Moderate Coverages Research Articles

First-principles studies of chiral step reconstructions of Cu(100) by adsorbed glycine and alanine

Adsorption of amino acids on Cu(100) is known experimentally to induce surface reconstructions featuring intrinsically chiral Cu(3,1,17) facets, but no information about the geometry of the molecules on these chiral facets is available. We present density-functional theory calculations for the structure of glycine and alanine at moderate coverages on Cu(3,1,17). As might be expected, molecules prefer to bind at the step edges on this surface rather than on the surface's (100)-oriented terraces. The adsorption of enantiopure alanine on Cu(3,1,17) is predicted to be weakly enantiospecific, with S-alanine being more stable on Cu(3,1,17)(S) than R-alanine. By comparing the surface energies of Cu(100) and Cu(3,1,17) in the presence of adsorbed glycine or alanine, our calculations provide insight into the driving force for chiral reconstructions of Cu(100) by amino acids.

The detachment of a viscous drop in a viscous solution in the presence of a soluble surfactant

When a buoyant viscous drop is injected into a viscous fluid, it evolves to form a distended shape that detaches via the rapid formation and pinching of a neck. The effects of surfactants in altering this process are studied numerically. In the absence of surfactants, surface contraction is fastest in the vicinity of the neck. Thus, when surfactants are present, they accumulate there and alter the ensuing dynamics by reducing the surface tension that drives the contraction. The surface tension is described by a nonlinear surface equation of state that accounts for the maximum packing of surfactant in a monolayer. When surfactant adsorption-desorption is very slow, interfaces dilute significantly during drop expansion, and drops form necks which are only slightly perturbed in their dynamics from the surfactant-free case. When adsorption-desorption dynamics are comparable to the rate of expansion, drops thin to form a primary neck at low surfactant coverage, to form both primary and secondary necks at moderate coverages, form only a secondary neck at higher coverages, or fail to neck at elevated coverages. When surfactant adsorption-desorption kinetics are rapid, the surface remains in equilibrium with the surrounding solution, and drops behave like surfactant-free drops with a uniform surface tension. These arguments are used to construct a phase diagram of drop neck shapes as a function of surfactant coverage. A map of neck/no-neck thresholds is also constructed as a function of surfactant coverage and sorption dynamics, suggesting that drop detachment can be used as a means of characterizing surfactant dynamics.

Highly Bendable, Transparent Thin‐Film Transistors That Use Carbon‐Nanotube‐Based Conductors and Semiconductors with Elastomeric Dielectrics

Transparent flexible thin-film transistors (see Figure) have been made using single-walled carbon nanotube networks of high and moderate coverages as the conducting and semiconducting layers. Electrical (e.g., good performance on plastic), optical (e.g. transparency to visible wavelengths), and mechanical (e.g. extreme flexibility) characteristics that would be difficult, or impossible, to achieve with conventional materials are reported. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2006/c1740_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Quantitative study of adsorbate-adsorbate interactions of hydrogen on the Si(100) surface

The equilibrium spatial distribution of hydrogen atoms adsorbed on the clean $\mathrm{Si}(100)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ surface under ultrahigh vacuum conditions has been investigated by means of scanning tunneling microscopy (STM). Singly occupied dimers, doubly occupied dimers, and clusters of adjacent doubly occupied dimers along a dimer row are observed. Through the evaluation of large-area STM images, a quantitative assessment of the prevalence of the different adsorbate configurations has been obtained for a range of hydrogen coverages from 0.03 to 0.59 monolayers. At moderate coverages, most of the hydrogen adatoms are found in doubly occupied dimers, while a relatively small number are present as singly occupied dimers or as part of clusters of doubly occupied dimers. The interaction between doubly occupied dimers was examined by determining the experimental size distribution of clusters and the spatial correlation function for the doubly occupied dimers. A nearest-neighbor interaction model is compared with experiment within analytic approximations and through full Monte Carlo simulations. The model is found to agree well with the experimental results. An effective pairing energy of $\ensuremath{\varepsilon}=0.31\ifmmode\pm\else\textpm\fi{}0.04\mathrm{eV}$ and an effective clustering energy of $\ensuremath{\omega}=0.04\ifmmode\pm\else\textpm\fi{}0.01\mathrm{eV}$ are inferred. These results are independent of any hypothesis concerning the adsorption/desorption pathways, but have implication for possible pathways and their kinetics.

