Ag Overlayers Research Articles

Polarization-dependent electronic structure of Ag quantum well states on the MoS2 (0001) surface using ARPES and DFT studies

The electronic band structure of clean and Ag-induced quantum well states on the ${\mathrm{MoS}}_{2}$(0001) surface has been investigated using polarization-dependent angle-resolved photoemission spectroscopy (ARPES) along with first-principle-based density functional theory (DFT) calculations. Using selective linearly polarized light, the orbital symmetry and $k$ dispersion of the surface localized electronic states have been unambiguously determined. The quantum well states (QWS) originating from the Ag ${p}_{z}$ orbitals are confined within the Ag overlayer, while the quantum well resonance states (QWRS) are hybridized with substrate Mo $4{d}_{{z}^{2}}$ and S $3{p}_{z}$ orbitals. The photon energy dependence of the dispersion characteristics confirms the two-dimensional (2D) nature of these quantum states while their momentum dependence is a consequence of the finite lateral coherence length of the electrons. Our ARPES experiments using incident circularly polarized light on ${\mathrm{MoS}}_{2}$(0001) surface show circular dichroism effects (CD-ARPES), similar to recent studies where it is used to probe the initial state orbital angular momentum or the Berry curvature. While the QWS on the Ag overlayer does not exhibit any intensity asymmetry with the helicity of the incident light, the Ag QWRS show circular dichroism characteristics. The observed circular dichroism from the QWRS is attributed to the hybridization of these quantum states to the substrate states. We compare our ARPES results with the calculated orbital projected band structure to obtain a microscopic insight into the electronic behavior, which is critically important for manipulating wave functions for potential device applications.

Amorphous Carbon Films with Embedded Well-Dispersed Nanodiamonds: Plasmon-Enhanced Analysis and Possible Antimicrobial Applications

An amorphous carbon film with embedded detonation nanodiamond (DND) particles (a-C:ND) was produced by magnetron sputtering of nanodiamond powder. An Ag film was deposited on the carbon structure by radiofrequency magnetron sputtering. The silver film was irradiated with a 150 eV Ar+ to form plasmonic-active nanoparticles (NP) on the surface of the a-C:ND. The structure of the obtained a-C:ND and a-C:ND/Ag structures were studied by scanning and transmission electron microscopy, electron energy-loss spectroscopy, UV–Visible absorption spectroscopy, Raman spectroscopy, and fluorescence lifetime imaging at two-photon excitation. The analysis revealed 76% of sp3-carbon and a good dispersion of diamond nanoparticles in the a-C. Surface-enhanced Raman scattering (SERS) was applied to investigate the a-C:ND/Ag structure, allowing for the observation of SERS from the sp2-carbon species and the absence of significant a-C:ND damage after Ar+ irradiation of the Ag overlayer. A plasmonic-metal-enhanced luminescence was observed at one- and two-photon excitations, revealing a two- to five-fold intensity increase. The activity of the used DNDs was tested using the agar diffusion method and observed against the bacteria of Bacillus subtilis, Staphylococcus aureus, and Escherichia coli and the fungi of Aspergillus niger, Aspergillus fumigatus, and the yeast of Candida albicans, showing DND activity against all the test strains of fungi.

Three-Dimensional Thermal Analysis of an SFCL REBCO Coil Immersed in Liquid Nitrogen

A resistive type superconducting fault current limiter (SFCL) using REBCO tapes has shown its advantages in power applications. However, the hotspot problem, which can be caused by the uniformity of critical current density JC over long-length of REBCO tapes, is still a threat to the SFCL safety. In this paper, we have studied the limiting performance and transient temperature rise distribution of a nonintersecting type non-inductive solenoidal SFCL REBCO coil using three-dimensional electromagnetic and thermal coupled FEM analysis. Firstly, we modelled a local degradation of JC and studied its influence on the transient temperature rise distribution during the over current. The results indicated that a narrow and severe JC degradation leads to the concentration of current and heat generation in the normal JC area, which is opposite to the case of a relatively wider and slighter JC degradation area. Furthermore, REBCO tapes with different thickness of substrate and Ag over-layer were analysed to study the influence on limiting performance and temperature rise. A thicker substrate leaded to a slower temperature rise because of its higher heat capacity, and the Ag over-layer of 4 μm showed better limiting performance and thermal stability because of its better thermal conductivity as well as uniform temperature rise and SN transition.

