Using an individual molecule to reproduce a mechanical function becomes now possible, essentially due to the progress in atomic-scale manipulations. Recently, L. Soukiassian et al synthesized a linear molecule, the polyaromatic 1,4''-paratriphenyldimethylacetone, nicknamed Trima, commensurate to the Si(001)2x1 surface [1]. This molecule involves three benzene rings and two COCH3 groups at the ends to bind the molecule to the Si(001) surface. The length of the molecule corresponds exactly to the space between five dimers in a row on this surface. The Trima molecule chemisorbs on the surface by its two ends, and the idea was that the central aromatic part of the molecule could be rotated in a controlled manner by injecting electrons with the tip of a scanning tunnelling microscope (STM).
In the framework of Density Functional Theory calculations, we investigate the Trima chemisorption on the Si(001) surface and particularly focus on the possibility of a central aromatic ring rotation.
Our results emphasize a preferential adsorption via the oxygen atom, involving a reaction between the ketone group (CH3-C=O) and the µ orbital of a silicon dimer. This leads to an enol (CH2-C-O-Si) located on one silicon of the dimer and a dissociated hydrogen atom on the other silicon atom of the same dimer. We thus confirm the results of the NEXAFS study reported in Ref 1. W also show that the central ring is able to rotate around the Trima molecule axis and evidence the interaction between the aromatic rings and the silicon dimers located just below.
[1] L. Soukiassian et al., Journal of Chemical Phyics 122 (2005) 134 704
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