Adsorption and bonding of C1Hx and C2Hy on unreconstructed diamond(111). Dependence on coverage and coadsorbed hydrogen

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Abstract
The adsorption and bonding of CH3, CH2, CH, C2H, and C2H2 fragments to clean and hydrogenated diamond(111) surfaces are investigated in the framework of the atom superposition and electron delocalization molecular orbital method. Low coverage calculations are performed using large cluster models for the surfaces, and high coverages are examined with band calculations on thick two-dimensional slabs with every surface carbon covered by a hydrocarbon fragment (i.e., 1:1 surface coverage). For low coverage adsorption on clean and H-covered surfaces the adsorption energies are in the order C2H>CH ≃ CH2>CH3. In each case, the predominant component of bonding is covalent in character and is a result of overlaps between the sp-hybridized singly occupied dangling surface state orbital on the surface carbon and the sp-hybridized orbital on the fragment carbon atom. While the charge transfer contribution to bonding is nearly the same for CH3, CH2, and CH fragments, it is significantly larger for C2H due to a comparatively stable radical orbital on C2H. C2H2 binds to the surface on the di-σ site where both its ends form bonds to the surface atoms. Onefold adsorption to a H-covered surface is predicted to be unstable. The 1:1 CH3 coverage on diamond(111) is highly unstable because of steric repulsions between adsorbate fragments due to their spacial proximity. This finding is supported by a calculation of the cis-trans isomerization energy of di-t-butyl ethylene, including full structure relaxations. At low coverage CH3 can bind on adjacent surface sites by tilting away from one another. The 1:1 coverage for CH2, CH, and C2H fragments is predicted to be stable on this surface.