Manipulation of exciton dynamics in macrocycle molecules and inorganic semiconductor nanocrystals

The temporal dynamics of excitons and the evolution of excited states of a material system reflect both the excitation conditions and the final destination of the excitation energy. Precise control of material structure through modern nanofabrication provides nanostructures with well-defined relaxation paths of excitons, which can be manipulated and probed using external stimulation. In particular, electrostatic manipulation of exciton dynamics with external electric fields can be used to study electronic properties of novel material systems such as semiconductor nanocrystals and pi-conjugated molecules, which may be well suited for future applications in optoelectronic devices. In this work, electric field induced quenching of photoluminescence through generation of indirect excitons is performed on colloidal tetrapod heterostructure nanocrystals and a multichromophoric model molecular system. The dependence of quenching on optical excitation density, which shows opposite trends in these two material systems, reflects the specific origin of quenching in each system. The large reduction in decay lifetime of indirect excitons in the tetrapods also enables storage of optical information with external electric field, which can be observed using time-resolved spectroscopy. As a model light-harvesting system with efficient energy funneling from the arm to the core, the tetrapod is an ideal system to study impact of electric field on multiexcitons in the core and the "hot" excitons in the arm, thus providing insight on the effects of an electric field on intrapartical energy transfer. While energy transfer in the heterostructure tetrapods is through direct charge carrier thermalization, it is the coherent and incoherent energy transfer that couple chromophores in the multichromophoric molecules which mimic the intermolecular interactions in organic electronics. Both single molecule spectroscopy and time-resolved spectroscopy were employed to probe the structural dependent coherent and incoherent energy transfer. Briefly, this work consists of four main results. (1) Quenching in tetrapods is due to the localization of indirect excitons at trap sites which causes saturation of quenching at high excitation density. (2) Multiexcitons and arm excitons with fast decay lifetimes are not affected by an external electric field since electrostatic manipulation is not instantaneous. (3) Coherent coupling between chromophores causes changes in spectrum and decay lifetime, while the incoherent coupling leaves a dimer as a single quantum emitter and causes structural dependent emission depolarization. (4) Field induced quenching increases with the increase of excitation density and number of chromophores in multichorphoric molecules.

MANIPULATION OF EXCITON DYNAMICS IN MACROCYCLE MOLECULES AND INORGANIC SEMICONDUCTOR NANOCRYSTALS by Su Liu A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics Department of Physics and Astronomy The University of Utah August 2012 Copyright © Su Liu 2012 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL The following faculty members served as the supervisory committee chair and members for the dissertation of_________Su Liu_____________________________. Dates at right indicate the members' approval of the dissertation. ____________John M. Lupton_______________, Chair _05/09/2012______ Date Approved ____________Adam Bolton_________________, Member _05/09/2012______ Date Approved ___________Andrey Rogachev______________, Member _05/09/2012______ Date Approved ____________Yong-Shi Wu________________, Member _05/09/2012______ Date Approved ____________Michal Morse________________, Member _05/09/2012______ Date Approved The dissertation has also been approved by__________David Kieda_______________ Chair of the Department/School/College of___Physics and Astronomy_______________ and by Charles A. Wight, Dean of The Graduate School. ABSTRACT The temporal dynamics of excitons and the evolution of excited states of a material system reflect both the excitation conditions and the final destination of the excitation energy. Precise control of material structure through modern nanofabrication provides nanostructures with well-defined relaxation paths of excitons, which can be manipulated and probed using external stimulation. In particular, electrostatic manipulation of exciton dynamics with external electric fields can be used to study electronic properties of novel material systems such as semiconductor nanocrystals and pi-conjugated molecules, which may be well suited for future applications in optoelectronic devices. In this work, electric field induced quenching of photoluminescence through generation of indirect excitons is performed on colloidal tetrapod heterostructure nanocrystals and a multichromophoric model molecular system. The dependence of quenching on optical excitation density, which shows opposite trends in these two material systems, reflects the specific origin of quenching in each system. The large reduction in decay lifetime of indirect excitons in the tetrapods also enables storage of optical information with external electric field, which can be observed using time-resolved spectroscopy. As a model light-harvesting system with efficient energy funneling from the arm to the core, the tetrapod is an ideal system to study impact of electric field on multiexcitons in the core and the "hot" excitons in the arm, thus providing insight on the effects of an electric field on intrapartical energy transfer. While iv energy transfer in the heterostructure tetrapods is through direct charge carrier thermalization, it is the coherent and incoherent energy transfer that couple chromophores in the multichromophoric molecules which mimic the intermolecular interactions in organic electronics. Both single molecule spectroscopy and time-resolved spectroscopy were employed to probe the structural dependent coherent and incoherent energy transfer. Briefly, this work consists of four main results. (1) Quenching in tetrapods is due to the localization of indirect excitons at trap sites which causes saturation of quenching at high excitation density. (2) Multiexcitons and arm excitons with fast decay lifetimes are not affected by an external electric field since electrostatic manipulation is not instantaneous. (3) Coherent coupling between chromophores causes changes in spectrum and decay lifetime, while the incoherent coupling leaves a dimer as a single quantum emitter and causes structural dependent emission depolarization. (4) Field induced quenching increases with the increase of excitation density and number of chromophores in multichorphoric molecules. CONTENTS ABSTRACT…………………………………………………………………………….iii LIST OF FIGURES……………………………………………………………………viii ACKNOWLEDGEMENTS.............................................................................................xi CHAPTERS 1 INTRODUCTION....................................................................................................... 1 1.1 Colloidal Semiconductor Nanocrystals................................................................... 4 1.1.1 Structure Properties ..................................................................................... 6 1.1.2 Electronic Properties ................................................................................. 10 1.1.3 Optical Properties...................................................................................... 16 1.1.4 Multiexcitons ............................................................................................ 19 1.2 π-Conjugated Molecules ....................................................................................... 24 1.2.1 Pi-conjugation ........................................................................................... 25 1.2.2 Intrinsic Electrical Properties .................................................................... 28 1.2.3 Intrinsic Optical Properties ....................................................................... 31 1.3 Energy Transfer and Molecular Aggregates ......................................................... 33 1.3.1 Motivation of Using Model Systems ........................................................ 33 1.3.2 Incoherent Energy Transfer ...................................................................... 36 1.3.3 Coherent Energy Transfer ......................................................................... 39 1.3.4 Aggregate and Excimer Formation ........................................................... 43 1.4 Blinking, Spectral Diffusion and Photobleaching ................................................ 47 1.4.1 In Colloidal Nanocrystals ......................................................................... 47 1.4.2 In π-conjugated Organic Molecules .......................................................... 53 1.5 Electrostatic Manipulation of Electric and Optical Properties ............................. 56 1.5.1 Stark effect ................................................................................................ 57 1.5.2 In Nanocrystals ......................................................................................... 58 vi 1.5.3 In Organic Molecules ................................................................................ 62 2 EXPERIMENTAL METHODS .............................................................................. 65 2.1 Concepts of Time-Resolved Spectroscopy ........................................................... 65 2.1.1 Gated Spectroscopy .................................................................................. 66 2.1.2 Field Induced Intensity Quenching and Exciton Storage ......................... 71 2.2 Single Molecule Spectroscopy .............................................................................. 73 2.3 Materials ............................................................................................................... 74 2.3.1 CdSe/CdS Tetrapods ................................................................................. 74 2.3.2 Cofacial pi-Conjugated Dimers ................................................................ 76 2.4 Sample Preparation ............................................................................................... 79 2.4.1 Capacitor Device for Field Induced Effects .............................................. 79 2.4.2 Single Molecule Sample ........................................................................... 81 2.5 Experimental Setup ............................................................................................... 82 2.5.1 Setup for Measurement of Field Induced Effects ..................................... 82 2.5.2 Single Molecule Microscopy and Streak Camera Spectroscopy .............. 86 3 COHERENT AND INCOHERENT INTERACTIONS BETWEEN COFACIAL Pi-CONJUGATED OLIGOMER DIMERS IN MACROCYCLE TEMPLATES ... 90 3.1 Abstract ................................................................................................................. 91 3.2 Introduction ........................................................................................................... 92 3.3 Results and Discussion ......................................................................................... 94 3.3.1 Photophysical Characterization and Self-Assembly ................................. 94 3.4 Field Induced Quenching .................................................................................... 105 3.5 Conclusion .......................................................................................................... 110 3.6 Supporting Information ....................................................................................... 111 3.6.1 Photoluminescence of Bulk Film ............................................................ 111 3.6.2 Solvatochromism .................................................................................... 113 3.6.3 Fluorescence Depolarization ................................................................... 115 3.7 Acknowledgements ............................................................................................. 117 4 EXCITON STORAGE IN CDSE/CDS TETRAPOD SEMICONDUCTOR NANOCRYSTALS: ELECTRIC FIELD EFFECTS ON EXCITON AND MULTIEXCITON STATES…… ................................................................................ 118 4.1 Abstract ............................................................................................................... 118 vii 4.2 Introduction ......................................................................................................... 119 4.3 Results and Discussion ....................................................................................... 121 4.3.1 Dependence of Quenching on Excitation Density .................................. 121 4.3.2 Impact of Electric Field on Multiexcitons .............................................. 130 4.4 Acknowledgment ................................................................................................ 134 5 CONCLUSIONS ........................................................................................................ 135 5.1 Summary of Scientific Contribution ................................................................... 135 5.1.1 Intramolecular Interchromophoric Interaction ........................................ 135 5.1.2 Field Induced Effects .............................................................................. 136 5.2 Future Work ........................................................................................................ 138 REFERENCES………………………………………………………………………...141 LIST OF FIGURES 1.1 Structures of three colloidal heterostructure nanocrystals. ........................................... 9 1.2 Band diagrams of the molecule, nanocrystal and bulk semiconductor.. ..................... 11 1.3 Size dependence of absorption and emission spectra. ................................................ 12 1.4 Three types of band structures of heterostructure nanocrystals. ................................. 14 1.5 Temperature dependent PL decay of single NCs (redraw from Ref.64).. .................. 18 1.6 Generation of multiexcitons. ....................................................................................... 20 1.7 Increase of decay rate with increasing excitation density.. ......................................... 23 1.8 sp3 hybridization and sp hybridization. ....................................................................... 26 1.9 π-π* transitions in 1,3-butadiene. ................................................................................ 28 1.10 Schematic of the S0-S1 transition with vibrational levels.. ....................................... 32 1.11 Energy transfer between chromophores along a conjugated polymer chain. .......... 35 1.12 Four examples of model molecular systems.. ........................................................... 36 1.13 Splitting of excited state in the strong coupling regime. .......................................... 40 1.14 Comparison of incoherent and coherent energy transfer .......................................... 44 1.15 Comparison of three configurations in dimers and the corresponding emission under each configuration.. .................................................................................................. 45 1.16 Diagram of ground-state and excimer potentials and the emission of the excimer. . 46 1.17 Correlated blinking with spectral jumps of a single nanocrystal ............................. 50 1.18 Stepwise photobleaching and correlated fluorescence lifetime of a single dimer .... 55 1.19 Stark effect in a single core/shell nanorod. ............................................................... 59 ix 1.20 "write", "store" and ‘read-out" of excitons in double quantum wells. ..................... 61 1.21 Schematic of singlet-singlet annihilation. ................................................................. 