Electrical Resistivity and Stoichiometry of K χ C 60 Films

Electrical resistances of polycrystalline fullerene (C(60)) 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 K(chi) C(60) films in equilibrium near room temperature were measured. The resistance changes smoothly from metallic at chi approximately 3 to activated as chi --> = 0 or chi --> 6. The minimum resistivity for K(3)C(60) films is 2.2 microohm-centimeters, near the Mott limit. The resistivities are interpreted in terms of a granular microstructure where K(3)C(60) regions form nonpercolating grains, except perhaps at chi approximately 3. Stoichiometries at the resistivity extrema were determined by ex situ Rutherford backscattering spectrometry to be chi = 3 +/- 0.05 at the resistance minimum and chi = 6 +/- 0.05 at the fully doped resistance maximum.