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

Abstract

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 transitional adlayer grows in the neutral forms of glycine and glycylglycine, binding to their respective interfacial adlayer through hydrogen bonding. For glycine, the presence of head-to-tail N···H–O hydrogen bonding is indicated by a new N 1s feature at 401.4 eV binding energy, between those for neutral amino N at 400.6 eV and zwitterionic N at 402.1 eV. For glycylglycine, the existence of hydrogen bonding can be inferred from the considerable thermal stability of the transitional adlayer (at least to 200 °C). In the final stage, both glycine and glycylglycine grow continuously in the zwitterionic form into thick films. Thermal evolution studies of these as-grown glycine and glycylglycine zwitterionic films on Si(111)7×7 reveal the reverse trend, with the zwitterionic multilayer and transitional adlayer desorbing sequentially and the interfacial adlayer less affected below 250 °C. The glycylglycine film clearly exhibits a higher thermal resistance than the glycine film. The present work demonstrates the vital role of hydrogen bonding in the formation of the transitional adlayer in these important biomolecules. The intermediate bond strength of a hydrogen bond (between those of a covalent bond and the long-range van der Waals interaction) promises new bonding flexibilities for building multifunctional biomolecular structures for biosensor and bioelectronic applications.