Protected surface state in stepped Fe (0 18 1)

Carbon (C) surface segregation from bulk stabilizes the Fe(0 18 1) vicinal surface by forming a c(3sqrt{2} × sqrt{2} reconstruction with C zig-zag chains oriented at 45° with respect to the iron surface steps. The iron surface electronic states as measured by high resolution ARPES at normal emission with polarized synchrotron radiation split in two peaks that follow distinct energy dispersion curves. One peak follows the dispersion of the carbon superstructure and is photoexcited only when the polarization vector is parallel to the steps, the second peak disperses similarly to the pristine Fe(0 0 1) surface. Such surface electronic structure is robust as it persists even after coating with an Ag overlayer. The robustness of this surface electronic structure and its similarity with that of the clean Fe(0 0 1) surface make this system of interest for magnetic and spintronic properties such as magneto tunnel junctions based on Fe/MgO interface.

Electrodeposition of Ag Overlayers onto Pt(111): Structural, Electrochemical and Electrocatalytic Properties

Epitaxially grown Ag overlayers have been fabricated by electrochemical deposition onto Pt(111). The electrochemical behaviour of these Ag overlayers has been studied by cyclic voltammetry, and their adsorption properties are significantly influenced by the underlying Pt(111) substrate and markedly different from those of Ag(111). A characteristic voltammetric peak for OH adsorption on pseudomorphic Ag islands has been observed for alkaline solution. A deposition–dissolution hysteresis in the underpotential deposition region for the Ag bilayer suggests exchange processes between subsurface Ag and Pt atoms. Theoretical DFT calculations confirm the stability of a pseudomorphic Ag monolayer. However, it is shown for two and three Ag layers that the formation of sandwich structures is theoretically favoured, i.e. Ag layers tend to be separated by single Pt layers. While Ag displaces hydrogen adsorbed in the underpotential region, the activity of Ag monolayers for the hydrogen evolution reaction (HER) is very close to that of Pt(111). Also, Tafel slopes for HER on the first pseudomorphic Ag monolayer on Pt(111) and for blank Pt(111) are almost identical, whereas thicker overlayers are more Ag-like. It is shown by theoretical calculations for the case of an Ag monolayer on Pt(111) that hydrogen can be adsorbed on the Pt subsurface layer.

Electrochemical Silver Deposition Onto Pt(111) Surfaces

The underpotential deposition (upd) of Ag onto Pt(111) produces surface structures exhibiting several interesting structural and electrochemical properties: (i) a contracted pseudomorphic Ag monolayer, (ii) deposition-dissolution hysteresis, (iii) an expanded lattice for the second Ag monolayer, (iv) absence of Hupd, and (v) enhanced H2 evolution activity of Ag overlayers compared with Ag(111). Related measurements under ultrahigh vacuum conditions reveal similar structural transformations, and DFT calculations indicate that there is a thermodynamic driving force for the migration of the first Ag(111) overlayer deeper into the electrode, upon deposition of the second monolayer. We will discuss adlayer formation and the link between structure formation and reactivity of the resulting surface.

Growth and structural evolution of Sn on Ag(001): Epitaxial monolayer to thick alloy film

The growth and structure of Sn on Ag(001), from submonolayer to thick film coverages at room temperature, are studied using low energy electron diffraction, x-ray photoemission spectroscopy and angle-resolved photoemission spectroscopy (ARPES) techniques. The authors observe different growth modes for submonolayer Sn coverages and for higher Sn coverages. Systematic surface structural evolution, consistent with the substitution of surface Ag atoms by Sn atoms, is observed for submonolayer Sn coverages while an ordered Ag-Sn bulk alloy film is formed for higher Sn coverages with an Ag overlayer. For monolayer coverage of Sn, a pseudomorphic growth of a Sn layer without alloying is determined. ARPES results also confirm the presence of an ordered Ag overlayer on the bulk Ag-Sn alloy film, suggesting the formation of an Ag/Ag3Sn/Ag(001) sandwich structure at the surface for higher Sn coverages. The present results illustrate the complex interplay of atomic mobilities, surface free-energies, and alloy formation energies in determining the growth and structural properties of the system.

