Noble Metal Surfaces Research Articles

Exploring the bonding of large hydrocarbons on noble metals: Diindoperylene on Cu(111), Ag(111), and Au(111)

We present a benchmark study for the adsorption of a large pi-conjugated organic molecule on different noble metal surfaces, which is based on X-ray standing wave (XSW) measurements and density functional theory calculations with van der Waals (vdW) interactions. The bonding distances of diindenoperylene on Cu(111), Ag(111), and Au(111) surfaces (2.51 A, 3.01 A, and 3.10 A, respectively) determined with the normal incidence XSW technique are compared with calculations. Excellent agreement with the experimental data, i.e. deviations less than 0.1 A, is achieved using the Perdew-Burke-Ernzerhof functional with vdW interactions that include the collective response of substrate electrons (PBE+vdW^{surf} method). Noteworthy, the calculations show that the vdW contribution to the adsorption energy increases in the order Au(111) < Ag(111) < Cu(111).

Five-vertex Archimedean surface tessellation by lanthanide-directed molecular self-assembly

The tessellation of the Euclidean plane by regular polygons has been contemplated since ancient times and presents intriguing aspects embracing mathematics, art, and crystallography. Significant efforts were devoted to engineer specific 2D interfacial tessellations at the molecular level, but periodic patterns with distinct five-vertex motifs remained elusive. Here, we report a direct scanning tunneling microscopy investigation on the cerium-directed assembly of linear polyphenyl molecular linkers with terminal carbonitrile groups on a smooth Ag(111) noble-metal surface. We demonstrate the spontaneous formation of fivefold Ce-ligand coordination motifs, which are planar and flexible, such that vertices connecting simultaneously trigonal and square polygons can be expressed. By tuning the concentration and the stoichiometric ratio of rare-earth metal centers to ligands, a hierarchic assembly with dodecameric units and a surface-confined metal-organic coordination network yielding the semiregular Archimedean snub square tiling could be fabricated.

Optical Properties of ZnO Nanowires Decorated with Au Nanoparticles

One of the key technologies in future optoelectronics is control of excitons in oxide materials by the coupling with plasmons on noble metal surfaces. Optical properties of ZnO nanowires decorated with Au nanoparticles were studied to understand fundamental mechanism of the coupling and to develop optoelectronic devices with new functionalities. Light intensity at the main peak position in the photoluminescence (PL) spectra of ZnO nanowires was enhanced with the coverage of Au nanoparticles. Lifetime of excitons excited optically decreased by the decoration of Au nanoparticles. Understanding of the coupling between excitons and plasmons leads to optical control of excitons and will pave the way for new type of optoelectronic devices.

Effects of the atomic level shift in the Auger neutralization rates of noble metal surfaces

In this work we compare characteristics of Auger neutralization of He+ ions at noble metal and free-electron metal surfaces. For noble metals, we find that the position of the energy level of He with respect to the Fermi level has a non-negligible influence on the values of the calculated Auger rates through the evaluation of the surface dielectric susceptibility. We conclude that even though our calculated rates are accurate, further theoretical effort is needed to obtain realistic values of the energy level of He in front of these surfaces.

Scattering of surface electrons by isolated steps versus periodic step arrays

We investigate the scattering of electrons belonging to Shockley states of (111)-oriented noble metal surfaces using angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM). Both ARPES and STM indicate that monatomic steps on a noble metal surface may act either as strongly repulsive or highly transmissive barriers for surface electrons, depending on the coherence of the step lattice, and irrespectively of the average step spacing. By measuring curved crystal surfaces with terrace length ranging from 30 to 180 \AA{}, we show that vicinal surfaces of Au and Ag with periodic step arrays exhibit a remarkable wave function coherence beyond 100 \AA{} step spacings, well beyond the Fermi wavelength limit and independently of the projection of the bulk band gap on the vicinal plane. In contrast, the analysis of transmission resonances investigated by STM shows that a pair of isolated parallel steps defining a 58 \AA{} wide terrace confines and decouples the surface state of the small terrace from that of the (111) surface. We conclude that the formation of laterally confined quantum well states in vicinal surfaces as opposed to propagating superlattice states depends on the loss of coherence driven by imperfection in the superlattice order.

