arXiv2014
Recently emerging large-area single-layer MoS2 grown by chemical vapor deposition has triggered great interest due to its exciting potential for applications in advanced electronic and optoelectronic devices. Unlike gapless graphene, MoS2 has an intrinsic band gap in the visible which crosses over from an indirect to a direct gap when reduced to a single atomic layer. In this article, we report a comprehensive study of fundamental optical properties of MoS2 revealed by optical spectroscopy of Raman, photoluminescence, and vacuum ultraviolet spectroscopic ellipsometry. A band gap of 1.42 eV is determined by the absorption threshold of bulk MoS2 that shifts to 1.83 eV in monolayer MoS2. We extracted the high precision dielectric function up to 9.0 eV which leads to the identification of many unique interband transitions at high symmetry points in the MoS2 momentum space. The positions of the A and B excitons in single layers are found to shift upwards in energy compared with those of the bulk form and have smaller separation. A very strong optical critical point predicted to correspond to a quasi-particle gap is observed at 2.86 eV, which is attributed to optical transitions along the parallel bands between the M and gama points in the reduced Brillouin zone. The absence of the bulk MoS2 spin-orbit interaction peak at ~ 3.0 eV in monolayer MoS2 is, as predicted, the consequence of the coalescence of nearby excitons. A higher energy optical transition at 3.98 eV, commonly occurred in bulk semiconductors, is associated with a combination of several critical points.These optical transitions herein reported enhance our understanding of monolayer MoS2 as well as of two-dimensional systems in general, and thus provide informative guidelines for MoS2 optical device designs and theoretical considerations.