Au Surface Layer Research Articles

Ultrathin Fe Films Sandwiched Between Au Layers: CEMS- study

The STM studies of Fe films with thickness above 0.6 ML do not contribute significantly to the solution of the structural problems, because of the Au surface layer, which hinders details of the Fe sub-surface layers. The Mossbauer spectroscopy appears in this case as an exceptional method, which has the ability to solve structural and magnetic problems. Our discussion is based on a simplified spectrum analysis, assuming a discrete character of the spectral components. In reality, a distribution of the hyperfine magnetic field, as well as isomer shift and the quadrupole splitting is an obvious and natural consequence of the finite size system 'presented by the monolayer film. The exact fitting of all hyperfine parameters distribution is too difficult. Most of the fitting procedures treat the distributions of IS vs. Bhf and QS vs. Bhf as linearly dependent as described using the well-known Voigt-based method.

Improving the surface layer structure in opal-like plasmonic-photonic crystals for efficient excitation of optical Tamm states

We investigate the transmission of light through three-dimensional structures in opal-like plasmonic-photonic crystals with an Au surface layer of different geometries. We study how the inclusion of buffer layers of various forms affects the spectral characteristics of the structures. Three-dimensional photonic crystals have significant advantages over one-dimensional ones since they display a complete photonic bandgap and porosity. Controlling the spectral characteristics of the hybrid plasmonic-photonic mode is an important task that offers the possibility of creating tunable devices based on such structures. In this work we show that the optical properties of structures with different shapes of the surface layer do not depend on light polarization, except for structures with a corrugated Au layer and a corrugated buffer layer. The bandwidth of a structure with a corrugated Au layer is smaller and shows polarization sensitivity in contrast to a structure with a smooth Au layer. The optical characteristics of hybrid structures undergo significant changes when buffer layers of various shapes are introduced between the photonic crystal and the Au layer. The width of the peak in the transmission spectra changes and its intensity increases against the background after the improvement of the structure of the plasmonic-photonic crystal. Thus, opal-like plasmonic-photonic crystals with different structure of the surface layer have spectral features that can be used in integrated circuits, biosensors, and resonators. • The inclusion of buffer layers of various forms affects the spectral characteristics of the plasmonic-photonic crystal. • The optical properties of different structures do not depend on light polarization. • As an exception, the bandwidth of a structure with a corrugated Au&buffer layers shows polarization sensitivity. • The width and intensity of the peak changes after the improvement of the plasmonic-photonic crystal.

Adlayer growth vs spontaneous (near-) surface alloy formation: Zn growth on Au(111).

As part of an extensive effort to explore the function of Au/ZnO catalysts in the synthesis of methanol from CO2 and H2, we have systematically investigated the temperature dependent growth, structure formation, and surface intermixing of Zn on the herringbone reconstructed Au(111) surface and the thermal stability of the resulting surfaces by scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). After Zn deposition at low temperatures, at about 105 K (STM) or below (XPS), we observed nucleation and two-dimensional growth of Zn islands mainly at the elbow sites of the Au(111) herringbone reconstruction. This results in local perturbations of the reconstruction pattern of the Au(111) substrate, which can create additional nucleation sites. XPS data indicate that Zn dissolution into deeper layers is kinetically hindered under these conditions, while local exchange with the Au surface layer, in particular at the elbow sites during nucleation, cannot be excluded. Zn deposition at room temperature, in contrast, results in near-surface alloy formation with a strongly distorted pattern of the herringbone reconstruction and condensation of the Zn and exchanged Au adatoms at ascending steps, together with some loss of Zn into deeper layers. Upon annealing, Zn atoms diffuse to lower layers and eventually to the Au bulk, and the surface successively regains its original Au(111) herringbone structure, which is almost reached after 500 K annealing. Compared with previous reports on the growth of other metals on Au(111), Zn shows a rather high tendency for intermixing and near-surface alloy formation.

