Transition Metal Dichalcogenides (TMDs): Atomically Thin 2D Semiconductors

Transition metal dichalcogenides (TMDs) are 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). A monolayer is a single sheet of metal atoms sandwiched between two layers of chalcogen atoms; a molybdenum disulfide (MoS2) monolayer is only about 6.5 angstroms thick. Common examples include MoS2, tungsten diselenide (WSe2), WS2, MoSe2, and MoTe2. The key advantage over Graphene: The Single-Atom-Thick Carbon Sheet With No Bandgap is that TMD monolayers are genuine semiconductors with a usable bandgap. Interestingly, the bandgap changes character with thickness: bulk MoS2 has an indirect gap (~1.2 eV) that becomes a direct gap (~1.8-1.9 eV) when thinned to a single layer. Reported monolayer gaps include MoS2 ~1.78 eV, WS2 ~1.84 eV, and WSe2 ~1.52 eV. A direct gap also makes these materials useful for optoelectronics. Because they are atomically thin yet switchable, TMDs are leading candidates for continuing transistor miniaturization beyond silicon; MoS2 field-effect transistors have shown on/off ratios exceeding 1e8 at room temperature. A practical obstacle, identified in 2026 research from TU Wien (published in *Science*), is that when 2D semiconductors are paired with the insulating oxide a transistor gate needs, weak van der Waals bonding leaves a ~0.14 nm gap that reduces gate control; proposed "zipper materials" would bond the layers more strongly. See No Single Successor to Silicon: The Heterogeneous Future of Chip Materials for how TMDs compare to other silicon alternatives.