Chemisorbed Layer Research Articles

Low-temperature adsorption of CO on Cu(1 1 1) studied by RAIRS

We report here a detailed RAIRS study of carbon monoxide adsorbed on the Cu(1 1 1) surface at substrate temperatures of 15–20 K. Surface diffusion in this temperature regime is thermally limited; this results in the formation of a different, more disordered surface structure than those grown at higher temperatures. Both monolayer and multilayer physisorption is observed, and these layers are found to affect the underlying chemisorbed layer.

The structure of a coadsorbed layer of thiophene and CO on Cu([formula omitted

The influence of coadsorbed CO on the structure and bonding of thiophene (C4H4S), an aromatic heterocycle, on Cu(111) has been investigated using a combination of near edge X-ray absorption fine structure spectroscopy, normal incidence X-ray standing wavefield absorption (NIXSW) and temperature programmed desorption. The coadsorption of CO does not induce an ordering of the disordered chemisorbed thiophene layer. This lack of ordering contrasts with the behaviour of related benzene coadsorption systems where CO induces ordering of disordered layers. The results of NIXSW measurements show that CO and thiophene both adopt atop adsorption sites within the coadsorbed overlayer, the same site adopted by both in pure layers. The orientation of the thiophene within the coadsorbed layer is also similar to that adopted in a pure layer. However, the CO molecules within the coadsorbed layer are tilted, which contrasts with a linear geometry observed in pure CO layers of a similar coverage. It is suggested that the lack of any significant cooperative effects between the CO and thiophene within the coadsorbed overlayers is due to the relatively weak adsorbate–substrate interactions.

Cross section for removing chemisorbed oxygen from an aluminum target by sputtering

By chemisorption of oxygen on an aluminum target, an absolute target voltage increase is noticed which can be attributed to the reduction of the secondary electron emission from the target. By following the target voltage behavior during magnetron sputtering of the aluminum target precovered with a chemisorbed oxygen layer, the cross section for removing the chemisorbed layer could be calculated based on a proposed model that relates the measured target voltage to the target coverage.

From atomistic lattice-gas models for surface reactions to hydrodynamic reaction-diffusion equations.

Atomistic lattice-gas models for surface reactions can accurately describe spatial correlations and ordering in chemisorbed layers due to adspecies interactions or due to limited mobility of some adspecies. The primary challenge in such modeling is to describe spatiotemporal behavior in the physically relevant "hydrodynamic" regime of rapid diffusion of (at least some) reactant adspecies. For such models, we discuss the development of exact reaction-diffusion equations (RDEs) describing mesoscale spatial pattern formation in surface reactions. Formulation and implementation of these RDEs requires detailed analysis of chemical diffusion in mixed reactant adlayers, as well as development of novel hybrid and parallel simulation techniques. (c) 2002 American Institute of Physics.

Mesoscopic-scale growth of oxygen-rich films on Ru( [formula omitted]) investigated by photoemission electron microscopy

Photoemission electron microscopy (PEEM) has been used for monitoring the modifications of the Ru(0 0 0 1) surface as induced by surface oxidation. The PEEM images of the initial oxidation stages exhibit a distribution of bright grains appearing on a dark background. The bright features are assigned to a Ru x O y region considered as a precursor of an extended RuO 2 phase. The dark areas reflect the presence of the saturated chemisorption layer of oxygen terminating the Ru surface. Increase of the oxygen load is reflected in the images by the increase of both brightness and density of individual grains. The growth of the oxide areas from nucleation centers up to a thick oxide film is found to strongly depend on the oxidation temperature. Three modes differing by characteristic morphologies are distinguished. For low temperatures, randomly distributed small-size patches dominate the surface. In the high-temperature oxidation regime, the oxidation produces an image, which consists of very bright discs randomly distributed over large dark areas. For moderate temperatures, bright stripes appear and grow along the main crystallographic directions of Ru(0 0 0 1). This mode starts with the formation of nuclei, which appear as small bright grains barely detectable by PEEM (<2 μm). Further oxidation occurs as a continuous growth of linear stripes, following the symmetry of the hexagonal substrate. Finally, the pattern resembles an ordered network of stripes. For very high oxygen exposures (>10 6 L), all the images become very bright with hardly distinguishable patterns. The saturated oxygen-rich layers created in the low- and moderate-temperature oxidation regimes are thermally unstable. The corresponding PEEM images become gradually darker when keeping the sample at temperatures below the desorption onset but higher than the respective oxidation temperatures thus revealing a depletion of the oxygen concentration within the topmost layer due to oxygen transport into the bulk.

