Comparative study of structures and electrical properties in cobalt-fullerene mixtures by systematic change of cobalt content

A systematic study is performed for the Co content-dependent structure evolution and electrical properties in the cobalt (Co)-fullerene (C₆₀) mixtures in the broad content region. Raman analysis confirms that the mixture has a saturation composition of Co₅C₆₀ for the compound formation. An important role of the covalent Co-C bonds for the characteristic structure and the properties in the Co-C₆₀ compounds is concluded from the content-dependent peak shifts and the symmetry lowering of C₆₀. The Co-dilute mixtures exhibit a semiconductive nature represented by the activated transport from the shallow mid-gap states. The Co-dense mixtures are characterized by hopping conduction (including variable range hopping) attributed to the Co clusters/nanoparticles in the compound matrix and by the percolated metallic conduction, depending on the Co content.