Prof. Lucio C. Andreani University of Pavia, Italy Molecular plasmonics is a rapidly growing interdisciplinary science that aims at investigating the coupling, at the nanometer scale, between emitting molecules and metallic nanostructures. This metal molecule electromagnetic interaction involves the excitation of localized and/or delocalized surface plasmons and is of great interest for a variety of research disciplines such as sensing, optoelectronics, medical diagnosis, optical communications, nanoscience, and energy. This handbook presents a comprehensive overview on the physics of the plasmon emitter interaction, ranging from electromagnetism to quantum mechanics, from metal-enhanced fluorescence to surface-enhanced Raman scattering, and from optical microscopy to the synthesis of metal nanoparticles, filling the gap in the literature of this emerging field. It is useful for graduate students as well as researchers from various fields who want to enter the field of molecular plasmonics. The text allows experimentalists to have a solid theoretical reference at a different level of accuracy and theoreticians to find new stimuli for novel computational methods and emerging applications. Stefania D Agostino is a postdoc fellow and assistant lecturer in the Department of Physics, University of Pavia, Italy. Her research interests focus on computational solid-state physics, nanoscience, photonics, and molecular plasmonics. V148 ISBN 978-981-4303-20-0 Della Sala D Agostino Fabio Della Sala leads the theoretical and computational division of the National Nanotechnology Laboratory of CNR, Lecce, Italy. He is an expert in density functional theory methods, computational material science, organic and inorganic optoelectronics, and computational nano-plasmonics. HANDBOOK OF This is a well-conceived and well-organized book in the broad and highly active area of molecular plasmonics. It contains leading specialists contributions on theory and simulation, experimental techniques, and fabrication. Each chapter is structured in order that it can serve both as an introduction for a non-specialist reader and as a review reference for an active researcher in the field. The content achieves very good balance between tutorial and research aspects what can make the book an authoritative reference in the field for several years. edited by Fabio Della Sala MOLECULAR PLASMONICS This is a really complete book that covers topics ranging from the basics of light matter interaction through practical and theoretical methods to understand and characterize the optical response of nanoparticles and molecules, as well as their interaction and related surface enhancement effects. The theoretical description is complemented by experimental examples and applications. The book will be very useful to researchers entering the field of molecular plasmonics, particularly those who are less familiar with the basic concepts, which are usually skipped in more specialized books or reviews. Prof. Luis M. Liz-Marzán CIC biomagune, Spain Stefania D Agostino
H A N D B O O K O F MOLECULAR PLASMONICS
H A N D B O O K O F MOLECULAR PLASMONICS edited by Fabio Della Sala and Stefania D Agostino
Published by Pan Stanford Publishing Pte. Ltd. Penthouse Level, Suntec Tower 3 8 Temasek Boulevard Singapore 038988 Email: editorial@panstanford.com Web: www.panstanford.com British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Handbook of Molecular Plasmonics Copyright c 2013 Pan Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN 978-981-4303-20-0 (Hardcover) ISBN 978-981-4303-21-7 (ebook) PrintedintheUSA
Contents Preface Acknowledgment xiii xvii 1 Foundations of Molecular Plasmonics 1 F. Della Sala 1.1 Electromagnetic Waves 2 1.1.1 Propagation in a Homogeneous Medium 5 1.1.1.1 Poynting vector 7 1.1.2 Reflection and Transmission at an Interface 9 1.1.2.1 Normal incidence 13 1.1.3 Current and Charges as Sources of Fields 14 1.1.3.1 Green s function 15 1.2 Dielectric Function of Metals 17 1.2.1 The Drude Model 17 1.2.2 Noble Metals 22 1.2.3 Linear Response Theory 23 1.3 Delocalized Surface Plasmon Resonances 27 1.3.1 The Surface Plasmon-Polariton Wavevector 30 1.3.1.1 Propagation length 32 1.3.1.2 Field-penetration depths 33 1.4 Localized Surface Plasmon Resonances 33 1.4.1 Nanosphere in the Quasi-Static Approximation 35 1.5 Dipole Radiation 41 1.5.1 Radiation Pattern 45 1.5.2 Dissipated Power 46 1.5.3 Classical Approach 47 1.6 Scattering Theory 50 1.6.1 Scattering Cross-Section 52 1.6.2 Optical Theorem 54
