WS2

WS2, or tungsten disulfide, is a layered transition metal dichalcogenide with the chemical formula WS2. In bulk form it is a dark, graphite-like solid. Each layer comprises a plane of tungsten atoms sandwiched between two planes of sulfur atoms, and adjacent layers are held together by van der Waals forces, allowing exfoliation to thinner flakes.

The most common crystalline form is the hexagonal 2H phase, where tungsten is coordinated by six sulfur atoms in a trigonal prismatic geometry. A metastable 1T phase is metallic and can be induced by intercalation or strain; 1T can be converted back to 2H by annealing. Thin layers often transition from indirect to direct bandgap behavior, which is relevant for optoelectronic applications.

WS2 exhibits a combination of properties that make it useful across several fields. It has excellent lubricity and a low coefficient of friction, contributing to its role as a solid lubricant for high-temperature and vacuum environments. It is chemically relatively inert and thermally stable under operating conditions typical of mechanical and electronic devices. In electronics, monolayer WS2 is a direct-band-gap semiconductor with a bandgap near 2 eV, while bulk WS2 has an indirect gap around 1.4 eV. The material also shows strong excitonic effects, and is studied for transistors, photodetectors, and flexible electronics.

Production and processing methods include mechanical exfoliation, chemical vapor deposition, chemical vapor transport, and hydrothermal synthesis. WS2 is used as a catalyst support for hydrodesulfurization and hydrogen evolution reactions, and as an electrode material in batteries and supercapacitors. Ongoing research focuses on scalable synthesis, phase control between 2H and 1T, and integration into electronic, optoelectronic, and catalytic systems.

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