Structures of dense glycine and alanine adlayers on chiral Cu(3,1,17) surfaces

Density Functional Theory calculations have been used to predict the structures of dense glycine and alanine adlayers on Cu(3,1,17)(S). Facets of this chiral Cu surface result from adsorbate-induced surface reconstruction when glycine or alanine are adsorbed and annealed on Cu(100). We have calculated the surface energy changes associated with this surface reconstruction. Our results allow the enantiospecificity of this reconstruction following adsorption of enantiopure or racemic alanine on Cu(100) to be discussed. The overall stability of glycine and alanine adlayers on Cu(3,1,17)(S) arises from an interplay between the formation of chemical bonds with the Cu surface, deformations in the adsorbed molecules during adsorption, and intermolecular hydrogen bonds within the adlayer; none of these factors individually dominates.     

Racemic versus enantiopure alanine on Cu(110): an experimental study

The adsorption of racemic alanine on the Cu(110) surface has been compared to that of enantiopure alanine using low-energy electron diffraction (LEED), reflection absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM). No evidence of chiral resolution at the surface was observed for the racemic system, indicating that the formation of separate enantiopure areas is not preferred. Also, in contrast to the enantiopure system, no chirally organized phase was observed for the racemic system. LEED shows that both systems display a common (3 x 2) phase at high coverage. However, the pathway and kinetic barriers to this phase differ markedly for the racemic and the enantiopure systems, with the racemic (3 x 2) appearing at a temperature that is more than 100 K below that required for the enantiopure system. In addition, we report intriguing complexities for the (3 x 2) LEED structure that is ubiquitous in amino acid/Cu(110) systems. First, a common (3 x 2) pattern with a zigzag distortion can be associated with both the racemic and enantiopure systems. For the racemic system, the coverage can be increased further to give a "true" (3 x 2) LEED pattern, which is a transformation that is impossible to enact for the enantiopure system. Most importantly, STM images of the "distorted" and "true" (3 x 2) structures created in the racemic system show subtle differences with neither arrangement being fully periodic over distances greater than a few molecules. Thus, the (3 x 2) phase appears to be more complicated than at first indicated and will require more complex models for a full interpretation.     

First-principles studies of chiral step reconstructions of Cu(100) by adsorbed glycine and alanine

Adsorption of amino acids on Cu(100) is known experimentally to induce surface reconstructions featuring intrinsically chiral Cu(3,1,17) facets, but no information about the geometry of the molecules on these chiral facets is available. We present density-functional theory calculations for the structure of glycine and alanine at moderate coverages on Cu(3,1,17). As might be expected, molecules prefer to bind at the step edges on this surface rather than on the surface's (100)-oriented terraces. The adsorption of enantiopure alanine on Cu(3,1,17) is predicted to be weakly enantiospecific, with S-alanine being more stable on Cu(3,1,17)(S) than R-alanine. By comparing the surface energies of Cu(100) and Cu(3,1,17) in the presence of adsorbed glycine or alanine, our calculations provide insight into the driving force for chiral reconstructions of Cu(100) by amino acids.     

STM study of glycine on TiO2(110) single crystal surfaces

The adsorption of glycine (NH2CH2COOH) was examined by scanning tunneling microscopy (STM) on TiO2(110) surfaces at room temperature. A (2x1) ordered overlayer was observed on the TiO2(110)-(1x1) surface. The adsorption of acetic acid and propanoic acid was also investigated on this surface and their STM images were quite similar to that of glycine. Since acetate and propanoate are formed by dissociative adsorption of these acids on TiO2(110), it is proposed that glycine adsorbs in the same way to form a glycinate. The amino group in the glycinate adlayer structurally analogous to those formed from aliphatic carboxylic acids would be extended away from the surface and potentially free to participate in additional reactions. The underlying structure of the TiO2 surface is important in determining the structure of the glycinate adlayer; no ordering of these adsorbates was observed on the TiO2(110)-(1x2) surface.     

Homochiral conglomerates and racemic crystals in two dimensions: tartaric acid on Cu(110)

Two-dimensional lattice structures formed by racemic tartaric acid on a single crystalline Cu(110) surface have been studied and compared with the enantiopure lattices. At low coverage, the doubly deprotonated bitartrate species is separated into two-dimensional conglomerates showing opposite enantiomorphism. At higher coverage, however, a singly deprotonated monotartrate species forms a heterochiral, racemic crystal lattice. While the enantioseparated bitartrate system undergoes decomposition at the same temperature as the enantiopure system, the racemic monotartrate lattice has a lower thermal stability than the enantiopure lattice of identical periodicity and surface density. At monolayer saturation coverage, the pure enantiomers form a denser lattice than the racemate. This is in contrast to the three-dimensional tartaric acid crystals, where the racemate crystallizes in a lattice of higher density, which is also more thermally stable than the enantiopure tartaric acid crystals.     

