Microstructural Studies of Pulsed-Laser Irradiated Graphite Surfaces

The surface structures of laser-irradiated samples of highly oriented pyrolytic graphite (HOPG) have been investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). The samples were irradiated with a 30 ns ruby laser (λ = 6943 Å) pulse with energy fluences ranging from 0.1 to 3.0 J cm−2. Optically, the specimens show a damage region approximately 5 mm in diameter. The surface structure displays three characteristic regions: an outer boundary characterized by submicron carbon spheroids resting on the surface; an inner boundary characterized by both submicron spheroids and 1 to 5 µm ‘torn’ carbon layers which appear to have broken away from the graphite surface after irradiation; a central region characterized by a uniform density of spheroids and a pattern of surface upheavals which trace out a grain pattern similar to that of the pristine substrate. Electron diffraction patterns taken on the irradiated region indicate an ultra-fine grain 2-dimensionally ordered carbon. Qualitative trends in the areal density of different microstructural features are presented. In addition, a simple model explaining the observed features is given. All observations are consistent with the rapid solidification of liquid carbon.