Di-σ and dative binding of benzene and pyridine on a Si(5,5,12)-2 × 1

We investigated the adsorption of benzene and pyridine on Si(5,5,12)-(2 × 1) at 80 K by using variable-low temperature scanning tunneling microscopy and density functional theory calculations. The benzene molecule most strongly binds to two adatoms on the D3 and D2 units in a tilted butterfly configuration, which consists of di-σ bonds between C atoms and Si adatoms and two C=C double bonds in the benzene molecule. Pyridine molecules interact with adatom(s) on the D2 and D3 units through both Si-N dative bonding and di-σ bonds. The dative bonding through the lone pair electrons of N atom produces a vertical configuration (pyridine-like), which is more stable than di-σ bonds. Di-σ bonds can be formed either through Si-N¹ and Si-C⁴ or Si-C² and Si-C⁵.