Crystallinity Of Carbon Nanoparticles Generated By Laser Ablation In Supercritical Carbon Dioxide

We fabricated carbon nanoparticles in a series of pulsed laser ablation (PLA) experiments with a graphite target using an Nd:YAG laser. The carbon nanoparticles were synthesized in a supercritical carbon dioxide (CO2) medium at various temperatures and various pressures. We also investigated the effect of CO2 pressure (density) on the morphology of the fabricated nanocarbon particles under isothermal conditions at 308 K or maintaining a constant medium density. The carbon particles generated under the supercritical condition in the vicinity of 8 MPa exhibited a spherical shape with a peak diameter range of 20-30 nm. Some of the generated particles had a nano-crystalline, regular structure. The lattice spacings of these single crystals were 0.20 nm and 0.37 nm, which corresponded to lattice spacings of diamond and graphite, respectively. The difiraction pattern of a selected area of the samples confirmed that the synthesized fine particles had a cubic- diamond crystal structure. The coarse particles, meanwhile, had a regular hexagonal-graphite structure. Raman spectroscopy of the synthetized nanoparticle samples revealed a phase transition from an amorphous structure to a mainly graphite structure in association with changes of the isobaric specific heat capacity of the supercritical CO2. The temperature distribution profile in the plasma plume was presumed to have affected the crystalline fraction of the generated carbon nanoparticles.