Growth and characterization of potassium-doped superfulleride thin films

Growth conditions for the formation of thin films (100-300 Å) of potassium-doped superfullerides (K_xC₆₀, x>6) are examined. Thin films of these compounds are formed by depositing C₆₀ onto a potassium precovered single crystal quartz substrate maintained at 200 K or lower, in a proportion of K:C₆₀>12:1, followed by annealing the surface to the K-sublimation temperature (300 K). In situ measurements of electrical and optical properties are used to identify the compounds. The formation of superfullerides is confirmed by C₆₀ doping of these phases to check for the formation of insulating K₆C₆₀ with a characteristic absorption spectrum. The absorption spectrum of the superfullerides shows distinct features corresponding to the filling of the t_1g band. The presence of two superfulleride phases is suggested, a near-metallic superfulleride K_xC₆₀ (x≈11.2) and a more insulating K_xC₆₀ (x≈8-9).