Internal Photoemission Spectroscopy of 2-D Semiconductor Transition Metal Dichalcogenides

Semiconductor transition metal dichalcogenides MX₂ (M=Mo, W; X= S, Se) have received tremendous attention for electronic and optoelectronic applications, with their direct bandgaps in the near-infrared to visible region in their 2-D monolayer limit. In particular, MX₂ Van der Waals heterostructures only with a staggered band alignment have been proposed as a structure that can separate photoexcited electrons and holes in different layers by charge transfer. It is thus important to know what the form of the band alignment of these hetorostructures is in order to realize appropriate device designs. Here we present the internal photoemission spectroscopy (IPE) approach to determine the band offsets of exfoliated large-area MoS₂, MoSe₂, WS₂, and WSe₂ monolayer. Experimentally, the quality and uniformity of the MX₂ monolayers are confirmed by Raman mapping and photoluminescence spectroscopy. The band offsets are measured in relative to the Al₂O₃ conduction band minimum from which we can determine whether the corresponding heterostructures fabricated from these MX₂ monolayers have a staggered band alignment configuration.