Special Physics Colloquium Thursday, January 15, 2015, 10:30 am PS 160 Two-dimensional materials beyond graphene: Atomically thin semiconductors Professor Tony F. Heinz Departments of Applied Physics and Photon Science, Stanford University Graphene has attracted great attention worldwide because of its exceptional potential for novel science and technology. Recently, this interest has expanded to the much wider class of 2D materials that occur as layers of van-der-Waals crystals that exhibit weak interlayer coupling, but strong in-plane bonding. While preserving graphene’s flexibility and tunability by external perturbations, this broader set of materials provides access to more varied electronic and optical properties. In particular, the semi-metallic system of graphene is now complemented by stable, atomically thin 2D layers that are insulators and semiconductors. Here we review our understanding of the properties of atomically thin layers of semiconductors in the family of transition metal dichalcogenides, MX2 where M = Mo, W and X = S, Se, Te. In the limit
of atomically thin layers, these materials exhibit a transition from optically
dark indirect-gap materials to bright direct-gap materials. The 2D layers display remarkably strong many-body interactions, with exciton binding energies of 100’s of meV and stable charged excitons at room temperature.
These materials also provide new possibilities for control of the valley character of charge carriers. In particular, the helicity of light allows the selection of one of the two degenerate, but quantum mechanically distinct K and K’ valleys in the Brillouin zone. We will discuss these emergent properties of the 2D semiconductors, as well as new possibilities that arise by forming tailored heterostructures of these
materials.
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