Hierarchical Semiconductor, Metal and Hybrid Nanostructures and the Study of their Light-Matter Interactions

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1 Hierarchical Semiconductor, Metal and Hybrid Nanostructures and the Study of their Light-Matter Interactions By Anna Lee A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Chemistry University of Toronto Copyright by Anna Lee, 2012

2 Hierarchical Semiconductor, Metal and Hybrid Nanostructures and the Study of their Light-Matter Interactions Anna Lee Doctor of Philosophy Graduate Department of Chemistry University of Toronto 2012 Abstract The work presented in this thesis explores the optical properties of hierarchical structures composed of nanoscale building blocks ranging from metals to semiconductors and composites, organized through bottom-up design methods. 1) By following the dynamic generation of hot-spots in self-assembled chains of gold nanorods (NRs), we have established a direct correlation between ensemble-averaged surface-enhanced Raman scattering (SERS) and extinction properties of these nanoscale chains. Experimental results were supported by comprehensive finite-difference timedomain simulations (FDTD). The relationship established between the structure of nanorod ensembles and their optical properties provides a basis for producing dynamic, solutionbased, plasmonic platforms for applications ranging from sensing to nanoelectronics. 2) We report theoretical and experimental analyses of the optical properties of side-by-side assembled gold NRs. Comprehensive FDTD simulations showed a blue shift of the surface plasmon resonance in the side-by-side assembled NR structures and a reduction of electric field intensity as the number of NRs per stack increased. These results ii

3 were experimentally verified via extinction measurements and ensemble-averaged SERS spectroscopy. The experimental results and electrodynamic simulations were found to be in agreement. 3) The efficacy of hollow core photonic crystal fibers (HCPCF) as a platform for SERS spectroscopy was demonstrated. SERS measurements carried out using this platform showed the capability to monitor minute amounts of ligands on the surface of gold nanoparticles and SERS signals from HCPCF exhibited a 10-fold enhancement. Using the exchange of cetyltrimethylammonium bromide with α-methoxy-ω mercaptopolyethylene glycol on the surface of gold nanorods as an exemplary system, we showed the feasibility of using HCPCF SERS to monitor the change in surface chemistry of NRs. 4) Facile, solution-phase formation of ordered, lamellar quantum dot (QD) arrays exhibiting structural integrity and temporal stability, without the need for chemical crosslinking, was achieved. While micrometers in diameter, they are typically only two to three QD layers thick. These structures are capable of carrying a cargo of water-soluble ions, molecules, metal nanoparticles, or biomolecules. The photoluminescence of the host CdSe QDs were enhanced by the encapsulation of gold nanoparticles within the lamellae, demonstrating the ability to modulate their properties through the cargo they carry. 5) This chapter explores a bottom-up method to produce a metamaterial designed to function as an optical cloak in the visible range. A composite material consisting of an array of silver nanowires (NWs) in a dielectric host has been produced based on the theory of a non-magnetic optical cloak. The required radial array of silver NWs was achieved by electroless deposition of the metal into the channels of a porous alumina structure grown perpendicularly from the curved surface of a micrometer scale aluminum wire. The iii

4 functionality of the cloak was demonstrated by partial cloaking in the visible range (540 nm). iv

5 v Ad Maiorem Dei Gloriam

6 vi For My Family.

7 Acknowledgments I would like to begin by thanking Neil and Grace for introducing me to chemistry and for their unconditional support. I am grateful to Prof. Stynes for his guidance and for challenging me with the hard questions, even in my undergraduate research. Importantly, thanks to my supervisor Eugenia Kumacheva for her ongoing passion, support and encouragement. I gratefully acknowledge the critical contributions made by my collaborators throughout this work: Neil Coombs, Alex Brolo, Aftab Ahmed, Gustavo Andrade, Michelle Souza, Reuven Gordon, Gilbert Walker, Fatemeh Eftekhari, Amr Helmy and Ilya Gourevich. I would like to thank undergraduate students: Boryana and Mathiue, in particular, for his hard work and friendship. I would also like to thank graduate students: Lucy and Alex for their contributions and my labmates for valuable discussions. I am grateful for the financial support provided by Biopsys and the department of Chemistry. I was lucky to be surrounded by a great group of friends who provided support and laughter throughout this period, including but not limited to :-) Shun, Jeong-Ho, Aftab, Sasha, Jai-Il, Kyung, Yaser, Marcus, Tihanna, Megan, Sandeep, Igor, Thi, Raluca, Milos and my little brother Ethan. I would like to dedicate this work to my loving family: Mom, Dad, Neil, Grace, Margaret, Alan, Rona, Jina, Jay, Nam-Kyung, my Aunts and Uncles and our babies: Mango and Ji-Min. Finally, I would like to thank my Grandparents for instilling in me the value of knowledge, kindness and prayer. vii

8 Authors Contributions This thesis is based on key projects which have been either published, submitted or are in preparation for peer-reviewed scientific journals. All manuscripts were written by Anna Lee with critical comments and revisions by Eugenia Kumacheva and corresponding collaborators. The contributions of all authors are provided in detail below. Chapter 3: Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold Nanorods by Surface-Enhanced Raman Scattering Authors: Anna Lee, Gustavo F. S. Andrade, Aftab Ahmed, Michele L. Souza, Neil Coombs, Ethan Tumarkin, Kun Liu, Reuven Gordon, Alexandre G. Brolo, and Eugenia Kumacheva Contribution: A. Lee contributed to the project by designing and carrying out all experiments, data analysis, interpretation and article writing. A. Ahmed carried out all FDTD simulations and related data analysis. G. F. S. Andrade carried out initial SERS experiments with A. Lee and provided helpful discussions. M. L. Souza carried out the SERS measurements with A. Lee on the optimized system. N.Coombs developed and carried out experiments on correlating the structure and SERS with A. Lee. E. Tumarkin did statistical analysis on nanorod chain populations with A. Lee. K. Liu took a number of extinction measurements and prepared samples for TEM analysis at the beginning of the project. R. Gordon, A. G. Brolo and E. Kumacheva provided critical guidance and suggestions on data analysis, interpretation, and article writing. A. Ahmed and G. F. S. Andrade contributed equally as second authors. viii

9 Chapter 4: Probing Side-by-side Assembled Gold Nanorods via Ensemble-averaged Surface- Enhanced Raman Scattering Authors: Anna Lee, Aftab Ahmed, Diego P. dos Santos, Neil Coombs, Jai Il Park, Reuven Gordon, Alexandre G. Brolo and Eugenia Kumacheva Contribution: A. Lee contributed to the project by designing and carrying out all experiments, data analysis, interpretation and article writing. A. Ahmed carried out all FDTD simulations and related data analysis. D. P. dos Santos carried out initial SERS experiments with A. Lee. A. Lee carried out the final SERS measurements. N. Coombs carried out TEM imaging. J. I. Park did statistical analysis on nanorod populations. R. Gordon, A. G. Brolo and E. Kumacheva provided critical guidance and suggestions on data analysis, interpretation and article writing. Chapter 5: Surface-Enhanced Raman Spectroscopy in Hollow Core Photonic Crystal Fibers: a tool for exploring the surface chemistry of gold nanoparticles Authors: Fatemeh Eftekhari, Anna Lee, (co- first authors), Eugenia Kumacheva and Amr Helmy Contribution: A. Lee contributed to the project by designing and carrying out SERS experiments, data analysis, interpretation and article writing. F. Eftekhari developed the HCPCF-SERS platform and carried out SERS experiments, data analysis, interpretation and article writing. E. Kumacheva and A. Helmy provided critical guidance and suggestions on data analysis, interpretation and article writing. ix

10 Chapter 6: Lamellar Envelopes of Semiconductor Quantum Dots Authors: Anna Lee, Neil A. Coombs, Ilya Gourevich, Eugenia Kumacheva, and Gregory D. Scholes Contribution: A. Lee contributed to the project originating the concept and by designing and carrying out all experiments, data analysis, interpretation and article writing. N. Coombs collaborated on all experiments, data analysis and interpretation with A. Lee except the synthesis of quantum dots. Ilya Gourevich carried out the confocal experiments and provided useful discussions. E. Kumacheva provided critical comments on article writing and G. Scholes provided guidance and contributed on article writing. Chapter 7: Towards the Experimental Demonstration of 2D Visible Range Cloaking via a Bottom-up Approach Authors: Neil A. Coombs, Anna Lee (co-first authors), Aftab Ahmed, Ilya Gourevich, Reuven Gordon and Eugenia Kumacheva Contribution: A. Lee originated the idea for the project and designed the initial experiments on magnetic and vertical assembly. N. Coombs originated the idea of utilizing aluminum core/alumina/electroless deposition of silver and designed all experiments. A. Lee and N. Coombs contributed to the project by carrying out experiments, data analysis and interpretation. A. Ahmed wrote the code and carried out optical transmission measurements. R. Gordon provided guidance on the transmission measurements. I. Gourevich provided useful discussions. E. Kumacheva provided overall guidance and assistance with writing. x

11 Publications during Ph.D. Studies This following is a full list of publications arising from studies carried out in the preparation of this thesis including those published, submitted and in-preparation for peerreviewed scientific journals. A. Lee s specific contribution is summarized below each article listing. A.Lee, G.Andrade, A. Ahmed, M. Souza, E. Tumarkin, R. Gordon, A.Brolo, E. Kumacheva, Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold Nanorods by Surface-Enhanced Raman Scattering, J. Am. Chem. Soc., 133,7563 (2011) Contribution: Designed and carried out all experiments, data analysis, interpretation and prepared manuscript A. Lee, S. Dubinsky, E. Tumarkin, M. Moulin, A. A. Beharry and E. Kumacheva, Multifunctional Hybrid Polymer-based Porous Materials, Adv. Func. Mater., 21, (2011) Contribution: Designed and carried out SERS experiments, data analysis, interpretation and prepared manuscript E. Tumarkin, L. Tzadu, M. Seo, H. Zhang, A. Lee, R. Peerani, K. Purpura, P. Zandstra, E. Kumacheva, High-Thoughput Combinatorial Cell Co-Culture Using Microfluidics, Integrative Biology, 3, (2011) Contribution: Manuscript preparation M. Zhang, M.Wang, S. He, J. Qian, A. Saffari, A.Lee, S. Kumar, Y.Hassan, A. Guenther, G. Scholes, and M. A. Winnik, Sphere-to-Wormlike Network Transition of Block Copolymer Micelles Containing CdSe Quantum Dots in the Corona, Macromolecules, 43, (2010) Contribution: Quantum-dot synthesis and characterization xi

12 V. M. Huxter 1, J. Kim 2, S. S. Lo 2, A.Lee 2, P. S.Nair 2 and G. D. Scholes, Spin Relaxation in Zinc Blende and Wurtzite CdSe Quantum Dots, Chem. phys lett,491,187(2010) Contribution: Synthesis of monodisperse, size controlled Quantum-dots and characterization A.Lee, N.Coombs, I. Gourevich, E.Kumacheva, G. Scholes, Lamellar Envelopes of Semiconductor Nanocrystals, J. Am. Chem. Soc. 131, (2009) Contribution: Originated concept, designed and carried out experiments, data analysis interpretation and prepared manuscript V. Huxter, A. Lee, S. Lo, G. Scholes, CdSe Nanoparticle Elasticity and Surface Energy, Nano Lett., 9, 405 (2009) Contribution: Synthesized all materials for characterization Shen, L.; Soong, R.; Wang, M.; Lee, A.; Wu, C.; Scholes, G. D.; Macdonald, P. M.; Winnik, M. A., Pulsed Field Gradient NMR Studies of Polymer Adsorption on Colloidal CdSe Quantum Dots, J. Phys. Chem. B.,112, 1626 (2008) Contribution: Quantum-dot synthesis and characterization M. Wang, S. Kumar, A. Lee, N. Felorzabihi, L. Shen, F. Zhao, P. Froimowicz, G. Scholes M. Winnik, Nanoscale Co-organization of Quantum Dots and Conjugated Polymers Using Polymeric Micelles as Templates, J. Am. Chem. Soc.; 130, 9481(2008) Contribution: Quantum-dot synthesis, characterization and edited manuscript F.Eftekhari 1, A.Lee 1, E.Kumacheva, A. Helmy, Surface-Enhanced Raman Spectroscopy in Hollow Core Photonic Crystal Fibers: a tool for exploring the surface chemistry of gold nanoparticles, Submitted (2011) Contribution: Synthesized materials, designed and carried out all experiments, data analysis, interpretation and prepared manuscript xii

13 A. Lee, A. Ahmed, D. P. dos Santos, N. Coombs, J. I. Park, R. Gordon, A.G. Brolo and Eugenia Kumacheva, Probing Side-by-side Assembled Gold Nanorods via Ensemble-averaged Surface-Enhanced Raman Scattering, In prep (2011) Contribution: Designed and carried out all experiments, data analysis, interpretation and prepared manuscript N. A. Coombs, 1 A. Lee, 1 A. Ahmed, I. Gourevich, R. Gordon and E. Kumacheva, 2D Visible Range Cloaking via a Bottom-up Approach In prep (2011) Contribution: Originated the concept of the bottom-up design and carried out the experiments, data analysis and interpretation A.Lee, A. Stewart, S. Ip, E. Kumacheva and G. Walker, Controlled Nanorod Aggregates as a Surface-Enhanced Raman Scattering Probe In Prep (2011) Contribution: Designed all experiments, data analysis, interpretation and carried out the key experiments Talks and Presentations: (presenter is marked with an asterisk) 1. A. Lee*, E. Kumacheva et al, MRS Spring Meeting, San Francisco, U.S.A. (2011) 2. A. Lee*, E. Kumacheva et al, Gordon Research Conference, South Hadley, U.S.A. (2010) 3. A. Lee*, E. Kumacheva et al, The Canadian Society for Chemistry, Toronto, Canada, (2010) 4. A. Lee*, E. Kumacheva et al, Biopsys all network meeting, Toronto, Canada (2010) 5. V. Huxter*, A. Lee, S. Lo, G. Scholes, Gordon Research Conference,(2008) 6. A. Lee*, G. Scholes et al, Quantum-dot Workshop, Toronto, Canada (2008) 7. A. Lee*, G. Scholes et al, Excited state processes, Los Alamos, USA (2007) 8. A. Lee*, D. Stynes, Inorganic discussion Weekend, Ottawa, Canada (2006) xiii

14 Table of Contents Chapter Introduction Overview of Optical Properties of Metal, Semiconductor and Insulator Optical Properties of Metals The Dielectric Function of the Free Electron Gas Volume Plasmons, Surface Plasmon Polaritons, Localized Surface Plasmons Absorbing and Scattering of Light by Metal Nanoparticles Anisotropic Nanoparticles Interaction between Metal Nanoparticles Optical Properties of Semiconductor Quantum Dots Optical Properties of Dielectric Material Self-assembly of Nano-materials Overview of Metamaterials References...40 Chapter Materials and Methods Materials Methods Synthesis and Fabrication Synthesis of CdSe Quantum Dots and Nanorods Synthesis of Gold Nanorods Fabrication of Al 2 O 3 /Ag hybrid Cloaking Structure Self-assembly of Semiconductor and Metal Nanoparticles Assembly of Quantum Dot Lamellar Envelopes Assembly of Gold Nanorods Characterization Electron Microscopy Surface-Enhanced Raman Spectroscopy xiv

15 Confocal Microscopy Extinction Optical Transmission Measurements Finite-Difference Time-Domain Simulations References...59 Chapter Results: Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold Nanorods by Surface-Enhanced Raman Scattering Introduction Results and Discussion End-to-end Nanorod Assembly and their Extinction and TEM... Analysis Ensemble-averaged SERS Finite-difference Time-domain Simulations Summary and Conclusions...81 References...82 Chapter Results: Probing Side-by-side Assembled Gold Nanorods via Ensemble-averaged SERS Introduction Results and Discussion Finite-difference Time-domain Simulations Reduction of Electric Field Intensity Side-by-side Nanorod Assembly Extinction and TEM Analysis Ensemble-averaged SERS Electric Field Distribution on Nanorod Ensembles Summary and Conclusions References xv

16 Chapter Results: Surface-Enhanced Raman Spectroscopy in Hollow Core Photonic Crystal Fibers: a tool for exploring the surface chemistry of gold nanoparticles Introduction Results and Discussions Experimental Set-up Examination of the Limit of Detection of CTAB coated Gold... Nanorods Determination of the Enhancement Factor Study of Exchange of cetyltrimethylammonium bromide (CTAB)... with α-methoxy-ω-mercapto-polyethylene glycol (SH-mPEG) on... the Gold NRs Summary and Conclusions References Chapter Results: Lamellar Envelopes of Semiconductor Quantum Dots Introduction Results and Discussion Formation of NC Lamellae Structural Analysis of QD Lamellae Proposed Mechanism of Lamellae Formation Testing the Hypothesis and its Potential Applications Summary and Conclusions Reference Chapter Results: Towards an Experimental Demonstration of 2D Visible Range Cloaking via a Bottom-up Approach Introduction Metamaterials and Optical Cloaking via Transformation Optics Theoretical Design of Non-magnetic Optical Cloak Results and Discussions xvi

17 Experimental Rationale Route I and II Route III Fabrication of Cylindrical-shaped Dielectric Host Electroless Deposition of Ag NPs and Ag NWs into pores Optical Transmission Measurements Summary and Conclusions References Chapter Summary, Conclusions and Future Work Summary and Conclusions Future Work Appendix A1. Basics of Finite-difference Time-domain A1.1. Finite Differences A1.2. The Yee algorithm 191 A1.3. Finite difference expressions for Maxwell s Equations. 193 A2. The Drude Model xvii

18 Tables of Figures Chapter1 Introduction Figure 1.1. Energy band gap diagram for (A) conductors whose conduction band (CB) and valence band (VB) overlap slightly, (B) in semiconductors CB and VB are spaced and (C) in insulatorscb and VB are widely separated... 2 Figure 1.2. Volume plasmon-the collective longitudinal oscillations of the conduction electrons of a metal... 5 Figure 1.3. Schematic illustration of a surface plasmon polariton (SPP) propagating along the x direction. Electric field lines of a SPP wave on a single interface where the structure is invariant with respect to the y axis. 6 Figure 1.4. Schematic illustration of a non-propagating localized surface plasmon...7 Figure1.5. Schematic illustration of an isotropic sphere placed into an electrostatic field.8 Figure 1.6. Schematic illustration of near-field coupling between metal nanoparticles (MNPs). Two different polarizations (parallel and perpendicular to the MNP axis) are shown..13 Figure 1.7. (A) SEM image of arrays of gold nanoparticle (B) Dependence of the plasmon peak position on the interparticle spacing d for both the transverse and longitudinal excitation of the collective mode. The dotted line shows a fit to the d -3 dependence of coupling predicted by a point dipole interaction model.. 14 Figure 1.8. Extinction spectra of gold nanoparticles (height 14 nm, diameter 150 nm). Reprinted with permission from Reference 35. Copyright 2000, American Physical Society...15 Figure 1.9. Schematic illustration of (A) a bulk semiconductor: continuous conduction band (CB) and valence band (VB) which are separated by an energy gap (E gap ) (B) a semiconductor nanocrystal (NC): with discrete atomic-like energy states and sizedependent E gap Figure (A) Allowed optical transitions from hole quantized states resulting from mixing between valence sub-bands to CB for the case of CdSe QDs. (B) Absorption spectra of size-dependent as-synthesized CdSe QDs showing well-resolved optical transitions...18 Figure Schematic illustration of fine-structure splitting of the lowest exciton state for CdSe QDs with wurzite crystal structure. The band-edge 1S(e)-1S 3/2 (h) transition is xviii

19 induced by a strong electron and hole exchange interaction and shape and crystal field anisotropy...19 Figure Electric polarization in dielectrics showing ionic (or molecular) and electronic polarization Figure Schematic of the self-assembly of nanoparticles into a variety of hierarchical structures: chains, bi-layer, ring, and hexagonal arrays Figure TEM images of (A) Self-assembly of magnetic dipole dipole interactions by using 20 nm cobalt nanoparticles in the absence of an external magnetic field. Reprinted with permission from Reference 32. Copyright 1966, American Institute of Physics. (B) Formation of ring conformation under an applied magnetic field of T. Inset shows a ring with almost single-particle thickness. (Inset Scale bar is 100 nm). Reprinted with permission from Reference 51. Copyright 2008, American Chemical Society Figure (A) Schematic illustration of a charged gold NP interacting with a gold nanorod via electrostatic interactions. (B) Ratio of the interaction energies for the end and side configurations as a function of screening length. Reprinted with permission from Reference 24. Reprinted with permission from Reference 48. Copyright 2009, Small...29 Figure Fluorescence confocal microscope images of varying sizes of water droplets in toluene in which CdSe NPs show self-assembly at the liquid-liquid interface. Optical crosssectional images at various depths are shown on the left. Reprinted with permission from Reference 81. Copyright 2003, Science Figure1.17. (A) Naturally occurring conventional material with its atoms (B) Metamaterial artificially structured atoms Figure is adapted from reference Figure Permittivity(ε)/Permeability(µ) Diagram. The first quadrant, second, third and fourth quadrant are assigned as double- positive (DPS), epsilon-negative (ENG), double-negative (DNG) and mu- negative (MNG) respectively Figure Schematic illustration of the proposed structure for making a double-negative (DNG) material showing arrays of paired nanorods. The arrows show the direction of current flow. Figure is adapted from reference Figure The idea of a perfect lens with sub-wavelength resolution (A) A conventional lens only collecting the propagating waves: k t < k 0 (B) The loss of the evanescent waves in a conventional imaging system (solid line represents propagating modes whereas dashed lines represents evanescent modes) (C) The focusing ability of a DNG slab (D) The growth of evanescent waves in the DNG slab and the restoration of both the propagating and evanescent waves. Figure was adapted from Reference xix

20 Chapter 2 Materials and Methods Figure 2.1. Cross-sectional sample preparation for internal structure investigation of lamellae by STEM. (A) CdSe QD and nanorod lamellae were prepared on separate carbon coated indexed TEM grids. STEM was used to identify the locations of individual lamellae. (B) the indexed grids were then coated with approx. 10nm of carbon via evaporation to secure the structure. (C) Grids were then sputtered with a 20-30nm layer of Au which was used as a visual marker in imaging. (D) The indexed grids were then embedded in epoxy resin and 30nm cross-sections through individual lamellae were prepared by ultramicrotomy Figure 2.2. Optical transmission setup using super-continuum (SC) laser source along with an acousto-optic tunable filter (AOTF) for monochromatic illumination of the cloak sample...57 Chapter 3 Results: Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold Nanorods by Surface-Enhanced Raman Scattering Figure 3.1. Schematic of the generation of hot-spots via end-to-end self-assembly of gold NRs into chains. (a) Gold NRs stabilized with CTAB. (b) Ligand exchange of CTAB with SH- PS at the ends of the NRs. (c) End-to-end assembly of NRs triggered by adding water to the solution of NRs in DMF, in the presence of Raman reporter OX. The volume fraction of water in the DMF/water mixture is 20 vol %. Hot-spots are generated between the ends of adjacent NRs. The distance between the adjacent NRs in the chain is maintained constant. Schematic is not drawn to scale Figure 3.2. (a) Representative STEM images of the self-assembled chains of NRs. Diffuse grey regions between adjacent NRs indicate the presence of SH-PS globules forming in a poor solvent. Scale bar is 40 nm. (b) Variation in extinction properties of NRs in the course of their self-assembly in chains. The spectral position of LSPR shifts from 754 nm to 812 nm with the aggregation number of the NR chains changing from =1 at t < 5 min to = 8 at t =18 hr. Transverse LSPR is located at 514 nm. The peak at 660 nm corresponds to OX...66 Figure 3.3. (a) Evolution of normalized ensemble averaged SERS spectra in self-assembled NR chains. The average aggregation number of NR assemblies changes from =1 at t < 5 min (bright-red spectrum) to = 8 at t =18 hr (black spectrum). The SERS peaks at 563 and 604 cm -1 are normalized against the SERS peak of DMF at 659 cm -1 (indicated with astericks). (b) Variation in the normalized SERS peak intensity measured at 563 cm -1 plotted as a function of the average aggregation number of the NR chains. SERS variation (y error) is based on three measurements taken within 15 min. Approximately 1000 NRs (including individual species) were used in the calculations of number (x error). Laser excitation wavelength was 785 nm...68 Figure 3.4. SER spectra of oxazine 4 5M adsorbed on roughened gold substrate as a function of solvent environment (a) H2O, (b) DMF and (c) DMF/ H2O mixture containing 20 vol. % of H2O...70 xx

