Three-Stage Growth of Glycine and Glycylglycine Nanofilms on Si(111)7×7 and Their Thermal Evolution in Ultrahigh Vacuum Condition: From Chemisorbed Adstructures to Transitional Adlayer to Zwitterionic Films

Nanometer-thick glycine and glycylglycine film growth on Si(111)7×7 at room temperature in ultrahigh vacuum condition and their thermal evolution are investigated by X-ray photoelectron spectroscopy (XPS). In order to understand the XPS result of initial exposure, we also calculate equilibrium geometries and the adsorption energies of plausible glycine and glycylglycine adspecies on model 7×7 surfaces using density functional theory. N 1s spectra reveal three growth stages for both glycine and glycylglycine nanofilms. The first stage involves N–H dissociative adsorption of glycine and N–H and O–H dissociative adsorption of glycylglycine, forming N–Si and O–Si bonds at the interface, respectively. The experimental results are consistent with the most stable glycylglycine adsorption structure involving both the amino and amide N atoms bonded to a Si adatom-restatom pair or an amino N and a carboxyl O atoms bridging two Si adatoms across a dimer wall, in a bidentate configuration. In the second stage, a tran...