Noble Metal Surfaces Research Articles

Norepinephrine: material-independent, multifunctional surface modification reagent.

A facile approach for material-independent surface modification using norepinephrine was investigated. pH-induced oxidative polymerization of norepinephrine forms adherent films on vastly different types of material surfaces of noble metals, metal oxides, semiconductors, ceramics, shape-memory alloys, and synthetic polymers. Secondary biochemical functionalizations such as immobilization of proteins and growth of biodegradable polyester on the poly(norepinephrine) films were demonstrated.

First-principles study of benzene on noble metal surfaces: Adsorption states and vacuum level shifts

We have studied the interaction of benzene with Cu(1 1 1), Ag(1 1 1) and Au(1 1 1) surfaces using density functional theory (DFT) within a generalized gradient approximation (GGA) and the van der Waals density functional [vdW-DF; M. Dion, H. Rydberg, E. Schröder, D.C. Langreth, B.I. Lundqvist, Phys. Rev. Lett. 92 (2004) 246401]. The adsorption energies using vdW-DF are significantly more accurate than those using GGA, while the equilibrium adsorption distances between benzene and metal substrates ( Z C eq ) calculated by both GGA and vdW-DF are almost identical. The work function changes induced by the adsorption of benzene are significantly underestimated compared with the experimental values, as a result of the overestimation of Z C eq by both GGA and vdW-DF. Instead of determining the Z C eq values from first-principles calculations, we deduced the most probable adsorption distances in such a way as to reproduce the experimentally-observed work function changes. The deduced adsorption distance ( Z C ded ) is shortest on Cu(1 1 1) while it is longest on Ag(1 1 1), reflecting the strength of the interactions between benzene and the metal surfaces. It turns out that the substrate dependence of the work function change is mainly ascribed to the difference in the benzene–metal distance ( Z C). Charge transfer and work function changes by the adsorption of benzene were analyzed by means of the induced density of interface states (IDIS) model [H. Vázquez, R. Qszwaldowski, P. Pou, J. Ortega, R. Pérez, F. Flores, A. Kahn, Europhys. Lett. 65 (2004) 802], and compared with the self-consistent GGA calculations. The vacuum level shifts estimated by the IDIS model agree with the GGA results for Z C ⩾ 0.3 nm . On the other hand, the discrepancy between the two methods becomes larger for Z C ⩽ 0.3 nm , where the back donation from the metal substrates to the adsorbate becomes significant. We show that the IDIS model reasonably works well for benzene on Cu(1 1 1), Ag(1 1 1) and Au(1 1 1) surfaces because Z C ded ≈ 0.3 nm on all surfaces. However, our analysis reveals that the actual charge density redistribution induced by the adsorption of benzene is more complicated than that assumed in the IDIS model.

Influence of the substrate on the spin-orbit splitting in surface alloys on (111) noble-metal surfaces

We have studied by angle-resolved photoelectron spectroscopy the ordered XCu2 surface alloys formed by X=Sb and Bi on a Cu(111) substrate. We found clear analogies with the corresponding XAg2 alloys formed by the same elements on Ag(111). The electronic states near the Fermi level are similarly split by the spin-orbit (SO) interaction with the smaller splitting in XCu2 reflecting the smaller atomic SO parameter of Cu vs Ag. The charge transfer from the Bi and Sb adatoms to the substrate atoms and their outward relaxation are different for two substrates; both determine the Fermi energy of the two-dimensional electron gas.

Adenine− and Adenosine Monophosphate (AMP)−Gold Binding Interactions Studied by Surface-Enhanced Raman and Infrared Spectroscopies

Understanding the interactions of biomolecules with noble metal surfaces is critical to our development of functional biomedical nanodevices and accurate biosensors. Here we use surface enhanced Raman spectroscopy (SERS) and surface enhanced infrared absorption spectroscopy (SEIRA) on Au nanoshell substrates to study the interactions of adenine and two adenine derivatives, thiolated polyadenine single-stranded DNA (polyA) and adenosine monophosphate (AMP), with Au surfaces. pH-dependent conformational changes of these molecular species adsorbed on Au nanoshell surfaces were observed using SERS, and confirmed with SEIRA. The combined SERS-SEIRA spectra show significant pH dependence, consistent with adenine protonation and reduced Au−adenine binding at low pH. The spectra are also consistent with adenine binding “end-on” to the Au surface via a ring nitrogen, with the bond to the external NH2 group aligned near the surface normal. For AMP, spectral evidence indicates binding through either a ring nitrogen and/or the external NH2 group. Density functional calculations on adenine and comparisons with the literature allow us to assign the observed spectral features and to gain insight to the local binding geometry of the adsorbates.

