ELECTRICAL-RESISTIVITY AND STOICHIOMETRY OF BAXC60 FILMS

Electrical resistance measurements show that uniform intercalation of Ba into C60 films is much more difficult to achieve than in the case of the other alkaline earths (AE), Ca and Sr. BaxC60 films studied after annealing at 220-degrees-C show a resistivity minimum between 1 < x < 2, and another weak minimum at x = 5. The activation energies for the conduction process show a minimum at Ba1C60. The resistivities are rho(min)(x almost-equal-to 1.5) = 0.2 OMEGA cm and rho(min)(x = 5) = 3 X 10(-3) OMEGA cm. This latter value is the lowest yet reported in a doped granular C60 thin (almost-equal-to 200 angstrom) film and Ba5C60 exhibits a metallic (positive) temperature coefficient of resistivity. There is a maximum in the resistivity at x = 3. The results suggest that the conductivities observed in (AE)xC60 films (AE = Ca, Sr, Ba) between 0 < x< 3 are associated with the population of bands derived from the t1u LUMO levels of C60, whereas the most conducting hases have the composition x = 5 and involve energy bands derived from the t1g LUMO + 1 levels of C60.