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 K_xC₆₀ grains, as x is varied.