The reactive chemisorption of alkyl iodides at Cu(110) and Ag(111) surfaces: a combined STM and XPS study

The chemisorption of methyl and phenyl iodide has been studied at Cu(110) and Ag(111) surfaces at 290 K with STM and XPS. At both surfaces dissociative adsorption of both molecules leads to chemisorbed iodine, with the STM showing c(2 x 2) and (square root 3 x square root 3)R30 structures at the Cu(110) and Ag(111) surfaces, respectively. At the Cu(110) surface a comparison of coexisting c(2 x 2) I(a) and p(2 x 1) O(a) domains shows the iodine adatoms to be chemisorbed in hollow sites with evidence at low coverage for diffusion in the (110) direction. In the case of methyl iodide no carbon adsorption is observed at either the silver or the copper surfaces, but chemisorbed phenyl groups are imaged at the Cu(110) surface after exposure to phenyl iodide. The STM images show the phenyl groups as bright features approximately 0.7 nm in diameter and 0.11 nm above the iodine adlayer, reaching a maximum surface concentration after approximately 6 Langmuir exposure. However, the phenyl coverage decreases with subsequent exposures to PhI and is negligible by approximately 1000 L exposure, consistent with the formation and desorption of biphenyl. The adsorbed phenyls are located above hollow sites in the substrate, they are stabilized at the top and bottom of step edges and in paired chains (1.1 nm apart) on the terraces with a regular interphenyl spacing within the chains of 1.0 nm in the (110) direction. The interphenyl ring spacing and diffusion of individual phenyls from within the chains shows that the chains do not consist of biphenyl species but may be a precursor to their formation. Although the XPS data shows carbon present at the Ag(111) surface after exposure to PhI, no features attributable to phenyl groups were observed by STM.     

Molecular ordering and adsorbate induced faceting in the Ag{110}-(S)-glutamic acid system

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.     

Highly ordered adlayers of three calix[4]arene derivatives on Au(1 1 1) surface in HClO4 solution: in situ STM study

Highly ordered adlayers of three calix[4]arene derivatives have been prepared on Au(1 1 1) surface in HClO₄ solution. The adlayer structures are characterized by in situ scanning tunneling microscopy (STM). High-resolution STM images reveal the molecular orientation and packing arrangement in the ordered adlayers. All the molecules are upright on Au(1 1 1) surface in pinched cone conformation. The structural difference of these molecules is proposed to be from the different functional groups at the upper and lower positions. The structural models are tentatively proposed for three ordered adlayers.     

Self-organization of a self-assembled supramolecular rectangle, square, and three-dimensional cage on Au111 surfaces

The structure and conformation of three self-assembled supramolecular species, a rectangle, a square, and a three-dimensional cage, on Au111 surfaces were investigated by scanning tunneling microscopy. These supramolecular assemblies adsorb on Au111 surfaces and self-organize to form highly ordered adlayers with distinct conformations that are consistent with their chemical structures. The faces of the supramolecular rectangle and square lie flat on the surface, preserving their rectangle and square conformations, respectively. The three-dimensional cage also forms well-ordered adlayers on the gold surface, forming regular molecular rows of assemblies. When the rectangle and cage were mixed together, the assemblies separated into individual domains, and no mixed adlayers were observed. These results provide direct evidence of the noncrystalline solid-state structures of these assemblies and information about how they self-organize on Au111 surfaces, which is of importance in the potential manufacturing of functional nanostructures and devices.     

Characterization of enantiospecific chemisorption on chiral Cu surfaces vicinal to Cu(111) and Cu(100) using density functional theory

Surfaces of simple fcc metals such as Cu with nonzero and unequal Miller indices are intrinsically chiral. Density functional theory (DFT) calculations are a useful way to study the enantiospecific adsorption of small chiral molecules on these chiral metal surfaces. We report DFT calculations of seven chiral molecules on several structurally distinct chiral Cu surfaces. These surfaces include two surfaces with (111)-oriented terraces and one with (100)-oriented terraces. Calculations are also described on a surface that was modified to mimic the surface structures that typically appear on real metal surfaces following thermally driven fluctuations in step edges. Our results provide initial information on how variation in the surface structure of intrinsically chiral metal surfaces can affect the enantiospecific adsorption of small molecules on these surfaces.     

