Cotrollable growth of monolayer MoS<sub>2</sub> films and their applications in devices
Authors: Li Lu , Zhang Yang-Kun , Shi Dong-Xia , Zhang Guang-Yu ,   CAS Key Laboratory of Nanoscale Physics and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China,   Beijing Key Laboratory for Nanomaterials and Nanodevices, Beijing 100190, China,   Collaborative Innovation Center of Quantum Matter, Beijing 100190, China
Published: March 2022

Monolayer molybdenum disulfide (MoS<sub>2</sub>) is an emerging two-dimensional (2D) semiconductor material.The MoS<sub>2</sub> film has a natural atomic-level thickness, excellent optoelectronic and mechanical properties, and it also has the potential applications in very large-scale integration technology in the future. In this article we summarize the research progress made by our group in the studying of monolayer MoS<sub>2</sub> films in the past few years. The controlled growth of large-size MoS<sub>2</sub> single crystals is achieved by oxygen-assisted chemical vapor deposition method. By a unique facile multisource CVD growth method, the highly oriented and large domain size ML MoS<sub>2</sub> films are epitaxially grown on a 4-inch wafer scale. Almost only 0° and 60° oriented domains are present in films, and the average size of MoS<sub>2</sub> grains ranges from 100 μm to 180 μm . The samples exhibit their best optical and electrical quality ever obtained, as evidenced from their wafer-scale homogeneity, nearly perfect lattice structure, average room-temperature device mobility of ~70 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> and high on/off ratio of ~10<sup>9</sup> on SiO<sub>2</sub> substrates. By adjusting the oxygen doping concentration in the MoS<sub>2</sub> film through using an effective CVD technique, electrical and optical properties can be well modified, thereby greatly improving the carrier mobilities and controllable n-type electronic doping effects resulting from optimized oxygen doping levels of MoS<sub>2–<i>x</i></sub>O<sub><i>x</i></sub> . In terms of MoS<sub>2</sub> thin film devices and applications, the 4-inch wafer-scale high-quality MoS<sub>2</sub> monolayers are used to fabricate the transparent MoS<sub>2</sub>-based transistors and logic circuits on flexible substrates. This large-area flexible FET device shows excellent electrical performance with a high device density (1,518 transistors per cm<sup>2</sup>) and yield (97%), and exhibits a high on/off ratio (10<sup>10</sup>), current density (~35 μA·μm<sup>–1</sup>), mobility (~55 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>) and flexibility. Based on the vertically integrated multilayer device via a layer-by-layer stacking process, an individual layer of all-2D multifunctional FET is successfully achieved with nearly multiplied on-current density, equivalent device mobility, and persevered on/off ratio and subthreshold swing (SS) of the individual layer, the combined performance of the device is fully utilized, and the integration of “sensing-storing-computing” is realized. A two-terminal floating-gate memory (2TFGM) based artificial synapse built from all-2D van der Waals materials is prepared, the 2TFGM synaptic device exhibits excellent linear and symmetric weight update characteristics with high reliability and tunability. A large number of states of up to ≈3000, high switching speed of 40 ns and low energy consumption of 18 fJ for a single pulse are demonstrated experimentally. The introduction of structural domain boundaries in the basal plane of monolayer MoS<sub>2</sub> can greatly enhance its hydrogen evolution reaction performance by serving as active sites. The progress we have made in the preparation of monolayer MoS<sub>2</sub> films and the research on device characteristics is of guiding significance for the basic and application research of MoS<sub>2</sub>, and also is universal and instructive for other 2D transition metal dichalcogenides.