Supporting Information for: Heavy-Metal-Free Fluorescent ZnTe/ZnSe Nanodumbbells

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1 Supporting Information for: Heavy-Metal-Free Fluorescent ZnTe/ZnSe Nanodumbbells Botao Ji, Yossef E. Panfil and Uri Banin * The Institute of Chemistry and Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel * uri.banin@mail.huji.ac.il 1

2 Table S1. Details for the synthesis of ZnTe/ZnSe NDBs nanodumbbells (NDBs) with different amounts of ZnSe precursors nmol of ZnTe nanorods were dispersed in a three-neck flask with 1.25 ml of TOP and 0.75 ml of oleylamine. The solution was heated to 240 o C under argon. The zinc precursor (zinc stock solution) was zinc oleate (0.1 M). The selenium precursor (selenium stock solution) was TOP-Se (0.1 M). 't = 0 min' means the beginning of the reaction. Injection number ZnTe/1ZnSe ZnTe/2ZnSe ZnTe/3ZnSe ZnTe/4ZnSe 0.16 ml 0.16 ml 0.16 ml 0.16 ml (Zn, t = 0 min) (Zn, t = 0 min) (Zn, t = 0 min) (Zn, t = 0 min) 0.16 ml 0.16 ml 0.16 ml 0.16 ml (Se, t = 15 min) (Se, t = 15 min) (Se, t = 15 min) (Se, t = 15 min) 0.21 ml 0.19 ml 0.19 ml 0.19 ml (Zn, t = 30 min) (Zn, t = 30 min) (Zn, t = 30 min) (Zn, t = 30 min) 0.25 ml 0.19 ml 0.19 ml 0.19 ml (Zn, t = 60 min) (Se, t = 45 min) (Se, t = 45 min) (Se, t = 45 min) 0.29 ml 0.25 ml 0.22 ml 0.22 ml (Zn, t = 90 min) (Zn, t = 60 min) (Zn, t = 60 min) (Zn, t = 60 min) 0.34 ml 0.29 ml 0.22 ml 0.22 ml (Zn, t = 120 min) (Zn, t = 90 min) (Se, t = 75 min) (Se, t = 75 min) 0.38 ml 0.34 ml 0.29 ml 0.25 ml (Zn, t = 150 min) (Zn, t = 120 min) (Zn, t = 90 min) (Zn, t = 90 min) 0.38 ml 0.34 ml 0.25 ml (Zn, t = 150 min) (Zn, t = 120 min) (Se, t = 105 min) 0.43 ml 0.38 ml 0.34 ml (Zn, t = 180 min) (Zn, t = 150 min) (Zn, t = 120 min) 0.43 ml 0.38 ml (Zn, t = 180 min) (Zn, t = 150 min) 0.48 ml 0.43 ml (Zn, t = 210 min) (Zn, t = 180 min) 0.48 ml (Zn, t = 210 min) 0.54 ml (Zn, t = 240 min) 2

3 Figure S1. (A) Evolution of absorption during ZnTe nanorods synthesis grown with heating rate of 5 o C/min. B) TEM image of ZnTe nanorods; corresponding diameter and length histograms are shown in C and D. (E) TEM image of ZnTe nanorods with heating rate of 10 o C/min. 3

4 Figure S2. PL spectra after ZnS growth on ZnTe nanorods via (A) layer-by-layer method and (B) thermal cycling using single-source precursor. ZnTe/ZnS is a typical type I core/shell structure, in which both electrons and holes are confined in the core materials. This confinement induces the exciton away from the surface of nanocrystals and thereby is expected to improve their optical properties. A layer-bylayer method was first applied. A wide trap emission around 540 nm is observed when sulfur precursor (sulfur dissolved in TOP) for the third layer of ZnS is added (Figure S2A). The addition of zinc precursor (zinc oleate dissolved in TOP) eliminates the trap emission. Meanwhile a very weak band gap emission around 486 nm emerges. Another method named thermal cycling by using single-source ZnS precursor (zinc diethyldithiocarbamate) was tried for growth of ZnS on ZnTe nanorods. The first layer growth of ZnS results in very weak emission around 481 nm similar to the above (Figure S2B). Further growth of two additional ZnS layers completely quenches the fluorescence. 4

