High resolution electron microscopy of graphite defect structures after keV hydrogen ion bombardment

Structure changes of graphite due to keV-energy hydrogen ion bombardment are studied by means of high resolution electron microscopy (HREM). Highly graphitized vapor grown graphite fibers of diameters around 700 nm are irradiated side on with 1 keV H+ ion beam to various fluences ranging from 1x1014 to 1x1018 H+/cm2. The (002) lattice fringe images from the fiber side edges give ''edge-on'' projections of several tens of nanometers deep layers of the H+ implanted graphite basal face, thus enabling direct observation of defect profiles in the surface layer. In a 1 keV H+ ion bombardment at ambient temperature, interlayer spacing, d002, within 12 nm from the graphite surface increases from the initial value of 0.336 nm to larger values ranging from 0.40 to 0.53 nm at an ion fluence of 1x1017 H+/cm2. The d002 value ranges from 0.45 to 0.58 nm at an ion fluence of 1x1018 H+/cm2. Mean crystallite size, La, on the other hand, decreases with fluence from the initial value above 100 nm to an equilibrium at 0.5 nm after a 1 keV H+ irradiation to a fluence of 1x1017 H+/cm2. The increase in the interlayer spacing is attributed at least to an increased van der Waals radius of the hexagonal carbon networks due to chemical bonding between lattice carbon atoms and hydrogen atoms at interlayer positions. (orig.)