Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.

In this thesis, colloidal nanocrystalline silicon (ncSi) capped with allylbenzene (AB) groups, is created using a sol-gel method. This novel material, AB-ncSi, is size separated and its optical, structural, and electrical characteristics are investigated in detail. For optical characterization, the photoluminescence (PL) peak is located for each fraction of the nanoparticles to determine its diameter. The AB-ncSi samples have diameters ranging from 2.89 nm to 7.65 nm. Ellipsometry and scanning electron microscopy (SEM) are used to estimate the film thickness and average distance between the particles, respectively, for structural characterization. No correlation was found between AB-ncSi size and film thickness however the estimated average distance between the particles decreased with decreasing diameter. Finally, for electrical characterization, conductivity of size-separated samples is measured and the temperature dependent conductance is analyzed. The results emerging from these analyses suggest that the charge transport mechanism for AB-ncSi is nearest-neighbor hopping (NNH) albeit VRH is also a potential contributor.

In this thesis, colloidal nanocrystalline silicon (ncSi) capped with allylbenzene (AB) groups, is created using a sol-gel method. This novel material, AB-ncSi, is size separated and its optical, structural, and electrical characteristics are investigated in detail. For optical characterization, the photoluminescence (PL) peak is located for each fraction of the nanoparticles to determine its diameter. The AB-ncSi samples have diameters ranging from 2.89 nm to 7.65 nm. Ellipsometry and scanning electron microscopy (SEM) are used to estimate the film thickness and average distance between the particles, respectively, for structural characterization. No correlation was found between AB-ncSi size and film thickness however the estimated average distance between the particles decreased with decreasing diameter. Finally, for electrical characterization, conductivity of size-separated samples is measured and the temperature dependent conductance is analyzed. The results emerging from these analyses suggest that the charge transport mechanism for AB-ncSi is nearest-neighbor hopping (NNH) albeit VRH is also a potential contributor.

Synthesis of nanocrystals is one of the most rapidly advancing areas of nanoscience, and today nanocrystals can be produced with impressive control over their composition, size, shape, polydispersity, and surface chemistry. As such, they are ideal building blocks for fabricating hierarchical architectures with tailorable functionality on every level of the hierarchy. Here an evaporation-driven, template-assisted nanocrystal assembly (ETNA) technique is developed, providing a novel and general approach to fabricating freestanding, 3D, functional architectures using diverse combinations of colloidal nanocrystal species and porous templates of arbitrary geometry. Colloidal PbS (photoluminescent) and CoFe2O4 (superparamagnetic) nanocrystals are template-assembled to fabricate freestanding nanorods and inverse opals, which retain the size-dependent properties of their constituent building blocks while replicating the geometry and preserving the functionality of the templates. Further multifunctionality is demonstrated through mixed-nanocrystal architectures which exhibit the aggregate functionality of their building blocks.

Synthesis of nanocrystals is one of the most rapidly advancing areas of nanoscience, and today nanocrystals can be produced with impressive control over their composition, size, shape, polydispersity, and surface chemistry. As such, they are ideal building blocks for fabricating hierarchical architectures with tailorable functionality on every level of the hierarchy. Here an evaporation-driven, template-assisted nanocrystal assembly (ETNA) technique is developed, providing a novel and general approach to fabricating freestanding, 3D, functional architectures using diverse combinations of colloidal nanocrystal species and porous templates of arbitrary geometry. Colloidal PbS (photoluminescent) and CoFe2O4 (superparamagnetic) nanocrystals are template-assembled to fabricate freestanding nanorods and inverse opals, which retain the size-dependent properties of their constituent building blocks while replicating the geometry and preserving the functionality of the templates. Further multifunctionality is demonstrated through mixed-nanocrystal architectures which exhibit the aggregate functionality of their building blocks.

Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.

