Dichalcogenides

Dichalcogenides are chemical compounds containing two chalcogen atoms per formula unit. In solid-state chemistry the term often refers to transition metal dichalcogenides (TMDCs), a family with the general formula MX2, where M is a transition metal such as molybdenum (Mo) or tungsten (W), and X is a chalcogen such as sulfur (S), selenium (Se) or tellurium (Te). These compounds typically adopt layered structures in which a sheet of metal atoms is sandwiched between two sheets of chalcogen atoms.

Most TMDCs crystallize in layered, hexagonal structures with strong covalent bonds within layers and weak van der Waals forces between layers, enabling exfoliation to few-layer or monolayer samples. They exist in multiple crystallographic polytypes, most commonly the semiconducting 2H phase (trigonal prismatic coordination) and the metallic 1T phase (octahedral coordination); a distorted 1T' phase is also known. The electronic properties vary with composition and thickness: bulk MoS2 is an indirect-bandgap semiconductor around 1.2–1.3 eV, while monolayer MoS2 has a direct gap near 1.8 eV. Spin–orbit coupling and reduced symmetry in monolayers give rise to spin-valley phenomena.

Common synthesis methods include mechanical exfoliation from bulk crystals, chemical vapor deposition (CVD), and sulfurization or selenization of metal films. Phase engineering can convert a semiconducting 2H material to metallic 1T or 1T' phases, enabling tunable conductivity and catalytic activity.

Dichalcogenides have potential in electronics and optoelectronics, including transistors, photodetectors and flexible devices, as well as catalysis for the hydrogen evolution reaction and as lubricants or additives in composites. Notable examples are MoS2, WS2, MoSe2 and WSe2.