64 2.1 Schematic and gating mechanism of a gated ICCD.................................................... 67 2.2 A time series of PL spectra and corresponding intensity decay of dual-color NCs. .. 68 2.3 Schematic and operation principle of a streak camera. ............................................... 70 2.4 Setup for measurement of fluorescence anisotropy. ................................................... 70 2.5 Schematic of pulse sequences to isolate different effects. .......................................... 72 2.6 Properties of CdSe/CdS tetrapod heterostructures ...................................................... 75 2.7 Chemical structures of the monomer (1), the closed dimers (2-4) and the open dimer (5). ............................................................................................................................... 77 2.8 Properties of the monomer.. ........................................................................................ 78 2.9 Cartoon of the layer structure of a capacitor device. .................................................. 79 2.10 RC time constant of capacitor device.. ..................................................................... 81 2.11 Diagram of the setup for measurement of quenching effect. ................................... 83 2.12 SMS and streak camera setup. .................................................................................. 87 3.1 Structures of the phenylene-ethynylene-butadiynylene oligomer 1 and dimers 2-5. . 95 3.2 Photophysical characteristics of compounds 1-5 ........................................................ 96 3.3 STM images and molecular models of self-assembled adsorbate. ........................... 100 3.4 Fluorescence anisotropy decay. ................................................................................ 102 3.5 Low-temperature (5 K) single-molecule luminescence spectra as a function of time ........................................................................................................................... 104 3.6 Molecular structure and PL spectrum of 6. ............................................................... 107 3.7 Dependence of quenching on excitation density of 1 (black), 3 (red), and 6 (blue) at 20 K. .......................................................................................................................... 108 x 3.8 PL spectra and time-resolved luminescence of molecules 1, 2, 4, and 5 in bulk films .......................................................................................................................... 112 3.9 Normalized solution emission spectra of molecules 1, 2, 4, 5 in hexane (black), THF (red), chlorobenzene (green) and chloroform (blue). ................................................ 114 3.10 Fluorescence anisotropy decay for dilute solutions of the monomer 1, closed dimers 2-4 and the open dimer 5. ...................................................................................... 116 4.1 Separation and storage of excitons in an electric field. ............................................ 124 4.2 Dependence of PL intensity and quenching efficiency on excitation density at different external electric field strengths.. ............................................................... 126 4.3 Transient luminescence of a device for which the electrical pulse is applied after excitation by a laser pulse. ...................................................................................... 128 4.4 Multiexciton emission from tetrapods at high excitation densities. ....................... 131 ACKNOWLEDGEMENTS A Ph.D. thesis is not a single person task. I would not have completed mine without the guidance, help and support from various people and sources. Here are the people who make this work possible.  Prof. John M. Lupton, who was my supervisor since 2007. The past five years' learning and research experience with John has been a pure joy. He is always there when guidance is needed through the weekly group meetings or occasional one-to-one meetings. The broad knowledge John has makes it easy to stay on the right track of graduate study and research. The two things I like mostly about John's mentoring is his firm optimistic attitude toward research problems and sincere interest in science. He has the magic of turning the most disappointing lab results into the most promising ones, which creates a motivating environment for me and the whole group to do our best throughout the Ph.D. study. I also enjoyed and benefited from his high level of professionalism in maintaining a high scientific standard for both the publications and experimental infrastructures. In this respect, I am thankful to the hard work he put in writing grant proposals to keep my research projects funded and the lab updated, which also allowed me to go to conferences to let my work be known and meet people. I appreciate the opportunity he gave to me to join his wonderful group to learn to be a scientist and to experience a professional and encouraging research environment. xii  Prof. Adam Bolton, Prof. Andrey Rogachev, Prof. Yong-Shi, Wu and Prof. Michael Morse for serving on my thesis supervisory committee, their guidance in full completion of my graduate studies, and time spent on my behalf.  Dr. Nick J. Borys, who was a graduate student and then a postdoc in the group. He was the one who taught me mostly of lab conduct and experimental techniques. His willingness to help and enthusiasm for solving problems made my start in the group so much easier. I am in debt to Nick for his continuous efforts in keeping the labs as a well organized, functional and upgraded research place to work. I cannot thank Nick enough for writing the two papers together with me, during which he put no less than the effort I devoted and offered help when needed. He also proofread my thesis, which significantly improved the language and structure of it. Besides the scientific help he provided, Nick was also dedicated to keeping the group so integrated and hospitable that I enjoyed staying.  Kipp J. van Schooten for setting up the time-resolved spectroscopy lab together with me and purchasing the glovebox system used for device fabrication.  And all other members in the nanoscale optoelectronics group, Dr. Sebastian Bange, Dr.Debangshu Chaudhuri, Dr.Dongbo Li, Dr. Eyal Shafran, Douglas Baird and Alex Thiessen for creating a creative, supportive and amiable group.  Matt DeLong, Ed Munford, Randy Polson and Wayne Wingert for technical support.  Kathy Blair, Heidi Frank, Sara Gardner, Jackie Hadley, Vicki Nielsen, and Kathrine Skollingsberg for help with all the administrative matters. xiii  Dr. Golda Hukic and Dr. Maria Navas for doing this together with me and their friendship. For this thesis, close collaborations with the chemistry groups provided us the unique materials to work with. Special thanks are given to  Prof. Sigurd Höger and Daniela Schmitz for chemical synthesis of the macrocycle molecules used in this study and their work on self-assembly using STM.  Prof. Dimitri Talapin and Dr. Jing Huang for providing the CdSe/CdS tetrapod heterostructures used in this thesis. I would also like to thank my beloved parents, Hongli Pang and Yongmin Liu, for supporting me through this different journey and always believing in me. CHAPTER 1 INTRODUCTION In 1959, Dr. Feynman gave a visionary talk on phenomena at small length scales prior to in-depth investigation into this field. Feynman foresaw the "weird" effects that might happen at scales of just a few to a few thousand atoms. Today, 50 years later, scientists are driven by both the curiosity of understanding new science at this scale as well as the rapid decrease in size of semiconductor electronics to a regime when novel effects are already starting to be observed. One unique advantage of the nanoscale is the engineering of properties through the control of physical shape, size and surface properties.1 Through the collective work of chemists, engineers, physicists and even artists, the fabrication of nanostructures of controllable shape and properties is rapidly progressing. Nanofabrications through both bottom-up and top-down approaches have achieved single atomic layer precision in nanocrystals (NCs)2,3 and synthesis of molecules with designed structures.4,5 The precise control of structures enables the engineering of properties to realize desirable functions in a single nanostructure or an assembly of various functional nanostructures.2,6 Semiconductor NCs, often referred to as quantum dots, show significantly modified properties compared to the bulk counterparts that form the basis of all modern electronics. This modification resultes from spatial confinement of electron and hole wavefunctions in the nanoscale, which increases the band gap of NCs with decreasing size.7 Consequently, the absorption and emission spectra shift to shorter 2 wavelengths in smaller NCs.8 Additional structural properties, such as shape and surface morphology, have been shown to change the polarization, luminescence quantum yield and exciton dynamics of NCs.3,9-12 Synthesis of heterostructure NCs, which consist of multiple semiconductor materials, opens another route to engineer electronic structure, and consequently, properties.8 The difference between NCs and the bulk counterparts is even more drastic at the single particle level. The emission of a single NC undergoes both fluorescence intermittency and spectral diffusion due to charging and local electrical field fluctuation, respectively.13,14 Inspired by the latter effect, an external electric field has been applied and shown to change both emission intensity and energy of a single NC or an ensemble of NCs.15,16 The colloidal synthesis of NCs through wet-chemical processes is by far the simplest and most successful method, in terms of quality of NCs and precision in control over structure and composition.17 The simple and precise fabrication, large absorption cross section and photostability compared to organic dyes, in turn motivate application of NCs in various areas such as gain medium in lasers,18 biolabeling,2 photovoltaic devices,19,20 and data storage.16,21,22 Unlike inorganic colloidal semiconductor NCs, of which the properties are mainly engineered through control over size during synthesis, organic pi-conjugated molecules show properties that correspond to the chemical structures. They are a class of materials that demonstrate interesting optical and electronic properties that are similar to that of the inorganic semiconductors. In the meantime they also have advantages over the inorganic counterparts originating from their plastic nature. These advantages include easy processing, low cost, mechanical flexibility, and tunability of properties through chemical synthesis. Therefore, a new class of plastic electronics such as flexible large area 3 displays,23 organic solar cells,24 lasers,25 organic spintronicss,26 and organic field-effect transistors27 can be developed using cheap solution processing techniques. The nanotechnology comes into play in this field as a tool to study the complex processes of generation, transport, recombination and dissociation of charge carriers in organic films or crystals which are the active regions of organic electronics.28-30 Despite the successful application of both colloidal NCs and organic materials in (opto)electronic devices, there is a large amount of knowledge and plenty of mysteries of optical and electrical properties of these two material systems under practical conditions in devices and a great need for more study. For example, the exact process of separation and dissociation of excitons in NCs and organic molecules in an external electric field is still a mystery. Additionally, answers that address how properties of organic molecules are modified due to the presence of surrounding molecules are needed for optimization of devices. In this work, a systematic study of the interplay between photophysical properties and interchromophoric interaction (Chapter 3) and manipulation of excitons using an external electric field (Chapter 3 and 4) are performed, with the goal of revealing the dynamics of excitons in organic and inorganic semiconductors for practical applications. The scope of this thesis is as follows: Chapter 1 gives an introduction to the relevant properties of organic pi-conjugated molecules and inorganic colloidal semiconductor NCs. Both material systems exhibit optical and electrical properties that can be engineered through control of structural properties such as size, shape, or surface morphology. Studies of the dynamics of the excited states in these two materials systems provide insight into how excitons form, 4 relax, transfer, recombine and separate in the nanoscale systems. Manipulation of excitons with an external electric field through the Stark effect is shown to provide an additional channel to control separation and storage of charge carriers. To study optoelectronic and dynamical properties of NCs and pi-conjugated molecules, two gated spectroscopic setups are used in this work. Chapter 2 first gives a detailed introduction to the operation principle of gated spectroscopy and its applications in the study of dynamics. Then, an introduction to the two material systems-colloidal heterostructure NCs and a model molecular system, as well as the fabrication techniques of samples, are given. At the end, the operation of the two setups is described. Chapters 3 and 4 present detailed discussions of experimental results. Chapter 3 starts with a systematic investigation of interchromophoric intramolecular interactions and electric field induced intensity quenching using a model molecular system. Then, in Chapter 4, intensive study of exciton separation, storage and detrapping with an external electric field is performed on colloidal NCs. Chapter 5 summarizes the main results and the scientific contributions, which are followed by an overview of future work. 1.1 Colloidal Semiconductor Nanocrystals Among several methods to fabricate nanostructures, the wet-chemical process for the synthesis of colloidal NCs stands out as the most versatile one due to its simplicity and high controllability over the size and shape of as-synthesized II-VI semiconductor NCs.2,17,20 Thus II-VI colloidal semiconductor NCs are the most frequently studied systems, among which CdSe has become the model system which is frequently studied to reveal the optical and electrical properties of NCs. CdSe is also the core material of the 5 heterostructure NCs under investigation in Chapter 4. In addition, colloidal synthesis of III-V semiconductor NCs, such as InP,31 and nonepitaxial growth of metal-semiconductor hybrid NCs,6 have also been developed to exploit rich phenomena in these systems. Through careful selection of experimental conditions, such as temperature, reagent concentration, surfactant, etc., NCs of spherical, rod and branched shape can be synthesized with nearly atomic precision of the size of a few to tens of nanometers. 17 The nanometer spatial extent of NCs applies an extra boundary condition that confines the electron wavefunction in a region that is smaller or comparable to the exciton radius. This confinement causes NCs to differ from their bulk counterparts through the so called quantum confinement. As a direct consequence, size tunability of optical and electrical properties of NCs enables applications in the full visible-IR spectral range. For example, quantum dot based lasers of tunable wavelengths can be designed simply by exciting particles of different size.18 Another unique effect in NCs is fluorescence intermittency and spectral diffusion, which is due to trapping of charges on localized trap sites in NCs and the sensitivity of NCs to the fluctuations of local electric field, respectively.32 These temporal fluctuations in emission have long been an obstacle in the application of NCs in biolabeling, which has now been lifted by the successful synthesis of non-blinking quantum dots through various surface passivation methods,10,33,34 which remove surface defects that are the main sources of trap sites. Proper surface passivation can significantly reduce the number of surface defects and increase the fluorescence quantum yield of NCs. The remaining defects can still influence properties and the interaction with external stimuli such as an electric field. 