2428 微小往復摺動における油性剤の効果

An oiliness agent, for example stearic acid, makes reduction of friction coefficient by forming chemisorption layers. Predominant factors over this reduction effect are experimentally investigated in reciprocating friction with small amplitude. In this experimental condition, severe starvation of lubricant in a contact area proceeds with increasing into the friction pass. According as this starvation, defect of the chemisorption layers increases and thus the friction coefficient abruptly increases. Endurance time of chemisorption layer up to abrupt friction increase positively depends on both friction velocity and resting time of upper specimen. These results show that time to re-flow into the contact area and re-from the layer may predominantly influence the endurance of the layer.

Charge transfer photodissociation of phenol on Ag(111)

The photochemistry of phenol on Ag(111) has been investigated by post-irradiation temperature programmed desorption (TPD). Ultraviolet (UV) irradiation at 355 and 266 nm was found to affect only the chemisorption layer in direct contact with the metal surface, while leaving the multilayer virtually intact. The main photoinduced reaction was found to be photodissociation of the O–H bond of phenol. Two new peaks were observed at the mass of phenol in the post-irradiation TPD spectrum at 335 K and 455 K. These peaks were assigned to the recombinative desorption of phenoxy with the hydrogen from O–H bond photodissociation and from thermal C–H bond fission, respectively. The photodissociation cross section was measured at different wavelengths and coverages. A charge transfer type photodissociation mechanism was proposed, where hot electrons generated in the substrate by UV photons attach to the affinity level of the adsorbed phenol. The transition to the transient anionic potential then leads to facile dissociation of O–H bond. The affinity level of phenol has been estimated to lie at 3.2–3.5 eV above the Fermi level for the 1 ML case.

Chemical modification of the interfacial frictional properties of vanadium carbide through ethanol adsorption

The frictional properties of the (1 0 0) face of vanadium carbide have been measured with atomic force microscopy (AFM) as a function of ethanol exposure under ultrahigh vacuum conditions. Although exhibiting a small sticking coefficient, ethanol reacts upon adsorption at room temperature producing a thin chemisorbed layer. This reaction has been characterized through high resolution electron energy loss spectroscopy and temperature programmed desorption. These studies demonstrate the formation of a partially dehydrated species involving bonds to the surface through both oxygen and carbon atoms. The growth of this reaction layer film has been followed with X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and scanning tunneling microscopy. The results of these surface analytical measurements have been correlated with the frictional and adhesion properties measured with AFM. An approximate 40% reduction in frictional forces is observed upon the formation of a complete monolayer of a hydrocarbon reaction layer. Little change in the interfacial friction was detected at higher or lower exposures of ethanol. The origin of the friction reduction, which cannot be directly related to changes in interfacial adhesion, is assigned to a change in interfacial shear strength as a result of the growth of the ethanol decomposition species.

The interaction of gas molecules at atmospheric pressures with surfaces investigated with surface X-ray diffraction

The suitability of surface X-ray diffraction to study the structure of chemisorbed layers of light gas molecules is discussed. The sensitivity of the technique, for well ordered adlayers, is comparable to that of Auger spectroscopy (around 10-2 monolayers). Surface crystallography is illustrated for CO/Rh(111) under UHV and also for CO/Au(111) near atmospheric pressure.

XPS measurements on l-cysteine and 1-octadecanethiol self-assembled films: a comparative study

The adsorption of l-cysteine on Au(111) surfaces from the solution phase has been investigated by X-ray photoelectron spectroscopy (XPS). To check the reliability of the XPS analysis, we performed measurements on l-cysteine/Au(111) and alkanethiol/Au(111) interfaces paying particular attention to spectra time evolution during X-ray exposure. In both systems, X-ray irradiation increases the Au4f intensity, but while in l-cysteine the line shape of the S2p spectra does not change, in 1-octadecanethiol it does. The analysis of the Au4f and S2p core level regions indicates the occurrence of a dissociative molecular chemisorption on the metal surface for both systems. In the case of l-cysteine, the S2p spectra indicate the presence of a minor sulphur component attributed to the presence of unbound cysteine molecules physisorbed on the first chemisorbed layer.

