Ab initiostudy of biphenyl chemisorption on Si(001): Configurational stability

We present an ab initio energetical and structural study of the configurational stability of the biphenyl molecule adsorbed on the Si(001) surface. A number of models in biphenyl tight-bridge, butterfly, twisted, and tilted configurations are considered. For an undissociated biphenyl adsorption, the tight-bridge configuration is found to be the most stable one, slightly favored over the butterfly configuration. The effect on the stability of various parameters is investigated. The position with respect to the surface of the first phenyl ring atom (C1) on which the second ring is bound plays a determinant role. The tilted dimer under the second ring mainly acts in the biphenyl butterfly and tilted configurations, and a second ring location above a silicon down-atom favors stability. The effect of the second ring height above the surface is also discussed. Our results allow us to classify these different contributions by decreasing importance. By the hypothesis of a dissociative biphenyl adsorption, the calculated adsorption energies are clearly lower than in the corresponding undissociated model and the dissociated butterfly configuration is largely favored. Comparing our results to the experimental data, we propose some interpretations relative to the weakly and strongly chemisorbed biphenyl molecule observed in the scanning tunneling microscopy experiments.