6 In this section, the basic structural, optical and electrical properties of semiconductor NCs will be introduced, with emphasis on carrier delocalization and the generation of multiexcitons in heterostructure NCs. 1.1.1 Structure Properties Depending on whether the conduction band is empty, nearly full, or partially full (10-90%),35 solid state materials can be characterized into insulator, semiconductor and metal, respectively, based on their conductivity. Although categorizing materials is largely determined by the elements they consist of, the relative electrical and optical properties of each type of material are solely shown in the bulk solid state, not in the atomic level. Therefore, how each atom is periodically arranged to form bulk materials of certain crystal structure, also plays an important role in determining the collective properties of all the building blocks. A typical example arises from carbon based materials, where diamond and graphite have exactly the same chemical composition, but are characterized as insulator and semimetal respectively. As shown in more detail in Section 1.2, pi-conjugated molecules consisting of the same atoms can also have distinctive conducting properties due to differences in chemical structures. Similar structure-property relations also exist in semiconductor NCs due to spatial confinement where quantum phenomena are present. However, the quantum confinement applies in different ways in inorganic semiconductor NCs compared with organic pi-conjugated molecules. The wavefunction is confined by the size and the shape in the former system but is confined in broken conjugation segments in the latter case. Nanostructures of various sizes and shapes can be fabricated using modern nanofabrication techniques with atomic precision. 7 There are two approaches to fabricate nanostructures: the top-down and the bottom-up. The top-down methods for nanofabrication involve patterning and refining materials in the macroscopic scale into small subunits in the nanoscale. Lithography, masking and etching are three typical top-down techniques that are heavily used in traditional semiconductor industry. With the most state-of-the-art e-beam lithography,36 sub-10 nm spatial resolution has been achieved. In the opposite way, the bottom-up approach assembles building blocks, such as atoms, molecules and nanoparticles, into functional superstructures by chemical synthesis, layer-by-layer epitaxial growth, self-assembly, scanning probing microscope lithography etc.5,8,20,37-41 The latter approach goes beyond the achievements of the top-down methods, with capability of fabricating nanostructures with predictable, designable and controllable properties through control of size, shape, composition and morphology. This approach may potentially lift the limitation on fabrication of traditional electronics. Two of the well-established bottom-up techniques for fabrication of crystalline nanostructures or NCs are epitaxial growth and colloidal synthesis. The term epitaxial refers to growth of thin films on top of crystalline substrates, where the grown film is atomically arranged in the way accepted by the crystallographic structure of the substrate. Developed from the fabrication of quantum wells, strain driven formation of semiconductor NCs can be realized by depositing a layer of highly lattice mismatched film (e.g., InAs) on top of a substrate (e.g., GaAs). The uniform size and shape distribution is permitted by a controlled ripening process under selected deposition conditions.42 Thus-formed NCs have well-defined boundaries and are much more stable than the colloidal ones. They have been used for the study of nanoscale optical and 8 electrical properties,38,43 quantum coupling,44 spintronics,45,46 lasers,47 and data storage.21,22 Colloidal NCs are synthesized from hot solutions in simple glasswares, and can be ready to use in solution or freely dispersed into other media. This simplicity is a great advantage over the epitaxial growth of NCs bonded to substrates which also require sophisticated apparatus. The colloidal synthesis has been proven to be the most successful method also in terms of quality and monodispersity of synthesized NCs. This approach expands the scope of the parameter space that can be exploited in terms of shape, surface morphology and composition. With this great flexibility, a collection of NCs, ranging from homogeneous spherical particles48 to more complicated heterostructure nanotetrapods49 and core/shell/shell/shell nanoparticles,50 have been synthesized and studied. And the composition in a single NC has gone beyond pure semiconductor material, to hybrid organic-inorganic or semiconductor-metal.6,10,51 Colloidal NCs that are equipped with functional groups for either biocompatibility or targeting are successfully used in biolabeling and biosensing.2,51,52 More importantly, the easy realization of colloidal heterostructure NCs consisting of two and three semiconductor materials opens an additional channel to manipulate charge carrier thermalization. Figure 1.1 demonstrates a few examples of the structure of colloidal NCs. Generally, the colloidal synthesis is temporally separated into two steps: relatively rapid nucleation and the following slow growth, for narrow size distribution.48 Nucleation is initiated by quick injection of the precursor into the hot coordination solvent, resulting in decomposition of precursor regents and supersaturation of monomer that is relieved by the generation of nuclei. Then the growth of particles is followed by adding monomers to 9 the existing nuclei, which grow into large particles at lower temperature. Size is controlled by reaction temperature, precursor concentration, concentration of coordinating ligands and so on;17,53 shape can be controlled through selective adhesion of ligands to prevent growth in certain direction,53 suitable monomer concentration,54 or growth from seeds of specific crystal structure.49 Due to the high surface to volume ratio of NCs, surface defect emission can dominate the photoluminescence (PL) spectrum and significantly reduce PL quantum yield.55,56 Therefore, surface passivation using either organic ligands or an outer inorganic shell of large band gap has become a standard practice.1,3,10 Blinking and spectral diffusion are more delicate phenomena that are directly linked to surface states.32 Suppression of blinking can principally be achieved in large particles,46,57 or through various surface passivation techniques, such as alloy types of interface,34 thick epitaxially grown shell,33 or good organic capping ligands.10 The effect of defects, commonly termed "traps", on optical and electronic properties will be further discussed in Section 1.4.1. Figure 1.1 Structures of three colloidal heterostructure nanocrystals.12 10 1.1.2 Electronic Properties The most important parameter of a semiconductor material is the energy gap between the conduction band and the valence band. In bulk materials, atoms are arranged periodically according to the crystalline lattice to approximately infinity, where electrons move in the periodical potential of the lattice. To explain the existence of electronic bands and the band gap, a simple picture of just two atoms, each of which contributing one conduction electron, is a good place to start. Depending on whether the wavefunctions of the two electrons overlap (bonding) or repel (antibonding) each other, two energy levels form: the bonding state of lower energy and the antibonding state of higher energy, which are similar to the highest occupied molecule orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels in the molecular orbital theory (see Section 1.2.2). Without external stimuli, both electrons occupy the bonding state, which is the ground state. In the bulk semiconductor, 2N electrons fill up the lower energy bonding states of negligible spacing compared to the thermal energy, and thus form the filled valence band; in the meantime, the antibonding states of higher energy form the empty conduction band.35,58 In the language of the Bloch theorem, the two standing waves forming at the boundaries of the Brillouin zone, respectively, represent the anti-bonding state where electron density concentrates around atoms and the bonding state where electron density concentrates between atoms.35 An energy gap forms between the conduction band and the valence band where no energy state is allowed. The size of the band gap of a bulk semiconductor is a material constant that does not change with physical dimensions. However, in semiconductor NCs of finite spatial extent, an additional standing wave condition is imposed by the confinement boundaries in one 11 dimension or multiple dimensions and allows only a few transitions out of the continuous band in the bulk.7 Therefore NCs form discrete energy states of spacing larger than the thermal energy, similar to that of molecular compounds, as shown in Figure 1.2, rather than the continuous energy distribution as in the bulk. The energy gaps of NCs increases inversely with size R roughly following a function 1/R2 due to the increase of kinetic energy in smaller NCs, which is the reason of the size-dependent shift of absorption and emission spectra as shown in Figure 1.3.59-61 This effect in low dimensional semiconductor systems is referred to as Quantum confinement and was first investigated by Efros and Brus.7,62 Due to band filling or weak interaction, the quantum confinement effect sets in at a relatively larger size in semiconductors compared to metal, insulator or molecular crystals.1 The energy diagram of NCs shown in Figure 1.2 is very simplified based on the particle-in-a-sphere model for spherical particles. Each hole energy level is eightfold degenerate,61 which is lifted when taking into account the shape asymmetry, crystal field and the exchange interaction. Consequently, each Figure 1.2 Band diagrams of the molecule, nanocrystal and bulk semiconductor. Compared to continuous conduction and valence band that are separated by an energetic gap Eg in bulk, each molecular orbital is a discrete energy level in the molecules. Nanocrystals show features that bridge the gap of both extremes. 12 Figure 1.3 Size dependence of absorption and emission spectra. (a) The absorption and emission spectra shift to a higher energy as size of nanocrystals decreases. Several peaks (indicated with arrows) can be observed in the absorption spectrum due to the splitting of the band edge state.63 (b) The energy of the first excited state (indicated by the red arrow in (a) is plotted as a function of 1/R2.61 Black line is a guide to the eye. degenerate level can split into a set of sublevels, which explains the fine structure observed in the absorption spectrum and the existence of the "dark exciton."64,65 As a NC is optically or electrically excited, an electron is promoted from the 1Sh state to the 1Se or higher states, leaving a hole behind and forming a bound exciton with the hole. The exciton binding energy in the bulk semiconductor depends on the exciton Bohr radius r and can be defined as (1.1) 13 where e, ε0 and ε are the charge of a electron, vacuum permittivity, and the relative dielectric constant of the semiconductor, respectively. The exciton Bohr radius in bulk CdSe is 5.4 nm, corresponding to a binding energy of 16 meV.66 This small binding energy characterizes these weakly bound excitons as the Mott-Wannier type of excitons. Due to quantum confinement, the exciton binding energy in NCs increases with the decrease in size as the coulombic interaction between charge carriers gets stronger.66 For example, as the size of a CdSe nanoparticle decreases to 2.5 nm, the exciton binding energy increases to about 400 meV.67 The above binding argument works best in the region where the size of NCs is smaller or approaching the Bohr radius, where the coulombic interaction is the main force.66,68 In the strong quantum confinement regime, where size is smaller than the Bohr radius (the diameter of the CdSe cores of NCs in this study is 4 nm), the treatment of the electron and hole as a bound pair becomes difficult. Instead, it is more appropriate to treat them separately.60 But the size tunability of properties is still valid.66 An important effect arising from strong quantum confinement in NCs is the increased interaction between excitons due to the forced wavefunction overlap and the reduced dielectric screening.69 This effect results in formation of multiexcitons63 and rapid Auger processes at sufficiently high excitation density.70 The Auger processes can also result in charging of NCs which largely modify the electrical properties of NCs. Due to its importance to this study and the length consideration, a detailed discussion of charging will be given in Section 1.4.1. The above statements are made based on homogeneous semiconductor NCs, which consist of one semiconductor material. As mentioned in the proceeding text, higher band 14 gap materials are very often grown as an outer shell on top to passivate the inside core, which eliminates surface states, for example, in the CdSe/ZnS core/shell spherical NCs. In this type-I heterostructure, both electrons and holes are confined in the core of lower band gap as shown in Figure 1.4a.3 But the conduction band and the valence band of the shell can also be easily tuned to be either aligned, lower or higher than that of the core through size changes or use of different materials.49,71 Consequently, electron (hole) can localize in either the core or the shell, or delocalize over the entire NC.8,12,72 When electrons and holes are localized in separate regions within a single heterostructure NC, the type-II band alignment is achieved as shown in Figure 1.4b.71,73 One important application of type-II heterostructure NCs is in lasers18,71 due to the internal electric field induced by separated electrons and holes that lifts the degeneracy in the lowest excited state through the Stark effect. Therefore optical gain in the single exciton region can be obtained, which removes the obstacle of necessitating rapid optical gain decay due to nonradiative Auger recombination from the development of quantum dot lasers.18 Between the type-I and the type-II band alignment there lies an interesting region-the quasi type-II band alignment-where either conduction bands or valence bands of two different materials are aligned; as a consequence, one type of charge carrier is delocalized Figure 1.4 Three types of band structures of heterostructure nanocrystals. The electron is indicated as filled orange dot and the hole is indicated as blue open circle. 