  The temperature-induced shift of the Raman G line in epitaxial graphene on
SiC and Ni surfaces, as well as in graphene supported on SiO2, is investigated
with Raman spectroscopy. The thermal shift rate of epitaxial graphene on
6H-SiC(0001) is found to be about three times that of freestanding graphene.
This result is explained quantitatively as a consequence of pinning by the
substrate. In contrast, graphene grown on polycrystalline Ni films is shown to
be unpinned, i.e., to behave elastically as freestanding, despite the
relatively strong interaction with the metal substrate. Moreover, it is shown
that the transfer of exfoliated graphene layers onto a supporting substrate can
result in pinned or unpinned layers, depending on the transfer protocol.