Interpretation of valence band photoemission spectra at organic-metal interfaces

Adsorption of organic molecules on well-oriented single crystal coinage metal surfaces fundamentally affects the energy distribution curve of ultra-violet photoelectron spectroscopy spectra. New features not present in the spectrum of the pristine metal can be assigned as "interface states" having some degree of molecule-substrate hybridization. Here it is shown that interface states having molecular orbital character can easily be identified at low binding energy as isolated features above the featureless substrate sp-plateau. On the other hand much care must be taken in assigning adsorbate-induced features when these lie within the d-band spectral region of the substrate. In fact, features often interpreted as characteristic of the molecule-substrate interaction may actually arise from substrate photoelectrons scattered by the adsorbates. This phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large pi-conjugated organic molecules.

Hybridisation at the organic–metal interface: a surface-scientific analogue of Hückel's rule?

We demonstrate that cyclooctatetraene (COT) can be stabilised in different conformations when adsorbed on different noble-metal surfaces due to varying molecule-substrate interactions. While at first glance the behaviour seems to be in accordance with Hückel's rule, a theoretical analysis reveals no significant charge transfer. The driving mechanism for the conformational change is hybridisation at the organic-metal interface and does not necessitate any charge transfer.

Influence of Step Defects on Methanol Decomposition: Periodic Density Functional Studies on Pd(211) and Kinetic Monte Carlo Simulations

Methanol decomposition on noble metal surfaces is an important industrial process and prototype for understanding heterogeneous catalysis. Despite many advances, the role played by surface defects and structural sensitivity is still not fully understood. In this work, methanol decomposition on a stepped palladium surface, Pd(211), is investigated using periodic density functional theory (DFT). The activation barriers and thermochemistry for relevant elementary steps leading to the final decomposition products CO and H2 are obtained. Similar to the previous theoretical results on flat Pd surfaces, the initial C–H bond scission is preferred on Pd(211) because it has a lower barrier than those for the initial O–H and C–O scissions. It was also found that the barriers for the C–H or O–H bond scissions are lowered at the step sites. Finally, kinetic Monte Carlo simulations on a realistic Pd surface reproduce the temperature-programmed desorption spectrum for methanol decomposition but only when modified DFT da...

Conventional and acoustic surface plasmons on noble metal surfaces: A time-dependent density functional theory study

First-principles calculations of the conventional and acoustic surface plasmons (CSPs and ASPs) on the (111) surfaces of Cu, Ag, and Au are presented. The effect of $s-d$ interband transitions on both types of plasmons is investigated by comparing results from the local density approximation and an orbital dependent exchange-correlation (xc) potential that improves the position and width of the $d$ bands. The plasmon dispersions calculated with the latter xc-potential agree well with electron energy loss spectroscopy (EELS) experiments. For both the CSP and ASP, the same trend of Cu$<$Au$<$Ag is found for the plasmon energies and is attributed to the reduced screening by interband transitions from Cu, to Au and Ag. This trend for the ASP, however, contradicts a previous model prediction. While the ASP is seen as a weak feature in the EELS, it can be clearly identified in the static and dynamic dielectric band structure.

Stability of reference masses V: UV/ozone treatment of gold and platinum surfaces

The stability of reference masses has been a long-standing concern within the SI. More recently the requirements of potential non-artefact realizations of the kilogram have added gold and its alloys to the platinum alloys that have historically been the focus of attention.Previously we proposed UV/ozone cleaning of standard-mass surfaces to improve stability with respect to carbonaceous contamination. Since then both NPL and BIPM have constructed prototypes of UV/ozone apparatus for cleaning kilogram standards. We have therefore tested a combined solvent wash and UV/ozone procedure, UVOPS or ‘UV/ozone with pre-wash and stabilization’, on both platinum and gold surfaces. X-ray photoelectron spectroscopy (XPS) shows this to be very successful in removing even gross contamination from these noble metal surfaces. Oxidation is negligible, limited to the outermost layer of noble metal atoms, and terminates at a level that is within the uncertainty of mass comparisons at the 1 kg level. This oxide has been seen in some earlier XPS studies but not others—we show that decomposition of the oxide by x-rays at XPS energies may be responsible for this disparity.

Modifying the Cu(111) Shockley surface state by Au alloying

The deposition of submonolayer amounts of Au onto Cu(111) results in a Au-Cu surface alloy with temperature- and thickness-dependent stoichiometry. Upon alloying, the characteristic Shockley state of Cu(111) is modified, shifting to 0.53 eV binding energy for a particular surface Au${}_{2}$Cu concentration, which is a very high binding energy for a noble-metal surface. Based on a phase accumulation model analysis, we discuss how this unusually large shift is likely reflecting an effective increase in the topmost layer thickness of the order of, but smaller than, the value expected from the moir\'e undulation.

