The adsorption, isomerization, and decomposition of HCN on Si(100)-2 × 1 surface have been investigated by means of a density functional theory calculation using a double-dimer cluster model. The results revealed that both HCN and its HNC isomer can be readily adsorbed on a Si−Si dimer either dissociatively or molecularly in an end-on and a side-on configuration. Side-on adsorption occurs by the cycloaddition of the C⋮N group on to the Si−Si dimer, whereas dissociative adsorption gives rise to H(a) and CN(a) adspecies initially via the end-on configuration on the same dimer or across the two dimers. Adsorbate−adsorbate interactions and reactions have also been studied with two HCN molecules. For the end-on adsorption, the first HCN(a) exerts a significant effect on the adsorption geometry of the second HCN. In particular, a synergetic effect has been observed for the parallel adsorption of two HCNs with their CN groups bridging across the two dimers. For the side-on adsorption, the adsorbate−adsorbate interaction is negligible with minor effects on the adsorption geometry. H-migration between the two neighboring, side-on HCN admolecules can occur readily, leading to the formation of HCNH(a) and NC(a) surface species. The calculated vibrational frequencies of the HCNH(a) and NC(a) adspecies are in good agreement with the experimental HREELS data of the HCN/Si(100) system.

Ab initio molecular orbital and hybrid density functional theory calculations have been performed to study the adsorption, isomerization, and decomposition of HCN on Si(lOO)-2xl using the Si,H 12 cluster model of the surface. The results of our calculations indicate that the HCN can adsorb molecularly without a barrier onto the surface with both end-on (LM I) and side-on (LM2) positions. LMI can isomerize to LM2 with a small barrier of 8 kcal/mol. The isomerization ofLM2 by H-migration from C to the N atom, requires 76 kcal/mol activation energy (c.I. 47.5 kcal/mol in the gas phase) because of surface stabilization. Both HCN(a) and HNC(a) end-on adsorbates were found to dissociate readily, as concluded in our earlier experiment, to produce Hand CN adspecies. The computed vibrational frequencies of HCN, CN, and also HCNH adspecies agree reasonably well with those observed experimentally. HCNH was found to be stable, with either the C or the N attaching to the surface.

The CO-CO adsorbate interaction on Si(100)-2×1 has been investigated with ab initio molecular orbital and hybrid density functional theory calculations using cluster models of the surface. Different adsorption combinations for one and two CO molecules on single-and double-dimer cluster models, Si 9 H 12 and Si 15 H 16 , respectively, are described. Our calculations indicate that the second CO molecule is physisorbed on the same surface Si dimer where the first CO molecule is chemisorbed. The chemisorption of the first CO molecule induces a change in the charge of the surface Si dimer atoms which inhibits further adsorbate-surface interaction. The dissociation energy of the physisorbed second CO molecule is less than 1 kcal/mol. Adsorption of the second CO molecule on the second Si dimer is energetically preferred over coadsorption of CO on the same Si dimer. The 2OC-normal.d14 structure is the most stable configuration, with the two CO molecules adsorbed diagonally across the two Si dimers. The dissociation energy of the chemisorbed second CO molecule in the 2OC-normal.d14 structure is 13.8 kcal/mol, about 4 kcal/mol less stable than the first adsorbed CO. Adsorption of two CO molecules in a bridge configuration indicates a weak surface-adsorbate and/or adsorbate-adsorbate interaction. The dissociation energy of the single chemisorbed CO in the bridged state is 5.0 kcal/mol while that of the second CO is 1.7 kcal/mol. A mixed configuration, i.e., OC-normal with OC-bridge, was found to be unstable based on the Si 15 H 16 surface model.

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