe core, but the hole energy states were kept the same. These data and their implications will be discussed in detail in Chapter 6. That means the hole was localized in the core since the hole energy states did not change in the core and core/shell QDs. Consequently, CdTe/CdS core/shell QDs are the quasi-type II QDs since the electron is delocalized in the core and shell but the hole is localized in the core only.

159 159 Table 6. Different parameters values in wavefunctions calculation. Offset Energies between Conduction Band (U e 0 ) 0.35 ev Core and Shell Valence Band (U h 0 ) 1.31 ev Electron in Core( m ) 0.1* m 0 Effective Masses h Hole in Core ( m c ) 0.4* m 0 Electron in Shell ( m ) 0.19* m 0 Hole in Shell ( m ) 0.8* m 0 Mass of Electron( m 0 ) kg Dielectric Constants Core (ε 1 ) 11 Shell (ε 2 ) 8.9 Band Gap Energy of Core (E g,c ) 1.5 ev 151 Band Gap Energy of Shell (E g,sh ) 2.5 ev 151 Energy Gap between Conduction Band of Shell and Valence Band of Core (E g ) 1.15 ev h s e c e s

160 160 Figure 5-7. (a) Wavefunction of CdTe/CdS core/shell QDs. (b) Radial probability density of CdTe/CdS core/shell QDs. The dash line represents the edge of the core and shell at r = 2 nm.

161 Experimental Probes of Electronic Structure and Dynamics Photoluminescence Decay Time-resolved PL is a method in which the sample is excited by a laser pulse with a fixed wavelength. Then the decay of PL at a certain wavelength is measured by the detector with respect to time. The lifetimes of the PL decay are very important photophysical parameters for QDs to explore the process of electron-hole recombination. For this reason, PL decay measurement was used to study the photophysical properties of CdTe and CdTe/CdS core/shell QDs and will be described in Chapter 6 and 7. The QDs sample was placed in a cuvette and diluted with solvent. PL decay measurements were made using the time-correlated single photon counting method (shown in Figure 5-8). A mode-locked, diode-pumped Ti:sapphire laser as a laser source (Mai Tai, Spectra-Physics) produced a train of 120 fs pulses with a wavelength of 795 nm and repetition rate of 40 MHz. The laser beam was separated into two beams by a beamsplitter, with one traveling to a pulse detector while the other continues through an electro-optic pulse picker (Quantum Technology, Inc.), by which the repetition rate is decreased. There is a polarizer in the side of the pulse picker that is away from the laser source, and the horizontally-polarized laser beam is blocked by this polarizer. In order to let the laser pass the polarizer, the polarization of the laser beam has to be adjusted by the nonlinear crystal (such as potassium di-deuterium phosphate, potassium titanyl phosphate, or beta-barium borate (β-bab 2 O 4, BBO) et al.), which is controlled by a 300 V voltage. When the voltage is on, the polarization of the crystal is changed by the electric field, thus the laser polarization is also changed after it travels through the crystal. The pulse

162 162 repetition rate therefore can be controlled by a pulse picker controller. After the laser pulse passes the pulse picker, another beamsplitter separates the laser pulse into two directions: one beam travels to a reference photodiode and the other one follows the same direction to a BBO crystal and the photon energy is doubled by focusing the pulse train into this crystal. When the laser pulse is caught by the reference photodiode, the signal is transferred to the PC and it starts counting the time. The other laser beam hits a sample in a dark box, and then the sample emits the photons. A small monochromator is used to select the emission wavelength, and a fast PMT is used as the detector. This PMT detector is very light sensitive, and it records one data point at a specific time position only once a photon is measured. Millions of measurements can be done by the PMT detector in a second, and after a period of measurings, time-resolved PL decay is obtained by a Becker & Hickl SPC-430 single photon counting module that was used for data collection.

163 163 Figure 5-8. Scheme of the time-resolved photoluminescence decay experiment Transient Absorption Spectroscopy Transient absorption (TA) spectroscopy is a technique that can measure the excited absorption by pump-probe signal. The sample is excited by a certain wavelength laser beam (pump), and the femtosecond detector collects the absorbance change after being excited with the pump beam. The probe beam is used to measure the absorbance with sub-picosecond resolution. The electronic structure and charge carrier relaxation dynamics of QDs can be explored by TA spectroscopy. The different wavelength range selections of the probe beam allow researchers to investigate interband and intraband processes of excitons in QDs, as well as charge transfer between QDs and the charge acceptors. Therefore, TA spectroscopy as an advanced tool was used to investigate the

164 164 electronic structure of CdTe and CdTe/CdS core/shell QDs and described in Chapter 6 and 7. A concentrated QDs solution was placed in a small cuvette, which has 2 mm length. TA measurements were performed using a commercially available TA spectrometer (HELIOS, Ultrafast Systems). An amplified Ti:sapphire laser (Solstice, Newport/Spectra-Physics) was used for sample excitation. The Solstice provides 3 mj, 150 fs pulses at a repetition rate of 1 khz and 795 nm wavelength. This beam was split to produce separate pump and probe pulse trains. The pump pulses were produced by directing one half of the Solstice output into an optical parametric amplifier (TOPAS-C, Light Conversion) to generate pulses with widely tunable wavelength and pulse energies in the µj range. The probe beam was generated by focusing a small fraction of the Solstice fundamental onto a sapphire crystal to generate a white light continuum extending from approximately nm. Neutral density filters were used to reduce the pump intensity below the threshold for biexciton production in the QDs. Pumping was typically achieved using a wavelength of 400 nm and pulse energy of 50 nj. The pump beam was focused down to a spot size of approximately 1 mm diameter, and the sample pathlength was 2 mm. 5.4 Ultrafast Charge Carrier Dynamics of CdSe Quantum Dots by Femtosecond Transient Absorption Spectroscopy CdTe QDs have the similar physical and chemical properties to CdSe QDs. This brief review of ultrafast charge carrier relaxation dynamics of the CdSe QDs can help to understand the charge carrier relaxation of the CdTe QDs in our work. The ultrafast

165 charge carrier dynamics of the CdSe QDs with a variety of surface passivations have been studied by femtosecond TA spectroscopy in the visible and infrared spectral , 160 ranges. These quasi-zero-dimensional QDs have atomic-like, discrete energy spectra, which are size dependent, due to the three- dimensional quantum confinement The different sizes of the high quality CdSe QDs can be obtained by the chemical route. Therefore it is possible to study the ultrafast charge carrier dynamics of the CdSe QDs with different size and passivation layer. In the nano scale, because of the large energy level spacing and the increased surface-to-volume ratio, the charge carrier dynamics of QDs are substantially different from those of the bulk materials. Under the excitation state, the absorption of the QDs is different from that in the ground state, where some ranges have the bleach and some ranges have the photoinduced absorption. The bleach is due to the energy states being occupied by an excited electron so that other electrons can not be excited to that same state. This is called state filling effect. Also the electrons play more important role in the state-filling-induced absorption than the holes because of the significant difference between the electron and the hole masses (m * h/m * e 6), and the degeneracy of the valence band. After the electron is excited into the high energy level in conduction band, it starts to cool down to the ground state (1S e ) and meanwhile the hole relaxes to the valence band ground state (1S h ). Finally these ground states are occupied by the exciton, and the absorption from these transition states is decreased. Photo-induced absorption can also be observed. This is generally caused by biexiton interactions. The separated electron and hole can produce a

166 166 dipole in a QD, and then the energy states are shifted by this very intense electric field. The phenomenon that causes the absorption shift is referred to as the Stark effect. In ultrafast charge carrier dynamics study, the power control of the laser source is very important because high laser power can excite multi-excitons instead of single one. If the average number of the excitons per QD, <N 0 >, is too large, more than one e-h pair will be generated, for example, two e-h pairs (biexciton) are generated once 1 < <N 0 > < 2. Therefore the situation will be more complicated for the interactions between two pairs than the single charge carrier. In order to avoid producing multi-excitons, <N 0 > has to be small enough. If <N 0 > equals to 0.1, which means among the ten QDs only one QD can be excited, the probability of multi-excitons generating is very low. From <N 0 >, the laser power can be calculated by the following equations. < 0 N >= Φ σ. (5.6) In equation (5.6), Φ is the pump fluence (photons/cm 2 ), as shown in equation (5.7), and σ is the absorption cross section at the pump wavelength as shown in equation (5.8). E 1 E A Pulse Φ =, (5.7) Photon Beam In equation (5.7), 21 σ = ε λ. (5.8) E Pulse (J/pulse) is the energy of one pulse, as shown in equation (5.9), E Photon (J/photon) is the energy of one photon, as shown in equation (5.10), and

167 A Beam (cm 2 ) is the laser beam area ( cm 2 ). In equation (5.8), ε λ ( extinction coefficient at the pump wavelength. E L mol 167 ) is the cm P P =, (5.9) υ 500 Pulse = rep hc E Photon =. (5.10) λ P (W) is the power of the laser, and υ rep (pulse/sec) is the repetition rate of the laser pulse in equation (5.9). h is Planck s constant, c is the velocity of light, and λ is pump wavelength in equation (5.10). Finally the maximum desired laser power can be calculated by the equation (5.11), P < N 0 > max υ rep ABeam h c = 1. (5.11) ε λ In Section 5.5, there is an introduction about the determination of the maxima laser power by experimental method. Figure 5-9 illustrates the normalized UV-visible absorption spectra of the CdSe QDs with band edge energy at 2.1 ev. The spectra of the CdSe QDs give some wellresolved features, which correspond to the interband optical transitions coupling different electron and hole quantized states. And the electron energy states of the CdSe QDs are labeled by a letter to indicate the angular momentum of the envelope wave functions, for example, S for l = 0, P for l = 1, D for l = 2, etc. In this system, the lowest three energy states are 1S, 1P, and 1D following the order of increasing energy. The hole energy states of the CdSe QDs are denoted in similar fashion. The subscripts in the notation of the hole λ

168 168 energy states represent the total angular momentum. The lowest hole energy states are 1S 3/2, 1P 3/2, and 2S 3/2 following the order of the energy increasing. In Figure 5-10, the diagram shows the various band edges of the CdSe bulk semiconductor and QD. Because of the quantum confinement, the band edge of the CdSe QD is wider than that of the CdSe bulk semiconductor. Also, depending on the size of the CdSe QDs increasing, the band edge energy becomes smaller. The red, blue and green arrows in Figure 5-10 represent the transition of 1S (e) 1S 3/2 (h), 1S (e) 2S 3/2 (h), and 1P (e) 1 P 3/2 (h), respectively, by the order of the energy increasing. These lowest energy transitions can be assigned to the lowest bands in the UV-visible spectra of the CdSe QDs (Figure 5-9).

169 169 Figure 5-9. UV-visible absorption spectrum of CdSe QDs, the different features in the spectrum were assigned to 1S(e)-1S 3/2 (h), 1S(e)-2S 3/2 (h), and 1P(e)-1P 3/2 (h) transition. (The UV-visible absorption data was collected by Chris McCleese.)

170 170 Figure Energy level diagram of CdSe QDs (right two) and bulk semiconductor (left). The red arrow represents 1S (e) 1S 3/2 (h), the blue arrow represents 1S (e) 2S 3/2 (h), and the green arrow represents 1P (e) 1 P 3/2 (h). E g0 and E gqd (R) represent the band gap energy of the bulk semiconductor and QD, respectively. Time-resolved femtosecond TA spectroscopy was used to study the energy structures and the carrier dynamics of the CdSe QDs. 160, Figure 5-11 shows TA spectra (without chirp correction) of the CdSe QDs (band edge at 2.1 ev) at 0.5, 1, and 2 ps after excitation compared to the UV-visible absorption spectrum. The energy of the pump beam was 3.1 ev. The TA spectra were detected from 1.38 to 3 ev with a

171 171 femtosceond white-light continuum generated by a sapphire crystal. The spectra in Figure 5-11 were obtained at very low pump intensities, which the initial nanoparticles population <N 0 > was less than one e-h pair per nanoparticle. The red short dash dot line on the top of Figure 5-11(a) was the UV-visible absorption spectrum of the CdSe QDs, which was similar with the rest three TA spectra. The features in TA spectra matched those in UV-visible absorption spectrum. B 1, B 2 and B 3 represented the bleaches from the transitions of 1S (e) 1S 3/2 (h), 1S (e) 2S 3/2 (h), and 1P (e) 1P 3/2 (h), respectively. A 1 was the photo-induced absorption from the Stark shift of the 1S transition, and A 2 was the photo-induced absorption from the Stark shift of the 1P transition, which was due to the biexciton interactions effect. At 1 and 2 ps, B 1, B 2 increased because of the occupation of the 1S electron states (state-filling) but B 3 decreased due to electron relaxation from the 1P to the 1S electron state. Furthermore, Figure 5-11(b) shows the normalized kinetic traces of features A 1 and B 1, where the sign of the intensity for B 1 was reversed. In the beginning, the intensity of A 1 rose faster than B 1, because once the exciton was produced, the ground states in the bands had not occupied, so that the biexciton interactions effect happened earlier than the state filling. Then the ground states (1S) were occupied by the exciton after it relaxed, the intensity of A 1 reduced sharply to the zero and meanwhile the intensity of B 1 was still increasing because of the state filling, that the electron was hard to be excited from the 1S 3/2 (h) stated to the 1S (e) state.

172 172 Figure (a) TA spectra of CdSe QDs recorded at 0.5, 1, and 2 ps after excitation (without chirp correction) in comparison to the UV-visible absorption spectrum. (b) Normalized TA dynamics at the positions of the B 1 (solid line), and A 1 (dashed line) features. (These TA data were from Dr. Van Patten, and the CdSe QDs were synthesized by Chris McCleese.)

173 Laser Power Dependent Bleach Intensity In TA measurement, the laser power applied to excite QDs is very important; therefore, this laser power dependent bleach intensity experiment was done to find out the certain laser power, by which only one exciton can be produced in a QD, to excite the CdTe/CdS core/shell QDs. In the low laser power range, because of only one exciton generated the intensity of the bleach of the 1S 3/2 (h)-1s(e) transition in TA measurement is proportional to the laser power, otherwise more than one exciton can be produced so that the trend of the bleach intensity to the laser power is not linear any more. The ultrafast charge carrier dynamics study in the dissertation is only focused on single excition condition. Moreover any multiexcitons conditions make the charge carrier relaxation process complicated and are not discussed in this dissertation. The CdTe/CdS core/shell QDs (734 nm emission wavelength) sample were diluted in CHCl 3 and exposed to the air. The laser excitation was fixed at 400 nm, and laser power was adjusted by the filter. The measurement started from the lowest laser power (1.1 µw) and finished at the highest laser power (711 µw). The TA data for each laser power was collected from 500 ps to 700 ps, in which range the TA kinetic trace of 1S 3/2 (h)-1s(e) transition became flat, and the final bleach intensity of 1S 3/2 (h)-1s(e) transition was the average intensity from 500 ps to 700 ps. Figure 5-12 illustrates this trend via testing the CdTe/CdS core/shell QDs. Below the 50 µw laser power, the bleach intensity (the sign of the absorbance change was reversed) is almost proportional to the power; however depending on the laser power increasing, the plot became flat. Consequently the laser power lower than 50 µw can

174 174 generate the single exciton in the CdTe/CdS core/shell QDs; moreover if the power is over 50 µw, more than one exciton can be produced in the CdTe/CdS core/shell QDs and this multi-exciton makes the situation more complicated. In my research work, the laser power was always kept about µw to ensure only one exciton generated. Figure Plot of B1 bleach intensity in CdTe/CdS core/shell QDs versus pump fluence. Pump laser wavelength was 400 nm. Inset: data points are within 50 µw laser power.

175 Chapter 6 Ultrafast Exciton Dynamics in CdTe Quantum Dots and Core/Shell CdTe/CdS Quantum Dots 175 (Most content of this chapter is from our paper with same title published in The Journal of Physical Chemistry C, and authorization is in Appendix D.) 6.1 Introduction Colloidal CdTe QDs have been the subject of intense research interest because of the relatively narrow bulk band gap, the facile synthetic routes, and the compatibility with other readily synthesized, colloidal II-VI semiconductor materials. 105, QDs and nanoheterostructures containing CdTe appear to hold promise for multiple applications, including NIR lumophores and solar energy conversion materials. 4, 102, 174, 175 The valence and conduction bands of CdTe are offset from those of CdSe and CdS, so that it is 4, 111, 172 possible to prepare type-ii core/shell QDs from combinations of these materials. The type-ii excitonic structure has important consequences for the electronic and optical properties of the resulting heterostructures, including intraparticle charge separation along the radial coordinate, dramatic reduction of the radiative recombination rate, and lowering of the optical band gap energy sometimes below that which is observed in the component bulk materials. These effects could be harnessed to enable or improve charge carrier separation prior to carrier cooling, to improve the rates and efficiencies of photoinduced charge separation, and to tailor the optical response of these materials in the near infrared. Pump-probe transient absorption (TA) spectroscopy is a powerful tool for 135, 136, 138, studying the ultrafast photophysics of molecules and colloidal nanostructures.

