  Even weak van der Waals (vdW) adhesion between two-dimensional solids may
perturb their various materials properties owing to their low dimensionality.
Although the electronic structure of graphene has been predicted to be modified
by the vdW interaction with other materials, its optical characterization has
not been successful. In this report, we demonstrate that Raman spectroscopy can
be utilized to detect a few % decrease in the Fermi velocity (vF) of graphene
caused by the vdW interaction with underlying hexagonal boron nitride (hBN).
Our study also establishes Raman spectroscopic analysis which enables
separation of the effects by the vdW interaction from those by mechanical
strain or extra charge carriers. The analysis reveals that spectral features of
graphene on hBN are mainly affected by change in vF and mechanical strain, but
not by charge doping unlike graphene supported on SiO2 substrates. Graphene on
hBN was also found to be less susceptible to thermally induced hole doping.
