STRUCTURE AND ENERGETICS OF CLEAN AND HYDROGENATED DIAMOND (100) SURFACES BY MOLECULAR MECHANICS

The molecular mechanics (MM3) method has been applied to the clean and hydrogenated surfaces of diamond (100). Periodic boundary conditions are incorporated into the computational algorithm, permitting calculations comparable in size to modest-sized clusters but without complications from edge effects. The atomic structure and energetics of the clean (100)-(2 X 1), monohydride (100)-(2 X 1):H, full dihydride (100)-(1 X 1):2H, and intermediate dihydride (100)-(3 X 1):1.33H surface shave been determined. Pairs of surface carbon atoms form symmetric dimers on the reconstructed diamond (100)-(2 X 1), (2 X 1):H, and (3 X 1):1.33H surfaces, with dimer bond lengths of 1.46, 1.63, and 1.59 angstrom, respectively, corresponding to strained double or single bonds. The full (1 X 1):2H dihydride, with two hydrogen atoms per surface carbon atom, is highly-strained due to H-H repulsions, causing a reduction of the H-C-H bond angle and twisting about the surface normal, and is predicted to be thermodynamically unstable with respect to dehydrogenation to the monohydride. Some important gas-surface reactions involving hydrogen and the diamond (100) surface are discussed in light of the derived energetics.