15 over the entire heterostructure while the other is localized in a smaller region.12,16,74 This wavefunction engineering realized in heterostructure NCs offers a new route to manipulate charge carriers through external stimuli such as electric field and charges,21,75 which are a significant part of this study. One great advantage of heterostructure NCs is the separation of the light absorbing region from the emitting region, which enormously increases the luminescence quantum efficiency of NCs as a light-harvesting system.3,49 For example, in CdSe/CdS nanotetrapods of 50 nm long CdS arms, 99% of incident light is absorbed in the arms rendering PL efficiency above 75%.49 After exciting with incident light of photon energy higher than the band gap, charge carrier pairs are first generated in the absorbing region and then rapidly relax into a suitable region within the heterostructure following the band structure as illustrated in Figure 1.4. In the CdSe/CdS nanorods of quasi type-II band alignment, it takes about 650 fs for the hole to localize in the CdSe core when excited in the CdS arm.76 However, the delocalization of the electron to the CdS arm is almost instantaneous when excited in the CdSe core.76 The presence of trap states can also affect the thermalization of charge carriers in heterostructure NCs by localizing the electron or hole at the trap sites instead of relaxing to quantum confined states.77 To provide further independence of wavefunction engineering of different functional regions within a single heterostructure NC, a type of core/shell/shell(/shell) spherical NCs was developed. This NC demonstrates dual-color emission from both the core and a shell layer.50,78 Independent tunabilities of both the core and the shell emitting regions enables control over the direction of energy flow between the core and the shell and the optimization of overall emission color as a white light source.50,78 16 Due to the small dimension of NCs, the shape also has a large impact on properties. For example, the shape asymmetry lifts the degeneracy in the band edge states which then split into a few sublevels. Each of these sublevels forms an peak in the absorption spectrum as shown in Figure 1.3.63,79,80 In CdSe/CdS heterostructure nanorods, the "bulb" of excess CdS formed around the CdSe-CdS interface has been shown to affect the sign of exciton-exciton binding energy and the transfer of excitons from the CdS arm to the CdSe core.12,81 In branched NCs like tetrapods, small variations in arm-to-arm diameter can shape the wavefunction to be more or less delocalized towards a certain arm and thus affects the emission anisotropy.74 1.1.3 Optical Properties Similar to the electrical properties, optical properties of NCs also exhibit structural dependence. In the early days, the study of optical properties of colloidal semiconductors was significantly hindered by the low quality of the materials. The large linewidth of emission spectrum due to the broad size distribution60 and strong defect emission obscured the observation of any structure-property relation.82 As discussed in Section 1.1.1, high quality NCs can be synthesized through improved synthesis methods, which show narrow size distribution, defect free emission, and high luminescence quantum yield. Size dependent absorption and emission of NCs as predicted by theory can be clearly observed as shown in Figure 1.3. The Stokes shift, defined by the energy difference between the first absorption peak and the emission peak, also decreases with increasing size.60,83 The PL emission generally originates from the band edge states even when the NCs are excited with photons of energy that are higher than the band gap. Thus generated "hot" electrons quickly relax to the band edge state through quick intraband 17 relaxation within a few hundred fs.80,84,85 Emission from a higher excited state is made possible through state filling when excitation density is sufficiently high. The dependence of optical properties on the shape of NCs is highly linked to wavefunction engineering as mentioned briefly in the last section. When the shape of the NCs changes from 0-D sphere to 1-D rods, the emission becomes linearly polarized along the long axis.86 This effect was later attributed to the large dielectric contrast between the nanorod and the surrounding matrix.9,87 The PL decay dynamics of NCs is strongly affected by the optically forbidden band edge state- the "dark" state-which is one of the splitting band edge states due to the exchange interaction. At room temperature, the PL intensity of NCs shows a monoexponential decay with a lifetime of a few to a few tens of ns at short delay time before it evolves into a power-law decay mediated by trap states at long delay time.59,64,88 The decay lifetime depends strongly on both the size of NCs and the temperature. Several experiments have shown that the lifetime decreases with the increase of size.59,60 This observation was first ascribed to surface localization of holes which generates two surface states of high and low oscillator strengths respectively.60 However, later investigations revealed that surface modification of NCs shows no effect on the lifetime, which disagrees with the surface state argument.83,89 Further experiments performed at liquid helium temperature or under magnetic field,64,89 established that the size and temperature dependence of the decay lifetime of NCs can be explained through band edge fine structures. As a simplified model, the band edge transition 1S(e)-1S3/2(h) split into two transitions due to exchange interaction: the "dark" state (|D>) that is optically passive and 18 the "bright" state (|B>) that is optically allowed as shown in Figure 1.5. The exchange interaction is proportional to the overlap of the electron and hole wavefunction, which can be up to tens of meV in NCs compared to a few meV in the bulk.83 Therefore, the energy gap ΔEB-D between the bright and the dark state is size dependent and larger for smaller NCs,83 which is the origin of the size dependence of radiative lifetime of NCs.59 When the thermal energy is lower than ΔEB-D, the thermalization rate γth between the "bright" and "dark" states is slow, and it is mostly the dark state that is populated through intersystem crossing γ0.64,83 Therefore, a single NC shows biexponential decay. The initial fast decay and the following slow decay are attributed to the bright exciton and the dark exciton, respectively.64 As temperature increases to a point that the thermal energy is Figure 1.5 Temperature dependent PL decay of single NCs (redrawn from Ref.64). At the low temperature, the single NC shows biexponential decay (red) due to the initial rapid recombination of the "bright" excitons and the following slow recombination of the "dark" excitons, respectively. At the high temperature, the decay curve becomes monoexponential (black) as thermalization between the "bright" and the "dark" states is faster than the recombination rate KB. For comparison, an ensemble of NCs shows a multiexponential decay (blue) due to different decay rates of NCs of different sizes. 19 close to or bigger than ΔEB-D, both states are equally populated since γth is larger than the radiative decay rate KB of the "bright" state. In this case, the emission mostly stems from the "bright" state and shows a monoexponential decay with a lifetime that is twice the lifetime of the "bright" exciton.64 Mediated through the size dependence of ΔEB-D, the decrease of the "dark" exciton lifetime with increasing size is possibly due to a stronger mixing between the "bright" and "dark" state at a smaller energy gap.59 It has been shown that emission of NCs are extremely sensitive to local environmental fluctuations due to their small size.14,15,75 A single NC has been shown to randomly jump between an emissive "on" state to an less emissive "off" state in time ranges from milliseconds to minutes.32 Furthermore, the emission peak position randomly diffuses.14 But in general, NCs are more stable compared to organic dye molecules, which makes NCs very suitable for applications as biolabels,2 especially the non-blinking NCs.33,34 The conventional charging model provides a fairly good explanation to most of the blinking related behavior in single NC.13 But recent experiments suggest modification to this theory is needed to take into account new discoveries.90,91 1.1.4 Multiexcitons At low excitation density, the excitation pulse generates one electron-hole pair, which then relaxes to the lowest excited state and forms a single exciton as shown in Figure 1.6a. At higher excitation density, there is a considerable possibility of exciting more than one electron-hole pairs a single NC, which form multiexcitons. The two mechanisms that govern the generation of multiexcitons are state filling and coulombic interaction. Considering a scenario that involves no more than three electron-hole pairs in a single NC, a simplified four-level system is used here to discuss the generation of a single 20 Figure 1.6 Generation of multiexcitons. (a) At the low excitation density, only the single exciton X (red) is formed. (b) As excitation density increases, biexciton BX (green) starts to appear. Coulombic interaction between excitons shifts the energy of the biexciton by ΔXX (positive in this case) with respect to the single exciton. (c) At very high excitation density, the newly generated exciton occupies a higher excited state due to the Pauli exclusion principle, which is the triexciton TX (blue). The PL spectra at different excitation densities consist of emission of the populated exciton species. exciton, a biexciton and a triexciton. Because of the Stark effect resulting from the coulombic interaction between the pre-existing exciton and the newly generated one, the energy of the latter is shifted by Δxx from that of the first exciton as shown in Figure 1.6b. Together these two excitons form a biexciton. The binding energy Δxx can be either positive (when the exciton-exciton interaction is repulsive like in this case) or negative (when the exciton-exciton interaction is attractive) depending on the electronic structure of the NCs. Because the coulombic interaction is inversely proportional to the distance, the binding energy of the biexciton in NCs scales with size and increases as size decreases.63,92,93 When the excitation density increases to a point where the absorption rate equals the biexciton Auger recombination rate, the biexciton population saturates and higher excited states start to be occupied.94 Due to the Pauli exclusion principle and state filling, the third exciton is excited to a higher energy as illustrated in Figure 1.6c, which 21 emits as a triexciton.95 Note that the triexciton is sometimes defined as a charged biexciton.63 The number of excitons generated on a single NC is governed by a Poisson distribution (1.2) where P(N) is the probability of having N excitons, and <N> is the average number of excitations per NC as calculated from the absorption cross section and excitation density. Although the binding energy is usually negative in bulk semiconductors and large epitaxially grown quantum dots,96 recent experiments on GaN quantum dots have shown that the binding energy can switch from positive to negative with decreasing exciton emission energy.68 The underlying mechanism of this controversial trend is similar to the switch of exciton binding energy from negative in type-I heterostructure NCs to positive in (quasi) type-II heterostructure NCs.81,94,97 Experiments have suggested that this switch is due to two reasons: increased correlation interaction resulting from increased electron-hole separation,98 and repulsive coulombic interaction in fixed single-particle orbitals.94 This control mechanism offers another route to design NCs that are especially suitable for applications as gain medium for lasers.18,81 While separation of the electron and hole wavefunctions might be beneficial for biexciton yields due to suppressed Auger recomnination,99 it reduces the yields of the triexciton, since the oscillator strength of the triexciton is proportional to the overlap of the electron and hole wavefunctions.94,97 Multiexcitons can be identified by additional emission peaks in the PL spectrum appearing with increasing exciton density as shown in Figure 1.6.18,68,71 Depending on whether the biexciton binding energy is positive or negative, the observed biexciton peak 22 either shifts to the red or blue with respect to the single exciton peak.68,81,97 The assignment of the biexciton peak can be confirmed by a quadratic power dependence of emission intensity on excitation density since it is a two-photon process.63 The triexciton peak is always blue shifted compared to the single exciton peak since it originates from a higher excited state. An example of the evolution of PL spectrum with excitation density is shown in Figure 1.6. Due to the nonradiative Auger recombination that is intrinsically associated with multiexcitons, they usually decay within a few ten to a few hundred picoseconds in type-I hetero-NCs. Therefore, the PL decay rate of NCs increases with increasing excitation density as shown in Figure 1.7. This decay lifetime scales with the volume of NCs, which has been observed in both CdSe and PbSe systems18,100 and is consistent with theoretical predictions.70 Morphology is another route to control the Auger process. Elongated nanorods have been shown to have longer biexciton Auger lifetime101 and improved optical gain compared to the spherical NCs.102 In CdSe/CdS core/shell nanorods- where the electrons are delocalized into the CdS shell while holes are localized in the CdSe core, a strong suppression of Auger recombination and long lifetime of the optical gain is observed using the transient absorption measurement.99 The common feature in systems of extended Auger lifetimes is the separation of the electron and hole. As a consequence of the charge carrier separation, the Auger lifetime no longer scales with volume V, but is modified to scale with V·Γrad, since the radiative lifetime Γrad is greatly affected by this charge separation.94 As a consequence, the biexciton lifetime can be increased to 2 ns.94 To study the rapid decay of multiexcitons, fast spectroscopic tools are required. The two commonly used techniques are the transient absorption and the time-resolved 23 fluorescence spectroscopy. The former method can probe multiexcitons in the excited state in a time range from a few fs up to a few ns, during which formation of multiexcitons either results in additional peaks or a spectral shift of the single exciton bleaching peak.63 On the other hand, the latter technique records emission from multiexcitons from picoseconds up. This technique offers direct measurement of the energy of the multiexcitons, their decay lifetimes, and any signature of lasing. In this study, time-resolved spectroscopy was performed using two systems: a streak camera with a resolution of a few ps and a gated intensified charge-coupled device (ICCD) with a resolution of a few ns. Exciton (or carrier) multiplication- which is the generation of multiple excitons through the absorption of a single photon of an energy that is at least three times greater than the band gap- is another effect stemming from the strong coulombic interaction Figure 1.7 Increase of decay rate with increasing excitation density. The rapid increase of the initial decay rate is attributed to the fast nonradiative Auger recombination. 24 between carriers in NCs.100,103,104 However, this effect is still under heavy debate due to controversial experimental results,100,104 and lack of a solid theory.103 1.2 π-Conjugated Molecules In the last section, inorganic semiconductor NCs were introduced. There it was shown that NCs behaved like an emissive organic molecule of discrete energy levels by reducing the size of the particle to a few nm. Here we will see that organic macromolecules built from a few to thousands of atoms can also behave like semiconductors. Organic semiconductors can be built in two approaches. One approach is by arranging small organic molecules periodically to form crystalline structures, in which molecules are held together by the weak van de Waals force. Properties of this type of material systems largely depend on the collective interaction of the participating molecules.29,105 Study in this field since the 1960s has yielded the realization of organic field effect transistors, a cheaper alternative compared to the traditional Si based devices.27 The second approach is through synthesis of conjugated polymers of length scale from a few nm to μm, in which delocalized pi-electrons over the conjugated backbone set the basis of the electrical properties of these macromolecules. Since the pioneering work of A. J. Heeger, A.G. MacDiarmid and H. Shirakawa in 1978 on doped polyacetylene of metal-like conductivity, research on organic semiconductors has gone beyond scientific interests into commercial products. For example, conjugated polymer based organic lighting emitting diodes (OELDs) have been successfully used in color displays on cell phones, televisions and various other appliances;23 and organic solar cells are approaching the required efficiency to compete with amorphous silicon counterparts.24 At the same time, chemists are also working on designing and synthesizing materials of novel chemical 25 structure to give desirable electrical and optical properties, based on empirical data or quantum chemical calculations. In this section, the intrinsic optical and electrical properties of pi-conjugated molecules will be introduced starting from molecular building blocks-atoms, to functional moieties, then to a chromophore and an exciton forming on the chromophore. The chromophore serves as the fundamental unit of optoelectronic materials. 