Ag Overlayers Research Articles

Dynamics of a Ag overlayer on Ni(100).

We have examined the structure and the dynamics of a nearly incommensurate monolayer of Ag on Ni(100) for a large range of temperatures using molecular-dynamics simulations based on potentials from the embedded-atom method. On the average surface the structure is a nearly hexagonal c(2×8), as expected from previous experimental data. At 300 K we find the Ag overlayer to slide back and forth in the nearly incommensurate direction on the Ni substrate while the vertical height of the silver atoms oscillates between a lower value (∼2 A) at the hollow sites and a slightly higher value (∼2.4 A) at the bridge sites

Electronic structure of random Ag-Pd and Ag-vacancy overlayers on an fcc Pd(001) substrate.

The electronic structure and properties of two different types of random Ag overlayers on a nonrandom Pd(001) substrate have been studied. One overlayer is an Ag-Pd alloy and the other is Ag plus vacancies. Calculations were performed by means of a self-consistent, surface Green's-function technique based on the tight-binding linear-muffin-tin-orbital theory. The disorder was included via the coherent-potential approximation generated to inhomogeneous systems. The layer-resolved local densities of states and the work functions were obtained

Structure and dynamics of an Ag overlayer on Cu(100): a study using the embedded atom method

We present the results obtained from molecular dynamics simulations of an overlayer of Ag on Cu(100) using potentials from the embedded atom method. As in the case of an Ag overlayer on Ni(100), the overlayer slides back and forth on the substrate along the nearly incommensurate direction. The mean square vibrational amplitudes of the Ag atoms are also largest along this direction. The Goldstone mode has a frequency of 1.3cm −1 at 300 K. At higher temperatures the surface disorders and Ag and Cu atoms interdiffuse. The dispersion of the lowest frequency vertical mode is in good agreement with inelastic electron scattering data. We also present some explanations of the phonons at Λ and the structure factor of the system.

Structure and dynamics of an Ag overlayer on Cu(100): a study using the embedded atom method

Structure and dynamics of an Ag overlayer on Cu(100): a study using the embedded atom method

Photoemission Study of Ag on Hydrogenated Amorphous Silicon

We report a photoemission study of Ag overlayers deposited on a hydrogenated amorphous silicon (a-Si:H) film at room temperature, using synchrotron radiation as a probe. Unlike the two other interfaces, Au/a-Si:H and Cr/a-Si:H, where the metal deposits start to intermix with the a-Si substrate after a critical thickness is exceeded, the Ag/a-Si:H interface is abrupt without any indication of intermixing. The non-intermixing nature is reminiscent of its crystalline counterpart. More interestingly, we have found a unique behavior of small metal adatoms on a-Si:H; that is, they reside favorably on the dangling-bond sites. Strong hydrogen passivation on the surface prevents the gathered adatoms from coalescing.

Surface and interface properties of cleaned and Ag-covered YBa 2Cu 3O 7−x films

As-received YBa 2Cu 3O 7−x (YBCO) films are generally covered by approximately one monolayer of chlorine. Such YBCO films, which were first cleaned by annealing in oxygen or additionally by electron-stimulated desorption and subsequently covered with Ag at room temperature, were studied by using Auger electron (AES), low-energy electron energy-loss (LEELS), and photoemission spectroscopy (UPS). AES and LEELS signals of YBCO were detected even for nominal silver coverages Θ = 20 nm. This indicates that Ag overlayers grow extremely nonuniform. The formation of Ag islands is concluded from the coverage dependence of the AES lines of the substrate as well as of silver. For small Ag coverages, the Ag(MVV) Auger lines were found to be shifted by approximately −1.4 eV with respect to the Ag(350/355 eV) signals recorded with thick overlayers. On clean samples, we conclude the initial formation of Ag 2O up to gQ ≈ 0.2 nm and a growth of metallic Ag islands for Θ > 1 nm only. On Cl-contaminated YBCO surfaces, a distinctly different behavior is observed. We infer an initial formation of AgCl which prevents an oxidation. Accordingly, energy losses caused by bulk-plasmon excitations in silver metal appear on YBCO:Cl samples already for Θ ≈ 0.2 nm. Our UPS data indicate that the Ag clusters show a polycrystalline morphology.

