Band alignments, conduction band edges and intralayer bandgap renormalisation in MoSe 2 /WSe 2 heterobilayers

Stacking two semiconducting transition metal dichalcogenide (MX 2 ) monolayers to form a heterobilayer creates a new variety of semiconductor junction with unique optoelectronic features, such as hosting long-lived dipolar interlayer excitons. Despite many optical, transport, and theoretical studies, there have been few direct electronic structure measurements of these junctions. Here, we apply angle-resolved photoemission spectroscopy with micron-scale spatial resolution ( µ ARPES) to determine the band alignments in MoSe 2 /WSe 2 heterobilayers, using in-situ electrostatic gating to electron-dope and thus probe the conduction band edges. By comparing spectra from heterobilayers with opposite stacking orders, that is, with either MoSe 2 or WSe 2 on top, we confirm that the band alignment is type II, with the valence band maximum in the WSe 2 and the conduction band minimum in the MoSe 2 . The overall band gap is E G = 1.43 ± 0.03 eV, and to within experimental uncertainty it is unaffected by electron doping. However, the offset between the WSe 2 and MoSe 2 valence bands clearly decreases with increasing electron doping, implying band renormalisation only in the MoSe 2 , the layer in which the electrons accumulate. In contrast, µ ARPES spectra from a WS 2 /MoSe 2 heterobilayer indicate type I band alignment, with both band edges in the MoSe 2 . These insights into the doping-dependent band alignments and gaps of MX 2 heterobilayers will be useful for properly understanding and ultimately utilizing their optoelectronic properties.