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Transition Metal Dichalcogenide monolayers
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/Monolayer_TMDC_structure.jpg/640px-Monolayer_TMDC_structure.jpg)
Transition Metal Dichalcogenide (TMDC) monolayers are atomically thin semiconductors of the type MX2, with M a transition metal atom (Mo,W ...) and X a chalcogen atom (S, Se ...). One layer of M atoms is sandwiched between two layers of X atoms. An 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, TMDC bulk crystals are formed of monolayers bound to each other by Van-der-Waals attraction. TMDC monolayers have properties that are distinctly different from the semimetal grahene:
- TMDC monolayers MoS2, WSe2, MoSe2, WS2 have a direct band gap, and can be used in electronics as transistors and in optics as emitters and detectors. [1] [2][3]
- The TMDC mononlayer crystal structure has no inversion center, which allows to access a new degree of freedom of charge carriers namely the k-valley index and to open up a new field of physics : valleytronics [4] [5] [6][7][8]
- The strong spin-orbit coupling in TMDC monolayers leads to spin splittings in the hundreds of meV range in the valence band and a few meV in the conduction band, which allows to control the electron spin by tuning the excitation laser photon energy.
The work on TMDC monolayers is an emerging research and development field since the discovery of the direct bandgap [1] [2] and the potential applications of the very peculiar electron valley physics. [6][7] [8]