176 148, 160, 167, It has been used with tremendous success in detailed studies of CdSe 176 QDs to understand ultrafast carrier dynamics. 135, 138, 148, 160, 167, 177, Unlike the CdSe system, the CdTe system has not been fully characterized on ultrafast time scales. While several groups have reported ultrafast TA measurements on nanoheterostructures containing CdTe, 155, 156, 176, 180, 182, major gaps remain in our understanding of this system. Studying the spectral and dynamic characteristics of these nanoheterostructures will provide insight into their photophysical response, including charge carrier cooling and localization (i.e. intraparticle charge transfer) dynamics. A strong understanding of the behavior of pure CdTe QDs will be required before it is really possible to understand and interpret measurements on CdTe-containing nanoheterostructures. As mentioned above, the ultrafast photophysics of CdSe QDs has already been extensively explored, and a complex picture of the excitonic behavior of those QDs is beginning to emerge. Interactions between charge carriers in specific excitonic and biexcitonic states induce transient shifts in energy levels and in the associated optical transitions observed in TA spectra. Furthermore, the intraband carrier relaxation (a phenomenon that has primary importance in the development of solar cells and lasers) is apparently governed by a complex interplay of exciton-phonon interactions, electron-hole interactions, carrier interactions with surface states, and carrier interactions with vibrational energy levels of the organic molecules (i.e. ligands and solvent) just outside the QD surface. 138, 148, 160, 189, 198, 199 Many of the previous studies on CdSe QDs have examined carrier cooling and trapping kinetics as a function of QD size and/or surface functionalization. By altering QD composition, surface reactivity, and crystal structure, it

177 177 should be possible to learn more about these mechanisms and to ascertain whether the behavior of CdSe is universal and representative of a wide range of QD systems. There are several differences between CdSe and CdTe that might affect ultrafast carrier dynamics. For example, electron cooling rates in CdSe QDs are mediated in large part by Auger interactions in which hot conduction band electrons undergo intraband transitions by transferring excess energy to the valence band hole, which has a higher density of states than the conduction band electron. CdTe has a larger dielectric constant 158, 200 than CdSe (11 in CdTe versus approximately 6 in CdSe), and the additional dielectric screening could reduce the Coulombic coupling that governs Auger processes in NCs. In addition, the density of hole states near the valence band edge should be reduced in CdTe relative to CdSe due to the very large (0.9 ev) spin-orbit interaction. The strong spin-orbit interaction shifts the J = ½ hole states far from the valence band edge, and this reduced density of low energy hole states could further reduce the rate of electron cooling via Auger interactions. The reduced density of low-energy valence band states in CdTe may not only affect the relaxation of conduction band electrons, but may also reduce the relaxation rate of valence band holes in comparison with CdSe. It has been shown that hole relaxation in CdSe QDs occurs faster than would be expected for phonon-mediated processes and that the relaxation rates are nearly independent of particle size. 148, 189 Those results have been rationalized on the basis of non-adiabatic processes in which the intraband transitions of valence band holes are coupled to vibrational modes of surface-bound ligands. Depending on the mechanism that predominates in CdTe NCs, the size-dependent density

178 178 of valence band states may play an important role in determining the rate of cooling for photoexcited holes. If relaxation of holes bound within the QD core is retarded in CdTe relative to CdSe, it may enhance the importance of hot hole trapping at the QD surface or on surface-bound hole acceptors. The role of surface trapping may be further altered in CdTe by differences in the surface reactivity patterns of tellurides compared with selenides. In this paper we report and analyze size-dependent, visible transient absorption spectra and dynamics from CdTe QDs and CdTe/CdS core/shell NCs. Assignments of the most prominent features, including the three lowest energy bleaches are made in accordance with previous experimental and theoretical work. The effect of the shell on the energy level spacings in the conduction band and valence band is examined. The results are consistent with a delocalization of the conduction band electron into the CdS shell and strong confinement of the valence band hole within the CdTe core. The TA bleach assignments, in combination with kinetic analysis of the individual peaks, provide evidence for size dependent changes in hole relaxation pathways using information from the visible spectral region. In the present experiments, samples were excited well above the band gap energy with a single pump wavelength (400 nm). Carrier cooling dynamics observed in these experiments arise from a combination of processes involving several excitonic states, so the observed cooling rates may differ from those observed when near-band-edge excitation or state-selective pumping is used. On the other hand, these measurements provide a look at the ultrafast carrier dynamics that prevail in CdTe QDs under conditions

179 179 of non-specific photoexcitation with a large excess photon energy. The results can be directly compared with earlier TA results obtained by Klimov and coworkers 198 on CdSe QDs in order to gauge the differences between these two material systems. In the core/shell systems, a new, high energy bleach is reported that is believed to be associated with the penetration of the conduction band electron into the shell material. The spectral position of this bleach is near the ground state absorption of the CdS shell. The signal associated with this bleach feature should be exclusively dependent on electron occupation of the conduction band, and should not be strongly influenced by hole dynamics. As mentioned above, the analysis of ultrafast time evolution of multiple peaks in the TA data can be used to glean information about hole dynamics. Similarly, the analysis of this high energy bleach together with the lower energy bleaches should provide a means to isolate and follow electron dynamics in the excited core/shell NCs. Distinguishing contributions of electrons and holes to state-filling-induced bleaches in TA spectral data has not been routinely achieved. In the past, analysis of single carrier dynamics in colloidal semiconductor QDs has usually relied on TA measurements in the mid-infrared spectral region. 135, 160, 198, 199, In recent years, Kambhampati and coworkers have extracted electron and hole dynamics in low-lying excitonic levels through analysis of carefully-designed experiments that selectively pump specific initial excitonic states. 148, Indeed, they were the first to emphasize the importance of the differences between the two lowest energy absorption features. Their experiments typically involve measurements using two different pump wavelengths, allowing extraction of the individual carrier dynamics by subtraction of kinetic traces at

180 specific probe wavelengths. An advantage of those experiments is that specific state-tostate transition rates can often be determined, allowing for estimation of individual carrier cooling rates through the lowest few excitonic levels. Weiss and coworkers have isolated and followed individual carrier dynamics by a different method. They have found that analysis of TA measurements in the near infrared spectral range can be used to directly 205, 206 follow the individual carrier dynamics in CdSe NCs. 6.2 Electronic States in Colloidal CdTe Quantum Dots 180 Understanding the electronic states in CdTe QDs is crucial before the ultrafast exciton dynamics study in CdTe QDs and Core/Shell CdTe/CdS QDs. Zhong et al. have reported the size-dependent electronic states of CdTe QDs using 2D photoluminescence excitation (PLE) spectroscopy. 207 They synthesized CdTe QDs with seven different sizes (diameters from 1.85 nm to 6.6 nm) via hot injection in ODE, which was very similar with one of our procedures to synthesize the CdTe QDs. For their 2D PLE measurements, a 2 nm bandpass was used for the 1.85 nm CdTe QDs, 1 nm bandpass for the 2.1 nm QDs, and 0.75 nm bandpass for the 2.8 nm-6.6 nm QDs. These slit widths provided optimum resolution and signal-to-noise ratio. Figure 6-1(a) shows the various energy levels from the seven different sizes CdTe QDs. This diagram combined the seven 2D PLE maps together. Here, the circles of the different colors represent the different sizes of CdTe QDs, and the numbers of the lines mark the energy states from low to high levels following increasing number. For example, line 1 is the 1S 3/2 (h)-1s(e) transition (band edge), line 2 is the 2S 3/2 (h)-1s(e) transition, and line 3 is the 1P 3/2 (h)-1p(e) transition. The assignment of the lines followed effective

181 181 mass calculations from Efros et al. 208 The red lines in Figure 6-1(b) are the energy levels calculated by Efros et al., and the black lines from the experimental data in the report of Zhong et al. were very close to the effective mass calculation results (red lines). Consequently, the exact excitation energy of the different energy levels can be found by the certain PLE energy of the CdTe QDs from this diagram. In our work, we used this diagram to assign the features in the CdTe QDs TA spectra.

182 182

183 183 Figure 6-1. (a) Size-dependent energy levels of seven CdTe NCs, shown in different colors. Straight lines overlie the data to illustrate the correspondence between different samples. (b) Dark line: size-dependent energy levels replotted with the excitation energy of the states in relation to the first dark excited state versus the PL energy. Red line: effective mass calculations published by Efros et al. 207 (Reproduced from Zhong, H.; Nagy, M.; Jones, M.; Scholes, G. D., Electronic States and Exciton Fine Structure in Colloidal CdTe Nanocrystals. Journal of Physical Chemistry C 2009, 113, (24), Used with permission) 6.3 Measurements Except for the case of the aqueous core/shell samples, the samples were kept under argon for absorption, PL, and TA spectroscopic measurements. The samples were prepared by diluting the crude QD samples in argon-saturated CHCl 3. These solutions were then loaded into an airtight cuvette and removed from the glovebox. SAXS, TEM, powder XRD, and ICP-OES measurements were performed on purified, air-exposed QD samples as described below. For SAXS measurements, the crude core and core/shell QDs were removed from the glovebox and purified in air by precipitating (one time only) as described above (Section 3.3.1). Approximately 50 nmol of QDs were purified and dissolved in 1 ml toluene for each measurement. SAXS measurements were made using a bench-top instrument (SAXSess, Anton Paar GmbH). A Cu K-α X-ray line source was used as the probe. The particles were measured using a flow-through sample cell in transmission mode. The scattered X-rays were first recorded by a two-dimensional (2D) detector, and

184 184 these 2D patterns were converted into 1D patterns after subtracting background and desmearing. The 1D SAXS patterns were analyzed by Prof. Gang Chen (OU Physics & Astronomy) using the Irena software package. 209 Size distributions of the particles were derived by fitting the SAXS patterns using a total non-negative least squares method. 209 For the CdTe QDs, a spheroid shape model was applied, while for the CdTe/CdS coreshell particles, a core-shell shape model was used for the fitting. For TEM measurements, the samples were precipitated three times following the procedures described in Section 3.3.1, and a very small amount was dissolved in toluene. A drop of the toluene suspension was deposited onto a carbon-coated copper TEM grid (Electron Microscopy Sciences) and allowed to air-dry overnight. Transmission Electron Micrographs were collected on a JEOL JEM-1010 operating at an accelerating voltage of 100 kv and at 100,000 direct magnification. For XRD measurements, the crude core and core/shell QDs were removed from the glovebox and purified in air by precipitating three times. After the third precipitation, the QDs were allowed to dry thoroughly on a planar glass substrate. Powder diffraction measurements were made on a Rigaku Miniflex diffractometer using a Cu K-α rotating anode source operating at 30 kv and 15 ma. 6.4 Results Basic Characterization CdTe and CdTe/CdS QDs of various sizes were prepared and studied. The trends described in this article apply to all the QD samples studied, but for simplicity and clarity, the results reported here focus on a selected group of these samples. Most of the data

185 185 presented here come from four particular samples, designated small core, small core/shell, large core, and large core/shell. Figure 6-2 shows absorption (solid line) and emission (dotted line) spectra of these four QD samples. Figure 6-2. Nomalized UV-visible absorption and PL emission spectra of (a) small CdTe core QDs, (b) small CdTe/CdS core/shell QDs, (c) large CdTe core QDs, and (d) large CdTe/CdS core/shell QDs. Solid lines are the UV-visible absorption spectra; dot lines are the PL emission spectra. PL quantum yield (Φ PL ) is also reported for each sample.

186 186 The sizes of CdTe and CdTe/CdS QDs used in this work were measured using three different methods, UV-visible absorption, TEM, and SAXS. Absorption measurements can provide a useful measure of particle size for CdTe cores using the empirical formula reported by Yu et al. 13 Figure 6-2 presents UV-visible absorption spectra (solid lines) and PL emission spectra (dotted lines) of the four QD samples. On the basis of the absorption spectra, the particle size for the small cores and large cores are estimated to be 3.2 and 3.9 nm, respectively. Unfortunately, absorption measurements cannot be directly used to determine the size or shell thickness of CdTe/CdS core/shell particles. For this reason, TEM and SAXS were also used to measure particle size. TEM images of the CdTe core QDs (Figure 6-3(a)) reveal uniform, monodisperse particles with a narrow size distribution. The particles self-organize on the TEM grid into a close-packed monolayer, and local hexagonal symmetry is evident in the packing arrangement. Compared with the cores, the core/shell QDs Figure 6-3(b) exhibit a broader size distribution and a variety of shapes. The size and shape variation is probably due to non-uniform CdS growth caused by the 11.5% lattice mismatch 115 between the core and shell materials. Diameters of a few hundred particles were measured directly from TEM images of both core and core-shell samples. For the CdTe cores, the mean size was always very close to the value estimated from absorption spectra. Core/shell particles had mean diameters that were slightly larger than the cores from which they were grown.

187 187 (a) (b) Figure 6-3. TEM images of (a) CdTe core and (b) CdTe/CdS core/shell QDs. Although the TEM images always showed that core/shell samples had slightly larger mean sizes than their parent cores, the distribution of core/shell sizes was broadened significantly. The particle size distribution of the core/shell particles extended beyond that of the core-only particles both on the large particle end of the distribution and on the small particle end of the distribution. Through extensive experimentation, we were able to confirm that the smaller particles in the distribution did not arise from nucleation of CdS particles but rather originated from the parent CdTe cores. Nucleation of CdS particles was observed if temperature and the rate of precursor addition were not carefully controlled, but CdS nucleation was avoided in the samples discussed here. In addition, attempts to grow very thick shells generally resulted in gross morphological changes (for example, tetrapod or rod formation).

188 188 In order to corroborate the particle size information obtained from TEM, SAXS measurements were performed on colloidal suspensions of the QDs in toluene. The SAXS data were largely consistent with the TEM data, but compared with TEM measurements, SAXS measurements provide better statistical averaging over the entire QD ensemble. SAXS measurements are also less subject to difficulties in the measurement of non-spherical particles or in selecting a representative sample of particles for imaging and measurement. For these reasons, SAXS data has been used here as the primary measure of particle size for the purpose of identifying core sizes and shell thicknesses. To calculate mean particle size from the SAXS data, the volume-weighted size distribution obtained from SAXS was converted to a number-weighted distribution, 210 and the mean value of the resulting number-weighted distribution was computed. The particle sizes as determined by SAXS were 3.1 nm and 4.2 nm for the small and large core particles, and 3.7 and 4.8 nm for the small and large core/shell particles. Like the absorption and TEM data, the SAXS measurements suggest the growth of a thin CdS shell. For the small core/shell particles, the measured shell thickness was approximately 3 Å, while for the large core/shell particles, it was 3.5 Å. The SAXS measurements were not in perfect agreement with the size estimates based on absorption data. For example, the SAXS and absorbance measurements of the large cores differed by 3 Å. Nevertheless, the values obtained by all three methods were in reasonable agreement, and most importantly, all three methods showed that the core/shell particles were, on average, larger than the parent cores.

189 189 In order to confirm that sulfur was incorporated into the QDs during the shell growth process, the QDs were analyzed via ICP-OES. These measurements showed definitive incorporation of sulfur in the QDs. For example, in large core/shell samples (similar to sample D in Figure 6-2), the Cd:Te:S ratio was found to be 19:9:7, which corresponds to a Te:S ratio of 56:44. Powder XRD was used to further probe the structural changes that accompanied the shell growth process. For the thin-shelled particles used in many of the optical studies, the XRD pattern failed to show clear evidence for a separate CdS phase; however, by growing thicker shells it was possible to demonstrate that a separate CdS phase was forming and to rule out the possibility of alloying. The XRD pattern of large core QDs (Figure 6-4, black line) matched the zincblende pattern for CdTe (PDF # ). Addition of the sulfur precursors caused negligible changes in these CdTe reflexes, but produced an entirely new set of reflexes that could be readily identified with wurtzite phase CdS (Figure 6-4). The appearance of two distinct diffraction patterns demonstrates that the material is not alloyed, but is instead composed of two distinct phases. In contrast to some previous studies on the CdTe/CdS system, 155, 211 these XRD patterns provide convincing evidence for growth of a CdS shell on the outside of the CdTe cores.

190 190 Figure 6-4. Measured powder diffraction patterns from CdTe core (bottom) and CdTe/CdS core/shell (top) QDs. Vertical bars at top and bottom of the figure show the patterns for zincblende CdTe (blue dotted lines, PDF ) and wurtzite CdS (red solid lines, PDF ), respectively Photophysics During shell growth, a significant red shift was observed in the lowest energy absorption peak (Figure 6-2). In the case of the small cores, the absorption peak shifted from 2.26 to 1.97 ev during shell growth. For the large cores, the peak shifted from 2.00 to 1.77 ev. Similar large shifts have been previously reported for CdTe/CdS core/shell QDs, 4, 111, 112 and the origin of these shifts is believed to be due to a type II band

191 alignment in CdTe and CdS. The conduction bands of CdTe and CdS are close in 191 energy, 154 and this close alignment results in delocalization of the conduction band electron through the core and shell, with subsequent reduction of that carrier s confinement energy. Figure 6-2 reports the PL quantum yield (Φ PL ) of the four samples along with their respective emission spectra. The results show that the quantum yield of the CdTe cores increases during the growth process. The quantum yield of the small cores was 0.30 while the large cores showed a quantum yield of Experiments with numerous coreonly samples showed that the quantum yield increases immediately after nucleation but then passes through a maximum before beginning to decline again as the particles continue to grow. The size at which Φ PL reaches its maximum depends in a complex manner on the reaction conditions, as has previously been reported for CdSe. 109 Deposition of a CdS shell on the CdTe cores increased both the quantum yield and the resistance to quenching upon exposure to air. These effects are often associated with the deposition of a wide band gap shell on a narrow band gap QD core. 20, 212 For the small QDs, addition of the CdS shell increased Φ PL (measured under argon) from 0.30 to 0.68, while Φ PL for the large QDs showed a more modest increase, from 0.40 to 0.45 upon addition of the shell. Figure 6-5 shows emission decay traces for the core QD samples. Consistent with the quantum yield trends, the large cores show longer decay times than the small cores. Neither decay resembles a single exponential, but both were well fit using a

192 192 multiexponential decay function with 3 lifetimes. After fitting, the average lifetime, τ, was calculated from the decay fit function according to 2 τ = a i τ i, (6.1) i where the a i are the coefficients of the exponential terms in the fit function (amplitudes), and the τ i are the lifetimes of the respective components. The larger particles generally showed slower decays than the smaller particles. It is clear from the figure that the addition of the shell led to a modest increase in the measured PL lifetime. In the case of the small particles, the average lifetime increased from 29 to 39 ns upon deposition of the CdS shell. This 35% increase in average lifetime is significantly less than the 125% increase in quantum yield that accompanied shell deposition. This rather modest increase in observed lifetime coupled with a much larger increase in quantum yield might appear to indicate that the radiative rate constant increased in these samples upon shell deposition. Ordinarily, the quantum yield, Φ, can be related to the radiative rate constant, k rad, and to the rate constant for non-radiative decay, k nrad, according to: Φ = k rad /(k rad +k nrad ) = k rad /k obs = k rad τ obs. In this treatment, it is assumed that the PL decay lifetime observed experimentally, τ obs, is simply the reciprocal of the sum of the radiative and non-radiative rate constants, k rad +k nrad = k obs. Since, in the present case, the relative increase in Φ is greater than the relative increase in τ obs, it might appear that k rad must also have increased. However, such an increase in k rad would be inconsistent with formation of a quasi-type-ii core/shell structure because the separation of excited carriers into separate QD domains should reduce the rate of radiative recombination as a

193 193 consequence of the decreased spatial overlap of the carrier wavefunctions. It will be shown later that for the core QDs there are ultrafast non-radiative processes that are too fast to observe in these PL decay experiments. This means that the average exciton lifetime in the small cores is actually much shorter than estimated from the PL decay shown in Figure 6-5. The same argument applies to the large core and core/shell samples. These samples showed a slight increase in both quantum yield (from 40% to 45%) and average lifetime (from 45 ns to 50 ns) upon shell deposition. However, the presence of some ultrafast non-radiative decay in the cores prevents extraction of the radiative rate constant from the quantum yield and PL decay data. We expect that radiative recombination is slower in all the core/shell samples discussed here than in the core-only QDs from which they were grown. Figure 6-5. Photoluminescence emission decays of CdTe cores (black solid lines) and CdTe/CdS core/shell QDs (red squares). The left panel depicts decay traces from small core and core/shell QDs and the right panel depicts decay traces from large core and core/shell NCs.

