Tunable electronic structure in gallium chalcogenide van der Waals compounds

Transition-metal monochalcogenides comprise a class of two-dimensional materials with electronic band gaps that are highly sensitive to material thickness and chemical composition. Here, we explore the tunability of the electronic excitation spectrum in GaSe by using angle-resolved photoemission spectroscopy. The electronic structure of the material is modified by in situ potassium deposition as well as by forming GaSxSe1-x alloy compounds. We find that potassium-dosed samples exhibit a substantial change of the dispersion around the valence-band maximum (VBM). The observed band dispersion resembles that of a single tetralayer and is consistent with a transition from the direct-gap character of the bulk to the indirect-gap character expected for monolayer GaSe. Upon alloying with sulfur, we observe a phase transition from AB to AA′ stacking. Alloying also results in a rigid energy shift of the VBM towards higher binding energies, which correlates with a blueshift in the luminescence. The increase of the band gap upon sulfur alloying does not appear to change the dispersion or character of the VBM appreciably, implying that it is possible to engineer the gap of these materials while maintaining their salient electronic properties.