Surface-Enhanced Raman Scattering of Silicon Nanocrystals in a Silica Film.

Surface-enhanced Raman scattering (SERS) is an intriguing effect, efficiency of which depends on many factors and whose applicability to a given system is not obvious before the experiment. The motivation of the present work is to demonstrate the SERS effect on silicon nanocrystals (Si-nc) embedded in silica, the material of high technological importance. Using the Ag overlayer method, we have found the SERS effect for this material. The best result is obtained for Ag layers of a weight thickness of 12 nm, whose surface plasmons are in a resonance with the laser wavelength (488 nm). The enhancement obtained for the Raman signal from 3–4-nm Si-nc in a 40-nm SiOx film is above 100. The SERS effect is about twice stronger for ultra-small Si-nc (~1 nm) and/or disordered silicon compared to Si-nc with sizes of 3–4 nm. The SERS measurements with an Ag overlayer allow detecting silicon crystallization for ultra-thin SiOx films and/or for very low Si excess and suppress the Raman signal from the substrate and the photoluminescence of the film.

LEED and photoemission study of growth and structural evolution of Sn on Ag(001)

Growth of tin (Sn) on Ag(001) substrate has been studied at elevated temperatures using low energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) measurements. A c(2 × 2) ordered surface is observed for very low coverages (0.1 ML) of Sn which subsequently transforms into a p(2 × 2) ordered surface for 0.25 ML Sn coverage, suggesting formation of a substitutionally ordered surface alloy. For higher coverages of Sn beyond 0.5 ML, no further structural evolution was observed with a constant p(1 × 1) LEED pattern. Our XPS results indicate diffusion of Ag atoms into Sn layers and forming Ag–Sn alloy film. Along with LEED and XPS, our ARPES results confirm the presence of an ordered Ag overlayer on the alloy film, suggesting a sandwich structure. Our studies are important in understanding the growth mechanism and electronic properties of Sn films on noble metal substrates.

Structure of Cr monolayer on Ag(001): A buried two-dimensionalc(2×2)antiferromagnet

The growth, morphology, and magnetic structure of ultrathin Cr films grown on a Ag(001) substrate are studied using low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), and ab initio density functional theory (DFT) calculations. The presence and temperature dependence of $c(2\ifmmode\times\else\texttimes\fi{}2)$ half-order spots in the LEED pattern, for low electron energies, along with the presence of characteristic Cr $3d$ bands in the ARPES spectra, confirm the existence of antiferromagnetic ordering for the Cr monolayer case. Our experiments are also consistent with the presence of a $p(1\ifmmode\times\else\texttimes\fi{}1)$ Ag overlayer on top of the Cr layer, suggesting the existence of a Ag/Cr/Ag(001) sandwich structure at the surface. Our DFT calculations confirm that this is the most favored geometric and magnetic structure of the system. The Cr layer is found to retain a ``two-dimensional'' character with enhanced Cr $3d$ magnetic moments, despite being buried below a Ag monolayer, due to the absence of significant hybridization between Cr $3d$ and Ag $4d$ electronic states. The coverage dependence of the magnetic ordering indicates a maximum ordering above the expected monolayer coverage, possibly due to intermixing between Ag and Cr atoms in the overlayer.

Surface Growth and Diffusion Energetics of Ag Monolayers on Cu (001)

The growth of Ag monolayers on Cu (001) was studied by periodic Density Functional Theory (DFT). Despite the limited solid solubility of Ag in Cu, the growth of a single Ag overlayer on Cu (001) was predicted as feasible. In contrast, the growth of consecutive Ag monolayers was found to be energetically forbidden. Inter-diffusion of Ag monolayers into Cu was raised as a possibility but it was dependent on the sequence in which the Ag monolayers were introduced into the Cu bulk. The Ag layers preferred to be kept neither too far apart nor too close to each other, the optimum spacing between two Ag monolayers determined to be that of two consecutive Cu layers. Ag diffusion mediated tensile stress in the Cu cell by causing an increase of the unit cell constant by as much as 22%. Interactions between the Ag and Cu species also involved a degree of covalency. In general, progression of a surface Ag monolayer into the Cu bulk involved charge depletion over the Ag species and a simultaneous charge concentration over neighboring Cu atoms; this mechanism was found to influence Cu up to a depth of four surface layers.

