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Transition metal dichalcogenide (TMDC) 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, or Te.). One layer of M atoms is sandwiched between two layers of X atoms. 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, 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 those of the semimetal graphene: * TMDC monolayers MoS2, WS2, MoSe2, WSe2, MoTe2 have a direct band gap, and can be used in electronics as transistors and in optics as emitters and detectors. * The TMDC monolayer 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 * The strong spin-orbit coupling in TMDC monolayers leads to a spin-orbit splitting of hundreds meV in the valence band and a few meV in the conduction band, which allows control of 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〔〔 and the potential applications of electron valley physics.〔〔〔 TMDCs are often combined with other 2d materials like graphene and hexagonal boron nitride to make van der Waals heterostructure devices. ==Crystal structure== In the macroscopic bulk crystal, or more precisely, for an even number of monolayers, the crystal structure has an inversion center. In the case of a monolayer (or any odd number of layers), the crystal has no inversion center. Two important consequences of that are: * nonlinear optical phenomena, such as second-harmonic generation. When the crystal is excited by a laser, the output frequency can be doubled. * an electronic band structure with direct energy gaps, where both conduction and valence band edges are located at the non-equivalent K points (K+ and K-) of the 2D hexagonal Brillouin zone. The interband transitions in the vicinity of the K+ (or K-) point are coupled to right (or left) circular photon polarization states. These so-called valley dependent optical selection rules arise from inversion symmetry breaking. This provides a convenient method to address specific valley states (K+ or K-) by circularly polarized (right or left) optical excitation.〔 In combination with strong spin-splitting, the spin and valley degree of freedom are coupled, enabling stable valley polarization. These properties indicate that TMDC monolayers represent a promising platform to explore spin and valley physics with the corresponding possible applications. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Transition metal dichalcogenide monolayers」の詳細全文を読む スポンサード リンク
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