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.