Rotation possibility of a molecule adsorbed on the Si(001) surface: Ab initio study

We present an ab initio study of the adsorption of the 1,4^″ -paratriphenyldimethylacetone molecule (trima) involving three phenyl rings and two COCH₃ groups at the ends to anchor the molecule to the Si(001) surface. A preferential adsorption via the oxygen atom is found, confirming earlier near-edge x-ray fine-structure studies. Considering the trima molecule adsorbed on the surface, we propose a classification of the investigated configurations in two families which present similar characteristics in the external ring displacements and very different behavior concerning the central ring rotation. This central ring rotation is particularly investigated. Our systematic study allows further understanding of the experimental data: the phenyl-ring-underlying dimer tilt interaction is expected to play a major role in the configurational modifications observed in scanning tunneling microscopy experiments. The asymmetry of the adsorbed trima molecule with respect to the dimer rows largely stabilizes the type-I configurations in which the three phenyl rings present the same orientation, the tilt of the dimer beneath favoring a larger amplitude of the central ring rotation.