Invited Paper GR+EM+NS+SS+TF-ThA8
Electronic and Optical Properties of MoS₂ at Monolayer Thickness

Thursday, October 31, 2013, 4:20 pm, Room 101 A

Inspired by the fascinating properties and application potential of graphene, interest within the community has extended to a broader class of stable, atomically thin materials. In this paper, we will discuss recent advances in the study of atomically thin layers of MoS₂ as representatives of the class transition metal dichalcogenides. Although the structure of the monolayer of MoS₂ is similar to that of graphene, the A and B sublattice are occupied either by Mo atoms or by a pair of S atoms, rather than just by C atoms. This difference in symmetry allows MoS₂ to be a semiconductor with a significant band gap. Through characterization of the optical properties of the material as a function of thickness, we show that quantum confinement effects lead to a crossover in MoS₂ from an indirect gap semiconductor in the bulk to a direct gap semiconductor at monolayer thickness.

As expected for lower-dimensional materials, excitonic effects are very strong in monolayer MoS₂. This allows for the existence of charged exciton with binding energies sufficient to produce stable states even at room temperature. Another distinctive feature of this material is the possibility of creating long-lived valley polarization in which one of the two inequivalent, but energetically degenerate K and K’ valleys is populated in preference to the other. We demonstrate how this can be achieved using optical excitation with circularly polarized radiation and monitored through the retention of the polarization in fluorescence emission. The pronounced change in symmetry from the non-centrosymmetric monolayer to the centrosymmetric bilayer has also been examined through its influence on the production of valley polarization and of optical second-harmonic radiation.