Molecular Ordering and Adsorbate Induced Faceting in the Ag{110}−(S)-Glutamic Acid System

Abstract

The adsorption of the amino acid, (S)-glutamic acid, was investigated on Ag{110} as a function of coverage and adsorption temperature using the techniques of scanning tunneling microscopy, low energy electron diffraction, and reflection absorption infrared spectroscopy. In the monolayer, (S)-glutamic acid was found to adsorb predominantly in the anionic glutamate form. Several discrete ordered adlayer structures were observed depending on preparation conditions. In addition, (S)-glutamic acid was found to induce both one- and two-dimensional faceting of the Ag{110} surface. In some cases, evidence was found that the 2-D faceting involved the creation of a chiral facet distribution. A comparison is made of the Ag/(S)-glutamic acid system with analogous studies of amino acids on Cu.

Adsorption and self-assembly of bio-organic molecules at model surfaces: A route towards increased complexity

“…It should be noted that the extent of ordering is relatively limited (the authors estimate that 10-20% of the surface consists of ordered domains, whose typical diameter is ~10 nm). This lack of long-range order has been reported for (R,R)-tartaric acid on Ni(111) [49] and Ni(110) [277] and contrasts to the behaviour observed for (S)-Glu on Ag(110) [51] and for (R,R)-tartaric acid [261], succinic acid [278] and other amino acids on Cu surfaces [69; 101; 158; 181]. The degree of surface ordering increases by annealing the surface to 350 K (Figure 68c), while further heating to 400 K (Figure 68d) changes the nature of the adlayer: molecules tend to order in 1-D structures extending along the close packed Ni direction, but no large 2-D ordered domains are observed.…”

“…For vibrational spectroscopy on metal surfaces, either infrared reflection absorption spectroscopy (IRAS or RAIRS -see Figure 3a) [47][48][49][50][51] or high resolution electron energy loss (HREELS -see Figure 3b) [52][53][54] detection can be used. The former technique relies on the amplification of the electric field at the surface, in the direction normal to it; like basic IR spectroscopy, IRAS underwent a rapid technical evolution over the last years, so that detection of adsorbates at a few percent of monolayer coverage is now feasible thanks to Fourier Transform-based acquisition (FT-IRAS).…”

“…Data taken from refs. [53] (column a); [54] (column b); [51] (column c); [250] (column d) and [251]. Figure 51b and c reports the parallel experiments performed on Ag(100) and Ag(111) and monitored by XPS, which allows for a much more straightforward interpretation of the results.…”

“…Note that the model applied is quite rigid and does not keep into account that the O1s binding energy may be affected also by other effects, e.g. by hydrogen bonds [51]. Theoretical DFT-D modelling of the Glu layers forming on Ag(100) at T = 350 K confirms however this empirical analysis, by suggesting the existence of two stable structures consisting of neutral molecules [59] and of a mixture neutral and anionic molecules in equal amounts [253], respectively.…”

“…The role of the deposition temperature in the self-assembly process was investigated in detail for the case of Glu adsorption at Ag(110) [51] and Ag(100) [54]. Glu molecules organize on Ag (110) in different ordered structures depending on coverage and adsorption temperature 300 K  T ev  to 475 K. Such geometries are determined by the combined analysis of LEED superstructures and STM images and are often associated to adsorbate-induced faceting of the surface.…”