Clean Ag Surface Research Articles

Optical properties of silicene, Si/Ag(111), and Si/Ag(110)

We present a state-of-the-art study of the optical properties of free-standing silicene and of single-layer Si 1D and 2D nanostructures supported on Ag(110) and Ag(111) substrates. Ab initio simulations of reflectance anisotropy spectroscopy (RAS) and surface differential reflectivity spectroscopy (SDRS) applied to the clean Ag surface and Si/Ag interfaces are compared with new measurements. For Si/Ag(110) we confirm a pentagonal nanoribbon geometry, strongly bonded to the substrate, and rule out competing zigzag chain and silicenelike models. For Si/Ag(111) we reproduce the main experimental features and isolate the optical signal of the epitaxial silicene overlayer. The absorption spectrum of a silicene sheet computed including excitonic and local field effects is found to be quite similar to that calculated within an independent particle approximation, and shows strong modifications when adsorbed on a Ag substrate. Important details of the computational approach are examined and the origins of the RAS and SDRS signals are explained in terms of the interface and substrate response functions. Our study does not find any evidence for Si adlayers that retain the properties of freestanding silicene.

First principles study of ortho–para H2 conversion on the O2(0.25 ML)/Ag(111) system

We investigated the ortho–para (o–p) H2 conversion on the O2/Ag(111) system, combining first principles calculations based on spin-polarized density functional theory with perturbative calculations. In this study, we calculated the o–p H2 conversion rate, using the standard Fermi's Golden Rule and compared the contribution efficacy of the magnetic dipole interaction and the Fermi contact interaction. The calculated o–p H2 conversion times induced by the magnetic dipole interaction and by the Fermi contact interaction, corresponding to the reciprocal of the o–p H2 conversion rates, are in the orders of 100–103 s and 100–106 s, respectively. In comparison with the o–p H2 conversion time of 610 s on clean Ag surfaces, these results indicate that the magnetic dipole interaction and the Fermi contact interaction enhance the o–p H2 conversion in the vicinity of O2. In addition, the magnetic dipole interaction and the Fermi contact interaction can be separately observed with the different distances between H2 and O2.

Theoretical study of chlorine adsorption on the Ag(111) surface

We study the adsorption of chlorine on the Ag(111) surface with full potential gradient corrected density functional calculations. When considering a root3 x root3 R30degree pattern, we find that the fcc hollow is the most favorable adsorption site. We obtain an Ag-Cl bond length of 2.62 Angstrom which is intermediate between two controversial experimental results. We discuss the differences of core level energies and densities of states for the different adsorption sites. We find the Cl-Ag interaction to be more consistent with an ionic, rather than covalent, picture of the bonding. In addition, we compute energetics and related properties of Ag bulk and the clean Ag surface comparing the local density approximation and the generalized gradient approximation.

The effect of electric fields on Ag(001)

The electronic structure of Ag(001) with an adsorbed overlayer of Xe is calculated in external electric fields of varying strength. The screening mostly takes place on the top of the surface Ag atoms, with polarization of the Xe atom. The variation in the centroid of the screening charge (the image plane) with field strength is studied, and it is found that the adsorption of Xe reduces the non-linear response compared with that of the clean Ag surface. The distribution of screening charge between the various atoms is found using a generalized effective charge, and it is found that the screening charge on Ag atoms directly underneath Xe is enhanced. The generalized effective charge gives the derivative of the force at the surface with respect to the electric field, and the phenomena of field-induced chemisorption and field desorption are discussed in these terms.

Removal of Carbonaceous Contamination from SERS−Active Silver by Self-Assembly of Decanethiol

The presence of carbonaceous contamination on common surface-enhanced Raman scattering (SERS) surfaces has been a primary limitation to the analytical utility of SERS; background contamination significantly affects reproducibility and often determines the limit of detection. Due to the strong interaction between thiols and Ag or Au, self-assembly of alkanethiols onto these metals results in the displacement of "adventitious material" from the metal. The cleaning procedure presented here utilized self-assembly of decanethiol on an electrochemically roughened Ag foil for removal of surface contamination. The self-assembled layer was subsequently removed by rapid oxidation with ozone, followed by rinsing in methanol. The clean Ag surface exhibited a tremendously improved background and a 3-fold increase in the enhancement factor for a probe molecule.

