Band alignment at interfaces of two-dimensional materials: internal photoemission analysis

The article overviews experimental results obtained by applying internal photoemission (IPE) spectroscopy methods to characterize electron states in single- or few-monolayer thick two-dimensional materials and at their interfaces. Several conducting (graphene) and semiconducting (transitional metal dichalcogenides MoS₂, WS₂, MoSe₂, and WSe₂) films on top of thermal SiO₂have been analyzed by IPE, which reveals significant sensitivity of interface band offsets and barriers to the details of the material and interface fabrication, indicating violation of the Schottky-Mott rule. This variability is associated with charges and dipoles formed at the interfaces with van der Waals bonding as opposed to the chemically bonded interfaces of three-dimensional semiconductors and metals. Chemical modification of the underlying SiO₂surface is shown to be a significant factor, affecting interface barriers due to violation of the interface electroneutrality.