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Investigations of Structure-Dependent Photophysical Properties of Metal, Metal Oxide, and Semiconductor Nanostructures


On the nanoscale, size and structure are powerful dictators of optical and electronic response. As such, they may be rationally manipulated in order to obtain desired properties for specific applications. The dissertation projects herein are aimed toward understanding how structure affects the photophysical properties of three nanoparticle systems: hollow gold nanospheres (HGNs), doped α-Fe2O3 nanostructures, and PbS/CdS core/shell quantum dots (QDs). The dissertation is divided into two parts: Part I: The Highly Tunable Hollow Gold Nanosphere: Synthesis, Size, and Surface Morphology and Part II: Ultrafast Charge Carrier Dynamics of Hematite Nanostructures and PbS/CdS Quantum Dots

Part I focuses on synthetic control and characterization of HGNs, solvent-filled plasmonic shells of gold ranging from 20-200 nm in diameter. HGNs have shown promising performance in drug loading, targeted delivery, surface-enhanced Raman scattering, and photothermal therapy. Their optical properties are very sensitive to their aspect ratio, the ratio of diameter to shell thickness. As such, a well-controlled synthesis is highly desired. In Chapter 1, the HGN synthesis was updated to enable simultaneous control of both diameter and SPR while maintaining monodispersity and uniformity of the resultant shells. This was possible through a detailed and systematic investigation of the synthesis of the sacrificial cobalt-based scaffolds onto which HGNs are formed through galvanic exchange.

In Chapter 2, additional synthetic adjustments were introduced to systematically control the HGN surface morphology from smooth to very bumpy. Rugose structures are perhaps the most versatile nanoparticles, with an increased density of active sites for catalysis as well as local electric field enhancement around the surface features for sensing and detection. As hollow particles have displayed enhanced plasmonic performance in comparison with their solid counterparts for a number of applications, the combination of hollow cores and rugose surfaces is highly attractive. One of the most attractive applications of HGNs is plasmonic photothermal therapy (PTT). In this application, plasmonic nanoparticles are targeted to cancer cells where they convert incident light to heat, raising the temperature of their environment above the point of cell viability. A systematic comparison of HGNs with different surface morphologies revealed that bumpy HGNs retain the excellent photothermal conversion efficiency (PCE) of their smooth counterparts. Next, in Chapter 3, PCE was investigated for HGNs of different diameters, theoretically and experimentally. The findings revealed that 50 nm HGNs generate ~2 times the heat per µg gold as their 70 nm counterparts and ~1.5 times the heat per µg gold as their 30 nm counterparts. In vitro HGN-mediated PTT of oral squamous cell carcinoma was also carried out. Ongoing efforts are needed to assess the PCE and potential size dependence of HGNs in vitro and in vivo.

In Part II, transient absorption spectroscopy (TAS) is used to probe the charge carrier dynamics in α-Fe2O3 (hematite) and PbS/CdS nanostructures. In both systems, structural modification has been crucial for obtaining enhanced photophysical performance. Hematite is considered to be an especially promising material for solar water splitting due to its chemical stability, abundance, and non-toxicity. Additionally, its band gap of approximately 2.0–2.2 eV facilitates the absorption of about 40% of incident solar light. However, it suffers from a number of limitations which hinder its practical implementation as a photoanode material. Chapter 4 discusses these limitations and structural approaches to overcome them, including nanostructuring and doping to facilitate charge transfer and improve PEC performance. Charge carrier recombination dynamics are investigated before and after doping to aid understanding of the mechanism of performance enhancement in Zr and Ti-doped hematite films. Although nanostructuring and doping are beneficial, performance gains have been modest and a review of the current approaches for the rational design of hematite heterostructures is provided in Chapter 5. In these reports, TAS has been employed as a useful tool to gain deeper insight into the mechanisms of photogenerated electron-hole recombination and their relation to PEC performance.

Finally, in Chapter 6, the passivation of PbS QDs with a CdS shell, and the dependence of the passivation on particle size, is investigated. Semiconductor QDs like PbS are highly attractive components in solar cells, sensing, and detection due to their size-dependent optical and electronic properties. Broad absorption, narrow photoluminescence (PL), and high PL quantum yield arise from quantum confinement. The formation of a thin shell atop the QD core has become a common method for passivating the surface bonds and thereby enhancing and stabilizing resultant PL. In this work, it was found that surface passivation of PbS with a CdS shell is core-size-dependent; ultrasmall PbS QDs did not benefit from CdS passivation like their larger counterparts, but instead experienced a decrease in PL. Coupling TAS with steady state PL and QY measurements enabled a comprehensive understanding of the radiative and nonradiative relaxation pathways of the PbS/CdS nanosystem. Insights into the size-dependent variation in trap state density and relaxation pathways may serve as a guide for future structural modification.

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