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.

No / During the last decades of nanoparticles research, many nanomaterials have been developed for applications in the field of bio-labelling. For the visualization of transport processes in the body, organs and cells, luminescent quantum dots (QDs) make for highly useful diagnostic tools. However, intercellular routes, bio-distribution, metabolism during degradation or quantification of the excretion of nanoparticles, and the study of the biological response to the QDs themselves are areas which to date have not been fully investigated. In order to aid in addressing those issues, CdSe/CdS/ZnS QDs were radioactively labelled, which allows quantification of the QD concentration in the whole body or in ex vivo samples by gamma-counting. However, the synthesis of radioactively labelled QDs is not trivial since the coating process must be completely adapted, and material availability, security and avoidance of radioactive waste must be considered. In this contribution, the coating of CdSe/CdS QDs with a radioactive (65)ZnS shell using a modified, operator-safe, SILAR procedure is presented. Under UV illumination, no difference in the photoluminescence of the radioactive and non-radioactive CdSe/CdS/ZnS colloidal solutions was observed. Furthermore, a down-scaled synthesis for the production of very small batches of 5 nmol QDs without loss in the fluorescence quality was developed. Subsequently, the radio-labelled QDs were phase transferred by encapsulation into an amphiphilic polymer. gamma-counting of the radioactivity provided confirmation of the successful labelling and phase transfer of the QDs.

Made available in DSpace on 2014-06-12T15:51:31Z (GMT). No. of bitstreams: 2 arquivo5719_1.pdf: 2389208 bytes, checksum: fd3ee770e606c50de4354c931bb7ce22 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2011 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Nanocristais fluorescentes de semicondutores, quantum dots (QDs) vêm sendo obtidos para diversas aplicações por vários grupos de pesquisa em todo o mundo. A maioria das rotas sintéticas encontradas na literatura refere-se à produção desta classe de nanomateriais em meio orgânico e condições de alta temperatura. A síntese em meio aquoso consiste em uma metodologia simples para a obtenção de nanocristais com estrutura core-shell de CdSe/CdS altamente fluorescentes em regime de confinamento quântico. Estes QDs dispersos em água foram sintetizados através da adição de sal de cádmio, selênio reduzido e ácidos orgânicos com radicais tióis. Tais compostos orgânicos nesse caso são fundamentais para a estabilização das partículas coloidais, além de fornecerem íons sulfeto, que participam da formação da camada de passivação. A caracterização dos QDs obtidos foi realizada através de medidas espectroscópicas e análises estruturais. Foram feitos também planejamentos quimiométricos e avaliações temporais da luminescência para se saber quais os melhores parâmetros e quando os QDs apresentam a melhor intensidade de luminescência após serem sintetizados. Devido sua elevada luminescência, estabilidade em meio aquoso e baixa fotodegradação, os QDs de CdSe/CdS foram utilizados para a marcação de fluorescência de cromossomos metafásicos. Apesar do grande interesse e crescente aumento na produção de QDs, os mesmos apresentam muitas vezes, metais de elevada toxicidade, tais como o Cádmio. Assim, o tratamento dos resíduos oriundos da produção de quantum dots, que ainda é alvo de poucos trabalhos, é aqui abordado mais especificamente com relação ao tratamento através de processos oxidativos avançados, de rejeitos oriundos da síntese dos QDs de CdSe/CdS, objetos do presente trabalho

