Figure 1. (a) Argon plasma phase transition diagram. (b, c) Raman (b) and fluorescence characterization (c) for different phase change processing times. (d) XPS characterization before and after phase change. Figure 2. (a) STM characterization of original 2H molybdenum disulfide samples (b, c) STM characterization after phase change treatment (d, e) STS spectra before and after phase transformation. Figure 3. (a) Schematic diagram of three field effect transistors. (b) Transfer characteristic curve of three field effect transistor devices. Monolayered molybdenum disulfide is a typical two-dimensional transition metal chalcogenide. Due to its special energy band structure and semiconductor properties, it has broad application prospects in many fields such as nanoelectronic devices and optoelectronics. The monolayer of molybdenum disulfide is a stack of three atomic layers (sulfur-molybdenum-sulfur). The different stacking sequences make up two different phases, the 2H and 1T phases. Between the 2H phase layer and the layer, according to ABA stacking, the metal atom is trigonal column coordination and has a semiconductor property of 2.2 eV. 1T corresponds to the stacking method of ABC. The metal atoms are octahedral coordination and are metallic. Controllable realization of the phase transition between 2H and 1T is of great significance not only for studying the intrinsic physical properties of monolayer molybdenum disulfide but also for related applications. Recently, Zhu Jianqi, a doctoral student of the N07 group of the Nanophysics and Devices Laboratory of the Chinese Academy of Sciences Institute of Physics/National Laboratory for Condensed Matter Physics (CPC), collaborated with Peking University professor Jiang Ying under the guidance of researchers Zhang Guangyu and Shi Dongxia to use one. A simple surface treatment method successfully achieved a controllable transition from 2H to 1T phase. This method utilizes argon ions with a certain kinetic energy to bombard the surface of a single layer of molybdenum disulfide. The argon ions can effectively induce the torsion of the sulfur-molybdenum bond on the top layer of monolayer 2H phase molybdenum disulfide like an atomic bowling ball, resulting in the top layer sulfur atom as a whole. Slippage causes local phase transition of 2H-1T in molybdenum disulfide. Using scanning tunneling microscopy, they verified the simultaneous presence of a direct bandgap 2H phase and a metallic 1T phase, and confirmed that a very small amount of sulphur atom vacancies makes this 1T/2H phase coexistence mosaic structure stable. Compared with the traditional phase change method, this method does not introduce superfluous impurities and is fast and controllable. It can also use the patterning technology to make only the phase change in the specified area to construct a heterostructure. Using this phase change method in a field effect transistor can make the part in contact with the metal into a 1T phase, thereby reducing the contact resistance, double the current density, and greatly improve the switching characteristics of the transistor. This method based on the atomic bowling effect to induce monolayer molybdenum disulfide phase transition is clean, efficient, controllable, and compatible with existing large-scale integrated circuit manufacturing processes, and is expected to be applied to molybdenum disulfide electronics, optics, energy and other fields. . The work was published in the American Chemical Society (JACS. 139, 30 (2017)). The research work was supported by the national key R&D program, the National Natural Science Foundation of China, the Frontier Science Key Research Project of the Chinese Academy of Sciences, and the Chinese Academy of Sciences’ strategic pilot project.
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