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Two-dimensional semiconductors: The counterpart to graphene

Heinz, Tony; Depts. of Applied Physics and Photon Science, Stanford University, Stanford, CA 94305, USA and SLAC National Accelerator Laboratory, Menlo Park, CA USA 94025, USA

We will review recent progress in understanding the electronic and optical properties of a new class of 2D semiconductors, monolayers of transition metal dichalcogenides in the MX2 class (M=Mo, W; X=S, Se). These materials provide an excellent complement to the metallic 2D system of graphene, in which, unlike in the case of carbon nanotubes, it is challenging to create a large band gap. These 2D semiconductors, which are stable under ambient conditions, can be readily employed as field-effect transistors with large on-off ratios. They have, moreover, been shown, in contrast to their bulk counterparts, to exhibit direct-gap character as monolayers. They are thus efficient nanoscale light emitters. In addition, the materials exhibit remarkably strong and distinctive excitonic interactions, associated with the reduced dielectric screening in the two-dimensional limit. This is manifest in the stability not only of excitons, but also of three (trion) and four (biexciton) states. The materials have also been shown provide a route to access the valley degree of freedom through selections rules for valley-selective excitation using circularly polarized light. Further, the existence of an ever-widening class 2D van der Waals materials offers exciting possibilities for the creation of novel heterostructures of layers with tunable electronic properties different from either of the constituents.

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