Electron Transport Properties of Layered Semiconductors and Superconductors
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The discovery of graphene has stimulated enormous studies of two-dimensional (2D) materials due to its unique electronic properties and its importance in potential technological applications. However, the lack of a band gap in the electronic structure of graphene also limits its possible applications. Recently, a growing interest has been focused on other layered 2D semiconductors beyond graphene, such as metal chalcogenides, providing unique opportunities beyond silicon era for the next-generation electronics, photonics, and energy applications. Besides the widely studied MoS2, other important metal chalcogenides include SnS2 (with a band gap of 2.1 eV), and SnS (with a band gap of 1.1 eV), which are earth-abundant and environment-friendly materials particularly desirable for future sustainable electronic/photovoltaic applications. However, despite its technological importance, the synthesis of thin crystal arrays of such 2D semiconductors at designed locations on suitable substrates has not been realized. Here in this dissertation we focused on a novel approach to the controlled synthesis of thin crystal arrays of SnS2 and SnS at predefined locations on the chip, by integrating a top-down process—standard nanofabrication, and a bottom-up process—chemical vapor deposition. We also have demonstrated their application as fast photodetectors with photocurrent response time ∼ 5 µs. This opens a pathway for the large-scale production of layered 2D semiconductor devices, important for applications in integrated nano-electronic/photonic systems. Due to the remarkable changes in the electronic properties of layered semiconductors as their thickness is reduced, we have also studied the electron transport properties of a layered superconductor, β-PdBi2. The latest experimental data from previous work show a TC of 5.4 K for the bulk β-PdBi2 single crystal. The temperature dependence of the specificheat suggests that β-PdBi2 is a multiple-band/multiple-gap superconductor. However, there is no direct proof from scanning tunneling microscopy or other experiments. Here, we describe a novel experimental approach to point-contact spectroscopy for nanoplate superconductor and have employed this method to unveil the existence of two superconducting gaps in β-PdBi2, which is important for understanding its pairing mechanism.