Morphology and structure of C:Co, C:V and C:Cu nanocomposite films

The influence of transition metals (TM) type on the encapsulating carbon (C) medium during the growth of C:TM composite films is investigated. The C:V(~30 at.%), C:Co(~30 at.%) and C:Cu (~30 at. %) thin films have been grown by ion beam co-sputtering in the temperature range of RT-500°C. The films have been investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy at two excitation wavelengths (532 nm and 632 nm). For comparison reasons, pure carbon films have been grown in the same temperature range.
XRD reveals that the nanoparticles in C:V and C:Cu films are in carbidic and metallic state over the whole temperature range of this study, respectively. In contrast, in C:Co films a phase transition from carbide phase, present for growth temperatures of RT-300°C, to the metallic phase above 300°C occurs. According to the TEM observations, cobalt or cobalt carbide nanoparticles embedded in a carbon matrix have an elongated shape in the direction of the thin film growth with both length and diameter increasing with growth temperature (the latter from 2 up to 5 nm). However, in C:V films the nanoparticles exhibit globular shapes with diameters of ~ 2 nm at RT, which only slightly increases with temperature. Raman spectroscopy shows that all three metals enhance 6-fold ring clustering of the carbon phase independent of the nanoparticle type, size, shape and phase.