2HTMDs

2H-transition metal dichalcogenides (2H-TMDs) are a class of layered crystalline materials with the formula MX2, where M is a transition metal such as molybdenum (Mo) or tungsten (W), and X is a chalcogen such as sulfur (S) or selenium (Se). They crystallize predominantly in the 2H polytype, a hexagonal structure characterized by layers where a metal atom is sandwiched between two chalcogen atoms. Each layer features trigonal prismatic coordination of the metal, and the unit cell contains two MX2 layers with a specific AB stacking pattern.

In the 2H polytype, neighboring layers align in a way that preserves hexagonal symmetry and yields semiconducting behavior. This contrasts with other polytypes of TMDs, such as the 1T phase, which is typically metallic. The bandgap of 2H-TMDs depends on thickness: monolayers often exhibit direct bandgaps in the visible range (for example, monolayer MoS2 has a direct gap around 1.8 eV), while bulk forms have indirect gaps and smaller effective gaps. They also display strong spin-orbit coupling and valley polarization effects, which are of interest for nanoelectronic and optoelectronic applications.

Common examples of 2H-TMDs include MoS2, MoSe2, WS2, and WSe2. These materials are typically prepared by exfoliation from bulk crystals or synthesized via chemical vapor deposition, enabling scalable integration into devices. Applications span field-effect transistors, photodetectors, light-emitting devices, and catalysis, notably hydrogen evolution reactions at exposed edge sites. Challenges remain in achieving uniform large-area monolayers, controlling defects, and optimizing contact resistance for device performance.

See also: transition metal dichalcogenides; 2H polytype; MoS2.