Femtosecond Laser Induced sp3 Bonds and Nanodiamonds Formation in Carbon Materials

A polycrystalline graphite target was irradiated using a pulsed femtosecond laser with 120 fs pulse duration and 800 nm wavelength and different energies per pulse. Were performed ‘in-depth’ and surface investigations and... [ view full abstract ]

A polycrystalline graphite target was irradiated using a pulsed femtosecond laser with 120 fs pulse duration and 800 nm wavelength and different energies per pulse. Were performed ‘in-depth’ and surface investigations and on the sp3 bonds percentage increase and possible diamond crystal formation. X-ray photoelectron spectroscopy investigations have shown an increasing tendency of the sp3 percent in the low power irradiated areas and similarly ‘in the depth’ of the higher power irradiated zones. Multiple wavelength Micro-Raman investigations have confirmed this trend for high laser power densities (tens of J/cm2 ) and accomplish by the crystallite size ‘in-depth’ grow. Synchrotron X-ray diffraction pattern have shown the presence of a diamond peak in one of the irradiated zones.
Based on the wavelength dependent photon absorption into graphite, our investigations have shown that sp2-sp3 transitions is depending on the laser fluencies but not on photon adsorption rate per atom. Thus the observed effects are correlated with high density photon per atom and are attributed to the melting and recrystallization processes taking place tens of nanometers below the target surface. The presence of a diamond related peak observed in just one central zone of a multiple pulse irradiated spot, also suggests that diamond bonds formation is rather an accidental process based on the non uniform laser adsorption and respectively on the ablation related processes taking place within the polycrystalline graphite.