194 194 Figure 6-6 shows chirp-corrected transient absorption (TA) spectra of the four QD samples at three different probe delay times (0.2 ps, 0.5 ps and 2 ps). The spectra show multiple photoinduced absorption and bleach features (labeled with A and B designations, respectively). The three main bleach features, labeled B1, B2, and B3, will be assigned in the next section to the 1S 3/2 (h)-1s(e), 2S 3/2 (h)-1s(e), and 1P 3/2 (h)-1p(e) exciton states, respectively. The spectra of all these samples are very similar to those that 135, 185, 198 have been previously reported for high quality CdSe QDs.

195 195 Figure 6-6. Transient absorption spectra at three different pump-probe delays in (a) small CdTe core, (b) small CdTe/CdS core/shell, (c) large CdTe core, and (d) large CdTe/CdS core/shell QDs. Pump-probe delays were 0.2 ps (red squares), 0.5 ps (green triangles), and 2.0 ps (blue circles). At very short delay time ( 0.2 ps), a derivative-like feature is observed near the lowest energy peak in the ground state absorption spectrum. This feature, which is perhaps easiest to discern in the large core spectrum at 0.2 ps delay (Figure 6-6(c), red squares), is due to Coulombic interactions between the excited carriers in the QD. These carriers, which are initially excited high above the band edge by the 3 ev pump photons, produce an intense electric field and thus give rise to a Stark shift in the absorption

196 196 spectrum. 213 At slightly longer times (Figure 6-6(c), 0.5 ps, green triangles), the Stark effect becomes more pronounced, and its effect on higher-energy transitions in the spectrum becomes evident as well. As the hot carriers relax to the band edge within the first 1-3 ps, the derivative feature observed near 1.9 ev gives way to a prominent bleach at 2.0 ev that results from state-filling of the lowest exciton state. At higher energy, the spectrum contains additional bleach features that will be discussed further in the next section. After the first few picoseconds, the spectral shape ceases to evolve further; the TA signal at longer times is simply characterized by a monotonic decay that lasts much longer than the 3 ns time window afforded by our instrument. Fits to the longest decay component give an estimated lifetime of a few tens of nanoseconds, similar to the time scale observed in the PL decays. TA kinetic traces obtained at the position of the lowest energy bleach show interesting behavior at early delay times. The rising edge of the bleach is characterized by two distinct components. The first component is a steep increase in bleach intensity that is observed to occur on a time scale comparable to the laser pulse width (< 200 fs). The second component is usually somewhat slower and varies from sample to sample. A similar behavior has been observed in CdSe QDs, and the slower component of the rising edge in those samples has been ascribed to intraband relaxation of the excited electrons to the lowest conduction band level. As explained later, additional processes are involved in these CdTe QD samples. Figure 6-7 compares the rising edge TA kinetics of four core and core/shell QDs. (Note that the vertical axis has been inverted relative to those in Figure 6-6, and the traces

197 197 have been normalized to unit peak height to facilitate direct comparison.) Both core QD samples show the initial, fast rising edge, which accounts for between one-third and onehalf of the total peak amplitude. The large core sample also shows a much slower component, accounting for at least half the total amplitude, which continues to rise until a delay time of between 2 and 3 ps. In contrast, the rising edge of the small core sample is so fast throughout that a slower component is difficult to even discern. The rising edges were fit to biexponential functions convolved with a fixed pulse width of 190 ps (the approximate pump-probe autocorrelation width). These fits provided a quantitative and objective estimate of the time constant for the slower rise component. From the data shown in Figure 6-7, rise times of 150 fs and 430 fs were obtained for the small and large CdTe cores, respectively. The kinetic traces for small and large core/shell QDs look very similar over the first 6 ps. Both show relatively slow rises, and both reach their peak amplitude after 3 or 4 ps. Careful examination shows that the small core/shell trace has a slightly slower second rise component. Biexponential fits confirm this and yield a slow component rise time of approximately 610 fs for the small core/shell and approximately 530 fs for the large core/shell sample. It is interesting to compare the kinetics between large core and small core/shell QDs because these two kinds of QDs have similar band gap energies and similar absorption spectra. As shown in Figure 6-7, the rising edge of the bleach signal is faster for the large cores than for the small core/shell QDs (430 fs vs. 610 fs). It can also be seen that the subsequent decay of the bleach signal is faster for the large cores than for the small core/shell particles.

198 198 The inset of Figure 6-7 shows the same kinetic traces over a much longer, 3 ns time window. These data demonstrate that both samples exhibit a fast decay component of approximately 100 ps and much longer components of 10 ns or more. In addition, the small core sample has a faster decay component of 1.7 ps, which can even be observed in the main frame of the Figure 6-7.

199 199 Figure 6-7. Transient absorption kinetics at the B1 bleach (1S 3/2 (h)-1s(e)) position of four CdTe and CdTe/CdS QD samples. Inset shows the extended decay, out to 3 ns delay. Black, red, blue, and green curves represent small core, large core, large core/shell and small core/shell, respectively. Figure 6-8 compares the early kinetics of the three main bleach peaks, B1, B2, and B3, in the spectrum of the small CdTe cores. Unlike the B1 peak, the B2 peak from this sample shows clear evidence for a slow rise component like that which is seen in the B1 peak of the other three samples (see Figure 6-7). Additionally, the B2 peak does not show the rapid, 1.7 ps decay that is evident in the B1 peak. As will be discussed later,

200 these data provide evidence that surface trapping of valence band holes is an important process in the early TA dynamics of the small cores. 200 Figure 6-8. Early TA dynamics of the three principal bleaches in small CdTe cores. Figure 6-9 compares the early kinetics of the three main bleach peaks, B1, B2, and B3, in the spectrum of the large CdTe cores. In the kinetics of the large CdTe cores, the kinetics of bleach peaks, B1 and B2 are similar and fast decay component is not observed in kinetic of B1 bleach peak.

201 201 Figure 6-9. Early TA dynamics of the three principal bleaches in large CdTe cores. Figure 6-10 shows TA spectra at 0.2, 0.5, and 2 ps delay for a sample of large core/shell particles in phosphate-buffered saline (PBS) at ph 7.4. These particles were prepared from a different batch of cores than the large core/shell particles depicted in Figure 6-2, Figure 6-6, and Figure 6-7. These particles were capped with TGA using established ligand exchange procedures, and the measurements were conducted in air saturated solution. These particles are highly luminescent, with quantum yield (0.50) similar to that measured prior to ligand exchange. TA spectra and dynamics of these particles are also very similar to those observed for the large core/shell particles prior to ligand exchange. Like the large core/shell particle TA spectra shown in Figure 6-6, this sample shows a strong bleach at photon energies above 2.6 ev. This bleach develops relatively late, as it is completely absent at 0.5 ps probe delay time.

202 202 Figure (A) Transient absorption spectra of large core/shell QDs in PBS at three different pump-probe delays. (B) Kinetic traces at the position of the B1 bleach for large core/shell QDs in chloroform (black curve, triangles) and in PBS (red curve, circles).

203 203 Table 7 shows the information of the lifetimes of the CdTe and CdTe/CdS core/shell QDs under argon and air by TA measurement. The data contains the lifetimes of 1S-1S, 2S-1S, and 1P-1P transitions. Depending on the size of the CdTe QDs increasing, the rising time of 1S-1S transition of the CdTe QDs also increased. For example, the rising time of the small CdTe QDs is 148 fs and for the large ones the rising time is 428 fs. Moreover the 1S-1S transition rising lifetimes did not change too much in the core/shell QDs when they were exposed to the air, but for the CdTe QDs once they were quenched by the air, the rising time of 1S-1S transition decreased and compared to the CdTe QDs under argon, there was one more lifetime component in the CdTe QDs in the air, which is a fast decay in the early time scale.

204 204 Table 7. Lifetime of CdTe and CdTe/CdS QDs via transient absorption measurement. Exp QDs sm core Ar sm core Ar sm core Ar sm core Air sm core Air sm core Air Transiti on Em. (nm) a1 t1 (ps) a2 t2 (ps) a3 t3 (ps) a4 t4 (ps) a5 t5 (ps) 1S-1S S-1S P-1P S-1S S-1S P-1P lg core Ar 1S-1S lg core Ar 2S-1S lg core Ar 1P-1P lg core Air 1S-1S lg core Air 2S-1S lg core Air 1P-1P sm core/shell Ar 1S-1S

205 205 Table 7 (Continued) sm core/shell Ar sm core/shell Ar sm core/shell Air sm core/shell Air sm core/shell Air lg core/shell Ar lg core/shell Ar lg core/shell Air lg core/shell Air 2S-1S P-1P S-1S S-1S P-1P S-1S S-1S S-1S S-1S

206 Discussion Uncoated CdTe core QDs are air sensitive. 110, 214 The luminescence intensity of the core-only QDs degrades significantly over a period of a few minutes to hours after exposure to air. We have shown that the air-induced luminescence quenching is accompanied by significant changes in the ultrafast TA dynamics. 215 The degradation is ascribed to oxidation at the surface as has been previously reported and studied by Borchert et al. 214 To avoid complexities related to the surface oxidation processes, the measurements presented here have been made under argon atmosphere except where otherwise noted. Deposition of a thin CdS layer dramatically improves the stability of the particles in air with respect to exciton quenching, so that PL and TA measurements on CdTe/CdS core/shell particles in air are essentially indistinguishable from those under argon. Great care was taken during the TA measurements to limit pump intensity so that the system remained safely within the single excitation regime and so that photoinduced sample degradation could be avoided (see Section 5.5). The value of <N>, the number of excitons per QD in the sample, was held below 0.1 in all cases. This required low pump fluence, especially for some of the large core/shell samples, which had very high extinction coefficient at the pump wavelength due to the presence of the CdS shell, which absorbs strongly in the blue and UV. In order to avoid sample photodegradation on the time scale of the experiment, the sample was stirred rapidly during data collection. Absence of photodegradation during the experiment was demonstrated by collecting three full sequential scans for each experiment. The data reported is the average of these three

207 207 scans. After completion of each experiment, the three scans were compared to verify that the ratio between the peak OD value at short delay time and the OD value at the end of the scan (150 ps or 3 ns, depending on the experiment) was the same within the experimental noise for all three scans. No evidence for photodegradation of the samples was observed under the experimental conditions employed Assignments of TA Spectral Features Recently, Zhong et al. have mapped out the size-dependent excitonic transition energies for CdTe QDs using PLE spectroscopy. 207 Their data provide an excellent foundation for understanding the transient spectra of the CdTe cores reported here. Figure 6-6 (a) and (c) depict TA spectra for the small and large core samples. At delay times longer than 1 ps, the spectra are dominated by bleach features labeled B1, B2, and B3. In each case, the position of B1 corresponds to the lowest energy peak in the linear absorption spectra (Figure 6-2). This feature is due to state-filling of the 1S 3/2 1S e exciton state. Using the data plotted in Figure 3 of Zhong et al., 207 it is possible to assign the features labeled as B2 and B3. The B2 feature corresponds to the 2S 3/2 1S e state, and the B3 feature is the 1P 3/2 1P e state. The positions of these two features in both the small core and large core spectra are in perfect agreement with the previous report. The bleach at B3 vanishes very quickly due to the rapid cooling of carriers to the band edge. This behavior is consistent with the assignment as the 1P 3/2 (h)-1p(e) state. In less than 2 ps, the conduction band electron and valence band hole fall to the 1S(e) and 1S 3/2 (h) energy levels, respectively. This gives rise to the dominant bleach feature at B1. The feature at B2 is shifted to higher energy by ev, corresponding to the energy

208 208 required to excite the hole from the 1S 3/2 (h) to the 2S 3/2 (h) level. The partial bleach at this energy does not imply that excited holes are present in the system; rather it is due to the filling of the 1S(e) state by the conduction band electron. Because 2S 3/2 (h) 1S(e) is an allowed transition, the filling of the 1S(e) state leads to partial bleaching of this transition even though the 2S 3/2 (h) state is empty. In the core/shell QDs, the energy levels of the conduction band are modified by the presence of the CdS shell (Figure 6-11). The conduction band of bulk CdS is approximately 0.35 ev lower in energy than that of bulk CdTe, 154 allowing for delocalization of the excited electron throughout the core and shell. With a sufficiently thick shell layer, the electron could completely localize in the CdS shell, but the shells here are too thin for this to happen in these samples. Instead, the conduction band electrons are delocalized throughout the QD. In contrast, the valence band offset of 1.31 ev effectively contains the valence band holes within the CdTe cores. The offset bands produce a quasi-type-ii carrier distribution with partial charge separation along the radial coordinate.

209 209 Figure Energy level diagram showing the lowest two levels for conduction band electrons and lowest three levels for valence band holes in CdTe (left) and CdTe/CdS core/shell QDs (right). Dashed lines represent the carrier potential energies of the conduction and valence band due to the CdTe and CdS lattices. When the CdS shell is added, the confinement energy of conduction band electrons decreases as these carriers delocalize throughout the core and shell. Valence band holes remain confined to the core due to the large valence band offset between the two materials. Vertical arrows represent the transitions, B1 (blue), B2 (red), and B3 (green) that are discussed in the transient absorption spectra.

210 210 The quasi-type-ii carrier distribution alters the apportionment of confinement energy between the electrons and holes in the core/shell particles. For this reason, the exciton energy levels in the core/shell particles cannot be assigned exclusively on the basis of the data from Zhong et al. 207 As the CdS shell is added, the spectral red shift that is observed is due primarily to reduction in the confinement energy of conduction band carriers. The energies of the holes, which remain confined to the core, are only weakly affected. This slightly complicates assignment of the B2 in the TA spectra of the core/shell particles (Figure 6-6(b) and (d). For example, in Figure 6-6(d), the lowest energy bleach, B1, appears at 1.79 ev. Referring to Figure 3 of Zhong et al., 207 we find that the B2 [2S 3/2 (h) 1S(e)] transition in such a sample should appear no higher than 1.85 ev, which is measurably below the feature labeled B2 in Figure 6-6(d). In fact, at 1.85 ev, the B2 feature would be difficult to resolve from the main bleach feature. In our core/shell sample, the feature labeled B2 is nearer to 1.93 ev, which is very close to the predicted position of the B3 [1P 3/2 (h) 1P(e)] transition. However, this latter assignment can be ruled out based on the kinetic evolution of the spectrum. If the 1.93 ev bleach in Figure 6-6(d) corresponded to the 1P transition, it would disappear in less than 2 ps. Instead, that bleach feature persists for nanoseconds. On this basis, the bleach feature labeled B2 in the core/shell spectra (Figure 6-6(b) and (d)) can be safely assigned as the same 2S 3/2 (h) 1S(e) transition that was assigned in the core-only spectra. Comparison of the hole energy level spacings in the core and core/shell QDs provides some interesting information. Figure 6-12 shows a graph of E, the difference between the energies of the 1S 3/2 and 2S 3/2 hole levels, versus optical band gap (as

211 211 measured by the position of the 1S 3/2 (h) 1S(e) bleach). The E values were measured from the TA spectra as the energy spacing between the two lowest energy bleach features (i.e. energy difference between the 1S 3/2 (h) 1S(e) and 2S 3/2 (h) 1S(e) bleach positions). Blue circles in Figure 6-12 represent the E values for several sizes of CdTe cores (blue circles) and CdTe/CdS core/shell particles (magenta stars). Spectral positions of the individual bleach features were determined by a least squares fit of multiple Gaussian peaks to the 2 ps TA spectra. The solid black line represents the hole energy level spacings for CdTe QDs as determined experimentally by Zhong et al. 207 It is clear from the figure that the E values obtained from TA spectra of our CdTe core samples are in close agreement with the energy spacings measured via PL excitation measurements. Furthermore, the figure shows that for the core/shell particles, the hole confinement energies are systematically larger than those of CdTe cores with the same optical band gap (i.e. all magenta stars are well above the solid line). The reason is that the electron confinement energy in the core/shell particles has been decreased due to delocalization of the electron throughout the core and shell (see Figure 6-11), whereas the hole confinement energy has remained almost unchanged. This comparison of hole energy levels provides direct evidence for the differential localization of electrons and holes in the CdTe/CdS core/shell system.