Due to their unique properties, nanoparticles have been a focus of significant research interest for use in various opto-electronic applications, particularly in the field of solar energy generation. In order to realize a nanoparticle-based solar cell, it is important to be able to create thin films of organised nanoparticles and to be able to control their surface properties. In this work the use of a novel synthesis technique involving reaction at the interface between two immiscible liquids to synthesise thin films of lead sulfide nanoparticles on the order of ~10 nm in diameter is reported. The use of the liquid-liquid interface allows the synthesis of particles without the use of stabilising ligands, with sizes and morphologies determined by the conditions present at the interface. Variations in the precursor used, solvent height, and precursor concentration were explored. Films synthesised at various solvent heights displayed a decrease in particle size with increasing solvent height. This trend was seen to vary depending on the lead-containing precursor used. Changes in the precursor concentration resulted in changes in the morphology of the resulting particles as observed with transmission electron microscopy (TEM). Preferential growth along certain planes was observed for particles synthesised with the highest lead precursor concentration. Experiments with precursors with differing organic chain length displayed an increase in particle size with increasing chain length, as well as an increase in preferential growth observed by X-ray diffraction (XRD). Surface ageing was investigated using X-ray photoelectron spectroscopy (XPS) techniques, which showed that all samples followed a similar oxidation mechanism. Oxidised lead species, attributed to hydrated lead oxide, were determined to be the initial oxidation product, formed within a week of exposure to air. Sulfoxy species were observed to form over a greater length of time, with sulfate being determined to be the final oxidation product. An oxidation mechanism is proposed based on XPS analysis of films exposed to air for up to nine months.

