  The purity of graphene samples is of crucial importance for their
experimental and practical use. In this regard, the detection of the defects is
of direct relevance. Here, we show that structural defects in graphene samples
give rise to clear signals in the vibrational density of states (VDOS) at the
specific peaks at high and low frequencies.These can be used as an independent
probe of the defect density. In particular, we consider grain boundaries made
of pentagon-heptagon pairs, and show that they lead to a shift of the
characteristic vibrational D mode towards higher frequency; this distinguishes
these line defects from the Stone-Wales point defects, which do not lead to
such a shift. Our findings may be instrumental for the detection of structural
lattice defects using experimental techniques that can directly measure VDOS,
such as inelastic electron tunneling and inelastic neutron spectroscopy.