21 Figure 3.5. Correlation of the normalized intensity of SERS peak at 563 cm -1 (red circles) and the product of extinctions measured at 785 and 821 nm (blue circles), plotted as a function of the aggregation number of the NR chains. Top: y errors of the intensity of SERS peak (red squares) and the product of extinctions (blue squares) were calculated based on three measurements 72 Figure 3.6. Three-dimensional finite-difference time-domain (3D-FDTD) simulation of the end-to-end assembly of gold NRs. Electric field profile was calculated at the resonance wavelength of the co-linear NR chain at (a) 760 nm, (b) 782 nm, and (c) 802 nm. Polarization of the incident light is parallel to the long axes of the NRs (i.e., to the z- coordinate). Hot-spots between adjacent NRs show a maximum electric field intensity 4000 times greater than the incident field 74 Figure 3.7. FDTD simulation showing (a) Electric field intensity squared obtained from incorporating average NR aggregation number, as a function of wavelength (factoring in experimentally determined statistical data) (b) Normalized sum of electric field intensity squared over a small volume enclosing the NR chain, for ideal NR chain lengths (Standard deviation is equal to zero) ranging from 1 to 9 NRs as a function of wavelength. (c) Sum of electric field intensity squared over a small volume enclosing NRs, chain lengths (number of NRs ranging from 1 to 9) as a function of wavelength (not normalized). (d) Peak electric field intensity squared values plotted against their corresponding resonant wavelengths. Number of NRs increases from 1 to 9 (left to right).. 75 Figure 3.8. Three-dimensional finite-difference time-domain (3D-FDTD) simulation showing examples of electric field profiles for end-to-end assembled gold NR dimmers and trimers. Polarization of the incident light is parallel to z-coordinate. Angular variance of (a) 0 degrees (b) 20 degrees (c) 40 degrees (d) 60 degrees (e) 90 degrees (f - i) Calculated absorption, scattering, extinction cross sections and electric field intensity squared respectively of various angled NR dimmers and trimers as a function of wavelength. Electric field strength between adjacent NRs decreases as angle between adjacent NRs increases..78 Figure 3.9. Calculated absorption, scattering and extinction cross sections as a function of wavelength for various NR chain lengths ((a) to (c) respectively) and average NR aggregation number ((d) to (f) respectively). A total-field scattered field (TFSF) source is utilized for calculating the scattering and absorption cross-sections. Incident field polarization is parallel to the major rod axis (i.e. z), the bandwidth of source is from 600 nm to 1000 nm. Simulation domain is terminated with perfectly matched layer (PML). A mesh override region of (1 nm x 1nm x 1nm) mesh size is defined for better modeling of the circular rods in Cartesian coordinates. A 3-D time domain monitor is utilized for recording the field strengths as a function of time and a Fourier transform provides the frequency domain results. Extinction cross-section were calculated for different NR chain lengths and a certain factor (see main text) from each curve was added according to the experimental statistical data to lead figure (f). The heterogenity of NR chain size at each stage of the assembly is one of the contributing factors to variations in the observed amplitude..79 xxi

22 Chapter4 Results: Probing Side-by-side Assembled Gold Nanorods via Ensembleaveraged SERS Figure 4.1. Schematic illustration of gold nanorods (NRs) assembled in a side-by-side manner showing a reduction of electric field as the number of NRs increases in NR ensembles. 92 Figure 4.2. Calculated normalized absorption (a), scattering (b) and extinction cross section (c), all plotted as a function of wavelength for NR assemblies containing from 1 to 8 NRs. Simulations were carried out using three-dimensional finite-difference time-domain (3D-FDTD) simulation..94 Figure 4.3. Modes supported by side-by-side assembly of NRs. Mode shapes of surface plasmons of 1 to 3 NRs from left to right. The resulting effective index values are used for the calculation of propagation constant of surface wave in the different geometries. Fields are normalized to their maximum intensities...96 Figure 4.4. (a-d) Examples of electric field profiles produced via 3D-FDTD simulation for ensembles of side-by-side assembled NRs. Polarization of the incident light is at 45 degrees to the long axis (z-coordinate) of NRs. (e) Sum of electric field intensity squared of ensembles containing a different number of NRs Figure 4.5. Schematic illustration of side-by-side NR assembly. A thiolated polystyrene (SH-PS) is attached to the ends of cetyltrimethylammonium bromide (CTAB) coated gold NRs in THF via site-specific ligand functionalization. After the addition of the Raman reporter, side-by-side assembly was triggered by the addition of water (10 vol. %)...98 Figure 4.6. (a) A photograph showing the typical change in color of self-assembling NRs in solution as a function of time (Top left). Representative scanning transmission electron microscopy (STEM) images of NRs in various stages of self-assembly. Scale bar is 15 nm (b) Variation in extinction properties of NR ensembles over time Figure 4.7. FDTD simulations showing absorption, scattering, and extinction of 2NRs per stack for y and z directions of propagation of incident radiation. When wave vector k is inc parallel to the NR axis, a peak at 520 nm is observed corresponding to the transverse SPR 103 Figure 4.8. Representative scanning transmission electron microscopy (STEM) images of NRs in various stages of side-by-side assembly. Recorded on a Hitachi S-5200 scanning electron microscope operating in STEM mode. Note: as-synthesized NRs contain a small population of spheroids (~5%)..102 Figure 4.9. (a) Representative ensemble-averaged SERS spectra of Cresyl violet (CV), measured in the course of side-by-side assembly of the NRs as a function of time. The band xxii

23 at 900 cm -1 corresponds to THF which is used as an internal standard to normalize the SERS of CV at 535, 595 cm -1. (b) Normalized SERS intensity at 535 (red circle), 595 cm -1 (blue triangle) and control experiments without the assembly (black square, for SERS of CV at 595 cm -1 ) as a function of time. (c, d) SERS of CV on a roughened gold substrate in THF and water respectively. A 785 nm laser excitation was used Figure A sum over volume of the electric field intensity squared via FDTD simulations for various NR assemblies (number of NRs from 1 to 8) as a function of wave length (nm) (right figure). The total volume of the sum of E field intensity squared for the ends of NR ensembles show a decrease with increasing number of NRs. Blue: 1 NR, green: 2 NRs, Red: 3 NRs, light blue: 4 NRs, pink: 5 NRs, black: 6 NRs, dotted blue: 7 NRs, and dotted green: 8 NRs Chapter 5 Results: Surface-Enhanced Raman Spectroscopy in Hollow Core Photonic Crystal Fibers: a tool for exploring the surface chemistry of gold nanoparticles Figure 5.1. Schematic illustration of experimental set-up. A hollow core photonic crystal fiber (HCPCF) filled with gold nanorod (NRs) solution Figure 5.2. (A) SERS spectra of CTAB coated gold NRs detected through direct sampling in a cuvette and core-filled HCPCF. (B) Variation in the normalized SERS peak intensity measured at 178 cm -1 plotted as a function of concentration of CTAB coated gold NRs (the concentration of the NRs were determined by extinction measurements). 21 SERS variation (y error) is based on 3 measurements.118 Figure 5.3. SERS spectra of 3 µm Congo Red molecules by using (A) core-filled HCPCF (B) direct sampling from a cuvette. (C) Ordinary Raman spectrum of Congo Red molecules at the concentration of 560 µm. The spectra have been separated vertically for clarity..119 Figure 5.4. Normalized SERS spectra of CTAB coated gold NRs as a function of SH-mPEG concentration (A) CTAB coated NRs as a control system (B) 20 µm of PEG (C) 50 µm of PEG (D) 100 µm of PEG. The peak at 103 cm -1 was used to normalize the peaks. The spectra have been separated vertically for clarity nm of NRs were used Chapter 6 Results: Lamellar Envelopes of Semiconductor Quantum Dots Figure 6.1. (A and B) Scanning transmission electron microscopy (STEM) images of colloidal CdSe QDs and CdSe bullet-shaped nanorods as controls, deposited from toluene solution onto carbon coated TEM grids, exhibiting the typical short range order produced by evaporation 129 Figure 6.2. (A) Liquid state confocal fluorescence microscopy image of CdSe NC lamellae formed by the addition of 10% (v/v) water with subsequent 20 sec sonication. (B) Confocal image of the same preparation as (A) at less than 10 s sonication time. Confocal images were recorded using an oil immersion lens (excitation at 364 nm, detection 550 to 600 xxiii

24 nm). (C) Solution state Wet cell BSE image showing the existence of large lamellar structures (see 1) in solution along with small droplets (see 2) whose greatest signal exists at its periphery. (D) Solution state Wet cell BSE image overview of large lamellae along with disordered aggregates. (E) Image intensity profiling of a lamella (inset) showing uniform intensity consistent with a disk- or sheetlike structure 130 Figure 6.3. (A) Bright field STEM low magnification overview of NC pancake shaped lamellae created by the addition of nonsolvent (water) and subsequent sonication. (B) Dark field STEM image of an individual nanorod pancake shaped lamella created by a similar procedure, mounted on a TEM grid with a combination ultrathin/lacy carbon film. 133 Figure 6.4. (A) STEM image of a NC lamella. Inset 1 shows NC ovelap indicated by linear structures. Fourier transform (inset 2) indicates hexagonal symmetry. (B) STEM image of a nanorod lamella inset 1 shows nanorod ovelap indicated by fine lines subdividing individual NCs. A region of ordered hexagonal packing is confirmed by Fourier transform (inset 2). (C) SEM image of a lamella (see 1) mounted on an uncoated Cu TEM grid. The lamella ( 15nm thick) spans the dark void ( 15 μm) (see 2) in the grid without support. (D) Examples of folds and tears present in lamellae indicating their structural integrity. (E and F) Cross-sectional STEM images of NC tri- and bilayers. For all cross sections, the thickness is 30 nm. The capping Au overlayer is used as a location marker Figure 6.5. EDS line scan of a cross-sectioned QD lamellar tri-layer. A line scan showing the presence of Cd (solid line), Se (dot dot dash) and P (dot dash). Ti (dot), which has no spectral overlap with the elements of interest, is included as a background control. Cd, Se and P are all significantly above background. Coincidence of P with Cd and Se indicates the presence of TOPO Figure 6.6. (A) Liquid state confocal fluorescence microscopy images of NC lamellae formed in the presence of the water-soluble dye fluorescein isothiocyanate (FITC), water 10% (v/v). Both FITC and NCs were excited using the 488 nm line of an argon ion laser. Note the coincidence between FITC (green) (collection range nm) and NCs (yellow) (collection range nm) indicating that the water-soluble dye is associated with the lamellar structure. (B) Energy dispersive X-ray spectroscopic (EDS) line scans for CoCl2 6H2O incroporated into CdSe lamellae. The inset shows an HAADF STEM image with the line scan (yellow line) across the lamellar structure (scale bar: 10 μm). (C) EDS line scan of a cross-sectioned ( 70 nm thick) Co incorporated NC lamellar bilayer showing the presnce of Co within the structure. The inset shows corresponding HAADF STEM image (scale bar: 35 nm). (D)EDS data for ferritin incorporated into the lamellae. The inset shows a corresponding HAADF STEM image (scale bar: 500 nm). Note: Ti Kα or V Kα lines were used as backgrounds since they have no spectral overlap with the elements of interest 139 Figure 6.7. (A to D) EDS maps of Cd, Se, Au and Ti (background) respectively, corresponding to the structure presented in Figure 6.8.A showing that distribution of Au NPs is fully contained within the structure Figure 6.8. (A) Incorporation of Au NPs into CdSe NC lamellae. In the HAADF STEM image shown, the bright dots are individual Au NPs. (B) Crosssectional ( 30 nm thickness) xxiv

25 STEM image confirming the encapsulation of Au NPs inside the NC bilayer (as previously, an evaporated Au layer, upper portion of the image, is used as a marker). (C and D) Simultaneously recorded SEM and TEM images, respectively, confirming encapsualtion of Au NPs within the NC lamellae. (E) SEM image of control sample with Au NPs added after the NC lamellae formation. (F) Histogram showing 10 maximum photoluminescense intensity measurements for both NC lamellae and Au encapsulated NC lamellae. (G and H) Representative fluorescence confocal microscope images of CdSe NC lamellae and Au encapsulated NC lamellae, respectively..143 Chapter 7 Results: Towards Experimental Demonstration of 2D Visible Range Cloaking via a Bottom-up Approach Figure 7.1. Schematic illustration of a three-dimensional view showing the wave trajectories of a spherical cloaking system. Reprinted with permission from Reference 14. Copyright 2006, Science Figure 7.2. (A) Straight field line through a homogeneous medium against a Cartesian coordinate system (B) distorted field line travelling through a heterogeneous medium produced by varying the spatial distribution of permittivity and permeability. Reprinted with permission from Reference 14. Copyright 2006, Science.151 Figure 7.3. A two-dimensional cross-sectional view of wave trajectories of a spherical cloaking system where light is deviated around the object to be cloaked (radius a) within the annular cloak region (radius b a) and return to its original path. Reprinted with permission from Reference 14. Copyright 2006, Science Figure 7.4. The coordinate transformation of a cylindrical shell model. A cylindrical region r<b into a concentric cylindrical shell a <r < b. There is no variation along the z direction. Reprinted with permission from Reference 13. Copyright 2007, Nature Figure 7.5. Calculated plot of radial component of electric permittivity (ε radial ) as a function of cloak dimensions (A) a = 0.7 µm and b = 2 µm (B) a = 1.2 µm and b = 3.5µm. Both parameters result in the effective permittivity at operating wavelength of 500 nm. Silver nanoparticles with a radius of 10 nm were used for the calculations..156 Figure Schematic illustration of the non-magnetic cloak structure. Inner core (dark grey) is the cloak area surrounded by metal nanowires (NWs) in a dielectric host. A Radial array of NWs is perpendicular to the z-axis and must satisfy the filling factor such that the radial component of electric permittivity varies from 0 at a to 1 at the exterior surface. Spatial positions of NWs do not need to be periodic Figure Summary of explored routes for the fabrication of a non-magnetic optical cloak device Figure 7.8. Schematic showing two possible routes to produce the optical cloak. Route I: vertical assembly of gold nanorods on silica then subsequently embedded via silica xxv

26 deposition. Route II: radial assembly of binary metal NWs (eg, gold/nickel) around a cylindrical host directed by a controlled magnetic field Figure 7.9. Representative TEM image of vertical assembly of gold nanorods onto synthesized silica particles. The ends of gold nanorods were functionalized by the introduction of 3-mercaptopropyl trimethoxysilane. 162 Figure Schematic illustration of the electrochemical method used to produce binary nanowires (NWs) composed of nickel and gold 163 Figure (A) Backscattered SEM images of binary NWs showing various lengths of each component. Note: silver is used to fill the bifurcated pores to provide even deposition of gold and nickel (B) EDS mapping showing atomic composition of NWs Figure Magnetic field line simulations analogous to (A) Helmholtz and (B) anti- Helmholtz configurations Figure (A) Example of experimental set-up using annular magnets in an anti- Helmholtz arrangement producing a radial magnetic field in the central zone between magnets (B) Optical micrographs showing top-views of NW assemblies via the two different configurations, showing a radial alignment of NWs in the anti-helmholtz arrangement Figure Schematic of the proposed route to the fabrication of radial porous alumina as a dielectric host via anodization of aluminum (Al) wire. The cross-sectional view shows a metallic Al core surrounded by a porous alumina coating with a radial distribution of pores Figure SEM images of (A) Bare Aluminum wire after electropolishing. (B) Anodized aluminum oxide (AAO) grown as a cylindrical dielectric shell around an Al wire core. (C) Surface morphology of AAO shell showing a uniform pore structure (D) A cross-sectional view of radial porous AAO grown using 3 % Oxalic acid (nb, surface roughness shown is due to fracturing artifact) Figure (A) Variations of average pore diameter (blue circle) and average cell size (red square) as a function of applied potential. (B) Calculated pore volume fraction as a function of applied voltage. The oxide layer was electrochemically grown over 105 minutes using 3 wt. % of Oxalic acid in water as an electrolyte solution..171 Figure (A-B) Low and high magnification backscattered SEM images of the surface of AAO structures containing silver NWs. Inset shows superficial deposition of larger silver particles which were subsequently removed by diamond paste washing. (C) Backscattered SEM image of a silver NW loaded radial AAO structure. This example shows both the desired radial silver NW distribution in a dielectric host along with the required structural dimensions. (D) Calculated plot based on (C) showing r response for a = 0.6 µm and b = 1.75 µm. Operating wavelength is 500 nm. Radius is variable ranging from a to b..175 xxvi

27 Figure Optical images captured by CCD camera at a wavelength of 540nm for transverse electric (TE) and transverse magnetic (TM) polarization. Quantification of the intensity across the fabricated structure was carried out by a series of sequential diagonal scans over the wavelength range of 450 to 750 nm Figure Polarization-dependent normalized field intensity plotted as a function of wavelength via transmission measurement. Transverse electric illumination (TE) and Transverse magnetic illumination (TM) on the fabricated structure. Field intensity of TM shows enhanced transmission (blue) in the range 540 to 550nm Table 1.1. Van der Waals interaction energy and force between macroscopic bodies of different geometries with surfaces a distance of D apart where D<<R. R is the radius and A is the Hamaker constant Table Summary of required parameters for the fabrication of a non-magnetic optical cloak device..158 xxvii

28 List of Appendices Figure A1.1. The Yee mesh, the Yee s algorithm centers its E and H components in three dimensional space so that every E component is surrounded by four circulating H components and vice versa. Figure A1.2. Space and time distribution of E and H fields based on Yee mesh and the leap frog algorithm. xxviii

29 1 Chapter 1 Introduction The work presented in this thesis explores and utilizes a variety of building blocks including conducting, semiconducting and insulating materials both in isolation and combination to exploit the optical properties of designed, hierarchically assembled nanoscale structures. In this chapter, optoelectronic properties of the constituent materials used as building-blocks are described. This chapter concludes with a brief discussion of various examples of the self-assembly of nanomaterials and an overview of metamaterials Overview of Metals, Semiconductors and Insulators Within atoms, the energy of bound electrons is quantized and as such only discrete values of electron energy are permitted. The overlapping wave functions of electrons results in discrete, quantized energy level splitting. As the number of atoms increases (i.e., in a crystalline solid) the allowed energies form two distinct energy bands the valence band (VB) and the conduction band (CB). The VB consists of closely spaced levels which are mostly filled with electrons whereas the CB represents mostly unoccupied electronic levels at higher energies. At particular interatomic distances, CB and VB can be separated by a zone where electron energies are not permitted. These forbidden energies represent the band gap of a material. 1 Energy band gap is a core property which influences a material s

30 2 characteristics from optical and electronic to mechanical properties. Simplified energy band gap diagrams for conductors, semiconductors and insulators are shown in Figure 1.1. A conductor (e.g., a metal) contains a free electron gas which is mobile when an electric potential difference is applied to the system. Metals are opaque and highly reflective. These optical properties are governed by the collective behavior of electrons in metals. Unlike metals whose electrons are loosely held together due to partly filled energy bands, most semiconducting materials have their energy bands filled. In an insulator (i.e., a dielectric), allowed energy bands are either completely filled or empty. As such, electrons are not mobile in an electric field and dielectrics are characterized by a wide energy band gap (usually larger than 5 ev). Therefore, thermal generation of free carriers in dielectric materials is extremely weak and requires a large amount of energy to generate a minute amount of current. Figure 1.1. Energy band gap diagram for (A) conductors whose conduction band (CB) and valence band (VB) overlap slightly, (B) in semiconductors CB and VB are spaced and (C) in insulators CB and VB are widely separated.

31 3 In the following sections, concise summaries of the optical and electronic properties of conducting, semiconducting and dielectric materials and their unique behavior on the nano-scale are provided Optical Properties of Metals The Dielectric Function of the Free Electron Gas Optical properties of metals can be explained by a plasma model where a free electron gas moves against fixed positive ion cores. Metals have frequency-dependent optical responses. For example, at the low frequency region of the electromagnetic spectrum (i.e., microwave and far-infrared), metals are reflective and electromagnetic waves are unable to penetrate. At higher frequencies (i.e., near-infrared and the visible region), the field penetration increases significantly which results in increased dissipation. In the case of ultraviolet frequencies, fields can propagate into the metal resulting in a dielectric character which is dependent on the electronic band structures of the specific metal. For noble metals such as gold and silver, the transition between electronic bands results in strong absorption. The dispersive nature of metals can be described via a complex frequency dependent dielectric function ε(ω) of the Drude model (the Drude model is explained in Appendix A2): ( ) ( ) where p n is the plasma frequency at which the density of the free electron gas oscillates. The real and imaginary parts of the dielectric function ( ) ( ) ( ) are given by:

32 4 ( ) ( ) ( ) ( ) ( ) Although the behavior of noble metals is predominantly governed by free electron responses, details of lattice potential and bound state electrons are not taken into consideration in Equation (1). Instead, it is assumed that the effective optical mass m of electrons in the band structure oscillate under an electromagnetic (EM) field and their motions are damped via collisions with a characteristic collision frequency (damping constant and is the mean electron collision time). In the case of noble metals (e.g., gold and silver), the applicability of Equation (2) breaks down due to interband transitions resulting in an increase of at visible frequencies. 2 Therefore, the dielectric function of the metal should contain the Drude term for both free electrons and bound electrons 3 ( ( ) ): ( ) ( ) ( ) ( ) ( ) Volume Plasmons, Surface Plasmon Polaritons and Localized Surface Plasmons Figure1.2 is a schematic illustration of collective displacement of the electron cloud which results in surface charge density ±σ at the metal slab boundaries. As a consequence, an electric (E) field is produced inside the metal and displaced electrons experience a restoring force. Volume plasmon is the quanta of these charge oscillations.

33 5 Figure 1.2. Volume plasmon - the collective longitudinal oscillations of the conduction electrons of a metal. 2 In 1957, Ritchie predicted a special kind of surface wave that can exist at a metal/dielectric interface. 4 Surface plasmon polaritons (SPPs) are electromagnetic modes propagating at the interface between a dielectric and a metal with dielectric constants and respectively (Figure 1.3). The energy in this type of wave is shared between the electron charge density of the metal (plasmon) and the electromagnetic wave (photon) and is confined to the surface. SPPs are transverse magnetic plane waves which propagate along the x direction, that is, the structure is invariant with respect to the y direction (i.e., ). Thus, SPPs are evanescently confined in the direction normal to the interface. These electromagnetic surface waves arise via the coupling of EM fields to coherent surface oscillations of free electrons in the metal. 2,4-7 The EM field intensity reaches its maximum at the metal surface and decays exponentially away from the interface. The specific mode, shape and decay rate are dependent on the material involved and the geometry of the structures. The reason for the existence of such waves is the opposing signs of the dielectric constants of the two media involved (i.e., metal and dielectric).

34 6 Figure 1.3. Schematic illustration of a surface plasmon polariton (SPP) propagating along the x direction. Electric field lines of a SPP wave on a single interface where the structure is invariant with respect to the y axis. 8 By using Maxwell s equations and applying the necessary boundary conditions, the dispersion relation for a single interface is given as: 2,5,8 ( ) where is the free space wave vector, and are the dielectric constants of metal and dielectric respectively. Equation (1) shows that the propagation constant reaches infinity as approaches. This results in the confinement of the wave at the surface and the wave decays exponentially on both sides of the interface. 9,10 Using a Drude fit for the dielectric constant of the metal results in a surface plasmon frequency in which the propagation constant approaches infinity:

35 7 ( ) where is the bulk plasmon frequency of the metal. Unlike SPPs, localized surface plasmons are non-propagating excitations of the conduction electrons of metal nanoparticles (MNPs) coupled to the EM field (Figure 1.4). 8,11 These modes arise from the scattering of sub-wavelength conductive MNPs as a result of excitation of the conduction electrons which experience a restoring force due to the surface curvature of these particles. Therefore, resonance can arise leading to field amplification in both the inside and outside (near-field) of MNPs. This resonance condition is called localized surface plasmon resonance (LSPR). The spectral positions of the LSPRs for gold and silver MNPs are in the visible range. Figure 1.4. Schematic illustration of a non-propagating localized surface plasmon. 12 Interaction of a particle with an EM field can be analyzed by a quasi-static approximation (i.e., size of the particle is much smaller than the wavelength of light). In this condition, the phase of the harmonically oscillating EM field is constant over the volume of

36 8 the particle and as such, spatial field distribution can be obtained based on the assumption that the particle is in an electrostatic field. 2,3,10 Figure 1.5 shows a schematic illustration of a homogenous, isotropic sphere in an electrostatic field. 13 Figure 1.5. Schematic illustration of an isotropic sphere placed into an electrostatic field. In the electrostatic approximation, the fields can be derived using the Laplace equation,, where is the electric potential and the E field can then be obtained from the gradient of the potential as This results in the following relationship for the field inside (E in ) and outside (E out ) of the sphere: ( ) where and are the permittivity of the surrounding medium and metal respectively and is the magnitude of the incident field:

37 9 ( ) (4) where r is the radial distance for the point of observation from the center of the particle and the dipole moment is given by: 13 ( ) where a is the radius of the MNPs. Equation (5) shows that there is a resonant enhancement in the dipolar moment for the wavelength range where approaches. This resonant enhancement, in turn, enhances the fields both inside and outside of the particle. This field enhancement at the plasmon resonance is the phenomenon on which numerous optical applications such as surface enhanced Raman scattering (SERS) rely Absorbing and Scattering of Light by Metal Nanoparticles One of the important results of the resonantly enhanced polarization α is the greatly improved efficiency with which MNPs are able to scatter and absorb light. 14 Absorption and scattering cross sections, C abs and C sca are given by: 15 [ ] [ ] ( ) ( )

38 10 Equations (1 and 2) show that for small MNPs (i.e., a λ), 16 the contribution of absorption is relatively large as compared to scattering. The absorption efficiency is scaling with a 3, whereas scattering efficiency scales with a 6. The equations also show that scattering and absorption of a MNP are resonantly enhanced at its plasmon resonance (based on dipole approximation and Frӧlich condition Re[ ( )] ). 17 The expression for the extinction cross section which is the sum of absorption (transfer to heat) and scattering (re-radiation), C ext = C abs +C sca is: [ ] ( ) Anisotropic Metal Nanoparticles It should be noted that to date, no analytical solution exists for the scattering and absorption cross sections of nanorods (NRs). However, a very similar geometry that has been analysed in the electrostatic approximation is that of an ellipsoid. Consider an ellipsoid with three perpendicular principal axes a i (i=1,2,3). The polarizabilities along the three principal axes are given as: 15 ( ) ( ) where and are the permittivities of metal and surrounding medium respectively and depolarization factor is a geometry dependant factor given by: ( )(( ) ( ) ( ) ) ( )

39 11 For the case of a prolate spheroid where the two minor axes are equal ( ) further, if ( ), and as, therefore. The denominator of Equation (1) predicts two separate resonances in the polarizability for the prolate spheroid, depending on the incident E field polarization. The resonant condition is: ( ) ( ) It can be seen that for the incident polarization along the major axis the resonance is red shifted due to a small value of. It can also be noted that for the case of a sphere where all three principal axes are equal resulting in, the resonant condition acquires the familiar form of. The scattering and absorption cross sections can easily be extracted from Equations (1 and 2 from Section ) using Equations (1 and 2 described above) for an ellipsoid. We can analytically predict the two different resonant peaks corresponding to the longitudinal and transverse surface plasmons (SPs) in absorption and scattering of a NR, assuming that its response can be approximated by that of an ellipsoid. Consider a NR with dimensions nm and nm. From Equation (2) these dimensions results in and. By inserting these values in Equation (3) we expect the following two resonant conditions the NR corresponding to the longitudinal SP and for E field polarized along the long axis of for E field polarized along the short axis of the NR, that is the transverse SP. It can be seen that polarization along the long axis of the NR results in SP resonance at longer wavelengths. Assuming gold NRs are immersed in H 2 O (i.e., ), the two resonant wavelengths are 761 nm and 490 nm.

