  Combining monolayers of different two-dimensional (2D) semiconductors into
heterostructures opens up a wealth of possibilities for novel electronic and
optical functionalities. Exploiting them hinges on accurate measurements of the
band parameters and orbital hybridization in separate and stacked monolayers,
many of which are only available as small samples. The recently introduced
technique of angle-resolved photoemission spectroscopy with submicron spatial
resolution ({\mu}-ARPES) offers the capability to measure small samples, but
the energy resolution obtained for such exfoliated samples to date (~0.5 eV)
has been inadequate. Here, we show that by suitable heterostructure sample
design the full potential of {\mu}-ARPES can be realized. We focus on
MoSe2/WSe2 van der Waals heterostructures, which are 2D analogs of 3D
semiconductor heterostructures. We find that in a MoSe2/WSe2 heterobilayer the
bands in the K valleys are weakly hybridized, with the conduction and valence
band edges originating in the MoSe2 and WSe2 respectively. There is stronger
hybridization at the {\Gamma} point, but the valence band edge remains at the K
points. This is consistent with the recent observation of interlayer excitons
where the electron and hole are valley polarized but in opposite layers. We
determine the valence band offset to be 300 meV, which combined with
photoluminescence measurements implies that the binding energy of interlayer
excitons is at least 200 meV, comparable with that of intralayer excitons.

Mesoscale and Nanoscale Physics