vi Contents 1.6.3 Rayleigh Scattering by a Nanosphere 55 1.6.4 Radiative Damping 57 1.6.5 Mie Scattering 58 1.7 Dipole Radiation in Scattering Environments 60 1.7.1 Forced Damped Oscillator 61 1.7.2 Purcell s Effect 62 1.7.3 Fluorescence Lifetime and Quantum Yield 64 1.7.4 Fluorescence Emission Near a Metal Surface 66 1.7.5 Fluorescence Emission Near a Metal Nanosphere 72 1.7.5.1 The Gersten-Nitzan model 75 1.7.5.2 The Carminati model 76 PART I THEORY AND COMPUTATIONAL METHODS 2 Computational Approaches for Plasmonics 83 M. A. Yurkin 2.1 Introduction 83 2.2 Overview of Different Methods 84 2.2.1 Finite Scatterers in Homogeneous Medium 84 2.2.2 Periodic Scatterers 86 2.2.3 Scatterers Near an Infinite Plane Surface 87 2.3 Electric Permittivity 88 2.4 Theory of the DDA 90 2.4.1 General Framework 90 2.4.2 Numerical Scheme 95 2.4.2.1 Iterative methods to solve the linear system 95 2.4.2.2 Block-Toeplitz structure and FFT acceleration 99 2.4.2.3 Orientation averaging and repeated calculations 102 2.4.3 Existing Formulations 104 2.4.3.1 Interaction term 104 2.4.3.2 Polarizability prescription 106 2.4.3.3 Calculating measurable quantities 110 2.4.3.4 Decreasing shape errors 112
Contents vii 2.5 Practical Aspects of DDA Simulations 113 2.5.1 General Applicability 113 2.5.2 System Requirements 114 2.5.3 Free Parameters 115 2.5.4 Available Codes 117 2.6 Accuracy of the DDA 119 3 Size and Shape Dependence of Localized Surface Plasmon Resonances 137 S. D Agostino 3.1 Introduction 138 3.2 Size Dependence 139 3.2.1 Small Nanoparticles and Surface Damping 139 3.2.2 Large Nanoparticles and Higher-Order Modes 140 3.3 Shape Dependence 144 3.3.1 Radially Symmetric Nanoparticles 144 3.3.1.1 Ellipsoids and spheroids 144 3.3.1.2 Cylinders and disks 146 3.3.2 Prisms and Finite-Number Facets Nanoparticles 148 3.3.2.1 Cubes 148 3.3.2.2 Triangular prisms 150 3.3.3 Polyhedral Nanoparticles 152 3.3.3.1 Truncated cubes 152 3.3.3.2 Rounded, regular and marks decahedral nanoparticles 153 3.3.4 Multi-Tips Objects 156 3.3.4.1 Stars 156 3.3.4.2 Urchins-like nanoparticles 157 3.3.5 Void Nanoparticles 161 3.3.5.1 Nanoshells 161 3.3.5.2 Void cubes 164 3.4 Conclusions 165 4 Computational Molecular Photophysics 175 E. Fabiano 4.1 Introduction 175 4.2 Electronic Excitations 177
viii Contents 4.2.1 Electronic Absorption Spectrum 180 4.2.2 Vibrational Contributions 182 4.3 Photophysics 184 4.3.1 Semiclassical Description of Internal Conversion and Inter-System Crossing 187 4.4 Computational Methods for Excited States 190 4.4.1 Configuration Interaction 194 4.4.2 Time-Dependent Density Functional Theory 196 4.4.3 Linear Response TD-DFT 200 5 Metal Molecule Electrodynamic Coupling 213 S. Corni 5.1 Introduction 213 5.2 The Quasi-Static Limit 217 5.3 The Point-Dipole Model of the Molecule, and the Classical Metal Nanoparticle 219 5.3.1 Light Absorption 220 5.3.2 Light Emission 225 5.3.3 Light Scattering 234 5.3.4 The Dielectric Function of the Classical Metal Nanoparticle 237 5.4 Toward an ab initio Molecular Plasmonics 243 5.4.1 Coupling the ab-initio Description of the Molecule with a Continuous Metal 243 5.4.2 Fully ab initio Description of the Molecule Metal Nanoparticle Systems 246 PART II APPLICATIONS AND EXPERIMENTAL ASPECTS 6 Near-Fields in Assembled Plasmonic Nanostructures 261 P. K. Jain and C. Deeb 6.1 Introduction 262 6.2 Optical Properties of Metal Nanoparticles 262 6.2.1 Nanoplasmonic Field Enhancement 265 6.2.2 Tunability of the LSPR and Near-Field 266 6.3 Optical Properties of Coupled Nanoparticles 268 6.3.1 Nanoparticle Assemblies 268
Contents ix 6.3.1.1 LSPR response of assemblies 268 6.3.1.2 Field enhancement 268 6.3.1.3 Plasmonic wave-guiding 269 6.3.2 Polarization Dependence of Coupling in a Dimer 269 6.3.3 Dipolar-Coupling Model 271 6.3.4 Analogy to Exciton Coupling in Molecular Aggregates 273 6.3.5 Bonding and Anti-Bonding of Plasmons 275 6.4 Spatial Profile of the Near-Field 277 6.4.1 Distance-Dependence of the Near-Field 278 6.4.2 Size-Scaling of Near-Field Decay 278 6.4.3 Direct Mapping of the Near-Field 280 6.5 Applications of Near-Field Coupling Concepts 283 6.5.1 Plasmon Ruler 283 6.5.2 Metal Nanoshells 284 6.5.3 Coupling in Larger Arrays 285 6.5.4 Molecular Sensing 285 6.6 Future Outlook 286 7 Noble Metal Nanostructure Enhancement of Fluorescence 295 R. J. Phaneuf 7.1 Introduction 295 7.2 Nanostructure Size, Shape and Spacing Dependence 298 7.3 Role of Substrate 302 7.4 Standing Wave Surface Plasmons 306 7.5 Spacer Layer Effect 312 8 Surface-Enhanced Raman Scattering 321 M. Sun 8.1 Introduction 322 8.2 Electromagnetic Mechanism and Numerical Methods 323 8.3 Chemical Mechanism and Visualization Method of Charge Transfer 329 8.4 Synthesis and Experiment on SERS 333 8.5 Remote-Excitation SERS 339 8.6 Conclusions 346