Aptamer-based sensor for quantitative detection of mercury (II) ions by attenuated total reflection surface enhanced infrared absorption spectroscopy

A sensing platform based on the attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) technique and immobilized aptamer has been proposed herein for the selective detection of mercury ions (Hg2+). In the proposed platform, 5′ thiolated 32-mer DNA probes with methylene blue at the 3′ end were immobilized on a thin gold (Au) surface layer. Following Hg2+ ions interacting with T bases of the aptamer, T-Hg-T bonds are formed; resulting in a hairpin-shaped formation of the DNA and a detectable change in the IR absorbance of the sensing interface. Notably, the background noise produced by external molecules (e.g., water, non-specific binding molecules and bulk solution) is reduced to a negligible level by means of the ATR detection mode. It is shown that the proposed sensor has a linear response (R2 = 0.986) with high sensitivity and good selectivity over the Hg2+ range of 0.01 μM–50 μM.

Synthesis, structure, and properties of a Au/MnO x –CeO2 nanocatalyst for low-temperature oxidation of carbon monoxide

We have synthesized Au/MnOx–CeO2 nanocatalysts for the low-temperature oxidation of carbon monoxide. Gold nanoparticles applied by the deposition precipitation (DP) method were used as an active phase. The composition, structure, and textural characteristics of the materials and the charge state of the components of the catalysts were studied using X-ray diffraction, X-ray photoelectron spectroscopy, highresolution transmission electron microscopy, inductively coupled plasma mass spectrometry, and low-temperature nitrogen adsorption measurements. The carbon monoxide concentration in the catalytic oxidation products was determined by gas chromatography. The influence of calcination temperature on the charge state of the components of the surface layer of Au/ MnOx–CeO2 and the catalytic activity of the materials was examined. The catalytic activity of the materials was shown to be determined to a significant degree by the Mn3+, Au3+, and weakly bound oxygen concentrations in the surface layer.

Geometry and Electronic Properties of Glycerol Adsorbed on Bare and Transition-Metal Surface-Alloyed Au(111): A Density Functional Theory Study

Glycerol exists in large amounts owing to its role as a byproduct in biodiesel production, and thanks to its chemical composition, it can be converted into more high-value products, such as mono- and polyethers, esters, diols, acrolein, and others. Hence, predicting glycerol-reactive properties is of utmost importance for designing efficient catalytic processes for its selective (electro)catalytic transformations; however, such an understanding is still far from complete. In this work, we performed quantum chemical calculations to validate a range of dispersion-corrected functionals to accurately predict and interpret structural, electronic, and vibrational properties of glycerol adsorbed on bare and transition-metal surface-alloyed Au(111) surface. optB86b-vdW (van der Waals) was found to have the overall best agreement with experiments concerning lattice constant, bulk stress, surface energy, and methanol adsorption among PBE (Perdew–Burke–Ernzerhof), optB88-vdW, optPBE-vdW, vdW-DF (density functional), vdW-DF2 (density functional 2nd version), and vdW-BEEF (Bayesian error estimation functional). Glycerol adsorption energy is found to correlate well with the calculated d-band center of the transition-metal-containing Au(111) surface layer. O–H stretching vibrations are found to be very sensitive of the surface-alloy atom and resulted in large shifts toward lower wavenumbers, when compared to those on bare Au(111). The latter results clearly show that adsorption of glycerol to surface-alloy atoms can be monitored in situ by infrared spectroscopy.

Influence of adsorbates on the segregation properties of Au–Pd bimetallic clusters

In this work, we investigate the surface segregation phenomena of 923-atom Au–Pd bimetallic clusters upon CO and O adsorption by using Monte Carlo simulations. The metal–metal interaction was modeled by the Johnson’s embedded atom method (EAM) and the metal–adsorbate interaction potential was developed by density functional theory (DFT) calculations. It is found that Pd atoms would like to occupy the surface sites for Au–Pd clusters upon adsorption, which is different to the surface segregation of Au on the surface of free Au–Pd clusters. This phenomenon may be due to a competition between surface energies of Au and Pd metals and their interaction strength with CO and O for the clusters upon adsorption. It is expected that our results can be used to understand and explain the experimental results of Pd atoms occupying the surface layer of Au–Pd bimetallic clusters in reactive environments.