1.2.1 Pi-conjugation The term pi-conjugation is given to organic molecules with alternating single-double bond or single-triple bond, in which unhybridized p orbitals of adjacent sites overlap with each other and form larger delocalized pi-orbitals or pi-bonds as shown in Figure 1.8. Despite the term "organic semiconductors" that is associated with pi-conjugated organic materials, the optical and electronic properties of these materials is better described using molecular orbital model than the early band models such as the SSH model.106 π-orbitals form through sp2 or sp hybridization. By mixing the 2s and three 2p orbitals of a fully hydrogen saturated carbon atom, a set of four equivalent orbitals are created, referred to as sp3 hybridization. When two carbon atoms are linked by a single bond, the sp3 orbitals of an atom overlap with that of the other and form one sigma bond of cylindrical symmetry as shown in Figure 1.8. It is also possible for the 2s and 2p orbitals to form sp2 (sp) hybridization when only two (one) of the three p orbitals are mixed with the s orbital, while the remaining one (two) orbital remains unmixed as in the case of double (triple) carbon bond. Consequently, the pure p orbitals of neighboring carbon atoms can overlap with each other and form the pi-orbitals or pi-bonds in addition to the sigma-orbitals. Figure 1.8 illustrates sp hybridization using acetylene as an example, 26 Figure 1.8 sp3 hybridization and sp hybridization. Carbon atoms in ethane and the corresponding polymer (such as polyethylene) are linked by localized σ-bonds. In acetylene, two π-bonds and one σ-bonds (blocked from view) are formed. Alternation of single-triple bonds in the polymer leads to delocalization of the π-electrons along the conjugated backbone. where one sigma bond and two pi-bonds form between the carbon atoms. Delocalized electron density above and below the conjugation plane of carbon atoms is established by electrons occupying the π-orbitals. These π -electrons are sufficient in determining the electrical properties of conjugated materials and are also a very important factor in the optical properties. Extension of conjugated chains by linking additional unsaturated carbon atoms, for example through polymerization, further delocalizes electron density to the added units. Typical π -conjugated oligomers and polymers contain from several to thousands of conjugated units that are capable of forming delocalized π -bonds. The delocalization nature enables quasi-free π -electrons to contribute to the electrical conductivity. As a good example, doped polyacetylene shows 5.6×102 ohm-1 cm-1 electrical conductivity at 27 room temperature.106 But polyethylene, consisting of only hydrogen saturated carbons linked by localized sigma-bonds (also shown in Figure 1.8), shows very low electrical conductivity. The latter, a plastic commonly used in plastic bags and bottles, is a very good insulator and shows no interesting optical properties. To introduce the molecular orbital theory, a simple example of 1,3-butadiene is used as shown in Figure 1.9. Before excitation, electrons occupy the highest occupied molecular orbitals (HOMO), which are the bonding π orbitals. This is the ground state, S0, of Ag symmetry. After excitation, an electron is promoted from the HOMO to the lowest unoccupied molecular orbital (LUMO) which is an anti-bonding π *orbital. This one-photon allowed transition is referred to as a π-π * transition, which excites the molecule to the 1Bu excited state (of Bu symmetry). Any one-photon forbidden 2Ag excited states (of Ag symmetry) requires the promotion of an electron from the HOMO to LUMO+1or from the HOMO-1 to LUMO or collectively promotion of two electrons from the HOMO to the LUMO. These processes are illustrated in Figure 1.9. The one-photon forbidden 2Ag states are the lowest excited states in pure polyenic organic molecules, like the highly conducting polyacetylene, which are nonemissive. The switching from pure polyenic to polyenic-aromatic backbone, like in the poly(p-phenylene vinylene) (PPV) type of polymers, lowers the 1Bu state relative to the 2Ag state. Consequently the 1Bu state becomes the lowest excited state and is strongly emissive,107 which is the reason that primary candidates for polymer-based light emitting devices have mixed polyenic-aromatic backbones. Cross-linking of benzenes with alternating single-double bonds in the PPV type of polymers, or single-triple bonds in the poly(p-phenylene ethynylene) (PPE) type of polymers yields some of the most studied 28 Figure 1.9 π-π* transitions in 1,3-butadiene. In the ground state, all electrons occupy the HOMO levels. One-photon allowed π-π* transition promotes an electron from the HOMO level to the LUMO level and forms an excited state of Bu symmetry that is emissive. Symmetry of each molecular orbital is indicated in the parenthesis. Excited states of Ag symmetry are formed by one-photon forbidden transitions, which are nonemissive.107 optoelectronic pi-conjugated materials, where benzene units and linear bonds form cross-conjugation along the backbone.29,108-110 1.2.2 Intrinsic Electrical Properties Due to the molecular nature and the semiconducting electrical properties, the scientific community has been debating whether a molecular111,112 or band-like model,106 or even a model considering both,113 would be more suitable to describe how pi-electrons determine material properties. It is difficult to find a coherent description of optoelectronic properties of conjugated materials. The molecular theory treats the excited state as localized excitons, while the band theory views it as delocalized free electron-hole pairs. Since only the lowest excited state is considered in describing the intrinsic properties of materials, the molecular orbital theory is used in this section.107,113 Excited state energy is roughly inversely proportional to conjugation length to the power of two. In conjugated polymers, even in the ideal chain, π-electrons are not 29 delocalized over the entire conjugated backbone. Instead, due to twist or defect sites, typical conjugation lengths only extend over 4-10 repeat units.107,114,115 Each of these broken conjugation segments along the backbone is a single chromophore,116 where electrically injected or optically excited excitons are localized. According to the particle-in-a-box model, electrons of mass m are confined in the one-dimensional potential well of length L with infinitely high boundaries at the ends of the chromophore. To zeroth-order approximation, the energy of the nth energy state is (1.3) where h is the Planck constant. 2N electrons in a chromophore fill orbitals to the EN energy state according to the Pauli exclusion rule, forming the HOMO level or π orbital in molecular orbital language as shown in Figure 1.9. This HOMO level is the ground state of the chromophore. The next energy level EN+1 is the LUMO level, or the π* orbital, which is the lowest excited state. In the semiconductor terminology, the energy difference between N+1 and N states (ΔE = EN+1-EN ) can be defined as the band gap. Excitons are formed on a chromophore after optical excitation which causes a π-π* transition. After absorbing a photon, an electron is excited into the LUMO level and leaves a vacancy (or hole) behind in the HOMO level. Coulombic interaction between the electron and hole binds the electron-hole pair together into an exciton in a localized configuration within the chromophore. The binding energy of the exciton is estimated as (1.4) 30 where e is the charge of a single electron, ε0 is the vacuum permittivity, r is the exciton Bohr radius, and ε is the dielectric constant. The exciton binding energy in pi-conjugated molecules is larger than that of inorganic semiconductors due to the three-times-smaller-dielectric constant in organic compounds. So-formed excitons are more tightly localized within a chromophore of size 20-25 Å via a process called exciton self-trapping.117,118 During this process, excitons quickly relax through vibronic coupling from an absorbing state delocalized over the entire chromophore to a more localized emitting state, which finally generates excitons of 1-2 nm size.117,119,120 Therefore, a tightly bound Frenkel exciton is formed in isolated pi-conjugated molecules, compared to the Mott-Wannier exciton formed in inorganic NCs. This localized exciton model invokes the hopping nature of energy transfer from a high energy chromophore to the neighboring lower energy chromophore along the backbone as discussed in Section 1.3.2.111,121 However, the Frenkel exciton model breaks down either by exciting the molecules into a very high energy state, or by strong electronic coupling that delocalizes the exciton over nearby chromophores.4,113,122,123 Aside from localized Frenkel excitons, polaron pairs or charge-transfer excitons are another frequently generated electron-hole pair type in π-conjugated molecules, which are loosely bound and easy to dissociate compared to excitons.124 These charge carrier pairs are of great importance in organic photovoltaic devices as the goal there is to improve the efficiency of exciton separation. Polaron pairs are formed when the electron and hole are delocalized on different chromophores on the same or different polymer chains.109,124 They are also believed to be the precursor states for excitons after charge injection in an electroluminescent device, like organic light emitting diodes 31 (OLED).125,126 Due to the delocalized nature of polaron pairs, the Wannier exciton model is more useful in studying their properties.105 1.2.3 Intrinsic Optical Properties Following the same molecular-orbital theory used in the previous sections, light absorption drives π-π* transition in π-conjugated molecules, which generates singlet excitons by exciting the molecule from the ground state S0 to the excited state Sn (n=1, 2, 3…) as shown in Figure 1.10. Optical excitation does not generate triplet excitons since the optical transition operator-which is the electric dipole moment operator-only appears in the electronic parts of the wavefunction in perturbation theory. The total energy of a molecule is approximately the sum of electronic and vibrational energy by neglecting the rotational energy since it is relatively small. The origin of electronic energy has been introduced in the previous section, and the vibrational energy Ev comes from coupling of nuclei with electronic transitions, also known as the vibronic coupling of electron and phonon. Based on the Born-Oppenheimer approximation, there is an equilibrium potential energy surface associated with each electronic state with a set of vibronic levels, labeled ν= 0, 1, 2… representing ascending vibrational energy. As electronic transitions takes place in less than 10-15s compared to the much slower nuclei reconfiguration time of about10-13s,105 the electronic transition from one electronic state to another is finished without change of nuclei position, which is denoted as the vertical transition in Figure 1.10, or a Franck-Condon transition. The excited state has a short lifetime and returns to the ground state within a few ps to μs depending on the dipole selection rule for a specific transition. For molecules that obey Kasha's rule,127 radiative transitions happen from the lowest excited state after nonradiative internal conversion in less than a few fs from upper 32 Figure 1.10 Schematic of the S0-S1 transition with vibrational levels. Each electronic state (S0 or S1) is associated with a potential energy curve in the nuclear coordinate. Besides the pure electronic transition (0-0), transitions from the vibrational levels (ν=1, 2) result in vibronic peaks in the mirror-symmetry absorption and emission spectra. 33 levels, emitting a photon in the form of fluorescence, or radiating heat.105 The above processes all happen within the singlet spin manifold, however, strong emission from the forbidden triplet states is also observed in the form of phosphorescence in organic materials.128 With the presence of heavy metal atoms,129,130 phosphorescence is especially strong due to increased intersystem crossing between singlet and triplet spin manifolds. The triplet state constantly lies about 0.7 eV below the singlet state in nearly all homopolymers where singlet and triplet excitons are localized to the same extent.131 However, in certain heteropolymers, the exchange gap can be reduced through delocalizing singlet excitons over localized triplet excitons.128 It is hard to overestimate the importance of spectroscopy in studying optical properties as can be easily seen from the symbolic spectrum drawn in Figure 1.10. All spectroscopic techniques involve detecting of a signal at certain energy, which can be optical as in all absorption and emission spectra, electrical like in the case of photocurrent measurement, or magnetic as in optically detected magnetic resonance (ODMR).126 Each spectral peak in an optical spectrum corresponds to a transition between two energetic states. Unfortunately, assignment of spectral peaks can be complicated by the existence of defects,132 dark states,133 and thermal broadening, which is why different experimental techniques are performed jointly to probe the full parameters of the material properties for cross proof of conclusions. 