This dissertation contains six chapters detailing recent advances that have been made in the synthesis and characterization of metal-semiconductor hybrid nanocrystals (HNCs), and the applications of these materials. Primarily focused on the synthesis of well-defined II-VI semiconductor nanorod (NR) and tetrapod (TP) based constructs of interest for photocatalytic and solar energy applications, the research described herein discusses progress towards the realization of key design rules for the synthesis of functional semiconductor nanocrystals (NCs). As such, a blend of novel synthesis, advanced characterization, and direct application of heterostructured nanoparticles are presented. Additionally, for chapters two through six, a corresponding appendix is included containing supporting data pertinent to the experiments described in the chapter. The first chapter is a review summarizing the design, synthesis, properties, and applications of multicomponent nanomaterials composed of disparate semiconductor and metal domains. By coupling two compositionally distinct materials onto a single nanocrystal, synergistic properties can arise that are not present in the isolated components, ranging from self-assembly to photocatalysis. While much progress was made in the late 1990s and early 2000s on the preparation of a variety of semiconductor/metal hybrids towards goals of photocatalysis, comprehensive understanding of nanoscale reactivity and energetics required the development of synthetic methods to prepare well-defined multidimensional constructs. For semiconductor nanomaterials, this was first realized in the ability to tune nanomaterial dimensions from 0-D quantum dot (QD) structures to cylindrical (NR) and branched (TP) structures by exploitation of advanced colloidal synthesis techniques and understandings of NC facet reactivities. Another key advance in this field was the preparation of "seeded" NR and TP constructs, for which an initial semiconductor QD (often CdSe) is used to "seed" the growth of a second semiconductor material (for example, CdS). These advances led to exquisite levels of control of semiconductor nanomaterial composition, shape, and size. Concurrently, many developments were made in the functionalization of these NCs with metallic nanoparticles, allowing for precise tuning of metal nanoparticle deposition position on the surface of preformed semiconductor NCs. To date, photoinduced and thermally induced methods are most widely used for this, providing access to metal-semiconductor hybrid structures functionalized with Au, Pt, Ag2S, Pd, Au/Pt, Ni, and Co nanoparticles (to name a few). With colloidal nanomaterial preparation becoming analogous to traditional molecular systems in terms of selectivity, property modulation, and compositional control, the field of nanomaterial total synthesis has thus emerged in the past decade. With a large toolbox of reactions which afford selectivity at the nanoscale developed, to date it is possible to design a wider array of materials than ever before. Only recently (the past ~ 5 years), however, has the transition from design of model systems for fundamental characterization to realization of functional materials with optimized properties begun to be demonstrated. The emphasis of chapter 1 is thus on the key advances in the preparation of metal-semiconductor hybrid nanoparticles made to date, with seminal synthetic, characterization, and application milestones being highlighted. The second chapter is focused on the synthesis and characterization of well-defined CdSe-seeded-CdS (CdSe@CdS) NR systems synthesized by overcoating of wurtzite (W) CdSe quantum dots with W-CdS shells. 1-dimensional NRs have been interesting constructs for applications such as solar concentrators, optical gains, and photocatalysis. In each of these cases, a critical step is the localization of photoexcited excitons from the light-harvesting CdS NR "antenna" into the CdSe QD seed, from which emission is primarily observed. However, effects of seed size and NR length on this process remained unexplored prior to this work. Previous work had demonstrated that, for core@shell CdSe@CdS systems, small CdSe seed sizes (< 2.8 nm in diameter) resulted in quasi-type II alignment between semiconductor components (with photoexcited electrons delocalized across the structure and holes localized in the CdSe seed), and large seed sizes (> 2.8 nm) resulted in type I alignment (with photoexcited electrons and holes localized in the CdSe seed). Through synthetic control over CdSe@CdS NR systems, materials with small and large CdSe seeds were prepared, and for each seed size, multiple NR lengths were prepared. Through transient absorption studies, it was found that band alignment did not affect the efficiency of charge localization in the CdSe core, whereas NR length had a profound effect. This work indicated that longer NRs resulted in poor exciton localization efficiencies owing to ultrafast trapping of photoexcited excitons generated in the CdS NR. Thus, with increasing rod length, poorer efficiencies were observed. This work served to highlight the ideal size range for CdSe@CdS NR constructs targeted towards photocatalysis, with ~ 40 nm NRs exhibiting the best rod-to-seed localization efficiencies. Additionally, it served to expand the understanding of exciton trapping in colloidal NC systems, allowing development of a predictive model to help guide the preparation of other nanorod based photocatalytic systems. The third chapter describes the synthesis of Au-tipped CdSe NRs and studies of the effects of selective metal nanoparticle deposition on the band edge energetics of these model photocatalytic systems. Previous studies had demonstrated ultrafast localization of photoexcited electrons in Au nanoparticles (AuNP) (and PtNP) deposited at the termini of CdSe and CdSe@CdS NR constructs. Also, for similar systems, the hydrogen evolution reaction (HER) had been studied, for which it was found that noble metal