The influence of halides on the initial selective dissolution of Cu3Au (1 1 1)

We report an electrochemical in situ X-ray diffraction (XRD) and ex situ atomic force microscopy (AFM) study on the influence of the halide additives bromide and iodide in comparison to chloride in 0.1M H2SO4 solution on the selective dissolution of Cu3Au (111). Our results suggest that the addition of these halides have a pronounced effect on the dealloying process and lead to peculiar changes in the resulting surface morphologies. For an additive of bromide we observed (similar to an earlier study with chloride) a sequence of specific Au surface layers with an initial ultra-thin stacking-inverted Au-rich layer, subsequent pure Au islands, and finally substrate oriented nanometer-scale ligaments (nanoporosity). These structures occur with a negative shift in the transformation potentials compared to pure sulfuric acid solution. With an additive of iodide no stable passive Au film is formed and a porous surface was observed at relatively short times and low potentials. A precipitation of copper iodide (CuI) visible in the ex situ AFM images leads to the formation of surface porosity with bimodal length scales.

Growth of Highly Single Crystalline IrO2 Nanowires and Their Electrochemical Applications

We present the facile growth of highly single crystalline iridium dioxide (IrO2) nanowires on SiO2/Si and Au substrates via a simple vapor phase transport process under atmospheric pressure without any catalyst. Particularly, high-density needle-like IrO2 nanowires were readily obtained on a single Au microwire, suggesting that the melted surface layer of Au might effectively enhance the nucleation of gaseous IrO3 precursors at the growth temperature. In addition, all the electrochemical observations of the directly grown IrO2 nanowires on a single Au microwire support favorable electron-transfer kinetics of [Fe(CN6)]4–/3– as well as Ru(NH3)63+/2+ at the highly oriented crystalline IrO2 nanowire surface. Furthermore, stable pH response is shown, revealing potential for use as a miniaturized pH sensor, confirmed by the calibration curve exhibiting super-Nernstian behavior with a slope of 71.6 mV pH–1.

DFT study of the M segregation on MAu alloys (M = Ni, Pd, Pt) in presence of adsorbed oxygen O and O2

Segregation phenomena of group 10 (M=Ni, Pd, Pt) transition-metals substituted in Au (111) surface and sub-surface layers are investigated by DFT periodic calculations in presence of adsorbed atomic and molecular oxygen. In contrast with vacuum conditions, where the metal impurities M prefer to be in the bulk of gold, in the presence of adsorbed O or O2, the impurities mainly segregate to the surface. The analysis of oxygen adsorption trends and electronic surface structures explain the change in the local atomic arrangement which is expected to occur on the surface of alloys during reaction conditions.

Selective Binding and Detection of Magnetic Labels Using PHR Sensor via Photoresist Micro-Wells

We have developed a novel platform for selective binding of magnetic labels on planar Hall resistance sensor (PHR) for biosensing applications. The photoresist (PR) micro wells were prepared on the PHR sensor junctions to trap the magnetic bead at specified locations on the sensor surface and thin layer of Au was sputtered in the PR wells immobilize bimolecular. The Au surface is functionalized with single-stranded oligonucleotide and further biotin was used to immobilize streptavidin coated magnetic labels (Dynabeads Myone 1.0 microm, Invitrogen Co.). After removal of the PR wells on the sensor surface the non specific binding magnetic labels were successfully removed and only the chemically bounded magnetic labels were remained on the Au surface for detection of biomolecules using PHR sensor. We controlled the number of magnetic labels on the PHR sensor surface by using different sizes of the PR well on the junctions. The specifically bounded magnetic labels were successfully detected by characterizing the individual PHR sensor junctions. This technique enables the complete control over the magnetic labels for selective binding of biomolecules on the sensor surface for increasing the sensitivity of the PHR sensor as well as removal of the non specific bindings on the sensor surface.