ToF-SIMS analysis of poly(l-lysine)-graft-poly(2-methyl-2-oxazoline) ultrathin adlayers

Understanding of the interfacial chemistry of ultrathin polymeric adlayers is fundamentally important in the context of establishing quantitative design rules for the fabrication of nonfouling surfaces in various applications such as biomaterials and medical devices. In this study, seven poly(l-lysine)-graft-poly(2-methyl-2-oxazoline) (PLL–PMOXA) copolymers with grafting density (number of PMOXA chains per lysine residue) 0.09, 0.14, 0.19, 0.33, 0.43, 0.56, and 0.77, respectively, were synthesized and characterized by means of nuclear magnetic resonance spectroscopy (NMR). The copolymers were then adsorbed on Nb₂O₅ surfaces. Optical waveguide lightmode spectroscopy method was used to monitor the surface adsorption in situ of these copolymers and provide information on adlayer masses that were then converted into PLL and PMOXA surface densities. To investigate the relationship between copolymer bulk architecture (as shown by NMR data) and surface coverage as well as surface architecture, time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis was performed. Furthermore, ToF-SIMS method combined with principal component analysis (PCA) was used to verify the protein resistant properties of PLL–PMOXA adlayers, by thorough characterization before and after adlayer exposure to human serum. ToF-SIMS analysis revealed that the chemical composition as well as the architecture of the different PLL–PMOXA adlayers indeed reflects the copolymer bulk composition. ToF-SIMS results also indicated a heterogeneous surface coverage of PLL–PMOXA adlayers with high grafting densities higher than 0.33. In the case of protein resistant surface, PCA results showed clear differences between protein resistant and nonprotein-resistant surfaces. Therefore, ToF-SIMS results combined with PCA confirmed that the PLL–PMOXA adlayer with brush architecture resists protein adsorption. However, low increases of some amino acid signals in ToF-SIMS spectra were detected after the adlayer has been exposed to human serum.

Dependence of molecular recognition of fullerene derivative on the adlayer structure of zinc octaethylporphyrin formed on Au(100) surface

Adlayers of ZnOEP were prepared on reconstructed Au(100)-(hex) and unreconstructed Au(100)-(1 x 1) surfaces by immersing into a benzene solution containing ZnOEP molecules, and the adlayer structures were characterized by scanning tunneling microscopy (STM). A hexagonally arranged ZnOEP array was formed on an Au(100)-(hex) surface, whereas a rectangularly arranged ZnOEP array was found on an Au(100)-(1 x 1) surface. The adlayer structure of ZnOEP was dependent upon the underlying Au atomic arrangements. Furthermore, an investigation of the spuramolecular assembly for these modified surfaces was carried out by using an open-cage C(60) derivative (opened C(60)). A supramolecular assembled adlayer with a 1:1 composition of opened C(60)/ZnOEP was formed on Au(100)-(hex), whereas aggregates of opened C(60) were found on the ZnOEP-modified Au(100)-(1 x 1) surface. Electrochemical responses of opened C(60) were significantly influenced by underlying ZnOEP arrays. This finding suggests that precise control of underlying ZnOEP adlayers with the Au atomic structure is important to recognize the opened C(60) on them.     

Scanning Tunnelling Microscopy Study of Chloride Adsorbed on Cu(110) Surfaces in Hydrochloric Acid Solution

Fundamental studies of the interaction of chloride with copper is of great interest from both scientific and technological view points since it is related to the copper electroplating and also the halogen etching processes^[1]. In this paper, in-situ scanning tunneling microscopy (STM) has been employed to study chloride adsorption on Cu(110) electrode in hydrochloride acid aqueous solution. The atomic resolution images of chloride adsorbed on Cu(111) surface have been obtained over the potential range from -400 mV to -100 mV (vs. SCE). A (l×l) structure for Cu(110) substrate was observed at the potential negative than -450 mV in which the chloride anions are desorbed. Our results indicate that the chloride anions are very strongly adsorbed on the Cu(110) surface at the potential positive than -400 mV. The images taken over a large region show parallel rows on the terraces and monolayer straight steps, which run along the[001] direction of the substrate lattice. The parallel row structures observed here are suggested to attribute from the chloride adlayers. The different corrugation height and periodical modulation in the height along[110] direction observed indicates that chloride anions are located at physically non-equivalent binding sites. Models are proposed to interpret the structures with three-fold periodicity and four-fold periodicity along[110] direction with a (4×1) structure containing three chloride anions and a (5×1) structure containing four chloride anions, respectively.     

