ROLE OF SP(3) DEFECT STRUCTURES IN GRAPHITE AND CARBON NANOTUBES

THE discovery of fullerenes1,2 and carbon nanotubes3,4 has led to the realization that it should be possible to tailor the properties of gra phitic sheets if their geometry can be controlled5-7. In exploring the se possibilities, we have found that the folding and tearing of graphi tic sheets follow well defined patterns which seem to be governed by t he formation of sp3-like line defects in the sp2 graphitic network. Ou r studies with the atomic force microscope and scanning tunnelling mic roscope reveal that these folds and tears occur preferentially along t he symmetry axes of graphite, and that 'ripples' are observed in the c urved portions of the folds. We also see ripples in deformed carbon na notubes. They lie along the directions for which sp3-like line defects can form most easily to relieve strain. Our ab initio molecular orbit al calculations indicate that the ripples stabilize the pi-electronic energy in the bent structures with the total energy balance being dete rmined by the amount of nuclear repulsion. These results provide insig ht into the geometries that graphitic structures will preferentially a ccommodate, and the properties that might ensue.