212 212 Figure Energy spacing, E, between 1S 3/2 and 2S 3/2 valence band hole levels plotted versus band gap (measured as 1S 3/2 (h) 1S(e) energy) for CdTe cores (blue circles) and CdTe/CdS core/shell QDs (magenta stars). The black line shows the relationship between E and lowest-energy PL excitation peak determined experimentally by Zhong et al for CdTe core NCs Charge Transfer Bleach The large core/shell sample (Figure 6-6(d)) exhibits a significant transient bleach feature at the high energy end of the spectrum. This feature, appearing at photon energies above 2.6 ev, develops at long probe delay times, greater than 500 fs. The feature has

213 213 been observed in multiple large core/shell samples, and is absent in all CdTe core samples examined so far. It is also evident in the TA spectra of thiol-capped, aqueous CdTe/CdS NCs, as shown in Figure 6-10, which were produced from a different batch of core/shell QDs. The spectral position of the high energy feature suggests that it is associated with the absorption of the CdS shell. As the CdS shell is added, the ground state absorption increases significantly in the short wavelength region due to the deposition of the wide band gap CdS. Control experiments were performed to determine whether this high energy bleach is observed when longer wavelength excitation is used. Even when the QDs are pumped well to the red of the high energy bleach (2.2 ev), this transient bleach is observed. This observation excludes the possibility that the feature could be due to pure CdS nuclei that are present in the system. If the feature is due to bleaching of the ground state CdS absorption, then its presence in the TA spectrum and its late appearance (after t = 1 ps) could be associated with charge redistribution that occurs as the conduction band electron wavefunction spreads into the shell. This intraparticle charge transfer process is not yet well understood, but other groups have also noted spectral dynamics occurring on the 1 ps time scale in nanoheterostructures and attributed these observations to intraparticle charge transfer events. 176, 180, 192, A curious characteristic of the high-energy bleach is that its evolution does not seem to be associated with noticeable changes in the lowest energy bleach. If the high-energy bleach is related to charge redistribution, then some red shift of the lowest energy bleach might be expected as the conduction band electron relaxes to its fully delocalized state; however, no spectral shift is evident in the spectra shown here. Further investigation and

214 214 experimentation is needed to definitively assign and fully understand the nature of the high-energy bleach Intraband Relaxation and Ultrafast Hole Trapping Figure 6-7 shows the rising edge of the 1S 3/2 (h)-1s(e) bleach for the CdTe and CdTe/CdS samples. The small core sample is noteworthy because of its very fast rise (τ = 150 fs) and decay (τ = 1.7 ps) compared with the other samples. The rising edge of the lowest energy bleach is often identified with the intraband relaxation of the conduction band electron from the 1P(e) to the 1S(e) energy level. 187, 198 This 1S 3/2 (h)-1s(e) bleach feature contains information about both the conduction band electron and valence band hole. At early delay times, there is also some contribution from the excitation-induced Stark effect, but this contribution decays rapidly. The electron is regarded as the main contributor to this lowest energy bleach signal because the electron degeneracy is lower than that of the hole, and the lowest energy hole levels are mixed with higher-lying states that are close in energy. 135 Furthermore, the electron usually dominates the rising edge kinetics of the 1S bleach because its effective mass is lower than that of the hole, thus giving a low density of states near the conduction band edge. This low density of states produces a phonon bottleneck, 146 which leads to slow electron kinetics. For these reasons, the electron cooling rate is often the rate-limiting factor in the development of the 1S bleach. This phonon bottleneck may be broken if other processes, such as Augermediated energy transfer between carriers, 146, 185 facilitate carrier cooling; however, even in these cases, the electron cooling is ordinarily rate-limiting.

215 215 Klimov and co-workers demonstrated that in high quality CdSe QD samples the rising edge kinetics of the 1S 3/2 (h)-1s(e) bleach matched the decay kinetics of the 1P 3/2 (h)-1p(e) bleach. 185, 198 This cross-verification provided compelling evidence that the observed rising edge was indeed due to the arrival of the conduction band electron in the 1S(e) energy level and that the electron was arriving directly from the 1P(e) level without being trapped in an intermediate state. Taken together, these results seemed to be most consistent with the Auger-dependent model for intraband relaxation that had previously been proposed by Efros et al. 146 Examination of the 1P kinetics of the small CdTe cores (B3 bleach, Figure 6-8) shows that the 1P decay approximately matches the rise of the 2S 3/2 (h) 1S(e) bleach (B2) for all the core particles. For example, for the large cores, the 1P(e)-to-1S(e) transition times extracted from fits to the B2 and B3 features were 670 fs and 740 fs, respectively. For all samples, the B2 bleach recovery and B3 rise gave satisfactory agreement with each other given the uncertainties in the fits. Surprisingly, the 1S 3/2 (h) 1S(e) rise time in CdTe cores did not typically match either the 1P decay time or the 2S 3/2 (h) 1S(e) rise. This behavior is in contrast to that which has been observed in numerous CdS and CdSe QD samples. 201 The difference was particularly striking for smaller core samples, as shown in Figure 6-8. That figure shows the obvious difference between the B1 rise time, which is 150 fs, and the B2 rise time and B3 decay time, which are both approximately 500 fs. In addition, the B1 feature shows a rapid decay component of 1.7 ps that is not observed in the B2 feature. The close match between the B3 decay and the B2 rising edge in every sample suggests that these

216 216 dynamics represent the same process, namely, the cooling of the electron to the 1S(e) level. However, this conclusion implies that the 1S 3/2 (h) 1S(e) rising edge does not accurately yield the electron cooling, and raises a question regarding the origin of the observed kinetics in the B1 feature. Since the 1S 3/2 (h) 1S(e) transition contains a bleach contribution from both carriers, but 2S 3/2 (h)-1s(e) contains a bleach contribution only from the conduction band electron, (see Figure 6-11) the difference in the dynamics between these two bleach features can be understood in terms of the fate of the valence band hole. Specifically, the absence of a slower rising edge component on the order of 500 fs is evidence for ultrafast hole trapping that competes with relaxation to the band edge. In addition, the fast 1.7 ps decay observed in the 1S 3/2 (h) 1S(e) bleach is indicative of surface trapping of the fully relaxed hole. Since the 2S 3/2 (h)-1s(e) kinetic essentially contains only information about the occupation of the 1S(e) level, that bleach must give the correct information about the 1P(e)-to-1S(e) relaxation process. The close agreement between the B3 decay time and the B2 rise time strongly support this argument. Analysis of the B2 and B3 dynamics in these small core QDs would appear to suggest much slower electron cooling rates for CdTe core QDs than for CdSe QDs of similar size. For example, Klimov et al. using similar experimental conditions and pump wavelength, measured 1P(e) decay times and 1S(e) build-up times of approximately 120 fs in CdSe QDs that were similar in both size (3.4 nm diameter) and in band gap ( 2.3 ev) to our small CdTe cores. 198 This represents a four-fold difference in observed electron cooling times between the small CdTe cores and similar CdSe NCs. However, it is important to remember that the ultrafast hole trapping discussed above would be

217 217 expected to slow down electron relaxation by turning off the Auger mechanism for electron cooling. Thus, the observation of slow dynamics in the B2 and B3 features is consistent with the notion of sub-picosecond hole trapping that removes the primary pathway for electron cooling. Hole trapping on the single picosecond time scale is consistent with previous results from CdSe NCs. 160, 188, 190, 199, 201, 219 Some of those reports have employed holetrapping ligands to predictably alter the QD response; however, in recent work, McArthur et al. 206 observed hole trapping with a 1.6 ps time constant in un-modified CdSe QDs using NIR TA spectroscopy. The small amplitude of the fast decay component in the B1 feature and the moderate PL quantum yield of the small core sample would seem to suggest that ultrafast hole trapping occurs in a relatively small fraction of the small core NCs. However, it may be premature to draw such a conclusion since (i) the holes are generally expected to make only a minority contribution to the amplitude of the B1 bleach feature, and (ii) very recent work by Knowles et al. 205 has shown that, at least in CdSe NCs, trapped holes can still participate in radiative recombination that is spectrally indistinguishable from the band edge emission. In the small cores, the ultrafast hole trapping results from high wavefunction amplitude at the surface in the smaller particles and from the abundance of defects that arise due to the very short growth and annealing period (2 min.) used in the synthesis of these NCs. Hole trapping undoubtedly occurs to some extent in the large core sample, too; indeed, surface carrier trapping is probably the primary process that limits the PL

218 218 quantum yield in the core samples. In the large cores, however, the rate of hole trapping is significantly diminished in comparison with the small core sample: there is no evidence that the process competes with carrier cooling in the large cores. Accordingly, it seems reasonable to expect that the Auger mechanism for electron cooling is fully active in the larger core QDs throughout the cooling process. The large cores and all core/shell samples exhibited B1 dynamics that reflected the electron cooling process. Like the small cores, the large cores, whose band gap is 2.0 ev, can be compared with literature reports on CdSe QDs of similar band gap. Klimov et al. reported a 1S build up time of 390 fs for 5.6 nm CdSe QDs with a band gap of approximately 2.0 ev. 198 This is quite comparable to the 430 fs measured for the large cores here (see Figure 6-7). Although further work is needed to fully characterize the size dependence of the cooling rate in CdTe NCs, these initial results appear to show close agreement with the values in CdSe NCs. An interesting question in the present system is whether the differential distribution of electron and hole wavefunctions within the quasi-type-ii core/shell QDs has a significant effect on the electron cooling rates. Since electron cooling in QDs is believed to depend on Auger-type energy transfer from electron to hole, the electron cooling rate would be expected to decrease as the Coulomb interaction between the carriers decreases. As the carriers become spatially separated in the core/shell QDs due to the quasi-type-ii exciton structure, this Coulomb interaction should decrease. The results presented in Figure 6-7 show that the electron cooling rate does indeed decrease as the CdS shell is added. For example, the B3 1P(e)-to-1S(e) decay time increased from

219 219 approximately 500 fs to 700 fs as the shell was deposited on the small cores. A similar increase was observed in the large NCs. Perhaps the best measure of the effect of carrier separation in the core/shell system, however, comes from a comparison of the large core and small core/shell NCs. These have similar band gap energies, so the excess photon energy is nearly the same 1.1 ev in both cases. The B1 rise time measured in the small core/shell QDs is approximately 40% longer (610 fs vs. 430 fs) than that measured in the large core NCs. This is in spite of the fact that TEM and SAXS data both indicate that the diameter of the large cores (4.2 nm) is more than 10% larger than that of the small core/shells (3.7 nm). Based on these results, it might be possible to use type-ii systems to slow down carrier cooling and thus enhance efficiencies of hot carrier extraction. Using their state-selective pump-probe experiments, Sewall et al. have previously shown that the B1 bleach in CdSe QDs arises almost exclusively from signals attributable to the conduction band electron in that material. 187 Our results suggest that the situation may be different in CdTe; differences in crystal symmetry and spin-orbit interactions may alter the effective degeneracies of the valence band levels near the band edge and may thus increase the contribution of valence band holes to bleaching near the band edge. Additional experiments are underway to answer this question. Since the electron and hole overlap depends in a complex manner on the core size and shell thickness, it would be interesting to analyze the dependence of the carrier cooling rates in terms of the detailed shell structure. Unfortunately, we have not yet established sufficient control over the shell thickness to enable systematic study of this question in the CdTe/CdS. Growth of very thin shells, on the order of 0.5 nm, has been

220 220 achieved routinely using both SILAR and slow injection methods. The shell growth typically saturates after the deposition of 1-2 monolayers of CdS on the surface of the core. Additional CdS can be deposited with the continued addition of Cd and S precursors in large excess, but the particle shapes become irregular and non-uniform. Estimating the spatial overlap of electron and hole in such samples would be problematic because of the ill-defined structure of the QDs and the sample heterogeneity. When this shell growth is driven to extreme, tetrapods are often formed, which presumably comprise zincblende CdTe cores decorated with wurtzite CdS arms Effects of Air, Ligand Exchange, and Aqueous Solvent on CdTe/CdS NCs Recently, Ghosh and co-workers 156, 196 reported TA spectra from colloidal CdTe and CdTe/CdS QDs. Their samples were prepared in aqueous media using hydrophilic thiols as capping ligands, according to a widely used synthetic procedure. We note that the TA spectra and excited state dynamics reported by those researchers are qualitatively very different from those presented here. Samples prepared in aqueous media typically have broader size distributions, and the excitation densities used by Ghosh et al. are much higher than those used in our experiments. In order to determine if hydrophilic thiol capping ligands or the aqueous environment had a significant impact on CdTe/CdS TA spectra, we transferred some CdTe/CdS QDs into phosphate buffered saline at ph 7.2 after replacing the native capping ligands with thioglycolic acid. Figure 6-10 shows that the TA spectra were not greatly altered by transfer into aqueous solvent or capping with hydrophilic thiols. (Note that this is a different batch of core/shell particles than those shown in Figure 6-6. The absence of the B2 bleach is a feature of this sample that stems

221 221 from the larger core size, which results in collapse of the spacing between the hole states near the band edge. It is not a consequence of the transfer into water.) Furthermore, the decay kinetics of the main B1 bleach after ligand exchange and solvent transfer are almost identical to the kinetics in chloroform prior to ligand exchange. It appears, then, that the differences noted in the TA spectra of Ghosh and co-workers stem from something other than the aqueous environment and the thiol capping ligands. The particles may be strongly affected by structural defects that are present in samples prepared at relatively low temperature, or there may be interactions between the particles and some of the reactants such as excess borohydride, which is used to prepare the NaHTe precursor. 6.6 Conclusions Ultrafast carrier dynamics have been explored in CdTe and CdTe/CdS core/shell QDs. The bleaches in the TA spectra have been assigned, and analysis of the energy spacings between the 1S 3/2 (h) and 2S 3/2 (h) levels supports the prevailing picture of quasitype-ii carrier distribution in the core/shell particles, with delocalized electrons and corelocalized holes. In addition, a new understanding of the rising edge kinetics of the lowest energy bleach (B1) has emerged for these systems. While the rising edge kinetics of the lowest energy bleach in semiconductor QDs are often associated with intraband relaxation of conduction band electrons, it is shown here that at least in the case of the small CdTe cores, the fast rise of the B1 feature reflects surface trapping of the valence band holes.

222 222 The effect of the CdS shell on the electron cooling dynamics was also investigated by comparing the sub-picosecond bleach dynamics in CdTe cores and CdTe/CdS core/shell QDs. The results show that arrival times for conduction band electrons in the 1S(e) level after excitation high (1.1 ev) above the band gap are approximately 40% longer for core/shell QDs than for the core-only QDs. This difference is likely due to reduction of the Auger coupling between electron and hole due to the quasi-type-ii excitonic structure, which leads to some spatial separation of the carriers in the core/shell structure. Finally, a new, high-energy bleach feature has been identified in the large CdTe/CdS core/shell samples, and this feature has tentatively been assigned as a bleach of the CdS absorption. This feature could, in principle, be used to isolate and study the dynamics of the conduction band electron individually in certain types of nanoheterostructures, but further study of the nature of this feature is necessary first.

223 223 Chapter 7 Spectroscopic Investigation of Oxygen Sensitivity in CdTe and CdTe/CdS Quantum Dots (Most content of this chapter is from our paper with same title published in The Journal of Physical Chemistry C, and authorization is in Appendix D.) 7.1 Introduction Colloidal semiconductor QDs possess interesting and unique electronic and optical properties, which have identified them as candidate materials and interesting 2, 97, model systems for a wide range of technologies. CdTe has become an interesting material for colloidal nanostructures because of its electronic properties and 4, 23, 105, 110, 173, 224 the existence of facile routes for preparation of high-quality CdTe QDs. These characteristics, combined with its electronic properties have made CdTe a favorite 4, 169, 172, 174, 176, 180, for inclusion in nanoheterostructures with other II-VI semiconductors. 197, CdTe can be combined with CdSe or CdS to produce type-ii or quasi-type-ii nanoheterostructures, 111, 172 which hold interesting possibilities for optical gain media, 229 bioimaging applications, 172, 174, , 176, 180, 192, 225 and solar energy conversion. The first few picoseconds after photoexcitation is an eventful time in semiconductor QDs, as carriers relax to the band edge through multiple pathways and may be trapped at the surface. 135, 147 Epitaxial growth of a wide band gap inorganic shell onto a semiconductor QD is generally expected to alter carrier trapping probabilities and rates at the QD surface. In some cases, the shell might also affect intraband relaxation rates, but this effect can be effectively removed from femtosecond TA data by pumping near the band edge. In contrast, surface trapping events would be rather difficult to

224 224 exclude from the experiments. For this reason, it is important to consider any possible shell-related changes in surface reactivity when comparing ultrafast dynamics between core and core/shell systems. Surface reactivity and carrier trapping tendencies in the core QD may significantly affect the ultrafast dynamics and thus confound comparison of the core and core/shell systems. This point is illustrated in the present work using CdTe, a common component in nanoheterostructure systems. We show that the surface reactivity of CdTe cores toward oxygen requires that special care be exercised in drawing comparisons between ultrafast dynamics in core and core/shell systems. CdTe QDs undergo surface reactions and luminescence quenching in the presence of air. The sensitivity of CdTe QDs to oxidation and quenching in air has been noted and investigated previously by other research groups. 180, 230 However, the dynamics of the airinduced quenching and its effect on the ultrafast photophysical response of CdTe QDs is unexplored. Furthermore, little is known about the extent to which the air sensitivity can be controlled through the synthetic reaction route, through choice of surface capping ligands, or through encapsulation within an ultrathin shell made of another semiconductor material. We report here a study on the air sensitivity of CdTe QDs with emphasis on the immediate and significant photophysical consequences of air exposure and the remarkable protection offered by even a very thin CdS shell layer. For the present study, CdTe QDs were produced using several different synthetic methods, 4, 110, 111, 224 all of which involved rapid injection of precursors at high temperature into high-boiling organic solvents. Various methods were also used for deposition of thin CdS shells onto

225 225 the CdTe QDs. 4, 111, 116, 215 The resulting core and core/shell QDs were subjected to a series of spectroscopic measurements to probe the universality of the CdTe QDs sensitivity to air. Femtosecond TA measurements were used to probe the changes in ultrafast exciton dynamics that accompany air-induced quenching. The results show that even brief exposure of CdTe QDs to air can significantly alter their ultrafast dynamics relative to samples coated with even a monolayer-thin CdS shell. 7.2 Experimental CdTe cores were prepared by several different methods, all of which have been previously reported. 4, 110, 111, 215, 224 In some cases, CdS shells were deposited onto the CdTe cores. The shells were likewise grown by multiple, previously-reported methods. These included SILAR methods 4, 116 as well as gradual, simultaneous injection of Cd and S precursors. 111, 215 The shell growth procedures were matched to the respective core synthesis methods. After synthesis, the QDs were transferred via syringe to a sealed, argon-filled container and placed in an argon-filled glovebox for storage. Spectroscopic measurements. Samples for air-free spectroscopic measurements were prepared inside the glovebox. To prepare these samples, the crude QD suspension was diluted with argon-saturated chloroform, loaded into a cuvette, and sealed with a rubber septum to avoid air exposure. The sealed cuvette was then removed from the glovebox for spectroscopic measurements. Steady-state absorption and PL spectra and time-resolved PL decays were collected using 10-mm pathlength quartz cuvettes. TA spectra were collected using 2-mm pathlength quartz cuvettes.