Direct Molecular Imaging of NO Monomers and Dimers and a Surface Reaction on Ag{111}

Molecular and atomic resolution STM images are presented of NO monomers and dimers and the reaction products of the dimer thermal decomposition, N2O and O, on a Ag{111} surface. The weakly chemisorbed NO monomer is formed at low coverages and temperatures between 4 and 65 K and is imaged as a 0.25 A deep depression. Continued adsorption leads to multilayer dimer cluster formation at 4 K and smoother multilayer dimer packing at 65 K, with imaging of each component of some of the dimers in the top layer. Removal of the multilayer by evacuation or heating leaves the dimer chemisorbed layer, where each dimer is imaged as a protrusion. Large domains of four different ordered phases are present, which we designate as α, β, χ, and δ. This layer is quite stable at 65 K. Images including clean patches of Ag{111} showing atomic resolution of the top Ag layer enable us to present structures for each of the dimer phases, with full site assignments. The dimers are adsorbed across bridge sites at low coverage and acros...

Ethylxanthate Adsorption on Copper Sulfide Influence of the Copper Sulfide Composition

The adsorption of ethylxanthate upon copper sulfide has been studied by electrochemical impedance spectroscopy. Prior to each experiment being performed, the copper sulfide composition was fixed by imposing a constant potential. The spectra with and without ethylxanthate in the solution show no difference below the potential threshold, but above this potential the spectra differed dramatically. The shapes of the impedance diagrams were interpreted by formation of a bilayer structure at the surface of the mineral which develops in two steps at the same potential. The first layer corresponds to a chemisorbed layer while the porous second layer comprises copper xanthate. An explanation for the suppressor effects of the anions is also given in terms of the redox potential for the redox couple. © 2000 The Electrochemical Society. All rights reserved.

Mesoscopic-scale pattern formation induced by oxidation of Ru(0001)

Photoemission electron microscopy (PEEM) has been utilized for monitoring the modifications of a Ru(0001) surface induced by surface oxidation. The PEEM images of the initial oxidation stages reveal bright Ru x O y regions surrounded by dark areas formed by the saturated oxygen chemisorption layer. For a narrow particular temperature interval the oxidation starts with the appearance of light nuclei or grains (<2 μm), which expand as long stripes along the main crystallographic directions of Ru(0001). Further oxidation proceeds via growth of linear as well as star-shaped islands, which follow the hexagonal symmetry of the substrate and eventually form an ordered network. These mesoscopic-scale structures reflect the atomic-scale arrangement of the clean Ru substrate. The temperature region where the hexagonal arranged stripes exist shifts towards higher temperatures with increasing oxidation pressure. The stripes are thermally stable up to the onset of oxygen desorption. The amount of atomic oxygen incorporated in the micrometer-scale structures can be depleted by exposing the surface to CO at elevated temperatures.

Combining ellipsometry and electron microscopy for identifying the initial stages of aluminum oxidation II. Ellipsometry

The interaction of aluminum with the residual atmosphere at room temperature and in a vacuum of 3×10−7 tor is studied by means of laser ellipsometry with the data of Auger spectroscopy taken into account. In terms of the model of the layer-by-layer growth, an optical model of the reaction zone near the surface (a mixture of the metal and dissolved oxygen) is proposed. The model allowed us to distinguish the following stages of the process: the formation of a supersurface chemisorption layer, the formation of a system of subsurface chemisorption layers with the resulting reconstruction of the surface, the formation of aluminum pseudooxide, and the growth of an islet-type oxide.

