Surface site specificity on the basal plane of graphite: 1.06 μm laser damage threshold and reactivity with oxygen between 350 and 2300 K

Abstract

Scanning tunneling microscopy (STM) has been used to document changes in the nanometer-scale morphology of the basal plane of highly oriented pyrolytic graphite after in situ exposure to 7 ns, 1064 nm Nd:YAG, and to ms Nd:Glass laser pulses producing surface temperatures up to 2300 K, and after ex situ exposure to temperatures as low as 350 K in air. Laser damage produced by the ns pulses was visible by STM at fluences far below those that produce melting and effects visible by other imaging techniques. Damage appears first on step edges and consists of exfoliation of graphite layers and recession of steps through removal of mono- or multilayer patches. ms and long-term heating leads to reaction with oxygen, which proceeds by etching of both crystalline boundaries and step edges. In addition a high concentration of flat-bottom shallow pits (two monolayers deep) and a smaller number of conical pits (greater than 30 Å deep) can clearly be identified. The results give for the reaction at the grain boundaries an activation energy of 36 kcal/mole while for steps and shallow pits it is 53 kcal/mole, within the energy range found in global, not site-specific reaction studies. Site-specific reaction probabilities as low as 10−11 are measured at 350 K.