Direct Imaging of Hydrogen Bond Formation in Dissociative Adsorption of Glycine on Si(111)7x7 by Scanning Tunneling Microscopy

Adsorption of glycine on a Si(111)7x7 surface at room temperature has been studied by scanning tunneling microscopy (STM). The observed STM images provide strong evidence for dissociative adsorption of glycine through N-H bond cleavage (and N-Si bond formation) as reported in our recent X-ray photoemission study. In particular, the dissociated H atom is found to anchor on a restatom while the N-H dissociated glycine molecule adsorbs on an adatom in a tilted, unidentate geometry. STM study for higher exposures further reveals that the second adlayer is mediated by vertical hydrogen bonding, in excellent accord with our recent X-ray photoemission results. In addition to this vertical hydrogen bonding between a glycine molecule and the N-H dissociated glycine adsorbate, we also observe horizontal hydrogen bonding, not seen before, between two N-H dissociated glycine adsorbates at two neighboring adatom sites. These hydrogen-bonded adstructures, as implicated in the STM images, have been corroborated with our computational DFT/B3LYP/6-31++(d,p) results by using the two largest model surfaces: a Si16H18 cluster to simulate an adatom-restatom pair and a Si26H24 cluster to model a double adatom-adatom pair across the dimer wall of the 7x7 surface. Furthermore, statistical analysis of the STM images for different exposures shows that the center adatom is more reactive than the corner adatom and that the faulted half is more reactive than the unfaulted half. The horizontal hydrogen bonding appears to be favored at a lower exposure than the vertical hydrogen bonding, which becomes dominant at a higher exposure as formation of the second adlayer proceeds. The present work illustrates the importance of hydrogen bonding in the early growth and site-specific chemistry of glycine on Si(111)7x7 surfaces.