Structure Evolution and Corresponding Electrical Properties in Weakly Bound Co-C 60 Mixture
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Structure Evolution and Corresponding Electrical Properties in Weakly Bound Co-C60 Mixture Seiji Sakai, Hiroshi Naramoto, Yonghua Xu, Tri Hardi Priyanto, Vasily Lavrentiev and Kazumasa Narumi Advanced Science Research Center, Japan Atomic Energy Research Institute, Watanuki 1233, Takasaki, Gunma 370-1292, Japan ABSTRACT Mixture films of Co and C60 with various compositions were fabricated by co-evaporation under UHV condition. As for the saturation composition with C60-based phase, Co5C60 is deduced from the systematic Raman analysis. It is reasonably interpreted that the C60-based phase can be realized by less than one electron transfer per a Co atom into C60 and the contribution of a covalent bonding which induces a differently coordinated structure of the C60 molecules from the fcc-C60. The electrical conduction features are dependent on the Co content. In the C60-based phase up to Co5C60, the conduction mechanism seems to be described by the band concept modified with the disorder, followed by the hopping conduction through the precipitated Co clusters or particles and then, by the metallic one with increasing Co content. INTRODUCTION Atomistic mixture between two building units with small binding energies comparable to others can be a kind of model-substance to study about the structure evolution because the competition between two processes with the comparable energies could be important as a driving force to realize the highly organized structures spontaneously. C60 molecules and transition metal atoms can be cited as most interesting building units due to their electronically complementary partners. These two units can be stabilized by the charge transfer, and in some case, it is not absurd to expect that C60 molecules with the internal energy could be converted into other important carbon allotropes under the moderate condition with the help of catalytic effect. In our study, cobalt atoms are chosen as electron-donating units with the small miscibility with carbon atoms. Recent preliminary study has confirmed the formation of nano-diamonds and carbon nanotubes in the mixture films of C60 and Co just after the deposition at room temperature [1]. Here in the present study, the systematic approach from the sample preparation to the characterization of the properties is employed not to be influenced by the possible contamination. The mixture films of C60 and cobalt, CoxC60, are prepared in the broad composition range of x = 1-200, which makes it possible to perform the semi-quantitative analysis on Raman spectra. EXPERIMENTAL DETAILS Mixture films of C60 (99.99%) and cobalt (99.99%) were fabricated by co-deposition under high vacuum condition of 5, which suggests that the C60-based phase is kept to be unchanged basically beyond this Co content, and x = 5 (Co5C60) is surmised as the saturating, or critical composition of the C60-based phase. In the followings the spectral features are discussed. Concerning the Hg(7) and Hg(8) peaks, neither a heavy dumping nor a remarkable broadening is observed with i
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