Electrical conductivity of germanium selenide nanosheets in oxygen and butane
Authors: Dai Fang-Bo , Yuan Jian-Mei , Xu Kai-Yan , Guo Zheng , Zhao Hong-Quan , Mao Yu-Liang ,   School of Physics and Optoelectronic, Xiangtan University, Xiangtan 411105, China,   School of Mathematics and Computational Science, Xiangtan University, Xiangtan 411105, China,   Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China
Published: June 2021

As a type of two-dimensional (2D) semiconductor material, 2D germanium selenide (GeSe) exhibits excellent optoelectronic properties, and has potential applications in optoelectronic devices. The GeSe is a layered material with weak van der Waals interaction. Because of the high brittleness of GeSe, it is not easy to obtain 2D GeSe samples only by mechanical peeling technique. In order to obtain a thinner GeSe sheet, we use heat treatment to thin the bulk GeSe at a high temperature in vacuum. The GeSe samples obtained by mechanical peeling are placed in a tubular furnace with a pressure of 5 × 10<sup>-4</sup> Pa for high temperature heating and thinning. In order to explore the better thinning effect, we set four temperatures to be at 320, 330, 340 and 350 ℃, respectively. After high temperature thinning, the samples are characterized and observed by atomic force microscope (AFM), scanning electron microscope (SEM), Raman spectrometer and photoluminescence (PL) spectrometer. From the above experiments, the GeSe nanosheet with a thickness of about 5 nm is prepared by mechanical peeling and high temperature thinning technology. Then, the electrical conductivities of GeSe nanosheets in oxygen (O<sub>2</sub>) and butane (C<sub>4</sub>H<sub>10</sub>) with different concentrations are evaluated by our designed experimental device. The results show that with the increase of oxygen concentration, the electrical conductivity of GeSe nanosheets increases. When the GeSe nanosheet is in butane gas, its conductivity under the same voltage decreases with the increase of the concentration of butane gas. In order to further analyze the mechanism of gas adsorption on GeSe nanosheets, we carry out the first-principles calculations. Our calculation results show that the adsorption energy of GeSe nanosheets for oxygen and butane is –4.555 eV and –4.865 eV, respectively. It is shown that both adsorption systems have a certain stability. The adsorption energy of C<sub>4</sub>H<sub>10</sub> is smaller than that of O<sub>2</sub>, which corresponds to the smaller layer spacing of C<sub>4</sub>H<sub>10</sub> than that of O<sub>2</sub> on GeSe surface. From Bader analysis, it is shown that 0.262<i>e</i> is transferred from the surface of GeSe nanosheet to O<sub>2</sub> molecule, which is much larger than 0.022<i>e</i> transferred from GeSe to C<sub>4</sub>H<sub>10</sub> molecule. It can be inferred that the bond formed between GeSe and O<sub>2</sub> molecule is covalent bond, while GeSe adsorption C<sub>4</sub>H<sub>10</sub> is very fragile hydrogen bond adsorption. In an ideal condition (single atomic GeSe layer, no Se vacancy, and the device preparation process is vacuum), our calculation results show that C<sub>4</sub>H<sub>10</sub> still has a weak ability to obtain electrons from the GeSe nanosheet. However, the complex conditions such as the actual layer thickness, the appearance of Se vacancy and the adsorption of O<sub>2</sub> molecules on the surface leads to the difference between the experimental results and the theoretical calculations, which can be attributed to the adsorption of O<sub>2</sub> molecules on the GeSe surface from the air during the processing of GeSe thinning and device fabrication. Owing to the high density of Se vacancies in the thin film, the high density of O<sub>2</sub> adsorption is caused. Thus, butane gas is easy to lose electrons on the GeSe surface due to the O<sub>2</sub> adsorption. In other words, electrons are transferred from butane gas molecules to the surface of GeSe film and neutralized with holes, which reduces the concentration of carriers and the concentration of holes in GeSe film, thus reducing the conductivity. Our research will contribute to the application of GeSe nanosheets in optoelectronic devices at the atmosphere of oxygen and butane.