Image and adsorbate state dependence on the adsorbate coverage in the Na/Cu( [formula omitted]) system

A model is developed to describe the adsorbate coverage dependence of excited state (image states and adsorbate-localised states) on a metal surface. It is applied to the case of Na adsorbates on a Cu(1 1 1) surface. It allows to define in a consistent way (i) an effective potential, which is only function of the electron-surface distance representing the average perturbation introduced by a large set of alkali adsorbates on the surface and (ii) the local perturbation induced around each adsorbate. It can be applied up to moderate coverages, typically for Na-induced work function changes below 1.5 eV. In this range, its predictions reproduce the experimental findings of Fischer et al. [Surf. Sci. 314 (1994) 89] for the energy of the image states and Na-localised state.

Adsorbates in phonon bath: moderate coverage of Cu(2 1 1) by CO molecules

Using molecular dynamics combined with semiempirical potentials, we have modeled, in the presence of a phonon bath, the adsorption of CO molecules on Cu(2 1 1) stepped surfaces at moderate coverages. Our calculations confirm the scanning tunneling microscopy results by Meyer et al. [Chem. Phys. Lett. 240 (1995) 379] that, at a coverage of 1/2 ML, the CO molecules are adsorbed on top, at the border of steps forming a (2 × 1) structure. At higher coverages, and/or temperatures, the overlayer structure is more complicated and CO molecules adsorb at step borders near a bridge position, seldom in the middle of a terrace. For the first time, relaxation of the substrate in the presence of the adsorbates is accounted for. At low temperature, the vibrational analysis of the (2 × 1) overlayer structure shows a mode at 61 cm −1 due to collective vibration of CO molecules. The CO molecules vibrate mainly perpendicular to the step borders of Cu(2 1 1) surface out of phase. Above the bulk cut-off frequency, the local phonon spectral density of the first two layers of the substrate displays a coupling mode between CO molecule and Cu(2 1 1) substrate, vibration consistent with the adsorption at the border of the steps.

Solvation and Ionization Stages of HCl on Ice Nanocrystals

The study focuses on acid adsorption on cold ice particle surfaces. The investigation encompasses HCl, DCl, and HBr adsorbate spectroscopy, Monte Carlo simulations of molecular HCl adsorbate on a model ice particle, and ab initio studies of HCl solvation and ionization in mixed acid−water clusters. It is shown that ice nanocrystal surfaces offer a range of adsorption sites, in which HCl freezes in different recognizable solvation stages. These stages were identified spectroscopically and assigned, with the help of calculations, to weakly and strongly stretched HCl molecules, and to Zundel and hydronium ions that are products of proton transfer. At moderate submonolayer coverages in the 50−60 K range, the acid adsorbate is predominantly molecular. Heating promotes formation of contact hydronium − chloride ion pairs. Near 90 K, an ionization burst is observed, resulting in an ionic surface hydrate layer rich in Zundel cations.

Adsorption and separation of hydrogen isotopes in carbon nanotubes: Multicomponent grand canonical Monte Carlo simulations

Adsorption isotherms of hydrogen isotopes from molecular simulations in carbon nanotubes and interstices are presented. The adsorption of pure isotopes follows Henry’s law up to moderate coverages. A modified path integral grand canonical Monte Carlo (PI-GCMC) technique for mixture adsorption is presented and applied to adsorption of isotope mixtures in carbon nanotubes. Adsorption isotherms of H2–T2 mixtures in nanotubes and interstices are determined at 20 and 77 K. Selectivities for T2 over H2 are calculated over a range of pressures. Selectivity in the nanotubes and interstices increases with pressure until the nanotube is saturated. Comparison of simulation results with predictions based on ideal adsorbed solution theory (IAST) shows good agreement up to moderate loadings. At higher loadings, selectivities determined from multicomponent simulations remain roughly constant, whereas IAST predicts continued increase in selectivities. Isotherms for H2–D2 and the selectivities of D2 over H2 are determined for adsorption in (10,10) nanotubes and in the interstitial channels of closed (10,10) nanotubes.