Nanoscale Electrodeposition of Ga on Au(111) from Ionic Liquids

Electrocrystallization of Ga on Au(111) in two different ionic liquid electrolytes has been investigated by in situ electrochemical STM imaging with partly nanometer resolution. In chlorogallate-containing melts underpotential deposition (UPD) leads to surface alloying whereby a Ga−Au surface layer is formed on top of Au(111). In AlCl3−[C4mim]Cl + 5 mM GaCl3 melts, 2D-electrochemical phase formation is driven by conventional nucleation and growth of 2D Ga islands. Prior to the UPD processes anion adsorption of GaCl4− and AlCl4−, respectively, occurs which is characterized by well-ordered anion layer structures with comparable lattice constants. At potentials negative of the UPD region, 3D Ga clusters are formed. On the basis of their shape and the coexistence of small and large clusters in contact, it is concluded that the clusters in the size range 5−30 nm are solid. Taking into account that the temperatures during electrodeposition ranged up to 313 K—above the melting point of α-Ga of 303 K—this observation is remarkable.

2D-isothermal compressibility of deposited gold layer on Au(1 1 1) surface

Based on thermodynamic arguments, a simple formula is derived that relates the 2D-isothermal compressibility, κ 2D, directly to ( ∂L D / ∂E) T and ( ∂L D / ∂N) T , where L D is the stripe separation of the Au(1 1 1) surface, E is the applied electrode potential and N is the surface concentration of deposit. It accounts for the deposition-induced compression of the top most Au(1 1 1) surface layer during gold deposition. Furthermore, the value of κ 2D for the deposited gold layer on Au(1 1 1) surface has been experimentally determined. The results reveal that the κ 2Dvalue for gold does not match the reported trend for other metals.

Alloy formation in the Au{1 1 1}/Ni system – An investigation with scanning tunnelling microscopy and medium energy ion scattering

Using the techniques of scanning tunnelling microscopy (STM) and medium energy ion scattering (MEIS), we examine the growth and annealing behaviour of ultrathin Ni films on Au{1 1 1} at 300 K. As has been shown previously, submonolayer growth of Ni on Au{1 1 1} is strongly influenced by the presence of the herringbone reconstruction with two-dimensional clusters nucleating at herringbone elbows. Second layer growth commences prior to the completion of the monolayer. After multiple layers have been deposited, the surface morphology retains a similar cluster-like appearance. Annealing produces surfaces exhibiting long range Moiré structures and, at higher temperature, triangular misfit dislocations. We use MEIS to examine the composition and structure of these surface alloy phases and conclude that in each case, they consist of an essentially pure Au surface layer on a bimetallic second layer.

Composition and Order-Disorder Transition in the Cu3Au (001) Surface Layer Investigated with the Use of DAES and DEPES

Composition and Order-Disorder Transition in the Cu₃Au (001) Surface Layer Investigated with the Use of DAES and DEPES

Modified herringbone reconstruction on Au(1 1 1) induced by self-assembled Azure A islands

Upon submonolayer deposition of Azure A molecules onto reconstructed Au(1 1 1), two-dimensional quasi-rectangular islands are observed by low-temperature scanning tunneling microscopy. Strikingly, the 22 × √3 reconstruction with alternating hexagonal close packed (hcp) and face centered cubic (fcc) domains of the native Au(1 1 1) surface layer is found to be strongly modified underneath the self-assembled islands of Azure A molecules. The bridge–hcp–bridge–fcc–bridge period of 63 Å on clean Au(1 1 1) is expanded by about 12% and occasionally reaches even 22%. This increase of the unreconstructed fcc areas underneath the molecular islands reflects a partial lifting of the reconstruction caused by the interaction between the adsorbed molecules and the outermost Au atoms at the molecule–metal interface.