40 Interactions between Metal Nanoparticles Optical properties of metal nanoparticle (MNP) ensembles exhibit unique surface plasmon resonance (SPR) shifts as compared to the SPR of individual MNPs. This is due to electromagnetic interactions between the localized plasmon modes. The interaction effect between plasmonic nanostructures have been investigated experimentally and theoretically for a variety of arrangements and shapes of MNPs. Specifically, studies of the coupling effect of dimers (e.g., ellipsoids, 18 spheres, 19,20 nanodisks, 21 nanorods, or nanoantennas 25,26 ), and also many-nanoparticle systems such as nanorod assemblies, 27 linear arrays of nanocylinders, 28 and two- or three-dimensional MNP arrays have been the subject of studies. For MNPs, SP interactions are of a dipolar nature and MNP ensembles can be treated as an ensemble of interacting dipoles in a first approximation. Let us consider an ordered array of MNPs. The optical response depends on the size of MNPs a and the interparticle distances d between adjacent MNPs. There are two regimes based on the magnitude of d: (i) for closely spaced MNPs where d λ, the arrays of interacting MNPs can be described as dipolar near-field interactions with a distance dependence of d -3. In this case, a strong localized field enhancement occurs in the gap between MNPs 33 and thus can serve as a hot spot for surface-enhanced Raman scattering. These interparticle interactions shift the spectral position of the SPR. The direction of the SPR shifts can be determined by the Coulomb forces associated with the polarization of MNPs. As shown in Figure 1.6, the restoring forces acting on the coherent oscillation of electrons of each MNP can be either increased or decreased by the charge distribution of adjacent MNPs. Depending on the

41 13 direction of the polarization of incident light, the SPR wavelength of MNP ensembles can either be red-shifted or blue-shifted. For example, if the incident light is polarized parallel to the MNP axis, a red-shift of SPR can be observed. On the other hand, when the incident light is perpendicular, a blue-shift of SPR is seen. In the case of end-to-end NR dimers, if d between the ends of NRs is reduced, a red-shift of the longitudinal SPR occurs while a decrease of d perpendicular to the long axis of NRs results in a small blue-shift of the resonance. 23,24 Figure 1.6. Schematic illustration of near-field coupling between metal nanoparticles (MNPs). Two different polarizations (parallel and perpendicular to the MNP axis) are shown. Figure 1.7(A) shows arrays of 50 nm gold MNPs with varying interparticle distances. The dependence of the spectral position of the SPR on interparticle distance for both longitudinal and transverse polarization is shown in Figure 1.7(B). 34 These experimental results showed that when d>150 nm, the SPR of the arrays of the MNPs showed spectral

42 14 features similar to individual MNPs. This is due to the strong coupling strength with d -3 dependence. The spectral position of SPR via near-field coupling is also dependent on the chain length of MNPs. Figure 1.7. (A) SEM image of arrays of gold nanoparticle (B) Dependence of the plasmon peak position on the interparticle spacing d for both the transverse and longitudinal excitation of the collective mode. The dotted line shows a fit to the d -3 dependence of coupling predicted by a point dipole interaction model. Reprinted with permission from Reference 34. Copyright 2002, American Physical Society. (ii) For large particle separation, the arrays of interacting MNPs can be described as a dipolar far-field interaction with a distance dependence of d -1. These coupling effects have been investigated for both two-dimensional arrays 31 and one-dimensional chains. 28 For example, Figure 1.8 shows extinction spectra of two-dimensional gold MNPs with a diameter of 150 nm and height of 14 nm. 35 Far-field coupling of these MNPs shows influences on both spectral position of the SPR wavelength and spectral peak width. This

43 15 observed spectral peak width is due to the decay time of the plasmon oscillations influenced by radiative damping. Figure 1.8. Extinction spectra of gold nanoparticles (height 14 nm, diameter 150 nm). Reprinted with permission from Reference 35. Copyright 2000, American Physical Society Optical Properties of Semiconductor Quantum Dots Semiconductor nanocrystals (NCs) are colloids whose size ranges from a few to tens of nanometers. Unique size- and shape-dependent optoelectronic properties of NCs arise because their dimensions are comparable with or smaller than the exciton Bohr radius a B. The Bohr radius is given by the distance of electronic excitations between an electron and hole in bulk materials. 36,37 Unlike a bulk material which possesses continuous conduction and valence bands separated by an energy gap, one of the distinct features of NCs is the

44 16 discrete structure of their energy levels. A simplified schematic illustration of electronic states for bulk and NC semiconductors is shown in Figure 1.9. Figure 1.9. Schematic illustration of (A) a bulk semiconductor: continuous conduction band (CB) and valence band (VB) which are separated by an energy gap (E gap ) (B) a semiconductor nanocrystal (NC): with discrete atomic-like energy states and size-dependent E gap. In the case of quantum dots (QDs), all their dimensions are smaller than a B such that the motion of the electron and the hole are confined in all directions and described as a three-dimensional particle in a sphere problem: 38 E m ( )

45 17 where a, m and are the radius, the effective mass of electron or hole and the nth root of the lth order spherical Bessel function. Equation (1) shows that the energies of electron and hole exhibit 1/a 2 dependence. If a > a B, the confinement effect is weak whereas if a < a B, the confinement effect is strong. In a QD, the size of a B is of the same order as the nanocrystal itself and as such charge carriers (electron and hole) are confined by a potential which is infinite at the surface of the QDs. The optical properties of QDs fall within the regime of strong confinement (e.g., a B ~ nm). An important consequence is that due to a forced overlap of electronic wave functions, a significant enhancement of Coulomb interactions between charge carriers occurs. In this regime, the energies of the optical transitions (the optical gap) are given by: 38 E E ( ) E ( ) E ( ) ( ) Equation (2) shows that the optical gap is governed by the energies of electron and hole ( 1/a 2 ) and the Coulomb interaction between electron and hole is also size-dependent and scales as 1/a. Electronic energies depend on the extent of the spatial confinement of electronic wave functions and therefore, on QD dimensions (known as the quantum-size effect). As such, the band gap energy can be tuned by adjusting QD size which leads to the control of the color of emission and the spectral position of absorption. After light is absorbed, the charge carrier population in excited states relaxes back to the lowest exciton state resulting in radiative and non-radiative decay. Absorption at the lowest exciton state is called bandedge absorption (radiative decay times for band-edge emission at room temperatures are

46 18 ~ 20 ns). 39,40 It is important to note that the simplified electronic states shown in Figure 1.9 offer a reasonable description of CB. However, due to quantum confinement, energy states of VB leads to mixing within sub-bands. 41,42 This results in greater complexity of the lowest hole states (e.g., 1S 3/2, 1P 3/2, and 2S 3/2 shown in Figure 1.10(A)). 41 Figure 1.10(B) shows size-dependent absorption spectra of CdSe QDs that we synthesized and the corresponding optical transitions of these electron and hole states. The emission properties of QDs are thus far understood by considering the fine structure of the band-edge 1S(e)-1S 3/2 (h) transition indicated in Figure 1.10(B-a). Figure (A) Allowed optical transitions from hole quantized states resulting from mixing between valence sub-bands to CB for the case of CdSe QDs. (B) Absorption spectra of sizedependent as-synthesized CdSe QDs showing well-resolved optical transitions. Figure 1.11 shows the fine structure of the lowest exciton states. The lowest exciton state is eight-fold degenerate and consists of the two-fold degenerate (spin ) lowest

47 19 electron state and the four-fold degenerate (spin ) lowest hole state. Because of strong electron-hole exchange interactions in QDs (e.g., up to tens of mev as compared to bulk materials), 43 eight-fold degeneracy is broken by the fine structure splitting of the band-edge exciton. This results in two manifolds of N states (total angular momentum) =1,2 where N =1 corresponds to an optically allowed bright exciton whereas N=2 is a dark exciton. 36 The degeneracy of these states is further split into five sublevels due to crystal structure and QD morphology (e.g., as-syntheiszed QDs shown in Figure 1.10(B) have hexagonal wurtzite structure). Figure Schematic illustration of fine-structure splitting of the lowest exciton state for CdSe QDs with wurzite crystal structure. The band-edge 1S(e)-1S 3/2 (h) transition is induced by a strong electron and hole exchange interaction and shape and crystal field anisotropy.

48 Optical Properties of Dielectric materials Dielectric materials have found a broad range of applications in optical components and optical devices. The range of refractive indices offered by dielectric materials, allows for manipulation of light as it passes through a chosen medium. The optical properties of dielectric materials are dependent on their electronic structures. The photon energy of visible light (ranging from 1.5 ev to 3 ev) is insufficient to bridge between the VB and CB in common insulators and as such dielectrics are typically transparent in the visible range. However, there are exceptions to this general rule. In the case of indium tin oxide (ITO), for example, the material possesses both electrical conductivity and optical transparency. Therefore, electronically, ITO acts as a metal but optically acts as a dielectric. Upon excitation of electrons in the VB by incoming photons, photon energy and the band gap of the material define the critical wavelength (i.e., shortest wavelength) at which the dielectric remains transparent: ( ) ( ) where h and c are Plank s constant and the speed of the light in a vacuum respectively. It is important to note that some semiconductors can also be treated as dielectric materials depending on their lossiness and absorptivity as a function of wavelength. (and ) for some common dielectric and semiconducting materials such as diamond, Si and ZnO 2 are 5.50 ev (0.23 nm), 1.12 ev (1.10 nm), and 3.44 ev (0.36nm) respectively. 1,44

49 21 Unlike metals, the charges in a dielectric (insulator) are bound charges, and thus are not free to move under the influence of an externally applied field. Although their atoms and molecules are macroscopically neutral, when subjected to an external electric field, these charges slightly shift their centroids giving rise to local electric dipoles 45 as shown in Figure1.12. Figure Electric polarization in dielectrics showing ionic (or molecular) and electronic polarization. 45 The dipole moment for one such induced dipole p is given as: (2) where Q is the magnitude of the electric charge and d is the distance between the positive and negative charges. To account for all the dipole moments, these individual dipoles are

50 22 summed over a microscopically large volume V (as compared to the charges) but macroscopically small (as compared to the incident wavelength), the result is the polarization vector P: ( ) The electric field density D can then be found as: (4) where is the dielectric constant of free space, = F/m. Induced electric dipoles alter the applied field and cause the local field to differ from the applied field. It should be noted that there is a large number of these induced dipoles per unit volume and each of these dipoles will effectively influence its neighboring dipoles, therefore the neighboring dipole is subjected to a different applied field. This problem was solved by Lorentz by considering that the molecular dimensions are much smaller than the wavelength of the incident light. Lorentz argued that in the case of a non-polar dielectric material, the applied field must be replaced by a field which is now known as the Lorentz field E L given as: 46,47 ( ) When the applied field Ea is replaced by E L in the harmonic oscillator model of the atom, a reduction in the resonant frequency is observed, thus dielectric materials have their resonance in the infrared region of the spectrum while in the visible region of the spectrum the permittivity is almost constant. For example, in the visible region as discussed in Chapter 7, dispersion of a metamaterial originates from a metallic component whereas the

51 23 dielectric component remains transparent, that is, considered to be constant which is an important factor when designing optical metamaterials. Otherwise, as a consequence of electron and photon resonances, significant losses may occur adversely affecting the performance of the metamaterial Self-assembly of Nano-materials Self-assembly of nanoparticles (NPs) offers a simple, cost-efficient method for producing ensembles of NPs, as well as the ability to fabricate nanostructures on nonplanar substrates. Assembly of NPs into hierarchical structures to exploit their collective properties (e.g., electronic, optical, mechanical) have entered into a mature stage in the field of nanochemistry. This section provides a concise overview of the self-assembly of NPs and examines the various interparticle forces used in their assembly. Assembly of NPs can be achieved for example, by exploitation of various forces such as electrostatic attraction, hydrogen bonding, covalent bonding and dipole-dipole interactions. 48 Specifically tailored interparticle interactions caused by these forces have resulted in a variety of hierarchical structures (Figure 1.13). For example, by modifying the surface of the NPs with photoisomerizable molecules, assembly and disassembly of NPs can be realized via molecular dipole-dipole interactions 48 and nanorods can be assembled into chains via hydrogen bonding of DNA linkers. 49 Molecular dipole-dipole interactions are relatively weak. The typical value of molecules with a permanent electric dipole moment is 0-4 Debye. However, when several molecules are tethered to the surface of NPs, these molecular dipole interactions can

52 24 become strong enough to induce NP self-assembly. The total interaction energy U dd due to molecular dipoles is approximately: 48 U dd u dd N dd (3) where u dd is the thermally averaged interactions between freely rotating dipoles and N dd is the number of interacting dipole dipole pairs between the two particles at contact ( dd A eff where A eff is an effective area of contact between two spherical NPs with radius a ( 2 σ ) and is particle s surface density). For example, in the case of 3 nm Au NPs functionalized with cis-azobenze terminated alkane thiols, 50 N dd may have a value as large as ~40 with a resultant total interaction energy of approximately 40 kt. Thus, when acting together, these molecular dipole interactions have sufficient influence to induce self-assembly. Figure Schematic of the self-assembly of nanoparticles into a variety of hierarchical structures: chains, bi-layer, ring, and hexagonal arrays.

53 25 Another example is a magnetic dipole-dipole interaction induced by using spherical magnetic NPs. This type of assembly exploits the directionality of the dipolar interactions between particles. This dipolar interaction of NPs is dependent on both the conformation and size of NPs. The attraction is the strongest when NPs are aligned in an in-line configuration and assembly into chains and rings can be achieved For example, polyisobutene-coated iron NPs with a diameter of 12 nm can form into a string conformation and the magnitude of the interaction is approximately 15kT (i.e., ). It should be noted that these assemblies were formed at a relatively low concentration of NPs. At higher concentrations, the major driving force of the assembly is due to entropic effects rather than magnetic dipolar dipolar interactions. In addition, higher dipolar interactions can be realized in the presence of an applied field shown in Figure ,55 Figure TEM images of (A) Self-assembly of magnetic dipole dipole interactions by using 20 nm cobalt nanoparticles in the absence of an external magnetic field. Reprinted with permission from Reference 51. Copyright 1966, American Institute of Physics. (B) Formation of ring

54 26 conformation under an applied magnetic field of T. Inset shows a ring with almost singleparticle thickness. (Inset Scale bar is 100 nm). Reprinted with permission from Reference 55. Copyright 2008, American Chemical Society. Hydrogen bonding can be an important driving force for the formation of nanoscale hierarchies. One important example is the formation of 3D NP arrays, resulting in facecentred or body-centred cubic superlattice structures achieved through the utilization of hybridization of complementary DNA molecules on the surface of NPs. 59,60 It has also been demonstrated that by varying the temperature above and below the melting point of the DNA oligomers (typically, ~40 C < T m < 80 C for ~20 bases in a solution with 50 mm cations) 61 allows the interactions to be turned off and on, giving precise control over assembly The melting temperature T m is defined by the condition under which half of the DNA strands in solution are in the double-helix state while the remainder is in the single-strand state. The estimated melting temperature is: 66 ( ) where and H are the total entropy and enthalpy of DNA hybridization respectively and C T is the molar concentration of DNA. T m depends on the concentration and the length of the strands, and on the nucleotide sequence. DNA-mediated attractive NP interactions, U DNA with a steric repulsive potential is given by: 67 ( ) ( ) ( ) ( )

55 27 where, c, L, and h are surface density of DNA ligands, concentration, separation distance between NPs and height of the grafted DNA respectively. Equation (5) is valid and qualitatively in good agreement with experiment when h < L < 2h. These highly specific DNA-mediated interactions are desirable for the self-assembly of nanoscale components and represent an important example of the role of hydrogen bonding in mediating the formation of nanostructures. Electrostatic interactions between NPs are useful for both ensuring colloidal stability in solution and guiding their self-assembly into hierarchical structures such as mono- and multi-layer arrays and binary superstructures Electrostatic attractions can either be repulsive or attractive between like-surface charges of NPs or opposite charges respectively and can also be directional with regard to NPs whose surface charge distribution is asymmetric. Also, electrostatic attraction can be tuned by the dielectric constant of the chosen solvent and concentration of surrounding counter ions. One of the most ubiquitous NP interactions is through van der Waals (vdw) forces which arise from electromagnetic fluctuations due to the continuous movements of positive and negative charges between material bodies. vdw forces can be divided into three groups: (1) thermally averaged dipole-dipole interactions, (2) dipole-induced dipole interactions and (3) London dispersion interactions between transient dipoles of polarizable bodies. The attractive vdw force between the atoms is proportional to 1/r 7, where r is the distance between the atoms. The point interactions describing the empirical potential (Lennard-Jones potential) is given by: 72 ( ) [( ) ( ) ] ( )

56 28 where A and C are attractive interaction and repulsive component respectively, represents the characteristic energy of the interaction between the bodies and is the collision diameter. In the case of macroscopic bodies, vdw forces are significant at a distance of a few nm to tens of nms. The interaction between different geometries, such as two spheres, a sphere and surface, or two crossed cylinders, can be calculated by integration. Table 1.1 shows vdw interaction free energies between bodies of different geometries that were calculated based on the Hamaker Summation Method. 72 Table 1.1. Van der Waals interaction energy and force between macroscopic bodies of different geometries with surfaces a distance of D apart where D<<R. R is the radius and A is the Hamaker constant. 72 Geometry of Bodies Van der Waals Interactions Energy Force Sphere near flat surface E =AR/6D F =AR/6D 2 Two identical spheres E = AR/12D F =AR/12D 2 Cylinder near flat surface Two identical parallel cylinders Two identical cylinders 90 to each other E =AR/6D F =AR/6D 2 The assembly of NPs via vdw forces is predominately limited to surface interactions. However, there may be additional body forces which occur between materials that possess permanent magnetic or electric polarization. These forces are usually weak and may be screened in electrolyte solutions. Figure 1.15 shows an example of preferred orientation of nanorods and spherical NPs as a function of screening length. 48 Whereby the screening length is determined by the electrostatic potential: 47,73

57 29 ( ) ( ) where and kt/e 25 mv at room temperature describes the electrostatic potential in the presence of dissolved ions. Therefore, is defined as: 73 ( ) ( ) where c s is the salt concentration. represents the length scale that approximates the exponential decay of electrostatic potential when moving away from charged bodies in solution (e.g., ). Figure (A) Schematic illustration of a charged gold NP interacting with a gold nanorod via electrostatic interactions. (B) Ratio of the interaction energies for the end and side configurations as a function of screening length. Reprinted with permission from Reference 48. Copyright 2009, Small.

58 30 One of two possible arrangements, side or end is preferred based on the range of the electrostatic interactions. When is significantly smaller than the dimensions of the NP, a spherical, charged NP will interact in preference with the sides of an oppositely charged nanorod, rather than with either of its ends. The electrostatic interaction between a nanorod (radius a r, length h) and a sphere (radius a s ) scales as U side ~ a s a 1/2 r /(a s +a r ) 1/2 for the side configuration and U end ~ a s a r /(a s +a r ) for the end configuration. Therefore, the side arrangement is preferred. It should be noted that vdw forces are only effective from a distance of a few nm to tens of nms. At a large separation, retardation effects occur due to the finite speed of light. This retardation effect causes the force to fall more rapidly with distance than in the short-range vdw limit 1/r 6 (~1/r 7 ). It is worth mentioning that casimir forces can arise from the confinement of the zero point energy fluctuations of the electromagnetic field between two bodies. 74,75 This casimir effect can be present as a force in binary liquids where liquid fluctuations confined between two surfaces change the equilibrium thickness of wetting layers. Also, it can be present as a force between colloidal particles due to confinement of solvent between the particles leading to aggregation. Assembly of NPs at interfaces, e.g., liquid-liquid or liquid-solid, can be achieved via the adsorption of NPs to a particular interface, 80,81 Langmuir-Blodgett deposition technique, and sedimentation and evaporation methods For example, adsorption of CdSe NPs at oil-water interfaces is controlled by reduction in the total free energy of the system. Figure 1.16 shows a confocal microscope image of a water droplet, whose surface is decorated with tri-n-octylphosphine oxide capped CdSe NPs. 80,88 Surface energy reduction E due to the assembly of a single particle at the oil-water interface is:

59 31 ( ( )) ( ) where are three contributions to the interfacial tensions at the oil-water, particle-water, and particle-oil interfaces respectively and r is the effective radius of a NP. An estimate of is approximately -5 kt for 2.8 nm NPs. The competition between interfacial energy and thermal fluctuations results in size-dependent NP self-assembly, due to the fact that depends on r 2. Therefore, when the NP is smaller (diameter 2.8 nm), the energy gain is relatively smaller as compared to the larger NPs (diameter 4.6 nm) and as such the larger NPs replace the smaller NPs. 88 In general, nanoscale assembly relies on the premise that careful design of components e.g., size, geometry, composition and their interactions will allow for the creation of desired 2D and 3D hierarchical structures. However, in practice, this is a challenging task and often there is more than one major driving force involved in the assembly. For example, assembly of 2-(dimethylamino)ethanethiol capped CdTe NPs into 2D sheets is in practice realized by balancing the dipole moment, surface charge and directional hydrophobic attractions. 89 Whether we are considering metals, semiconductors or dielectrics, an understanding of their optical and electronic properties both individually and collectively, is a critical first step in the design and formation of functional nanoscale ensembles. From CdSe semiconductor QD lamellae (Chapter 6), to chains and stacks of gold nanorods (Chapter 3 and 4 respectively), the common theme is the bottom-up creation of architectures whose compositional and geometrical design affords the possibility of new insights and potential applications on the nanoscale.

60 32 Figure Fluorescence confocal microscope images of varying sizes of water droplets in toluene in which CdSe NPs show self-assembly at the liquid-liquid interface. Optical cross-sectional images at various depths are shown on the left. Reprinted with permission from Reference 81. Copyright 2003, Science Overview of Metamaterials Metamaterials are artificially structured composites tailored for their specific ensemble electromagnetic (EM) properties that are not found in naturally occurring media and are not observed in the constituent materials components The term metamaterial is derived from the Greek word, in which meta means beyond. Thus metamaterials are materials having properties beyond those of conventional materials. The heterogeneity of these materials exists on a length scale less than the wavelength of interest. Thus, the EM response of the material is a function of the collective behavior of the

61 33 materials components. Figure 1.17 shows a schematic illustration of both a conventional material and metamaterial. 95 In a conventional material, the response to an EM field is determined by the atoms and molecules, whereas in a metamaterial, the response is engineered through many identical structural units whose dimensions are less than the wavelength of the operating frequency. One subgroup of metamaterials that have received considerable attention, are referred to as negative refractive index materials. 92,96,97 These complex materials possess negative real values for both permeability and permittivity at certain frequencies These materials have their conceptual beginnings with Sir Arthur Schuster who, in 1904 stated Energy can be carried forward at the group velocity but in a direction that is antiparallel to the phase velocity. and the deviation of the wave entering such a medium is greater than the angle of incidence. 101 In 1967, Veselago theoretically investigated planewave propagation in a material whose permeability and permittivity were assumed to be simultaneously negative. 102 For a monochromatic uniform plane wave in such a medium, the direction of the Poynting vector is anti-parallel to the direction of the phase velocity. In recent years, Smith and Schultz et al constructed such a composite medium for the microwave region and demonstrated experimentally the presence of unusual refraction. 100,103

62 34 Figure1.17. (A) Naturally occurring conventional material with its atoms (B) Metamaterial artificially structured atoms Figure is adapted from reference 87. More generally, it is well known that the response of a system to the presence of an EM field is determined, to a large extent, by the properties of the materials involved. We describe these properties by defining the macroscopic parameters of their permeability and permittivity. Figure 1.18 shows permeability and permittivity diagrams for the classification of a medium separated into four quadrants.