x Contents 9 Parabolic Mirror Assisted Gap-Mode Optical Ultramicroscopy 355 D. Zhang and A. J. Meixner 9.1 Introduction 355 9.2 Principles 356 9.2.1 Instrumentation 359 9.2.1.1 Optics layout 359 9.2.2 PM Optics 361 9.2.3 Tip-Sample Distance Control and Image Recording 365 9.3 Different Types of Gap-Modes 367 9.3.1 Gap-Mode of Metallic System 367 9.3.1.1 Au tip and Au substrate 367 9.3.1.2 Au tip and monolayer adsorbates/au substrate 372 9.3.1.3 Au tip and single molecule/au substrate 377 9.3.2 Gap-Mode of Metal-Organic Semiconductor System 379 9.3.2.1 Au tip and diindenoperylene molecule 379 9.3.2.2 Au tip and organic solar cell blends 379 9.3.2.3 Gap-mode of metal-inorganic semiconductor system 381 9.4 Conclusion 387 10 Wet-Chemical Synthesis Techniques for Colloidal Plasmonic Nanostructures Assisted by Convective or Microwave Dielectric Heating 395 L. Carbone 10.1 Introduction 395 10.1.1 Wet-Chemical Synthesis: Basic Principles 397 10.1.1.1 Hybrid nanoarchitecture formation 401 10.2 Synthesis under Conventional Convective Heating 402 10.2.1 Hard-Templated Growth 402 10.2.2 Precipitation-Promoted Growth 403 10.2.3 Electrochemical and Shape-Controlled Growth 405
Contents xi 10.2.4 Photochemical and Shape-Controlled Growth 408 10.2.5 Galvanic Displacement-Mediated Growth 409 10.2.6 Seed-Promoted Growth 416 10.2.6.1 Homogeneous nucleation 417 10.2.6.2 Heterogeneous nucleation 421 10.3 Synthesis under Microwave Dielectric Heating 425 10.3.1 Basic Principles of Microwave-Promoted Growth 425 10.3.2 Mono-Metallic Nanostructures 429 10.3.3 Bi-Metallic Hetero-Nanostructures 434 10.4 Conclusions 440 Index 463
Preface The term molecular plasmonics refers to a rapidly growing interdisciplinary science which aims at investigating the coupling, at the nanoscale level, between emitting molecules and metallic nanostructures. Collective oscillations of the conduction electrons, named surface plasmons, can strongly enhance the electromagnetic field around metallic nanoparticles and nano-patterned surfaces: the optical responses (absorption, fluorescence, lifetime, and Raman scattering) of molecules close to the metal are thus strongly modified. Despite these effects have been known since the seventies, it is only with the recent progress in the synthesis and fabrication of nanosystems as well as innovations in the characterization procedures and spectroscopies that interest in molecular plasmonics has been boosted. Surface-enhanced Raman spectroscopy (SERS), localized surface plasmon resonance (LSPR) spectroscopy, and metalenhanced fluorescence (MEF) find large applications in biology, to realize plasmonic biosensors or to detect molecular-binding events, as well as in medicine, for molecular-specific imaging, detection, and photothermal therapy of cancer. Surface plasmons and MEF are also widely used in organic opto-electronics, photonics, and energy-conversion applications. In addition, different theoretical approaches and modeling tools have been developed in recent years to describe both organic molecules and metal nanoparticles as well as their interactions, with increased accuracy and efficiency. Molecular plasmonics thus has great interdisciplinary appeal, attracting researchers from fields as diverse as telecommunication engineering (as emitting molecules behave like electromagnetic antennas), inorganic chemistry (to synthesize metal nanoparticles), quantum mechanics (to describe optical properties of molecules and
