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Influence of the transition metal affinity on the encapsulating carbon medium during the growth of carbon:transition metal nanocomposite films
Berndt, M.; Krause, M.; Abrasonis, G.; Mücklich, A.; Munnik, F.; Kolitsch, A.; Möller, W.;
The understanding of the interactions of carbon (C) atoms with transition metals (TM) is of particular importance as these interactions are involved in many processes in nanoscience. However, the underlying fundamental mechanisms are still not well understood. In this work, the influence of the TM chemical affinity to C on the encapsulating carbon nanostructuring is studied during the growth of C:TM nanocomposite films. C:Co and C:V nanocomposites with metal content of ~ 15 and ~ 30 at.% have been grown by ion beam co-sputtering in the temperature range of RT-500°C. The so-grown films have been investigated by the means of elastic recoil detection analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy at two excitation wavelengths (532 nm and 785 nm). In order to highlight the influence of the transition metal on the encapsulating matrix, the results are compared with the pure carbon films deposited at the same temperatures.

Nanocomposite structure is observed for all the C:TM films which consists of metal (or metal carbide) nanograins embedded in carbon medium. The C:V films consist of spherical grains of the diameter of ~2 nm whose size or shape is independent of the growth temperature. Vanadium is in the carbidic state over the whole temperature range of this study. The Co nanoparticles in C:Co films grown at RT also exhibit a spherical shape with a diameter of ~ 2 nm. However, with increasing deposition temperature the Co nanoparticles become elongated the long axis coinciding with the direction of the film growth. A carbide phase is identified for growth temperatures of RT-300°C, while a metallic phase is formed above 300°C.

The embedding carbon phase resembles that of the amorphous carbon at lower growth temperatures, while at elevated growth temperatures (>= 300°C) curved graphenic sheets encapsulating the metal nanoparticles can be identified. Raman spectroscopy shows that both metals enhance 6-fold ring clustering of the carbon phase since the D peak intensity related to the 6-fold ring breathing vibrations is increased in comparison to the pure carbon films. This enhancement occurs independently on the nanoparticle type, size, shape and phase. Besides the D and G peak, a third peak at ~ 1100 cm-1 is identified, which shows a resonance enhancement for visible laser excitation. This peak is absent in pure carbon films. Its intensity in relation to D-G band decreases when the growth temperature increases, while it increases concomitantly with the metal content. Moreover, the position of this peak is independent on the metal type indicating that it is an inherent feature of carbon.
Keywords: composites, ion beam sputtering, metallic and carbon nanostructures
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Publ.-Id: 11867 - Permalink