Probing enantioselectivity on chirally modified Cu(110), Cu(100), and Cu(111) surfaces

Temperature programmed desorption methods have been used to probe the enantioselectivity of achiral Cu(100), Cu(110), and Cu(111) single crystal surfaces modified by chiral organic molecules including amino acids, alcohols, alkoxides, and amino-alcohols. The following combinations of chiral probes and chiral modifiers on Cu surfaces were included in this study: propylene oxide (PO) on L-alanine modified Cu(110), PO on L-alaninol modified Cu(111), PO on 2-butanol modified Cu(111), PO on 2-butoxide modified Cu(100), PO on 2-butoxide modified Cu(111), R-3-methylcyclohexanone (R-3-MCHO) on 2-butoxide modified Cu(100), and R-3-MCHO on 2-butoxide modified Cu(111). In contrast with the fact that these and other chiral probe/modifier systems have exhibited enantioselectivity on Pd(111) and Pt(111) surfaces, none of these probe/modifier/Cu systems exhibit enantioselectivity at either low or high modifier coverages. The nature of the underlying substrate plays a significant role in the mechanism of hydrogen-bonding interactions and could be critical to observing enantioselectivity. While hydrogen-bonding interactions between modifier and probe molecule are believed to induce enantioselectivity on Pd surfaces (Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2007, 129, 15240-15249), such critical interactions may be missing on Cu surfaces where hydrogen-bonding interactions are believed to occur between adjacent modifier molecules, enabling them to form clusters or islands.     

On the existence of ordered organic adlayers at the Cu(111)/electrolyte interface

We have reinvestigated the behavior of a Cu(111) electrode in pure and cinchonidine containing aqueous 0.1 M HClO4 solution by cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (STM). In contrast to previous publications by Wan et al. (Langmuir 2000, 19, 1958-1962 and references cited therein) on Cu(111) in pure 0.1 M HClO4 which claimed an adsorbate-free Cu(111) surface in the entire potential range, we have found a highly ordered hexagonal adsorbate structure with a (4 x 4) unit cell, which is stable in the potential range from hydrogen evolution at -350 to -150 mV (RHE). The adsorbate-free (1 x 1) Cu(111) surface is only visible in a fairly small potential range from -150 to +50 mV. A disordered surface structure is formed at more positive potentials which is interpreted by adsorption of an oxygen-containing species. Furthermore, the formation of a highly ordered cinchonidine adlayer on Cu(111) in 0.1 M HClO4 as reported by Wan et al. (J. Am. Chem. Soc. 2002, 124, 14300-14301) could not be reproduced here. In fact, the similarity of all structures reported by Wan et al. for a great variety of different organic adlayers on Cu(111) in HClO4 solution including cinchonidine with the (4 x 4) superstructure found here already in pure HClO4 solution (i.e., without organic solute) casts serious doubts on the validity of those previous results by Wan et al. in general.     

Sb在Cu单晶电极上欠电位沉积的电化学和现场STM研究

应用循环伏安法和现场扫描隧道显微镜研究了在HClO4和H2SO4两种溶液中Sb于Cu(111)和Cu(100)电极上的欠电位沉积.结果表明,不同的表面原子排列和强吸附阴离子的存在将明显影响Sb的欠电位沉积行为.在结构较为开放的Cu(100)表面,Sb形成的欠电位沉积层结构也较为开放,并且伴随着表面合金的形成;而在密堆积的Cu(111)表面上,Sb形成了致密的单层结构.又当Cu(111)表面存在强吸附的SO42-时,Sb原子首先在SO42-吸附层与Cu表面交接的新台阶处成核,随后通过取代SO42-向上一层晶面发展,表现出独特的成核—生长行为;而在弱吸附的HClO4溶液中,Sb的欠电位沉积系以在晶面上随机形成一些单原子层高度的Sb岛为特征.在Cu(100)表面,通过SO42-的诱导共吸附,欠电位沉积的Sb原子形成了开放性更大的(4×4)结构,不同于在HClO4溶液中所形成的(22×22)R45°结构.     

