Observation of the out-of-plane mode in the Raman scattering from the graphite edge plane

Abstract

We report a sharp band observed at 867 ${\mathrm{cm}}^{\mathrm{-}1}$ in polarized Raman scattering from the edge plane of highly oriented pyrolytic graphite (HOPG). Polarization measurements indicates that this band is a vibrational mode with “out-of-plane” atomic displacement which is absolutely unexpected from the factor-group analysis. This band can be assigned to the $k=0\mathrm{}$ zone-center mode originating from the Raman-inactive $B_{2g}$ mode which becomes Raman active by modification of crystalline point-group symmetry due to a slight rearrangement of lattice structure at the edge. The origin of the ${867-\mathrm{c}\mathrm{m}}^{\mathrm{-}1}$ band is fundamentally different from that for the 1355- and ${1620-\mathrm{c}\mathrm{m}}^{\mathrm{-}1}$ bands (D, $D^{\prime }$ band) which are peculiar to disordered graphite and are $k\ne 0$ modes becoming Raman active due to the breakdown of the wave-vector selection rule by loss of translational symmetry.