  Vertically stacked van der Waals heterostructures are a lucrative platform
for exploring the rich electronic and optoelectronic phenomena in
two-dimensional materials. Their performance will be strongly affected by
impurities and defects at the interfaces. Here we present the first systematic
study of interfaces in van der Waals heterostructure using cross sectional
scanning transmission electron microscope (STEM) imaging. By measuring
interlayer separations and comparing these to density functional theory (DFT)
calculations we find that pristine interfaces exist between hBN and MoS2 or WS2
for stacks prepared by mechanical exfoliation in air. However, for two
technologically important transition metal dichalcogenide (TMDC) systems, MoSe2
and WSe2, our measurement of interlayer separations provide the first evidence
for impurity species being trapped at buried interfaces with hBN: interfaces
which are flat at the nanometer length scale. While decreasing the thickness of
encapsulated WSe2 from bulk to monolayer we see a systematic increase in the
interlayer separation. We attribute these differences to the thinnest TMDC
flakes being flexible and hence able to deform mechanically around a sparse
population of protruding interfacial impurities. We show that the air sensitive
two dimensional (2D) crystal NbSe2 can be fabricated into heterostructures with
pristine interfaces by processing in an inert-gas environment. Finally we find
that adopting glove-box transfer significantly improves the quality of
interfaces for WSe2 compared to processing in air.