Fabrication of a single-atom electron source by noble-metal surface diffusion

For application as single-atom emitters, the authors have developed a new method for preparing atomic-scale pyramids with three {211}-facet sides (nanopyramids). In the new method, palladium covers the “backward” area of the tungsten tip, approximately 1 mm from the sharpened end, rather than the end itself. The palladium was deposited via surface diffusion promoted by elevating the temperature. Field ion microscopy exhibited typical signs indicating that, with the added annealing, identical nanopyramids grew spontaneously. The field emission characteristics of these nanopyramids were investigated and compared with those produced by the two established preparation methods. The authors found that field emission patterns for single-atom tips were narrow circles at low extractor voltages and three-pronged stars, indicating the presence of three ridges of the nanopyramid, at high voltages. The patterns are the same for tips prepared by whichever method. As for field emission stability, clear differences were also not seen across the preparation methods, except for a minute difference in fluctuation ratios obtained from time-dependent current data.

Calculating a local oscillating quasiparticle’s lifetime at screened defects in solids

We report on a novel approach to calculate quasiparticle lifetimes of localized initial states, which decay into the continuum of underlying quasi-free quasiparticle states in the vicinity of point defects and steps in solids. By using this interpretation of the inelastic damping of wavefunctions, the lifetime becomes a local property. In particular, we consider electrons, which are injected by a scanning tunneling microscope tip into the surface state of a noble metal surface. We investigate numerically the configuration of a single scatterer, a chain of scatterers, and a triangular quantum corral. As compared to an exponential increase of the damping from prior theories, we find an oscillating damping together with a linear background of the resulting measurement signal. The different configurations show increased lifetimes with increasing dimensionality as their scattering phase space is decreased.

Photoemission Study of the Morphology and Barrier Heights at the Interface between Benz[a]anthracene and Noble Metal (111) Surfaces

Various coverages of benz[a]anthracene (BA) thin films prepared by depositing the molecules in several steps on Au(111) and Ag(111) were investigated using ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). XPS and UPS were taken after each deposition. A Schottky junction formed between BA/Au(111) and BA/Ag(111). A hole barrier height of 1.89 eV in BA/Ag(111) was determined directly from the observed highly occupied molecular orbital (HOMO) peak cutoff however a HOMO peak cutoff in BA/Au(111) 1.63 eV was indirectly determined. When combined with previous measurements of the BA/Cu(111) interface, the slope S of the plot of hole barrier height as a function of metal workfunction (WF) was calculated to be 0.39, which is close to the S value of pentacene. Discussion on film growth mode, band bending, band diagram, and comparisons of interface parameter S of BA with S of other molecules condensed on metal surfaces are presented.

(2√3×2√3)R30∘induced self-assembly ordering by C60on a Au(111) surface: X-ray diffraction structure analysis

The (2$\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}2\ensuremath{\surd}3)R$30\ifmmode^\circ\else\textdegree\fi{} ordering of fullerene on the herringbone reconstructed Au(111) surface has been studied using grazing incidence x-ray diffraction. The interaction between C${}_{60}$ molecules and gold atoms removes the initial herringbone reconstruction, typical of the clean Au(111) surface, and models the topmost substrate surface layer with one atom vacancy per unit cell and C-Au distances equal to 2.49 \AA{}. The two topmost gold layers are slightly compressed. The molecule is sited on top of the vacancy and oriented with one of its hexagonal carbon rings parallel to the surface, optimizing the number of bonds between the molecule and the substrate with bond distances typical of bonds with a mixed covalent/ionic character. This configuration, that favors the formation of carbon-gold bonds, is similar to the one observed for C${}_{60}$/Pt(111) and C${}_{60}$/Ag(111) indicating that the adsorption of fullerenes on the (111) surface of noble metals comes with the formation of vacancies in order to minimize the system energy.

Preparation of Pd-based intermetallic targets for high intensity irradiations

A new method of intermetallic target preparation is described. Based on the molecular plating technique followed by “coupled reduction”, this method allows producing stable and homogeneous metallic targets for high intensity irradiations. In the first step, the target material is electroplated on a noble metal surface, ensuring homogeneous distribution of the desired element on the target backing. In the second step, the foil with the plated material is heated in a hydrogen flow. Due to the formation of an intermetallic compound with the noble metal support, reduction of the target material with hydrogen at high temperatures becomes thermodynamically possible.Nitrates of all six investigated elements (U, Th, Am, Gd, Nd, and Eu) were electroplated with excellent yields on Pd backing foils by the molecular plating technique and completely reduced in hydrogen atmosphere. A homogeneous distribution of the target material over the whole thickness of the Pd foil was observed suggesting a pronounced diffusion of the reduced metals into the backing material already during the reduction process. A first test irradiation experiment with a thin 3.5μm U/Pd intermetallic target is described.