Adsorption of TTF,TCNQ and TTF-TCNQ on Au(111): An <Emphasis Type="Italic">in situ</Emphasis> ECSTM study

The adsorption and adlayer structures of tetrathiofulvalene (TTF), tetracyanoquinodimethane (TCNQ) and TTF-TCNQ on Au(111) have been systematically investigated by in situ electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry in 0.1 mol L⁻¹ HClO₄. All the three molecules were found to form well-ordered adlayers in the double-layer potential region of Au(111). For TTF and TCNQ adlayers, (6×3) and (4×7) structures have been observed, respectively. A structural transition was observed on TCNQ adlayer at potential negative of 0.08 V vs. the reversible hydrogen electrode (RHE), and induced a new phase with (3 × 12) structure. On the other hand, the charge transfer complex, TTF-TCNQ, self-organized into ordered domains with a lamellar structure different from those of the pure TTF and TCNQ adlayers on Au(111). Its packing arrangement was comparable to surface structures of either single crystal or thin film of TTF-TCNQ. Supported by the National Natural Science Foundation of China (Grant Nos. 20673121, 20733004 & 20821003), the National Key Project for Basic Research (Grant Nos. 2006CB806101 & 2006CB932100) and Chinese Academy of Sciences     

A step toward the wet surface chemistry of glycine and alanine on Cu{110}: destabilization and decomposition in the presence of near-ambient water vapor

The coadsorption of water with organic molecules under near-ambient pressure and temperature conditions opens up new reaction pathways on model catalyst surfaces that are not accessible in conventional ultrahigh-vacuum surface-science experiments. The surface chemistry of glycine and alanine at the water-exposed Cu{110} interface was studied in situ using ambient-pressure photoemission and X-ray absorption spectroscopy techniques. At water pressures above 10(-5) Torr a significant pressure-dependent decrease in the temperature for dissociative desorption was observed for both amino acids, accompanied by the appearance of a new CN intermediate, which is not observed for lower pressures. The most likely reaction mechanisms involve dehydrogenation induced by O and/or OH surface species resulting from the dissociative adsorption of water. The linear relationship between the inverse decomposition temperature and the logarithm of water pressure enables determination of the activation energy for the surface reaction, between 213 and 232 kJ/mol, and a prediction of the decomposition temperature at the solid-liquid interface by extrapolating toward the equilibrium vapor pressure. Such experiments near the equilibrium vapor pressure provide important information about elementary surface processes at the solid-liquid interface, which can be retrieved neither under ultrahigh vacuum conditions nor from interfaces immersed in a solution.     

Ambient Pressure X-ray Photoelectron Spectroscopy Study of Oxidation Phase Transitions on Cu(111) and Cu(110)

The surface structure effect on the oxidation of Cu has been investigated by performing ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Cu(111) and Cu(110) surfaces under oxygen pressures ranging from 10⁻⁸ to 1 mbar and temperatures from 300 to 750 K. The APXPS results show a subsequential phase transition from chemisorbed O/Cu overlayer to Cu₂O and then to CuO on both surfaces. For a given temperature, the oxygen pressure needed to induce initial formation of Cu₂O on Cu(110) is about two orders of magnitude greater than that on Cu(111), which is in contrast with the facile formation of O/Cu overlayer on clean Cu(110). The depth profile measurements during the initial stage of Cu₂O formation indicate the distinct growth modes of Cu₂O on the two surface orientations. We attribute these prominent effects of surface structure to the disparities in the kinetic processes, such as the dissociation and surface/bulk diffusion over O/Cu overlayers. Our findings provide new insights into the kinetics-controlled process of Cu oxidation by oxygen.     

In situ scanning tunneling microscopy of molecular assemblies of cobalt(II)- and copper(II)-coordinated tetraphenyl porphine and phthalocyanine on Au(100)

Molecules of copper(II) and cobalt(II) 5,10,15,20-tetraphenyl-21H,23H-porphine (CuTPP and CoTPP) and cobalt(II) phthalocyanine (CoPc) are spontaneously adsorbed onto reconstructed Au(100) substrate from a benzene solution containing each individual complex. In situ scanning tunneling microscopy (STM) was used to examine the real-space arrangement and the internal molecular structure of each of the individual molecules in 0.1 M HClO4 under potential control. The adsorption of CuTPP and CoTPP produced the same highly ordered square array with an intermolecular spacing of 1.44 nm on a reconstructed Au(100) surface. These molecular superlattices and the underlying reconstructed Au(100) predominated between 0 and 0.9 V, but lifting of the reconstructed Au(100) surface and elimination of the ordered adlayers occurred at more positive potentials. Molecular resolution STM revealed propeller-shaped admolecule with its center imaged as a protrusion for Co(II) and a depression for Cu(II). In contrast, the spontaneous adsorption of CoPc molecules resulted in a rapid phase transition from the reconstructed Au(100) surface to the (1 x 1) phase, coupled with the production of locally ordered, square-shaped arrays with an intermolecular distance of 1.65 nm. This molecular adlayer and the Au(100)-(1 x 1) remained unchanged when the potential was modulated between 0 and 1.0 V. These results indicate that the subtle variation in the molecular structure of adsorbate influenced not only its spatial arrangement but also the structure of the underlying Au(100) substrate.