Growth and structure of fullerene-like CN_(x) thin films produced by pulsed laser ablation of graphite in nitrogen

摘要

The growth and structure of fullerene-like CN_(x) films produced by laser ablation of graphite in low pressure nitrogen were investigated. Deposition conditions were selected based on investigations of CN and C_(2) concentration at the condensation surface, vibrational temperature of CN radicals, and kinetic energies of atomic and molecular species. Films were characterized with x-ray photoelectron spectroscopy, Raman spectroscopy, high-resolution transmission electron microscopy, nanoindentation, and stress analyses. The nitrogen content in CN_(x) films directly depended on the concentration of CN radicals at the condensation surface. Formation of fullerene-like structures required a high vibrational temperature of these radicals, which was maximized at about 4 eV for depositions at 10 mTorr N_(2) and laser fluences of ～7 J/cm~(2). The presence of C_(2) had only a minor effect on film composition and structure. Optimization of plasma characteristics and a substrate temperature of 300℃ helped to produce about 1-μm-thick solid films of CN_(x) (N/C ratio≈0.2-0.3) and pure carbon consisting of fullerene-like fragments and packages. In contrast to carbon films, fullerene-like CN_(x) films exhibited a high elastic recovery of about 80 in using a Berkovich tip at 5 mN load and indentation depths up to 150 nm. Their elastic modulus was about 160 GPa measured from the unloading portion of an indentation curve, and about 250 GPa measured with a 40 Hz tip oscillation during nanoindentation tests. The difference was related to time dependent processes of shape restoration of fullerene-like fragments, and an analogy was made to the behavior of elastomer polymers. However, unlike elastomers, CN_(x) film hardness was as high as 30 GPa, which was twice that of fullerene-like carbon films. The unusual combination of high elasticity and hardness of CN_(x) films was explained by crosslinking of fullerene fragments induced by the incorporated nitrogen and stored compressive stress. The study demonstrated laser ablation as a viable technique for the growth of fullerene-like CN_(x) films, which may be used as hard protective coatings resisting brittle fracture at high loads and extensive substrate deformations.