1.3 Energy Transfer and Molecular Aggregates 1.3.1 Motivation of Using Model Systems In the previous section, the intrinsic properties of organic π-conjugated oligomers and polymers in isolation from influences of the environment or other molecules were 34 summarized. Despite the successful demonstration of device concepts based on a single molecule,134,135 it is the bulk organic films or single crystals that serve as the active areas in the majority of organic optoelectronic and photovoltaic devices.136 In these bulk states, interchromophoric interactions between different conjugated segments of the same molecule or different molecules contribute largely to the final properties that a molecule presents.29,137-139 Unfortunately, the poor understanding of the intra- and intermolecular interaction remains an obstacle to the construction or optimization of practical devices based on organic materials.109 The intramolecular interaction is especially important in individual polymer chains, where the constituent chromophores are statistically situated in a local environment varying strongly from one site to another. As shown in Figure 1.11, energy transfer is favored from chromophores of high energy to chromophores of low energy until reaching the nearby chromophore of lowest energy where emission takes place. This hopping process leads to a redshift of the ensemble emission spectrum within picoseconds, which can be resolved in time-resolved spectroscopy.137,140 In addition, the fact that there are more absorbing chromophores than emitting chromophores suggests differences between the excitation polarization and the emission polarization and an energy transfer induced depolarization.141-144 The electronic and optical properties depend largely on the physical conformation of molecules and the way the molecules are packed together, which influence intermolecular interactions.29,109 These morphological properties vary with processing conditions such as: solvents, thermal annealing, and concentration.109 This vulnerability yields many controversial results in the literature.109 35 Figure 1.11 Energy transfer between chromophores along a conjugated polymer chain. Each chromophore shows a size dependent emission peak (narrow peaks). Energy transfer from the high energy chromophores to the low energy ones leads to a shift of the ensemble emission spectrum (broad peak) toward a longer wavelength with delay time after excitation. Intermolecular interaction through excitation energy (or exciton) transfer is the key process for the device applications. Similar to the incoherent exciton hopping between chromophores along the polymer chain shown in Figure 1.11, excitons can also hop between adjacent molecules. In photovoltaic devices based on blended materials of different HOMO and LUMO levels, such as the well studied P3HT/PCBM blend film used in organic solar cells,124 efficient exciton hopping is crucial to achieve exciton dissociation at the boundaries of two material domains.145 Coherent energy transfer is added to intermolecular interactions when the intermolecular distances are so close that strong electronic coupling results in direct excitation energy delocalization between molecules.119,139,146 Different interchain species, such as aggregates and excimers,109,138,147,148 are formed in this strong coupling regime depending on how the 36 molecules interact. The emission spectrum, decay lifetime and charge transport across the bulk are largely altered by the interchain species.105,149 To isolate these practical complications and gain insight into how molecules interact, model molecular systems are designed to mimic the intermolecular processes through intramolecular interchromophoric processes in controllable fashion. As illustrated in Figure 1.12, various model systems can be synthesized to contain chromophores arranged in different orientation and space. The use of model molecular system offers a toolbox to study the complex intermolecular interaction in (opto)electronic devices in a simplified and controllable way. 1.3.2 Incoherent Energy Transfer Fluorescence resonance energy transfer (FRET) or Förster transfer refers to a radiationless transition of energy from an excited donor to an acceptor in the ground or Figure 1.12 Four examples of model molecular systems. Identical chromophores (blue) are linked with various orientation and distances through covalent bonds (red) in linear dimers (a), dendrimers (b), and cofacial dimers (c). In the end-capped polymer (d), a different chromophore (orange) is added at one end of a polymer chain. 37 excited state. As the dominating long-range energy transfer mechanism, the electronic coupling in this regime can be approximated with only the coulombic interaction between the transition dipoles of donor and acceptor. The Förster theory is applicable to the weak coupling regime where the thermal equilibrium and internal relaxation finish before the transfer process occures.150,151 Therefore, FRET is incoherent and irreversible. The dipole approximation is made valid by assuming the center-to-center distance between the molecules is much larger than the size or shape of molecules. Thus, according to theclassic electrodynamics theory of dipole-dipole interactions, the FRET rate κ scales with the inverse of distance R6 between donor and acceptor:152 (1.5) where ΦD is the fluorescence quantum yield of the donor, is the decay lifetime of the donor in isolation, n is the refractive index of the medium, and I is the spectral overlap between donor emission and acceptor absorption. R0 being the Förster radius at which FRET efficiency drops to 50% is consequently written as (1.6) The FRET is particularly suitable when dealing with energy transfer between non-identical molecules as spectral overlap between the donor emission fD (λ) and acceptor absorption aA(λ) is required. This term is defined as integral I (1.7) 38 The FRET rate is in the range of a few tens of ns-1 to a few hundreds of ps-1 depending on the distance between donor and acceptor.150,151,153 Both spectral overlap between the donor and the acceptor and the refraction index of the dispersing medium can strongly affect the radius of FRET which is generally above 0.5 Å. Below this distance, FRET is beyond the weak coupling limit where Förster theory is no longer valid.151 The above theoretic descriptions of FRET indicate a clear dependence of the efficiency of energy transfer on the separation between molecules, or on molecule stacking in films and crystals. The energy transfer rate is increased by one order of magnitude in film compared to solution, which suggests a lower interchromophoric exciton hopping rate along the polymer chain than the interchain transfer rate in the film.151 A recent modification was made to the Förster theory when the distance R is sensitive to the exact position of donor and acceptor, in which case averaging over coupling between wavefunctions should replace the traditional averaging over wavefunction, and subsequent coupling.151-153 This modification is especially suitable for the intramolecular energy hopping between chromophores along a polymer chain where the donor and acceptor are arranged in the head-to-tail configuration instead of parallel configuration as shown in Figure 1.12a. FRET can also act as one cause of fluorescence blinking. The specific pair of donor and acceptor is subjected to local field fluctuation, therefore their emission and absorption follows a subsequent change. In consequence, the spectral overlap I(t) and FRET rate κ(t) become time dependent, which causes a type of blinking that is correlated with the spectral diffusion.141 39 Förster's theory is applicable in limited cases under conditions like no energy transfer induced changes in lifetime, line shape or oscillator strength in addition to the requirement of the dipole-dipole approximation. Therefore a new theory is needed in cases of molecular aggregates when both absorption and emission spectra and decay lifetime are altered due to perturbation of the wavefunction of the interacting molecules. This effect is observed in closely bridged dimers as shown in Figure 1.12,123,154 dendrimers, 155,156 and some interchain species in films and crystals. The last case leads to luminescence quenching in films forming ordered π-π stacking between molecules of a few Å spacing as a consequence of formation of weakly emissive interchain aggregates.138,157 The next two sections will focus on this topic. 1.3.3 Coherent Energy Transfer In contrast to requirements of spectral overlap for FRET in the weak coupling limit, coherent energy transfer in the strong coupling regime requires direct spatial overlap between the wavefunctions of donor and acceptor, which is similar to the condition of the strong quantum confinement of charge carriers in NCs. In this regime, the distinction between donor and acceptor becomes meaningless since the interaction is mutual among all interacting molecules or chromophores. Instead of being localized on one molecule, the excitation energy mixes and oscillates coherently between two molecules in a timescale as fast as intraband vibronic relaxation. Therefore, both the electronic and vibronic structures change in the strong coupling regime as shown in Figure 1.13b. Using the dimer as an example for strong coupling between two molecules, the energy of the newly formed molecular system of mixed and delocalized electronic structure is derived using quantum mechanics as shown in Figure 1.13a. The sharing of 40 Figure 1.13 Splitting of excited state in the strong coupling regime. (a) When the interaction term V12 between the two monomers is nonzero, the excited state of the dimer shifts by W' and then splits into two levels E(+) and E(-). In this diagram, the resonance energy β>0. (b) Splitting of the absorption spectrum in the dimer (red and blue) with respect to that of the monomer (black). excitation energy can be described by a wavefunction that is the linear combination of the unperturbed states of the constituent molecules (1 and 2) in the ground state (Φ1, Φ2) and the excited state (Φ1*, Φ2*). Thus the excited state of the dimer can be written as105 (1.8) 41 In the case of dimers of identical molecules, the normalization coefficients are , and the excited state energy of each molecule is equal (E1*= E2*). The splitting of dimer energies E(±) is given by (1.9) where β is a resonance interaction energy that causes the splitting of the energy of the dimer: (1.10) and is the coulombic energy of the interaction of the charge distribution of the excited state of molecule 1 with the ground state of molecule 2 (and vice versa): (1.11) V12 is the term representing the intermolecular interaction energy, which only represents the electronic interaction in β and . The exchange interaction that Dexter type of energy transfer is attributed to is neglected here and will be discussed separately at the end of this section. Figure 1.13a demonstrates how the energy of the dimer changes with respect to the monomer according to the above formulas. Equation 1.9 indicates the splitting of excited state energy due to delocalization of excited state wavefunctions between molecules when the intermolecular distance R becomes small enough (typically R 0.5Å) due to the resonance energy term β. This splitting of energy is observed as split of the absorption lines of linearly covalently bound 42 dimers, trimers and dendrimers as illustrated in Figure 1.13b.4,139,153,158 Both experimental and quantum chemical calculations reveal the decrease of delocalization with increase of the intermolecular distance, angle, and the size of the bound molecules.4,139,153 These trends cause a decrease of energy splitting (2β). Single molecule spectroscopy of linearly bridged dimers (shown in Figure 1.12a) and trimers shows correlated spectral shift and superradiance compared to the corresponding monomer, which adds further proof of electronic coherence.123,159,160 In contrast to the above covalently bound chemical dimers, the physical dimers- which are formed in bulk films or crystals when molecules are close to one another-also cause a redshift or blueshift of excited state energy.29,138,148 Compared to incoherent energy transfer, coherent energy transfer through direct delocalization of wavefunction is very fast. For example, the decay of fluorescence polarization anisotropy due to rapid coherent energy transfer always precedes that due to slower incoherent energy transfer (see Section 2.1.1).161,162 Quantum chemical calculations, such as the collective-electronic oscillator (CEO) procedure, can directly reveal the delocalization of charge density from one atom to an adjacent one. Therefore, it is particularly useful to study coherent energy transfer as mentioned above.119,136 The CEO procedure calculates the linear absorption spectra and the relevant transition density matrices which connect optical response with the underlying electronic motion. Upon optical excitation, an electronic transition from a ground state to an excited state is represented by a transition matrix and contributes to one absorption line which closely reproduces the experimentally observed absorption spectrum.119,139,158 43 The Dexter exchange integral that contributes to energy transfer through electron exchange between two molecules, i.e., Dexter energy transfer,163 decreases exponentially with distance. Therefore, Dexter energy transfer is short ranged and typically sets in at 1 nm.118 In some extended polymer chains, the strong interchromophoric coupling leads to fast energy transfer, which is dominated by Dexter transfer.164,165 Although spectral overlap is not required for Dexter energy transfer, coupling between the donor and the acceptor can be mediated by the intervening bonds or chromophores that serve as the bridge states to extend the exchange interaction to a longer range than the conventional Dexter transfer.152,166,167 The distinction between the long range energy transfer through spectral overlap and the short range energy transfer through wavefunction overlap is demonstrated in Figure 1.14. However, it is very common to find both types of energy transfer appearing at short and long timescales, respectively, in a single molecular system. 153,161 1.3.4 Aggregate and Excimer Formation Strong electronic coupling between molecules separated by a few Å in amorphous films, single crystals and even solution, populates a large collection of interchain species as introduced previously.29,109,168 Among these interchain species, aggregates (H- aggregate of parallel geometrical arrangement as shown in Figure 1.15b and J-aggregate of head-to-tail geometrical arrangement as shown in Figure 1.15a) and excimers are particularly interesting because of the rich optical properties presented in them.4,105,138 The theory introduced in the last section on the basis of a dimer offers a generalized solution for both J- and H-aggregates without distinguishing the relative orientation of the transition dipoles of the participating molecules. The attractive (repulsive) interaction 44 Figure 1.14 Comparison of incoherent and coherent energy transfer. (a) For incoherent energy transfer, a spectral overlap between donor emission (blue) and acceptor absorption (red) is required. (b) For coherent energy transfer, a direct spatial overlap between wavefunctions of the donor and acceptor is essential for excitation energy delocalization. between the interacting dipoles selects the E(-) (E(+)) split energy level as the excited state for the dimer, while the selection rule of the absorption transition is determined by the vector sum of the transition dipoles in a specific arrangement. It turns out that only in-phase arrangement of the dipoles presents allowed transition (the solid arrow in Figure 1.15) since the wavelength of the excitation photon is much larger than the size of the molecules, thus molecules in a dimer absorb most effectively under simultaneous in- phase perturbation.150 Due to this argument, the high energy transition E(+) is the allowed transition in the H-aggregate which shows blueshifted absorption and emission spectra compared to the monomer. H-aggregates also shows very slow decay rate as energy relaxes from the high energy E(+) state to the low energy forbidden state E(-) as shown in Figure 1.15b. In contrast, the low energy transition E(-) is the allowed transition in the J- aggregate which shows redshifted absorption and emission spectra and faster decay compared to the monomer as shown in Figure 1.15b. The oblique dimer yields split 45 Figure 1.15 Comparison of three configurations in dimers and the corresponding emission under each configuration. Because the excitation wavelength is larger than the size of molecules, the allowed transition happens in the in-phase configuration of dipoles in the dimer. Therefore, in a J-aggregate (a), the allowed transition takes place between the low energy split state and the ground state, which shifts the dimer absorption to a lower energy with a faster decay rate compared to the monomer. In an H-aggregate (b), the allowed transition takes place between the high energy split state and the ground state, which shifts the dimer absorption to a higher energy with a lower decay rate compared to the monomer. (c) In an aggregate of oblique orientation between dipoles, both split energy states are allowed, which causes splitting of the dimer absorption. energy levels of nonzero transition moments but mutually perpendicular polarization of light absorption ( as shown in Figure 1.15c. This configuration is frequently found in linearly bridged dimers that show split absorption peaks.139,169 An excimer is formed between a molecule in the excited state and another molecule in the ground state and only exists in the excited state. Therefore, the excimer exhibits the optical absorption characteristic to a monomer, but reveals a broad and structureless emission spectrum that shows characteristics of a dimer as shown in Figure 1.16b. The 46 Figure 1.16 Diagram of ground-state and excimer potentials and the emission of the excimer. (a) r0 is the ground-state distance between molecules (or monomers) that form the excimer. S1 is the excited state of the monomers. Solid arrow indicates possible S0-excimer absorption, while dashed arrow indicates excimer emission. The