Study of the kinetics of potassium diffusion in thin C60 films

We present a quantitative analysis on the electron energy loss spectra (EELS) of potassium (K) doped C60 thin films collected as a function of time to explore the diffusion of K atoms at and near the surface at 100 K. The spectral features of K3C60, K4C60, K6C60 and pure C60 phases were observed to coexist before reaching saturation doping. The ratio of these phases can be controlled by the K deposition rate. At low deposition rate, we found no trace of K3C60 at the surface. A quantitative analysis, based on fits using Lorentzian functions characteristic of each structural phase, is used to monitor the time and dose variation of the multiphase-film. The analysis gives the dissipation rates of K4C60 and K6C60 phases at the surface together with the peak broadening of the LUMO–LUMO+1 transitions of K4C60 and K6C60 structures as a function of grain size. The results give insights into the relative kinetic stability of KxC60 grains, as x is varied.