Theoretical study of the formation of closed curved graphite-like structures during annealing of diamond surface

In recent high resolution transmission electron microscopic studies we have found that high temperature vacuum annealing (1200-1800 K) of ultradispers ed (2-5 nm) and micron size diamond produces fullerene-like graphitic speci es, namely, onion-like carbon and closed curved graphite structures (multil ayer nanotubes and nanofolds), respectively. Here we undertake theoretical studies to help in the understanding of the experimental data for these sys tems. (1) Calculations of cluster models by a standard semiempirical method (MNDO a software package) are used to explain the preferential exfoliation of {111} planes over other low index diamond planes. (2) The same approach suggests the likelihood that the graphitization is initiated by a signific ant thermal displacement of a single carbon atom at temperatures close to t he Debye temperature. (3) At the diamond-graphite interface we have observe d the formation of two curved graphitic sheets from three diamond {111} pla nes. We suggest that the evolution of this interface proceeds by a "zipper" -like migration mechanism with the carbon atoms of the middle diamond layer being distributed equally between the two growing graphitic sheets. (4) Th e observed mosaic packaging of closed curved graphite structures during the diamond surface graphitization is suggested to be a self-assembling proces s. This process is explained in terms of the "stretching" of a bowed graphi te hexagonal network. The stretch is due to the fact that, if relaxed, the network would be smaller than the initially transformed hexagonal diamond ( 111), and to the increased separation between the separated sheet and the s urface. The initial phase of the process is studied quantitatively using a molecular mechanics simulation. (C) 1999 American Institute of Physics. [S0 021-8979(99)02514-1].