exciton-excimer hopping indicates transition from an excited monomer to an excimer. (b) Emission from the excimer with longer lifetime is broad and redshifted compared to the monomer. ground state of the pair of molecules forming an excimer is dissociative, thus the ground state energy increases with decreasing distance as shown in Figure 1.16a, which explains the redshifted and broad emission of excimers.170 The dashed arrow in Figure 1.16a represents the low quantum yield and slow decay rate of the excimer emission as also shown in Chapter 3.138,147 The study of aggregates on the microscopic level based on single molecule spectroscopy has yielded comprehensive results. Both J-aggregates and H-aggregateshave been experimentally identified as correlated spectral shift with decay lifetime as theory predicted.122,123 Chemical dimers consisting of covalently bonded monomers of fixed separation and orientation show experimental results agreeing with theory and calculations.4,136 Furthermore, the stepwise photobleaching of single dimers or trimers (as shown in Figure 1.18) shows correlation of the emission spectrum and decay lifetime with fluorescence intensity that changes collectively from a high level to a lower 47 level when one monomer is photobleached while the rest are still emissive.159,160 This stepwise photobleaching represents an optically induced transition from aggregates to monomer. The observation of excimer on the single molecule level remains controversial due to complications from the keto defects.132,171 The study of aggregation in bulk films and crystals is much more complicated due to the number of parameters that affect aggregation (formation of interchain species, not agglomeration) and the number of aggregation species that coexist.29,109 Aggregation is more commonly formed between chromophores on distinct polymer chains than on the same chain, which, as expected, increases with increasing concentration of polymers in solution used for film casting.109 Similarly, aggregation in solution is increased when the solvent favors open coil over tight coil conformation.168 The appearance of redshifted and broad emission is a typical signature of the formation of aggregation in bulk films when accompanied by changes of decay rate or quantum yield.109,138,147,157 Rare cases of observation of H-aggregates are reported in crystals where some molecules can be periodically arranged in a cofacial fashion.29,148 1.4 Blinking, Spectral Diffusion and Photobleaching 1.4.1 In Colloidal Nanocrystals Trap states play a key role in many aspects of the properties of NCs, such as fluorescence intermittency,172 emission color,173 carrier localization,77 multiexcitons,174 and intensity decay.11 Ref. 66, Ref. 70 and Ref. 175175 give a good review on both the experimental and theoretical progresses on the study of trap states and its impact on optical and electronic properties of NCs. 48 Trap states in NCs originate from several sources: one major type of trap state is the surface traps states originating from dangling anion and cation bonds, which act as electron and hole trap sites.176 Due to the large surface to volume ratio, NCs are abundant of surface states. For example, a NC of 4 nm diameter has 30% of the constituent atoms at the surface.177 In the early stages of the development of colloidal NCs, strong emission from the surface trap states appeared as a broad emission band that was red shifted compared to the emission of the band edge states. Significant reduction of this broad and red shifted emission band through surface passivation suggested its origin in surface trap states. 3,55 Good passivation of a NC surface with both organic ligands or an inorganic shell has been shown to largely remove these surface trap states,3,55 but still there are remaining sites that affect more subtle effects such as blinking, fluorescence lifetime and exciton storage.11,15,16 The second source of trap states is the impurity or dopant which might introduce a state in the band gap.174,178 Implanting trap sites into NCs through doping has been a particularly hard task due to the small size of NCs since even doping of a NC with a single dopant would have exceeded the normal doping level in bulk semiconductors.66 Successful doping of NCs with metal atoms has shown formation of trap states in or out of the band gap, which results in localization of excitons around these defects and controlled energy transfer from band edge states to the defect states.19,174,178 The third type of trap states is the intrinsic gap state of semiconductors which originates from the Shockley-like surface state.66 This type of trap state localizes on the surface in large NCs, but delocalizes to the entire volume in small NCs. In this section, only surface trap states will be focused on. 49 Trap states can affect properties of NCs on the ensemble as well as on the single particle level. In an ensemble of NCs, the delayed emission is dominated by direct trap emission or band edge emission through exciton transfer from the trap states.88,179 The trap emission only shows up in delayed emission due to its long lifetime.88,180,181 The energetic distribution of trap states results in different decay rates.88,179 Consequently, the decay of the emission intensity of an ensemble of NCs follows a power-law function for delay times above 1 ns 182 and depends strongly on temperature.11,88 With increased excitation density, the decay rate of NCs increases, too, as shown in Figure 1.7. This effect is attributed to increased fast nonradiative Auger recombination due to the presence of photogenerated charges that are localized at the trap sites.183 The microscopic mechanism of this effect is the same as the blinking behavior observed on single NCs. The blinking behavior observed in a single NC shows as intensity fluctuations over time between the "on" state of high intensity and the "off" state of low intensity,32,61,70 which is often referred to as "fluorescence intermittency." The widely accepted charging model of blinking proposed by Efros and Rosen13 is based on nonradiative Auger recombination of a single carrier charged NC which keeps the NC in the "off" state until the neutral state recovers. When a charged NC absorbs a photon which then generates an exciton, it becomes a three-particle system as shown in Figure 1.17a. Instead of emitting a photon, recombination energy of the exciton is transferred to the charge trapped on the NC through a rapid (~100ps) Auger process70 that is faster than the radiative lifetime of the exciton (10 ns to 1us).11 Therefore, a charged NC is a "dark" NC since it emits no light. The reasons for the blinking behavior being observed exclusively in NCs but not in bulk materials are enhanced Auger recombination resulting from strong coulombic 50 Figure 1.17 Correlated blinking with spectral jumps of a single nanocrystal. (a) Emission of a single NC jumps from low to high intensities, when the NC switches from charged "dark" state with a trapped hole (orange), to neutralized "bright" state.70 (b) Two types of spectral diffusion in the emission of single NCs are shown here. A large spectral jump (from long to short wavelength) follows a "dark" period. Random spectral jitter happens during the entire "bright" period. interaction between charge carriers and relaxed requirements of momentum conservation of Auger type processes at abrupt surfaces of NCs.70 The charged NC becomes neutralized by obtaining a charge carrier of opposite sign from the surrounding matrix and becomes bright again. But the neutralization process might leave a net dipole moment in the NC which then causes a correlated spectral jump with blinking as shown in Figure 1.17b. The "on" and "off" periods change from a few millisecond to minutes.32,70 Regardless of composition, size, shape or surface of the studied NCs,184-187 the probability of both "on" and "off" time distribution P(t) follows a universal power-law statistics as188 (1.12) 51 where α is the power-law exponent. α is close to 1.5 for both "on" and "off" time distributions. However, the "on" time distribution does deviate from the power-law statistics at a time called the "truncation point." This stands in contrast to a persistent power-law functionality of "off" time statistics over the whole experimental time range.186,189,190 The truncation point shifts to a shorter time with increase of excitation density and temperature or removal of passivation layer,32,189 which is directly reflected as stronger blinking. Although blinking traces are usually collected from a single NC, a clear interplay between the slow decay dynamics of ensemble NCs with the single particle intermittency can be identified by investigating blinking statistics of a collection of NCs.191,192 This interplay indicates the microscopic mechanism of the power-law like intensity decay of NCs, which is associated with the blinking behavior. The independence of both the power-law statistics of blinking and its exponent on temperature, excitation density and NC morphology all suggests that the power-law statistics of "on" and "off" blinking is governed by a tunneling process. 186 To explain the power-law statistics, several models based on a distribution of trapping and neutralization rate were proposed.186,188,193 The diffusion based model suggests that the trap state energy undergoes a random walk, with a transition from a bright state to a dark (trap) state happening when the energy of these two states is the same.186 Therefore the blinking period is given by the diffusion time it takes for the trap state to drift in and out of resonance with this condition, which naturally assumes a power-law distribution.186,188 This diffusion model is then further developed by Tang and Marcus through the diffusion-controlled electron transfer model.194,195 The other model assumes a uniform spatial distribution of traps in the matrix around a NC and an exponential decay of 52 trapping and recovery rate with distance between the NC to a trap.193 The deviation of the "on" time distribution from the power-law after truncation point indicates a secondary process involving on-to-off transitions that is temperature, excitation density and morphology dependent as discussed above. This process might be an optical or thermal charge ejection due to Auger ionization. Another phenomenon that can be observed in single NC spectroscopy measurements is random jumps or jitter of the spectral position over time, i.e., spectral diffusion.15,75,196 The spectral diffusion is found to be correlated with blinking events in a way that spectral jump follows recovery of intensity from an "off" event as shown in Figure 1.17b. 14,196 By placing a NC in an external electric field, the same spectral jumps are reproduced, which indicates that spectral diffusion is related to local field fluctuations.14,15 The correlation relation found between emission intensity and spectral position of a single NC is of similar origin as the correlated intensity and spectral changes. Both correlation relations are induced by electric fields which leads to quantum confined Stark Shifts (see Section 1.5.1).15,75,197 Therefore, qualitative models propose that the spectral diffusion and intensity fluctuation are due to local charges that are present on the surface of NCs. These charges exert a local electric field on the NCs which then changes the emission peak position and emission intensity as in the case of an applied external electric field.14,197 Consequently, the observed correlation between spectral diffusion and blinking can be explained as an effect of spectral diffusion- an effect induced by the remaining electric field from the charging event that cause blinking.70,196 53 1.4.2 In π-conjugated Organic Molecules As demonstrated in inorganic semiconductor NCs in the last section, the fluorescence of a single quantum emitter is subject to intensity blinking and spectral diffusion under continuous wave excitation, which is also observed in pi-conjugated organic molecules of single or multiple chromophores in a similar fashion.121,198 One well-studied cause of blinking in organic molecules is the transition from an emissive singlet state to a dark triplet state through intersystem crossing.133,199-201 Despite the presence of dark states of similar nature in semiconductor NCs, this type of blinking is hard to observe due to both the smaller singlet-triplet energy gap and shorter decay lifetime of the dark state (shorter than the experimental resolution) in NCs compared to that in organic molecules. The fast blinking due to the triplet dark state usually follows exponential statistics.202 Besides this fast blinking, long-lived dark states with "off" time much longer than that of the triplet state have also been reported in organic molecules.133,203 In the case of organic molecules, the formation of such dark states is thought to originate from the formation of reversible nonemissive photo-oxidation products,204 electron tunneling processes, 203 charged molecules (radical anion or cation) after exciton dissociation.202 Such dark states have led to power-law like blinking statistics of "on" and "off" time distribution as in NCs. Other sources of blinking such as energy transfer to defect sites133 and exciton blockade of energy transfer to acceptor chromophore have also been reported in literature.205 Aside from the temporary loss of emission, single molecule spectroscopy also suffers from photobleaching, a photochemical reaction such as oxidation that yields nonemissive products.204 Single molecule studies on stepwise photobleaching of a trimer demonstrates collective reversible "off" events due to the formation of charged radicals preceding the 54 irreversible photobleaching in most cases,133 which suggests the cause of photobleaching as long lived radicals. Interestingly, the "off" time distribution extracted from a single molecule fluorescence time trace in this study also follows a power-law distribution of exponent -1.5 like in the semiconductor NCs, and a similar power-law distribution of "on" time is also reported.202 This observation points to a close link between the fluorescence intermittency of organic and inorganic single quantum emitters with stochastic time dependencies.199 Single molecule spectroscopy of coupled multichromophoric molecular systems reveals important features of aggregates. As discussed in Section 1.3.4, the formation of J-aggregates shortens fluorescence lifetime due to a gain of oscillator strength through exciton delocalization between multiple units such as chromophores. This superradiance signature can be easily observed in correlated time traces of stepwise photobleaching and lifetime measurement.159,160,200 This effect is illustrated in Figure 1.18 using a dimer for example. The drop of intensity is due to photobleaching of one chromophore when the other one is still emitting. Therefore, lifetime increases through the loss of the superradiance condition. The similar correlation is also observed in other molecular systems of coupled chromophores.133,160 Other than this coherent intramolecular interchromophoric coupling that changes emission energy and lifetime, weak coupling through incoherent Förster type energy transfer (FRET) between chromophores can also be identified through single molecule spectroscopy. It is shown that only one chromophore is emissive when two chromophores are brought to be within the proximity of the FRET radius even if they are both excited.206 In another words, the coupled multichromophoric system is a single emitter as proved by the photo-antibunching 55 measurement.200 Spectral diffusion is another common feature observed in both single NCs and single organic molecules. As a direct probe of variation of energy gap, spectral diffusion can be caused by conformation changes by local heat dissipation and by nonradiative decay or excess energy dissipation through phonon-coupled relaxation. The other cause of spectral diffusion like in the case of NCs is local field fluctuation. Changes of local environment by charges sitting on the side chains cause a change of the local electric field which subsequently perturbs the energy level through the Stark effect and cause schange of emission energy.28 Correlated blinking and spectral diffusion can be observed under the Figure 1.18 Stepwise photobleaching and correlated fluorescence lifetime of a single dimer. (a) Time trace of intensity initially shows a high level (red) from the dimer fluorescene. After photobleaching of a unit, the dimer becomes a monomer of lower emission intensity. The inset shows a representational molecular structure of a linearly bridged dimer.160 (b) The fluorescence lifetime increases upon the switch from a dimer to a monomer.160 56 energy transfer condition as spectral diffusion leads to a time dependent overlap between donor and emitting acceptor as discussed previously. 