Silicon surfaces as electron acceptors: Dative bonding of amines with Si(001) and Si(111) surfaces

The bonding of them trimethylamine (TMA) and dimethylamine (DMA) with cryst alline silicon surfaces has been investigated using X-ray photoelectron spe ctroscopy (XPS), Fourier transform infrared spectroscopy, and density-funct ional computational methods. XPS spectra show that TMA forms stable dative- bonded adducts on both Si(001) and Si(111) surfaces that are characterized by very high N(1s) binding energies of 402.2 eV on Si(001) and 402.4 eV on Si(111). The highly ionic nature of these adducts is further evidenced by c omparison with other charge-transfer complexes and through computational ch emistry studies. The ability to form these highly ionic charge-transfer com plexes between TMA and silicon surfaces stems from the ability to delocaliz e the donated electron density between different types of chemically distin ct atoms within the surface unit cells. Corresponding studies of DMA on Si( 001) show only dissociative adsorption via cleavage of the N-H bond. These results show that the unique geometric structures present on silicon surfac es permit silicon atoms to act as excellent electron acceptors.