63 35 Figure Permittivity(ε)/Permeability(µ) Diagram. The first quadrant, second, third and fourth quadrant are assigned as double- positive (DPS), epsilon-negative (ENG), double-negative (DNG) and mu- negative (MNG) respectively. A medium with both permeability (µ) and permittivity (ɛ) greater than zero ( ) will be designated a double-positive (DPS) medium. Most naturally occurring media, e.g., dielectrics, fall under this designation. A medium with permittivity smaller than zero and permeability greater than zero ( ) will be designated an epsilon-negative (ENG) medium. In certain frequency regions, noble metals, e.g., Au and Ag, exhibit this characteristic. For a medium with permittivity and permeability less than zero ( ) several names and terminologies have been suggested, such as "left-

64 36 handed" media; media with a negative refractive index; "backward-wave media" (BW media); and "double-negative (DNG) materials. 104,105 A medium with permittivity greater than zero and permeability less than zero ( ) will be designated a mu-negative (MNG) medium. It is important to note that at optical wavelengths, all naturally occurring media show no magnetic response and as such, light interacts with the material via its electric field only. Therefore, µ is always taken to be unity in the visible region. In the past few years, a number of researchers have proposed methods for the fabrication of metamaterials. The manipulation and combination of material falling within these quadrants (Figure 1.18), allows for the fabrication of a variety of metamaterials with applications ranging from DNG materials to cloaking (Chapter 7). 96, Regarding DNG materials, one possible metamaterial design is shown in Figure The building block is a pair of nanorods. The unit cell, that is, the metamaterial atom, should be very small relative to the wavelength of interest. The unit cell of the metamaterial is designed in such a way that an electric field is parallel to the long axis of the nanorod as shown in Figure 1.19, inducing parallel currents in the paired nanorods. The magnetic field of the EM wave (perpendicular to the long axis of the nanorods) causes anti-parallel currents in the nanorods. The path of current flow is completed by the displacement current at the edges of the nanorods. Thus the structure showed in Figure 1.19 forms an LC resonant circuit, thereby enabling the metamaterial to interact with the magnetic component of light. This enables the control of light in an unconventional way such that this anti-parallel current causes the magnetic response of the system. 113

65 37 Figure Schematic illustration of the proposed structure for making a double-negative (DNG) material showing arrays of paired nanorods. The arrows show the direction of current flow. Figure is adapted from reference 106. Advances in the fabrication of structures with small (smaller than the wavelength of visible light) dimensions, have paved the way for creating metamaterials with potential applications such as new types of beam steerers, modulators, band-pass filters, lenses, microwave couplers, and antenna radomes. 114 One of the interesting features of DNG media is their ability to provide phase compensation or phase conjugation due to their negative refraction. This feature can lead to interesting applications in device and component designs including sub-wavelength cavities, phase-compensated, time-delayed, wave guiding systems and waveguides with lateral dimensions below diffraction limits. 98,114 Another interesting potential application that results from negative-refraction properties, is sub-wavelength focusing, 98 which offers the possibility of a "perfect lens" or focusing beyond the diffraction limit. This was first recognized by Veselago 115 and the properties of such a slab were first analyzed by Pendry who showed that a slab with a

66 38 refractive index n 1 in vacuum results in the imaging of objects with sub-wavelength focus. 98 In his analysis of the image formation process in a slab of lossless DNG material, the evanescent spatial Fourier components can be ideally reconstructed, in addition to the reconstruction of all of the propagating spatial Fourier components. The evanescent wave reconstruction is due to the presence of the "growing exponential effect" in the DNG slab. This effect, in theory, leads to the formation of an image with a resolution higher than the conventional limit. 98 Waves scattered by an object have all the Fourier components: ( ) The propagating waves are limited to: (2) where k t is the transverse wave vector. To resolve features of size we need a wavelength smaller than, that is, (3) From Equation (1), if the required is greater then (the free space wave number), the waves become evanescent and therefore these features cannot be resolved. The DNG material provides a possible solution, if the optical losses are overcome. The evanescent waves are re-grown in a DNG slab and are fully recovered at the image plane. Figure 1.20 summarizes this effect. A detailed discussion of this effect can be found in the field of Fourier Optics.

67 39 Figure The idea of a perfect lens with sub-wavelength resolution (A) A conventional lens only collecting the propagating waves: k t < k 0 (B) The loss of the evanescent waves in a conventional imaging system (solid line represents propagating modes whereas dashed lines represents evanescent modes) (C) The focusing ability of a DNG slab (D) The growth of evanescent waves in the DNG slab and the restoration of both the propagating and evanescent waves. Figure was adapted from Reference 91. Beyond the potential of DNG materials, the manipulation of ε from 0 to 1, while keeping µ = 1 (DPS quadrant Figure 1.18) offers the possibility of optical cloaking by the careful combination of metal and dielectric components on the nanoscale. Chapter 7 discusses in detail, one particular bottom-up route for the achievement of this goal. 72

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76 48 Chapter 2 Materials and Methods 2.1. Materials Tri-n-octylphosphine oxide (TOPO, tech. grade 90%) and Tri-n-octylphosphine (TOP), Cadmium Oxide (CdO, ~99%), Selenium (powder, %), Steric acid, Cadmium acetate dihydrate (~98%) were purchased from Sigma-Aldrich and used as received. Hexadecyltrimethylammoniumbromide (CTAB, 98%) was purchased from Fluka. Sodium borohydride (99%) and l-ascorbic acid were purchased from Sigma-Aldrich. Thiolated Polystyrene was purchased from Polymer Science. 25 micrometer diameter Aluminum wire (99.999%) was purchased from Alpha-Aesar. Perchloric, Oxalic acid, Silver Nitrate, Sodium Sulfide (anhydrous) and Ethlyene Glycol were purchased from Sigma-Aldrich. Deionized water (18 MΩ) from a Millipore Milli-Q water purification system was used in all the experiments.

77 Methods Synthesis and Fabrication Synthesis of CdSe Quantum Dots and Nanorods TOPO capped CdSe quantum dots (QDs) were synthesized through an established organometallic approach at high temperature. 1,2 In a typical CdSe QD synthesis, CdO (63 mg) was dissolved in a mixture of steric acid (1g) and 3g of TOPO by first evacuating at 80 C for 2 h, followed by heating at 340 C under an Ar atmosphere. Once an optically clear solution was achieved, selenium in tri-n-octylphosphine (Se-TOP; 37 mg in 1.0 ml) was injected rapidly at 310 C. QDs were then allowed to grow for 1-20 min at 280 C and an aliquot was taken out to examine the quality of the CdSe QDs. Once CdSe QDs of desired dimensions were grown, QD growth was terminated by removal of the heating mantle, and at 50 C, anhydrous methanol (MeOH) was added to the mixture to precipitate the QDs. Purification was achieved by centrifugation (2 ). As synthesized TOPO-capped CdSe QDs were dissolved in toluene (good solvent) and temporal stability is typically many months. CdSe nanorods were prepared as described elsewhere 3 with the exception that a mixture of equimolar amounts (9 mmol) of Cd(OAc) 2 2H2O and NH 4 (OAc) was used in the multiinjection step. In a typical synthesis, CdO (0.096 g), TDPA (0.430 g), and TOPO (3.88 g) were loaded into a 25 ml round-bottom flask. The mixture was heated to 320 C under an argon flow. Once a colorless homogeneous solution was formed, a solution of TOPSe in TOP (0.51 ml, 1 M) was then injected into the solution. Subsequent multiple injections were performed over 5 min using an automated syringe pump and samples were taken out

78 50 at 3 and 6 min. Purification of CdSe bullet-shaped rods was carried out using the procedure for CdSe QDs as described above Synthesis of Gold Nanorods Gold NRs were prepared by the seed-mediated growth method devised by El- Sayed 4. Briefly, seed gold NPs were synthesized by the reduction of HAuCl 4, dissolved in an aqueous solution of cetyltrimethylammonium bromide (CTAB), with cold sodium borohydride (NaBH 4 ). The growth solution was prepared by dropwise addition of ascorbic acid in an aqueous solution of HAuCl 4, CTAB and AgNO 3. A seed solution aged for 5 min was added to the growth solution and the NR growth was initiated. The color of the solution mixture changed from clear to deep-purple after incubation for 10 hr at 27 C. The resultant CTAB-coated gold NRs were purified by two centrifugation cycles (8500 rpm for 30 min) Fabrication of Al2O3/Ag hybrid Cloaking Structure 25 micrometer diameter aluminum wire was first degreased in a 1:1 mixture of acetone and propanol. The wire was then electropolished in a 1:5 mixture of perchloric acid and propanol. The electropolishing was carried out at a constant potential of 20 V. In order to produce the desired final diameter of 3 to 4 µm, the wire was repeatedly dipped and removed from the electropolishing bath to create a tapered profile. The final desired wire diameter was confirmed by light microscopy. Following electropolishing the wire was washed in distilled water then rinsed with acetone.

79 51 Porous Al 2 O 3 growth was carried out using a 3 % oxalic acid solution at 21 C. In order to produce the desired radial oxide growth with the appropriate gradation in pore volume, the applied potential was varied from 30 V to 18 V over a period of 100 minutes. Specifically, voltage was held at 30 V for 20 mins, 24 V for 30 mins and 18 V for 50 mins. At each voltage transition, a rate of 1V per min was used. The wire was removed from the oxalic acid solution and washed in distilled water then rinsed with acetone. Electroless deposition of Ag nanowires was carried out by placing the AAO coated aluminum wire into ethylene glycol containing 12 % AgNO 3 and 30 µmol of Na 2 S. The solution containing the wire was first sonicated for 5 min at 42 khz then placed in an oven at 155 C for 30 minutes. After removal from the solution, the wire was then paced in a slurry of propanol and diamond polishing powder and sonicated for 5 minutes at 42 khz. This step is necessary to remove superficial Ag crystalline deposits Self-assembly of Semiconductor and Metal Nanoparticles Assembly of Quantum Dot Lamellar Envelopes Trioctylphosphine oxide (TOPO) capped CdSe quantum dots (QDs) and nanorods were synthesized through an established organometallic approach at high temperature and described in section As-synthesized TOPO capped CdSe QDs or nanorods were precipitated with methanol three times to remove free TOPO and then dissolved in toluene. For optimmized lamellar formation, 10 μl of deionized water (10% v/v) were added into CdSe QDs ( mol/l) dissolved in toluene (20% v/v water was used for nanorods). The mixtures were sonicated from 15 to 30 s at 42 khz. A cloudy, colored suspension

80 52 formed immediately following sonication. The sample was withdrawn via micropipette for further characterization. For all encapsulation experiments, 10 μl (10% v/v) of watersoluble species (CoCl2 6H2O, M; EuCl3 6H2O, M; Au NPs 37.8 nm, and ferritin 53mg/mL in 0.15 M NaCl) were added as described above for water Assembly of Gold Nanorods For end-to-end NR assembly, 760 μl of tetrahydrofuran (THF) were evaporated from the stock NR solution. The dried NRs were re-dissolved in 2.45 ml of dimethylformamide (DMF). A solution of Raman reporter molecule of Oxazine 720 (OX) in DMF (4 µm) was added dropwise under shaking to the NR solution in DMF. Following a 30 min agitation under gentle vortex conditions, the mixture was incubated for 1 hr. The end-to-end selfassembly of the NRs was triggered by a dropwise addition of the DMF-water mixture containing 20 vol. % of water. Side-by-side assembly of gold NRs was similar to that described above with the exception that Cresyl Violet (CV) was introduced (3 µm) as a Raman reporter and 10 vol. % of water was introduced into the THF solution containing NRs. All subsequent physical measurements were carried out on the same batch of NRs. As soon as NR self-assembly began, we carried out at regular time intervals, parallel extinction and SERS measurements, as well as the preparation of samples for electron microscopy experiments to maximize the fidelity of results.

81 Characterization Electron Microscopy For the characterization of the structure of lamellar QD Arrays (see chapter 6), scanning transmission electron microscopy (STEM) was used. A single droplet of solution ( 25 μl) was applied to 200 or 400 mesh carbon coated copper TEM grids and also to 1000 mesh uncoated TEM grids. Images in both bright field and high angle annular dark field (HAADF) were recorded using the Hitachi HD-2000 dedicated STEM operating at 200 kv. For the cross-sectional analysis, samples were first prepared on carbon coated indexed TEM grids. STEM was used to identify the locations of individual lamellae. Subsequently, the indexed grids were coated with 10 nm of carbon using an Emitech high vacuum carbon coater. Grids were then coated with a nm layer of Au using a Denton Desk II sputter coater. The indexed grids were then embedded in epoxy resin, and 30 nm cross sections were prepared using a Leica Ultramicrotome (Figure 2.1).

82 54 Figure 2.1. Cross-sectional sample preparation for internal structure investigation of lamellae by STEM. (A) CdSe QD and nanorod lamellae were prepared on separate carbon coated indexed TEM grids. STEM was used to identify the locations of individual lamellae. (B) the indexed grids were then coated with approx. 10nm of carbon via evaporation to secure the structure. (C) Grids were then sputtered with a 20-30nm layer of Au which was used as a visual marker in imaging. (D) The indexed grids were then embedded in epoxy resin and 30nm cross-sections through individual lamellae were prepared by ultramicrotomy. For solution state imaging of lamellae, samples ( 15 μl) were injected into Quantomix WETSEM capsules and backscattered electron images recorded using a Hitachi S-3400 SEM operating at 20 kv and with the Hitachi TM-1000 table top variable pressure SEM operating at 15 kv. Energy dispersive X-ray spectroscopy (EDS) in both SEM and STEM was carried out using an Oxford Instruments INCA system. STEM EDS analysis of cross sections of QD lamellae was performed at -120 C to minimize electron beam damage

83 55 and contamination during X-ray acquisition. Simultaneous TEM and SEM were recorded using a Hitachi S-5200 field emission SEM equipped with a transmitted electron detector. Selected area electron diffraction (SAED) patterns of CdSe QDs and nanorods and their lamellar structures were recorded using an FEI Technai 20 operating at 200 kv. Subsequent analysis of polycrystalline ring patterns was carried out using the Process Diffraction software package Surface-Enhanced Raman Scattering Spectroscopy In SERS measurements, 1.5 ml of the solution of self-assembled NRs was placed in a vial. Raman spectra excited with a 785 nm laser line were acquired with a Renishaw InVia System spectrometer coupled to a Leica microscope. The laser power was set to 1% of the full power (approximately 80 µw). The laser beam was focused on the sample by a 5 x objective lens (NA= 0.12). The calculated interrogated volume was 6.46 nl. The spectra were measured with a 4 cm -1 resolution, using a 1 sec exposure and 25 scans. Control SERS experiments were conducted using a roughened gold substrate. A solid gold electrode with the surface area of 0.3 cm 2 was roughened with 20 successive oxidation-reduction cycles from -0.3 V to V at 100 mv/s in an aqueous 0.1 M KCl working solution. Then, the electrode was isolated from the electrochemical cell and exposed to a 4 µm solution of OX in water, in pure DMF or in the DMF/water mixture (20 vol. % of water). After 15 min exposure, the surface was rinsed with an appropriate solvent, that is, with water, DMF or the DMF/water mixture, respectively, and dried under

84 56 nitrogen flux. The SERS spectra of the OX adsorbed to the gold substrate were acquired using excitation at 785 nm (laser power 1%, 4 accumulations and 5 s exposure time) Confocal Microscopy Liquid state confocal fluorescence microscopy of CdSe QD lamellae was carried out using a Leica TCP SP2 upright confocal microscope with a oil immersion lens. The 364 nm line on an Innova 90C Argon ion laser was used for excitation with the signal collected over a range from 550 to 600 nm. Fluorescence intensity studies were carried out by recording a series of sequential fluorescence images in the range from 500 to 700 at 5 nm intervals. Again, the 364 nm line of an Innova 90C Argon ion laser was used for excitation (power: 130 mw ± 1 with 50 % laser power, pinhole: open, zoom 4, gain PMT1: 700V). For the analysis of dye (FITC 100 ng/ml) incorporation into the lamellae, fluorescent images were obtained using a Leica TCS SP2 confocal microscope with a oil immersion lens. Both FITC and QD lamellae were excited using the 488 nm line of an argon ion laser. FITC emission was collected in the range nm Ch1 (green). CdSe QD emission was collected in the range nm Ch2 (yellow) Extinction The progress of gold NR assemblies for both end-to-end and side-by-side conformations were monitored by using a Cary 500 UV/vis/near-IR spectrophotometer. Extinction measurements were recorded in the spectral range from nm at room temperature.

85 Optical Transmission Measurements To assess the performance of the fabricated cloaking structure, we used the transmission setup shown in Figure 2.2 which is comprised of a dual optical microscope (Olympus) coupled to a super-continuum source (Fianium SC-400-4) with a tunable filter. It allows for high spectral density (>1mW/nm), diffraction limited transmission imaging from 420 nm to 2000 nm. The output of the laser source is fed to an acousto-optic tunable filter (AOTF) to choose the desired wavelength from a broad spectrum. Output of AOTF having a spectral purity of 3nm to 7nm full width half maximum (FWHM) is passed through a polarizer to ensure TM illumination. A microscope objective (MO) is utilized to focus light on the sample and collect the scattered radiation. The collected light is fed to a CCD and spectrometer via a beam splitter (BS) to monitor the field profile and spectral quality of the laser source. Figure 2.2. Optical transmission setup using super-continuum (SC) laser source along with an acousto-optic tunable filter (AOTF) for monochromatic illumination of the cloak sample.

86 Finite-Difference Time-Domain Simulations The assembly of NR chains was simulated by the 3D finite-difference time-domain (3D-FDTD) method 6. The time and space derivatives were numerically modelled using the central difference scheme, which is the second order accurate representation of the analytical Maxwell s equations. Basics of FDTD are summarized in Appendix A1. Incident field polarization was parallel to the rod axis and simulation domain was terminated with perfectly matched layer (PML). The gold NRs were modelled using a fit to Johnson & Christy s experimental data. 7 To calculate the absorption and scattering cross-sections we employed the formalism of total field scattered field (TFSF). We introduced a set of 2D power monitors forming two closed surfaces enclosing the NRs, one inside the TF region (power monitor 1, (PM1)) and the other in the SF region (power monitor 2, (PM2)). PM1 was used to calculate the absorption cross-section by evaluating the net power flow into PM1, which represents the power lost in the NRs. Total power exiting PM2 was used for the calculation of scattering cross-section as follows where is the scattered power obtained from PM2 and is the source intensity. The calculated extinction cross-section is the summation of scattering and absorption cross-sections.

87 59 References 1. Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, Nair, P. S.; Fritz, K. P.; Scholes, G. D. Small 2007, 3, Nikoobakht, B.; El-Sayed, M. A. Chem.Mater.2003, 15, Labar, J. L. Ultramicroscopy 2005, 103, Taflove, A.; Hagness, S. C. Computational Electrodynamics: The Finite-Difference Time Domain Method, 2nd ed.; Artech House: Boston, Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6,

88 60 Chapter 3 Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold Nanorods by Surface Enhanced Raman Scattering Elements reprinted with permission from Journal of the American Chemical Society, 133, 7563, Copyright 2011, American Chemical Society. Continuing progress in the applications of self-assembled nanostructures depends critically on establishing a fundamental understanding of the relation between the properties of nanoparticle ensembles and their time-dependent structural characteristics. By following the dynamic generation of hot-spots in self-assembled chains of gold nanorods, we have established a direct correlation between ensemble-averaged surfaceenhanced Raman scattering and extinction properties of these nanoscale chains. Experimental results were supported by comprehensive finite-difference time-domain simulations. The relationship established between the structure of nanorod ensembles and their optical properties provides a basis for producing dynamic, solution-based, plasmonic platforms for applications ranging from sensing to nanoelectronics.

89 Introduction Hierarchical organization of individual nanoparticles (NPs) into complex nanostructures, including superlattices or small clusters, remains a frontier area of research in nanoscience. While individual NPs offer a multitude of scientific challenges and applications, ensembles of NPs show unique, coupled properties which may potentially be exploited in functional nanoscale devices. 1-6 Self-assembly of NPs offers a facile, low cost, solution-based route for producing ensembles of NPs, along with the ability to fabricate nanostructures on nonplanar substrates To date, self-assembled nanostructures composed of metal, semiconductor, and magnetic NPs have found applications in the areas of data storage, imaging, and sensing of chemical and biochemical species The exploitation of self-assembled nanostructures in other applications, is limited in comparison with those produced by nanofabrication techniques, largely because of the difficulty in forming defect-free structures with precisely controlled geometry and distance between adjacent NPs. Additionally, a fundamental understanding of the relationship between the properties of self-assembled NP clusters and their dynamic structural characteristics such as aggregation number, mutual NP orientation, and interparticle distance is required. With this framework of understanding in place, it should be possible to more accurately and reproducibly predict the properties of self-assembled structures, both theoretically and practically. In the case of metal nanocrystals, gold NPs in particular (with various shapes) have been organized into a variety of nanostructures including chains, two-dimensional sheets, and superlattices When compared to shape isotropic NPs, the self-assembly of anisotropic gold nanorods (NRs) provides potentially more useful applications, since it

90 62 offers the ability to exploit vectorial properties of the resultant nanostructures The optical properties of ensembles of gold NRs are typically characterized by measurements of extinction in the visible and near-infrared spectral ranges. In individual gold NRs, two types of localized surface plasmon resonances (LSPRs) are observed. These are due to the coherent oscillations of the conduction band electrons in directions that are parallel and perpendicular to the long NR axis. 30,31 When gold NRs are assembled into an end-to-end chain formation, coupling of alternating dipoles along the chain occurs resulting in a red shift of the longitudinal LSPR. In the case of side-by-side assembly of NRs, a blue shift of longitudinal LSPR and a red shift of the transverse LSPR occurs. 30,32,33 In chains of gold NRs, coupling of LSPRs results in the formation of a periodic array of enhanced electric fields (hotspots) in the spaces between adjacent NR ends. As such, the self-organization of gold NRs offers a tool for the study of the optical properties of ordered NR ensembles by surface-enhanced Raman scattering (SERS). Furthermore, the in solution, dynamic selfassembly of NRs into well defined, one-dimensional nanostructures provides a method to explore the influence of order in NR ensembles on their SERS properties. To date, the majority of studies of hot-spots have been carried out for NP assemblies with a limited degree of order As a result, there remains an insufficient understanding of the influence of the structural characteristics of aggregates on their SERS properties. In addition, research has focused on the study of isolated NP aggregates ( single particle SERS), and as such an understanding of the properties of a system comprised of multiple NP assemblies remains elusive. In order to achieve greater control over hot-spot generation in NP clusters, recent studies have focused on SERS of self-assembled dimers and trimers of spherical gold NPs in single-aggregate and ensemble-averaged systems

91 63 Currently, only a single report exists on the generation of hot-spots in self-assembled chains of gold NRs. However, this report is focused on the reorientation of analyte molecules in the gaps between NR ends. 40 In this Chapter, we report the results of experimental and theoretical studies of the relationship between the dynamic structural characteristics of self-assembled clusters of gold NRs and their ensemble-averaged SERS properties, resulting from the controlled generation of plasmonic electromagnetic hot-spots. To this end, we exploited the geometrical and chemical anisotropy of NRs to induce their assembly into chains in an endto-end arrangement. In the course of assembly, the dielectric environment and the spacing between adjacent NRs remained constant. The process was characterized by correlating the average aggregation number of NRs in the chains with their extinction and ensembleaveraged SERS signals. Up to this point, such a correlation has only been demonstrated for nanostructures fabricated by the top-down method. 41,42 Our experimental findings were supported by the results of finite-difference time-domain (FDTD) simulations of the optical properties of NR assemblies. This work establishes a strong link between experiment and theory, and it provides an important insight into the properties of hot-spots in ordered, solution-based nanostructures. In addition to the fundamental importance of these results, the established relationship between the structure of NR assemblies and their optical properties provides the basis for the development of new design rules for the formation of nanostructures with applications ranging from biomedicine to nanoelectronics.

92 Results and Discussion End-to-end NR Assembly and their Extinction and TEM Analysis The schematic in Figure 3.1 illustrates the site-specific functionalization of gold NRs and dynamic generation of hot-spots via end-to-end NR assembly. NRs with a mean length of 37.6 ± 4.4 nm and a mean diameter of 11.4 ±1.0 nm were used as the building blocks for end-to-end chain formation. In the ligand-exchange step, thiol-terminated polystyrene molecules (SH-PS) replaced cetyltrimethylammonium bromide (CTAB) at the ends of NRs, converting them into amphiphilic species. 28 These NRs were well-dispersed in DMF, a good solvent for both the CTAB molecules coating the long sides of the NRs and the SH-PS molecules attached to the ends of NRs. 28 End-to-end assembly of the NRs was triggered by introducing 20 vol% of water to the solution of amphiphilic NRs in DMF in the presence of the Raman reporter molecule, Oxazine (OX). Upon addition of water, the mixture became a poor solvent for the PS ligands localized at the NR ends but remained a good solvent for the hydrophilic CTAB ligands coating the long sides of the NRs. The NRs associated in an endto-end manner in order to avoid unfavorable contact of PS molecules with the DMF/water solution and to reduce the surface energy of the system.