Enhancement of upconversion luminescence of Er and Yb co-doped Y2O3 nanoparticle by Ag half-shell

Upconversion photoluminescence (PL) properties of single Y2O3 nanoparticles doped with Yb and Er (Y2O3:Yb,Er) with a Ag over-layer is studied. We traced the PL and light scattering images of individual nanoparticles by changing the thickness of a Ag over-layer. When the Ag thickness is relatively small and only the upper part of a nanoparticle is covered by Ag (Ag half-shell), the PL is strongly enhanced. On the other hand, when the Ag thickness is increased and a continuous Ag over layer is formed, the enhancement factor decreases. From the correlation between the enhancement factors of the upconversion PL and scattering intensities as well as the change of the PL lifetime, the mechanism of the PL enhancement is discussed.

Role of hybridization in the Rashba splitting of noble metal monolayers on W(110)

In contradiction to the nature of the spin-orbit driven Rashba splitting of surface states which increases with atomic number, Shikin et al. [Phys. Rev. Lett. 100, 057601 (2008)] have observed that the size of the splitting in Au overlayers on W(110) is smaller than for Ag overlayers. In the framework of first-principle density functional theory, we have studied the origin of the Rashba splitting at Au/Ag overlayers on the W(110) surface. We show how the asymmetric behavior of the wave function in the vicinity of the surface atom nucleus, in addition to the strength of the nuclear potential gradient, plays a crucial role for the size of the splitting. The influence of the electronic structure and spin dependent hybridization on the Rashba splitting is discussed. The asymmetric behavior of the surface wave function originates from the surface-interface $sp\ensuremath{-}d$ hybridization. We find that a spin dependent hybridization in the Ag overlayer influences strongly the size of the Rashba splitting.

First principles electronic structure of coincidence site epitaxial Ag/Si(111) interface

AbstractAg/Si(111) heterojunction having ∼25% lattice mismatch can be formed by placing four surface unit cells of Ag(111) on three surface unit cells of Si(111), thereby reducing the effective strain in the Ag film to ∼0.3% in this coincidence site epitaxial growth. We have carried out first‐principles investigation of such Ag/Si(111) interface to establish the interplay between the structural and electronic properties. While the Ag overlayer affects the reconstruction of the Si(111) surface, we find that the geometrical relaxation of the Ag atoms is influenced by the sub‐surface Si‐layer and the concomitant lattice mismatch. The electronic density of states show some oscillations near the Fermi energy, which have been compared with the scanning tunnelling spectroscopic (STS) experimental data. The signature of metal‐induced gap states (MIGS) for this metal–semiconductor heterojunction has been established from the evolution of localized gap states in the layer projected density of states. Our DFT estimated work function values for Ag(100), (110) and (111) have an excellent agreement with the available experimental results. Also, the p‐type Schottky barrier height (SBH) of this rectifying Ag/Si contact has been calculated from the Kohn–Sham estimates of EF − Ev modified by the interface induced dipole.

Electrocatalytic Behaviour of Epitaxial Ag(111) Overlayers Electrodeposited onto Noble Metals: Electrooxidation of d-Glucose

The electrooxidation of d-glucose in 0.1 M NaOH has been studied at epitaxially grown Ag monolayers, which have been fabricated by electrodeposition onto Au(111), Pt(111), and Pd(111) single-crystal electrodes. The electrochemical behaviour and the adsorption properties of these Ag monolayers are significantly altered by the underlying substrates. While the Ag monolayer on Pt(111) shows the most negative onset potential for glucose oxidation, Ag overlayers on Au(111) reveal the highest current densities for the systems under study. Interestingly, a Ag submonolayer on Au(111) shows the highest current densities for glucose oxidation under the experimental conditions. However, Ag overlayers on Au(111) with a thickness of two monolayers and more are poor electrocatalysts like massive Ag(111). The influence of OH adsorption and the relevance of the potentials of zero charge for the various Ag surfaces on the measured oxidation current densities are discussed.