xiv Preface metals), nano-photonics (to manipulate light at a length scale below the diffraction limit), and optical microscopy (to measure the nearfield around metallic objects). Handbook of Molecular Plasmonics is intended for a broad readership and contains both high-level specialized chapters and introductory chapters as well as theoretical and experimental reviews. The main idea underlying this project is to create a useful feedback between theory and experiments, giving a theoretical reference to experimentalists and, at the same time, new inputs to theoreticians for further developments. This handbook is organized in 10 chapters that reflect the current status of this evolving scientific field, discuss the most recent developments, and identify the directions of future research. Chapter 1 introduces the basic foundations of molecular plasmonics. It is a self-contained chapter, starting with Maxwell s equations and concluding with the derivation of the radiative and non-radiative decay rates of emitting molecules near metal surfaces and nanoparticles. After this introductory chapter, the handbook is subdivided in two parts: the first one describes the computational and theoretical methods of interest in molecular plasmonics, while the second is entirely dedicated to the most relevant applications and experimental techniques. Both parts contain precious contributions from international experts to ensure a plurality of points of view. Part I, Theory and Computational Methods, opens with a chapter by M. A. Yurkin (Russia) who describes in detail the Discrete Dipole Approximation (DDA) approach, which is an efficient method to study the absorption and scattering of metal nanoparticles of arbitrary shapes. This chapter will serve as an important reference for theoreticians to model metal nanoparticles. Chapter 3 reports DDA results for nanoparticles of different sizes and shapes. This systematic analysis, inspired by recent literature, should represent an important reference for both experimentalists and theoreticians to verify and compare the absorption and scattering spectra of different nanoparticles. While these first two chapters are completely dedicated to metal nanoparticles, Chapter 4 introduces the discussion about the molecular counterpart. In this chapter E. Fabiano (Italy) sheds light on the optical and photophysical
Preface xv properties of organic molecules as well as theoretical methods of quantum mechanics. The interaction between metals and organic molecules is then analyzed in detail in Chapter 5 by Stefano Corni (Italy). This chapter represents the synthesis of the concepts of all previous chapters. Part II, Applications and Experimental Aspects, starts with a chapter by P. K. Jain (Illinois) and C. Deeb (Illinois). They describe the coupling between two plasmonic nanostructures and concepts and applications of the optical near-field. This chapter serves as a link between theory and experiments. In Chapter 7, R. J. Phaneuf (Maryland) discusses recent results on MEF. Both localized and delocalized surface plasmon resonances are presented, including comparison between theory and experiments. Chapter 8 is dedicated to SERS, and M. Sun (China) gives special emphasis to theoretical calculations of charge-transfer effects and to experimental results on remote SERS excitation, a treatment which should be useful for both theoreticians and experimentalists. In Chapter 9, D. Zhang and A. J. Meixner (Germany) describe nearfield microscopy and the techniques based on the parabolic mirror confocal microscope. Different types of gap modes are discussed and relevant experimental images and results are presented. The last chapter is devoted to the colloidal synthesis of metal nanoparticles. L. Carbone (Italy) presents a complete review on different synthetic approaches, ranging from convective to microwave heating. This chapter should prove very useful not only to chemists but also to readers from other backgrounds as a reference for all types of nanoparticles that can be realized experimentally. It has been a great pleasure to work with all these leading scientists of this field, and we thank all of them for their support. We hope this handbook proves to be a helpful reference manual and a useful tool for both students and experts in this field, and we encourage readers to give their valuable comments/suggestions so that the book can be improved further. Fabio Della Sala Stefania D Agostino Summer 2013
Acknowledgment We would like to thank all our collegues of the National Nanotechnology Laboratory who have collaborated with us in the field of molecular plasmonics. F. D. S. thanks his wife Erika for her constant support and for proofreading the book. S. D. thanks the Photonics and Nanostructures group of the Physics Department of Pavia for the precious friendship and support received during the period of the project.