Modelling of benzene-1,4-dithiol on a Au(1 1 1) surface

The bonding of benzene-1,4-dithiol to a Au(1 1 1) surface is studied using a Au₂₅–SC₆H₄SH model. We find that the most favorable structure has the S directly above an Au atom, i.e., an on-top site. If the molecule is constrained to be perpendicular to the surface, the on-top site is the least favorable site and the threefold hollow is the most favorable one.     

Effects of underpotentially deposited TI and Pb submonolayers on the surface-enhanced Raman scattering (SERS) from pyridine at Ag electrodes

SERS from pyridine on an Ag electrode is almost completely quenched by underpotential deposition of TI to a surface coverage of ≈ 3%. Removal of TI by anodic stripping does not restore the original SERS intensity. These results suggest that only a small fraction of adsorbed pyridine molecules is SERS active, and TI is specifically deposited on such a SERS active site, which then undergoes irreversible destruction. Pb appears to be non-specifically deposited on the Ag surface, but exhibits a similar irreversible quenching of SERS.     

Open circuit stability of underpotentially deposited Pb monolayer on Cu(1 1 1)

The stability of underpotentially deposited (UPD) Pb layer on Cu(1 1 1) is investigated by conventional electrochemical techniques in perchlorate solution at open circuit potential (OCP). In situ scanning tunneling microscopy (STM) is employed to monitor and ascertain structural and morphological changes at characteristic potentials. A corrosion-like mechanism associated with UPD layer stripping powered by reduction processes is found to operate in the system of interest in absence of potential control. OCP transients suggest strong dependence of the Pb layer stability upon the concentration of oxidizing agents, such as oxygen and/or nitrate ions, present in the solution. It is found that the increase of the oxidizing agent concentration results in a proportional decrease of the Pb UPD layer stripping time at OCP. The concentration of the dissolved oxygen is found to affect the UPD layer behavior in the entire range of underpotentials in accordance with the strong affinity of the Pb²⁺/Cu(1 1 1) system to the oxygen reduction reaction (ORR). In contrast to oxygen, nitrate ions appear to play active role only in the potential range positive to the UPD peaks where mostly bare Cu surface is in contact with the solution. Specifically adsorbing Cl⁻ ions are examined as a possible inhibitor of the reduction processes operating in the Pb underpotential range. Concentrations of Cl⁻ ions as low as 1 × 10⁻⁴ M are found to stabilize the Pb UPD layer by a factor of 2.5.     

Long-range effects on palladium deposited on Pt(1 1 1)

Two-dimensional long-range effects have been pointed out electrochemically for palladium adlayers on Pt(1 1 1) in the absence of anion specific adsorption. The observation of these effects requires well ordered electrode substrates. In this way, the characteristic features are not observed on stepped electrodes with 20 atoms wide terraces. The formation of a homogeneous adlayer is also required and the effects are pointed out only when the palladium coverage is close to the completion of the full monolayer.     

In situ and real-time surface differential diffraction (SDD) study of Cd underpotential deposition on Au(1 1 1)

In situ and real-time surface differential diffraction (SDD) has been used to study the underpotential deposition (UPD) of Cd on Au(1 1 1) in sulfuric acid media. Comparison of SDD results in sulfate electrolytes with and without the presence of Cd²⁺ ions reveals that the surface reconstruction associated with the sulfate adsorption and desorption dominates the structural effect. It is also found that the reconstructed gold surface is stable upon Cd UPD process. In the initial stages of UPD, Cd atoms bind to the surface in bridge sites. This is followed by an adlayer structure with Cd adsorption in threefold hollow sites before Au/Cd intermixing takes place.     

Electronic structure of the Au–Si(1 1 1) interface as a function of Au coverage

We have investigated the Au–Si(1 1 1) interface as a function of the Au coverage by the core-level photoemission spectroscopy. With increasing the Au coverage, the spectral features in the Si 2p core-level changed remarkably and some fine structures in both Si 2p and Au 4f spectra were observed. Based on the curve fitting analysis, the Si 2p and Au 4f spectra at more than 20 Å Au coverage were decomposed into three chemically different components, respectively. The assignments of their components were performed. In addition, we have compared these results for the Au–Si(1 1 1) interface with our previous study for the Au–Si(1 0 0) interface. It was found that the electronic structures for the Au–Si(1 1 1) interface is essentially identical to those of the Au–Si(1 0 0) interface except at the initial Au deposition.     

