title: Controlling the Electronic and Optical Properties of Low-Dimensional Materials
Creators: name: (:Unkn) Unknown
Publisher: name: George Mason University
Year: 2020

description: Low-dimensional materials provide a means for controlling the optical and electronic properties of matter. This has tremendous potential for next-generation electronics with reduced size and improved energy efficiency. Here, we explore this concept primarily in two-dimensional (2D) transition metal dichalcogenides (TMDs) through alloy engineering. We also investigate the resistive switching of one-dimensional (1D) Ni/NiO core/shell nanowire junctions and Forster resonance energy transfer in DNA-based molecular photonic wires (MPWs). In the first study, the TMDs zirconium disulfide (ZrS2) and zirconium diselenide (ZrSe2) are alloyed to produce ZrSxSe2-x. We conduct the first low-temperature Raman spectroscopy measurements of this system and compare them with density functional theory (DFT) calculations. This analysis reveals that substitutional doping renders infrared (IR) modes to be Raman-active due to the large ionicity of the ZrSxSe2-x bonds. Spectroscopic ellipsometry measurements demonstrate continuous control of the direct exciton energies across near-infrared (NIR) wavelengths with alloy composition x, and strong light-matter interactions with low optical loss in the NIR suggest these alloys are ideal for photodetector and photovoltaic applications. In the second study, the TMD molybdenum ditelluride (MoTe2) is alloyed with tungsten ditelluride (WTe2) to produce Mo1-xWxTe2. We use Raman spectroscopy, X-ray diffraction, scanning transmission electron microscopy, and DFT to develop the phase diagram of this alloy system. 2H semiconducting, 1T' semimetallic, and Td Weyl semimetal phase regions, as well as a two-phase 1T'-Td region, are identified, indicating the potential of this system for use in phase change memories. Alloy disorder is found to cause asymmetric broadening of the Raman lineshapes, which are fit with the phonon confinement model to extract the phonon correlation length, and we also find disorder-enhanced two-phonon scattering and disorder-activation of infrared modes. In the third study, we examine how alloys of tungsten diselenide (WSe2) and WTe2, which produce WSe2(1-x)Te2x, can expand valley properties that may enable novel computational architectures. This is the first exploration of valley phenomena, which couple interband transitions with polarized light, in a phase change candidate system (WSe2 and WTe2 occupy the different semiconducting H and semimetallic Td phases, respectively). Low-temperature Raman and photoluminescence (PL) measurements, combined with DFT, allow alloy phase identification. We track the evolution of phonons across the semiconductor-semimetal phase transition and correlate these observations with DFT to identify alloy-only Raman modes as W-Te vibrations. PL measurements reveal band gap tunability across NIR wavelengths and show that valley polarization and coherence survive in alloys at high Te concentrations and are more robust against temperature than pure WSe2. Electronic transport studies of single Ni/NiO core/shell nanowire junctions demonstrate resistive switching that highlight the potential for non-volatile memory. Low-temperature electrical measurements show that conductive filament geometry is crucial to the transport characteristics, and we discover a third filamentary state with a semiconducting nature that acts as a quantum point contact. In the final study, we explore DNA MPWs, which provide a simple and economical means for capturing and directing photonic energy for optoelectronics. We show that DNA MPWs can be spin-coated in a polymer matrix onto silicon substrates, where they exhibit a 5-fold increase in photonic transfer efficiency over solution-based MPWs. The transfer efficiency of the MPWs is further increased to 40 - 240 %, depending on their length, when cooled to cryogenic temperatures. The findings of this dissertation lay the foundation for engineering novel functionalities in low-dimensional platforms for advanced electronics.