93 65 Figure 3.1. Schematic of the generation of hot-spots via end-to-end self-assembly of gold NRs into chains. (a) Gold NRs stabilized with CTAB. (b) Ligand exchange of CTAB with SH-PS at the ends of the NRs. (c) End-to-end assembly of NRs triggered by adding water to the solution of NRs in DMF, in the presence of Raman reporter OX. The volume fraction of water in the DMF/water mixture is 20 vol %. Hot-spots are generated between the ends of adjacent NRs. The distance between the adjacent NRs in the chain is maintained constant. Schematic is not drawn to scale. Figure 3.2.a shows representative scanning transmission electron microscopy (STEM) images of the NR assemblies in various stages of chain growth. STEM imaging was carried out at low voltage (30kV) which is better suited to the imaging of organic macromolecules. All images were recorded without recourse to staining. The diffuse gray regions between the ends of adjacent NRs in the chains correspond to globules of SH-PS

94 66 molecules, which formed in the poor DMF-water solvent. Using image analysis, we determined that the average distance between the ends of adjacent NRs throughout the self-assembly process remained at 8.9 ± 1.5 nm. We note that while STEM images are valuable in determining the average aggregation number of NRs in the chain and the inter- NR spacing, the conformation of the chain may be influenced by the drying process. Therefore the precise geometry of the chains in solution cannot be directly inferred by the micrographs presented in Figure 3.2.a. Figure 3.2. (a) Representative STEM images of the self-assembled chains of NRs. Diffuse grey regions between adjacent NRs indicate the presence of SH-PS globules forming in a poor solvent. Scale bar is 40 nm. (b) Variation in extinction properties of NRs in the course of their self-assembly in chains. The spectral position of LSPR shifts from 754 nm to 812 nm with the aggregation number of the NR chains changing from =1 at t < 5 min to = 8 at t =18 hr. Transverse LSPR is located at 514 nm. The peak at 660 nm corresponds to OX.

95 67 The growth of NR chains in the course of self-assembly was characterized by the change in the average aggregation number, : ( ) where and are the number of NR chains and the total number of NRs in the system, respectively, and is the number of chains containing NRs. The values of were calculated by analyzing STEM images of nanostructures formed during the course of selfassembly experiments. The growth kinetics of the NR chains resembled the evolution of polymer chains in reaction-controlled step-growth polymerization, as reported in our earlier work. 116 However, the self-assembly of the NRs in the presence of OX occurred at a greatly increased rate, in comparison with an OX-free system. The extinction and SERS measurements were carried out throughout the course of self-assembly (t from 5 min to 18 hrs) with concurrent collection of samples for STEM imaging. Figure 3.2.b shows the evolution of the extinction spectra of the system undergoing self-assembly. In the course of chain growth, the longitudinal LSPR peak shifted from 754 to 812 nm, due to the coupling of alternating dipoles along the NR chain. The end-to-end arrangement led to the reduction in resonance energy with respect to individual NRs. 24,27, In the course of self-assembly, the width of the longitudinal LSPR peak broadened by 12 % when increased from 1 to 8, which was substantially narrower than for solution-based aggregates of gold NPs reported to date. The absorption peak at 659 nm corresponded to OX molecules. This peak was not measurably shifted or

96 68 diminished in intensity for a period of greater than 18 hr, which suggested good structural and temporal stability of OX during NR self-assembly Ensemble-averaged SERS Figure 3.3.a shows the evolution of the ensemble-averaged SERS spectra of OX through the course of NR self-assembly. The most enhanced bands at 563 and 604 cm -1 (assigned to vibrational modes of the phenoxazine ring of the dye) 121,122 were consistent with the Raman spectrum of the solution of OX in DMF. The same values of vibrational frequencies of OX adsorbed on the surface of NRs and of the solution of OX in DMF suggested that the reporter molecule was physisorbed onto the gold surface. 123 Figure 3.3. (a) Evolution of normalized ensemble averaged SERS spectra in self-assembled NR chains. The average aggregation number of NR assemblies changes from =1 at t < 5 min (brightred spectrum) to = 8 at t =18 hr (black spectrum). The SERS peaks at 563 and 604 cm -1 are normalized against the SERS peak of DMF at 659 cm -1 (indicated with astericks). (b) Variation in the normalized SERS peak intensity measured at 563 cm -1 plotted as a function of the average

97 69 aggregation number of the NR chains. SERS variation (y error) is based on three measurements taken within 15 min. Approximately 1000 NRs (including individual species) were used in the calculations of number (x error). Laser excitation wavelength was 785 nm. Next, we considered the local environment of the OX molecules that provided the main contribution to the overall intensity of SERS. It is possible that OX could be localized within the CTAB layer (on the NR sides) and/or be associated with SH-PS molecules at the NR ends. In the first instance, hydrophobic interactions with hydrocarbon chains of CTAB, could dominate localization of OX since the localization of positively charged OX in the vicinity of the cationic groups of CTAB was less likely. In the second case, OX could be associated with the PS, such that the nonpolar component of the dye would interact with hydrophobic PS molecules while its polar head would remain in the solvent environment. To address the question of the localization of the dye within the NR chains which made a predominant contribution to SERS signals, a series of control SERS experiments were carried out with OX dissolved in several solvents; water, DMF and the DMF/water mixture (water content 20 vol. %). The experiments were carried out using a roughened gold substrate. The SERS frequency of the strongest OX band in the cm -1 region was dependent on the solvent type: in water, the SERS band was centered at 595 cm -1, similar to previously reported results, 121,122 and in DMF the spectral position of the peak was 567 cm -1 (Figure 3.4.). In the DMF/water mixture the SERS spectrum of OX featured two peaks located at 595 and 567 cm -1 (Figure 3.4). For the self-assembled NR chains, the main SERS peak of OX was measured at 563 cm -1, that is, very close to 567 cm -1, suggesting that OX was located in a DMF environment. We note that the absence of a shoulder at 595 cm -1 (Figure 3.4)

98 70 suggested that no appreciable interactions existed between OX and water. Therefore, it was reasonable to conjecture that we were probing OX predominately localized within the hotspot region between the ends of the NRs from which water is largely excluded. We note that this did not rule out the possibility of OX molecules being located in the hydrophobic environment in the CTAB layer. However, the species outside the hot-spots did not significantly contribute to the overall SERS signal. Furthermore, no change in vibrational frequency of OX at 563 cm -1 in the course of assembly indicated that the location of physisorbed OX remained unaltered. The relative intensities of the bands of OX at 563 and 604 cm -1 also remained constant throughout the process of self-assembly, suggesting that OX retained its orientation and geometry with respect to the NR surface without any appreciable molecular re-orientation. 124,125 Figure 3.4. SER spectra of oxazine 4 µm adsorbed on roughened gold substrate as a function of solvent environment (a) H 2 O, (b) DMF and (c) DMF/ H 2 O mixture containing 20 vol. % of H 2 O.

99 71 The change in the SERS intensity was determined over the course of NR self-assembly by using the intensity of the peak corresponding to DMF vibration at 659 cm -1 as an internal standard. Figure 3.3.b shows the variation in the ratio of intensities of the peak at 563 cm -1 of OX to the intensity of the peak at 659 cm -1, plotted as a function of. Importantly, the change in ensemble-averaged normalized SERS intensity was not monotonic: it increased for 1 3, leveled off when 3 5, and increased again for 5 8. To understand this non-linear behavior, we considered only SERS arising from the electromagnetic effect. Under resonance conditions, the incident light absorbed by the nanoparticles generates localized surface plasmons, thereby creating a strong local electromagnetic field, E loc (ω exc ), close to the surface of the NRs. This effect leads to the enhancement in intensity of the Raman scattered light by the OX molecule which is assumed to be a point dipole. The scattered light also excites localized surface plasmons and generates an enhanced field, E loc (ω RS ), at the Raman Stokes frequency. The field enhancement G SERS is proportional to the square of the product of the local field at the incident frequency and the local field at the scattered Raman Stokes frequency, that is: G SERS E loc (ω exc ) E loc (ω RS ) 2 (2) As discussed above, light extinction (absorption + scattering) at wavelengths matching the resonances of the nanostructure, generate LSPR that leads to field localization. Therefore, a correlation between the SERS efficiency and the product of extinctions at ω exc and ω RS should be expected.

100 72 We plotted the variations in the normalized SERS intensity and the product of the extinctions measured at the excitation wavelength of 785 nm (ω exc ) and at the wavelength of the Stokes-shifted radiation of 821 nm (ω RS ) versus the average aggregation number,, of the NR chains (Figure 3.5). In the course of NR self-assembly, the product of extinctions varied, since the spectral position of the longitudinal LSPR gradually red-shifted (see Figure 3.2). Figure 3.5. Correlation of the normalized intensity of SERS peak at 563 cm -1 (red circles) and the product of extinctions measured at 785 and 821 nm (blue circles), plotted as a function of the aggregation number of the NR chains. Top: y errors of the intensity of SERS peak (red squares) and the product of extinctions (blue squares) were calculated based on three measurements. Figure 3.5 shows a strong correlation between the variation in ensemble-averaged SERS intensity and the product of extinctions, plotted as a function of. Such correlation

101 73 indicated that the variation in SERS properties with NR assembly, indeed, originated from the inherent electromagnetic properties of the self-assembled nanostructure, rather than from chemical effects. In addition, the results shown in Figure 3.5 suggested a narrow distribution of hot-spots, confirming the high level of organization of the dynamic selfassembled system in solution. We note that a direct correlation between extinction and SERS is qualitatively followed in simple nanostructures such as individual gold spheres and organized arrays, 59 but it falls apart dramatically in strongly coupled random systems with a large distribution of spatially localized resonances. 60 Therefore, the results from Figure 3.5 confirm the high level of organization of our system Finite-difference Time-domain Simulations In order to further highlight the relationship between SERS enhancement and the dynamic evolution of hot-spots, we conducted comprehensive FDTD simulations by numerically solving Maxwell s curl equations by iteration over time. 131 Figure 3.6 shows examples of electric field (E field) profiles corresponding to different wavelengths for the chains of co-linearly assembled NRs. The field inside individual NRs rapidly decayed, while hot-spots between adjacent NRs exhibited a maximum E field intensity 4000-fold greater than the intensity of the incident field.

102 74 Figure 3.6. Three-dimensional finite-difference time-domain (3D-FDTD) simulation of the end-toend assembly of gold NRs. Electric field profile was calculated at the resonance wavelength of the co-linear NR chain at (a) 760 nm, (b) 782 nm, and (c) 802 nm. Polarization of the incident light is parallel to the long axes of the NRs (i.e., to the z-coordinate). Hot-spots between adjacent NRs show a maximum electric field intensity 4000 times greater than the incident field. In our work, the self-assembled NR chains were characterized by the distribution of their aggregation numbers and the variation in the angle between adjacent NRs within the chain. In order to examine the role of the distribution in of the NR chains, we performed FDTD simulations for chains with different lengths, and then used experimentally determined aggregation statistics to calculate the E field intensities (Figure 3.7). For = 8 the sum of E field intensity squared had the largest value when normalized with respect to the aggregation number of the chain (Figure 3.7a), which was consistent with experimental results shown in Figure 3.3. When the populations of NRs are not mixed (ideal case) the longer chains had a lower maximum intensity, they also had less variation in their maximum intensity wavelength, compared to the shorter chains (Figure 3.7b). The highest E field intensity observed when = 8 may be due to the relatively high spectral purity at this degree of the assembly. When the distribution in the aggregation numbers was not taken into account, the field intensity was highest for chains containing three NRs when normalizing to the number of NRs in the chain (Figure 3.7b). This effect originated from the trade-off between

103 75 the local field enhancement and optical absorption (that is, loss) in the NR chains. A figure of merit that compares the local field enhancement to the loss is the ratio between the real and imaginary parts of the relative permittivity of the NRs. For gold, this figure of merit is at maximum at ~770 nm, which was close to the resonance wavelength of the trimer. Figure 3.7. FDTD simulation showing (a) Electric field intensity squared obtained from incorporating average NR aggregation number, as a function of wavelength (factoring in experimentally determined statistical data) (b) Normalized sum of electric field intensity squared over a small volume enclosing the NR chain, for ideal NR chain lengths (Standard deviation is equal to zero) ranging from 1 to 9 NRs as a function of wavelength. (c) Sum of electric field intensity squared over a small volume enclosing NRs, chain lengths (number of NRs ranging from 1 to 9) as a function of wavelength (not normalized). (d) Peak electric field intensity squared values plotted

104 76 against their corresponding resonant wavelengths. Number of NRs increases from 1 to 9 (left to right). Since the SERS and LSPR properties of the NR assemblies depend on the co-linearity of the NRs with respect to the long axis of the chain, 32,33,48 we have performed FDTD calculations for dimers and trimers of NRs with different orientations with respect to each other (Figure 3.8). The deviation from co-linearity at the angles between the long axes of the NRs of 20, 40, 60 and 90 o resulted in a significant reduction of extinction and thus, decrease in E field intensity (Figure 3.8). This result implied that in ensemble measurements, the greatest contribution in extinction and SERS arose from co-linear chain conformations, with a minor influence on E field intensity from off-axis NRs.

105 77

106 78 Figure 3.8. Three-dimensional finite-difference time-domain (3D-FDTD) simulation showing examples of electric field profiles for end-to-end assembled gold NR dimmers and trimers. Polarization of the incident light is parallel to z-coordinate. Angular variance of (a) 0 degrees (b) 20 degrees (c) 40 degrees (d) 60 degrees (e) 90 degrees (f - i) Calculated absorption, scattering, extinction cross sections and electric field intensity squared respectively of various angled NR dimmers and trimers as a function of wavelength. Electric field strength between adjacent NRs decreases as angle between adjacent NRs increases. For the calculation of scattering and absorption cross-sections, we employed the TFSF method for the separation of the scattered field from the incident radiation. Our calculations showed that the extinction cross-section mainly originated from NR absorption, whereas the contribution from scattering was minor (Figure 3.9).

107 79 Figure 3.9. Calculated absorption, scattering and extinction cross sections as a function of wavelength for various NR chain lengths ((a) to (c) respectively) and average NR aggregation number ((d) to (f) respectively). A total-field scattered field (TFSF) source is utilized for calculating the scattering and absorption cross-sections. Incident field polarization is parallel to the major rod axis (i.e. z), the bandwidth of source is from 600 nm to 1000 nm. Simulation domain is terminated

108 80 with perfectly matched layer (PML). A mesh override region of (1 nm x 1nm x 1nm) mesh size is defined for better modeling of the circular rods in Cartesian coordinates. A 3-D time domain monitor is utilized for recording the field strengths as a function of time and a Fourier transform provides the frequency domain results. Extinction cross-section were calculated for different NR chain lengths and a certain factor (see main text) from each curve was added according to the experimental statistical data to lead figure (f). The heterogenity of NR chain size at each stage of the assembly is one of the contributing factors to variations in the observed amplitude. Experimental and calculated results obtained for the extinction (Figure 3.9) and variations in the spectral position of the longitudinal LSPR (Figure 3.10a) were in good agreement when NR number distribution was considered. Figure 3.10b shows the change in the calculated extinctions (product of 785 and 821 nm) and E field intensity squared as a function of. A strong correlation between the two trends was consistent with the relationship expressed in Equation (2). Significantly, the experimental results (Figure 3.5) and theoretical results shown in Figure 3.10(b) were found to be in good qualitative agreement. Figure (a) Variation in experimentally measured (blue circles) and calculated (red squares) spectral position of the longitudinal LSPR plotted as a function of the average aggregation number,, of the NR chains. (b) Variation in the calculated product of the squares of the electric field

109 81 (intensity) at 785 nm and 821 nm (red circles) and the product of the extinction cross-sections at 785 nm and 821 nm (blue circles), plotted vs Summary and Conclusions Following dynamic generation of hot-spots via controlled, solution-based self-assembly of gold NR chains, (1) we have established a direct relationship between extinction and SERS properties of the chains. An important aspect of our work included the formation of ensembles with well-defined, invariant distance between adjacent NRs. (2) Our FDTD calculations showed that as NR chains get longer (Xn > 3) the sum of E-field intensity squared decreases due to a trade-off between the local field enhancement and optical absorption in the NR chains. However, measured ensemble-averaged SERS intensity was the highest at = 8 as a consequence of a reduced NR population spread resulting in higher spectral purity. (3) Factoring in the observed chain length distribution over time, it was possible to reconcile observation and calculation data. (4) The observed ensemble-averaged SERS signals were intentionally modest in order to probe the dynamic generation of hot-spots in solution state assembly. This was achieved by carefully balancing experimental parameters. Our work opens the way for studies of optical properties of other geometry-dependent dynamic plasmonic systems, e.g., ensembles of side-by-side gold NRs and compartmentalization of molecules or particles using hot spots. Building from the correlation between SERS and extinction, practical applications of self-assembled structures ranging from chemical and biological sensing to nanoelectronics, e.g., plasmonic circuits, become a step closer.

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115 87 Chapter 4 Probing Side-by-side Assembled Gold Nanorods via Ensemble-averaged SERS There is often a misconception that aggregates of metal nanoparticles are better surface-enhanced Raman scattering (SERS) probes than individual nanoparticles. We show that for asymmetric particles such as gold nanorods (NRs) this is not always the case as the plasmonic behavior of NR ensembles depends on the architecture of the ensemble, that is, on the mutual orientation of the NRs. We report experimental and theoretical analysis of the optical properties of side-by-side assembled gold NRs. Extinction measurements and ensembleaveraged SERS spectroscopy showed a blue shift of the surface plasmon resonance and a reduction of SERS intensity respectively. Comprehensive Finite-Difference Time-Domain (FDTD) simulations showed a reduction of electric field intensity as the number of NRs per cluster increased due to the radial component of electric field cancellation leading to destructive interference. A further understanding of a blue shift of the surface plasmon resonance in the side-by-side assembled NR structures is explained by the propagation constant of the surface plasmon mode. The present work expands our understanding of configuration-specific optical behavior of gold NRs in a solution state. Furthermore, this study offers guidance towards

116 88 the establishment of design rules for the development of colloidal NR systems for plasmonic sensing applications Introduction The utilization of light-metal interactions on the nanoscale has shown great promise in nanoantennas, 1-5 extraordinary transmission, 6,7 and plasmonic waveguides, 8-10 all achieved through precise top-down control of geometry, size and composition of metallic nanostructures. In parallel to these fabrication approaches, bottom-up methods for employing colloidal metallic nanoparticles (MNPs) are receiving increasing attention, because of their relative low cost and potential use for bio-sensing in-vivo. One of the important applications of the plasmonic properties of MNPs is their utilization in surface-enhanced Raman scattering (SERS). SERS provides structural information about analytes adsorbed on the surface of MNPs and exceptionally high sensitivity as compared to ordinary Raman scattering The amplification of Raman intensity arises from local electromagnetic fields resulting from the surface plasmon resonance (SPR) of MNPs. The magnitude of the Raman scattering from analytes adsorbed or close to the surface of MNPs is approximately proportional to the fourth power of the local field when the electromagnetic mechanism of SERS is considered. 11,14,15 The spectral position of the SPR of MNPs can be tuned by varying the dimensions and shapes of MNPs, along with the nature of the medium surrounding them. There is also significant interest in ensembles of MNPs, owing to the coupling of plasmons of adjacent particles. 9,16 Structural, configuration-dependent, plasmonic behavior of ensembles of

117 89 MNPs can affect both the intensity and the spectral position of SPR wavelength, thereby providing greater insight into the plasmon coupling phenomena. Gold NRs are of particular interest because of their intrinsic shape-anisotropy. They exhibit transverse and longitudinal SPR modes corresponding to the coherent electron oscillations perpendicular and parallel to the long NR axis, respectively. 17 This NR feature offers spectral tunability of the longitudinal SPR in the near-ir region, thereby offering potential applications in the biological window range. 18,19 In addition, in their ensembles, gold NRs can exhibit a well-defined mutual alignment in a side-by-side or end-to-end manner. The plasmon coupling between gold NRs in their ensembles depends not only on the interparticle distance but also, on the mutual orientation of the NRs with respect to each other. Therefore, gold NRs are well-suited to study the optical properties of aggregated NP systems. To date, the majority of experimental studies of SERS properties of clusters of MNPs have been carried out for irregular, aggregated systems. These aggregate structures led to significant variability of their optical properties. As a result, there remains an insufficient understanding of the influence of the architecture of aggregates of MNPs on their SERS properties, particularly in a solution state. As such, research has focused on the study of isolated NP aggregates. Specifically, the optical properties of isolated dimers of NRs in a dry state, that were arranged in the end-to-end, side-by-side, L-shape or T-shaped manner have been investigated via single-particle scattering measurements. 20,21 The experimental results showed that the scattering intensity of dimers is dependent on the polarization direction of the incident light, so that the maximum intensity is reached when the polarization is parallel to the long axis of the NRs. These experimental findings were in

118 90 good agreement with calculations of plasmon hybridization and Finite-Difference Time- Domain (FDTD) simulations. 20 A study of the effect of the angular orientations of NRs in the dimers fabricated by electron beam lithography with separations no smaller than 20 nm showed plasmon coupling dependence on NR orientation, separation, induced dipole strength and the dielectric constant of the medium. 22 Single-particle measurements of scattering, extinction and SERS of NR dimers and a theoretical explanation of geometry-specific NR assembly 23 have provided deeper insights into the optical behavior of isolated NP aggregates. However, the understanding of the properties of ensemble-averaged system behavior, factoring in aggregate populations, SPR shift and E field distribution in a dynamic solution state, remains challenging. A small piece of this puzzle has been addressed in Chapter 4, where the optical properties of ensembles of NRs arranged in an end-to-end manner were explored. 24 We studied the highly localized electric field (E field) intensity regions ( hot spots ) that were generated between the ends of adjacent NRs. 25,26 We showed a direct correlation between extinction and ensemble-averaged SERS as a function of the average aggregation number of NR end-to-end assembly. 24 The calculated, normalized E field intensity showed a decrease as the average length of NR chains increased, which may be due to a trade-off between the loss and the local field enhancement. For this end-to-end configuration where the E field is oriented along the NR axis, the near E fields couple in a manner analogous to a bonding interaction resulting in a red-shift of the longitudinal SPR. 20,21 The side-by-side assembly of gold NRs has been achieved via attractive forces between the ligands coating the long side of the NRs, namely, chelating agents, 27 an antibody of a toxin molecule (microcystin-lr), 28 and the addition of anions via electrostatic

119 91 interactions (citrate). 29 Currently, a single experimental study exists that reports on the solution state SERS properties of side-by-side assembled NRs. This study showed a 10-fold increase in intensity of SERS signals by using a resonant dye as compared to the SERS of the individual NRs. 29 Additionally, 3D multilayered stacks and 2D sheets of side-by-side assembled NRs are highly SERS active. 27,30,31 We note that 3D and 2D superstructures give a distinct E-field distribution and consequently, different SERS properties as compared to side-by-side assembled clusters in solution. For example, a micrometer-sized 3-D NR stack consisting of ~15 layers of NRs can be realized as a highly efficient SERS substrate. The simulated E field intensity is higher for the 3 layered NR stacks as compared to the 1 layered. This may be due to the field enhancement in the gap between the layers. 31 Another report showed that a side-by-side assembled sheet-like micrometer -sized structure (2D) showed a high SERS activity likely due to the very large population of constituent NRs thus, producing larger number of hot-spots 27. We hypothesized that in clusters of side-by-side aligned NRs, since plasmons of NRs interact with each other in an anti-bonding mode (leading to a blue shift of the longitudinal SPR of the NRs in their ensembles), the local E-field should decrease (Figure 4.1). Consequently, the intensity of SERS should also decline, if it originates from electromagnetic effects. To test our hypothesis, we examined theoretically and experimentally the optical properties of gold NRs assembled in the side-by-side fashion. We simulated E-field distribution and extinction (absorption + scattering) of the clusters of side-by-side assembled NRs via the Finite-Difference Time-Domain (FDTD) method. We numerically solved Maxwell s curl equations by iteration over time. 32 Experimentally, we

120 92 examined the extinction and ensemble-averaged SERS properties of the NR clusters following their assembly in a solution state. Figure Schematic illustration of gold nanorods (NRs) assembled in a side-by-side manner showing a reduction of electric field as the number of NRs increases in the NR ensembles.

121 Results and Discussion Finite-difference Time-domain Simulations Figure 4.2 (a-c) shows FDTD calculations of the change in the normalized absorption, scattering and extinction cross-sections, all plotted as a function of wavelength for NR assemblies in side-by-side conformations containing from 1 to 8 NRs. For the complex permittivity of gold, we used the experimental data of Johnson and Christy. 33 The simulation domain was terminated by perfectly matched layer to ensure minimum reflections. Total field scattered field (TFSF) source was used for the determination of absorption, scattering and extinction cross-sections of assembled NRs. TFSF divides the simulation domain into two regions. In one region, only scattered fields are present while the other region contains both the incident and scattered fields. Scattering cross-section was determined by calculating the Poynting vector over a closed surface surrounding the NRs in the scattered field region. Absorption cross-section was calculated by determining the net flow of power into a closed surface surrounding the NRs in the total field region. The incident plane wave was polarized at 45 degrees to the long axis of the NRs and as such, this polarization probes both the transverse and longitudinal modes. The mesh override region was defined with a mesh size of 1 nm for accurate modeling of the cylindrical structure in a Cartesian coordinate system.

