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,
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
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
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.
To the left below
is the next stage of the calculation of DeVita et al and to the right below
results. Observe here and below
that in both cases there is graphitization from the surface
and also within the bulk.
To the left below
is the third stage of the calculation of DeVita et al and to the right below
our third stage
To the left below
is the final stage of the calculation of DeVita et al and to the right below
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
Some other parts of this project and
are avaliable as a
files on our
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 (
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.
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
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.
In the figure below the lattice is shown after displacement of the atom in a different (off-axis) direction.
The stability of this defect
had been predicted from first principles, but the creation mechanism under
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
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.)
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