Local critical current measurements on (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tape with an electromagnetic probe

The use of induced currents from a small noncontacting electromagnetic probe to determine the critical current density in a (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x//Ag tape on a local scale is described. The technique forces full field penetration into the tape locally and infers the critical current density from the Bean critical state model, accounting for the Ag overlayers. Critical current images of the tape can be obtained by scanning the probe over the tape surface with spatial resolution on the order of 1.0 mm. Results for tapes with different microstructures are discussed. >

The adsorption of Ag on CO, O, and H covered W(110) I. Overlayer formation and stability

Abstract The adsorption of Ag mono- and multilayers on O/W(110), CO/W(110), and H/W(110) was investigated, from 90 K to Ag desorption temperatures. Auger measurements indicate that Ag growth occurs layer by layer at 90 K on all substrates. For Ag/O/W(110) dewetting, i.e. partial balling up of Ag occurs from 200 to 400 K. At T ⩾ 500 K segregation sets in, with O forced into p(1 × 1) islands, and Ag pushed into islands which can be several layers thick. This behavior is similar to that seen previously with O and Cu and O and Pd on W(110). There is also a slight amount of O left trapped under the Ag covered surface. In the case of CO partial dewetting also seems to set in at 200–300 K. Ag adsorption does not cause CO desorption. However, with an Ag overlayer there is a slight decrease from 400 to 350 K in the virgin-CO desorption peak, and the ratio of virgin to β-desorption increases with Ag coverage. This suggests that Ag competes with remaining CO for sites which could be used for dissociative, i.e. β-adsorption. CO is only very weakly adsorbed on Ag 1 /W(110); this explains the relatively small effect on v-CO desorption, since there is no driving force for CO-Ag interchange, as in the case of Cu/CO/W(110), where CO is still strongly bound on Cu 1 /W(110). The effect of Ag on H 2 desorption from H/W(110) is quite dramatic, with the appearance of a sharp low temperature peak at 150 K and a second sharp peak at 400 K, also lower than the broad main peak for H/W(110). This behavior is very similar to what was seen previously for Hg/H/W(110) and Pd/H/W(110) and may result from a modification of H binding sites on W(110) by the overlayer metal, even if the latter does not interact directly with H.

The adsorption of Ag on CO, O, and H covered W(110): I. Overlayer formation and stability

The adsorption of Ag mono- and multilayers on O/W(110), CO/W(110), and H/W(110) was investigated, from 90 K to Ag desorption temperatures. Auger measurements indicate that Ag growth occurs layer by layer at 90 K on all substrates. For Ag/O/W(110) dewetting, i.e. partial balling up of Ag occurs from 200 to 400 K. At T ⩾ 500 K segregation sets in, with O forced into p(1 × 1) islands, and Ag pushed into islands which can be several layers thick. This behavior is similar to that seen previously with O and Cu and O and Pd on W(110). There is also a slight amount of O left trapped under the Ag covered surface. In the case of CO partial dewetting also seems to set in at 200–300 K. Ag adsorption does not cause CO desorption. However, with an Ag overlayer there is a slight decrease from 400 to 350 K in the virgin-CO desorption peak, and the ratio of virgin to β-desorption increases with Ag coverage. This suggests that Ag competes with remaining CO for sites which could be used for dissociative, i.e. β-adsorption. CO is only very weakly adsorbed on Ag 1/W(110); this explains the relatively small effect on v-CO desorption, since there is no driving force for CO-Ag interchange, as in the case of Cu/CO/W(110), where CO is still strongly bound on Cu 1/W(110). The effect of Ag on H 2 desorption from H/W(110) is quite dramatic, with the appearance of a sharp low temperature peak at 150 K and a second sharp peak at 400 K, also lower than the broad main peak for H/W(110). This behavior is very similar to what was seen previously for Hg/H/W(110) and Pd/H/W(110) and may result from a modification of H binding sites on W(110) by the overlayer metal, even if the latter does not interact directly with H.