226 226 Once data collection on the argon-protected samples was completed, the seals were removed from the cuvettes and the samples were exposed to air. A 1 ml syringe was filled with air, and this air was bubbled into the sample over a period of 5-10 seconds. This process was repeated four more times in rapid succession. After the air was bubbled through the sample, the cuvette was re-sealed to minimize evaporation of chloroform, and the spectroscopic measurements were repeated on the air-exposed sample. TA measurements were performed using equipment that has been described in detail previously. 215 Neutral density filters were used to limit the pump beam intensity and thus avoid biexciton production in the QDs. The pump wavelength was 400 nm, and the pump fluence at the sample was typically 6 µj/cm2 (1 mm pump beam spot size). The sample pathlength was 2 mm. 7.3 Results and Discussion Figure 7-1(a) shows the UV-visible absorption spectra of the CdTe QDs before (solid black curve) and after (dashed red curve) air exposure. The lowest energy absorption peak appeared at 2.00 ev (620 nm) for the argon-protected sample, and this peak was observed to blue shift approximately 0.01 ev after exposure to air. Other than the small blue shift, the spectra under argon and air are very similar. The extinction coefficient at the lowest energy peak is almost unchanged. Additional experiments in which much more air was bubbled through the sample over a period of 3 minutes showed increased blue shifts. The mechanism for the observed blue shift is not certain. Oxidative etching of the QD surface is one possible explanation, but previous studies have also shown that oxygen can bind to the surface of CdTe QDs, oxidizing exposed Te ions to

227 227 form surface-bound TeO x species. 230 This surface oxidation could effectively reduce the size of the CdTe QD and thus produce a confinement-induced blue shift. Either way, as will be shown below, exposure to air produces irreversible photophysical changes that are consistent with permanent chemical modification of the CdTe QD surface.

228 228 Figure 7-1. (a) UV-Vis spectra of the CdTe QDs under argon (solid black curve) and after brief air exposure (red dashed curve). (b) PL emission spectra of the CdTe QDs under argon (solid black curve) and after brief air exposure (red dashed curve). PL emission continued to decrease futher with continued exposure to air, until the quenching reached 100%.

229 229 Figure 7-1(b) illustrates the dramatic PL quenching of the CdTe QDs that occurs within approximately 15 minutes after bubbling 5 ml of air through the sample. For this particular sample, the air exposure quenched the PL intensity approximately 70% within 15 minutes. With additional time and/or with additional exposure to air, the quenching continues to increase, effectively reaching 100% within several hours. We attribute the quenching to the formation of TeO x species on the surfaces of the QDs, as described above, which has been previously correlated with reduced quantum yield in CdTe QD samples. 230 The CdTe core QDs prepared by different methods and with different capping ligands showed slightly different behavior in our experiments. These differences were noted only in the kinetics of the surface reaction, not in the ultimate fate of the CdTe cores. All the samples showed significant quenching within a day of being exposed to air. Certain ligands, such as HDA and oleic acid, were able to reduce the rate of PL degradation upon exposure to air. When HDA was included in the CdTe synthetic reaction mixture, the time scale for degradation was shifted from a few minutes to over an hour. Oleic acid, when used in place of TDPA or stearic acid, was effective at preserving PL intensity for up to 1 day. Unfortunately, substitution of oleic acid for TDPA significantly alters the QD growth kinetics, generally resulting in broader size distributions and lower quality QD samples. For samples prepared without OA and HDA, it was observed that brief exposure of the CdTe QDs to air caused rapid (within minutes) PL quenching; however, quickly

230 230 washing the QDs via standard precipitation/resuspension procedures could partially restore the PL intensity. After the washing, however, further rapid quenching was observed, and this quenching became irreversible within an hour. In some cases, the decline in intensity was noticeably accelerated after washing the QDs. The partial recovery of intensity after washing the QDs was not universal, but it implies the existence at least in some cases of a fast, reversible step in the reaction with air that involves one or more ligands bound to the surface. The removal of the affected ligands via washing caused a partial recovery in the PL intensity. In spite of the differences noted above, the various ligands employed all failed to preserve the PL emission intensity after continued air exposure. In all cases, air exposure ultimately led to quantitative, irreversible PL quenching in the CdTe cores. The quenching phenomenon was observed on different sizes of CdTe NCs. The data presented in Figure 7-1 were collected from 3.9 nm CdTe cores, but the quenching phenomenon was observed in a wide range of sizes. The results were qualitatively the same, and comparing the extent of quenching in these samples is not particularly informative since all effectively reached 100% quenching with prolonged air exposure. We did, however, notice differences in the ultrafast quenching kinetics in different sized NCs. These data will be discussed in further detail below. Experiments were performed to differentiate the effects of oxygen and water vapor on the CdTe NCs.First, a flask of deionized water was bubbled with argon for 30 minutes to remove other dissolved gases. Then the stream of [water-saturated] argon exiting the water flask was directed into a vessel containing the argon-protected CdTe

231 231 QDs. After 5 minutes of bubbling water-saturated argon through the QD mixture, the PL quantum yield was unchanged. Another sample of argon-protected QDs was bubbled with pure, dry oxygen for 5 minutes, and the PL was quenched by more than 80%. The quenching by molecular oxygen is consistent with the aforementioned idea that the quenching may be due to the reaction of O 2 with Te atoms on the QD surface. Figure 7-2 shows normalized PL emission decays of the CdTe QDs under argon (black curve) and after air exposure (magenta curve). The emission decays of the two samples are nearly identical. Average lifetimes for these two samples, as determined from a multiexponential fit using three lifetimes, were 47 ns for the argon-protected sample and 43 ns for the air-exposed sample. The slight difference in these two values is likely due to the very slight variance between the two curves at long time delays, and this slight variance is within the uncertainty of the background subtraction that was used to correct for the dark counts accumulated during the measurements. Taking this uncertainty into consideration, the decays are essentially indistinguishable, and the air exposure appears to have no impact on the observed luminescence lifetime of the NCs.

232 232 Figure 7-2. Normalized time-resolved emission decay of the CdTe QDs under argon and the air. The invariance of the PL decay rate after air exposure implies that the excited state is quenched on an ultrafast time scale, yielding essentially binary toggling between unquenched and completely quenched states. Following excitation, the QDs return to the ground state (or proceed to a non-emissive intermediate state) on a time scale that is too fast to observe with our PL decay measurements.

233 233 Table 8 lists the emission decay lifetimes information of the CdTe and the CdTe/CdS core/shell QDs. Although the air quenching of the CdTe QDs was observed, there was no difference in the emission decay lifetimes of the CdTe QDs under argon (28.6 ns) and air (28.9 ns). That means the quenching happened in a very short time scale, and some QDs were totally quenched but some are still as normal. The CdTe QDs in different sizes had the various emission decay lifetimes, and the small CdTe QDs have the short lifetime and the large CdTe QDs have the longer lifetime. For instance, the lifetime of the small CdTe QDs (emission: 561 nm) is 28.6 ns, and the lifetime of the large CdTe QDs (emission: 636 nm) is 47.4 ns. Furthermore, there was no obvious difference between the fresh washed and crude CdTe/CdS core/shell QDs; however the lifetimes of the CdTe/CdS core/shell QDs after being washed 9 days (14.9 ns) were much shorter than the original one (36.2 ns). This could be the etching happened on the CdTe/CdS core/shell QDs, and the CdS shell was removed so that the CdTe core lost the protection, or the quencher (MeOH) touched on the CdTe/CdS core/shell QDs surface.

234 234 Table 8. Emission decay lifetime of CdTe and CdTe/CdS core/shell QDs. Exp. Batch QDs Wavelength (nm) Model a1 t1 (ns)/components (%) 1 12/30/2010 CdTe+Ar 561 Reconv. Exponential / /30/2010 CdTe+Air 561 Reconv. Exponential / /30/2010 CdTe+Washed 561 Reconv. Exponential / /18/2011 CdTe+Ar 636 Reconv. Exponential / /18/2011 CdTe+Air 636 Reconv. Exponential / /18/2011 CdTe+Washed 636 Reconv. Exponential / /30/2010 CdTe/CdS+Ar 641 Reconv. Exponential / /30/2010 CdTe/CdS+Air 641 Reconv. Exponential / /30/2010 CdTe/CdS+Washed 641 Tailfit Exponential / /30/2010 CdTe/CdS+Washed after 9 days 641 Reconv. Exponential / /22/2010 CdTe/CdS+Ar 741 Reconv. Exponential / /22/2010 CdTe/CdS+Air 741 Reconv. Exponential / /22/2010 CdTe/CdS+Washed 741 Reconv. Exponential /68.72

235 235 Table 8. (Continued) Exp. a2 t2 (ns)/components (%) a3 t3 (ns)/components (%) Ave. t (ns) / / / / / / / / / / / / / / / / / / / / / / / / /

236 236 In order to monitor the ultrafast dynamics of the quenched QDs, we employed femtosecond pump-probe TA spectroscopy. Figure 7-3 shows TA kinetic traces of CdTe QDs under argon (black triangles) and after exposure to air (red circles). Panel (a) depicts data collected for CdTe cores of approximately 4 nm diameter while panel (b) shows data collected on smaller cores, approximately 3 nm in diameter. In both cases, the kinetic traces were recorded at the wavelength corresponding to the 1S 3/2 (h) 1S(e) bleach, so they depict the population dynamics of carriers near the band edge. Two distinct phenomena contribute to the rising edge of the kinetic traces in Figure 7-3. These two distinct phenomena can be seen most clearly in the larger core data shown in Figure 7-3(a). Absorption of a 3.1 ev photon excites the QDs to a high-lying excited state, and transitions between this excitonic state and the biexcitonic state are shifted relative to the ground state absorption features due to Coulombic interactions between the excited carriers. 135, 187 These shifts in the transition energies appear instantaneously and thus produce a fast rising bleach component at the probe wavelength used in this experiment. This fast rise component matches the temporal characteristics of the instrument response function (approximately 190 fs FWHM). Immediately after excitation, the highly excited electron and hole begin to relax to the lowest exciton state, and as they reach the lowest energy exciton state, further bleaching is observed due to state filling. 135 This is the process represented by the slower rising component in the curves. The time scale for this carrier cooling process is dependent on the material, and fs is typical for CdTe QDs in the strong confinement regime. 215

237 237 Figure 7-3. TA kinetic traces of (a) 4-nm CdTe QDs and (b) 3-nm CdTe QDs under argon (black triangles) and after exposure to air (red circles). The inset shows the same data over a longer pump probe delay time (up to 150 ps).

238 238 In Figure 7-3(a) it is clear that the fast-rising components for the argon-protected and air-exposed QDs are nearly identical. Note that the data have not been normalized or re-scaled in this figure. Since the ground state absorption of the sample is the same before and after air exposure, and the excitation conditions are identical, the two curves can be directly compared using the same vertical axis scaling. It was confirmed that the maximum amplitude of the Stark effect peak (observed as a photoinduced absorption on the long wavelength side of the 1S 3/2 (h)-1s(e) bleach) was the same before and after air exposure. In contrast, the peak intensity of the 1S 3/2 (h)-1s(e) bleach is reduced by approximately 25% after exposure to air. Since the ground state absorption is nearly unchanged after air exposure (Figure 7-1), this decrease in bleach amplitude measured at the wavelength corresponding to the emitting state implies that a fraction of the excited QDs fail to reach the emitting state due to some ultrafast quenching process. In addition to the reduced bleach intensity, the air-exposed sample exhibits a fast decay component that is not present in the argon-protected sample. The data for the smaller cores, shown in Figure 7-3(b) show changes upon air exposure that are qualitatively similar to those observed in the larger cores; however, the faster dynamics seen in the smaller cores under argon makes the air-induced change less obvious. In order to better understand the air-induced changes in the TA kinetics, the data from the larger cores (Figure 7-3(a)) were fit to multiexponential functions, including rising edge components convolved with a 190 fs instrument response function. The fit parameters are summarized in Table 9. As shown in the table, the dynamics of the argonprotected samples could be described by a function with one rise time and two decay

239 239 lifetimes. The rise time was approximately 430 fs, which is similar to values previously recorded for high quality CdTe and CdTe/CdS NCs. 147 The decay was composed of two major components, one of which was approximately 50 ps, and one of which was approximately 1 ns. The latter lifetime includes significant relative uncertainty because of the limited time window used in the fits. After exposure to air, additional fast decay pathways were introduced into the kinetics. The first of these can be observed qualitatively in Figure 7-3(a) as a reduction in the maximum bleach amplitude. In the fit parameters, this change can be observed as a decrease in the amplitude of the rising component as well as a decrease in the measured lifetime of the rise component. This decrease in the lifetime of the rising edge component is likely due to a competing, ultrafast process (such as carrier trapping at the QD surface) that quenches the excited state before the carriers can relax to the lowest excitonic energy level. In addition, the data from the air-exposed sample exhibit an extra decay component with a very short (2.6 ps) lifetime. The significant effect of these additional decay pathways on the excited state dynamics is revealed in the inset of Figure 7-3. Within 150 ps, the bleach intensity of the air-exposed QDs falls to approximately half that of the argon-protected QDs. The kinetics of the two samples on longer time scales was observed to be very similar, in accordance with the data from the time-resolved PL measurements.

240 Table 9. Multiexponential fit parameters (amplitudes and lifetimes) for argon-protected and air-exposed CdTe QDs. 240 argonprotected a rise τ rise (ps) a 1 τ 1 (ps) a 2 τ 2 (ps) a 3 τ 3 (ps) air-exposed As mentioned above, the TA data in Figure 7-3 showed that after the first 100 ps, the bleach intensity of the air-exposed sample was reduced as compared with the argonprotected sample at the same delay time. However, the decrease in the bleach intensity was less than 50%. In contrast, the PL quenching was much greater than 50% in the airexposed sample. This apparent discrepancy between the TA and PL results may be rationalized by considering that the quenching mechanism may be due to a process other than direct ground state recovery. For example, surface trapping of an individual carrier in a given QD could quantitatively quench PL in that QD, but the trapped carrier state could still produce some bleach intensity in the TA measurements. Thus, the fast processes that lead to rapid decay of the TA signal in the air-exposed sample may not be a direct ground state relaxation, but probably instead involve an intermediate state such as a surface-trapped carrier. We have previously shown evidence for size-dependent hole trapping in CdTe cores, even under argon. 215 In that work, fast hole trapping was reported in CdTe cores that were smaller than about 3.2 nm in diameter. The CdTe cores investigated here are large enough (4.0 nm) that the intrinsic hole-trapping tendency is suppressed.

241 241 Similar PL quenching experiments were performed on core/shell CdTe/CdS QDs. Like the CdTe cores, the CdS shells were synthesized by several different methods drawn 4, 111, 116, 215 from the literature. In some cases, CdS shells were grown via slow, simultaneous injection of Cd and S precursors using a syringe pump, while in other cases they were grown via the SILAR method. An interesting and important aspect of the SILAR approach is that the thicknesses of the resulting shells can be finely controlled since the reagents are delivered one shell layer at a time. The CdS shells provided remarkable protection from quenching for the CdTe QD. Quantum yields of the core/shell samples routinely reached 65-70%, and the quantum yields were typically decreased by no more than a few percent after long-term (several days) air exposure. Figure 7-4 shows the TA kinetics of the CdTe/CdS QDs under argon (black triangles) and under air (red circles). These core/shell QDs were grown from the same cores represented in Figure 7-3(a). The kinetic traces are very similar, with only a slight decrease in amplitude for the air-exposed QDs. Multiexponential fits to these samples both yielded three components with very similar lifetimes. The inset shows that the difference between the two TA traces remains small even after a 3 ns pump-probe delay.

