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

Abstract

In recent high resolution transmission electron microscopic studies we have found that high temperature vacuum annealing (1200–1800 K) of ultradispersed (2–5 nm) and micron size diamond produces fullerene-like graphitic species, namely, onion-like carbon and closed curved graphite structures (multilayer nanotubes and nanofolds), respectively. Here we undertake theoretical studies to help in the understanding of the experimental data for these systems. (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 significant thermal displacement of a single carbon atom at temperatures close to the Debye temperature. (3) At the diamond–graphite interface we have observed the formation of two curved graphitic sheets from three diamond {111} planes. 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) The observed mosaic packaging of closed curved graphite structures during the diamond surface graphitization is suggested to be a self-assembling process. This process is explained in terms of the “stretching” of a bowed graphite 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 surface. The initial phase of the process is studied quantitatively using a molecular mechanics simulation.

Photoemission study of onionlike carbons produced by annealing nanodiamonds

“…3 and 4; Tables II and III͒. To explain this shift, we note that previous HRTEM studies 25,26,43 showed that at lower temperatures of graphitization the first 1-3 graphite layers exfoliating from diamond surfaces merge with these surfaces, possess significant curvature, and contain defects. Graphitization of the smallest ND particles ͑2-3 nm in diameter͒ at lower temperatures results in the formation of OLC particles with 2-3 fullerenelike shells with the size of inner shells corresponding to the size of the C 60 fullerene molecule.…”

“…1͒. 32,43 The defects within the curved layers are five or seven atom rings, 3 "Y" junctions of two basal graphite planes 7,25 or an interstitial plane between two basal planes, 7 and vacancies. 32 The x-ray emission spectroscopy study performed in Ref.…”

“…According to HRTEM data and measurements of the sample densities, the graphitization of ND starts at temperatures above 1400 K. 26 The graphitization under annealing begins from the surface and proceeds towards crystal bulk. 25,43,44 Figure 1 presents HTRTEM images and schematic diagrams of the products of graphitization of ND. The schematics summarize the results of our previous HRTEM investigations of OLC and intermediates of ND to OLC transformation.…”

“…The schematics summarize the results of our previous HRTEM investigations of OLC and intermediates of ND to OLC transformation. 25,26,32,43 Due to the strong aggregation of the initial particles of ND the resulting OLC ͑shown in the schematic for annealing to 1800-1900 K͒ mostly consist of aggregates of carbon onions. OLC particles are connected together by joint curved graphite layers.…”

Structural Stability of Clean, Passivated, and Partially Dehydrogenated Cuboid and Octahedral Nanodiamonds Up to 2 Nanometers in Size

“…The intersection of the bucky-diamond and OLC stability was found to be very close to the intersection for nanodiamonds and fullerenes at ∼1100 atoms (∼1.9 nm), suggesting that above this size an sp 3 -bonded core becomes more favorable than a sp 2 -bonded core, irrespective of surface structure. 35 It was also determined using semiempirical 36 and first principles methods 37 that the nanodiamond-toonion phase transition is initiated by the presence of the diamond C(111) surface, and does not occur on the C(110) and C(100) surfaces. 38 A related study by Barnard and Snook 39 found that single-walled carbon nanotubes are the most energetically preferred form for 1-D pure nanocarbon structures, but that there may exist a narrow window of stability for nonhydrogenated diamond nanowires between about 450 atoms (2.7 nm diameter) and 870-930 atoms (3.7-3.9 nm) per unit length.…”