TMDCs

TMDCs, or transition metal dichalcogenides, are a family of layered materials with the formula MX2, where M is a transition metal (such as molybdenum or tungsten) and X is a chalcogen (sulfur, selenium, or tellurium). They consist of covalently bonded layers of MX2 held together by van der Waals forces, allowing exfoliation to single or few-layer sheets that act as two-dimensional semiconductors with properties that depend strongly on thickness.

The common semiconducting phase is the 2H structure, featuring hexagonal stacking and trigonal prismatic coordination of the metal atoms. A metastable 1T phase with octahedral coordination is metallic. Phase and composition engineering, including alloying (for example MoS2xSe2(1−x)) and vertical or lateral heterostructures, enable tuning of bandgaps, carrier type, and electrical conductivity.

In monolayer TMDCs, many materials exhibit direct bandgaps in the visible range, while their bulk forms have indirect gaps. This thickness dependence, together with strong spin–orbit coupling and reduced dielectric screening, leads to pronounced excitonic effects and valley physics. Excitons and charged excitons (trions) dominate optical responses, producing strong photoluminescence and tunable optical properties.

Synthesis methods include mechanical exfoliation for high-quality samples and scalable approaches such as chemical vapor deposition and molecular beam epitaxy for large-area films. TMDCs can form vertical and lateral heterostructures with other 2D materials like graphene and hexagonal boron nitride, enabling diverse device architectures.

Applications span field-effect transistors, photodetectors, light-emitting devices, and flexible electronics, as well as catalytic uses such as hydrogen evolution at exposed edges. Challenges include achieving uniform large-area quality, defect control, contact optimization, and environmental stability. Notable examples are MoS2, MoSe2, WS2, and WSe2.