Simultaneous Photoluminescence and SERS Observation of Nanodiamond at Laser Deposition on Noble Metals

The metal-modified luminescence and surface-enhanced Raman scattering (SERS) occurring near nanostructured surfaces of noble metals recently have been observed for different kinds of nanocrystals associated with the metal nanostructures. In the present work, the photoluminescence and Raman scattering of diamond nanocrystals of sizes 100 and 300 nm patterned on Ag and Au thin nanostructured films via laser accelerated deposition using a femtosecond laser are discussed. The laser accelerated deposition forms ordered periodical nanodiamond–metal nanostructures and allows adjusting the interaction between nanodiamond and metal by varying the laser acceleration parameters as well as by using different metals (Ag and Au), and varying the structure of the metal film. Correspondingly, the spectroscopic properties of the system determined by interaction between nanoparticles and metal are tuned. The enhancement of nanodiamond photoluminescence together with SERS of graphite fraction and disordered carbon of nanodiamonds are observed for nanodiamond–Ag structures at 488- and 532-nm excitations, while for the nanodiamond–Au structure some characteristic SERS effects are observed at 785-nm excitation. The mechanisms of enhancement are discussed considering the nanodiamond–metal interaction and laser acceleration effect on nanodiamond.

Theoretical predictions of properties of group-2 elements including element 120 and their adsorption on noble metal surfaces

Trends in properties of group-2 elements Ca through element 120 and their M(2) and MAu dimers were determined on the basis of atomic and molecular relativistic density functional theory calculations. The relativistic contraction and stabilization of the ns AO with increasing atomic number were shown to result in the inversion of trends both in atomic and molecular properties in group 2 beyond Ba, so that element 120 should be chemically similar to Sr. Due to the same reason, bonding in (120)(2) and 120Au should be the weakest among the considered here M(2) and MAu. Using calculated dissociation energies of M(2), the sublimation enthalpy, ΔH(sub), of element 120 of 150 kJ/mol was estimated via a correlation between these quantities in group 2. Using the M-Au binding energies, the adsorption enthalpies, ΔH(ads), of element 120 of 172 kJ/mol on gold, 127 kJ/mol on platinum, and 50 kJ/mol on silver were estimated via a correlation with known ΔH(ads) in the group. These moderate values of ΔH(ads) are indicative of a possibility of chromatography adsorption studies of element 120 on these noble metal surfaces.

Method for calculating the contribution of inelastic electron-electron scattering lifetimes of electronic states on (110) noble metal surfaces

A method for calculating the decay rate of surface states on the (110) noble metal surfaces within the GW approximation is described in detail. The band structure is described by a two-dimensional pseudo-potential model. As an example, the calculated lifetimes of surface states at the $ \bar{Y} $ point on Cu(110) are presented. The calculated lifetime of the occupied state is approximately 2 times greater than that of the unoccupied state. The result obtained for the occupied surface state is in good agreement with the photoemission data.

Gold nanoparticle-coated Ni/Al layered double hydroxides on glassy carbon electrode for enhanced methanol electro-oxidation

We report a facile electrochemical strategy for the synthesis of Ni/Al layered double hydroxides (LDHs) and gold nanoparticle (AuNPs)-coated glassy carbon electrode (GCE). The new electrode is named LDH/AuNPs/GCE. The new electrode is named LDH/AuNPs/GCE. The electrocatalytic activity of LDH/AuNPs toward methanol electro-oxidation was studied by cyclic voltammetry and chronoamperometry. Compared to the Ni/Al-LDH modified GCE without AuNPs film (LDH/GCE), the LDH/AuNPs/GCE exhibits remarkably higher catalytic activity for methanol electro-oxidation, e.g. the lower oxidation potential (0.57 V vs. SCE) and the higher current density (6-fold). The enhancement may be attributed to the higher electrocatalytic activity of Ni/Al-LDHs in the presence of AuNPs, the synergy effect between them, or both. The results presented here may be of broad interest not only for developing fuel cells but also for understanding of OH− electro-generated on noble metal surfaces.