Monolayer molybdenum disulfide (MoS ₂) is an emerging two-dimensional (2D) semiconductor material.The MoS ₂film 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 ₂films in the past few years. The controlled growth of large-size MoS ₂single 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 ₂films 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 ₂grains 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 ²·V ^–1·s ^–1and high on/off ratio of ~10 ⁹on SiO ₂substrates. By adjusting the oxygen doping concentration in the MoS ₂film 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 _2–xO _x. In terms of MoS ₂thin film devices and applications, the 4-inch wafer-scale high-quality MoS ₂monolayers are used to fabricate the transparent MoS ₂-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 ²) and yield (97%), and exhibits a high on/off ratio (10 ¹⁰), current density (~35 μA·μm ^–1), mobility (~55 cm ²·V ^–1·s ^–1) 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 ₂can greatly enhance its hydrogen evolution reaction performance by serving as active sites. The progress we have made in the preparation of monolayer MoS ₂films and the research on device characteristics is of guiding significance for the basic and application research of MoS ₂, and also is universal and instructive for other 2D transition metal dichalcogenides.