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 C 60 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 K 3 C 60 , K 4 C 60 , K 6 C 60 and pure C 60 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 K 3 C 60 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 K 4 C 60 and K 6 C 60 phases at the surface together with the peak broadening of the LUMO–LUMO+1 transitions of K 4 C 60 and K 6 C 60 structures as a function of grain size. The results give insights into the relative kinetic stability of K x C 60 grains, as x is varied.