Preparation and characterisation of platinum- and gold-coated copper, iron, cobalt and nickel deposits on glassy carbon substrates

Abstract Pt- and Au-coated Cu, Fe, Co and Ni deposits have been formed on glassy carbon (GC) substrates by electrodeposition of controlled amounts of the core metal onto the substrate and its subsequent partial replacement by Pt or Au upon immersion into a chloroplatinic or chlorolauric solution. This process resulted in a thin Pt or Au shell over the bimetallic particle as electrochemical evidence suggests, while indicative sputter-etch Auger Electron Spectroscopy points to the coexistence of the two metals in the particle core. SEM/EDS characterisation of the deposits revealed the existence of Pt(M) and Au(M) extensively agglomerated nanoparticles (M: Cu, Fe, Co, Ni). The surface electrochemistry of the deposits (hydrogen adsorption/desorption on Pt and oxide formation/stripping on Au) proved the complete coverage of the bimetallic particles by Pt or Au and allowed an estimate of their electroactive surface area. The study of hydrogen evolution at these deposits points to a modification of the electronic properties of the Pt and Au surface layers by the core metal (due to strain effects and/or ligand–electronic interactions) and further confirmed that these layers form a very thin outer shell.

In situ diffraction studies of electrode surface structure during gold electrodeposition

Surface X-ray scattering (SXS) in transmission geometry provides a valuable tool for in situ structural studies of electrochemical interfaces under reaction conditions, as illustrated here for homoepitaxial electrodeposition on Au(1 0 0) and Au(1 1 1) electrodes. Employing diffusion-limited deposition conditions to separate the effects of potential and deposition rate, a mutual interaction between the interface structure and the growth behavior is found. Time-dependent SXS measurements during Au(1 0 0) homoepitaxy show with decreasing potential transitions from step flow to layer-by-layer growth, then to multilayer growth, and finally back to layer-by-layer growth. This complex growth behavior can be explained within the framework of kinetic growth theory by the effect of potential, Cl adsorbates and the Au surface structure, specifically the presence of the surface reconstruction, on the Au surface mobility. Conversely, the electrodeposition process influences the structure of the reconstructed Au surface, as illustrated for Au(1 1 1), where a significant deposition-induced compression of the Au surface layer as compared to Au(1 1 1) surfaces under ultrahigh vacuum conditions or in Au-free electrolyte is found. This compression increases towards more negative potentials, which may be explained by a release of potential-induced surface stress.

Effects of spontaneous surface buckling on the diffraction performance of an Au-coated elastomeric grating

Stretchable silicone gratings show promise as dynamic beam scanners but are usually limited to transmission applications because of the low reflectivity of the material. We modify an elastomeric replicated grating for use in reflection geometries by adding a 100 nm surface layer of Au. Diffraction efficiency in the sample increases by a factor of 50. Scanning of the first-order diffraction is performed by stretching the grating in one direction, with Δ θ = 2.34° for normal incidence at a maximum applied strain of 7.6%. Distortion of the deflected beam is minimal. Buckles on the Au surface perpendicular to the grating grooves spontaneously appear in response to compressive stress resulting from elongation. Diffraction efficiency decreases with increasing strain. This is attributed to reflectivity loss and secondary diffraction from the buckling structures. The initial efficiency and its variation with strain are dependent on the specific area of the grating being used. We are able to describe the observed behavior of efficiency vs. strain in terms of an inverse relationship between efficiency and compressive stress.

Electrocompression of the Au(111) Surface Layer during Au Electrodeposition

In situ grazing-incidence x-ray diffraction studies of reconstructed Au(111) electrodes in aqueous electrolyte solutions are presented, which reveal a significantly increased compression of the Au surface layer during Au electrodeposition as compared to Au(111) surfaces under ultrahigh vacuum conditions or in the Au-free electrolyte. The compression increases towards more negative potentials, reaching 5.3% at the most negative potentials studied. It may be explained within a simple thermodynamic model by a release of potential-induced surface stress.