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Synthesis, Characterization, and Ultrafast Dynamics of Metal, Metal Oxide, and Semiconductor Nanomaterials

  • Author(s): Wheeler, Damon Andreas
  • Advisor(s): Zhang, Jin Z.
  • Chen, Shaowei
  • et al.
Abstract

SYNTHESIS, CHARACTERIZATION, AND ULTRAFAST DYNAMICS OF METAL, METAL OXIDE, AND SEMICONDUCTOR NANOMATERIALS

ABSTRACT

The optical properties of each of the three main classes of inorganic nanomaterials, metals, metal oxides, and semiconductors differ greatly due to the intrinsically different nature of the materials. These optical properties are among the most fascinating and useful aspects of nanomaterials with applications spanning cancer treatment, sensors, lasers, and solar cells. One technique which is central to understanding the optical properties and subsequently allowing exploitation of them is femtosecond transient absorption (TA) which specifically aids in explicating the kinetics of photoexcited charge carriers. In this dissertation, their respective syntheses, optical (e.g., steady-state) characterizations, and TA (e.g., time-resolved) dynamics were explored in order to obtain a fundamentally deeper understanding of their photophysical properties and to provide a deeper understanding of the nature of the underlying photophysics of each class of nanomaterial. For hollow gold nanospheres (HGNs), their ability to absorb high-peak-power femtosecond laser pulses was investigated and was found to be dependent upon average power of the laser. Additionally, gold nanostars (AuNS) were found to exhibit coherent vibrational oscillations arising from phonon relaxation following hot electron relaxation of ~2 ps, dependent upon peak power. Metal oxides likewise were explored with TA techniques in order to probe their excited state kinetics. Specifically, TA was used to study the ultrafast charge carrier dynamics of hydrogen-treated TiO2 nanowires which contains a singly-occupied oxygen vacancy within the bandgap unlike pristine TiO2. Following UV irradiation, the charge carrier recombination of the hydrogen-treated TiO2 NWs was slower than that of pristine TiO2. These ultrafast results indicate that the presence of the oxygen vacancy significantly slows the charge carrier recombination within the hydrogen-treated TiO2 relative to pristine TiO2. The combination of metal oxides and metal, in the form of Fe3O4-Au core-shell NPs, combined the modalities of magnetism from the metal oxide and surface plasmon resonance from the metal. This was done to explore the surface enhanced Raman scattering (SERS) properties of the resultant core-shell nanomaterial, in which magnetically-induced aggregation of the NPs was observed to enhance greatly their SERS response by a factor of 7x via the generation of more "hot spot" sites as well as improved scattering from the aggregates overall. Finally, TA was utilized to study the exciton dynamics and relaxation pathways of porous silicon nanowires, for which a power-dependence was found and the initial recombination increased in amplitude and decreased in time for increasing laser power, due to exciton-exciton annihilation. Similar results were found for PbS and PbS/CdS core/shell quantum dots. Knowledge of the charge carrier relaxation dynamics in these semiconductor materials may lead to an improved understanding of the origin of the photoluminescence (PL) and how to optimize better and exploit more fully that property for future applications.

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