  We report an investigation of the graphene/substrate interface morphology in
large-area polycrystalline graphene grown by chemical-vapour deposition and
wet-transferred onto Si wafers. We combined spectroscopic ellipsometry, X-ray
photoelectron spectroscopy and atomic-force microscopy in order to yield
morphological and chemical information about the system. The data showed that
wet-transferred samples may randomly exhibit nanosized relief patterns
indicative of small water nanopockets trapped between graphene and the
underlying substrate. These pockets affect the adhesion of graphene to the
substrate, but can be efficiently removed upon a mild annealing in high vacuum.
We show that ellipsometry is capable of successfully and reliably detecting,
via multilayer dielectric modelling, both the presence of such a spurious
intercalation layer and its removal. The fast, broadly applicable and
non-invasive character of this technique can therefore promote its application
for quickly and reliably assessing the degree of adhesion of graphene
transferred onto target substrates, either for ex-post evaluation or in-line
process monitoring.