Isotopic studies of the reaction of NO on silver surfaces

The isotopomers 14N 16O and 15N 18O were used to obtain information on the mechanisms of NO reaction on silver layers on Ru(001), by observation of the isotopic distributions of the products. NO adsorption on Ag in the 75–100 K range produces adsorbed N 2O and oxygen atoms. Very different isotopic distributions of product N 2O are observed, depending on whether the NO isotopomers are dosed simultaneously or sequentially. The observed N 2O distributions favor a non-dissociative reaction pathway with a NO dimer as the reaction intermediate. The isotopic distribution of a high temperature NO desorption state can be explained by dissociation of a NO 2 surface species that is formed at a low temperature. The N 2O which is produced by reaction of NO on the surface is found to have a higher desorption temperature than N 2O adsorbed on either the clean Ag surface or on an oxygenated Ag surface. This is attributed to a strong binding interaction with surface NO 2.

Isotopic studies of the reaction of NO on silver surfaces

Abstract The isotopomers 14 N 16 O and 15 N 18 O were used to obtain information on the mechanisms of NO reaction on silver layers on Ru(001), by observation of the isotopic distributions of the products. NO adsorption on Ag in the 75–100 K range produces adsorbed N 2 O and oxygen atoms. Very different isotopic distributions of product N 2 O are observed, depending on whether the NO isotopomers are dosed simultaneously or sequentially. The observed N 2 O distributions favor a non-dissociative reaction pathway with a NO dimer as the reaction intermediate. The isotopic distribution of a high temperature NO desorption state can be explained by dissociation of a NO 2 surface species that is formed at a low temperature. The N 2 O which is produced by reaction of NO on the surface is found to have a higher desorption temperature than N 2 O adsorbed on either the clean Ag surface or on an oxygenated Ag surface. This is attributed to a strong binding interaction with surface NO 2 .

Interaction of SO 2 with single crystal metal surfaces

Studying the interaction of SO 2 with metal surfaces under UHV conditions, a question of central interest is whether the molecule dissociates (leaving back the catalyst poison sulphur on the surface) or not. A spontaneous or a thermally activated dissociation of SO 2 occurs on Fe, Rh, W, Ni, Pd and Pt. On Cu and Ag a strong chemisorption, but only a partial dissociation induced by defects or coadsorbed alkalis, and on Au no chemisorption at all were observed. In this paper a comparison of our results obtained for the chemisorption and multilayer adsorption of SO 2 on Cu(111), Ag(111), Ag(100) and Ag(110) in the temperature range between 80 K and 900 K is given. By combining highly resolved TPD-measurements, isothermal and temperature-programmed ΔΦ-experiments after different stages of exposure and molecular beam backscattering measurements (MBBS) —assisted by LEED, AES and isotope mixing experiments — a destinction between ordinary desorption and desorption after a reorientation process during the heating procedure could be made. Whereas on clean Ag surfaces adsorption and desorption of SO 2 are observed only below 300 K, on Cs-precovered Ag desorption of SO 2 takes place even above 600 K. Finally, results concerning the different stages of SO 2 multilayer adsorption (bi-, tri-, multilayers) are presented showing a characteristic dependence of the layer growth on the adsorption temperature, the impinging SO 2 flux density and on the surface structure.

Formation and reaction of allylic species on silver surfaces: bond-order conservation Morse-potential analysis

The reaction energetics, particularly the intrinsic activation barriers for possible reactions involving allylic species C 3H 5X, X = H, OH, O, Cl on clean and oxygen-preadsorbed Ag surfaces, have been calculated by using the bond-order conservation Morse-potential (BOC-MP) method. The calculations were made for low coverages of C 3H 5X with qualitative corrections for higher coverages. On clean Ag surfaces, propylene C 3H 6 and allyl alcohol C 3H 5OH are projected to desorb without dissociation, in contrast to allyl chloride C 3H 5C1, which is projected to desorb only at high coverages but to dissociate at low coverages forming a stable π-allyl (and atomic chlorine). It is found that the intrinsic activation barrier for dimerization of π-allyl into 1,5-hexadiene is very small and the apparent barrier should be mainly of diffusional character. In the presence of preadsorbed hydroxyl OH s, π-allyl is projected to undergo various transformations producing allyl alcohol, allyl alkoxide, acrolein, and propylene, when most recombination and disproportionation reactions have low intrinsic activation barriers. The BOC-MP model projections are in good agreement with experiment, particularly with the recent HREEL and TPD studies of C 3H 5C1.