Resonance Raman contribution to the D band of carbon materials: Modeling defects with quantum chemistry

Polycyclic aromatic hydrocarbons (PAHs) are employed to model the Raman features that are generally associated with sp2 nanostructures in carbon materials or with disorder and defects in graphitic materials. To this end molecular parameters (geometry changes upon electronic excitation, vibrational normal modes, and displacement parameters) are computed with semiempirical quantum-chemical methods for a series of PAHs ranging from 6 to 384 carbon atoms, and Raman intensities are evaluated according to Albrecht’s formalism restricted to the A term. The computed preresonance and resonance Raman intensities are compared with available experimental data for hexa-peri-hexabenzocoronene and for pyrene. For the latter compound, simulations carried out at semiempirical and ab initio levels of theory are shown to be of comparable quality. Finally, the collection of displacement parameters computed for the sample of conjugated molecules is used to model the effect of disorder and defects in the Raman response of a carbon material containing sp2 islands. It is shown that the computed D-band frequency dispersion, with respect to excitation wavelength, reproduces closely the experimental data measured for sp2 hybridized carbon materials.