Optical study of theK-pointπ-band dispersion in graphite-H2SO4

We report the results of optical reflectivity studies of stage-1 and -2 graphite-${\mathrm{H}}_2$ ${\mathrm{SO}}_4$ in the range 0.2–6.0 eV. These particular graphite intercalation compounds are unusual because they can easily be prepared as a continuous series of compounds with differing charge transfer for fixed stage index. We have studied the end-point compounds in each series which exhibit the lowest and highest charge transfer. An analysis of the free-carrier contribution to the data supports a two-dimensional (2D) rigid-band model for the carbon $p_z$ valence band(s) (for fixed stage index). Assuming a value for the hole density consistent with the charge passed in the electrochemical reaction to synthesize the respective compounds, we find that the free-carrier absorption data are in good agreement with the linear-combination-of-atomic-orbitals model of Holzwarth (stage 1) as well as the tight-binding model of Blinowski et al. (stages 1 and 2), provided a stage-independent value of the principal band parameter ${\gamma }_0$=2.93 eV is used in this latter model. The differences in these 2D rigid-band models, as they pertain to the interpretation of low-energy (E≲3 eV) optical data in these compounds, are discussed. Both models fail to satisfactorily explain the position of the threshold for valence-to-conduction band absorption—which is observed downshifted by ∼0.5 eV from the predicted position (∼2$E_F$).