The potentials of zero charge of Pd(1 1 1) and thin Pd overlayers on Au(1 1 1)

The potential of zero charge (pzc) of Pd(1 1 1) has been determined in dilute NaF solutions by measuring the Gouy–Chapman minimum of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of −0.12 V vs. SCE has been found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) have also been determined. While the pzc of the first, pseudomorphic Pd layer on Au(1 1 1) is −0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc's and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/water interface is estimated.     

Gap-mode SERS studies of azobenzene-containing self-assembled monolayers on Au(1 1 1)

Self-assembled monolayers of azobenzene-containing thiols on smooth Au(1 1 1) surfaces were studied by gap-mode surface-enhanced Raman spectroscopy (gap-mode SERS). By adsorption of colloidal Au nanoparticles on top of the organic adlayer highly reproducible spectra with strongly enhanced intensities are obtained. The observed bands indicate a trans conformation of the azobenzene moieties and are in agreement with structural data for the molecular layer. A characteristic dependency on the terminal and the spacer groups of the molecules is found. Samples prepared during illumination with UV light show pronounced spectral differences that can be attributed to azobenzene in cis conformation.     

The oxidation of trivalent chromium at Au(1 1 1) electrode

Electro-oxidation of Cr(III)–Cr(VI) species was investigated on the (1 1 1) plane of a gold single crystal in a highly alkaline solution. This process was found to be structure sensitive due to the involvement of the OH⁻ anion in the rate-determining step. It was noticed additionally, that in the ionic system the lifting of the reconstruction could be due to the coadsorption of the anions from the supporting electrolyte and the anions supplied from the electro-oxidation process.     

Surface plasmon resonance and Raman scattering effects studied for layers deposited on Spreeta sensors

The possibility to detect and prospectively to characterize deposited organic layers directly on Spreeta sensors by Fourier transform Raman spectroscopy was studied. A special holder of sensors that enabled measurement of FT Raman spectra was developed. The effects of various angles of incidence of the laser beam on the sensor surface were studied with respect to the intensity of Raman spectra and to the risks of artifacts. No effect of measurement of FT Raman spectra on SPR functionality of sensors was proven. The key role of the surface morphology of the sensing gold layer on repeatability of SPR curves and the possibility to check the surface by optical microscopy was demonstrated.     

Dynamics of the dissolution of an underpotentially deposited Cu layer on Au(111): a combined time-resolved surface-enhanced infrared and chronoamperometric study

It is well known that an ordered phase consisting of 2/3 monolayer (ML) of Cu and 1/3 ML of sulfate is formed by the underpotential deposition of Cu on Au(111) in sulfuric acid. Dynamic processes involved in the dissolution of the ordered phase for a potential step have been investigated by time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS) and chronoamperometry. The results demonstrate that the dissolution occurs through two successive steps. The initial step involves the desorption of 1/3 ML of Cu and 1/3 ML of sulfate via a Langmuir-type kinetics and the remaining 1/3 ML of Cu is desorbed in the subsequent step via a nucleation-and-growth kinetics. The observation is partly consistent with dynamic Monte Carlo simulations reported in the literature.     

NEXAFS study of a Pt-containing rod-like organometallic polymer (Pt-DEBP): molecular orientation onto HOPG, Au/Si(1 1 1), Cr/Si(1 1 1) and Si(1 1 1) surfaces

The influence of different substrates on the molecular orientation of organometallic polymer Pt-DEBP, [Pt(PBu₃)₂CCC₁₂H₈CC]_n, has been investigated by NEXAFS spectroscopy. Thin films were deposited on HOPG, Au/Si(1 1 1), Cr/Si(1 1 1), Si(1 1 1) and stainless steel. The assignment of the spectral features has been carried out on the basis of previous STEX calculations performed on phenylacetylene model molecule in gas phase and adsorbed on Pt(1 1 1) and Cu(1 0 0). Angular dependent analysis of the π* resonance occurring at 285.50 eV photon energy deriving by the benzene carbon orbitals showed a polarisation effect for all substrates. A preferential molecular orientation at nearly 40° to the surface normal was observed. This result might be explained by the strong interaction between sp and sp² carbons of the organic diethynylbiphenyl DEBP moiety contained in close chains, leading to polymer self-assembling.     