Melting of Two-Dimensional Adatom Superlattices Stabilized by Long-Range Electronic Interactions

The melting transition of Ce adatom superlattices stabilized by long-range substrate-mediated electronic interactions on Cu(111) and Ag(111) noble metal surfaces has been investigated by low-temperature scanning tunneling microscopy, density functional theory calculations, and kinetic Monte Carlo simulations. Intriguingly, owing to the interaction between Ce adatoms and substrate, these superlattices undergo two-dimensional melting to a liquid without transition through the hexatic phase. The crucial parameters for this direct solid to liquid transition are identified.

Magnetic behavior of one- and two-dimensional nanostructures stabilized by surface-state electrons: a kinetic Monte Carlo study

Recent experiments have demonstrated that it is possible to create macroscopic-ordered one- and two-dimensional nanostructures on (111) noble metal surfaces exploiting long-range substrate-mediated interaction. Here, we report on the systematic theoretical studies of magnetic properties of these atomic structures in the externally applied magnetic field. The spin dynamics is investigated by means of the kinetic Monte Carlo method based on transition-state theory. For the characteristic values of (i) magnetic anisotropy energy of adatoms and (ii) exchange coupling between adatoms in the considered class of nanostructures, we reveal the hysteresis-like behavior at low temperatures (typically at 1–3 K).

Toward Robust High Resolution Chemical Imaging

Abstract Resonant plasmon excitations at the surface of noble metals can localize and amplify an electromagnetic field in a very small volume and are the enabling element of surface enhanced optical microscopies. Tip enhanced Raman spectroscopy (TERS) combines scanning probe microscopy (SPM) with Raman spectroscopy, taking advantage of this enhancing mechanism. So far a 20 nm lateral resolution in chemical imaging of a surface has been achieved. So far a 20 nm lateral resolution in chemical imaging of a surface has been achieved.

Study of Oxide Growth on Noble Metal Surfaces by Dynamic Electrochemical Impedance Spectroscopy

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Electron-hole spectra created by adsorption on metals from density functional theory

Nonadiabaticity in adsorption on metal surfaces gives rise to a number of measurable effects such as chemicurrents and exoelectron emission. Here we present a quantitative theory of chemicurrents on the basis of ground-state density functional theory (DFT) calculations of the effective electronic potential and the Kohn-Sham band structure. Excitation probabilities are calculated both for electron-hole pairs and for electrons and holes separately from first-order time-dependent perturbation theory. This is accomplished by evaluating the matrix elements (between Kohn-Sham states) of the rate of change of the effective electronic potential between subsequent (static) DFT calculations. Our approach is related to the theory of electronic friction, but allows for direct access to the excitation spectra. The method is applied to adsorption of atomic hydrogen isotopes on the Al(111) surface. The results are compatible with the available experimental data (for noble metal surfaces); in particular, the observed isotope effect in H versus D adsorption is described by the present theory. Moreover, the results are in qualitative agreement with computationally elaborate calculations of the full dynamics within time-dependent density functional theory, with the notable exception of effects due to the spin dynamics. Being a perturbational approach, the method proposed here is simple enough to be applied to a wide class of adsorbates and surfaces, while at the same time allowing us to extract system-specific information.

Adsorption of 3,4-ethylenedioxythiophene (EDOT) on noble metal surfaces: A photoemission and X-ray absorption study

The adsorption of 3,4-ethylenedioxythiophene (EDOT) on Au and Pt surfaces is studied by core level and valence band photoemission using synchrotron radiation and by near edge X-ray absorption spectroscopy. To closer simulate real applications, the films are grown from aqueous solution at room temperature and are compared to an ‘ideal’ film prepared by dosing the molecules from the vapour phase on a clean Au(1 1 1) single crystal. The S 2 p, C 1 s and O 1 s levels show multiple components which are associated to molecular fragmentation. NEXAFS confirms that fragmentation takes place at the surface. Thiophene species as well as alkyl chains and S n species are identified as the most probable fragmentation products.

