Raman scattering from intercalated donor compounds of graphite

Raman spectra of vibrational excitations in graphite intercalated with alkali donor atoms have been obtained at room temperature in the backscattering configuration using argon ion laser excitation. Stage 1, 2, and 3 cesium donor compounds prepared from highly oriented pyrolytic graphite (HOPG) were studied as were stage 1 and 2 HOPG potassium donor compounds. Second stage cesium and potassium intercalates exhibit a single Raman band unshifted from the E2g intralayer mode of HOPG which occurs at 1582 cm-1. In contrast, the third-stage cesium intercalate produces a doublet spectrum with one component upshifted from and the other at the same position as the HOPG intralayer mode. The above described observations for stage-2 and higher-stage donor compounds are explained by an analysis which considers (a) the nearest-layer configurations which a given carbon layer can experience in a given stage of intercalation, (b) the change in C-C bond distance with intercalation, and (c) the effect of intercalation on the carbon interlayer interaction. The spectra of stage-1 cesium and potassium intercalates are almost identical and each contains two anomolously shaped bands, one near to but down shifted from the interlayer mode of HOPG and one at 560 cm-1, both being superposed on a continuum background. These bands are attributed to Breit-Wigner or Fano resonances between an electronic Raman scattering continuum and the E2g intralayer mode and an out of plane intralayer vibration, respectively. The parameters, q, and governing the Breit-Wigner resonance are determined by fitting the observed line shapes of each of the two bands in the cesium and potassium intercalate spectra.