Electrical Resistivity and Stoichiometry of K χ C 60 Films

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Abstract
Electrical resistances of polycrystalline fullerene (C60) films were monitored while the films were being doped in ultrahigh vacuum with potassium from a molecular-beam effusion source. Temperature- and concentration-dependent resistivities of Kχ C60 films in equilibrium near room temperature were measured. The resistance changes smoothly from metallic at χ ≈ 3 to activated as χ → = 0 or χ → 6. The minimum resistivity for K3C60 films is 2.2 microohm-centimeters, near the Mott limit. The resistivities are interpreted in terms of a granular microstructure where K3C60 regions form nonpercolating grains, except perhaps at χ ≈ 3. Stoichiometries at the resistivity extrema were determined by ex situ Rutherford backscattering spectrometry to be χ = 3 ± 0.05 at the resistance minimum and χ = 6 ± 0.05 at the fully doped resistance maximum.