Li atoms deposited on single crystalline MgO(0 0 1) surface. A combined experimental and theoretical study

In this Letter the interaction of Li atoms with terrace sites of MgO(0 0 1) surfaces is characterized by combining EPR spectroscopy with theoretical calculations. The Li atoms adsorbed to MgO terrace sites show a reduction of the isotropic hyperfine coupling constant by approximately 50% with respect to Li atoms in the gas phase. In combination with theoretical calculations it can be shown that this reduction of the hyperfine coupling constant is not due to a charge transfer between Li and MgO but can be understood by a polarization of the Li atoms which are essentially neutral.     

Gold adatom as a key structural component in self-assembled monolayers of organosulfur molecules on Au(1 1 1)

Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent formation of the self-assembled structure. Gold adatom chemistry is proposed here to be a unifying theme that explains various aspects of the alkanethiol self-assembly and reconciles experimental evidence provided by scanning probe microscopy and spectroscopic methods of surface science. While several features of alkanethiol self-assembly have yet to be revisited in light of the new adatom-based models, the successes of alkanethiol SAMs suggest that adatom-mediated surface chemistry may be a viable future approach for the construction of self-assembled monolayers involving molecules which do not contain sulfur.     

The influence of chloride on the initial anodic dissolution of Cu3Au(1 1 1)

We report a detailed in situ X-ray diffraction study of the influence of chloride on the atomic structure evolution at the solid-electrolyte interface during the selective dissolution of Cu from a Cu₃Au(1 1 1) surface immersed in 0.1 M H₂SO₄. We disclose that the formation of the initial ultrathin Au-rich (1 1 1) with an inverted stacking sequence, as recently observed at Cu₃Au(1 1 1) in contact with pure 0.1 M H₂SO₄, is strongly influenced by adding 5 mM HCl. The main finding is a negative shift of about 150 mV of the critical potential at which the ultra-thin Au-rich layer transforms into thicker Au islands. The presented results support the view that it is not a thermodynamic driving force, but rather the rate of surface diffusion that dominates the formation of the structures of the metallic layer.     

Adsorption of alkali metals on Ge(0 0 1)(2 × 1) surface

Ab initio total energy calculations have been performed for Na, K and Rb adsorption on Ge(0 0 1)(2 × 1) surface. It was found that the adsorption site of AM is AM size dependent. Structural analysis showed that the Ge–Ge dimer bond becomes stronger with increasing AM size. As the coverage increases from 0.5 to 1 ML it turns out that no depolarization effect occurs upon Na adsorption, while this effect becomes more important with increasing AM size. We also found that for all adsorption systems investigated the germanium surface is metallic and semiconducting for the coverage of 0.5 and 1 ML, respectively.     

EC-STM observation on electrochemical response of fluidic phospholipid monolayer on Au(1 1 1) modified with 1-octanethiol

Electrochemical scanning tunneling microscopy (EC-STM) was applied to observe phospholipid layers over thiol-modified gold substrates as a model biological cell membrane. On a monolayer of 1-octanethiol on Au (1 1 1), a synthetic lipid, 1,2-dihexanoyl-sn-glycero-3-phosphocholine, was introduced in a neutral 0.05 M NH₄ClO₄ buffer solution. The lipid molecules formed a fluidic layer at 0.0 V vs. RHE of the substrate electrode potential. By cycling the electrode potential between +0.2 V and −0.2 V, the lipid layer reversibly changed over between the fluidic phase and a striped/grainy structure. This structural change might involve partial decomposition and oligomerization of phospholipids. This method will contribute for molecular biology by revealing the nanometer-scale structure of cell membrane.     

STM tip-induced nanostructuring of Zn in an ionic liquid on Au(1 1 1) electrode surfaces

In this communication, the “jump-to-contact” based STM tip-induced nanostructuring is extended to BMIBF₄ ionic liquid for the first time. It is demonstrated successfully that Zn, as an example of less noble metal and being hard to deposit from aqueous solutions, can be nanostructured on Au(1 1 1) surfaces in the ionic liquid. Due to the large effective tunnel barrier in the ionic liquid, the Z-pulse required to create Zn nanoclusters in ionic liquid is about twice as large as for Cu nanoclusters of similar size in aqueous solutions. Patterns as well as large-scale arrays consisting of 100 × 100 Zn nanoclusters have been produced. The present work demonstrates the feasibility for surface nanostructuring a new category of systems that have not been possible in aqueous solutions, which could open up new opportunities for studies of nanoscopic effects from various aspects.