nanoparticle tips were necessary to extract photoexcited electrons from the NR constructs and drive catalytic reactions. However, in these studies, energetic trap states, generally ascribed to surface defects on the NC surface, are often cited as contributing to loss of catalytic efficiency. In this study, we found that the literature trend of assuming the band-edge energetics of the parent semiconductor NC applies to the final metal-functionalized catalyst did not present a complete picture of these systems. Through a combination of ultraviolet photoelectron spectroscopy and waveguide based spectroelectrochemistry on films of 40 nm long CdSe NRs before and after AuNP functionalization, we found that metal deposition resulted in the formation of mid-gap energy states, which were assigned as metal-semiconductor interface states. Previously these states had only been seen in single particle STS studies, and their identification in this study from complementary characterization techniques highlighted a need to further understand the nature of the interface between metal/semiconductor components for the design of photoelectrochemical systems with appropriate band alignments for efficient photocatalysis. The fourth chapter transitions from NR constructs to highly absorbing CdSe@CdS TP materials, for which a single zincblende (ZB) CdSe NC is used to seed the growth of four identical CdS arms. These arms act as highly efficient light absorbers, resulting in absorption cross sections an order of magnitude greater than for comparable NR systems. In the past, many studies have been published on the striking properties of TP nanocrystals, such as dual wavelength fluorescence, multiple exciton generation, and inherent self-assembly owing to their unique geometry. Nonetheless, these materials have not been exploited for photocatalysis, primarily owing to challenges in preparing TP from ultrasmall ZB-CdSe seed size (owing to phase instability of the zincblende crystal structure), thus preventing access to quasi-type II structures necessary for efficient photocatalysis. In this study, we successfully break through the type I/quasi-type II barrier for TP NCs, reclaiming lost ground in this field and demonstrating for the first time quasi-type II behavior in CdSe@CdS TPs through transient absorption measurements. This was enabled by new synthetic protocols for the synthesis and stabilization of ultrasmall (1.8 – 2.8 nm) ZB-CdSe seeds, as well as for the synthesis of CdSe@CdS TPs with arm lengths of 40 nm. Easily scalable, TPs were prepared on gram scales, and the quasi-type II systems showed dramatically enhanced rates of selective photodeposition of AuNP tips under ultraviolet and solar irradiation. These are promising materials for photocatalytic and solar energy applications. The fifth chapter continues with the study of CdSe@CdS TPs, and elaborates on a new method for the selective functionalization of the highly symmetrical TP construct. Previous studies had demonstrated that access to single noble metal NP tips was vital for efficient photocatalytic HER from NR constructs. However, TP materials have been notoriously difficult to selectively functionalize, owing to their symmetric nature. Using a novel photoinduced electrochemical Ostwald ripening process, we found that initially randomly deposited AuNPs could be ripened to a single, large (~ 7 nm) AuNP tip at the end of one arm of a type I CdSe@CdS TP with 40 nm arms. To demonstrate the selectivity of this tipping process, dipolar cobalt was selectively overcoated onto the AuNP tips of these TPs, resulting in dipolar Au@Co-CdSe@CdS TP nanocrystals. These particles were observed to spontaneous self-assemble into 1-D mesoscopic chains, owing to pairing of N-S dipoles of the ferromagnetic CoNPs, resulting in the first example of “colloidal polymers” (CPs) bearing bulky, tetrapod ("giant t-butyl") pendant groups. The sixth chapter elaborates further on the preparation of colloidal polymers, further extending the analogy between molecular and colloidal levels of synthetic control. One challenge in the field of colloidal science is the realization of new modes of self-assemble for compositionally distinct nanoparticles. In this work, it was found that Au@Co nanoparticle dipole strength could be systematically varied by tuning of AuNP size on CdSe@CdS nanorods/tetrapods. In the first example of a colloidal analogue to reactivity ratios observed for traditional chain growth polymerization systems, highly disparate AuNP tip sizes (and thus final Au@Co NP dipole strength) were found to result in segmented colloidal copolymers upon dipolar self-assembly, whereas similar AuNP tip sizes ultimately led to random dipolar assemblies. Clearly visualized through incorporation of NR and TP sidechains into these colloidal polymers, this study presented a compelling case for continued exploration of colloidal analogues to traditional molecular levels of synthetic control.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21590号 / 理博第4497号 / 新制||理||1646(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 寺西 利治, 教授 島川 祐一, 教授 吉村 一良 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

In this diploma thesis we will study the luminescence properties of nanocrystals. We will summarize the basic division according to size and standard method of theoretical description of semiconductor and metal nanocrystals. We will describe the luminescence properties of nanocrystals and the influence of the surrounding environment. In the se- cond part of the thesis we will follow up the construction of an apparatus for imaging luminescence spectroscopy intended for the measurement of individual nanoobjects in a liquid environment. Finally, we will study luminescence properties of organixally passi- vated metal clusters in a liquid environment with changes in temperature and excitation intensity. We compare the luminescence of gold nanocrystals with and without long po- lyethylene glycol chains bound on the surface. 1