Composition-Tunable Electrical and Thermoelectric Properties of 2D ReS2xSe2(1-x) Alloys

• Main Authors: Syu, Ming-Cheng, 許銘城
• Other Authors: Jian, Wen-Bin
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/pj52qu

碩士 === 國立交通大學 === 電子物理系所 === 107 === In our experiments, we focus on physical properties of 2D transition metal dichalcogenides (TMDs) of ReS2xSe2(1-x) alloys, including ReSe2, ReS0.6Se1.4, ReSSe, and ReS2. ReS2xSe2(1-x) alloys were synthesized with different sulfur and selenium compositions. By using mechanical exfoliation from the bulk, we could easily make few-layer flakes and place them on the substrate. The few-layer flakes were attached with source and drain electrodes to form field-effect transistors (FETs) for electrical and thermoelectric property measurements. We then investigated transport characteristics including mobility, on-off ratio, threshold voltage, and current density of few-layer ReS2xSe2(1-x) FETs to study the variation of physical properties due to different chemical compositions. Our experiments and discussions are separated to three sections. In the first section, we study carrier transport behaviors at temperatures in the range from 300 K to 80 K. We use the Mott’s two-dimensional variable range hopping theory to analyze the temperature dependent resistances when the few-layer ReS2xSe2(1-x) FETs show an insulator behavior. Furthermore, when the devices are gated to have high carrier concentrations, both ReSe2 and ReS2 FETs reveal a metal-insulator transition behavior at low temperatures. We extract the critical conductivity of ReSe2 and ReS2 and discuss the dependence of chemical compositions. In the second section, we study the anisotropy of few-layer ReS2xSe2(1-x) FETs. ReS2xSe2(1-x) exhibits a distorted octahedral phase 1T’ with asymmetric and significant in-plane anisotropy. We make devices with 12 gold electrodes circulating on ReS2xSe2(1-x) flakes. Each gold electrode is isolated and spaced with an angle of 30° so as to investigate the anisotropy. We study the anisotropy behavior and compare the variations on few-layer ReS2xSe2(1-x) with different chemical compositions. In the last section, we design and make thermoelectric devices on few-layer ReS2xSe2(1-x) flakes. We measure the Seebeck coefficient and study its variation for ReS2xSe2(1-x) alloys with different chemical compositions.