Nanoscopic observations of stress-induced formation of graphitic nanocrystallites at amorphous carbon surfaces

Abstract

The formation of graphitic nanocrystallites at the surface of amorphous carbon under large mechanical stresses was examined by using micro-Raman spectrometry, transmission electron microscopy and in-situ compressions. In the Raman analyses of severely deformed (above a strain energy density criterion of 5.9J/m2) surface regions of nanoscratched and nanoindented amorphous carbon films, two additional sharp and narrow peaks, DGr and GGr at 1330 and 1580cm−1, appeared from the main unchanged broad spectra, revealing the transformation of some small-range amorphous carbon to nanocrystalline graphite. Transmission electron microscopic images presented the formation of surface shear layer within which dispersed graphitic nanocrystallites (a size of about 3nm) were formed in the remaining amorphous matrix. The in-situ nanoscopic observation of amorphous carbon nanopillars under compressions confirmed the formation of graphitic nanocrystallites at pillar edge surfaces. The formed graphite (001) and (100) lattices were well oriented along maximum resolved shear stresses, being an evidence of lattice reconstruction and suggesting a possibility of stress-induced graphitization of amorphous carbon in the absence of heat.