Electron energy loss studies of potassium intercalated graphite

Abstract

Using a scanning transmission electron microscope equipped with a magnetic electron energy analyzer, electron energy loss spectra at various momentum transfers, q, have been obtained from single crystal samples of pristine graphite, stage 1 potassium intercalated graphite, KC 8, and stage 2 potassium intercalated graphite, KC 24. By comparison with pristine graphite, KC 24 and KC 8 show additional small- q energy loss peaks at 1.8 and 2.5 eV, respectively. They are assigned as plasmons of free carriers donated by potassium atoms. The 7.0 eV energy loss peak in graphite shifts to 6.7 and 6.3 eV in KC 24 and KC 8, respectively, and still disperses quadratically with q. In graphite, the 27 eV energy loss peak develops into double peaks as q increases, while in both KC 24 and KC 8, double peak features are observed around 27 eV at small q which merge into single peaks as q increases. By Kramers-Kronig analysis, the π-electron critical point transition energies are found to be 4.5, 4.2 and 3.8 eV for graphite, KC 24, and KC 8, respectively. These results are consistent with the prediction of theoretical band structure calculations.