  The pressure evolution of the Raman spectrum of graphene grown by chemical
vapour deposition on polycrystalline copper is investigated with the use of a
polar and a non-polar pressure transmitting medium (PTM). The G and 2D Raman
bands exhibit similar pressure slopes for both PTM irrespectively of any
unintentional initial doping and/or strain of the samples. Our analysis
suggests that any pressure-induced charge transfer effects are negligibly small
to influence the pressure response of graphene. This is determined by the
mechanical stress due to the pressure-induced substrate contraction and the
transfer efficiency of the latter to the graphene layer, as well as the
PTM-graphene interaction. For the non-polar PTM, a peculiar pressure behavior
of graphene is observed in the PTM solidification regime, resembling that of
free standing graphene.
