Overtone and combination features of G and D peaks in resonance Raman spectroscopy of the C78H26 Polycyclic Aromatic Hydrocarbon

We have studied the Raman spectra of C78H26, a polycyclic aromatic hydrocarbon with D2h symmetry point group resembling a longitudinally confined graphene ribbon (or a graphene island) with armchair edge. The experimental spectra recorded with several excitation laser lines have been compared with the results from a theoretical analysis of the resonant Raman response based on density functional theory calculations. Compared to previous investigation the spectra show better signal-to-noise ratio, which allows determining previously unresolved weak spectroscopic features. We have extended our analysis to the overtone and combination region (i.e., above 2000 cm-1) demonstrating the presence of signals attributable to 2G, G+D, 2D, Dj+Dk and G + acoustic-like modes. Moreover, we have measured the temperature dependence of the G peak position, which turns out to show a similar behavior with respect to that of graphene/graphite.