School of Chemistry - Theses
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ItemTowards large-scale fabrication of plasmonic nanomaterials by fluid-mediated forces( 2017)Nano-assembly promises powerful technological advances, yet there is a persistent need to make such assembly more economical before we see these benefits in daily life. Scalable and reliable nano-assembly of any kind has remained elusive to researchers worldwide. This thesis focuses on Capillary Force-assisted Assembly (CFA), particularly for plasmonic structures and materials. CFA assembles nanoparticles out of solution via liquid-mediated forces over a nano-template. It is significantly cheaper and faster than standard nano-fabrication routes. CFA-assembled optical plasmonic materials have not been demonstrated over sizes larger than square-micrometers. An early goal of this work is to demonstrate centimetre-squared assembly. An unexpected hindrance emerged in the reliable manufacture of large-area nano-templates. Despite this, scaled-up assembly has been demonstrated, producing an optical two-tone metamaterial from plasmonic nanorod pixels. Harnessing the CFA protocol is difficult. Experimenters rely heavily on the current CFA model to guide their efforts. This model is widely accepted yet has received little critical analysis. In pushing the envelope of CFA, the conventional CFA model is inadvertently put to the test, and it is found wanting. A much needed thorough review dismantles the conventional model from experimental and theoretical standpoints. An as-yet unmentioned type of capillary force is suggested to apply on nanoparticles in CFA, and its form is mathematically derived. A new framework for CFA is proposed. In this framework, contact line pinning on template cavities, and the subsequent local enhancement of convective flows, are the primary assembly drivers. This model draws well from the limited existing nano-wetting theory, and conforms well to high-level experimental expectations. However, for reliable and scalable CFA, core dynamics must be quantified and relevant experimental parameters ascertained. This is non-trivial, requiring knowledge of the time-resolved three-dimensional (3D) meniscus form during single CFA-assembly events. No techniques exist to non-invasively probe such dynamics. Therefore, a novel experimental technique is developed to rapidly and non-invasively profile a meniscus shape in 3D on the micro-scale. Meniscus dynamics are observed over templated cavities with millisecond resolution, showing that the meniscus pins and closes down over cavities, acting to clamp particles in place. Moreover, evaporative dynamics can be modelled on the 3D micro-meniscus profiles, paving the way for crucial thermodynamic analysis. Characterisation of the new model therefore begins by abundantly probing pinning dynamics on cavities. Surface tension is revealed to direct pinning dynamics down to the nano-scale; inertial/viscous forces become negligible. Interpolatable trends allow us to theoretically reconstruct meniscus forms under a range of experimental conditions at any moment during a single CFA assembly event. From this versatile CFA-meniscus model, and armed with the ability for thermodynamic analysis, convective flows during assembly events are estimated and validated against extant literature. The subsequent kinematic behaviour of a nanoparticle in a cavity’s vicinity during a CFA-assembly event is simulated under a variety of experimental conditions. This sheds valuable light on which experimental parameters are most crucial to optimise CFA dynamics, and why. This new and arguably successful model is called the pinned-convective model of nano-CFA.
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ItemUltrafast spectroscopy of nanostructures( 2017)This thesis presents studies of ultrafast laser spectroscopy of semiconductor and gold nanostructures, aiming to advance our understanding of, and consequently control, photoinduced charge carrier dynamics in nanostructures to further improve their performance in practical applications. Artificial nanostructures have drawn significant attention in applications such as optoelectronic devices, photo-catalysts, and solar cells. Compared to bulk materials, nanostructures provide unique optical properties, which more importantly can be directly and easily tailored through changing size or shapes of the structures, during their synthesis procedures. Photoinduced charge carrier dynamics in the nanostructures play an important role in the photon conversion processes. However, in contrast to the fast development of nanostructure-based devices, the mechanisms of these processes are still being experimentally unravelled. In this study, a range of ultrafast optical spectroscopy methods have been applied to investigate the carrier dynamics, with a focus on the electron transfer (ET) process. Semiconductor nanoparticles, or quantum dots (QDs), of core/shell heterostructures are promising for their good photostability and high photoluminescence quantum yields. The ET dynamics from the 1S$_\mathrm{e}$ electron state to adsorbed methyl viologen electron acceptors, in CdSe/CdS and CdSe/CdS/ZnS QDs, were studied using femtosecond transient absorption and time-resolved photoluminescence spectroscopy. By changing shell thickness or alloying the shell interface, significant modulation of the ET dynamics was observed. In CdSe/CdS QDs, the 1S$_\mathrm{e}$ ET dynamics exhibited a hole-coupled effect, which is ascribed to the Auger-assisted ET process. In CdSe/CdS/ZnS QDs, the formation of alloyed shell interfaces at elevated shelling temperatures reduced the shell potential barrier, leading to an observed greater ET rate. Photoinduced ET processes from gold nanorod and nanowire structures to TiO$_{2}$ were also investigated, using a visible pump-NIR probe transient absorption spectroscopy method. Partially embedded Au nanorods on a TiO$_{2}$ layer exhibited an enhanced but directional ET process. An Au nanowire grating supported on a TiO$_{2}$ layer structure underwent the plasmon-waveguide hybridisation mechanism. The ET dynamics from the split states showed a dependence on the light-matter coupling effect that can be varied with the Au grating period. In summary, this thesis shows the great ability of ultrafast optical spectroscopy to reveal photoinduced processes in nanostructures. Results indicate ways for rational design of nanostructure-based devices. A greater understanding in underlying physics leads to better control of the performance of these nano-systems in potential practical applications.