Metal chalcogenide and arsenide nanowires

Abstract

As the size of material decreases to nanoscale, the material exhibits exotic properties different from the bulk-scale materials, such as large surface area to volume ratio and quantum size effect. Because of the novel characteristics, nanomaterials have been actively studied for decades and can be applied to various fields including chemistry, electronics, optics, materials science, and biology. In particular, nanomaterials are extensively investigated in quantum physics because quantum phenomena such as quantum confinement, quantum interference, ballistic transport and phase coherence occur well in the low dimensional systems. Nanomaterials offer a unique possibility to control dimensions, chemical composition, and doping during the growth process, which makes it possible to control the physical properties of the material, allowing for a variety of studies.

In this dissertation, we focus on the nanomaterials with strong spin-orbit coupling, which are metal chalcogenide (Ag$_2$Se$_x$Te$_{1-x}$, Bi$_{2-x}$Sb$_x$Se$_{3-y}$Te$_y$) and metal arsenide (InAs) nanowires, and study the quantum phenomena in the nanomaterials. The spin-orbit coupling, which is a relativistic interaction of electron spin and orbital angular momentum, facilitates the manipulation of electron spin and induces topological states in materials, enabling applications to spintronics and quantum computing. We present the synthesis and structural characterization of the nanomaterials with strong spin-orbit coupling and demonstrate their electronic states and physical properties via electron transport measurement and electromechanical motion measurement.

In chapter 1, we introduce the quantum phenomena in nanomaterials and the nanomaterials with strong spin-orbit coupling. In chapter 2, we present the synthesis of single-crystalline nanostructures—nanowires, nanoribbons, and nanoplates—via chemical vapour transport method and their structural characterization. We synthesize multicomponent Ag$_2$Se$_x$Te$_{1-x}$ and Bi$_{2-x}$Sb$_x$Se$_{3-y}$Te$_y$ nanostructure with tunable chemical composition. By controlling the precursor molar ratio or the reaction temperature, we adjust the chemical composition and dimension of the multicomponent nanostructure. Without using surfactants, the synthesized nanostructures have atomically smooth surfaces and ordered structures with definite stoichiometric chemical composition. Materials with single crystal structures and clean surfaces are required for the study of intrinsic material properties, and chemically synthesized nanostructures satisfy such conditions. In addition, the chemical composition and dimension adjustment of nanomaterials enables the manipulation of physical properties such as band structure, thus providing enormous opportunities for studying the quantum phenomena in materials.

In chapter 3, we report the electron transport properties of ternary silver chalcogenide, Ag$_2$Se$_{0.5}$Te$_{0.5}$, nanowire as a new ternary topological material. Magnetoresistance measurements reveal Aharonov-Bohm oscillation and weak anti-localization, which are expected quantum interference phenomena in topological materials due to the helical electron transport along the surface of nanowire. Shubnikov-de haas oscillation is also observed, indicating that ternary Ag$_2$Se$_{0.5}$Te$_{0.5}$ nanowires present topological surface states with a higher electron mobility and longer mean free path compared to binary silver chalcogenides. First-principles calculations verify that Ag$_2$Se$_{0.5}$Te$_{0.5}$ is a topological insulator and the observed enhancement in transport properties originates from the modified band structure and the reduced bulk carrier contribution in the new ternary silver chalcogenide. Via chemical component engineering in Ag-based topological materials, our results demonstrate that the band structure can be manipulated, thereby enabling control of the topological states and adjustment of chemical potential.

In chapter 4 and chapter 5, we study the electromechanical properties of Bi$_2$Se$_3$ nanowire and InAs nanowire, respectively. We demonstrate a novel characterization of quantum interference in topological insulator Bi$_2$Se$_3$ nanowire and quantum dot properties in InAs nanowire via nanomechanical resonance measurements. In this measurement system, the nanowire is configured as an electromechanical resonator such that its mechanical vibration is associated with quantum capacitance, i.e. the internal charging capacity of the nanowire. We conduct simultaneous measurements of DC conductance and resonant frequency shifts of nanowire and confirm the strong coupling between electron transport and mechanical motion, which is explained by the quantum capacitance effect. In Bi$_2$Se$_3$ nanowire resonator of chapter 4, the surface electronic states of nanowire is reflected in the resonant frequency, thereby revealing Aharonov–Bohm oscillations. The Aharonov–Bohm oscillation in the resonant frequency shift gets more pronounced as the chemical potential of noaniwre approaches the Dirac point. Similarly, in InAs nanowire resonator of chapter 5, the charge transfer in quantum dot is reflected in the resonant frequency, thereby showing Coulomb blockade features. Our results demonstrate that the nanomechanical detection scheme can be used to probe the electronic states of nanomaterials through measurement of shifts in resonant frequency and thus, this technique would be generally applicable to the characterization of the Dirac electronic structures, where the strength of electronic signal is vanishingly small near Dirac point due to its low density of states.