Glycine Adsorption on Pt(111) in the Monolayer and Multilayer Regime

Abstract

Glycine (GL) overlayers of varying thickness were deposited in situ on the clean Pt(111) surface by evaporating commercially available glycine powder. The temperature-stabilized evaporation source, made of Cu, was kept at 420 K during evaporation. The distance between Pt and the evaporation source was about 240 mm. The deposition rate and the total amount of glycine deposited were monitored by a quartz crystal microbalance that could be moved to intersect the GL beam approximately 40 mm downstream compared to the Pt position. The total amount of glycine in the reproduced spectra was 34 (~0.5 ML) and 340 ng/cm2 (~ 5 ML), respectively. We discuss the spectra with reference to both the neutral and zwitterionic states. There are two glycine-related C 1s peaks at both low and high GL coverages and a shoulder at ~ 285 eV most probably due to partial decomposition of glycine inside the evaporation source, at least for a thicker GL layer. The C 1s peak at lower binding energy is usually attributed to the C 1s carbon, while the peak at higher BE is supposed to stem from the carboxyl group. The energy of the peaks shifts with coverage, mainly due to the differences in charge transfer between Pt-GL (low coverage) compared to the GL-GL (high coverage). Slight charging of the sample (3.7 meV/ML) takes place, but results in a virtually uniform shift of all the GL levels involved. In the N 1s spectral region there appear two different N 1s components. The intensity of the peak at lower BE levels off with increased coverage from ~25% of the total N 1s intensity at low coverage to ~8% at higher coverages. The energy position of this lower BE peak is characteristic of NH2-like bonding. The position of the second N 1s peak suggests NH3+-like bonds. At high GL coverages the N 1s peak at low BE, and a C 1s shoulder at 285 eV, stem mainly from a slight decomposition of GL inside the evaporation source (below 8%). At low glycine coverages (<1 ML) there exist two different GL states on the surface: a majority of the amino acid molecules form zwitterions, while the remaining part is either neutral or decomposes into fragments some of which contain the NH2 group.