Laser-irradiation-induced structural changes on graphite

Pyrolytic graphite has been irradiated with high-energy density laser pulses (248 nm wavelength, 20 ns duration, and up to 250 J/cm(2) energy density). Craters with depth up to 5 mu m have been obtained by irradiating with a single pulse and the bottom of the craters has been analyzed with many techniques to investigate the possible morphological and structural modifications induced in the irradiated material where temperature and pressure fields, appropriate to the graphite-diamond phase transition, may be achieved. Melting of the surface layers of the irradiated graphite was realized, as established with scanning electron microscopy, while the estimated and measured pressure attained a maximum value on the order of 2.5 GPa. The temperature profile depends on the depth below the irradiated surface and the observed structural modifications are associated with different depths. Just below an ordered sp(2) hybridized nanocrystalline graphite surface layer, a disordered graphitic layer was formed, within which diamond particles with spheroidal geometry are embedded.