Conducting films of C₆₀ and C₇₀ by alkali-metal doping

Abstract

THE recent syntheses^1,2 of macroscopic quantities of C₆₀ have suggested possible applications in host–guest and organic chemistry, tribology, electrochemistry and semiconductor tech-nology. Here we report the preparation of alkali-metal-doped films of C₆₀ and C₇₀ which have electrical conductivities at room temperature that are comparable to those attained by n-type doped polyacetylene. The highest conductivities observed in the doped films are: 4Scm⁻¹ (Cs/C₆₀), 100 (Rb/C₆₀), 500 (K/C₆₀), 20 (Na/C₆₀), 10 (Li/C₆₀), 2 (K/C₇₀). The doping process is reversed on exposure of the films to the atmosphere. At high doping levels, the films become more resistive. We attribute the conductivity induced in these films to the formation of energy bands from the π orbitals of C₆₀ or C₇₀, which become partially filled with carriers on doping. The smaller alkali metal ions should be able to fit into the interstices in the lattice without disrupting the network of contacts between the carbon spheroids. In the case of C₆₀, this would allow the development of an isotropic band structure, and we therefore propose that these materials may constitute the first three-dimensional 'organic' conductors.