122 94 Figure 4.2. Calculated normalized absorption (a), scattering (b) and extinction cross section (c), all plotted as a function of wavelength for NR assemblies containing from 1 to 8 NRs. Simulations were carried out using three-dimensional finite-difference time-domain (3D-FDTD) simulation Reduction of Electric Field Intensity The FDTD simulation showed a blue shift of the resonance wavelength from 779 to 653 nm, as the number of NRs per ensemble increased from 1 NR to 8 NRs, respectively. The resonance wavelength of a single NR or a stack of NRs arranged in a side-by-side manner, is determined by two variables: the propagation constant of the radial SP mode

123 95 and the phase of light reflection from the ends of the NRs. At resonance, the following condition must be satisfied: where L is the NR length, is the phase of reflection from the end of the NRs, and is the ref propagation constant ( where is the effective index of the system containing NRs and the surrounding medium). The analytical expression for the phase of reflection from flat-ended NRs has recently been derived, 34 whereas only numerical solutions for phase of reflection for NRs with hemispherical ends (common in colloidal systems) are currently available. (1) The value of is a geometry-dependent variable and it can be obtained from the mode shape of the radial SP wave. Figure 4.3 shows mode shapes of 1 NR and side-by-side assemblies of 2 and 3 NRs. By using modal solutions, we numerically verified that as the number of NRs increases in the ensembles, decreases, thereby reducing. Specifically, for 1, 2 and 3 NRs, the resultant was 11.81, 9.29 and 8.13, respectively, and the corresponding value of was , and m -1, respectively. This reduction occurred when the NRs are brought into close proximity to each other in the side-by-side ensembles. Consequently, the mode shape of interacting NRs changed as compared to an individual NR. The radial components of the E field of the neighboring NRs cancel each other due to destructive interference, resulting in a reduction of the local E field. In order to satisfy Equation (1), the reduction in must be compensated by a proportional amount of reduction in the resonant wavelength. Therefore,

124 96 a blue shift occurs as the number of NRs increases in side-by-side assembly. It should be noted that which is weakly wavelength-dependent to first order approximation. 34 Figure 4.3. Modes supported by side-by-side assembly of NRs. Mode shapes of surface plasmons of 1 to 3 NRs from left to right. The resulting effective index values are used for the calculation of propagation constant of surface wave in the different geometries. Fields are normalized to their maximum intensities. Figure 4.4 (a-e) shows examples of FDTD simulations of E field profiles of ensembles containing from 1 to 5 NRs at their corresponding resonance wavelengths. We note that the field intensity in the gap between two neighboring NRs is significantly smaller, compared to the field intensity for a single NR. Figure 4.4 (e) shows the normalized sum of E field intensity squared as a function of wavelength. As the number of NRs per stack increases from 1 to 8, the sum of the intensity squared decreases. The reduction of the field originates from cancellation of the radial component of the SP mode leading to the destructive interference shown in Figure 4.3.

125 97 Figure 4.4. (a-d) Examples of electric field profiles produced via 3D-FDTD simulation for ensembles of side-by-side assembled NRs. Polarization of the incident light is at 45 degrees to the long axis (z-coordinate) of NRs. (e) Sum of electric field intensity squared of ensembles containing a different number of NRs Side-by-side Nanorod Assembly We hypothesize, based on FDTD calculations that side-by-side assembled NRs will have reduced SERS due to reduced local fields, and we tested this hypothesis experimentally. We carried out side-by-side assembly of gold NRs in a solution state. The NRs capped with cetyltrimethylammonium bromide (CTAB) were synthesized following the procedure described elsewhere. 35 By exploiting the anisotropy of the NRs in shape,

126 98 surface energy and crystal facet, we carried out site-specific CTAB exchange at the NR ends with an SH end-terminated polystyrene (SH-PS) (the molecular weight of SH-PS was 12,000 g/mol and polydispersity index was 1.09). 39 These PS-functionalized NRs can be viewed as an amphiphilic building block with a hydrophilic long side and hydrophobic end-groups. Later in the text, we refer to the PS-functionalized NRs as NRs, unless otherwise stated. To these NRs dissolved in tetrahydrofuran (THF), a solution of Raman reporter, Cresyl Violet (CV) was introduced dropwise to a final CV concentration of 3 µm, and the solution mixture was equilibrated for 1 hr. We triggered the self-assembly of the NRs in a side-by-side manner by changing the quality of solvent for the ligands. We note that THF is a poor solvent for CTAB, and the stability of the NRs in THF was attributed to the polystyrene ligands (the values of the second virial coefficient, A 2, is mol cm 3 g 2, equivalent to Flory Huggins interaction parameter 0.4). 39 We added water to the THF solution of the NRs to a total concentration of water of 10 wt %, thereby reducing the solubility of the PS ligands. As a result, the NRs assembled in the side-by-side manner (Figure 4.5). Figure 4.5. Schematic illustration of side-by-side NR assembly. A thiolated polystyrene (SH-PS) is attached to the ends of cetyltrimethylammonium bromide (CTAB) coated gold

127 99 NRs in THF via site-specific ligand functionalization. After the addition of the Raman reporter, side-by-side assembly was triggered by the addition of water (10 vol. %) Extinction and TEM Analysis Figure 4.6 (a) shows photographs of the solutions of NRs following their selfassembly over time. The typical color of the solutions changed from reddish-purple to blue, due to an anti-bonding interaction 20,23 suggesting the side-by-side organization of the NRs. The side-by-side assembled NRs showed colloidal and temporal stability for 3 months. Figure 4.6 (b) shows the evolution of extinction spectra of NRs self-assembling in solution, plotted over the course of NR self-assembly. The consistent blue-shift of the longitudinal SPR peak indicated that the NR assemblies in solution were relatively monodisperse without the presence of peak broadening, typically seen with larger, irregular NR aggregates. The longitudinal SPR showed a blue shift from 770 to 715 nm, exhibiting the trend presented in the FDTD simulations. The blue shift occurred due to the parallel alignment of dipole modes of individual NRs. 20 This configuration has a higher energy, in comparison with the bonding mode, that is, the NRs in the end-to-end alignment. The transverse SPR showed a relatively small red shift from 510 to 519 nm also in agreement with the FDTD results. It should be noted that the relatively low intensity of the transverse SP modes, which interact attractively, leads to the observed red-shift. Representative scanning transmission electron microscopy (STEM) images of the NR assemblies in various stages of self-assembly are shown in Figure 4.6 (a). 40

128 100 Figure 4.6. (a) A photograph showing the typical change in color of self-assembling NRs in solution as a function of time (Top left). Representative scanning transmission electron microscopy (STEM) images of NRs in various stages of self-assembly. Scale bar is 15 nm (b) Variation in extinction properties of NR ensembles over time. A low magnification and additional STEM images of the NR assemblies are shown in Figure 4.8. Using image analysis, we determined that throughout the self-assembly process the average distance between the long sides of adjacent NRs remained at 1.97 ± 0.48 nm. The inter NR spacing was smaller than would be expected for four layers of CTAB ligands (~ 4 nm, assuming capping of the NR sides with a CTAB bilayer). 41,42 It is known that when the solvent evaporates, the NRs are brought together and CTAB molecules are forced to overlap or inter-digitate driving van der Waals interactions. 41,43-45 In addition, STEM presents projected inter-nr distance, rather than the actual distance. As such, since the majority of side-by-side NR stacks will not deposit on the substrate such that they are perpendicular to the incident electron beam, the observed interparticle distance may not reflect that present in solution. We also note that the precise geometry of the stacks in solution state may not be directly inferred by the STEM images.

129 101 By comparing the experimental and simulation results for the case of 1 NR, we observed a small difference in spectral position and relative intensity of the transverse SPR. The SPR modes are dependent on the direction of propagation of incident radiation. We explain this difference as follows. The SPR modes are dependent on the direction of propagation of incident radiation. The simulations were carried out for one direction of propagation incident wavevector (k inc ) which was perpendicular to the long axis of the NRs. This was not the case for the extinction experiments carried out for the NR solution. To verify the dependence on the propagation direction, we conducted simulations with k inc parallel to the NR axis that is z-propagation. Figure 4.7 shows a comparison of simulation results carried out with two different directions of propagation that is parallel (z-propagation) and perpendicular (y-propagation) to the long axis of NRs. These simulations were carried out for a stack of 2 NRs. It should be noted that for the case of the parallel propagation, only transverse SPR is excited. The results of absorption, scattering and extinction crosssections and the sum of intensity squared of 2 NRs showed the presence of the transverse SPR at 520 nm.

130 102 Figure 4.8. Representative scanning transmission electron microscopy (STEM) images of NRs in various stages of side-by-side assembly. Recorded on a Hitachi S-5200 scanning electron microscope operating in STEM mode. Note: as-synthesized NRs contain a small population of spheroids (~5%).

131 103 Figure 4.7. FDTD simulations showing absorption, scattering, and extinction of 2NRs per stack for y and z directions of propagation of incident radiation. When wave vector kinc axis, a peak at 520 nm is observed corresponding to the transverse SPR. is parallel to the NR We did not attempt to calculate the average aggregation number of the NR ensembles because it is well-known that upon evaporation of the solvent on a carbon coted TEM grid, gold NRs form stacks in which they are aligned in the parallel manner. In this case, NRs interact via van der Waals forces. 43 Based on Hamaker integral approximation, 46 assuming NR separation distance d is small ( where a is 1/2 of the diameter of the NR and L is the length of the NR), side-by-side configuration is preferred if the ratio of van der Waals potential for side-by-side and end-to-end configuration is greater than unity: 47 ( )

132 104 where and (where A is Hamaker coefficient ). In the case of NRs used in this study (L = 32 nm and a = 4 nm), the ratio is ~2.9. Therefore, the sideby-side alignment of the NRs driven by van der Waals interactions is possible upon solvent evaporation. While the extinction spectra shown in Figure 4.6 (b) indicate that the parallel arrangement of the NRs occurs in solution state, we were unable to differentiate these structures from those that occur upon drying in preparation for TEM imaging. Nonetheless, inspection of the TEM analysis showed a general trend in which the number of individual NRs decreased in the time course of the assembly process. For example, 24 h after triggering the assembly, we observed 57 % of single NRs, 28 % of dimers, 10 % of trimers and 3 % of tetramers, whereas for t = 216 h, the fractions of these species were 30 %, 44 %, 18 % and 8 %, respectively Ensemble-averaged SERS Figure 4.9 (a) shows representative spectra of the ensemble-averaged SERS of CV, measured over the course of side-by-side assembly of the NRs. These spectra were recorded concurrently with the extinction measurements. The most enhanced SERS bands of CV appeared at 535 and 595 cm -1. These signals were caused by in plane vibrational ring modes. 48 The band at 900 cm -1 corresponds to the ring breathing mode of THF. 49 This band was used as an internal standard. The position of the bands shown in the SERS spectrum were in accordance with the ordinary Raman spectrum of CV, which suggested that CV was physically adsorbed on the surface of the NRs. Figure 4.9 (b) shows the

133 105 variation in the normalized SERS peak intensity of CV at 535 and 595 cm -1, plotted as a function of self-assembly time. We observed a gradual reduction of both peak intensities of CV from t = 5 min to t = 216 h. However, the spectrum of the control system which contained the same amount of NRs and CV, without the assembly, showed relatively constant peak intensities at 595 cm -1 for the duration of 216 h. It should be noted that NRs used were from the same batch and all measurements were done in parallel in order to ensure the fidelity of the results. Next, we identified the local environment of the CV molecules that provided the main contribution to the overall intensity of SERS. We conducted control SERS experiments by adsorbing CV from THF or water on a roughened gold substrate. A solid gold electrode was roughened with 25 successive oxidation and reduction cycles from 0.3 to 1.2 V in an aqueous 0.1 M KCl working solution. A platinum wire was used as a counter electrode and the reference electrode was Ag AgCl KCl(sat). The roughened gold electrode was then isolated from the electrochemical cell and CV in THF or water was introduced for the SERS measurements. Figure 4.9 (c-d) shows solvent-dependent SERS spectra of CV. In THF, the spectral positions of the CV peaks (591 cm -1 ) were in concordance with those observed for CV co-assembled with NRs shown in Figure 4.9 (a). Whereas, in the water environment, CV exhibited significant peak variations (541 and 600 cm -1 ) as compared to the CV in THF. The observed changes may be due to H-bonding interactions between CV molecules and water, which cause the change in the average orientation of the CV molecules with respect to the surface of NRs. These results suggest that we are probing CV in a THF environment where water is largely excluded. This is most likely the hot-spot region which is located at the ends of NRs where the surface of curvature is the highest (Figure 4.4).

134 106 Figure 4.9. (a) Representative ensemble-averaged SERS spectra of Cresyl violet (CV), measured in the course of side-by-side assembly of the NRs as a function of time. The band at 900 cm -1 corresponds to THF which is used as an internal standard to normalize the SERS of CV at 535, 595 cm -1. (b) Normalized SERS intensity at 535 (red circle), 595 cm -1 (blue triangle) and control experiments without the assembly (black square, for SERS of CV at 595 cm -1 ) as a function of time. (c, d) SERS of CV on a roughened gold substrate in THF and water respectively. A 785 nm laser excitation was used.

135 Electric Field Distribution on Nanorod Ensembles To further support our finding of the reduction of SERS intensity arising from probing at the ends of NRs, we numerically examined location-specific E field variation at the ends of NRs. The normalized sum of E field intensity squared showed a decrease as a function of the number of NRs (Figure 4.10). This reduction of E field suggests that SERS of CV should decrease as the number of NRs per ensemble increases, based on considering only the electromagnetic effect of SERS which is the predominant factor in this case. Figure A sum over volume of the electric field intensity squared via FDTD simulations for various NR assemblies (number of NRs from 1 to 8) as a function of wave length (nm) (right figure). The total volume of the sum of E field intensity squared for the ends of NR ensembles show a decrease with increasing number of NRs. Blue: 1 NR, green: 2 NRs, Red: 3 NRs, light blue: 4 NRs, pink: 5 NRs, black: 6 NRs, dotted blue: 7 NRs, and dotted green: 8 NRs.

136 Summary and Conclusions The present study provides an important insight into the optical properties of sideby-side assembled gold NRs. We showed that as the number of NRs increase in the NR ensembles, the normalized sum of the E field intensity decreased. The observed reduction of E field intensity is due to the cancelation of the radial component of SP modes as a consequence of the side-by-side conformation. Calculated and measured extinction showed a blue shift of the longitudinal SPR resulting from the reduction of effective index as the number of NRs per ensemble increases. A previous study showed an increase of SERS in the assembly and followed the general assumption that aggregates are better SERS platforms. However, our experimental work showed the reduction of ensemble-averaged SERS signals which was in concordance with comprehensive FDTD calculations. Although, there has been significant progress in the control of colloidal side-by-side assembly of NRs which has been achieved by a variety of methods, the next challenge is to exploit the functionality of these plasmonic ensembles. While the observed spectral shift of SPR of NRs has found application in colorimetric based sensors, our results suggest that side-by-side ensembles may not be suitable as a highly sensitive SERS based platform. Fundamentally, this study expands our understanding of the interplay between geometry, assembly and the optical properties of plasmonic nanoparticles.

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141 113 Chapter 5 Surface-Enhanced Raman Spectroscopy in Hollow Core Photonic Crystal Fibers: a tool for exploring the surface chemistry of gold nanoparticles In this study, we have demonstrated the efficacy of hollow core photonic crystal fibers (HCPCF) as a platform for surface-enhanced Raman scattering (SERS) spectroscopy. SERS measurements carried out using this platform showed the capability to monitor minute amounts of ligands on the surface of gold nanoparticles. The SERS signals from HCPCF exhibited a 10-fold enhancement compared to that in direct sampling using a cuvette. Using exchange of cetyltrimethylammonium bromide (CTAB) with α-methoxy-ω mercaptopolyethylene glycol (SH-mPEG) on the surface of gold nanorods as an exemplary system, we showed the feasibility of using HCPCF SERS to monitor the change in surface chemistry of nanoparticles.

142 Introduction Gold nanoparticles (NPs) have a broad range of applications owing to their intrinsic electromagnetic properties and the chemistry of their surfaces. In one important application, gold NPs are used as surface-enhanced Raman scattering (SERS) substrates for biomedical applications. 1,2 SERS offers exceptionally high sensitivity and can provide structural information (vibrational properties) of analytes adsorbed on noble metal substrates. SERS addresses a number of limitations of fluorescence-based sensing of analytes, such as photobleaching, sensitivity and the broad emission spectrum of organic dyes which hinders multiplexing. 2,3 The root cause behind the high sensitivity achieved by SERS arises from the enhancement of the local electromagnetic fields of metal NPs, due to excitation of localized surface plasmons. 1,4 Modifying the surface of NPs with mixtures of ligands is essential in rendering NPs functional, stable, and target-specific. In particular, a thorough understanding of the surface chemistry of gold NPs is fundamental to their efficient application in materials science and biomedical fields. Using gold NPs as a biological SERS probe requires control of their surface chemistry in order to make them biologically compatible. For example, because as synthesized gold nanorods (NRs) are coated with cytotoxic CTAB, an exchange with biocompatible ligands is required. 5 Current techniques used to identify the presence of surface ligands on NPs include fluorescence displacement methods, Fourier transform infrared (FTIR), reflection absorption infrared (RAIR) spectroscopies, contact angle measurements and transmission electron microscopy. 6,7 These techniques are either destructive, or have insufficient sensitivity for low concentrations of surface ligands. Although SERS measurements conducted for colloidal

143 115 gold NPs offer a sensitive and nondestructive alternative method, full exploitation of local electric field enhancement is commonly achieved using aggregates of gold NPs. 8,9 In addition, using low concentrations of NPs in a solution state can be challenging, due to the commensurate poor signal to noise ratio. In order to overcome these barriers, enhanced analytical approaches are needed. Hollow core photonic crystal fibers (HCPCF) serve as a promising platform for various sensing applications. The waveguide characteristics and accessibility to the hollow cladding channels of HCPCF have opened up many possibilities for chemical and biochemical sensing utilizing fluorescence, 10 Raman scattering and SERS The application of HCPCF can result in the enhancement of the Raman signal strength by approximately two orders of magnitude. 18 Initial SERS measurements in HCPCF have been conducted and showed a significantly higher sensitivity by incorporating colloidal solutions of Au or Ag NPs either by being coated on the inner surface of the fiber hollow channels or mixed with a solution and loaded within a limited segment of the HCPCF In this chapter, we discuss the feasibility of a non-destructive and direct route to identifying minute amounts of ligand adsorbed on the surface of gold NRs via SERS in HCPCF. As an initial demonstration of SERS enhancement in HCPCF, adsorption of Congo Red onto NRs is studied and the limit of detection is determined. We also show the suitability of HCPCF as a SERS probe to monitor a ligand exchange process on the surface of NRs by investigating PEGylation of colloidal NRs.

144 Results and Discussions Experimental Set-up The schematic diagram of the experimental setup is shown in Figure 5.1. The excitation source was a 632 nm HeNe laser in conjunction with a JY Horriba HR800 LabRam Raman spectrometer. The excitation light was coupled into the core of HCPCF from the top end through an objective lens, while the other end of the HCPCF was immersed into the solution of NRs. Figure 5.1. Schematic illustration of experimental set-up. A hollow core photonic crystal fiber (HCPCF) filled with gold nanorod (NRs) solution.

145 117 The cladding holes were sealed at the probing end of the fiber using the fusion splicing technique. As such, the solution studied only filled the central core of the fiber via capillary forces. The periodic air holes of the cladding confined the excitation laser inside the core of the HCPCF through both photonic bandgap effects and total internal reflection. The wellconfined excitation interacted directly with the sample, while propagating along the length of the HCPCF. The back-scattered Raman from the sample propagated through the fiber back to the top end and was collected through the same objective lens. The 50 objective was chosen with a numerical aperture that best matched that of the HCPCF to provide maximum coupling and detection efficiency. The HCPCF (Crystal Fiber model HC1060) had a 9.5 µm diameter core, surrounded by a silica microstructured cladding with an air fill fraction > 90 %. The wavelength region where photonic bandgap guiding was present for this fiber coincides with the HeNe laser wavelength, when filled with aqueous solutions. Minimal interference from the Raman spectrum of the silica of the HCPCF was observed. All the measurements reported here were carried out using 7.0 cm-long segments of HCPCF. CTAB-coated gold NRs with an average length and width of 38.4 ±2 nm and 8.8 ±0.6 nm respectively prepared by the method reported elsewhere 19 and used as a SERS substrate. Transverse and longitudinal localized surface plasmon resonance wavelengths were centered at 510 and 773 nm, respectively.

146 Examination of the Limit of Detection of CTAB coated Gold Nanorods Figure 5.2.A shows SERS spectra of the 14.4 nm aqueous solution of the NRs obtained by focusing the laser beam onto the core of a core-filled HCPCF and by focusing the laser beam directly into the NR solution in a cuvette. The detected signal from HCPCF was nearly 40 times stronger than that from the direct sampling of NRs in the cuvette. The enhanced band centered at 178 cm -1 was attributed to the Au-Br vibrational mode that originates from the CTAB capping molecules of gold NRs. 20 We used this Raman mode to determine the limit of detection of SERS of CTAB coated NRs in HCPCF by varying the concentration of the NRs shown in Figure 5.2.B. We found the limit of detection was 0.14 nm for the solution of CTAB coated gold NRs. Figure 5.2. (A) SERS spectra of CTAB coated gold NRs detected through direct sampling in a cuvette and core-filled HCPCF. (B) Variation in the normalized SERS peak intensity measured at 178 cm -1 plotted as a function of concentration of CTAB coated gold NRs (the concentration of the NRs were determined by extinction measurements). 21 SERS variation (y error) is based on 3 measurements.

147 Determination of the Enhancement Factor A Raman reporter, Congo Red (3 µm) was introduced to a NR solution and used to study SERS enhancement in HCPCF. Figure 5.3 (A and B) shows the SERS spectra of Congo Red, which were acquired from the core filled HCPCF and direct sampling from a cuvette respectively. In order to verify that the detected signal is caused by SERS rather than ordinary Raman scattering, a Raman spectrum of Congo Red of 560 µm was acquired, as can be seen in Figure 5.3.C. No discernable vibrational modes of Congo Red were detected from that Raman spectrum. Figure 5.3. SERS spectra of 3 µm Congo Red molecules by using (A) core-filled HCPCF (B) direct sampling from a cuvette. (C) Ordinary Raman spectrum of Congo Red molecules at the concentration of 560 µm. The spectra have been separated vertically for clarity.

148 120 A comparison of the spectra presented in Figure 5.3 shows that with the use of HCPCF, the noise level was considerably reduced and well-resolved Raman modes of Congo Red were revealed. The resolved peaks of Congo Red matched its characteristic peaks reported in the literature The bands around cm -l region are due to N=N stretching mode and naphthalene stretching modes and 1598 cm -1 is assigned to phenyl and naphthalene ring modes. In addition, the peak height of the 1167 cm -1 Raman mode (corresponding to phenyl-n vibrational modes) obtained from the SERS in HCPCF was enhanced by 10-fold, as compared to the spectrum acquired by direct sampling in the cuvette. We attribute the signal enhancement to the increased interaction length and efficient collection of the Raman scattering signal, via the bandgap effect over this extended interaction length. By confining both the excitation laser and the liquid sample along the length of the HCPCF, a larger length for light-matter interaction was achieved compared to that associated with the conventional Raman spectroscopy scheme. In direct sampling, the excitation laser was focused directly into the NR solution, therefore the effective interaction volume was limited by the spot size of the pump laser and the depth of field. Raman scattering is omni-directional, and due to the limited numerical aperture of the collecting objective lens, in this optical arrangement only a fraction of the scattered Raman signal could be collected. Since the scattered Raman wavelengths were only slightly shifted from the excitation laser wavelength, with the use of HCPCF, Raman signal propagation was also confined inside the central core of the fiber and was collected more efficiently by the objective lens. The collection efficiency of HCPCF can be viewed as the solid angle that collects the Raman signal inside the fiber. It is governed by the numerical aperture of the fiber or the refractive index difference between the core and the cladding. Since the

149 121 effective refractive index of HCPCF cladding is close to 1, a greater collection angle is obtained from HCPCF, in comparison with other capillary waveguides, such as Teflon capillary tubes Study of Exchange of cetyltrimethylammonium bromide (CTAB) with α-methoxy-ω-mercapto-polyethylene glycol (SHmPEG) on Gold NRs To further highlight the sensitivity of the proposed HCPCF SERS platform, we investigated a ligand exchange process on the surface of NRs by replacing CTAB with SHmPEG (molecular weight 12,000 g/mol). Gold NRs are considered to be suitable as a SERS probe for biological applications, due to the spectral position of the localized surface plasmon resonance, which is located in the near-ir region. However, CTAB is cytotoxic requiring ligand exchange with bio-compatible ligands for applications in biology. 25 In particular, replacement of CTAB with SH-mPEG provides biocompatibility, reduced enzymatic degradation, non-immunogenicity and stability in both highly ionic media and in blood circulation systems. 26 Surface replacement of CTAB occurs as a result of chemisorption of the thiol moiety of PEG (the binding energy of sulfur to gold is approximately 167 kj mol -1 ). 27 Figure 5.4 shows the variation of SERS spectra of CTAB as a function of the concentration of SH-mPEG in solution. The solution of SH-mPEG (with concentrations of 20, 50, and 100 µm) was introduced into the solution of CTAB-coated gold NRs, so that the final concentration of the NRs was maintained at 0.54 nm. We emphasize that at this concentration of NRs they are not detectable via direct solution

150 122 sampling in a cuvette. In contrast, via HCPCF SERS, we observed that as the concentration of SH-mPEG increased, the intensities of CTAB Raman modes at 178, 1155, 1387, and 1512 cm -1 sequentially decreased indicating the gradual replacement of the CTAB capping layer with the SH-mPEG polymer (Figure 5.4). We estimated that the surface area of an individual NR is 1300 nm 2, based on the assumption that the NR has two hemispheres on the ends of the cylinder. The area per molecule of CTAB and SH-mPEG (assuming a brush-like configuration) is 0.22 nm 2 and 10.8 nm 2 respectively. 28 Therefore, each NR would carry approximately CTAB (assuming a bilayer) and PEG molecules on its surface, representing of CTAB and of PEG molecules at a 0.5 nm concentration of NRs. Figure 5.4. Normalized SERS spectra of CTAB coated gold NRs as a function of SH-mPEG concentration (A) CTAB coated NRs as a control system (B) 20 µm of PEG (C) 50 µm of PEG (D) 100

151 123 µm of PEG. The peak at 103 cm -1 was used to normalize the peaks. The spectra have been separated vertically for clarity nm of NRs were used. This result demonstrated the capability of the HCPCF SERS probe to effectively monitor the changes of the surface of NRs that cannot be detected using direct solution sampling. In contrast to other techniques, we emphasize that this extremely low concentration of NRs was not detectable via direct solution sampling methods due to poor signal to noise ratio. Further, the minute amounts of solution (sampling volume is approximately cm 3 ) that can be analyzed may offer advantages in biological sensing including forensics. By using this fiber methodology, there is the potential for portable sensing and ultra-compact devices.

