Raman spectroscopy of nanographites

Abstract

This work presents the application of the Raman spectroscopy for the study and characterization of nanographite systems. We report the ¯rst detection of nanographite ribbons on a highly oriented pyrolytic graphite substrate by Raman spectroscopy. We found a way to di®erentiate the Raman signal of the ribbon from that of the substrate, the Raman signal of the ribbon having the same order of magnitude as the one of the substrate, despite the much smaller number of illuminated carbon atoms (»10¡3). The results show that these structures present van Hove singularities in the electronic density of states due to quantum con¯nement into their 1D structure. In another experiment, we use micro-Raman scattering to determine the arrangement of carbon atoms in a graphite edge. Theedge of a semi-in¯nite plane can be considered as a one-dimensional defect, leading to unusual defect-induced Raman scattering that turns out to be structurally selective. In this case, Raman scattering can be used to de¯ne the orientation of the carbon hexagons with respect to the edge of a graphite plane, in the so called armchair and zigzag arrange-ments. These two experiments involve the ¯rst detection of the anisotropy in the optical absorption of graphite, giving strong support to previous theoretical predictions. We also present a systematic study of the ratio between the integrated intensities of the disorder-induced D band and the ¯rst-order allowed G band (ID=IG) in the Raman spectra of nanographite samples with di®erent crystallite sizes (La) and using di®erent excitation laser energies (wavelengths). The crystallite size La of the nanographite samples were obtained both by X-ray di®raction using synchrotron radiation and directly from scan-ning tunneling microscopy images. A general equation for the determination of La using any laser energy in the visible range is obtained. Moreover, we performed measurementsof the absolute intensities of individual features in the Raman spectra of nanographites,showing the dependence of these features on the excitation laser energy and crystallite size.