Photoemission studies of layered transition-metal dichalcogenides: MoS2

Abstract

Ultraviolet photoemission spectroscopy (UPS) has been used to study the valence-band electronic structure of $2H-\mathrm{M}\mathrm{o}{\mathrm{S}}_2$. The samples were cleaved or heated in ultrahigh vacuum of ∼ 1 × ${10}^{-10\mathrm{}}$ Torr to achieve atomically clean surfaces. A spherical retarding-field analyzer was used to measure the angle-integrated photoelectron energy-distribution curves (EDC) and Fermi levels were located by the use of a copper reference sample. In $2H-\mathrm{M}\mathrm{o}{\mathrm{S}}_2$, EDC structure is observed primarily at constant initial-state energies but with relative strengths being a strong function of photon energy. Arguments are presented that these data are indicative of bulk Mo${\mathrm{S}}_2$ electronic structure. The data can thus provide a sensitive test for a detailed calculation of Mo${\mathrm{S}}_2$ bulk electronic structure and transition probabilities. The UPS data are combined with other reported data, and a rather complete band model for Mo${\mathrm{S}}_2$ is developed. Two key points of this model are a semiconducting band gap in excess of 1 eV and an overlap of the two valence bands. This model is compared with earlier models (such as Wilson and Yoffe's) as well as more recent "first-principles" band-structure calculations.