ELECTRICAL-RESISTIVITY AND STOICHIOMETRY OF KXC60 FILMS

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 K(x)C60 films in equilibrium near room temperature were measured. The resistance changes smoothly from metallic at x almost-equal-to 3 to activated as x --> 0 or x --> 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 x almost-equal-to 3. Stoichiometries at the resistivity extrema were determined by ex situ Rutherford backscattering spectrometry to be x = 3 +/- 0.05 at the resistance minimum and x = 6 +/- 0.05 at the fully doped resistance maximum.