Structural phase of femtosecond-laser-melted graphite

We have used Raman scattering to investigate the effects of intense laser pulses on the structure of resolidified graphite. Graphite was irradiated with 0.325–3.25-J/${\mathrm{cm}}^2$, 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 ≥0.82 J/${\mathrm{cm}}^2$) 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-${\mathrm{cm}}^{\mathrm{-}1}$ and the broad 1360-${\mathrm{cm}}^{\mathrm{-}1}$ and 1600-${\mathrm{cm}}^{\mathrm{-}1}$ vibrational bands. The interplanar structure of our nanocrystalline graphite is also studied quantitatively via the low-frequency shear mode at 42 ${\mathrm{cm}}^{\mathrm{-}1}$. 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 ${\mathit{sp}}^2$ bonding character.