242 242 Figure 7-4. TA kinetic traces of CdTe/CdS core/shell QDs under argon (black triangles) and after exposure to air (red circles). The inset shows the same data over a longer pumpprobe delay time (up to 3 ns). These core/shell QDs were prepared from 4 nm CdTe cores similar to those represented in Figure 7-3(a). SILAR and slow injection shell growth experiments demonstrated that even a single monolayer of CdS is sufficient to provide full protection from air-induced quenching. In fact, as we have reported elsewhere, 215 the shell growth seems to terminate after a very thin shell is deposited unless the system is driven hard with high temperature or highly reactive monomers. Several avenues of characterization were used to confirm the ultrathin nature of the shell deposited in these experiments. The measurements and

243 243 typical results on these ultrathin shell measurements have been reported elsewhere. 215 CdTe cores and CdTe/CdS core/shell QDs were measured directly from transmission electron micrographs. The average diameter of the QDs typically grew by 6 8 Å during shell growth, indicating a shell thickness of no more than 3 4 Å. The shell thicknesses could have been a bit less than that estimated from the change in diameter since Ostwald ripening may also have contributed in part to the increase in QD size during shell growth. The particle size distribution was also measured via small angle X-ray scattering (SAXS). The size distribution estimated from SAXS data was very similar to that determined from TEM. The peak locations in the particle size distribution of the core/shell QDs showed a diameter increase of 6.5 Å during the shell growth. Elemental analysis by ICP-OES indicated that the quantity of sulfur in the core/shell QDs was insufficient even to provide a full monolayer of coverage. We note that these results are consistent with other results published in the literature. In particular, Wang et al. showed strong increases in luminescence quantum yield (up to 80%) after addition of only 1 monolayer of CdS to CdTe NCs. 112 In that report, the quantum yield reached its optimum level (92%) after only 2 monolayers. Growth of additional CdS layers beyond the second monolayer led to significant reduction of the quantum yield. The effectiveness of a single CdS monolayer in protecting the CdTe core implies that direct binding of oxygen to the CdTe surface is a requirement for quenching, in agreement with the Te oxidation model discussed above. The effectiveness of a single CdS monolayer in protecting the CdTe cores is remarkable since the SILAR reaction yield is expected to be less than 100%, and the shell uniformity is likely to be imperfect.

244 244 From these considerations it follows that some bare CdTe probably remains exposed after the first monolayer of CdS shell is added. That the remaining exposed CdTe does not lead to quenching suggests that the Te surface atoms most susceptible to oxidation react preferentially with the CdS shell precursors. This idea opens the possibility that factors such as shape control, surface annealing during particle synthesis, and/or surface stoichiometry 231 may allow some control over the air sensitivity of CdTe QDs. However, our experience with numerous synthetic procedures failed to yield any CdTe QD batches that were truly resistant to air oxidation. The effects of CdS shell growth on the ultrafast exciton dynamics of CdTe QDs can be appreciated by directly comparing the data shown in Figure 7-3(a) and Figure 7-4. At the same time, we can examine the importance of making this comparison under an inert atmosphere. Figure 7-5 shows comparisons of TA kinetic traces from CdTe cores and CdTe/CdS core/shell QDs (a) under argon and (b) under air. The experiment run under air shows that the shell strongly modifies the ultrafast exciton dynamics, whereas the experiment run under argon shows only subtle differences between the two samples. When the experiment is run in the presence of air, the data seem to suggest that the core/shell sample possesses additional relaxation processes on the 1 ps time scale; such changes could be attributed to a reduced rate of intraband carrier cooling or to an intraparticle charge separation. However, the comparison under argon demonstrates that the observed changes are really due to protection of the QD surface against oxygen by the CdS shell. Furthermore, these results show that even brief exposure to air is sufficient to significantly affect the ultrafast TA dynamics in CdTe NCs. For this reason, great care

245 must be exercised in the interpretation of TA data collected from air-exposed CdTe QDs and in comparisons between these and related (e.g. nanoheterostructure) systems. 245 Figure 7-5. Comparisons of TA kinetic traces from CdTe cores (black triangles) and CdTe/CdS core/shell QDs (red circles) under (a) air and (b) argon. 7.4 Conclusion In summary, unprotected CdTe QDs are highly sensitive to quenching upon exposure to air. The quenching is caused by oxygen binding to the QD surface, and it is irreversible. The rate of the reaction with oxygen depends on the details of the procedure used to synthesize the CdTe QDs, but in all cases, significant quenching is observed within a period of minutes to hours. TA measurements showed that the quenching was caused by ultrafast relaxation processes in the air-exposed samples that were not observed in argon-protected samples. These relaxation processes were sufficiently fast to compete with intraband carrier cooling and thus prevented many of the QDs from

246 reaching the emitting state. Even those that did reach the lowest exitonic level showed an ultrafast (2.6 ps) decay to a dark state. Because the quenching occurs on such a fast time scale, the measured PL decay is unchanged; only the unquenched population of QDs is observed in the PL decay measurement. On the other hand, the TA rising edge and ultrafast decay dynamics were both strongly altered by the quenching. From these results it is clear that meticulous exclusion of oxygen is required in order to reliably measure and interpret ultrafast exciton dynamics in bare CdTe NCs. In addition, comparisons of , 197, between excited state dynamics of CdTe QDs and CdTe-containing heterostructures 215 must take into account these differences in oxygen sensitivity.

247 247 Chapter 8 Emission Wavelength Shift of CdTe/CdS Core/Shell QDs by Temperature In this chapter, an independent experiment about the emission wavelength shift of the CdTe/CdS core/shell QDs will be introduced. The experiment showed the emission wavelength of the CdTe/CdS core/shell QDs in CHCl 3 shifted with the changing temperature. Figure 8-1 presents the normalized emission spectra of the CdTe/CdS core/shell QDs at different temperatures (5 to 55 C). The results showed that from 5 to 55 C the FWHM of the emission peak did not change (FWHM ~ 40 nm) but the wavelength of the emission peak increased as the temperature raised. The cause of this trend was that the lattice length of the QD was longer at high than at low temperature. Therefore the radius of the QDs is higher at higher temperature, and the band gap energy is lower once the QDs radius increases. This phenomenon can be used for the thermo measurement.

248 248 Figure 8-1. Normalized PL emission spectra of the CdTe/CdS core QDs at different temperatures. Three methods (center of mass, average of FWHM, and the highest point of derivative) were tried to build a relationship between the emission spectra shift and temperature. Table 10 lists the emission wavelengths at different temperatures using various methods. For center of mass method, the wavelength was at the position of the half of the spectrum area. The wavelengths obtained by center of mass method shifted to red when the temperature increased. The data was fitted by linear function (emission wavelength was set as y, and temperature was set as x), and the slope was 0.14±0.07 and

249 249 the intercept was 750.4±2.3 in 95% confidence level. However, if emission wavelength is used to calculate temperature, temperature is in a range from 6 to 76 C. Hence the uncertainty of calculated temperature is too large to be determined by emission wavelength. The second method was average of FWHM method, and the wavelengths were at the middle of FWHM. The main trend was still an increase of wavelength with the rising temperature. The slope was 0.086±0.03 and the intercept was 756.4±0.9 when the data was fitted by the linear function in 95% confidence level (emission wavelength was set as y, and temperature was set as x). The value of temperature changes from 24 to 76 C when the emission wavelength is applied to calculate temperature. Therefore, the uncertainty of calculated temperature is also too large by average of FWHM method. The last method was the highest point of derivative, and the wavelengths were picked at the highest point of derivative of the smooth emission spectra. There was no trend by the highest point of derivative method, and the wavelengths were changing from 740 to 744 nm randomly. Consequently, a major trend was observed that the emission wavelength of the CdTe/CdS core/shell QDs shifted to red by the rising temperature; however the plots of emission wavelength and temperature by center of mass, average of FWHM are very flat which means even significant temperature changing can only cause minor emission wavelength shifting. Therefore, temperature cannot be discriminate by emission wavelength of CdTe/CdS QDs.

250 250 Table 10. Emission wavelengths of CdTe/CdS core/shell QDs at different temperature by various methods. 5 C 15 C 25 C 35 C 45 C 55 C Center of Mass Average of FWHM Highest Point of Derivative nm

251 251 Chapter 9 Conclusions The CdTe QDs are able to simply be synthesized by the hot-injection by organometallic route in organic solvent, ODE. The second injection of the Cd, Te precursors can be applied to reduce the size distribution and increase the size of the CdTe QDs. Furthermore the CdS and ZnS shells can be added by the various methods such as the slow injection, SILAR, and TC-SP methods. The high quality CdTe/CdS core/shell QDs (770 nm emission wavelength) are able to be synthesized by the slow injection method, which is one pot approach, simple operation, and short time synthesis. The highest PL QY of these CdTe/CdS core/shell QDs (770 nm) was around 70%, and the narrowest PL emission distribution was smaller than 40 nm. These high quality CdTe/CdS core/shell QDs are able to be transferred into aqueous phase (PBS, basic buffer, or DI water) via the surface modification method, which switches ligands with some thiol ligands. The short chain monothiolate ligands, TGA, give the best quality for the water soluble CdTe/CdS core/shell QDs (PL QY~ 70%, desirable solubility and stability in aqueous phase). Besides TGA, the biocompatible compound PEG ligands can be attached on the CdTe/CdS core/shell QDs through the surface modification. However these PEG ligands can quench the CdTe/CdS core/shell QDs when they mix together. The PL QY of the core/shell QDs decreased from 70% to 10%. Unfortunately, not only the PEG ligands can quench the CdTe/CdS core/shell QDs but also some other organic compounds or common solvents are able to quench these core/shell QDs. Some experiments also showed that the PL QY of the PEG ligands capped core/shell QDs was affected by the

252 252 bright white light. Xenon arc lamp was employed to excite the PEG ligands-capped- CdTe/CdS core/shell QDs for 250 minutes, in the first 60 minutes the PL QY was increasing and started decreasing from 150 minutes. On the other hand, it has been proved that the stability of the polythiolate ligands (PEG ligands)-capped-core/shell QDs is better than that of the monothiolate ligands (TGA)-capped-core/shell QDs via the dialysis. Also the hydrodynamic diameter of the CdTe/CdS core/shell QDs are affected by the ligands deeply. In PBS the hydrodynamic diameter of the short molecule TGAcapped-QDs (13 nm) is much smaller than that of the long chain PEG ligands-capped- QDs (45 53 nm). In addition, the size of the core/shell QDs is proportional to the temperature and the bond among the aggregation of the core/shell QDs is the weak physical bond. The quenching issue also happens to the unprotected CdTe QDs since they are extremely air sensitive. The quenching process by the oxygen binding to the CdTe QDs surface is irreversible. The different capping ligands of the CdTe QDs can determine the rate of the quenching but the considerable quenching is observed in minutes or hours in the end. In the TA measurements, ultrafast relaxation processes were noticed in the airexposed samples instead of the sample under argon-protected. These relaxation processes are so fast that many of the QDs can not reach the emitting state. Although some of the CdTe QDs reached the lowest exitonic level, an ultrafast (2.6 ps) decay showed they were in the dark state. Because the quenching occurs so quickly, this process can not be observed in PL decay. Since the TA rising edge and ultrafast decay dynamics were both

253 253 strongly affected by the oxygen quenching, the meticulous exclusion of oxygen is required in order to reliably measure and interpret ultrafast exciton dynamics in the bare CdTe QDs. And the oxygen sensitivity effect must be considered for the excited state dynamics of CdTe QDs or CdTe-containing heterostructures. The bleaches in the TA spectra of the CdTe and CdTe/CdS core/shell QDs under argon have been assigned, and analysis of the energy spacings between the 1S 3/2 (h) and 2S 3/2 (h) levels supports the prevailing picture of quasi-type-ii carrier distribution in the core/shell particles, with delocalized electrons and core-localized holes. Additionally the rising edge kinetics of the lowest energy bleach in QDs are often associated with intraband relaxation of conduction band electrons, and it is shown here that at least in the case of the small CdTe cores, the fast rise of the B1 feature reflects surface trapping of the valence band holes. Therefore this analysis shows that it is possible to isolate and study individually the population dynamics of conduction band electrons and valence band holes using only information from the visible TA spectra. Interestingly, a new, highenergy bleach feature has been identified in the large CdTe/CdS core/shell samples, and this feature has tentatively been assigned as a bleach of the CdS absorption. Nevertheless, many issues of the CdTe and CdTe/CdS core/shell QDs need to be further investigation. The quenching issue hinders the CdTe/CdS core/shell QDs being used in the bio-image. To avoid it, some polymers can be applied as micelle to encapsulate the QDs inside. The quenching processes of the different quenchers to the core/shell QDs are still not clear. By femtosecond TA spectroscopy, it is possible to examine the quenching mechanisms of the various quenchers even if they occur on a

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287 287 Appendix A: Details of Synthesis of CdTe Core, CdTe/CdS Core/Shell and CdTe/CdS/ZnS Core/Shell/Shell Quantum Dots Table 11. Synthesis of CdTe QDs by one injection. TDPA was applied as the ligands for the cadmium precursors. Exp.: Number of the different experiments of synthesis of CdTe QDs. ODE 1 : The solvent for the cadmium precursors. ODE 2 : The solvent for the tellurium precursor. Exp. CdO (mmol) TDPA (mmol) ODE 1 (g) Te (mmol) 0. 1 TBP (mmol) ODE 2 (g) Injection T ( C) Reaction Time (min) Reflux T ( C) Date /2/ /3/ /17/ /31/ /6/ /25/ /28/ /1/ /13/ /2/2008

288 288 Table 11. (Continued) /7/ /5/ /8/ /9/ /11/ /11/ /3/2009 Table 11. Exp. Results Comments 1 PL: 550nm and bright, FWHM=33 nm Before injection the solution was cloudy 2 PL: 517, 568 nm and bright Before injection the solution was cloudy 3 PL: 555 nm and bright, FWHM=31 nm Before injection the solution was clean 4 Two PL peaks 5 PL: 539 nm, FWHM=32 nm CdO was hard to dissolve in ODE, before injection the solution was cloudy Before injection the solution was clean, injection and stir must be fast to prevent two size of CdTe QDs 6 PL: 571 nm, FWHM=29 nm Less Cd, longer emission wavelength

289 289 Table 11. (Continued) 7 PL: 623 nm, FWHM=40 nm Less Cd, longer emission wavelength 8 PL: 546, 638 nm Stir paused PL: 549 nm FWHM=35 nm (1 min), 580 nm FWHM=34 nm (30 mins), 591 nm FWHM=41 nm (60 mins), 610 nm (90 mins) PL: 542 nm FWHM=36 nm (1 min), 575 nm FWHM=32 nm (12 mins), 577 nm FWHM=34 nm (35 mins),580 nm FWHM=35 nm (55 mins),592 nm FWHM=49 nm (115 mins),591 nm FWHM=60 nm (109 mins) black precipitate, no PL PL: 595 nm FWHM=31 nm (1 min), 622 nm FWHM=42 nm (10 mins), 676 nm (20 mins),no PL (30 mins) PL: 602 nm FWHM=35 nm (1 min), 610 nm FWHM=35 nm (20 mins), 616 nm FWHM=36 nm (30 mins),620 nm FWHM=38 nm (40 mins) PL: 601 nm FWHM=31 nm (5 min), 608 nm FWHM=34 nm (10 mins), 612 nm FWHM=36 nm (15 mins), 620 nm FWHM=39 nm (20 mins) PL: 600 nm FWHM=33 nm (5 min), 613 nm FWHM=38 nm (10 mins), 626 nm FWHM=45 nm (15 mins), 641 nm FWHM=58 nm (20 mins), 669 nm FWHM=69 nm (25 mins) PL: 592 nm FWHM=38 nm (1 min) PL: 665 nm FWHM=37 nm (30 mins) There was a shoulder in emission spectrum of 90 mins, reflux at 290 C Set 180 C to avoid nucleation of new CdTe, and did the distribution vs. time More Te made reaction more vigorous Up scale

290 290 Table 12. Synthesis of CdTe QDs by one injection. Oleic acid was applied as the ligands for the cadmium precursors. Exp. CdO (mmol) OA (ml) ODE 1 (g) Te (mmol) TBP (mmol) ODE 2 (g) Inject T ( C) Reaction T ( C) Reaction Time (min) /14/ /21/ /21/ /16/ /17/ /18/ /8/2009 Date Table 12. Exp. Results Comments 1 PL: 683 nm, not very bright, FWHM=52 nm 2 PL had two peaks Made a mistake when doing injection 3 PL was very broad 4 PL was very broad Injection at high temperature was not good for this method

291 291 Table 12. (Continued) 5 PL: 660 nm, (1 min), 750 nm FWHM=69 nm (5 mins) 6 PL: 721 nm FWHM=76 nm 7 PL: 711 nm FWHM=48 nm Table 13. Synthesis of CdTe QDs by one injection. HDA was applied as the ligands for the cadmium precursors. And TOP was used as the ligands for the tellurium precursors. Exp. CdO (mmol) TDPA (mmol) HDA (mmol) ODE 1 (g) Te (mmol) TOP (mmol) ODE 2 (g) Inject Temp ( C) Reaction T ( C) Reaction Time (min) Results Date PL: 592 nm, FWHM=35nm 1/11/2010 Table 14. Synthesis of CdTe QDs by two injections. Exp. Injection Order CdO (mmol) TDPA (mmol) ODE 1 (g) Te (mmol) TBP (mmol) ODE 2 (g) Inject T ( C) Reaction Time (min) Reaction T ( C) Injection Rate Dropwise by hand

292 292 Table 14. (Continued) Dropwise by hand Dropwise by hand Dropwise by hand µl/min µl/min µl/min µl/min µl/min

293 293 Table 14. (Continued) µl/min µl/min µl/min µl/min µl/min µl/min µl/min µl/min

294 294 Table 14. (Continued) µl/min µl/min µl/min for the first half,and 100 µl/min for the second half 50 µl/min for the first half,and 100 µl/min for the second half 50 µl/min for the first half,and 150 µl/min for the second half µl/min

295 295 Table 14. (Continued) µl/min µl/min for the first half,and 150 µl/min for the second half 50 µl/min for the first half,and 150 µl/min for the second half µl/min µl/min for the first half, after 60 mins 150 µl/min 50 µl/min for the first half, after 60 mins 150 µl/min

296 296 Table 14. (Continued) µl/min µl/min µl/min µl/min µl/min µl/min µl/min µl/min µl/min

