Valence and core excitation spectra in K, Rb, and Cs alkali-metal stage-1 intercalated graphite

Abstract

Excitation spectra of the alkali-metal stage-1 intercalated graphites K${\mathrm{C}}_{8}$, Rb${\mathrm{C}}_{8}$, and Cs${\mathrm{C}}_{8}$ have been measured by high-resolution (0.1 eV) electron-energy\char22{}loss spectroscopy. The $\ensuremath{\pi}$ intraband and interband plasmon excitations in the region below 10 eV are the same in all three compounds. However, in the valence region between 15 and 30 eV, small peaks are observed in Rb${\mathrm{C}}_{8}$ and Cs${\mathrm{C}}_{8}$ which are weaker than those previously observed in K${\mathrm{C}}_{8}$. In K${\mathrm{C}}_{8}$, these structures were identified as both excitations to the backfolded graphite bands created by the introduction of the intercalant and also as characteristic metal-atom core excitations. Within this interpretation, the weakening of the structure due to the backfolded bands with intercalant atomic number is consistent with weakening the coupling far above the Fermi level between the graphite and intercalant atoms. The carbon $1s$ core shell excitation at \ensuremath{\sim}285 eV is also reported. We find that both the spectral shape and threshold energy are unaffected by the choice of alkali-metal intercalant (K, Rb, Cs), although the resulting spectrum differs from that of pristine graphite. Hence the graphite $p$ states just above the Fermi level which are probed by the C $1s$ excitation are identically perturbed by the K, Rb, and Cs intercalations, suggesting that the degree of hybridization and charge transfer in these materials is the same.