Carbon Atom, Dimer and Trimer Chemistry on Diamond Surfaces from Molecular Dynamics Simulations
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53 Mat. Res. Soc. Symp. Proc. Vol. 389 0 1995 Materials Research Society
potential is relatively unique in its ability to recognize different hybridization environments which is essential for these carbonaceous of systems. Next a system of 160 carbon atoms in a diamond lattice is terminated with 16 hydrogen atoms. The (111) surface is exposed to the various gas species after several annealing steps. To start a trajectory I use the following sequence of steps: 1. Anneal the pristine (111) surface 2. Choose a random position for the center of mass of the incoming gas atom or molecule 3. Choose a random set of velocities for the incoming species. 4. If the impinging species is a molecule, run 10 annealing steps to randomize the rotational and vibrational degrees of freedom 5. Run the full system trajectory
Typically for a system of 4x4 lattice constants the duration of the trajectory is far shorter than the interval between collisions. It is neither computationally feasible nor physically insightful to follow this intervening time. Step 1 has a second purpose, that being to substitute for the time interval between collisions. During that step, the system becomes randomized into a statistically independent state. This is akin to a numerical form of annealing. RESULTS We consider the results of each of the three species of interest, carbon monomers, dimers and trimers. They are only considered separately, never in combination, although they often appear that way in many practical systems. Our purpose here is to look at the reactivities of these three species exclusive of other complications. C1 As reported previously, 7 atomic carbon will not add to a pristine diamond (111) surface (based on 20 trajectories). See Figure 1.
----.. ..,,' adds nonreactive
Sradical
site
FIGURE 1. Schematic of a surface on which atomic carbon is impinging. The arrows are meant to represent trajectories of carbon atoms. Trajectories striking outside of the radical site (inner box) are always nonreactive. 85 % of the trajectories striking the radical site are nonreactive, but the rest add to the surface.
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This seems likely because of the large difference in bond energy between a CH molecule 8 and a C-H bond in a diamond (111) surface. The later is around a volt more stable. However if a radical site is somehow formed by another species, MD simulations with the Brenner potential estimate (based on 30 trajectories) that the carbon atom will add with probability 0.15. Ifthe radical sites are produced by atomic hydrogen, the probability for a reactive site being available is approximately 0.08. Taking the product of these two probabilities gives the overall reaction probability of approximately 0.012. Once a carbon atom has added, presumably it quickly becomes saturated with hydrogen from incoming atomic and/or molecular hydrogen. How additional atomic carbon reacts with such surface species is unknown at this time. We restrict our attention here to elementary events. As a sanity check, we estimate a growth rate for an atmospheric pres
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