Carbide Formation and Optical Properties in Carbon: Transition Metal Nanocomposites Thin Films

Transition metals with carbon deposited by physical vapor deposition techniques, lead the formation of metal nano-clusters or nanocrystalline metallic carbides embedded in a carbon matrix. Interstitial carbides are stable at high temperature, have high melting points and possess a high reflectivity. In contrast, the resulting carbon: transition metal nanocomposites show optical selective properties such as good absorptance in the visible with high reflectance in the infrared. These properties make them very attractive for applications where high temperature resistant materials with selective optical properties are required.
In this study, carbon: transition metal nanocomposites were grown using a physical vapor deposition system incorporating two pulsed filtered cathodic arc sources, one provided with a graphite cathode and the other with a metallic cathode (Zr, V or Mo). The metal content in the composite was controlled by adjusting the pulse ratio between the two sources, and determined by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). Comprehensive structure characterization was carried out using a combination of X-ray diffraction (XRD), Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Optical characterization has been done using both ellipsometry and spectrophotometer measurements in order to obtain the optical constants and the reflectance spectra of the samples.
Together with experimental characterization, a computer program is used to simulate the reflectance spectra of different carbon: transition metal films. Bruggeman effective medium theory was used to average the dielectric functions of the two components which compose the film. According to our simulations, the resulting reflectance of the nanocomposite films is strongly affected by the metal content, independently if it results in metallic nano-clusters or nanocrystalline metallic carbides. Simulated spectra were compared with the measured reflectance of the deposited films obtaining good agreement between simulations and experimental results.