152 Summary and Conclusions In this study, we demonstrated an efficient platform for SERS spectroscopy by using a core filled HCPCF. We achieved a 10-fold enhancement in the SERS signal in HCPCF, compared to that achieved by direct sampling in a cuvette. Therefore, this platform can be applied as a useful method for enhanced detection of vibrational modes of chemical and biological molecules. The great potential of HCPCF for optical sensing originates from the increased light-matter interaction volume and the efficient accumulation of SERS scattering along the extended length of the HCPCF. Using the exchange of CTAB with SH-mPEG on the surface of gold NRs as an exemplary system, we showed the feasibility of using HCPCF SERS to monitor the change in surface chemistry of NRs, which can be extended to studies of in-situ cytotoxicity of different kinds of NPs.

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155 127 Chapter 6 Lamellar Envelopes of Semiconductor Quantum Dots Elements reprinted with permission from Journal of the American Chemical Society, 131, 10182, Copyright (2009) American Chemical Society. The study presents the solution-phase formation of ordered lamellar nanocrystal (NC) arrays. These semiconductor lamellae exhibit structural integrity and temporal stability, without the need for chemical crosslinking. While they can be micrometers in diameter they are typically only two to three NC layers thick. These structures are capable of carrying a cargo of water-soluble ions, molecules, metal nanoparticles or biomolecules. Notably, the photoluminescence of the host CdSe NCs is enhanced by the encapsulation of gold nanoparticles within the lamellae. Critically, these NC envelopes are easy to prepare yet their properties can be modulated through the integration of a vast array of watersoluble species. This approach marks a step toward ordered, compartmentalized, NC-based complexes with controlled architectures.

156 Introduction In many fields of science there is immense interest in organization of complex nanoscale suprastructures, both natural and synthetic. Nanoscale building blocks such as polymer molecules, colloidal particles, surfactants, proteins, and liquid crystals can conceptually be viewed as atoms or molecules that, when assembled, form the basis of new classes of materials. 1-7 As building blocks, colloidal semiconductor nanocrystals (NCs) are interesting materials because of their particular size, shape and composition-dependent optical and electronic properties The organization of NCs into large arrays is of interest both fundamentally and practically. Controlled assembly of NCs has been pursued by a number of techniques including the three layer oversaturation technique for the formation of three-dimensional NC superlattices 11 and by the assembly of free-floating monolayer NC sheets by electrostatic and hydrophobic interactions in conjunction. 12 NC assembly has also been accomplished at liquid/liquid interfaces where reduction of interfacial energy drives the formation of NC monolayers at the spherical surface of micrometer-diameter droplets. 13,14 Controlled evaporation of a NC suspension onto a substrate has produced some of the most highly ordered NC superlattice structures to date. 15,16

157 Results and Discussion Formation of NC Lamellae The CdSe NCs (mean diameter 4.0 nm) and CdSe bullet-shaped nanorods (mean length 19 nm, width 11 nm) (Figure 6.1) were capped with trioctylphosphine oxide (TOPO) molecules whose hydrophobic tails stabilize nanoparticles in nonpolar solvents. Figure 6.1. (A and B) Scanning transmission electron microscopy (STEM) images of colloidal CdSe QDs and CdSe bullet-shaped nanorods as controls, deposited from toluene solution onto carbon coated TEM grids, exhibiting the typical short range order produced by evaporation. Addition of a controlled amount of water to a suspension of NCs in toluene, followed by sonication, led to the formation of pancake-shaped lamellae. After 20 s sonication, three distinct phases were observed: the majority phase consisting of large disk-like quantum dot (QD) lamellae (mean diameter 20 μm), smaller droplets coated with QDs, and larger disordered aggregates (Figure 6.2.A). At shorter (less than 10 s) sonication times two distinct phases were observed: QD-coated droplet-like structures (the major phase) (Figure 6.2.B) and the QD lamellae. Given the inherent shallow depth of focus of confocal

158 130 fluorescence microscopy, our observations clearly differentiated the flat, pancake-like morphologies from the surrounding droplets which consistently exhibited the strongest fluorescence intensity at their periphery. Figure 6.2. (A) Liquid state confocal fluorescence microscopy image of CdSe NC lamellae formed by the addition of 10% (v/v) water with subsequent 20 sec sonication. (B) Confocal image of the same preparation as (A) at less than 10 s sonication time. Confocal images were recorded using an oil immersion lens (excitation at 364 nm, detection 550 to 600 nm). (C) Solution state Wet cell BSE image showing the existence of large lamellar structures (see 1) in solution along with small droplets (see 2) whose greatest signal exists at its periphery. (D) Solution state Wet cell BSE image overview of large lamellae along with disordered aggregates. (E) Image intensity profiling of a lamella (inset) showing uniform intensity consistent with a disk- or sheet-like structure.

159 131 To confirm the liquid state fluorescence microscopy results, we carried out wet-cell scanning electron microscopy (SEM) where solution-state samples ( 15 μl), contained within a sealed chamber, were viewed through a polymer based electron transparent window. Backscattered electron (BSE) imaging was used. BSE images, where signal intensity is atomic number-dependent, clearly showed CdSe NC lamellae, droplets, and disordered aggregates (Figure 6.2.C and 6.2.D). Energy dispersive X-ray spectroscopy (EDS) of a single lamellar structure, in solution, confirmed the presence of Cd and Se. Additionally, a BSE video was recorded showing the existence of the lamellae floating in solution. BSE signals are not only atomic number dependent but also sensitive to sample thickness. Image intensity profiling across a single lamella revealed a uniform, flat, greyscale value for the structure (Figure 6.2.E), which were in qualitative agreement with those observed in confocal fluorescence microscopy. The lamellae were found to be temporally stable as indicated by solution samples imaged 3 months after original selfassembly experiments which contained intact lamellae. Interestingly, the lamellar structures are not unique to CdSe NCs but were also formed using PbS NCs capped with oleic acid and CdTe NCs capped with tetradecylphosphonic acid. The formation of NC lamellae was found to be dependent on two main factors; sonication time and the percentage water added. The effect of sonication time was examined by varying it from 5 to 180 s at a fixed sonication frequency (42 khz ±6 %) and energy (70 W). For times shorter than 10 s, assembly of NCs was dominated by the formation of micrometer-size droplets, similar to those produced by mechanical agitation, 14 with the exception that the average droplet size in this work was measurably smaller. At sonication times in the range s, lamellae were the predominant

160 132 structures. The transition from droplets to lamellae is reminiscent of intermediate disk-like pancake structures induced by sonication of liposomes. 17 At longer sonication times, greater than 150 s, we observed small, dense spheroidal structures and disordered precipitates of NCs. With respect to water concentration, we observed that at 5 vol % water and below, small, poorly ordered disk-like structures formed, which had no structural integrity. Under these conditions, the absence of lamellae could be statistical: although lamellae might form, they were present in too small numbers to be readily detected by microscopy. In contrast, addition of greater than 20 vol % water produced disordered aggregates and dense spheroidal NC structures. Under these conditions, it is likely that at a constant concentration of NCs, the number of NCs per unit area of toluene/water interface decreases, resulting in incomplete interfacial coverage and structural instability of the lamellae. Drawing on these observations, we optimized lamellar formation by the addition of 10 vol % of H 2 O to the solution of NCs Structural Analysis of QD Lamellae Scanning transmission electron microscopy (STEM) images of pancake-shaped lamellae (Figure 6.3.) confirmed that the self-assembled structures were preserved during the drying process necessary for STEM imaging. High magnification views indicated a significant degree of order within the lamellar structures formed by both NCs (Figure 6.4.A) and nanorods (Figure 6.4.B).

161 133 Figure 6.3. (A) Bright field STEM low magnification overview of NC pancake shaped lamellae created by the addition of nonsolvent (water) and subsequent sonication. (B) Dark field STEM image of an individual nanorod pancake shaped lamella created by a similar procedure, mounted on a TEM grid with a combination ultrathin/lacy carbon film. Furthermore, NC overlap, indicated by apparent linear features in images of NC lamellae (Figure 6.4.A inset 1) and by the fine lines subdividing individual NCs in images of nanorod lamellae (Figure 6.4.B inset 1), suggested the presence of multiple layers of NCs. Fast Fourier transform analyses for both systems indicated a clear tendency to hexagonal symmetry in the NC packing, perpendicular to the plane of the lamellae (Figure 6.4.A and 6.4.B, inset 2). The arrays displayed a high degree of order, similar to that achieved using controlled solvent evaporation. 15,16,18 The NC lamellae exhibited clear signs of structural integrity: when mounted on uncoated TEM grids they spanned the voids in the grid structure and withstood imaging by the high-energy electron beam without deformation (Figure 6.4.C). Given that these lamellae appear to be only a few NC layers in thickness, it was surprising that they could span distances 3 orders of magnitude larger, up to 15 μm, without support. Folds and tears observed in STEM images of individual NC lamellae

162 134 further indicated their structural integrity (Figure 6.4.D). The strength of the NC lamellae resembled elastic membranes formed by non-crosslinked monolayers of Au NPs, which spanned voids with dimensions of up to 1 μm. 19 The results of confocal fluorescence microscopy experiments (Figure 6.2.A) and wet cell SEM (Figure 6.2.C and 6.2.D) in combination with the observed NC overlap in the lamellae of spherical nanoparticles and nanorods (Figure 6.4.A and 6.4.B) suggested the presence of a sheet-like structure with more than one layer of NCs. To explore the crosssectional structure of the lamellae, they were deposited on indexed TEM grids, stabilized by the deposition of an 10 nm thick carbon coating and a nm thick gold layer and microtomed to 30 nm thick sections. The STEM images of both NC and nanorod lamellae showed well-ordered bi- and tri-layers (Figure 6.4.E and 6.4.F). Figure 6.5. shows EDS line profiling across a tri-layer structure of NCs confirming the presence of Cd, Se, and P. The P signal demonstrated that TOPO was present within the structure.

163 135 Figure 6.4. (A) STEM image of a NC lamella. Inset 1 shows NC ovelap indicated by linear structures. Fourier transform (inset 2) indicates hexagonal symmetry. (B) STEM image of a nanorod lamella inset 1 shows nanorod ovelap indicated by fine lines subdividing individual NCs. A region of ordered hexagonal packing is confirmed by Fourier transform (inset 2). (C) SEM image of a lamella (see 1) mounted on an uncoated Cu TEM grid. The lamella ( 15nm thick) spans the dark void ( 15 μm) (see 2) in the grid without support. (D) Examples of folds and tears present in lamellae indicating their structural integrity. (E and F) Cross-sectional STEM images of NC tri- and bilayers.

164 136 For all cross sections, the thickness is 30 nm. The capping Au overlayer is used as a location marker. Figure 6.5. EDS line scan of a cross-sectioned QD lamellar tri-layer. A line scan showing the presence of Cd (solid line), Se (dot dot dash) and P (dot dash). Ti (dot), which has no spectral overlap with the elements of interest, is included as a background control. Cd, Se and P are all significantly above background. Coincidence of P with Cd and Se indicates the presence of TOPO Proposed Mechanism of Lamellae formation To reconcile the plan view and the cross-sectional structure of the NC lamellae (Figure 6.4.A and Figure 6.4.E, F, respectively) we propose the following interpretation of our results. For a NC bilayer in plan view, projections of individual NCs are observed in a hexagonal arrangement, surrounded by linear structures that follow the same symmetry (Figure 6.4.A). These discrete, individual NC projections correspond to two NCs with vertical alignment with AA-type layer stacking, whereas the linear structures are likely the

165 137 result of AB-type layer stacking. Supporting this observation, cross-sectional images (Figure 6.4.E, F) show both regions of discrete single NC alignment along with extensive regions of NC overlap. While it is also possible that coverage in the second layer is incomplete, and NCs lie at the 2-fold sites, instead of the normal 3-fold sites for hard sphere stacking, the cross section STEM images of lamellae argue against this since they show extensive areas of continuous multilayers of NCs. The NC center-to-center spacing of 6.5 nm in the plane of the lamellae was similar to that observed for control samples (CdSe NCs deposited from toluene solution onto carbon coated TEM grids and assembled via evaporation). However, the NC-to-NC spacing between the layers was only ca. 4.5 nm (Figure 6.4.E and 6.4.F). We conjecture that water may play a role in the preferential vertical compression between NC layers by inducing asymmetries in either ligand distribution or conformation, driven by hydrophobic effects. In either case, the reduction in interlayer distance between adjacent NCs is likely to increase their dipole interactions. 12,18 This may be an important factor in explaining the structural integrity of these lamellae (the ground state dipole in the crystal is 100 D). 20 We note that even if dipole-dipole interactions and hydrophobic forces between the QDs are sufficient to stabilize the lamellae, the question of the driving force for their formation remains. Assembly of QDs at the surface of aqueous droplets, driven by the reduction of interfacial energy, 14,21 occurred at short (<10 s) sonication times. It is likely that these droplets served as precursors to the lamellar structures formed at longer sonication times (15-30 s). We propose that exposure to ultrasound plays a key role by generating a mechanical force in the system. Acoustic cavitation, a major effect of low frequency sonication, induces high magnitude shear around oscillating bubbles. 22,23 It has been

166 138 demonstrated that shear forces tend to elongate and stretch the emulsion droplets (while surface tension forces try to oppose this effect). 24 We conjecture that, with sufficient elongation, the opposing walls of a droplet are brought into sufficiently close proximity that dipole-dipole and hydrophobic forces acting between the QDs stabilize the resulting 2D lamellar structure. From a broader perspective, at both the molecular and supramolecular level, acoustic cavitation effects are gaining attention as a new route to the production of novel architectures that cannot be obtained otherwise Testing the Hypothesis and its Potential Applications If, in the course of formation of the lamellae, water is confined between the NC sheets, then not only is this important for the formation of the structure of the lamellae but it also offers an opportunity to incorporate, in the envelopes, a wide variety of watersoluble species. To test this hypothesis, we prepared NC lamellae in the presence of the water-soluble dye, fluorescein isothiocyanate (FITC). Liquid state confocal fluorescence microscopy showed matching localization of photoluminescence (PL) from FITC and CdSe in the NC lamellae (Figure 6.6.A). To build from this observation and exploit its implications, we incorporated a variety of water-soluble ionic, molecular, nanocrystalline and biomolecular species into CdSe NC lamellae. EDS line scans across individual lamellae showed that cobalt (Figure 6.6.B) introduced as salts in H2O (10 % v/v), were strongly partitioned within the NC lamellae. Furthermore, EDS line scans of cross sections of lamellae prepared with Co salts confirmed the presence of Co within the lamellar structure (Figure 6.6.C). The incorporation of larger molecules into the lamellae interior was demonstrated for water-soluble tris (2,2 -bipyridyl)ruthenium(ii) (results not shown) and

167 139 ferritin, a biological macromolecule which is an iron-storage protein consisting of a protein shell surrounding a nanocrystalline iron complex core. 29 EDS line scans confirmed the presence of Fe and by implication, the presence of ferritin (Figure 6.6.D). The very strong Fe signal is an indicator of the high loading potential of these NC lamellae. Figure 6.6. (A) Liquid state confocal fluorescence microscopy images of NC lamellae formed in the presence of the water-soluble dye fluorescein isothiocyanate (FITC), water 10% (v/v). Both FITC and NCs were excited using the 488 nm line of an argon ion laser. Note the coincidence between FITC (green) (collection range nm) and NCs (yellow) (collection range nm) indicating that the water-soluble dye is associated with the lamellar structure. (B) Energy dispersive X-ray spectroscopic (EDS) line scans for CoCl2 6H2O incroporated into CdSe lamellae. The inset shows an HAADF STEM image with the line scan (yellow line) across the lamellar structure (scale bar: 10 μm). (C) EDS line scan of a cross-sectioned ( 70 nm thick) Co incorporated NC lamellar bilayer showing the presnce of Co within the structure. The inset shows corresponding

168 140 HAADF STEM image (scale bar: 35 nm). (D)EDS data for ferritin incorporated into the lamellae. The inset shows a corresponding HAADF STEM image (scale bar: 500 nm). Note: Ti Kα or V Kα lines were used as backgrounds since they have no spectral overlap with the elements of interest. As a specific example of the potential functionality of CdSe NC lamellae, water soluble citrate-capped Au spheroidal nanoparticles (NPs) with a mean diameter of 10 nm were incorporated. Consistent with the other water-soluble species studied, the Au NPs were found exclusively within the lamellae (Figure 6.8.A). EDS mapping showed the uniform distribution of Cd and Se and well dispersed discrete Au signals indicating the presence of Au NPs (Figure 6.7). Figure 6.7. (A to D) EDS maps of Cd, Se, Au and Ti (background) respectively, corresponding to the structure presented in Figure 6.8.A showing that distribution of Au NPs is fully contained within the structure.

169 141 Two approaches were taken to confirm the internalization of the Au NPs. First, cross sectional STEM imaging (Figure 6.8.B) showed the presence of Au NPs within the lamellar structure. Second, simultaneous high resolution surface (SEM) and transmitted (TEM) images of the same lamellar region were recorded. These results indicated that the Au NPs were encapsulated (Figure 6.8.C and 6.8.D), in contrast to the control experiment (Figure 6.8.E) where Au NPs were added after lamellar formation and clearly present on the surface. When semiconductor NCs are coupled with metal NPs, plasmon-exciton interactions result in either enhancement or quenching of NC photoluminescence. 30,31 Enhancement of emission occurs due to the plasmon-induced field enhancement effect, whereas quenching of NC emission is due to energy transfer from NC to metal NPs. 32,33 This plasmon-exciton coupling interaction is of both practical and theoretical interest in the research areas of light emitting devices, nanoscale lasing, and solar cells etc. To study the effects of encapsulation of Au NPs on the luminescence of CdSe lamellae, photoluminescence (PL) spectra were measured using confocal fluorescence microscopy for 10 randomly selected lamellae with compartmentalized Au NPs. Lamellae formed by CdSe NCs only were used in control experiments. Representative confocal fluorescence microscopy images of control and Au NP encapsulated CdSe lamellae are presented in Figure 6.8.G and 6.8.H, respectively. To compare the relative PL yield from each of these structures, we plotted two histograms of the integrated PL intensity. Figure 6.8.F shows a near doubling of the PL intensity for the Au encapsulated lamellae as compared to the control sample. This PL enhancement is likely due to electronic interactions between the Au NPs and the CdSe NCs in the hybrid lamellae. 34 This interaction could result from plasmonic enhancement of the NC radiative rate, 35 or NC PL could be sensitized by

170 142 resonance energy transfer from Au NPs. 36 A slight blue shift of PL maximum (582 to 574 nm) was detected similar to that observed in previous studies. 37,38 In keeping with the argument of Lee et al., 37 the blue shift may arise due to decreased exciton diffusion length as a result of the increased radiative decay rate caused by exciton-plasmon interactions. To further optimize luminescence enhancement of the lamellar structure, control of experimental parameters, e.g., the type of encapsulated NPs, choice of QDs, their concentrations, shape, dimensions, and variation of interparticle distances will be required. 32,33,39-41

171 143 Figure 6.8. (A) Incorporation of Au NPs into CdSe NC lamellae. In the HAADF STEM image shown, the bright dots are individual Au NPs. (B) Crosssectional ( 30 nm thickness) STEM image confirming the encapsulation of Au NPs inside the NC bilayer (as previously, an evaporated Au layer, upper portion of the image, is used as a marker). (C and D) Simultaneously recorded SEM and TEM images, respectively, confirming encapsualtion of Au NPs within the NC lamellae. (E) SEM image of control sample with Au NPs added after the NC lamellae formation. (F) Histogram showing 10 maximum photoluminescense intensity measurements for both NC lamellae and Au encapsulated NC lamellae. (G and H) Representative fluorescence confocal microscope images of CdSe NC lamellae and Au encapsulated NC lamellae, respectively.

172 Summary and Conclusions This study demonstrates a facile, solution-based, method that involves controlled addition of a nonsolvent (water) combined with sonication to trigger the formation of micrometer-size lamellar sheets of ordered NC arrays. The most striking characteristics of these structures are that, while they are many micrometers in diameter, they are typically only two or three NC layers in thickness ( 15 nm), yet they exhibit structural integrity without recourse to chemical cross-linking. In addition, the properties of the lamellar structures can be modified by internalizing water soluble species including but not limited to ionic salts, metal nanoparticles (NPs), and biomolecular complexes. The possibility of tuning the optical properties of the NC lamellae was demonstrated by enhancing PL intensity via incorporation of Au NPs. We speculate that these ions or molecular complexes may be useful in photoelectrochemical processes, e.g., in forming redox couples in an ordered, nanoparticle solar cell More fundamentally, these structures mark a step toward ordered, compartmentalized, NC-based synthetic complexes whose properties can be modulated by the cargo they carry.

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176 148 Chapter 7 Towards an Experimental Demonstration of 2D Visible Range Cloaking via a Bottom-up Approach This chapter explores a bottom-up method to produce a metamaterial which can potentially function as an optical cloak in the visible range. A composite material consisting of an array of silver nanowires (NWs) in a dielectric host has been produced based on the theory of a non-magnetic optical cloak. 1 The required radial array of silver NWs was achieved by electroless deposition of the metal into the channels of a porous alumina structure grown perpendicularly from the curved surface of a micrometer scale aluminum wire. While the required architecture and dimensions may require further adjustment, the functionality of the cloak in the visible range has been demonstrated. Fundamentally this metamaterial structure represents an important step forward in the production of tunable, optically functional, complex three dimensional architectures through the bottom-up approach.

177 Introduction Metamaterials and Optical Cloaking via Transformation Optics Metamaterials are artificially constructed composite materials which exhibit ensemble electromagnetic (EM) properties not present in the constituent materials. 2-8 The heterogeneity of these materials exists on a length scale smaller than the wavelength of interest. Thus, the EM response of the material is a function of the collective behavior of a material s components (an overview of metamaterials is provided in Chapter 1). As a consequence of their tunable EM properties, metamaterials have become a focus in the area of transformation optics. 4,7,9-12 Transformation optics explores the control of light paths via manipulation of the spatial distribution of permittivity (ε) and permeability (μ) 13 within metamaterials. In effect, transformation optics 7,12 describes the conditions necessary to warp light space in a manner analogous to warping space-time in general relativity. Within the realm of transformation optics, the possibility of optical cloaking (i.e., invisibility) has sparked scientific curiosity in recent years. 1,14-19 In a perfect optical cloak, the object to be rendered invisible will create no reflection, scattering or absorption. Figure 7.1 illustrates an optical cloak in a spherical coordinate system where light is bent around an object and redirected to its original trajectory. The object being cloaked is to be placed within the inner sphere whereas the region between the inner and outer sphere constitutes the cloaking device.

178 150 Figure 7.1. Schematic illustration of a three-dimensional view showing the wave trajectories of a spherical cloaking system. Reprinted with permission from Reference 14. Copyright 2006, Science. In any naturally occurring material, light rays will bend toward the center of the sphere due to the material s higher refractive index in accordance with Snell s law. 20 To diffract light rays away from the center, a material whose refractive index is less than 1 is required. For example, one way to achieve this is by employing thin metallic wires in a dielectric host which acts to dilute the metal and thus reduce the plasma frequency to obtain ε less then unity, for a desired wavelength. 21 One exploitation of this notion, is the design of an optical cloak which was introduced by Pendry 18 and Leonhardt et al. 7,18 In this model, the path of the EM wave was controlled by using a specific spatial profile of ε and μ to make light avoid a particular region in space. Figure 7.2 shows an example of the transformed media using ε and μ tensors. In a homogenous medium where ε and μ are constant, a straight field line is produced. However, by varying the spatial distribution of ε and μ, a distorted field line results as it travels through the heterogeneous medium.