Optimal strictly localized basis sets for noble metal surfaces

The properties of the (111) surfaces of Cu, Ag, and Au are evaluated using a first-principles approach with strictly localized basis sets. These surfaces present metallic and extended properties that are a priori difficult to describe with a local-basis approach. We explore methodologies to enhance the basis sets of the surface atoms in order to accurately describe surface properties such as surface energies, surface states, and work functions. In this way, the advantages of local-basis computations (namely, efficiency, optimum size scaling, and a natural description of bonding in real space) can be retained, while keeping the accuracy in the description of the properties of the surface at a very good level.

Co impurities on Ag and Cu: Kondo temperature dependence on substrate orientation

The surface Kondo effect of a single Co impurity adsorbed on different noble-metal surfaces is revisited. The magnetic system mapped onto the Anderson impurity model is solved within standard noncrossing approximation techniques using data coming from electronic structure calculations. The observed experimental trends of the Kondo temperature are explained in terms of the particular features of the hybridization function and of the $d$ impurity level position.

Electronic structure of [121]tetramantane-6-thiol on gold and silver surfaces

The mechanism for electron photoemission of [121]tetramantane and its functionalized compound [121]tetramantane-6-thiol adsorbed on different noble metal surfaces has been investigated by density functional theory calculations. It is found that good chemical bonding between molecules and metal surfaces is a helpful but not a necessary condition for electron photoemission. A lower work function and weaker hybridization between the molecule and the metal could lead to much more efficient electron photoemission. It is observed that, neglecting final state effect, a simple ground state picture cannot result in negative electron affinity for the systems under investigation. Calculations have shown that by exciting an electron in the lowest unoccupied molecular orbital, the highest singly occupied molecular orbital of the molecule can be shifted above the vacuum level, resulting in negative electron affinity and emission of the accumulated electrons.

Binding Energy of Adsorbates on a Noble-Metal Surface: Exchange and Correlation Effects

We discuss the adsorption of xenon and of PTCDA on the silver (111) surface within a first-principles approach, focusing on the adsorbate-substrate interaction energy as a function of distance. We combine exact exchange with correlation energy from the adiabatic-connection fluctuation-dissipation theorem. At a large distance Z from the surface, the correlation causes a van der Waals attraction [approximately -C3/(Z - Z0)3]. At a closer distance, the attraction deviates from its asymptotic form and, combined with the repulsive exact-exchange energy, yields an equilibrium in close agreement with experiment.

Plasmons in the presence of Tamm-Shockley states with Rashba splitting at noble metal surfaces

Au(111) or similar noble metal surfaces feature Tamm-Shockley surface states that are known to possess considerable spin-orbit splitting of the Rashba type of order $\ensuremath{\Delta}=0.1\text{ }\text{eV}$. When interacting with an electromagnetic field such states are expected to have resonances when the frequency of the field is near the energy of the spin-orbit splitting $\ensuremath{\Delta}$. They originate from the intersubband transitions between spin-split subbands and can be observed in the frequency dependence of the surface impedance. Plasmons in thin metal films are gapless and can be strongly affected by these spin resonances, acquiring significant modification of the spectrum when it intersects the $\ensuremath{\omega}=\ensuremath{\Delta}$ line. Finally, an interesting demonstration of the intersubband resonances can be achieved when metal films are coated with ionic dielectrics that have a frequency of longitudinal or transverse optical phonons above or below $\ensuremath{\Delta}$. The dielectric function between the two optical phonon frequencies is negative which forbids propagation of conventional plasmon polaritons. However, the presence of spin-orbit-split surface states allows plasmon polaritons to exist in this otherwise forbidden range of frequencies.

Characterization of Thermophilic Bacteria Using Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) can provide molecular-level information about the molecules and molecular structures in the vicinity of nanostructured noble metal surfaces such as gold and silver. The three thermophilic bacteria Bacillus licheniformis, Geobacillus stearothermophilus, and Geobacillus pallidus, a Gram-negative bacterium E. coli, and a Gram-positive bacterium B. megaterium are comparatively characterized using SERS. The SERS spectra of thermophilic bacteria are similar, while they show significant differences compared to E. coli and B. megaterium. The findings indicate that a higher number of thiol residues and possible S-S bridges are present in the cell wall structure of thermophilic bacteria, providing their stability at elevated temperatures. Incubating the thermophilic bacteria with colloidal silver suspension at longer times improved the bacteria-silver nanoparticle interaction kinetics, while increased temperature does not have a pronounced effect on spectral features. A tentative assignment of the SERS bands was attempted for thermophilic bacteria. The results indicate that SERS can be a useful tool to study bacterial cell wall molecular differences.

