What is a transition metal dichalcogenide (TMD)?

Transition metal dichalcogenide (TMD) monolayers are atomically thin semiconductors of the type MX2, with M a transition metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, etc.). One layer of M atoms is sandwiched between two layers of X atoms. For example, a MoS2 monolayer is 6.5 Å thick.

The discovery of graphene shows how new physical properties emerge when a bulk crystal of macroscopic dimensions is thinned down to one atomic layer. Like graphite, TMD bulk crystals are formed of monolayers bound to each other by Van-der-Waals attraction. TMD monolayers have properties that are distinctly different from those of the semimetal grahene.

For more information about TMDs, see http://en.wikipedia.org/wiki/Transition_metal_dichalcogenide_monolayers.