Optical Spectroscopy of the Lattice Modes in Graphite Intercalation Compounds

Abstract

The weak coupling between carbon and intercalate layers in graphite intercalation compounds (GIC) [1,2] results in a lattice mode picture consisting of perturbed graphitic modes, and new modes derived from the intercalate layer. Lattice modes in GICs have been studied using Raman scattering, IR reflection spectroscopy and inelastic neutron scattering. Neutron studies are typically limited to the frequency range ω<;400 cm−1 and require much larger samples, such as can be prepared from polycrystalline pyrolytic graphite. The crystallites are preferentially aligned in this material, resulting in a well defined sample c-axis. Neutron scattering studies have therefore been largely concerned with the low frequency interlayer c-axis modes. The strong intralayer bonding between relatively light carbon atoms gives rise to high frequency optic branches which have been extensively studied using Raman and infrared (IR) spectroscopy. Most of these spectra are taken with radiation incident on the c-face (cleavage face). A limited number of optical experiments have been carried out [3] on a-faces, however. The a-face is important for studying interlayer modes which are excited when the electric vector \(\vec E\) is parallel to the graphitic c-axis \((\vec E\parallel \vec C)\)