Scanning tunneling microscopy study of a Ag monolayer on Cu(111)

Abstract The structure of submonolayer Ag deposited on Cu(111) has been examined with scanning tunneling microscopy. The long diffusion length of Ag leads to the formation of large (111)-like islands nucleating from the “downhill” side of Cu step edges. The growth fronts of the Ag islands are mostly straight and lie along the 〈110〉 directions. The Ag overlayer is apparently slightly compressed relative to bulk Ag, and forms a mixture of superstructures rotated slightly relative to the substrate.

Scanning tunneling microscopy study of a Ag monolayer on Cu(111)

The structure of submonolayer Ag deposited on Cu(111) has been examined with scanning tunneling microscopy. The long diffusion length of Ag leads to the formation of large (111)-like islands nucleating from the “downhill” side of Cu step edges. The growth fronts of the Ag islands are mostly straight and lie along the 〈1̄10〉 directions. The Ag overlayer is apparently slightly compressed relative to bulk Ag, and forms a mixture of superstructures rotated slightly relative to the substrate.

Nitrobenzene adsorption on clean and NaCl-covered Cu(100), Ni/Cu(100), Ag/Cu(100) and Na/Cu(100) surfaces studied by XPS and XAES

Adsorption of nitrobenzene on clean and NaCl-covered Cu(100), Ni/Cu(100), Ag/Cu(100) and Na/Cu(100) surfaces has been studied by X-ray photoelectron spectroscopy and X-ray-induced Auger electron spectroscopy. Dissociative adsorption of nitrobenzene into nitrosobenzene and phenylnitrene occurs at a lower temperature on a 1.5 monolayer (ML) Ni/Cu(100) surface than on a Cu(100) surface, and the dissociation is significantly attenuated by a 2.0 ML Ag overlayer. A portion of NaCl deposited on Cu(100), Ni/Cu(100) and Ag/Cu(100) gives disrupted or loosely bound NaCl. The metallic character of the Na of the NaCl is indicated by the formation of the nitrobenzene anion after exposure of the NaCl- and Na-covered surfaces to nitrobenzene at 176 K and 110 K, respectively. The Auger parameter (2066.2 eV) for Na atoms (0.5 ML) on Cu(100) (near the value of metallic Na) changed to 2062.7 eV (Near the value of NaNO 2) after the formation of the anion. This shows a localization of a large positive charge on Na after nitrobenzene adsorption. A mechanism for the occurrence of the metallic character of the Na from NaCl is discussed.

Structure and dynamics of an Ag overlayer on Ni(100) : comparison of embedded atom and pair potential results

We present a comparison of the results obtained from molecular dynamics simulations of an overlayer of Ag on Ni(100) using potentials from the embedded atom method, on the one hand, and central, pair-wise potentials, on the other. At room temperature there is qualitative agreement between the most striking result obtained from the two types of potential, namely that the Ag atoms slide back and forth over the Ni substrate, maintaining an average distance above the nickel of 2.15 Å. The mean square vibrational amplitudes of the atoms exhibit interesting similarities and differences at 300 K. At 1200 K there emerge also qualitative differences in the predictions from the two methods, alluding to appearance of anharmonic effects.

Structure and dynamics of an Ag overlayer on Ni(100): comparison of embedded atom and pair potential results

Structure and dynamics of an Ag overlayer on Ni(100): comparison of embedded atom and pair potential results

A SERS investigation of adsorption of Fe(bpy) 2+3 complex on silver

Spectroelectrochemical measurements have been carried out with the system Fe(bpy) 2+ 3 + 0.1 M KCl/Ag by using laboratory-built equipment optimizing the separation from the noise and collection of the Raman signals to study adsorption of the complex of 2,2-bipyridil with Fe 2+ on Ag. Specific surface roughness necessary to measure large SERS signals has been successfully modelled by latex spheres covered with a thin Ag overlayer. The results suggest an irreversible adsorption of the complex on Ag and compare with those obtained with a “standard” SERS procedure. The role of surface morphology in the enhancement process is discussed.