5 Figure S3. ZnSe tip growth on ZnTe nanorods by injecting Se precursor (TOP-Se, for 6 monolayers of ZnSe) in Zn precursor solution (Zn oleate dissolved in the mixture of TOP and oleylamine) which contained ZnTe nanorods. The reaction temperature was 260 o C and the reaction time was 60 minutes. Evolution of (A) absorption and (B) emission spectra, and (C) TEM image of ZnTe/ZnSe nanoparticles at the end of synthesis. The quantum yield of the obtained nanoparticles is typically smaller than 5%. The NDBs structure could be recognized but poorly defined. 5

6 Figure S4. TEM images of ZnTe/ZnSe NDBs by adding different amounts of ZnSe precursors. (A) 1 monolayer; (B) 2 monolayers; (C) 3 monolayers; (D) 4 monolayers. Corresponding length histograms and the average length are shown below the TEM images. 6

7 Table S2. Relative concentrations of Zn, Se and Te measured by ICP-MS for ZnTe nanorods and for ZnTe/ZnSe NDBs with different amounts of ZnSe precursors as shown in Figure S4. The measured and calculated ZnSe sizes of the dimension perpendicular to c-axis of ZnTe nanorods are also shown. To simplify the calculation, the ZnSe tip is structure is approximated as a cylinder. The cylinder height 'H' is extracted with the lengths of the ZnTe nanorods and ZnTe/ZnSe NDBs. The volume of ZnTe nanorod is considered as a constant. The ZnSe tip volume 'V' is then obtained based on the Se/Te ratio and the volume of ZnTe nanorods. Then ZnSe tip size 'D' is calculated by V = 2π ( D 2 )2 H. Zn Se Te Se/Te Measured ZnSe size from TEM Calculated ZnSe size from ICP ZnTe rods ZnTe/1ZnSe ±0.3 nm 4.2 nm ZnTe/2ZnSe ±0.4 nm 5.3 nm ZnTe/3ZnSe ±0.4 nm 6.4 nm ZnTe/4ZnSe ±0.5 nm 6.5 nm 7

8 Figure S5. Evolution with time of PL wavelength (A) and quantum yield (B), for ZnTe/3ZnSe NDBs with different zinc treatments after the third injection of selenium precursor. Compared to the case without further zinc precursor additions, adding more zinc oleate induces larger red-shift and higher quantum yield. However, when ZnCl 2 - TDPA solution is introduced, the PL wavelength does not shift to the red any more, accompanied by the significant quantum yield enhancement. Figure S6. (A) The energy dispersive X-ray (EDX) measurement of the ZnTe/3ZnSe NDBs confirmed the presence of Zn (59%), Te (20%) and Se (20%). (B) HAADF- STEM image of ZnTe/3ZnSe NDBs. 8

9 Figure S7. (A) Te 3d and (B) Se 3d XPS spectra of ZnTe nanorods, and ZnTe/3ZnSe NDBs. The Te 3d spectra can be seen in both ZnTe nanorods and ZnTe/3ZnSe NDBs. The relative intensity does not greatly decrease after the ZnSe growth, which can be explained by the formation of dumbbell structure. In the case of core/shell quantum dots, several layers of full shell growth significantly decrease or even completely block the signals from the core. The Se 3d spectra are detected in ZnTe/ZnSe NDBs. Figure S8. Quantum yield evolution of ZnTe/3ZnSe NDBs with different zinc treatments for the last two injections of zinc precursor (from the point marked in the circle, time = 180 min). Compared to zinc oleate, the obtained ZnTe/ZnSe NDBs with the addition of ZnCl 2 and ZnCl 2 -TDPA display significantly enhanced quantum yield. All three samples show similar emission wavelength at the end of synthesis. 9

10 Figure S9. (A) Absorption and emission spectra, (B) TEM image, of ZnTe/3ZnSe NDBs with chloride treatment. Figure S10. (A) PLE photoselection measurements and corresponding fluorescence anisotropy of ZnTe/3ZnSe NDBs. A known property of nanorods is their linearly polarized absorption and emission. Here we also examine the emission polarization of ZnTe/ZnSe NDBs by using the excitation photoselection method. The sample of ZnTe/ZnSe NDBs dispersed in hexane (sealed in a cuvette) was excited by a vertical light, followed by the measurements of photoluminescence excitation (PLE) spectra parallel (I VV ) and perpendicular (I VH ) to the excitation light. The anisotropy was then extracted according to r = I VV I VH I VV + 2I VH The ZnTe/ZnSe NDBs show a typical anisotropy between 0.07 and 0.1 at the measured wavelength range, which is higher than spherical QDs and lower than CdSe/CdS dot-in-rod or rod-in-rod nanoparticles. 10

