Covalent and Hydrogen Bonding in Adsorption of Alanine Molecules on Si(111)7×7

Molecular adsorption bonding configurations and specific interfacial chemistry of alanine on Si(111)7×7 have been determined by combining the results from scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) with ab initio calculations based on the density functional theory (DFT). XPS spectra of the N 1s region show that alanine molecules bind to the 7×7 surface by N-Si covalent bonding, while STM imaging reveals that such N-H dissociative adsorption of alanine occurs on an adjacent Si adatom-restatom pair, with the dehydrogenated alanine moiety and dissociated H atom occupying the Si adatom and restatom sites, respectively. At a sample bias above +2 V, the dehydrogenated alanine appears as a bright round protrusion, slightly off-center from a Si adatom site and leaning toward the opposite Si adatom across the dimer wall. The off-center character can be attributed to an electrostatic attraction between the electron-rich carbonyl O of the dehydrogenated alanine and electron-deficient nearest Si adatom across the dimer wall. Our DFT calculation also shows that the monodentate O-Si bonding configuration resulting from O-H dissociative adsorption is more thermodynamically favorable than the experimentally observed N-Si bonding configuration, suggesting that the interfacial dissociative adsorption reaction is a kinetically controlled rather than a thermodynamically driven process. Alanine molecules in the second adlayer (transitional layer) are found to attach to those in the first adlayer (interfacial layer) by N···HO hydrogen bonding, as supported by the presence of the N 1s feature at 401.0 eV. An alanine molecule H-bonded to a dehydrogenated alanine in the first adlayer has also been observed in STM as a brighter and larger protrusion close to the expected location of the free OH group in the dehydrogenated first-adlayer alanine. No thick zwitterionic alanine film can be obtained at room temperature possibly due to steric constraint caused by the methyl group.