Polymeric Co-C60 Compound Phase Evolved in Atomistically Mixed Thin Films

The interactions between C60 and transition metal elements have attracted much attention for their possible formation of new functional materials, and it has been shown that the interesting structures are evolved in the atomistically mixed thin films of C60 and transition metals with the poor or the negligible solubility with carbons. However, the detailed studies of the microstructures have been limited due to the difficulty in the precise control of sample composition, the decomposition of C60 during deposition and the possible contamination from atmosphere. Recently, the mixtures of Co and C60 with the broad composition range of 1 < x < 700 (x: the number of Co atoms per a C60 molecule) have been successfully fabricated without the decomposition of C60, and the Raman analysis and the characterization of volume-change were performed under the well-defined conditions. The systematic study suggests the formation of a new compound-phase between Co and C60 (C60-based phase, hereafter) with the saturating composition of Co5C60 in the Co-dilute mixtures (x 5) and the precipitation of the excess Co atoms in the Co-dense mixtures (x > 5). The Raman analysis of the C60-based phase shows the symmetry-lowering of C60 and a relatively small amount of charge transfer (about 0.2 electrons/Co atom) from the bonded Co atom to a C60 molecule. 7) In addition, the considerable volume-dilatation was detected in the corresponding mixtures with increasing x (x 5). The purpose of this study is to confirm the atomic configuration and the bonding character in the suggested C60-based phase. Both of the Co-K edge X-ray absorption fine structure (XAFS) analysis and positron annihilation study are performed to know the short-range structure around the bonded Co atoms together with the size of the intermolecular free-volumes reflecting the spatial configuration of the short-range structure.