Layer dependence of the electronic band alignment of few-layer Mo S2 on Si O2 measured using photoemission electron microscopy

Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultrathin, flexible, and efficient. The 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already shown favorable characteristics for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer structures is still lacking in experimental studies, which hinders development of novel devices based on TMDs. Here, we report on the local band alignment of monolayer, bilayer, and trilayer MoS2 on a 285-nm-thick SiO2 substrate using an approach to probe the occupied electronic states based on photoemission electron microscopy and deep-ultraviolet light. Local measurements of the vacuum level and the valence band edge at the Brillouin zone center show that the addition of layers to monolayer MoS2 increases the relative work function and pushes the valence band edge toward the vacuum level. We also deduced n-type doping of few-layer MoS2 and type-I band alignment across monolayer-to-bilayer and bilayer-to-trilayer lateral junctions. Conducted in isolation from environmental effects owing to the vacuum condition of the measurement and an insulating SiO2 substrate, this study shows a metrology to uncover electronic properties intrinsic to MoS2 semiconducting layers and emerging 2D crystals alike.