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Diese Arbeit untersucht die Photo-Lumineszenz (PL)-Unterbrechung (Blinken) einzelner in Polymer-Nanopartikeln eingebetteter CdSe/CdS Halbleiter-Nanokristalle (Quantenpunkte) im Einfluss elektrischer Gleich- und Wechselfelder mittels Weitfeld-Mikroskopie. Hierbei emittieren die einzelnen Quantenpunkte trotz kontinuierlicher Anregung mit einer zwischen hellen An- und dunklen Aus-Zuständen variierenden PL-Intensität. Die Ergebnisse zeigen, dass die Dynamik dieses Blinkens durch Wechselfelder stark beeinflusst wird und von deren Feldstärke, teilweise auch deren Feldfrequenz abhängt. Für zunehmende Feldstärken lässt sich ein schnellerer Wechsel zwischen An- und Aus-Zuständen (erhöhte Blinkfrequenz) beobachten, der von einer reduzierten Häufigkeit langer An- und Aus-Ereignisse begleitet wird. Der Verlauf der An-Zeit-Verteilungen bei kleinen Zeiten wird zunehmend (monoton) flacher, während die Verteilungen der Aus-Zeiten zunächst ebenfalls einem analogen Trend folgen, ab einer bestimmten und von der Feldfrequenz abhängenden Feldstärke jedoch wieder steiler verlaufen. Ein solcher Monotonie-Wechsel in der Blinkdynamik im Fall einer gleichbleibenden Variation einer äußeren Bedingung wurde bei Halbleiter-Nanokristallen so erstmalig beobachtet. Für Gleichfelder zeigen sich hingegen nahezu keine Auswirkungen. Lediglich die An-Zeit-Verteilungen sowie die Blinkfrequenz im Fall hoher Feldstärken werden modifiziert. Die Ergebnisse werden im Kontext verschiedener aktueller Modelle zur PL-Unterbrechung wie dem trapping-Modell, dem self-trapping-Mechanismus oder dem Modell multipler Rekombinationszentren diskutiert und diese entsprechend erweitert. Dabei stehen die dielektrischen Eigenschaften und die Relaxationsdynamik der lokalen Quantenpunkt-Umgebung im Mittelpunkt, deren Reaktion auf die externen Felder durch eine zeitabhängige Ausrichtung permanenter Dipole beschrieben werden kann.

Diese Arbeit untersucht die Photo-Lumineszenz (PL)-Unterbrechung (Blinken) einzelner in Polymer-Nanopartikeln eingebetteter CdSe/CdS Halbleiter-Nanokristalle (Quantenpunkte) im Einfluss elektrischer Gleich- und Wechselfelder mittels Weitfeld-Mikroskopie. Hierbei emittieren die einzelnen Quantenpunkte trotz kontinuierlicher Anregung mit einer zwischen hellen An- und dunklen Aus-Zuständen variierenden PL-Intensität. Die Ergebnisse zeigen, dass die Dynamik dieses Blinkens durch Wechselfelder stark beeinflusst wird und von deren Feldstärke, teilweise auch deren Feldfrequenz abhängt. Für zunehmende Feldstärken lässt sich ein schnellerer Wechsel zwischen An- und Aus-Zuständen (erhöhte Blinkfrequenz) beobachten, der von einer reduzierten Häufigkeit langer An- und Aus-Ereignisse begleitet wird. Der Verlauf der An-Zeit-Verteilungen bei kleinen Zeiten wird zunehmend (monoton) flacher, während die Verteilungen der Aus-Zeiten zunächst ebenfalls einem analogen Trend folgen, ab einer bestimmten und von der Feldfrequenz abhängenden Feldstärke jedoch wieder steiler verlaufen. Ein solcher Monotonie-Wechsel in der Blinkdynamik im Fall einer gleichbleibenden Variation einer äußeren Bedingung wurde bei Halbleiter-Nanokristallen so erstmalig beobachtet. Für Gleichfelder zeigen sich hingegen nahezu keine Auswirkungen. Lediglich die An-Zeit-Verteilungen sowie die Blinkfrequenz im Fall hoher Feldstärken werden modifiziert. Die Ergebnisse werden im Kontext verschiedener aktueller Modelle zur PL-Unterbrechung wie dem trapping-Modell, dem self-trapping-Mechanismus oder dem Modell multipler Rekombinationszentren diskutiert und diese entsprechend erweitert. Dabei stehen die dielektrischen Eigenschaften und die Relaxationsdynamik der lokalen Quantenpunkt-Umgebung im Mittelpunkt, deren Reaktion auf die externen Felder durch eine zeitabhängige Ausrichtung permanenter Dipole beschrieben werden kann.