Self-assembling hybrid nanoparticles during simultaneous deposition of Co and C60 on sapphire

The Co-C60 nano-composite film prepared by simultaneous deposition of Co and C60 at room temperature (RT) on sapphire with high content of cobalt (50 at.% Co) is recognized as ensemble of the fcc-Co crystals (5-6 nm in size) separated by the C60-based matrix. It is shown formation of internal stress in the composition arising due to the phase separation. The internal stress causes the phenomena sufficiently influencing structure of the nano-composite. One of them is locking the Co atoms within the C60-based matrix (retained Co atoms) occurring during the separation process. Analysis of the Raman spectrum argues that the retained Co atoms are included in the Co-C60 polymer dominating in the matrix of nano-composite. It is suggested importance of the internal stress for the polymer formation. Another phenomenon is the structural relaxation releasing the internal stress. These phenomena are tested through applying different thermal treatments. Raman spectrum of the mixture film deposited at 200 degrees C shows the lower polymerization efficiency in the C60-based matrix due to the more complete phase separation decreasing number of the retained Co atoms. Post-deposition annealing of the RT-deposited Co-C60 mixture film done at 300 degrees C for 1 hour induces the structural relaxation as conversion of fullerene into the regular carbon structure. According to the Raman analysis the regular carbon structure corresponds to the single-wall carbon nanotubes (SWNT) doped by cobalt. Similar analysis of the 200 degrees C-deposited mixture film treated by the following annealing reveals formation of SWNT only after much longer annealing. These experiments designate the Co diffusion as a main process driven by the carbon nanotube formation. The results demonstrate remarkable opportunity to control structure of the Co-C60 nano-composite using proper thermal treatments.