THE SPATIALLY-RESOLVED COMPOSITION OF K-DOPED C-60 FILMS

The Raman spectra of thin films of C-60 were recorded as a function of potassium coverage under ultrahigh vacuum conditions. Both clean and air-exposed C-60 films were studied. The growth of the K3C60 and K6C60 phases are observed but no spectroscopic evidence of a K4C60 phase is found. The spectra indicate that potassium and C-60 co-exist as distinct C-60, K3C60 and K6C60 phases as opposed to solid solutions with variable dopant concentrations. Facile potassium diffusion into vacuum-sublimed C-60 films is observed even at 50 K resulting in the formation of K3C60 and K6C60. The electrical resistivity of films were recorded in situ at 373 K as a function of K exposure. Raman spectra recorded periodically at various levels of K exposure and at multiple locations on the film indicate that the doped C-60 films are granular with average grain sizes of the order of one to several micrometers. The occasional simultaneous detection by Raman of all three phases indicates that a small quantity of undoped C-60 remains even at levels of potassium exposure sufficiently high to form K6C60. This implies that while potassium infiltration into C-60 crystallites is rapid, intercrystallite potassium exchange is slow.