Browsing by Subject "Electronic properties"
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Item Electronic properties and electron-electron interaction effects in transition metal dichalcogenides(2018-08-09) Larentis, Stefano; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; MacDonald, Allan H; Register, Leonard F; Shi, LiTransition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter.Item Molecular doping of carbon nanotube conductors(2018-06-19) Chin, Khai Yi; Fahrenthold, Eric P.Carbon nanotubes (CNTs) show attractive electronic properties that have been studied extensively, including interest for cabling and wiring applications. Specifically, CNTs may provide an advantage over conventional materials, such as copper, due to their lightness and flexibility, which are properties demanded in naval and aircraft applications. Using molecular doping with the potassium tetrabromoaurate molecule (KAuBr₄), doped nanowires with enhanced electrical properties may be obtained. This thesis presents the first comprehensive modeling effort on KAuBr₄ doping of CNTs, including doping of SWNT junctions. The results showed that the dopants had an overall positive effect on SWNT based conductors. The conductance of K doped junctions was similar, regardless of doping configuration, while the conductance for the AuBr₄ doped junction was heavily reliant on the doping configuration. The AuBr₄ doping fragment showed a unique characteristic: it eliminated the dependence of the junction conductance on nanotube overlap. A nanowire model was developed and used as a metric for comparison with experimental studies of KAuBr₄ doped CNTs. The nanowire model provided a reasonable comparison of the computational results with previous experimental work. Overall, results presented in this thesis show the promise of doped SWNTs as potential candidates for the replacement of conventional copper conductors.Item The search for electrocatalysis of carbonyl reductions on tin oxide semiconductor electrodes covalently modified with NAD⁺ coenzyme analogs(1984) Seguin, Russell Joseph; N/ASeveral N-alkyl-3-carboxamidopyridinyl halides were prepared to serve as NAD⁺ coenzyme model compounds. By varying the electron-withdrawing properties of the alkyl moiety, the electronic properties of these quaternized nicotinamides (i.e., ring current and reduction potential) could be manipulated, as was observed by Nuclear Magnetic Resonance Spectrometry and Cyclic Voltammetry (CV). Several propyltriethoxysilane derivatives were also prepared through linkage to the pyridine's amide moiety. A series of the quaternized nicotinamides were covalently attached to the surface of tin oxide semiconductor electrodes by several silanization methods. The surface coverage was confirmed by X-Ray Photoelectron Spectroscopy and by the appearance of the corresponding CV reduction wave on the electrode's surface. The reduction of several electron deficient carbonyl compounds was investigated upon the NAD⁺ analog electrodes; however, no significant catalysis of the reductions by the enzyme mimic was observed