Electron-phonon coupling and structural phase transitions in early transition metal oxides and chalcogenides
Farley, Katie Elizabeth
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Pronounced nonlinear variation of electrical transport characteristics as a function of applied voltage, temperature, magnetic field, strain, or photo-excitation is usually underpinned by electronic instabilities that originate from the complex interplay of spin, orbital, and lattice degrees of freedom. This dissertation focuses on two canonical materials that show pronounced discontinuities in their temperature-dependent resistivity as a result of electron—phonon and electron—electron correlations: orthorhombic TaS 3 and monoclinic VO 2 . Strong electron-phonon interactions in transition metal oxides and chalcogenides results in interesting structural and electronic phase transitions. The properties of the material can be changed drastically in response to external stimuli such as temperature, voltage, or light. Understanding the influence these interactions have on the electronic structure and ultimately transport characteristics is of utmost importance in order to take these materials from a fundamental aspect to prospective applications such as low-energy interconnects, steep-slope transistors, and synaptic neural networks. This dissertation describes synthetic routes to nanoscale TaS 3 and VO 2 , develops mechanistic understanding of their electronic instabilities, and in the case of the latter system explores modulation of the electronic and structural phase transition via the incorporation of substitutional dopant atoms. We start in chapter 2 with a detailed study of the synthesis and electronic transport properties of TaS 3 , which undergoes a Peierls’ distortion to form a charge density wave. Scaling this material down to the nanometer-sized regime allows for interrogation of single or discrete phase coherent domains. Using electrical transport and broad band noise measurements, the dynamics of pinning/depinning of the charge density wave is investigated. Chapter 3 provides a novel synthetic approach to produce high-edge-density MoS 2 nanorods. MoS 2 is a promising catalyst for electrocatalytic water splitting and can catalyze the hydrogen evolution reaction that is utilized within photoelectrochemical cells. Chapters 4 and 5 delve into the synthesis and doping of VO 2 , which undergoes a metal to insulator transition. Chapter 4 develops a detailed understanding of the influence of doping on the MIT and reports the activation energies of the monoclinic→rutile (insulator→metal) and rutile→monoclinic (metal?insulator) transitions. The dynamical effects of doping on hysteresis are considered for both Mo- and W-doped VO 2 . Chapter 5 reports the development of synthetic route to produce optical grade VO 2 with considerable size control. Smart window applications for this material require small particle sizes in order to reduce visible light scattering. This chapter systematically explores hydrothermal syntheses for the preparation of VO 2 and allows for development of mechanistic postulates for obtaining size control.