LARGE-SCALE PERIODIC FEATURES ASSOCIATED WITH SURFACE BOUNDARIES IN SCANNING TUNNELING MICROSCOPE IMAGES OF GRAPHITE

Large-scale periodic features (6-12 nm), which appear as two-dimensional waves, have been observed in four separate examples with scanning tunneling microscope imaging of pyrolytic graphite. The features have approximate hexagonal arrangement and cover regions of up to mu-m dimensions. At least one edge boundary was observed in all regions showing the large features. Atomic-scale imaging on the waves gave the usual atomic resolution image of graphite on some parts of the waves (near maxima), but substantially different images on other parts. The orientation of the large lattice with respect to the atomic lattice was found to be 29 +/- 1.5-degrees and corresponds closely to the covalent bond directions. Damaging a 20 x 60 nm area by increasing the bias to 3 V removed the waves from that area, but not the surroundings. The nature of these periodic features is not yet established. IBM researchers have proposed that such large-scale waves are Moire patterns due to rotational misorientation of the top two graphite atomic layers [APS Bulletin 35, 485 (1990)]. However, some features of the wave images presented here are not readily accounted for by rotational Moire effects and thus indicate that additional phenomena may be present. Another possibility is a constrictional Moire pattern caused by increased double bond character in the top layer, which has additional valence electrons from the broken bonds at the edges.