Optoelectronic, structural, and topological properties of van der Waals layered materials under extreme conditions
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The concept of Internet of Things (IoT) has been discussed extensively in the recent years, where billions of smart devices and sensors communicate with each other and provide ubiquitous service. Two-dimensional (2D) materials for such application could be exposed to extreme conditions that IoT devices may experience, such as mechanically stressing, chemically reactive, high-temperature, and/or radiative environment. Therefore, it is crucial to understand the materials' properties under extreme conditions, and further engineer the properties from the acquired knowledge. In this dissertation, we focus on the effects of oxygen/moisture condition on air-sensitive 2D materials, and effects of hydrostatic pressure on 2D and other layered materials. In Chapter 2, we report detailed study on air-degradation of few-layer phosphorene films and field effect transistors, as well as an effective encapsulation method that enhances the stability of devices up to several months. In the later parts we explore the effects of hydrostatic pressure on layered materials, where the anisotropic van der Waals structure exhibit remarkably large pressure-modulation of material properties. In Chapter 3, pressure effects on Raman modes in bulk Mo₀.₅W₀.₅S₂ alloy are examined to discover strengthening of inter-layer interactions under pressure. In Chapter 4, pressure-induced structural transition of bulk WTe₂ is discussed, where layer sliding introduces inversion symmetry, similar to the case in monolayer WTe₂. In Chapter 5, evolution of optical band gaps of monolayer WS₂ and Mo₀.₅W₀.₅S₂ are studied, where we show different pressure-behaviors of band edges according to the composition. In Chapter 6, structural, vibrational, and topological electronic properties of Bi₁.₅Sb₀.₅Te₁.₈Se₁.₂ topological insulator alloy is explored, to show that the topological states could be modulated by pressure, without transitions in the crystal structure.