Dissociative versus molecular adsorption of phenol onSi(100)2×1: A first-principles calculation

Abstract

We investigated the competitive adsorption of a bifunctional molecule, phenol, on Si͑100͒2 ϫ 1 by ab initio calculations. We performed geometry optimizations of phenol adsorbed either molecularly or dissociatively, on five possible sites ͑top, bridge, valley bridge, cave, and pedestal͒, in the low coverage regime. We found that the dissociative adsorption of phenol on top of a silicon dimer is the most favorable adsorption configuration. In the group of dissociative adsorption the phenol initially placed on the bridge or the valley-bridge sites ends up as a toplike local minima. The pedestal and cave sites remain as low-adsorption energy "open" sites. In the group of molecular adsorption, a higher adsorption energy is associated to the adsorption through an addition reaction and loss of the aromatic character ͑bridge, valley-bridge, and pedestal sites͒. Standard butterfly or diagonal butterfly are the corresponding optimized geometries. Retention of aromatic character and lower adsorption energy are associated to the adsorption on the top and cave sites. The ordering of adsorption sites according to the adsorption energy shows a mixture of the dissociative and the molecular sites. In the case of adsorption on the top site, the adsorption energies after a rotation of the phenoxy fragment along the bonding axis and hydrogen migration on the surface are very similar. The bend of the phenoxy fragment on the surface, instead, is not favored ͑the adsorption energy is 1.004 eV lower compared to the vertical position͒. Different electron density maps were calculated for different adsorption sites and modes. Finally, we investigated the possibility that molecularly adsorbed phenol behaves as a precursor for the dissociative one by nudged elastic band calculations. We found a barrier of the same order of magnitude of the thermodynamic energy at room temperature for the conversion of the valley-bridge molecular into the top dissociative site.

Coverage effects on phenol adsorption on Si(100)2×1 as: A first principle calculation

“…1c-n along with the reference single adsorbate models as obtained in Ref. [9] (Fig. 1a is the single molecule on-top adsorption, 1b the bridge molecular one).…”

“…the bridge and the on-top ones, respectively [9], were used as starting models for the multi-molecules adsorption. The geometrical outcome of the optimization is reported in Fig.…”

“…This triggered comparative studies of phenol also on Si(1 0 0)2 Â 1. In a previous theoretical investigation [9] we examined the possibility that phenol adsorbs either molecularly or dissociatively on Si(1 0 0)2 Â 1, we determined the favourite adsorption sites and whether a conversion between adsorption types is possible. We found that, at low coverages, the favourite molecular adsorption occurs in a CSi di-r bond in a bridge configuration.…”

Adsorption structures of phenol on theSi(001)−(2×1)surface calculated using density functional theory

“…This trend has been found previously for benzene 31 and for chloro-and dichlorobenzene. 32 For a 0.5 ML coverage, the energetically most favorable structure is F, which is by 0.8-0.9 eV lower in energy than the next lowest energy structure C. However, according to the revPBE calculations D is 0.15 eV more favorable than E, whereas according to PW91 and PBE calculations E is 0.22-0.23 eV more favorable than D. Carbone et al 5 used firstprinciples calculations with the BLYP functional 14,15 to study structures A and D on various adsorption sites and found the adsorption energies of 0.55 eV and 2.56 eV, respectively. This is in good agreement with our B3LYP results and the small differences are presumably due to the differences between BLYP and B3LYP functionals and the different coverages of 1 6 ML and 0.5 ML used in their and the present calculations, respectively.…”

“…The theoretical study by Carbone et al 5 focused on possible adsorption sites for the non-dissociated butterfly structure ͑structure A in Fig. 1͒ and the above-mentioned dissociative structure.…”