Chemisorption of pentacene on Si(111)-7 x 7 studied via scanning tunneling microscopy and density functional theory

The chemisorption of pentacene on Si(111)-7 x 7 surface was studied using scanning tunneling microscopy and density functional theory calculations. On the basis of the STM results, three binding configurations, each involving four C-Si covalent bonds, can be identified from the orientations of pentacene with respect to the underlying substrate atoms. One of the configurations involves the unique engagement between three adatoms and one rest atom on the surface with four of the C atoms on pentacene such that the double bonds conjugation on pentacene is preserved despite of a loss in the aromaticity. Pentacene in another configuration binds to two center adatom-rest atom pairs within one-half of the surface unit cell. The third configuration entails a twisted pentacene structure as the molecule terminally bridges two oppositely oriented adatom-rest atom pairs that span across the faulted and unfaulted halves of a unit cell. The different binding configurations modified the electronic structures of the adsorbed pentacene as evidenced by their STM images, which were modeled by theoretical simulation. Structural modeling and adsorption energies calculations were carried out using DFT method. Factors affecting the relative thermodynamic stabilities based on the calculation results and the relative populations of pentacene in the different binding configurations as observed experimentally were discussed.