179 151 Figure 7.2. (A) Straight field line through a homogeneous medium against a Cartesian coordinate system (B) distorted field line travelling through a heterogeneous medium produced by varying the spatial distribution of permittivity and permeability. Reprinted with permission from Reference 14. Copyright 2006, Science. For the design of an optical cloak, in the case of a cylindrical coordinate system, the region 0 < r < b is transformed into < r < b by using the following transformation: ( ) where a and b are the radius of the core (i.e., region of invisibility) and the distance from the center of the core to the outer diameter (i.e., perimeter of the cloak) respectively. r and r are the radial coordinates in the original and transformed system respectively z and θ are the coordinates of the transformed system respectively. The transformed region extends from a to b only, shown in Figure 7.3. The transformation can be obtained by the following space profiles of ε and μ for the case of a z- polarized incident field: ( ) ( ) ( )

180 152 It should be noted that only has a gradient as a function of radius and thus this type of transformation is known as a magnetic cloak. However, this type of transformation still suffers from non-zero scattering. The first practical demonstration of a cloak based on the above mentioned transformation was performed by Schurig et al at microwave frequencies in Figure 7.3. A two-dimensional cross-sectional view of wave trajectories of a spherical cloaking system where light is deviated around the object to be cloaked (radius a) within the annular cloak region (radius b a) and return to its original path. Reprinted with permission from Reference 14. Copyright 2006, Science Theoretical Design of a Non-magnetic Optical Cloak The magnetic cloak discussed above cannot be practically scaled down in dimensions for the development of a cloak in the visible spectrum range as indicated by Klein et al. 22 An alternative theoretical solution for a non-magnetic cloak which can operate at optical frequencies (390 to 750 nm) was proposed by Cai and Shalaev et al. 1 Our bottomup approach for the production of the cloaking structure illustrated in the following sections is based on this model. In the model, ε has a gradient as a function of radius. Figure 7.4 illustrates a coordinate transformation of this cylindrical shell model. The proposed

181 153 design should meet the following space profile for transverse magnetic (TM) illumination (i.e., the magnetic field polarized along the z-axis): ( ) ( ) ( ) ( ) where, and are the azimuthal and the radial dielectric permittivity and the z-axis magnetic permeability respectively. Figure 7.4. The coordinate transformation of a cylindrical shell model. A cylindrical region r<b into a concentric cylindrical shell a <r < b. There is no variation along the z direction. Reprinted with permission from Reference 13. Copyright 2007, Nature. It should be pointed out that although this design can produce the desired trajectory of light, impedance mismatch at the outer boundary of the cloak occurs. This results in a certain amount of scattering determined by the ratio of a to b. The main advantage of this theoretical design is its ease of fabrication due to the fact that no magnetic resonance is required. Further is a constant and is larger than unity which can be easily achieved through a variety of dielectric materials. The largest design challenge is the choice and control of an appropriate metamaterial where the profile of varies from 1 to 0 as the wave propagates from the outer boundary of the cloak b to the inner a.

182 154 The space profile of ε r and ε θ dictates an interaction between the media and applied field in the radial direction only, whereas negligible interaction in the azimuthal direction is desirable to provide a constant ε θ. The choice of metallic rod-like structures (e.g., arrays of nanowires or nanoparticles) is well-suited for this purpose since it lends itself to predictive modeling via the effective medium theory. 23,24 Let us consider a metal ellipsoid with three perpendicular semiaxes a i (i=1,2,3). The depolarization factor q 1 for incident field polarized along a 1 is given as: ( ) ( ) ( ) ( ) Similar expressions for q 2 and q 3 can be obtained with cyclic changes. In the case of a long elliptical cylinder, a 2 =a 3 and a 1 >> a 2 which will result in q 1 0 and as q 1 +q 2 +q 3 =1, therefore q 2 =q 3 =0.5. The screening factor k which indicates the strength of interaction between the wire and the applied field is given by: ( ) (7.5) Equation (7.5) shows that k 1 will achieve a very large value whereas k 2 = k 3 =1. Therefore, if the long axis of the metal nanowires or rods (i.e., a 1 ) is in the radial direction, we will observe a strong interaction in the radial direction whereas the interaction in the transverse direction will be negligible. The effective permittivity of such a composite media in a given direction is: [ ] ( )

183 155 where [( ) ] [ ( ) ]. As we expect no interaction in the azimuthal direction: ( ) ( ) where is the permittivity of the dielectric host. Thus the ratio of inner to outer radius of the cloak (R=a/b) is governed by : ( ) where and are the filling factors for the metal nanowires at the inner and outer surfaces of the cloak respectively and their ratio should satisfy the above relation. Considering the possible dimensions of the cylindrical shell i.e., a and b that can be achieved by the bottom-up approach, we have utilized radial arrays of silver nanowires (and nanoparticles). From a practical, experimental point of view, it is important to note that as long as the Equations from (7.4) to (7.8) are satisfied, the key parameters i.e., a, b and filling factors may be varied considerably. Given the complexity of the components involved in generating the necessary architecture, such flexibility is critical in producing the necessary structure. Based on these equations, examples of the design possibilities are presented in Figure 7.5 which shows the calculated variation of r as a function of design parameters e.g., a and b at the operating wavelength of 500 nm. For these examples, we used silver nanowires (NWs) in a dielectric host, alumina (See Route III, Section 7.2.3). The required variation of r from 0 to 1 for the interior and exterior surfaces of the cloak respectively are achieved by a = 0.7 µm and b = 2 µm (Figure 7.5 A) and a = 1.2 µm and b = 3.5 µm (Figure 7.5 B). In order to satisfy the equations, the corresponding filling factors are

184 156 f 0 =24.4 % and f 1 =9.4 % and values of R calculated from Equation (7.8) is 0.35 at the operating wavelength of 500 nm. Figure 7.5. Calculated plot of radial component of electric permittivity (ε radial ) as a function of cloak dimensions (A) a = 0.7 µm and b = 2 µm (B) a = 1.2 µm and b = 3.5µm. Both parameters result in the effective permittivity at operating wavelength of 500 nm. Silver nanoparticles with a radius of 10 nm were used for the calculations.

185 Results and Discussions Experimental Rationale The model proposed by Cai and Shalaev et al 1 represents a significant fabrication challenge for both top-down and bottom-up approaches. Figure 7.6 and Table 7.1. summarize the required parameters for the bottom-up approach of a 2D cloaking material at a visible frequency. Based on this model the target dimensions for our experiment for the object to be cloaked is approximately 1 µm in diameter surrounded by a cylindrical cloak with a wall thickness of approximately 1.5 µm. In addition, the metamaterial cloak consists of a radial array of metal NWs in an appropriate dielectric host. Since the refractive index of the cloak is required to be 0 at the interior and 1 at the exterior, it is critical that the filling factor of the metal within the dielectric host varies accordingly. For example, if the specific cloak dimensions described above are used, a filling factor of metal NWs (or a radial array of nanoparticles (NPs)) of 4% at the exterior and 12% at the interior of the cloak is needed. While structures in this dimensional range are relatively simple to achieve via top down methods, the most significant challenge is the formation of a radial array of metal NWs organized within the dielectric host. The NWs are required to be less than 1/10 in width of the incoming optical wavelength. For example, this would require a NW width of less than approximately 40 nm for blue light and less than 70 nm for red light. While the formation of metal NWs in this size range is possible using focused ion beam technology, 25 the device is required to be three-dimensional. Specifically, using top-down strategies, only one layer could be deposited at a time and multiple individual layers of

186 158 dielectric and subsequent radial metal deposition would need to be manufactured. The time and cost of this process rules it out as a practical approach. Figure Schematic illustration of the non-magnetic cloak structure. Inner core (dark grey) is the cloak area surrounded by metal nanowires (NWs) in a dielectric host. A Radial array of NWs is perpendicular to the z-axis and must satisfy the filling factor such that the radial component of electric permittivity varies from 0 at a to 1 at the exterior surface. Spatial positions of NWs do not need to be periodic. Table Summary of required parameters for the fabrication of a non-magnetic optical cloak device. Material Arrays of metal nanowires or nanoparticles (eg, gold or silver) Host for metal Dielectric (e.g., SiO 2, Al 2 O 3 ) Shape of host Cylindrical Metal orientation Radial Metal width nm or less Filling factors of metal Must satisfy ε rad 0 at inner boundary to ε rad = 1 at outer boundary. The filling factor varies depending on dimensions of host and metal (eg, for a = 1.4 µm, b = 4 µm and metal width ~10 nm, f 0 =18.6% and f 1 =6.6%). Additional practical notes (1) Diameter of object to be cloaked is ~ 30% of total diameter of cloaking device.

187 159 (2) Variations in metal length may not be important as long as filling factor is satisfied (i.e.,averaging effect). (3) System must be robust, processible, temporally stable and needs to be responsive and allow for fine tuning. Using a bottom-up approach, the general rationale would be to produce a NW (or NP) array which is stabilized by a dielectric host. A porous dielectric host with tunable dimensions can be realized and subsequently, the pores can be populated with appropriate metal NWs (See details in Section 7.2.3, Route III). Figure 7.7 is a summary of three possible routes which were explored for the fabrication of the optical cloak via bottom-up approaches. Figure Summary of explored routes for the fabrication of a non-magnetic optical cloak device.

188 Routes I and II Of the three approaches outlined in Figure 7.7, Route III (Section 7.2.3) allowed for the production of structures falling within the range of the desired parameters while Route I and II, failed to produce effective architectures. The preliminary experimental data for Routes I and II (Figure 7.8) will be discussed concisely in this section. Figure 7.8. Schematic showing two possible routes to produce the optical cloak. Route I: vertical assembly of gold nanorods on silica then subsequently embedded via silica deposition. Route II: radial assembly of binary metal NWs (eg, gold/nickel) around a cylindrical host directed by a controlled magnetic field.

189 161 In preliminary experiments via Route I, the concept is to produce a radial array of NWs or NRs which project vertically from the surface of dielectric spheres with subsequent embedding of the NWs through the deposition of an additional layer of dielectric. By repeating this process in a stepwise manner, multiple layers of radially arranged NWs in a dielectric host could be produced. The vertical assembly of NWs can be realized, in principle, by functionalizing their ends. However, one of the foreseen challenges of this approach is that since both ends of the NWs are functionalized, there is the possibility of undesired bridging of adjacent spheres by NWs or attachment of both ends of a single NW to the substrate. Silica spheres (and hollow silica spheres) 26 were chosen as the dielectric host and substrate for assembling metal NW arrays. Monodisperse colloidal silica (SiO 2 ) particles were prepared by controlled hydrolysis and condensation of tetraethylorthosilicate (TEOS) in ethanol to which water and ammonia were added (the Stöber method). 27 We produced highly monodisperse spherical SiO 2 particles with dimensions ranging from 300 to 500 nm. Gold nanorods (NRs) (L NRs = 40 ± 4 nm, H NRs= 13 ± 1 nm) were synthesized and the ends of NRs were functionalized with 3-mercaptopropyl trimethoxysilane to facilitate their vertical assembly on the surface of the SiO 2 spheres. Figure 7.9 shows the results of vertical gold NR assembly on the silica particles. Although the majority of NRs observed were close to vertical alignment, the number density of NRs on the surface of the spheres was consistently too low to move on to the next step of multiple layer formation. In addition, direct seed deposition of gold on the surface of SiO 2 particles was explored. In this approach, gold seed nanoparticles were obtained by the reduction of HAuCl 4 by NaBH 4. These seeds were then deposited and grown on the surface of the SiO 2 spheres. In both

190 162 cases, the required control of NR vertical assembly necessary for this hybrid structure to be effective as a cloak was not achieved. Figure 7.9. Representative TEM image of vertical assembly of gold nanorods onto synthesized silica particles. The ends of gold nanorods were functionalized by the introduction of 3-mercaptopropyl trimethoxysilane. As an alternate approach (Route II) an attempt was made to create a radial array of nickel-tipped gold NWs by the use of a magnetic field in an anti-helmholtz configuration. Figure 7.10 illustrates an electrochemical method for the production of binary NW structures consisting of gold and magnetically responsive nickel. A thin film of silver (~400 nm) was evaporated on a commercially available aluminum oxide filter (Anodisc TM, USA) and used as the cathode. A platinum wire served as an anode. A silver buffer layer was deposited first to fill the pores evenly, followed by gold and nickel deposition. The optimized plating conditions were 0.5 ma/cm 2 for deposition times of 10 to 60 min, depending on the desired length of each segment. Figure 7.10 shows examples of

191 163 representative SEM images of the resultant binary structures with varying lengths. Energy dispersive X-Ray spectroscopy (EDS) mapping confirms the composition. Figure Schematic illustration of the electrochemical method used to produce binary nanowires (NWs) composed of nickel and gold.

192 164 Figure (A) Backscattered SEM images of binary NWs showing various lengths of each component. Note: silver is used to fill the bifurcated pores to provide even deposition of gold and nickel (B) EDS mapping showing atomic composition of NWs. For the magnetic radial assembly, as synthesized binary Ni-Au NWs described above were used as a building-block. Figure 7.12 (A) shows the simulation of the magnetic field lines analogous to the Helmholtz configuration. In this configuration, the opposing poles of cylindrical annular magnets are facing each other such that a homogenous magnetic field can be produced within the central region between the magnets. However, when their dipole moments are aligned in the anti-helmholtz configuration i.e., the same poles are facing each other, the magnetic fields from two poles flow in opposite directions. This

193 165 results in a zero net magnetic field at the center surrounded by a radial field as indicated in Figure 7.12 (B). Figure Magnetic field line simulations analogous to (A) Helmholtz and (B) anti-helmholtz configurations. By exploiting the anti-helmholtz configuration, we hypothesized that radial assembly of the binary NWs could be achieved in this region where the radial magnetic field lines are compressed along the horizontal axis between two magnets due to the resulting pressure in the field by repulsion. Figure 7.13 (A) shows a photograph of our general experimental set-up. Annular magnets were arranged in an anti-helmholtz configuration and placed on a capillary tube (or a glass vial) containing binary NWs in an aqueous solution. This set-up should produce a radial magnetic field line in the central

194 166 zone between the magnets such that the magnetically responsive component, Ni, of the binary NWs should align within this field. Figure 7.13 (B) shows the top-views of NW assembly in the Helmholtz and anti-helmholtz configurations respectively. In the case of the Helmholtz configuration, most of the NWs were aligned parallel to the field and thus from the top view, we are looking down the long axis of the NWs. For the anti-helmholtz configuration, a distinctly different radial distribution of NWs was observed. Although the radial alignment of NWs was successful on the millimeter scale, there was insufficient control once the process was miniaturized to the micrometer scale. Further, while radial arrangement of NWs was possible, achieving precise radial control of the volume fraction via magnetic fields was problematic. Figure (A) Example of experimental set-up using annular magnets in an anti-helmholtz arrangement producing a radial magnetic field in the central zone between magnets (B) Optical

195 167 micrographs showing top-views of NW assemblies via the two different configurations, showing a radial alignment of NWs in the anti-helmholtz arrangement Route III Fabrication of a Cylindrical Shaped Dielectric Host Our third approach (Route III) was to use a porous host as a template for creating radial metal arrays. While the model proposed by Cai and Shalaev et al 1 (see section 7.1.2) considers silica as a dielectric host and silver NWs, a combination of other dielectrics and metals is possible. Of the available materials, porous alumina is particularly attractive since the band gap energy of alumina (Al 2 O 3 ) is approximately 9 ev and its critical wavelength (see Chapter 1) at room temperature is approximately 0.14 µm which makes alumina transparent in the visible range (note, band gap energy and critical wavelength of SiO ev and 0.15 µm respectively). 31,32 Dimensions of porous alumina can be controlled by the appropriate selection of applied voltage during anodization. 33 Further, since the starting material, aluminum, can be readily fabricated into wires with a diameter smaller than 20 µm, this metal offers a unique opportunity for a structure to produce its own cloak. Figure 7.14 is a schematic illustration of the method for the production of a radial porous alumina template as a dielectric host for the optical cloak.

196 168 Figure Schematic of the proposed route to the fabrication of radial porous alumina as a dielectric host via anodization of aluminum (Al) wire. The cross-sectional view shows a metallic Al core surrounded by a porous alumina coating with a radial distribution of pores. Anodized aluminum oxide (AAO) is a material whose thickness, pore size and pore volume fraction can be readily manipulated through chemistry and applied potential during oxide growth Porous AAO as a dielectric host should provide: (1) radial arrays of pores for metal deposition and (2) appropriate pore size and template dimensions to potentially satisfy the filling fraction of metal required by the theoretical model. 1 In our work, the anodization process was carried out on an aluminum wire (Al, %, purchased from Alfa Aesar) electropolished to be less than 5 µm in diameter. An Al wire should in principle, provide radial arrays of pores, since it is known that pores grow perpendicular to the Al metal surface. This is because there is an equilibrium of oxide dissolution and oxide growth at the interface between oxide/electrolyte and metal/oxide respectively. 37 The net reaction during anodization is:

197 169 2Al + 3H 2 O Al 2 O 3 +3H 2 Oxide growth is electrochemically driven by the movement of oxygen containing ions (e.g., O 2- /OH - ) at the metal/oxide interface from the electrolyte through the oxide layer at the bottom of the pore where the following reaction takes place: 2Al +3O 2- Al 2 O 3 +6e - By the hydration reaction, the dissolution of the oxide layer results. Al 3+ ions migrate through the oxide layer and eject into the electrolyte solution at the oxide/electrolyte interface. This loss of Al 3+ ions into the electrolyte is necessary for growth of the porous oxide: Al 2 O 3 +6H + 2Al 3+ +3H 2 O At the cathode, hydrogen gas evolution can occur by the ejection of electrons into the electrolyte solution: 6H + + 6e 3H 2 This balance between the growth of Al 2 O 3 and the loss of Al 3+ ions, is key to producing alumina s porous columnar structure. 37,38 We explored growth rate, pore density, pore regularity and overall integrity of the oxide layer by using a number of acids e.g., sulfuric, phosphoric and oxalic acids 33,34 to determine optimal conditions for radial oxide growth. In the case of sulfuric acid (15 v % in an aqueous solution at 10 C), the AAO growth rate was rapid (>1 µm in 5 min) and it produced a high pore volume fraction. Therefore, the commensurate pore diameter was too small to satisfy the model. 1 Phosphoric acid had a significantly slower growth yielding controllable pore size but suffered from bifurcation, that is, branching during the anodization process which suggested that control over pore volume by varying applied

198 170 potential would be impossible. Ultimately, oxalic acid (3 wt %, in aqueous solution at 21 C), was chosen because it formed straight, well defined pores with dimensions in broad compliance with those required by the model. Figure 7.15 shows representative SEM images of the AAO host formed in the presence of oxalic acid. A cylindrical oxide shell was grown with pores perpendicular to the Al wire surface. Relatively straight porous channels without bifurcation were formed. Figure SEM images of (A) Bare Aluminum wire after electropolishing. (B) Anodized aluminum oxide (AAO) grown as a cylindrical dielectric shell around an Al wire core. (C) Surface morphology of AAO shell showing a uniform pore structure (D) A cross-sectional view of radial porous AAO grown using 3 % Oxalic acid (nb, surface roughness shown is due to fracturing artifact).

199 171 Although the theoretical model requires a filling factor for silver NWs of 12 % for the interior and 4 % for the exterior of the cloak, 1 for the dimensions and wavelength of a 0.5 µm, b 2 µm and 633 nm respectively, the model affords significant flexibility. This is because the required filling factor varies with the dimensions of the host and the object to be cloaked. Therefore, we investigated the effect of oxide growth and subsequent pore size, volume, and density by applying various potentials to the electrolyte. 33 Assuming a 2D dense packing of the pores, applied voltage dependent average pore size and cell size (that is the distance between adjacent pores) were measured by analyzing SEM micrographs. Figure 7.16 (A) shows the variations of average pore size and average cell size plotted as a function of applied potential. In the voltage range V, the AAO pore diameter showed only a modest decrease while a significant decrease of the average cell size was observed. This feature is critical in providing the template for the required filling factor control. Figure (A) Variations of average pore diameter (blue circle) and average cell size (red square) as a function of applied potential. (B) Calculated pore volume fraction as a function of

200 172 applied voltage. The oxide layer was electrochemically grown over 105 minutes using 3 wt. % of Oxalic acid in water as an electrolyte solution. Figure 7.16 (B) shows the variation in the pore volume fraction as a function of applied voltage. The volume fraction was calculated as V = 78.5P 2 /C 2 from the pore diameter (P) and the cell size (C) based on the assumption that the cross-section of pores is cylindrical and constant. 33 As can been seen from Figure 7.16 (B), by choosing the appropriate applied voltage, it is possible to alter the volume fraction of the pores from greater than 15 % to less than 5 %. It is important to note that during the growth process of approximately 100 min the voltage has to be changed gradually to prevent bifurcation or the formation of steps which could influence subsequent metal loading. During preliminary studies with thicker wires of µm in diameter, severe cracking of the AAO was observed. The reason for cracking is unclear however based on our observations, it appeared to be related to the thickness of the AAO layer. This may be due to the fact that AAO occupies a measurably larger volume than the aluminum substrate from which it grows. Since significant compressive stresses are induced during the growth process 36 these are likely exacerbated by the high radius of curvature of our cylindricalshaped system. However, as we approached the target dimensions required for cloaking (e.g., a = 0.6 µm and b = 1.75 µm), cracking routinely terminated at a wire diameter of approximately 5 µm, with an uninterrupted porous radial array below this diameter.

201 Electroless Deposition of Ag NPs and Ag NWs into AAO pores Initially, a pulsed electrochemical approach using silver nitrate (AgNO 3 ) was employed. However, it typically produced polydisperse silver NWs with incomplete pore filling. Consequently, we focused on electroless deposition via polyol reduction of Ag + ions. In a typical polyol synthesis, ethylene glycol reduces AgNO 3 to produce Ag atoms by the following mechanism: 39 2HOCH 2 CH 2 OH 2CH 3 CHO + 2H 2 O 2Ag + +2CH 3 CHO CH 3 CO OCCH 3 + 2Ag +2H + Similar to the synthesis of quantum dots, nucleation and growth of silver nanostructures can be initiated once the concentration of silver atoms reaches the supersaturation point In order to enhance the rate of silver NW growth, a trace amount of sodium sulfide (Na 2 S) was added. 43 One of the challenges of this method was that the NW density was greater at the surface of the AAO, tapering off towards the interior the opposite of that required for the model. To overcome this challenge, first, a solvothermal reduction was used to deposit seeds of Ag at the base of the AAO pores. In order to produce a higher concentration of seeds at the base, the AAO was dipped in AgNO 3 solution then briefly rinsed in 1:1 ethanol/acetone solution prior to solvothermal reduction of the silver followed by polyol reduction. The resulting structure showed a much more uniform distribution throughout the AAO layer.

202 174 Even with uniform loading, another challenge originated from the excess silver present as large particles on the surface of the AAO. The formation of these undesired particles occurred during electroless deposition (Figure 7.17 (A) inset). This was resolved by sonicating the structure in a diamond paste slurry which led to uniformly clean surfaces (Figure 7.17 (B)) without disruption of the silver NWs deposited in the pores. We found that the optimal silver loading was achieved by using a 12 wt % solution of AgNO 3 in ethylene glycol. Figure 7.17 (C) shows silver NWs loaded in the AAO structure whose inner and outer diameters were 1.2 µm and 3.5 µm respectively (i.e., a =0.6 µm and b =1.75 µm). For this system, the calculated response of r as a function of the structure s dimensions (radius/a) is shown in Figure 7.17 (D). As an example, at an operating wavelength of 500 nm, the value of r exhibited the required variation from 0 to 1 for the experimentally produced cloak with the following parameters; filling factor of f 0 =24.4 % and f 1 =9.4 % and calculated R = 0.35.

203 175 Figure (A-B) Low and high magnification backscattered SEM images of the surface of AAO structures containing silver NWs. Inset shows superficial deposition of larger silver particles which were subsequently removed by diamond paste washing. (C) Backscattered SEM image of a silver NW loaded radial AAO structure. This example shows both the desired radial silver NW distribution in a dielectric host along with the required structural dimensions. (D) Calculated plot based on (C) showing r response for a = 0.6 µm and b = 1.75 µm. Operating wavelength is 500 nm. Radius is variable ranging from a to b Optical Transmission Measurements To validate the performance of the experimentally produced non-magnetic cloak structure, we used the optical transmission setup (shown in Figure 2.2, Chapter 2) where a super-continuum (SC) laser source was used which allowed us to cover a wide range of

204 176 wavelengths from 450 nm to 1100 nm. An acousto-optic tunable filter was used to select a specific wavelength from the broad spectrum of the SC source for the illumination of the cloak sample. The sample was illuminated by TM polarized monochromatic light (i.e., transverse magnetic illumination) and the transmitted light was collected and displayed on a CCD camera. At the chosen wavelength, with TM polarization, the cloak is expected to bend the rays of light around the object, in this case the Al wire core. 1 Ideally, the wavefront should be completely recovered behind the sample. In reality, this is not possible due to the fact that the cloak is not impedance matched to free space and thus a finite amount of scattering is inevitable which is determined by the ratio of a to b. The reflected power due to this mismatch can be given as [ ( )] where. Therefore we expect only partial recovery of the wavefront after passing through the cloak. 1 Figure 7.18 (A-B) shows examples of transmission optical images at a wavelength of 540 nm recorded on a CCD camera for the TE and TM polarizations respectively. Similar images are shown at wavelengths ranging from 450 to 750 nm in increments of 5 nm. In order to quantify the transmission through the fabricated structure, as a function of wavelength, we collected sequential intensity data across the structure in a diagonal manner at each chosen wavelength.

205 177 Figure Optical images captured by CCD camera at a wavelength of 540nm for transverse electric (TE) and transverse magnetic (TM) polarization. Quantification of the intensity across the fabricated structure was carried out by a series of sequential diagonal scans over the wavelength range of 450 to 750 nm.