Crystallinity of Carbon Nanoparticles Generated by Laser Ablation in Supercritical Carbon Dioxide

Abstract

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 diffraction 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.