Adsorption of Ethyl Pyruvate on Pt(111) Studied by XPS and UPS

The adsorption of ethyl pyruvate on Pt(111) at low temperature was investigated by XP and UP spectroscopy. The assignment of the photoelectron spectra was assisted by calculation of correlated ionization potentials. Comparison of the XP and UP spectra of the condensed and chemisorbed layer indicates a strong ethyl pyruvate adsorption bond in the latter. Upon chemisorption, the HOMO of ethyl pyruvate, which is a lone-pair orbital delocalized over both CO groups, is stabilized by about 0.7 eV with respect to the other orbitals, which is characteristic for a lone-pair bonding mechanism. The same bonding mechanism was found for coverages far below saturation. The XP spectra further indicate that the ketone CO is more strongly involved in the chemisorption bond than the carboxyl CO of ethyl pyruvate. The packing density of the saturated chemisorbed ethyl pyruvate layer, as determined by XPS, is high. This points toward an upright or tilted orientation of ethyl pyruvate in this layer, in line with the observed bonding mechanism.

Scanning tunneling microscopy of 'commensurate – incommensurate' structural phase transitions in the chemisorbed layers of halogens

Scanning tunneling microscopy of 'commensurate – incommensurate' structural phase transitions in the chemisorbed layers of halogens

Simulations of hydrogen diffusion on BCC metal (110) surfaces; coverage and temperature dependence

We study the coverage and temperature dependences of the tracer diffusion coefficient for a chemisorbed layer of interacting hydrogen atoms on a model of a bcc metal (110) surface. The surface is rigid, and the short bridge barriers between adjacent chemisorption wells are sufficiently low that hydrogen atoms diffuse actively across the surface, on a time scale compatible with our molecular dynamics simulations. We deduce the coverage dependence of the effective activation barrier and the prefactor. We also examine, as a function of coverage, the percentage of jumps from singly occupied to either empty or occupied chemisorption wells, and from doubly occupied wells to empty or singly occupied wells. Although the effective activation barrier deduced from the numerical data exhibits a weak dependence on coverage, as found in data on H diffusion on the W(110) surface, the percentage of jumps of the types mentioned varies dramatically. The prefactor in the diffusion constant extracted from the simulations agrees well with elementary expectations for the rigid surface, but is much larger than that found experimentally. Finally, the low coverage tracer diffusivity is found to be appreciably anisotropic. The anisotropy decreases substantially as coverage increases.

Adsorption Kinetics of Rigid 4-Mercaptobiphenyls on Gold

We have presented the first experimental evidence that the molecular dipole and the electron density on the S-atom affect the adsorption process of thiols on gold. The adsorption kinetics of five rigid 4-mercaptobiphenyls onto a polycrystalline gold surface has been studied by the quartz crystal microbalance (QCM) technique. The kinetics data cannot be fitted to the Langmuir equation because it does not take interadsorbate interactions into consideration. A new lattice-gas adsorption model was developed that approximates the chemisorbed layer of interacting mercaptobiphenyls as lattice-gas particles with pair interactions between nearest-neighbor sites. The interacting lattice-gas model produces much better fits to experimental data and provides quantitative estimates of the strength of the chemisorption potential and the dipolar interactions, as well as of the rate constants. The formation of self-assembled monolayers (SAMs) from mercaptobiphenyls in toluene solutions is faster than that of n-alkanethiol...

Self-organization of stressed surfaces: The role of local relaxations

We investigate, in a very simple form and using a continuum approach, the influence of the Frenkel-Kontorova mechanism on the self-organization of stressed chemisorbed layers on an anisotropic surface. The long-range substrate-mediated interactions are still the driving force for self-organization but short-range relaxations of the adsorbate domains play an important role. When the adsorbate film includes dislocations at high coverage, special values of the adsorbate domain size are favored. Surprisingly, short-range relaxations are still important for registered adsorbate films leading to modifications of Marchenko predictions.

Thermal and UV photo-induced decomposition of azomethane on Rh(111)

The thermal and UV photo-induced decomposition of azomethane, CH3NNCH3, was investigated by means of reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption spectroscopy (TPD). The RAIRS data revealed that azomethane adsorbs in the trans-configuration mode on Rh(111) at 90 K. During thermal treatment, azomethane decomposes exclusively by N–N bond scission, yielding H2, N2, C2N2 and traces of HCN. Upon UV irradiation at 90 K, adsorbed azomethane undergoes tautomerization, forming formaldehyde methylhydrazone, CH3NHNCH2. The important features of the post-irradiation TPD spectra are the significant suppression of C2N2 and N2 formation, and the appearance of the new products methylamine (CH3NH2) and CH4. C–N bond scission also occurred in the illuminated chemisorbed layer at 90 K.