1.5 Electrostatic Manipulation of Electric and Optical Properties As introduced in Section 1.1.1, state-of-the-art fabrication enables precise control of composition, shape and size of semiconductor NCs which show a large range of electronic structures. The electronic and optical properties of these interesting material systems are extensively studied for applications in various optoelectronic and photovoltaic devices.20 It is only natural to study the impact of electric fields on spectroscopic properties of these materials. The manipulation of excitons-which are the carriers of optical and electronic information with electric field-is one important field that attracts considerable attention.21,207 Concepts of devices for storage, read-out and transfer of excitons are explored using an electric field as manipulation force.21,22,208-210 The importance of manipulation of excitons is also applicable to pi-conjugated organic materials. Photoinduced charge carrier generation and transport are key processes underlying the application of organic semiconductors as the active materials of devices such as solar cells and diodes.149 These processes largely determine the performance of the corresponding devices. The central interest is in understanding the mechanism that governs the surmounting of the coulombic barrier which leads to charge carrier photogeneration. The manner in which to manipulate electronic and optical properties of semiconductor NCs and organic materials will be discussed in full detail in this section. 57 1.5.1 Stark Effect The "Stark effect" is the shifting and splitting of spectral lines of atoms and molecules under the influence of a static external electric field. The Stark effect in systems of size where quantum interaction applies is often called quantum confined Stark effect (QCSE) and is largely enhanced compared to the bulk due to the increased carrier coulombic interaction. To the second order approximation, the shift of energy caused by the Stark effect can be written as (1.13) where ε is the external electric field, and μ and α are the projections of the dipole and polarizability in the direction of the electric field. Empedocles and Bawendi reported the observation of the Stark effect in single CdSe spherical NCs as the shift of emission lines by as much as 75meV.14 In single NCs, variations from linear to quadratic dependence of Stark shift on electric field are observed.14,15 However, thus far, experiments on the electric field induced spectral shift of a single conjugated polymer chain reveal only a linear dependence of the Stark shift on the field, which is possibly due to the strong internal field of the molecules.28 Aside from spectral changes, PL intensity quenching, defined as the decrease of PL intensity when an electric field is on compared to the zero-field PL intensity, also arises when organic pi-conjugated materials and semiconductor NCs are under the influence of an external electric field.197,211 This effect is caused by the separation of excitons, which reduces the overlap of electron and hole wavefunctions and consequently the oscillator strength. The separated but still weakly bound exciton is often referred to as indirect 58 exciton or polaron pair of smaller binding energy and slower radiative decay rate.21,207,212,213A linear increase of quenching with increase of electric field is observed in CdSe/CdS tetrapods in this work, but this dependence can change into other complicated functionality in different NCs197,214,215 and pi-conjugated polymers.213,216 The separation of exciton and intensity quenching also shows an opposite dependence on excitation density in NCs compared pi-conjugated materials, which will be given in detail in Chapter 3 and Chapter 4. 1.5.2 In Nanocrystals The band structure of a bulk semiconductor dominate its optoelectronic properties since the interband transition is largely determined by the atomic-like Bloch parts of the wavefunction as introduced previously in Section 1.1.2. Thus it seems that strong interband transitions of short radiative lifetime are linked to the direct band gap semiconductors such as GaAs, while weak interband transitions of long radiative lifetime are associated with indirect band gap semiconductors like Si. Instead of manipulating the band structure in this momentum space which is hard to achieve practically, experimental approaches seek ways to achieve band-gap engineering in the physical space. The initial attempts of band-gap engineering focused on quantum superlattices of alternating n and p types of doping regions for the separation of electrons and holes spatially into these two types of regions, respectively.217 This method changed the electronic structure by manipulating composition and formed quantum wells of separated space charges. There were also attempts to manipulate the electronic structure through the application of electric-field waves across the sample to achieve spatial separation of 59 charge carriers which can be entirely controlled using a switch, unlike the former static method.207,218 The electronic structure of semiconductors can also be manipulated using an external electric field. This method is based on the tilt of the conduction and valence bands along the electric field and the subsequent shift of electron and hole wavefunctions to the opposite directions. To illustrate this mechanism, a single core/shell nanorod heterostructure is used as an example as shown in Figure 1.19. The applied external electric field ( ) tilts the conduction and valence band in the way that the electron wavefunction shifts along the opposite direction of toward the CdS arm, while the hole wavefunction is still localized in the CdSe core due to the structure asymmetry. This increase of distance between electron and hole reduces emission intensity and increases Figure 1.19 Stark effect in a single core/shell nanorod. Normalized PL spectra at different electric fields are plotted here. Both emission intensity and emission energy decreases with increased field strength (redrawn from Ref.197). The inset demonstrates how band structure and electron-hole wavefunction changes with the applied electric field. 60 the radiative lifetime.16,197,212 As shown in Figure 1.19, the PL intensity of NCs when the electric field is on (red) is lower than the intensity without electric field (black). The quenching of emission intensity is correlated with redshifted emission due to a large reduction of correlation energy when the electron and hole wavefunctions are separated.15,197 This negative Stark shift is shown to be also correlated to emission linewidth broadening.14,15 This observation can be explained by attributing electric fields as additional sources of local field fluctuations, which causes broadening of energy state. The shape of NCs can also affect the strength of electric field induced changes. For instance, NCs of elongated shape exhibit larger quenching and Stark shifts compared to spherical NCs.14,15,197 Increase of radiative lifetime through separation of the electron and hole wavefunction indicates an storage mechanism of excitons. An overshot of emission intensity immediately after the switch-off of the electric field can be observed as excitons stored in the electric field recombine.16 The storage of excitons was first investigated in double quantum wells,21 in which exciton excited into one quantum well was separated and the electron and the hole were stored in different wells. These processes are illustrated as "write" and "store" steps in Figure 1.20a and Figure 1.20b respectively. The application of a "read" electric pulse tilts the band structure to allow the electron and the hole to be present in the same well and recombine as shown in Figure 1.20c. The corresponding electrical and laser pulses are plotted in Figure 1.20d. The storage time can be seconds during which there is small loss of the stored signal through slow indirect exciton recombination and thermal activation or tunneling.16,21,208 Optimization of this storage method, such as a reduction of response time,22 polarization preserving light storage,210 61 Figure 1.20 "write," "store" and ‘read-out" of excitons in double quantum wells.21 (a) The laser excitation pulse generates an exciton in the left quantum well, which is immediately separated by the electric field. The electron tunnels to the right quantum well, while the hole remains in the left well. (b) The electric field stores electron and hole in separated wells and prevents them from recombination. (c) Application of the read-out bias reverses the electric field. Therefore, the hole tunnels to the right well, where it recombines with the electron. (d) shows sequences of laser pulse, the electric field and the read-out signal in time. 62 and controllable exciton flow among different logic gates209 are recent focuses in this field. Exciton storage within isolated colloidal NCs was originally studied on CdSe/CdS nanorods in which the storage time was up to 100 microseconds.16 The long exciton storage time indicates that the lifetime of the indirect exciton is too long to be entirely attributed to reduction of electron and hole wavefunction overlap.197 In analogy to the case of double quantum wells, storage sites that provide an additional barrier to prevent charge carriers from recombination are needed for similar purposes as the high band gap material between the wells. These storage sites are attributed to long-lived trap sites in NCs. The above arguments are based primarily on experiments at relatively low excitation density to avoid charging of samples. However, the impact of electric fields on interesting phenomena such as multiexciton generation and high-density exciton storage can be studied only at high excitation density. Therefore, a study of the dependence of field induced intensity quenching and exciton storage on excitation density was conducted and further discussed in Chapter 4. 1.5.3 In Organic Molecules Two basic models are currently used to describe carrier generation in organic molecules. According to the direct carrier generation model based on the studies of PPV type of polymers, unbound electron and hole pairs are generated directly after photoabsorption.219,220 The other two-step model introduces the generation of the polaron pair, i.e. weakly bound excitons, as the intermediate step before final photogeneration of charge carriers.213,221,222 63 Electric field induced singlet exciton dissociation and generation of polaron pairs were reported in numerous polymers which resulted in quenching of fluorescence intensity137,213,216,223. The applied external electric field provides excess energy required for the formation of polaron pair of higher energy compared to the exciton. Field induced quenching is not instantaneous but instead evolves on the picosecond time scale during which dynamical processes of exciton dissociation occur.137 Gulbinas's work on poly(paraphenylene) (MeLPPP) using transient absorption demonstrates the electric field induced formation of charge carriers throughout the entire lifetime of S1-S0 transitions of a few hundred picoseconds.213 The dependence of the yield of charge separation of less than 1% on electric field follows the Poole-Frenkel model.213 The efficiency of photogeneration of charge carriers can be increased by increasing excitation photon energy224 or excitation density, which both supply excess of energy to overcome energy barriers of dissociation.213,225 An increase of excitation density significantly increases the population of excitons and promotes interaction between them in a similar way as in semiconductor NCs shown in Section 1.1.4. Fast exciton depopulation through nonlinear singlet-singlet annihilation at increased excitation density was reported to contribute to the generation of charge carriers.213,225 As illustrated in Figure 1.21, one singlet exciton (S1) transfers its recombination energy to the other one and promotes the latter to a higher energy state Sn. Excitons at Sn states have increased an chance of exciton dissociation and an increased contribution of photogeneration of charge carriers similar to the scenario where samples are excited with photons of high energy.224 The fact that intermolecular exciton species only appear at high exciton density also 64 indicates the increase of photogeneration of carriers through an increase of excitation density.226 The same study also demonstrates a dependence of field induced fluorescence quenching on molecular structures. It is shown that quenching increases with number of chromophores in the molecules. Similar results were also observed in MEH-PPV and its five-ring oligomers where quenching also was shown to decrease with decrease of chain length.227 Figure 1.21 Schematic of singlet-singlet annihilation. CHAPTER 2 EXPERIMENTAL METHODS 2.1 Concepts of Time-Resolved Spectroscopy Time-resolved spectroscopy is an important experimental tool to study the temporal dynamics of materials after an excitation pulse. The difference between time-resolved spectroscopy and traditional spectroscopy is that the former probes spectra at varying delay time following a pulse of excitation and therefore reveals the temporal evolution of excited states. As an important characterization tool, time-resolved spectroscopy distinguishes materials based on the decay lifetime, which also reflects the nature of the excited state. Furthermore, the capability of time-resolved spectroscopy to "freeze" a moment in time enables the probing of the temporal evolution that a process undergoes. Since its development in the 1970s, time-resolved spectroscopy has been applied in studies of excited state lifetime,63,212 energy transfer, 146,151,153 exciton transfer and storage,16,74,76 emission depolarization,142,143,228 phosphorescence,128,130 dynamics of proteins and membrane229 as well as several other fields.124,178,230 Accurate timing of detection with excitation necessitates usage of temporally pulsed excitation (usually a pulsed laser) in time-resolved spectroscopy. The various time-resolved techniques can be divided into two groups according to the gating mechanism of the temporal signal: the pump-probe method and the time gated method. With the pump-probe method, samples are first excited with a narrow and intense pump pulse followed by a weak probe pulse at 66 a certain delay that probes structural and energetic changes initiated by the pump pulse. Popular pump-probe techniques include transient absorption,231 time-resolved fluorescence upconversion,162 three pulse photon-echo measurements,232 and degenerate four-wave mixing.233 In contrast, time-gated techniques utilize a similar excitation pulse scheme, while the temporal evolution is resolved through the gating of detection which controls both the length of detection and the starting time with respect to the excitation pulse. This technique has been successfully applied in streak cameras, gated ICCDs, and time-correlated single photon counting (TCSPC) apparatuses. Generally, the pump-probe methods have higher time resolution but short delay ranges compared to the time gated methods. Since temporal dynamics of emission over both a short and a long delay range is the focus of this study, time-resolved PL spectra are collected by a streak camera and a gated ICC