11 Figure S11. (A) Absorption spectra and (B) emission spectra of ZnS growth on ZnTe/3ZnSe NDBs through thermal cycling method using single-source precursor (zinc diethyldithiocarbamate). The PL intensity is significantly decreased after the addition of the precursors for growth of the first ZnS layer and completely quenched with further additions. The corresponding TEM image shows the maintained dumbbell structure and a significant self-nucleation of ZnS. The PL quenching may be caused by lattice defects introduced by ZnS growth since the lattice mismatch between ZnS and ZnTe is large (11.4%). 11

12 Simulation Methods: We calculated the energy levels and the electron and hole wavefunctions of the ZnSe/ZnTe NDBs by using the multiphysics mode of Comsol 5.2a software. Electron and hole states are calculated with a 3D single-band effective mass Hamiltonian. Interacting electron and hole states are obtained by iterative resolution of the Schrodinger-Poisson equation, within a self-consistent Hartree procedure, taking into account the dielectric mismatch with the dielectric surroundings of the NC. 1 The dimensions we used in the calculation are based on experimental data. Material- dependent parameters such as effective masses m e,h (r) dielectric constants ε(r), and e,h conduction and valence band profiles (V confinment ) used in this calculation are summarized at Tables S3. The entire computational space extends further from the NDBs boundary allowing for electron and hole wavefunctions to extend outside of the NDBs boundaries and decay into free space. We are starting the simulation by computing the non-interacting electron and hole states by solving the Schrödinger equation: ( ħ2 2 ( 1 m e,h (r) ) + V e,h confinment i (r)) Ψ e,h (r) = E e,h Ψ e,h (r) We use von Neumann boundary-condition at the inner (between the ZnTe rod and the ZnSe tip) and outer boundaries of the NDBs in order to impose the BenDaniel-Duke condition. At the edge of the computational domain (around 100 nm away from the NDBs) we set the Dirichlet boundary-condition by setting the wavefunction to zero. After normalization of the wavefunctions, the Poisson equation is solved (with the same boundary conditions) to derive the hole or electron coulombic potential generated from the other particle φ e,h (r). (ε 0 ε(r) φ e,h (r)) = q e < Ψ e,h (r) Ψ e,h (r) > 2 With these potentials, Schrödinger equations are solved again for the electron and the hole with all of the contributions to the potentials: ( ħ2 2 ( 1 m e (r) ) + V e confinment(r) + q e φ h (r)) Ψ e (r) = E f e Ψ e (r) ( ħ2 2 ( 1 m h (r) ) + V h confinment(r) + q e φ e (r)) Ψ h (r) = E f h Ψ h (r) 12

13 This process is repeated iteratively until the electron and hole energies converge. In most cases, three iterations are sufficient to obtain a convergence. The emission energy calculated by: Type E emission = E g + E i e + E i h E coulomb E coulomb = E e i E f e + E i f h E h 2 Type Where E g (1.79 ev) 2 is the difference between the conduction band edge of the ZnSe tip and the top of the valence band of the ZnTe rod. E emission is calculated in a way which avoiding the accounting of the coulomb potential twice (both for the electron and both for the hole). Table S3. Material parameters which were used in the simulation. e V confinment ZnSe ZnTe Environment Units Ref [ev] 2,3 h V confinment [ev] 2,3 m e m 0 3 m h m 0 3,4 ε ,5 REFERENCES: (1) Grivas, C.; Li, C.; Andreakou, P.; Wang, P.; Ding, M.; Brambilla, G.; Manna, L.; Lagoudakis, P. Single-Mode Tunable Laser Emission in the Single-Exciton Regime from Colloidal Nanocrystals. Nat. Commun. 2013, 4, (2) Dmitrenko, K. A.; Shevel, S. G.; Taranenko, L. V.; Marintchenko, A. V. The Temperature Dependence (4.2 to 293 K) of the Resonance Energies of Excitonic Transitions in II VI Compounds. Phys. status solidi 1986, 134, (3) Wang, J.; Isshiki, M. Wide-Bandgap II--VI Semiconductors: Growth and Properties. In Springer Handbook of Electronic and Photonic Materials; Springer US: Boston, MA, 2007, (4) Madelung, O. Semiconductors Data Handbook. Springer: Berlin, (5) Web. 20 March,