Electrochemical kinetics and growth modes of silver deposits on polyfaceted platinum spherical electrodes

The Ag electrodeposition on Pt spherical substrates was investigated over a wide range of experimental conditions to establish a correlation between the kinetics of the process and the different growth modes of the Ag overlayer. At E d ≊ E rev, the electrodeposition of the first Ag layer obeys a combined adsorption-desorption and nucleation and 2-D growth process under diffusion control. At E d < E rev, the formation of the 2-D Ag layer proceeds through an adsorption mechanism, whereas the formation of the 3-D Ag layer fits a progressive nucleation and 3-D growth mechanism under diffusion control. Finally, when E d < E φ, a potential threshold related to Ag dendritic growth, the kinetics of the reaction apparently obeys a progressive nucleation and 1-D growth. This process is triggered at edges and corners of large Ag crystals. Dendritic growth takes place outside the diffusion layer defined around the Pt substrate sphere. In this case migration plays a substantial role in the process.

Mechanisms for adatom-induced disruption of Bi2Sr2CaCu2O8(001): Scanning-tunneling-microscopy studies of Ag-, Au-, and Cr-overlayer growth.

The growth of Ag, Au, and Cr overlayers on Bi 2 Sr 2 CaCu 2 O 8 (001) was studied with use of scanning tunneling microscopy (STM). Submonolayer depositions of Ag at 300 K produced ∼25-A-diam clusters that formed rows along the b axis. These clusters were derived, in part, from Ag atoms but, more importantly, they resulted from a conversion of the terminating Bi-O surface layer into Bi 2 O 3 -like structures. Such a conversion provides the driving force for substrate disruption, even for adatoms that are not chemically active with the constituents of the superconductor

Evaluation of the surface structure of diamond films prepared in a combustion flame by surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) was performed on the surface characterization of polycrystalline diamond films prepared in a C2H2-O2 combustion flame. The conventional Raman spectra of a diamond film showed only the peak at 1333 cm−1 corresponding to diamond. However, if Ag island films were coated onto the diamond surface (Ag overlayer method), the resultant spectra changed drastically. The peaks around 1580 and 1355 cm−1 were remarkably enhanced; the former is assigned to the sp2 graphite structure and the latter is assigned to the disordered microcrystalline graphite structure, respectively. This would indicate that the diamond surface has a sp2-like structure, which is different from the sp3 structure in bulk. This surface configuration is discussed from the viewpoint of the surface reconstruction.

X-ray scattering study of Ag/Si(111) buried interface structures.

Various interface structures formed between Si(111) and a {ital thick} Ag overlayer are investigated by grazing-incidence x-ray diffraction. The (7{times}7) reconstruction of Si(111) is preserved under a room-temperature deposited Ag film. Upon annealing to 250 {degree}C the interface becomes (1{times}1). This is contrasted by the ({radical}3 {times} {radical}3 ){ital R}30{degree} structure formed by annealing a {ital thin} Ag film on Si(111). By depositing a thick Ag film on this ({radical}3 {times} {radical}3 ){ital R}30{degree} Ag/Si(111) surface at room temperature, the ({radical}3 {times} {radical}3 ){ital R}30{degree} reconstruction is suppressed.

Oxidation of Si(100) surfaces with ultrathin metal overlayers in ozone atmosphere

Si(100) substrates with Ag, Au, Bi and Ba ultratin overlayers were heated in distilled ozone atmospheres. It was found that Ag, Au, Bi and Ba atoms remained on the Si(100) surfaces at substrate temperatures T s ≲ 550, 780, 300 and 650°C, respectively. Au atoms diffused to a depth of ∼ 200Åfrom the Si(100) surface at T s = 780°C. Silicon oxide layers with a thickness of 10–20Åwere formed in the Si(100) substrates with the metal overlayers. Clean Si(100) substrate was also oxidized at T s = 650°C under the same conditionns, and the thickness of resultant Si oxide in the Si(100) substrate was almost the same as in the Si(100) with the metal overlayers.