297 297 Table 14. (Continued) Exp. Results Comments Date 1 PL: 527, 562, 593, 634 (main peak) CdO did not totally dissolve in ODE 5/11/2008 PL: 560, 653 After the second injection, less peaks 5/11/ PL: 587 nm FWHM=35 5/24/2008 PL: 626 nm FWHM=30 (30 mins), 641 nm FWHM=35 (70 mins) 3 PL: 616 nm FWHM=42 4 PL: 648 nm FWHM=34 (30 mins), 668 nm FWHM=23 (60 mins) After second injection, a small peak was observed 5/24/2008 Forgot to take 2 ml Cd out of flask for the second injection 5/27/2008 No shoulder, distribution was good 5/27/2008 PL: green 5/29/ PL: 564 nm FWHM=33 nm 6/3/2008 PL: 568 nm FWHM=34 (30 mins), 572 nm FWHM=42 (60 mins), 587 nm FWHM=42 (85 mins) Injected 0.8 ml in the beginning, then added 0.3 ml after 60 min 6/3/ PL: 586 nm FWHM=44 nm 6/5/ PL: 609 nm FWHM=41 (30 mins), 664 nm FWHM=40 (60 mins) PL: 590 nm FWHM=32 nm (1 min), 615 nm FWHM=40 nm (12 mins), 615 nm FWHM=41 nm (30 mins) Second injection for 30 mins, then still reacted for 30 mins 6/5/2008 6/7/2008 PL: 664 nm FWHM=32 nm 6/7/2008

298 298 Table 14. (Continued) 8 PL: 608 nm FWHM=43 nm 6/8/2008 PL: 652 nm FWHM=46 nm (58 mins), 652 nm FWHM=44 nm (73 mins) After the second injection for 15 mins, the distribution was good 6/8/ PL: 623 nm FWHM=68 nm 6/9/2008 PL: 665 nm FWHM=40 nm (60 mins), 665 nm FWHM=35 nm (75 mins) 6/9/ /11/2008 PL: 646 nm FWHM=46 nm (50 mins), 667 nm FWHM=29 nm (110 mins), 668 nm FWHM=26 nm (125 mins), 668 nm FWHM=26 nm (140 mins) Stopped injecting at 110 mins 6/11/ /15/2008 PL: 724 nm very broad (165 mins), 717nm PL weak (180 mins), 767 nm PL weak (195 mins) Stopped injecting at 165 mins 6/15/ /28/2008 PL: 674 nm FWHM=30 nm (125 mins) Stopped injecting at 110 mins 6/28/ /1/2008 PL: 763 nm FWHM=54 nm (250 mins) Stopped injecting at 235 mins 7/1/ PL: 581 nm FWHM=29 nm (1 min), 740 nm broad (30 mins) 7/4/2008 no PL (115 mins) Stopped injecting at 115 mins 7/4/ /13/2008 PL: 620 nm FWHM=37 nm (10 mins), 640 nm FWHM=40 nm (20 mins), 672 nm FWHM=54 nm (35 mins) 7/13/2008

299 299 Table 14. (Continued) 16 7/14/2008 PL: 626 nm FWHM=37 nm (10 mins), 647 nm FWHM=40 nm (17 mins), 668 nm FWHM=56 nm (27 mins) Stopped injecting at 17 mins 7/14/ /15/2008 PL: 625 nm FWHM=39 nm (10 mins), 671 nm FWHM=57 nm (25 mins), 750 nm very broad (32 mins), no PL (35 mins) 7/15/ PL: 746 nm FWHM=85 nm (30 mins) 7/17/2008 PL: 752 nm FWHM=75 nm (1 min) Stopped injecting at 1 min 7/17/ /18/2008 PL: 689 nm FWHM=59 nm (70 mins), 751 nm FWHM=73 nm (110 mins), 759 nm FWHM=66 nm (119 mins) Stopped injecting at 114 mins 7/18/ /21/2008 PL: 762 nm broad (80 mins), 752 nm broad (105 mins), no PL (193 mins) Stopped injecting at 193 mins and saw black chunks, doubled injection rate at 105 mins 7/21/ /24/2008 PL: 723 nm broad (65 mins), 751 nm broad (105 mins), 774 nm broad (125 mins),no PL (175 mins) Stopped injecting at 160 mins 7/24/ /27/2008 PL: 764 nm FWHM=53 (80 mins), 770 nm broad (107 mins), 769 nm broad (110 mins),771 (125 mins) Stopped injecting at 110 mins 7/27/2008

300 300 Table 14. (Continued) 23 PL: 626 nm FWHM=43 (30 mins) 7/28/2008 PL: 700 nm broad (50 mins), 757 nm broad (95 mins),761 (110 mins) Stopped injecting at 95 mins 7/28/ PL: 641 nm FWHM=41 (30 mins) 7/30/2008 PL: 676 nm FWHM=43 nm (30 mins), 717 nm FWHM=74 nm (60 mins), 755 nm broad (75 mins), no PL (85 mins) Stopped injecting at 75 mins 7/30/ PL: 646 nm FWHM=52 (30 mins) 8/1/2008 PL: 677 nm FWHM=71 nm (30 mins), 730 nm broad (65 mins), 757 nm broad (100 mins), 766 nm broad (120 mins) no PL 130 mins) Stopped injecting at 120 mins 8/1/ PL: 658 nm (30 mins) 8/3/2008 PL: 730 nm (35 mins), 768 nm FWHM=40 (60 mins), 776 nm weak (90 mins) 8/3/ /7/2008 PL: 741 nm (60 mins), 752 nm (75 mins) 8/7/ /11/2008 PL: 773 nm (65 mins) 8/11/ /13/2008 PL: 776 nm (65 mins) 8/13/ broad, many peaks (30 mins) 9/8/2008 PL: 768 nm FWHM=39 nm (25 mins), 775 nm FWHM=39 nm (40 mins), 776 nm FWHM=46 nm (46 mins) 9/8/2008

301 301 Table 14. (Continued) 31 9/22/2008 PL: 691 nm broad (25 mins), 751 nm broad (35 mins), 763 nm FWHM=47 nm (41 mins), 768 nm FWHM=38 nm (51 mins), 774 nm FWHM=43 nm (62 mins) 9/22/ /23/2008 PL: 724 nm big shoulder (38 mins) 9/23/2008 PL: 750 nm (11 mins), 756 nm broad (17 mins), 762 nm FWHM=40 nm (23 mins) 9/23/ /21/2008 PL: 768 nm FWHM=42 nm(39 mins) 10/21/ /10/2009 PL: 687 nm FWHM=74 nm(35 mins) Up scale 3/10/ /16/2009 PL: 746 nm FWHM=64 nm(47 mins) 3/16/ /21/2009 PL: 740 nm (47 mins) 4/21/ /25/2009 PL: 742 nm, small peak at 684nm (40 mins) 5/25/2009

302 302 Table 15. Synthesis of the CdS shell growing on the CdTe core by slow injection. Exp. Batch of CdTe Amount of CdTe (nmol) Extraction Volume of ODE (ml) Cd Monomers (M) Cd Monomers (mol) S S Monomers Monomers (M) (mol) 1 50 y y y y /14/ y /21/ y /21/ y /28/ y /7/ y /13/ n y /29/ y /21/ y /16/ n

303 303 Table 15. (Continued) 15 2/4/ n /11/ n /24/ n /3/ n /3/ n /10/ n /10/ n /16/ n /21/ n /1/ n /5/ n /5/ n /14/ n /14/ n /24/ n /28/ n

304 304 Table 15. (Continued) 31 5/31/ n /4/ n /7/ n /26/ n /29/ n /30/ n /6/ n /9/ n /11/ n /14/ n /20/ n /27/ n /18/ n /8/ n /18/ y /dimethylcadmium in TOP /S in TOP

305 305 Table 15. (Continued) Exp. Additional Ligands Amount of Additional Ligands (mmol) 1 HDA HDA, TDPA 0.159, Reaction T ( C) Reaction Time (min) Results Comments Date CdTe turned to black chunks at 220 CdTe QDs were washed 12/17/2007 C CdTe QDs lost PL when they were redissolved in ODE, and when temperature reached 80 C, the solution turned to black and smoke was CdTe QDs were washed 2/13/2008 observed. The black precipitate was observed after 160 C and black chunks appeared after 200 C 3 HDA CdTe QDS lost PL Added 3-4 drops of Cd and S monomers 2/18/2008 Added 2 drops of Cd and S monomers, CdTe QDS lost PL, 4 HDA flask was purged by argon for 30 mins to 2/19/2008 and precipitated remove the air

306 306 Table 15. (Continued) 5 HDA Weak PL 6 HDA HDA HDA HDA HDA HDA HDA HDA HDA HDA PL: 688nm FWHM=37 nm PL: 565 nm FWHM=43nm PL: 623 nm FWHM=37nm PL: 752 nm FWHM=72nm PL: 752 nm FWHM=48nm PL: 752 nm FWHM=56nm PL: 776 nm FWHM=42nm PL: 771 nm FWHM=39nm PL: 768 nm FWHM=41nm PL: 707 nm FWHM=47nm Added 2 drops of Cd and S monomers, flask was purged by argon for 30 mins to remove the air, mixture of CdTe QDs, ODE and HDA still had PL Added 1 drop of Cd and S monomers at 150 C, and another one drop of Cd and S monomers at 160 C Added 2 drops of Cd and S monomers at 150 C, cloudy because of HDA Added 2 drops of Cd and S monomers at 150 C, cloudy because of HDA, transferred into water Added 2 drops of Cd and S monomers at 150 C Added 2 drops of Cd and S monomers at 150 C 2/20/2008 2/24/2008 3/2/2008 3/31/2008 8/12/2008 8/13/ µl/min 8/24/ µl/min 10/7/ µl/min, after injection kept reacting for 1 hr 50 µl/min, saw smoke at 160 C, at 180 C started injecting, at 200 C QDs still good 10/21/2008 1/16/ µl/min, started injecting at 150 C 2/5/2009

307 307 Table 15. (Continued) 16 TBP PL: 719 nm FWHM=48nm 50 µl/min, the solution was clear without HDA 2/12/ TBP PL: 700 nm FWHM=37nm 50 µl/min 2/26/ TBP PL: 720 nm FWHM=41nm 50 µl/min 3/3/ TBP PL: 701 nm FWHM=44nm 50 µl/min 3/4/ TBP PL: 715 nm FWHM=63nm 50 µl/min 3/11/ TBP PL: 739 nm FWHM=56nm 100 µl/min 3/13/ TBP PL: 770 nm FWHM=39nm 100 µl/min 3/17/ TBP PL: 770 nm FWHM=44nm 100 µl/min, small peak at 674 nm 4/23/ TBP PL: 746 nm 50 µl/min, small shoulder 584 nm 5/4/ TBP PL: 674 nm FWHM=29 nm 50 µl/min, small shoulder 620 nm 5/5/ TBP PL: 652 nm FWHM=33 nm (20 mins), 661 nm FWHM=33 nm (53 mins), 676 nm FWHM=29 nm (103 mins), 678 nm FWHM=31 nm (115 mins) 50 µl/min 5/13/2009

308 308 Table 15. (Continued) 27 TBP TBP TBP TBP TBP TBP TBP TBP TBP PL: 766 nm FWHM=40 nm shoulder at 716 nm PL: 761 nm FWHM=48 nm PL: 742 nm, small peak at 684nm PL: 746 nm FWHM=61 nm PL: 769 nm FWHM=37 nm PL: 775 nm FWHM=35 nm PL: 768 nm FWHM=42 nm PL: 764 nm FWHM=44 nm PL: 768 nm FWHM=42 nm 100 µl/min, small peak at 674 nm 5/14/ µl/min 5/16/ µl/min, using TDPA (0.5 mmol) instead of OA, TBP (2.07 mmol) was added into Cd monomers to make it liquid, emission peaks did change. 50 µl/min, after injection kept reacting running for 15 mins. 100 µl/min, CdTe with two peaks at 695 and 744 nm 100 µl/min, CdTe at 765 nm, refluxing for 10 mins after injection 5/25/2009 5/28/2009 5/31/2009 6/4/ µl/min, CdTe at 753 nm 6/7/ µl/min, CdTe at 724 nm, at 55 mins pause adding, at 85 mins adding again. 100 µl/min, CdTe at 677 nm, at 55 mins pause adding, at 85 mins adding again increased temperature to 270 C. 7/25/2009 7/29/2009

309 309 Table 15. (Continued) 36 TBP TBP PL: 770 nm FWHM=34 nm (55 mins), 776 nm FWHM=33 nm (85 mins), 777 nm FWHM=33 nm (115 mins), 796 nm FWHM=55 nm (147 mins), 801 nm FWHM=59 nm (180 mins), QY= 33% PL: 740 nm FWHM=46 nm (85 mins), 745 nm FWHM=44 nm (130 mins), 746 nm FWHM=43 nm (155 mins), 748 nm FWHM=44 nm (180 mins), 752 nm FWHM=43 nm (220 mins) 100 µl/min, CdTe at 769 nm, at 100 mins paused adding, at 115 mins adding again, at 150 mins paused adding, at 180 mins adding again, no PL when the CdTe/CdS QDs were transferred into water 100 µl/min, 25 µl/min after 55 mins, 100 µl/min after 115 mins, CdTe at 680 nm, at 85 mins paused adding, at 115 mins adding again, at 130 mins paused adding, at 140 mins adding again, at 155 mins paused adding, at 165 mins adding again, at 180 mins paused adding, at 190 mins adding again 7/31/2009 8/6/2009

310 310 Table 15. (Continued) 38 TBP TBP TBP TBP PL: 776 nm FWHM=39 nm (107 mins), 783 nm FWHM=50 nm (137 mins), 788 nm FWHM=57 nm (152 mins), 805 nm FWHM=62 nm (187 mins), 814 nm FWHM=60 nm (240 mins) PL: 771 nm FWHM=39 nm (135 mins), 775 nm FWHM=42 nm (170 mins), 775 nm FWHM=48 nm (205 mins), 786 nm FWHM=55 nm (253 mins) PL: 752 nm FWHM=52 nm (72 mins) PL: 770 nm FWHM=36 nm (100 mins) 100 µl/min, CdTe at 755 nm, at 57 mins paused adding, at 87 mins adding again, at 107 mins paused adding, at 137 mins adding again, at 152 mins paused adding, at 172 mins adding again, at 187 mins paused adding, at 207 mins adding again, no PL when the CdTe/CdS QDs were transferred into water 100 µl/min CdTe at 746 nm, at 55 mins paused adding, at 85 mins adding again, at 100 mins paused adding, at 120 mins adding again, at 135 mins paused adding, at 155 mins adding again, at 170 mins paused adding, at 190 mins adding again, at 205 mins paused adding, at 225 mins adding again 100 µl/min CdTe at 723 nm, no orange (CdS), at 58 mins stopped adding 100 µl/min, no orange (CdS), at 60 mins paused adding, at 75 mins restarted, at 85 mins stopped adding 8/9/2009 8/11/2009 8/14/2009 8/20/2009

311 311 Table 15. (Continued) 42 TBP TBP TBP Oleylamine PL: 762 nm FWHM=45 nm (75 mins) PL: 744 nm FWHM=66 nm (10 mins), 773 nm FWHM=68 nm (30 mins), 816 nm FWHM=57 nm 44 mins) PL: 774 nm FWHM=57 nm PL: 668 nm FWHM=36 nm (30 mins), 675 nm FWHM=28 nm (60 mins), 686 nm FWHM=40 nm (90 mins), 734 nm FWHM=82 nm (110 mins), 758 nm FWHM=64 nm (140 mins) 10 µl/min, CdTe at 762 nm, too slow injection not helpful for shell growth 8/27/ µl/min 11/18/ µl/min 12/8/ µl/min, after injection kept reaction running for 30 mins. still can be quenched by PEG ligands (2500) 2/19/2010

312 Table 16. Synthesis of the CdS shell growth on the CdTe core via SILAR method. The precursors used in the reaction were CdOA and S. Exp. Batch of CdTe/CdS Amount of CdTe (nmol) Extraction Volume of ODE (ml) Cd Monomers (M) 1st Cd Layers (mmol) 2nd Cd Layers (mmol) 3rd Cd Layers (mmol) 1 12/29/ n th Cd Layers (mmol) 312 5th Cd Layers (mmol) 2 1/7/ n Table 16. Exp. S Monomers (M) 1st S Layers (mmol) 2nd S Layers (mmol) 3rd S Layers (mmol) 4th S Layers (mmol) 5th S Layers (mmol) Reaction T ( C)

313 313 Table 16. (Continued) Exp. Reaction Time (min) Results Comments Date 1 Added Cd first, after 5 mins added PL: 674 nm FWHM=27 nm (1 S at 190 C, then heated up to 250 layer), 678 nm FWHM=27 nm CdTe 669nm C for 20 mins, decreased to 150 C (2 layers), 692 nm FWHM=37 FWHM=31 nm to add the second layer, the third nm (3 layers) layer 12/29/2009 Added Cd first, after 5 mins added 2 S at 190 C, then heated up to 250 C for 20 mins, decreased to 150 C to add the rest layers PL: two peaks CdTe 574nm 1/8/2010 Table 17. Synthesis of the CdS shell growth on the CdTe core via SILAR method. The precursors used in the reaction were dimethylcadmium and TOPS. Exp. Batch of CdTe/CdS Amount of CdTe (nmol) Extraction Cd Monomers (M) 1st Cd Monomers (mmol) 2nd Cd Monomers (mmol) 3rd Cd Monomers (mmol) 4th Cd Monomers (mmol) 5th Cd Monomers (mmol) 1 1/11/ y /29/ y

