Raman activation in disordered graphites of the A1′ symmetry forbidden k≠0 phonon: The origin of the D line

Abstract

The analysis of the tensors of polarizability derivatives, computed with quantum-chemical methods for several large polycyclic aromatic hydrocarbon molecules, allows to rationalize the origin of the Raman D line observed in disordered graphites as due to a relaxation of the phonon selection rules. It is shown that a minimal rearrangement of the atomic and electronic structure in a lattice different from that of a perfect graphite crystal causes a selective activation of a specific totally symmetric phonon whose frequency falls in the region of the D band and whose motion is dominated by the breathing vibration of aromatic rings. We numerically proof this conjecture on the basis of polarizability derivatives obtained with Density Functional Theory calculations.