In
electronic and optoelectronic devices made from van der Waals
heterostructures, electric fields can induce substantial band structure
changes which are crucial to device operation but cannot usually be
directly measured. Here, we use spatially resolved angle-resolved
photoemission spectroscopy to monitor changes in band alignment of
the component layers, corresponding to band structure changes of the
composite heterostructure system, that are produced by electrostatic
gating. Our devices comprise graphene on a monolayer semiconductor,
WSe2 or MoSe2, atop a boron nitride dielectric
and a graphite gate. Applying a gate voltage creates an electric field
that shifts the semiconductor bands relative to those in the graphene
by up to 0.2 eV. The results can be understood in simple terms by
assuming that the materials do not hybridize.