Importance of surface states on the adsorption properties of noble metal surfaces

The Shockley-type surface state on noble metal (111) surfaces can be used as a sensitive probe for different surface modifications, adsorption processes, and interactions between adsorbate and substrate. On the model systems of Cu and Au(111) surfaces covered with noninteger monolayers of Ag we demonstrate that angle-resolved photoelectron spectroscopy is able to identify the growing positions of adsorbates by the investigation of the Shockley state. Vice versa, we discuss in a simple model to what extent the surface states themselves influence the adsorption process and determine the favored surface for physisorbed closed-shell systems.

Adsorption of inert gases including element 118 on noble metal and inert surfaces from ab initio Dirac–Coulomb atomic calculations

The interaction of the inert gases Rn and element 118 with various surfaces has been studied on the basis of fully relativistic ab initio Dirac-Coulomb CCSD(T) calculations of atomic properties. The calculated polarizability of element 118, 46.3 a.u., is the largest in group 18, the ionization potential is the lowest at 8.91 eV, and the estimated atomic radius is the largest, 4.55 a.u. These extreme values reflect, in addition to the general trends in the Periodic Table, the relativistic expansion and destabilization of the outer valence 7p(3/2) orbital. Van der Waals coefficients C(3) and adsorption enthalpies DeltaH(ads) of Ne through element 118 on noble metals and inert surfaces, such as quartz, ice, Teflon, and graphite, were calculated in a physisorption model using the atomic properties obtained. The C(3) coefficients were shown to steadily increase in group 18, while the increase in DeltaH(ads) from Ne to Rn does not continue to element 118: The large atomic radius of the latter element is responsible for a decrease in the interaction energy. We therefore predict that experimental distinction between Rn and 118 by adsorption on these types of surfaces will not be feasible. A possible candidate for separating the two elements is charcoal; further study is needed to test this possibility.

Synthesis and Self-Assembly of Thio Derivatives of Calix[4]arene on Noble Metal Surfaces

Self-assembled monolayers (SAMs) provide a simple route to functionalize electrode surfaces with organic molecules. Herein we use cavity-containing derivatives of calix[4]arenes in SAMs. Bound to noble metal surface, the assembled molecules are candidates to serve as molecular sieves for H 2 molecules and H (+) ions, which could have relevance for fuel cell applications. Tetra- O-alkylated calix[4]arenes with thiolacetate and thiolamide wide-rim anchoring groups in cone and partial-cone conformations were designed, synthesized and self-assembled onto Au, Pt, and Pd surfaces. The resulting SAMs were systematically examined. Single crystal X-ray diffraction of 5,11,17,23-tetrakis(thioacetyl)-25,26,27,28-tetra- i-propoxycalix[4]arene confirmed the cone conformation and revealed the cavity dimensions of the SAMs that were formed by immersing noble metal substrates (Au, Pt and Pd deposited on Si-wafers) in solutions of calix[4]arenes. Surface characterization techniques including ellipsometry, cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) were used, indicating that the metal surface is terminated with a monomolecular layer. Experimental thicknesses obtained from the ellipsometry are consistent with the calculated values. CV results showed 50 to 80% physical passivation against the Fe(CN) 6 (3-/4-) couple, implying an overall relatively low concentration of defects and pinholes in the films. The binding energies of the S2p core level in the XPS were consistent with the literature values and revealed that up to 3.2 out of four anchoring groups were bonded to the noble metal surface.

Chlorine interaction with defects on the Au(111) surface: A first-principles theoretical investigation

Chlorine is an important element in promoting oxidation on noble metal surfaces. Here, we report a comprehensive theoretical study of chlorine interaction with defects on the Au(111) surface, using density functional theory calculations and periodic slabs to model the surface. We find that chlorine binds preferentially on steps, vacancies, and gold adatoms. The increase in binding energy per chlorine atom, compared to binding on the flat, defect-free surface, is 0.29 eV when the chlorine atom is on top of a gold adatom, 0.38 eV when it is at the edge of a step, and 0.19 eV when it is next to a single surface vacancy. An extensive study of chlorine interaction with different numbers of surface gold vacancies revealed that chlorine interacts the strongest with three vacancies.