STRUCTURAL PHASE OF FEMTOSECOND-LASER-MELTED GRAPHITE

We have used Raman scattering to investigate the effects of intense laser purses on the structure of resolidified graphite. Graphite was irradiated with 0.325-3.25-J/cm(2), 620-nm, 90-fs single-laser pulses causing it to melt and rapidly resolidify. Raman studies of the resolidified carbon in the crater show that the rapid annealing process (by pulses with energy fluences greater than or equal to 0.82 J/cm(2)) causes a breakdown in the ordered layers of hexagonal carbon rings and disorder in the intraplanar spacing upon resolidification into a nanocrystalline material. The thickness of the nanocrystalline-graphite near-surface layer increases with increasing fluence. Residual planar structure of the resulting material is observed for the various pulse-energy values by comparing the narrow graphitic 1581-cm(-1) and the broad 1360-cm(-1) and 1600-cm(-1) vibrational bands. The interplanar structure of our nanocrystalline graphite is also studied quantitatively via the low-frequency shear mode at 42 cm(-1). The Raman spectrum of our glassy carbon is found to be well described by planar ordering approximately 2 to 3 layers in extent using a simple correlation function approach. Our results indicate a layered morphology is present in our nanocrystalline graphite, confirming a strong sp(2) bonding character.