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COMPARISON BETWEEN TERSOFF POTENTIAL

AND AB INITIO RESULTS

FOR SURFACE GRAPHITIZATION OF DIAMOND

As part of an ongoing project to simulate radiation damage in carbon, David Saada, Joan Adler and Rafi Kalish have used classical molecular dynamics with Tersoff's carbon potentials to repeat the first principle study of DeVita, Galli, Canning and Car (Nature, 379, 523 (1996); Applied Surface Science 104 , 298 (1996)) of the surface graphitization of diamond.

We are very grateful to Dr Devita for forwarding us the exact coordinates of his samples so that we could use our own graphics to draw all the pictures with the same system. We use yellow to indicate three-fold coordinated carbon atoms and blue to indicate four-fold coordinated carbon atoms. As well as appearing in yellow-blue on this site and on a printer, these colors reproduce quite reasonably in white and grey respectively on a black and white printer. (In fact the colors were selected for this reason.)

The first figure below illustrates the initial state of DeVita et al (note the surface reconstruction) and therefore also of our study. In both cases boundary conditions are free at top and bottom and periodic on other sides, and Nose-Hoover thermostats were used to regulate the temperature at 2500 degrees.

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To the left below is the next stage of the calculation of DeVita et al and to the right below our results. Observe here and below that in both cases there is graphitization from the surface and also within the bulk.

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To the left below is the third stage of the calculation of DeVita et al and to the right below our third stage results.

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To the left below is the final stage of the calculation of DeVita et al and to the right below our final results.

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Please observe the excellent agreement between the calculations! This project is part of a detailed study, ``Computer Simulation of Damage in Diamond due to Ion-impact and its Annealing '', Phys. Rev. B 59 6650 (1999) available by ftp (SAK2)). Some other parts of this project and the visualizations are avaliable as a movie or mpeg files on our ftp server.

EARLIER RESULTS FROM RELATED PROJECTS

Our earlier results can be found in the paper ``Transformation of the Diamond (sp^3) to Graphite (sp^2) bonds by ion-impact'', (1998) International Journal of Modern Physics, C, 9, 61, which can be found on our ftp site ( ftp (SAK)), where we described our T=0 study of the formation of point defects in diamond induced by an energetic displacement of a carbon atom from its lattice site and the relaxation of the thereby disrupted crystal. The displacement energy for Frenkel pair creation was calculated to be 52 eV, in agreement with experiments. It was found that the <100> split-interstitial, with a bonding configuration which resembles graphite, was created by many different bombardments.

In the figure below left, a section of a 5000 atom carbon sample ordered in a diamond lattice is shown, before one atom (drawn in green) is kicked out of position. In the figure below right, the lattice is shown at a later stage when the atom has moved to the right and displaced the surrounding ones.

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In the figure below the lattice is shown after displacement of the atom in a different (off-axis) direction.

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The stability of this defect had been predicted from first principles, but the creation mechanism under bombardment was unknown. We ``discovered'' the split-interstitial in the final configurations by using color to highlight three-fold coordinated atoms and shortened bonds. It just jumped out at us once this color coding was introduced. In the uppermost figures no bond colorcoding was used, just a continuous variation of hues from blue (perfect diamond) to red (graphitic), with the displaced atom shown in green. In the figure shown immediately above green bonds indicate those of the split interstitial, and red those of graphitic length with threefold coordinated atoms in red, the displaced one in green and fourfold coordinated in blue. You may observe how the split interstitial is clearly highlighted. (We don't recommend that you try to print the red-blue colored graphics in black and white.)

DIFFUSION of HYDROGEN IN CARBON

As part of an ongoing project to simulate diamond carbon, David Saada, and Joan Adler and Rafi Kalish have studied the diffusion of Hydrogen in diamond. The hydrogen atom executes a random walk between some very special interstitial sites. Details are given in our new preprint, SAK3.

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