Optical Study of Charge Transfer in Graphite Intercalation Compounds.

The optical and electronic properties of several binary and ternary graphite intercalation compounds have been studied using optical reflectance, transmittance, x-ray diffraction and Raman scattering. The c-face optical reflectance of stage 1 and 2 graphite-H_2SO_4 (C_{rm p}^ {+}HSO_4^{ -}(H_2SO_4 )_{rm x}) has been studied in the energy range 0.2 -6.0 eV. The endpoint compounds in each stage (stage 1 p = 21, 28), (stage 2 p = 48, 60) which exhibit the lowest and highest charge transfer have been studied. The c-face optical reflectance of the K(NH _3)_{rm x}C _{24} (X = 0, 1.49, 4.11, 4.38) and K(NH_3)_{rm x} C_{36} (x = 0, 4.5) have been measured in the photon energy range 0.5 -6 eV. Kramers-Kronig analyses were performed to determine the in-plane dielectric function. It is proposed that the potassium atoms are ionized (K^{+ }) and the K(4s) electrons are donated to the carbon pi* band and localized states in the metal ammonia intercalate layers. A sharp absorption band at 1.85 eV is observed and attributed to transitions between localized states in the K(NH_3) _{rm x} layers associated with solvated electrons. The pi electron contribution to the dielectric function is analysed in terms of two-dimensional energy band models. These models and the experimental pi electron plasma frequency result in a value of f_ {rm K} ~ 0.79 +/- 0.07 electrons per intercalated K-atom residing in the carbon pi* band for the K(NH_3)_{rm x}C_{24} x = 4.11 and 4.38 compounds and ~ 0.62 +/- 0.07 for the K(NH_3)_ {4.5}C_{36}, in good agreement with recent NMR results. Similar optical studies have been carried out in the K(THF)_ {rm x}C_{24 } (x = 0, 1, 2) system and a much larger electron backdonation (f_{rm K} ~ 0.51) is reported. The optical transmission study through ~1000 A thick flakes of graphite and the potassium graphite intercalation compounds KC_8 and KC_{24} ie the photon energy range of 1-5.5 eV is reported. Structure in the transmission spectra associated with the Drude minimum, the threshold for inter-pi-band and the graphitic M-point absorption are observed. We can also show how the electronic absorption in the quasi two -dimensional carbon pi-bands influence the general form of the transmission spectra of graphite intercalation compounds. (Abstract shortened with permission of author.).