Theoretical study of hydrogen-covered diamond (100) surfaces: A chemical-potential analysis

Abstract

The bare and hydrogen-covered diamond (100) surfaces were investigated through pseudopotential density-functional calculations within the local-density approximation. Di erent coverages, ranging from one to two, were considered. These corresponded to di erent structures including 1 1, 2 1, and 3 1, and di erent hydrogen-carbon arrangements including monohydride, dihydride, and con gurations in between. Assuming the system was in equilibrium with a hydrogen reservoir, the formation energy of each phase was expressed as a function of hydrogen chemical potential. As the chemical potential increased, the stable phase successively changed from bare 2 1 to (2 1):H, to (3 1):1.33H, and nally to the canted (1 1):2H. Setting the chemical potential at the energy per hydrogen in H2 and in a free atom gave the (3 1):1.33H and the canted (1 1):2H phase as the most stable one, respectively. However, after comparing with the formation energy of CH4, only the (2 1):H and (3 1):1.33H phases were stable against spontaneous formation of CH4. The former existed over a chemical potential range ten times larger than the latter, which may explain why the latter, despite of having a low energy, has not been observed so far. Finally, the vibrational energies of the C H stretch mode were calculated for the (2 1):H phase.

Energetics of the dihydride phases on the diamond (100) surface

“…Previously, we investigated this canted model for both C and Si systems. 2,10 From the present work, we find that for the diamond case, a CH 2 canting by 10°increases the distance between H atoms in the neighboring dihydrides from 1.09 to 1.37 Å, thereby being 0.45 eV/(1ϫ1) lower in energy than the symmetric phase ͑see Table I͒. This energy difference is very close to the previous results.…”

“…This energy difference is very close to the previous results. 2 Similarly, for the silicon case, a SiH 2 canting by about 17°increases the distance between the H atoms in the neighboring dihydrides from 1.50 to 2.18 Å, thereby lowering the energy by 0.16 eV/(1ϫ1), which is also very close to the previous results. 10 WRH argued that the canted Si͑100͒ structure is sufficient to stabilize the dihydride units due to a larger distance between H atoms in the neighboring dihydrides, while the canted C͑100͒ is not sufficient due to the shorter lattice constant of diamond.…”

“…The former is expected to keep the carbon dimer reconstruction (2ϫ1), while the latter saturates both the dangling bonds of a surface carbon atom, yielding a structure likely to be (1 ϫ1) and high in energy due to the steric repulsion. Previously, 2 we studied several dihydride phases on the C͑100͒ surface including the symmetric and canted (1ϫ1) phases, and then found that the canted (1ϫ1):2H structure is lowest in energy among the dihydride phases. The top views of the symmetric and canted structures are shown in Fig.…”

“…Especially, the hydrogen morphology on the C͑100͒ surface has been extensively studied: see Ref. 2 and references therein. Upon adsorption of hydrogen atoms, possible configurations of the hydrogencovered C͑100͒ surface include the monohydride ͑one hydrogen attached to a carbon atom͒ and dihydride ͑two hydrogen atoms attached to the same carbon atom͒ arrangements.…”

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

“…1(D)). 58 (In theory a dihydride (2.0 ML) could de-reconstruct the C(100) surface but its existence is controversial, and the monohydride may be energetically preferred and more stable against spontaneous CH 4 formation; 61 1.25 ML, 62 1.33 ML, 63 1.5 ML, 58 1.75 ML, 62 and other polyhydrides 62 64 have also been proposed.) Partial dehydrogenation of monohydride-passivated C(100)-H(2 × 1) surface should not alter the existing lattice reconstruction geometry.…”