Broadband optical properties of large-area monolayer CVD molybdenum disulfide

Abstract

Recently emerging large-area single-layer MoS[subscript 2] 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, MoS[subscript 2] 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 paper, we report a comprehensive study of fundamental optical properties of MoS[subscript 2] 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 MoS[subscript 2] that shifts to 1.83 eV in monolayer MoS[subscript 2]. 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 MoS[subscript 2] momentum space. The positions of the so-called 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 because of the decreased interactions between the layers. A very strong optical critical point predicted to correspond to a quasiparticle gap is observed at 2.86 eV, which is attributed to optical transitions along the parallel bands between the M and Γ points in the reduced Brillouin zone. The absence of the bulk MoS[subscript 2] spin-orbit interaction peak at ~3.0 eV in monolayer MoS[subscript 2] is, as predicted, the consequence of the coalescence of nearby excitons. A higher energy optical transition at 3.98 eV, commonly occurring in bulk semiconductors, is associated with a combination of several critical points. Additionally, extending into the vacuum ultraviolet energy spectrum are a series of newly observed oscillations representing optical transitions from valence bands to higher conduction bands of the monolayer MoS[subscript 2] complex band structure. These optical transitions herein reported enhance our understanding of monolayer MoS[subscript 2] as well as of two-dimensional systems in general and thus provide informative guidelines for MoS[subscript 2] optical device designs and theoretical considerations.