Structure of diamondlike carbon films deposited by femtosecond and nanosecond pulsed laser ablation

摘要

The characterization of diamondlike carbon (DLC) films is a challenging subject, considering the diversity of carbon-based nanostructures depending on the deposition process. We propose to combine multiwavelength (MW) Raman spectroscopy and electron energy-loss spectroscopy (EELS) to probe the structural disorder and the carbon hybridizations of DLC films deposited by pulsed laser ablation performed either with a nanosecond laser (film labeled ns-DLC), either with a femtosecond laser (film labeled fs-DLC). Such deposition methods allow to reach a rather high carbon sp~3 hybridization but with some significant differences in terms of structural disorder and carbonaceous chain configurations. MW Raman investigations, both in the UV and visible range, is a popular and nondestructive way to probe the structural disorder and the carbon hybridizations. EELS allows the determination of the carbon plasmon energy in the low-loss energy region of the spectra, as well as the fine structure of the ionization threshold in the high-loss energy region. The paper shows that the combination of MW Raman and EELS is a powerful way to elucidate the nanostructure of DLC films. Complementary nanoindentation investigations allow to correlate the analytical results with the mechanical properties of the films. The ns-DLC film presents a stronger sp~3-bonded C character (74%-85%) with a significant content of sp~2 chains, whereas the fs-DLC contains less sp~3 bonds (35%-50%) with a significant content of sp~2-bonded C rings. The ns-DLC film exhibits a higher proportion of disordered sp~2 C mainly in the form of chains. Comparatively, the fs-DLC exhibits a predominance of more ordered sp~2 C structures in the form of graphitic aggregates whose size has been estimated near three aromatic rings. The film characteristics are in agreement with their mechanical properties. We also propose a correlation between the nanostructure and composition of the films with the deposition mechanisms. The difference in kinetic energy distribution in the plasma plume, together with an absence of interaction between the plasma plume and the femtosecond laser, are responsible for the observed differences in sp~3 C content and sp~2 C configuration ranging between a predominance of more ordered sp~2 rings in the fs-DLC film and a predominance of sp~2 chains in the ns-DLC film. These results are consistent with the mechanisms of subplantation occurring during DLC deposition.