PUBLIC FINAL EXAMINATION
Degree of Doctor of Philosophy
Investigation of the Fundamental Growth Mechanisms in Pulsed Laser Deposition Synthesis of Nanostructured Materials
By WILLIAM THOMAS MOZET
Thursday, September 29, 2016 10:00 am Engineering Building B-223
Dissertation Director: Stephen D. Tse
Abstract:
Studies are conducted to better understand growth mechanisms in pulsed laser deposition (PLD) synthesis of nanostructured materials, namely graphene and bismuth telluride (Bi2Te3). For graphene, as the substrate temperature increases, the order of the film increases, from an amorphous carbon film to nanocrystalline graphite and few-layer graphene (FLG). By using a high energy laser, the size and type of ablated species can be controlled to create films with smaller nanocrystalline domains. PLD allows the thickness of the films to be directly controlled by the deposition duration. Films can be grown on arbitrary substrates, unlike other methods which utilize surface chemistry. Substrate morphology also affects the samples, with higher surface roughness leading to larger D/G and 2D/G ratios. Polishing substrates prior to deposition can decrease these ratios by up to 15%. Here, the type of carbon source has little impact on sample growth, except in atmospheric growth of graphene, which may not be an optimal condition because of energy loss of the carbon species. In-situ plasma plume analysis is conducted to analyze the species being ablated from the target. Ablated species consist primarily of C+ ions, with some neutral C and C2 species. Ablated C+ ions are at temperatures as high as 12,000 K in vacuum and 10,000 K in 0.1 torr argon. For bismuth telluride, optimal growth conditions are found for the stoichiometric transfer of Bi2Te3, which can vary from system to system. In general, a deposition temperature of 200°C and a deposition pressure of 0.1 to 1.0 torr argon are required for stoichiometric transfer. Using a high energy laser for ablation leads to smaller grain sizes in the nanostructured films. In addition, using a nitrogen atmosphere instead of argon leads to increased gas-phase condensation prior to deposition, resulting in a highly featured surface. When outside of the ideal pressure range, the substrate material can significantly affect the surface morphology of the sample, ranging from smooth films to nanoparticles and nanorods. These morphologies affect the electrical properties of the material. In general, the lowest electrical resistance came from films grown using 532 nm laser irradiation, which leads to larger grain sizes and more featured surfaces. Films grown at slightly reduced pressure, which leads to more featured surfaces, are also low in electrical resistance. These films also have large Seebeck coefficients, both of which lead to a higher thermoelectric figure of merit.