314 314 Table 17. (Continued) Exp. S Monomers (M) 1st S Monomers (mmol) 2nd S Monomers (mmol) 3rd S Monomers (mmol) 4th S Monomers (mmol) 5th S Monomers (mmol) Reaction T ( C) Reaction Time (hr) Changed Changed 14 Table 17. Exp. Results Comments Date 1 PL: 656 nm FWHM=34 nm (1 layer), 668 nm FWHM=30 nm (2 layers), 675 nm FWHM=28 nm (3 layers), 686 nm FWHM=35 nm (4 layers), 763 nm FWHM=50 nm (5 layers) Added half layer each time, added Cd and 10 mins later added S, the first layer took 4 hours for each half layerm, reaction temp was 150 C, the rest layer took 30 mins for each half layer at 225 C. Switched ligands with PEG (2500) and TGA, PEG (2500) still quenched QDs, but TGA was good for them. 2 PL: 642 nm FWHM=34 nm (1 layer), 665 nm FWHM=350 nm (2 layers), 674 nm FWHM=32 nm (3 layers), 681 nm FWHM=33 nm (4 layers), 700 nm FWHM=41 nm (5 layers) Added half layer each time, added Cd and 10 mins later added S, the first layer took 4 hours for each half layer, reaction temp was 150 C, the rest layer took 30 mins for each half layer at 225 C. Switched ligands with PEG (2500) and TGA, PEG (2500) still quenched QDs, but TGA was good for them. 1/14/2010 2/1/2010

315 Table 18. Synthesis of the ZnS shell growth on the CdTe/CdS core/shell QDs by slow injection method. The precursors used in the reaction were ZnOA and S. Exp. Batch of CdTe/CdS Amount of CdTe/CdS (nmol) Extraction ODE (ml) Zn Monomers (M) Zn Monomers (mmol) S Monomers (M) S Monomers (mmol) Additional Ligands 1 10/5/ n TBP 315 Table 18. Exp. Amount of Additional Ligands (mmol) T ( C) Time (min) Results Comments Date Paused adding at 60 mins, PL: 800 nm FWHM=57 nm (60 mins), 811 nm restarted at 75 min, at 85 FWHM=59 nm (100 mins), QY= 42% mins finished, kept reacting 10/8/2009 for 15 mins

316 Table 19. Synthesis of the ZnS shell growth on the CdTe/CdS core/shell QDs by slow injection method. The ligands used in the reaction were zinc stearate and thioacetamide. Exp. Batch of CdTe/CdS Amount of CdTe/CdS (nmol) Extraction Volume of ODE (ml) Zn Monomers (M) Zn Monomers (mmol) S Monomers (M) S Monomers (mmol) Additional Ligands 1 9/3/ y TBP 316 Table 19. Exp. Amount of Additional Ligands (mmol) Reaction T ( C) Reaction Time (min) Results Comments Date PL: 757 nm FWHM=51 nm (60 mins), CdTe/CdS 752 FWHM=45 nm Put Zn and S precursors in flask and heated up around 110 C to dissolve them in ODE. If the temperature is too high, ZnS QDs will be produced. 9/8/2010

317 Table 20. Synthesis of the ZnS shell growth on the CdTe/CdS core/shell QDs via SILAR method. The precursors used in the reaction were ZnOA and S. Exp. Batch of CdTe/CdS Amount of CdTe/CdS (nmol) Extraction Zn Monomers (M) 1st Zn Monomers (mmol) 2nd Zn Monomers (mmol) S Monomers (M) 1st S Monomers (mmol) 2nd S Monomers (mmol) 1 12/8/ n /29/ n /5/ y /10/ y /18/ y

318 318 Table 20. (Continued) Amount of Additional Ligands Exp. Additional Ligands Reaction T ( C) Reaction Time (min) Results Date Added Zn first, after 5 mins added S at 150 C, PL: 818 nm then heated up to 250 C for 20 mins, decreased FWHM=69 nm to 150 C, added the second layer 12/8/ Added Zn first, after 5 mins added S at 190 C, PL: 748 nm then heated up to 250 C for 20 mins, decreased FWHM=51 nm to 150 C, added the second layer 12/30/ TBP 2.07 mmol oleic amine 6 ml Added Zn first, after 5 mins added S at 190 C, then heated up to 250 C for 20 mins, decreased to 150 C, added the second layer Added Zn first, after 5 mins added S at 190 C, then heated up to 250 C for 20 mins, decreased to 150 C, added the second layer Added Zn first, after 5 mins added S at 190 C, then heated up to 250 C for 20 mins, decreased to 150 C, added the second layer PL: 792 nm FWHM=72 nm (1 layer), 826 nm weak (2 layer) PL: 788 nm FWHM=66 nm (1 layer), 825 nm weak (2 layer) PL: 736 nm FWHM=49 nm (1 layer), 740 nm FWHM=49 nm (2 layer) 5/6/2010 5/10/2010 5/20/2010

319 Table 21. Synthesis of the ZnS shell growth on the CdTe/CdS core/shell QDs via TC-SP method. The precursor used in the reaction was Zn(DDTC) 2. Exp. Batch of CdTe/CdS Amount of CdTe/CdS (nmol) Extraction Volume of ODE (ml) Zn(DDTC) 2 (g) Oleylamine (ml) CHCl 3 (ml) 1st Layer (mmol) 2nd Layer (mmol) 1 5/24/ y /2/ y rd Layer (mmol) 3 6/2/ y /17/ y th Layer (mmol)

320 320 Table 21. (Continued) Exp. Reaction T ( C) Reaction Time (minutes) Results Comments Date Added precursors at 50 C for the first layer, then increased to 180 C to grow PL: 778 nm (1 layer), the shell, then decreased to 120 C to nothing (2 layers), Zn(DDTC) 2 was easy to add precursors for the second shell, then CdTe/CdS 745 nm dissolve in oleylamine 6/1/2010 increased temperature to 180 C for 20 FWHM=46 nm mins Added precursors at 60 C for the first layer, then increased to 180 C to grow the shell, then decreased to 120 C to add precursors for the second shell, then increased temp to 180 C for 20 mins Added precursors at 60 C for the first layer, then increased to 180 C to grow the shell, then decreased to 120 C to add precursors for the rest shell, then increased temp to 180 C for 20 mins Added precursors at 60 C for the first layer, then increased to 180 C to grow the shell, then decreased to 120 C to add precursors for the rest shell, then increased temp to 180 C for 20 mins PL: 740 nm FWHM=50 nm (1 layer), 740 nm FWHM=56 nm (2 layers), CdTe/CdS 741 nm FWHM=47 nm PL: 742 nm FWHM=46 nm (1 layer), 745 nm FWHM=48 nm (2 layers),, 745 nm FWHM=49 nm (3 layers), 743 nm FWHM=50 nm (4 layers) CdTe/CdS 741 nm FWHM=47 nm PL: 736 nm FWHM=69nm (2 layer) QY= 61%, CdTe/CdS 740 nm FWHM=56 nm UV-vis peak did not change, can be transferred into water by TGA, not PEG even with TMAH Zn(DDTC) 2 did not dissolve in CHCl 3 very well Zn(DDTC) 2 did not dissolve in CHCl 3 very well 6/4/2010

321 321 Table 22. CdTe QDs precipitation by different solvents. Exp. Appendix B: Details of Quantum Dots Water Transfer CdTe (ml) MeOH (ml) Hexane (ml) Acetone (ml) Results No precipitate A little CdTe QDs precipitate Cloudy, immiscible White precipitate and a little orange precipitate White precipitate and a little orange precipitate More white precipitate and orange precipitate Immiscible Immiscible Still had some CdTe QDs in the solution Almost all CdTe QDs precipitate (orange) A little CdTe QDs precipitate

322 322 Table 23. Precipitation Experiments of organic ligands capped CdTe/CdS core/shell QDs by different solvents. Exp. CdTe/CdS (ml) CHCl 3 (ml) MeOH (ml) Hexane (ml) Acetone (ml) Results Not good to precipitate Almost precipitate Almost precipitate Almost precipitate Table 24. Extraction of CdTe QDs. Exp. CdTe (ml) CHCl 3 (ml) MeOH (ml) Results Solution separated and the CdTe QDs were on the top layer Solution separated and the CdTe QDs were on the top layer Solution separated and the CdTe QDs were on the top layer

323 323 Table 25. Water transfer of CdTe/CdS core/shell QDs via two layers transfer method. Exp. Amount of CdTe/CdS (nmol) Volume of CHCl 3 (ml) Volume of TGA (µl) Base/Acid Results M NaOH 5 mins late the water phase turned to weak PL, 2 hrs late it was clear in CHCl 3 phase, after 1 day QDs almost in water phase M H 2 SO 4 After 2 hrs no change ph=5.8 No change ph=11.3 QDs were transferred into water Table 26. Water transfer of CdTe/CdS core/shell QDs via one layer transfer method with different hydrophilic ligands. Exp. Amount of CdTe/CdS (nmol) Volume of CHCl 3 (ml) Ligands Reaction Time (hr) TGA/150 µl dodacanethiol/300 µl 2 Sonic Time (hr) Solvent Results PBS (2mL), 1M NaOH QDs were transferred into dropwise PBS (2mL), 1M NaOH dropwise TGA/150 µl 2 ph=11.3 buffer TGA/150 µl 2 ph=11.3 buffer, 1M NaOH dropwise water No precipitate in the CHCl 3 QDs were transferred into water, and very bright 770 nm QDs were hard to be transferred into buffer

324 324 Table 26. (Continued) MA/some 2 ph=5.6 buffer, TGA/300 µl 2 ph=11.3 buffer, 1M NaOH dropwise TGA/600 µl 2 water TGA/1200 µl 2 water TGA/1200 µl 2 PBS TGA/600 µl 2 PBS TGA/600 µl 1 PBS TGA/600 µl 1 water MPA/600 µl 2 PBS MUA/0.26 g 2 PBS 770 nm QDs were hard to dissolve in buffer 770 nm QDs lost PL when 1M NaOH was added 770 nm QDs were not dissolved in water 770 nm QDs were not dissolved in water 770 nm QDs dissolved in PBS very well, washed by 2 ml CHCl 3 again 770 nm QDs dissolved in PBS very well, washed by 2 ml CHCl 3 again 770 nm QDs dissolved in PBS very well, washed by 2 ml CHCl 3 again 770 nm QDs dissolved in PBS not very well, washed by 2 ml CHCl 3 again 770 nm QDs dissolved in PBS, washed by 2 ml CHCl 3 again 770 nm QDs dissolved in PBS bad, washed by 2 ml CHCl 3 again

325 325 Table 26. (Continued) DHLA/300 µl 45 ph=11.5 buffer TGA/600 µl 2 PBS TGA/300 µl 3 PBS added some acetone to ppt, washed by CHCl 3, dissolved in buffer well, but still quenching, 811 nm QDs with TGA, no PL 811 nm QDs with TGA, washed by 1mL CHCl 3, good dissolve in PBS, QY=20% Table 27. Water transfer of CdTe/CdS core/shell QDs with MPA and TMAH. Exp. Volume of QDs (ml) MPA (ml) TMAH (g) CHCl 3 (ml) Results One small brown liquid ball on the top of liquid Saw bubbles, 41 hrs later there was a small brown ball, added water the ball disappeared and dissolved in water layer, under UV light the water layer looked darker than CHCl 3 layer Dissolved in water

326 326 Table 28. Water transfer of CdTe/CdS core/shell QDs with PEG ligands. Saturated PEG Ligands in CHCl 3 (ml) Sonic Time (min) Reaction Time (min) Exp. PEG Type QDs QDs (nmol) CHCl 3 (ml) PPT Solvent 1 From Germany Washed CdTe/CdS Hexane 2 From Germany Washed CdTe/CdS Hexane 3 From Germany Washed CdTe/CdS Hexane , PEG (1500), fresh , PEG (1500), fresh Washed CdTe/CdS Hexane Washed CdTe/CdS Hexane 6 PEG (2500) Washed CdTe/CdS Hexane 7 PEG (2500) Washed CdTe/CdS Hexane 8 9 9/23/2009 PEG (1500) 10/2/2009 PEG (2500) Washed CdTe/CdS Hexane Washed CdTe/CdS Hexane Results Cloudy in water but had red PL Partially dissolved in PBS Partially dissolved in PBS Dissolved in PBS very well, quenching QDs Dissolved in PBS very well, quenching QDs The ppt did not dissolve in PBS looked like film The ppt did not dissolve in PBS Dissolved in water, still had weak PL Dissolved in water, still had weak PL

327 327 Table 28. (Continued) /2/2009 PEG ( 2500) 10/2/2009 PEG (2500) 9/23/2009 PEG (1500) 13 PEG (2500) 14 PEG (2500) 15 PEG (2500) Washed CdTe/CdS Hexane Washed CdTe/CdS/ZnS Washed CdTe/CdS/ZnS Washed CdTe/CdS 800nm Washed CdTe/CdS/ZnS Washed CdTe/CdS/ZnS 12/30/2009 Dissolved in water, still had weak PL, QY= 10% Hexane Quenched Hexane Quenched Hexane Hexane Hard to dissolve in 11.5 buffer, QY=2% Some QDs dissolved in PBS QY=1.3% Hexane Cloudy in water Table 29. Water transfer of CdTe/CdS core/shell QDs with CTAB. Exp. CTAB (mm) QDs (nmol) Heat T ( C) Solvent Results Water Hard to dissolve in water Water It was cloudy, some QDs dissolved in water, most of them did not dissolve in water, the emission peak was at 770 nm

328 328 Table 30. Water transfer of CdTe/CdS core/shell QDs with MHA ligands. Exp. QDs (nmol) Mass of MHA (g) Purified MHA Heat T ( C) Volume of CHCl 3 (ml) Reaction Time Solvent Results n 70 1 over night Water Can not be dissolved in water y mins Water Added 1 ml t-butanol, shaked, still cloudy, filtered by 0.2um filter to get clear solution but it was very light, y mins Water Can not be dissolved in water y hrs Water Can not be dissolved in water Table 31. Hydrodynamic diameter of the CdTe/CdS core/shell QDs capped by different ligands. Exp. QDs Solvent Radius (nm) T ( C) Date 1 TGA capped CdTe/CdS PBS /12/ TGA capped CdTe/CdS PBS /12/ Organic ligands capped CdTe/CdS CHCl /12/ PEG2500 capped CdTe/CdS CHCl /14/ PEG2500 capped CdTe/CdS Water /14/ PEG2500 capped CdTe/CdS Water /14/2010

329 329 Table 31. (Continued) 7 TGA capped CdTe/CdS for 3 days PBS /14/ TGA, PEG2500 capped CdTe/CdS Water /14/ TGA, PEG2500 capped CdTe/CdS over night Water /16/ TGA capped CdTe/CdS over night PBS /16/ TGA capped CdTe/CdS for 5 days PBS /19/ TGA, PEG2500 capped CdTe/CdS for 5 days Water /19/ TGA capped CdTe/CdS for 7 days PBS /21/ TGA, PEG2500 capped CdTe/CdS for 7 days Water /21/ TGA capped CdTe/CdS for 8 days PBS /22/ TGA, PEG2500 capped CdTe/CdS for 8 days Water /22/ TGA capped CdTe/CdS for 5 mins PBS /22/ TGA capped CdTe/CdS for 20 mins PBS /22/ TGA capped CdTe/CdS for 60 mins PBS /22/ TGA capped CdTe/CdS for 120 mins PBS /22/ TGA, PEG2500 capped CdTe/CdS (fresh) Water /26/ TGA, PEG2500 capped CdTe/CdS (30 mins) Water /26/ TGA, PEG2500 capped CdTe/CdS (60 mins) Water /26/2010

330 330 Table 31. (Continued) 24 TGA, PEG2500 capped CdTe/CdS cool down Water /26/ PEG2500 capped CdTe/CdS (fresh) Water /26/ PEG2500 capped CdTe/CdS (fresh) Water /26/ PEG2500 capped CdTe/CdS heated up to 60 C then cooled down to 20 C PEG2500 capped CdTe/CdS heated up to 60 C then cooled down to 20 C, and heated to 60 C again PEG2500 capped CdTe/CdS heated up to 60 C then cooled down to 20 C, and heated to 60 C again, cooled down to 20 C TGA, PEG2500 capped CdTe/CdS heated up to 60 C then cooled down to 20 C TGA, PEG2500 capped CdTe/CdS heated up to 60 C then cooled down to 20 C, heated up to 60 C again TGA, PEG2500 capped CdTe/CdS heated up to 60 C then cooled down to 20 C, heated up to 60 C again, cooled down to 20 C TGA, PEG2500 capped CdTe/CdS exp.32 after one day TGA, PEG2500 capped CdTe/CdS exp.32 after one day sonic PEG2500 ligands (dialysis of PEG 2500 ligands) inside Water /27/2010 Water /27/2010 Water /27/2010 Water /27/2010 Water /27/2010 Water /27/2010 Water /28/2010 Water /28/2010 Water /14/2010

331 331 Table 31. (Continued) PEG2500 ligands (dialysis of PEG 2500 ligands) outside PEG2500 ligands + NaBH 4 (dialysis of PEG 2500 ligands) inside PEG2500 ligands + NaBH 4 (dialysis of PEG 2500 ligands) outside Water /14/2010 Water /17/2010 Water /17/2010 Table 32. Water transfer of CdTe/CdS/ZnS core/shell/shell QDs with TGA. Batch Amount of CdTe/CdS/ZnS (nmol) Volume of CHCl 3 (ml) Volume of TGA(uL) Sonic Time (hr) Solvent Results 12/8/ PBS Dissolved well, QY=12% Table 33. Water transfer of CdTe/CdS/ZnS core/shell/shell QDs with PEG. Exp. PEG Type QDs 1 PEG2500 Washed CdTe/CdS/ZnS QDs (nmol) Saturated PEG Ligands in CHCl 3 (ml) CHCl 3 (ml) TMAH Sonic Time (min) PPT Solvent A little 30 Hexane Results Dissolved in PBS but totally quenched

332 332 Appendix C: List of Publications (1) Yueran Yan, Gang Chen, and P. Gregory Van Patten. Ultrafast Exciton Dynamics in CdTe Nanocrystals and Core-Shell CdTe-CdS Nanocrystals. J. Phys. Chem. C (Accepted) (2) Yueran Yan, Lei Wang, Chery Vaughn and P. Gregory Van Patten. Spectroscopic Investigation of Oxygen Sensitivity in CdTe and CdTe-CdS Nanocrystals,. J. Phys. Chem. C (Accepted)

333 Appendix D: Copyright Permission 333

334 334

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