Prediction of reconstruction in heteroepitaxial systems using the Frenkel-Kontorova model

Many heteroepitaxial metal-on-metal systems reconstruct into patterns of alternating domains of stacking faults separated by partial misfit dislocations. Here, we use two approaches to investigate the question of whether these can be predicted and controlled: (i) We map the system onto a one-dimensional Frenkel-Kontorova model, and then obtain a simple criterion to determine whether or not the surface will reconstruct; this had earlier been done for homoepitaxial systems, but is here generalized to the heteroepitaxial case. (ii) The two-dimensional Frenkel-Kontorova model is solved numerically by performing quenched molecular dynamics simulations. The necessary microscopic parameters are obtained by performing ab initio density functional theory calculations on the unreconstructed systems. The systems considered are overlayers of Fe, Co, Pt, Ag, Au, and Pb on a Ru(0001) substrate, as well as clean Ru(0001). The predictions of the two approaches agree with one another as well as with experiment. Both the presence and periodicity of the reconstruction are very sensitive to the value of ``chemical potential'' $\ensuremath{\Gamma}$. Accordingly, we suggest that $\ensuremath{\Gamma}$ can be used to tune the periodicity so as to obtain a desired nanotemplate for subsequent growth of self-organized nanostructures.

Growth and crystalline structure of Ag layers on Au(111)

The Ag/Au(111) adsorption system was investigated by Auger electron and directional elastic peak electron spectroscopies (DEPES). The change of the Auger peak heights for Au (NVV transition, 74 eV) and Ag (MNN transition, 360 eV) recorded during the continuous Ag deposition show the formation of the second layer before the completion of the first monolayer. Further evaporation leads to the nucleation of islands on the not-completed second layer. The intensity of elastically backscattered electrons was measured as a function of the incidence angle of the primary electron beam to obtain stereographic DEPES distributions at energies Ep = 1.7 keV and 2.0 keV. About 8 ML of Ag on Au(111) raises intensity contrast of the total map and of the particular peaks associated with crystallographic directions proving well ordered Ag(111) overlayer formation. The quantitative analysis of the contrasts for experimental and theoretical DEPES maps, the latter obtained by using the multiple scattering theory, suggests the coexistence of the reconstructed and nonreconstructed domains on the Au(111) surface.

Growth and structure of Ag on bilayer Co nanoislands on Cu(111)

We have studied the growth of Ag on bilayer high Co nanoislands on Cu(111) using scanningtunneling microscopy. Noble metal capping of magnetic nanostructures is known toinfluence the magnetism and knowledge of the growth is therefore important. We find thatAg preferentially nucleates on the Co nanoislands, initially leaving the free Cu sites clean.Furthermore we observe that those Co islands which are capped with Ag are almost completelycapped, thus making a perfect multilayered system of Ag/Co/Cu(111). We observe a (9 × 9) reconstruction of the Ag overlayer on Co/Cu(111).

Hydrogen on and in Selected Overlayer Near-Surface Alloys and the Effect of Subsurface Hydrogen on the Reactivity of Alloy Surfaces

The interaction of hydrogen with the close-packed facets of seventeen transition metals overlaid with 1 ML of five transition metals (Au, Ag, Cu, Pt, and Pd) has been studied using periodic self-consistent (GGA-PW91) density functional theory (DFT) calculations. For noble metal overlayers (Au, Ag, and Cu), hydrogen at the host-metal/overlayer interface (subsurface hydrogen) is more stable than subsurface hydrogen in the pure host. For certain Au and Ag overlayers, subsurface hydrogen is more stable than surface hydrogen in the same system. The presence of subsurface hydrogen was found to have a significant effect on the electronic structure of the overlayer, resulting in its modified surface reactivity.

Potential energy landscape of metallic Moiré patterns

The modulation of the electronic structure of cobalt islands on Cu(111) by the Moiré pattern of an Ag overlayer is investigated. Acquisition of tunneling spectroscopy maps reveals a local modification of both the energy and the amplitude of the cobalt-related d-states on the length scale of the periodicity of the Moiré superstructure. The modulation of the energy can be rationalized by the spatially varying hybridization of the cobalt atoms with the silver sp-bands. We propose that Moiré modulated d-states at metal interfaces with a periodicity of a few nanometers may provide a novel route to manipulate the chemical reactivity of surfaces and might serve as a template which modulates magnetic properties such as the spin density.