The reliability of vibrational spectroscopy as a means of identification of the structures of chemisorbed species on metal surfaces: the cases of CO, NO and C 2 hydrocarbon surface species

A review of the literature shows that vibrational spectroscopy has been extremely successful in anticipating the structures of C 2 chemisorbed species that are later evaluated by the more direct methods of photoelectron diffraction (PED), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). More difficulties have attended these comparison procedures as applied to the spectra of adsorbed CO and NO, where the diffraction methods have shown that on (1 1 1) planes the 3-fold bridged species on ‘hollow’ sites occupy a wider range of coverages than had been anticipated spectroscopically. Exceptional coverage dependence of the positions of the νCO and νNO absorption bands leads to overlap of the characteristic wavenumber ranges for the 2-fold bridge and 3-fold bridged surface species except at low coverages. The high coverage-dependence of these spectra is associated with the exceptionally high intensity of these absorptions leading to vibrational dipolar coupling and to sensitivity to static electronic interactions with neighbouring adsorbed molecules. Such factors are minimal in the spectra of the C 2 species. It is concluded that the 3-fold bridged CO surface species has an extended wavenumber range 1950–1800 cm −1 (high to low coverage), which overlaps at moderate coverages with that of the 2-fold species between 2000 and 1870 cm −1. An exhaustive literature survey has yielded improved νNO wavenumber ranges for adsorbed NO species at low coverages, namely M–NO linear (on-top site), 1800–1720 cm −1; M–NO bent, 1700–1620 cm −1; 2-fold bridge, 1570–1500 cm −1; 3-fold bridge (hollow site) 1470–1380 cm −1.

Preferential cluster nucleation on long-range superstructures on Al 2O 3/Ni 3Al(1 1 1)

The room temperature growth of metal clusters on well-ordered alumina films on Ni 3Al(1 1 1) was investigated by scanning tunneling microscopy (STM). The electronic superstructures that appear in the STM images of the bare oxide films, constitute nucleation centers for the cluster growth. For Ag clusters this pinning operates only at low metal coverage, whereas higher coverages lead to large agglomerates of the clusters. This is attributed to the relatively weak bonding of Ag to the alumina film. Conversely, in the case of Mn clusters the pinning persists even for moderate metal coverages owing to the much stronger bond between metal and oxide film. Finally, at high metal coverage the pinning is lost, the Mn clusters do, however, not agglomerate.

Uranium(VI) Sorption Complexes on Montmorillonite as a Function of Solution Chemistry

We have investigated the effect of changes in solution chemistry on the nature of uranyl sorption complexes on montmorillonite (SAz-1) at different surface coverages (1.43–53.6 μmol/g). Uranyl uptake onto SAz-1 between pH 3 and 7 was determined in both titration and batch-mode experiments. These pH values result in solutions that contain a range of monomeric and oligomeric aqueous uranyl species. Continuous-wave and time-resolved emission spectroscopies were used to investigate the nature of U(VI) sorbed to SAz-1. A discrete set of uranyl surface complexes has been identified over a wide range of pH values at these low to moderate coverages. For all samples, two surface complexes are detected with spectral characteristics commensurate with an inner-sphere complex and an exchange-site complex; the relative abundance of these two species is similar over these pH values at low coverage (1.43–2.00 μmol/g). In addition, surface species having spectra consistent with polymeric hydroxide-like sorption complexes form at the moderate coverages (∼34–54 μmol/g), increasing in abundance as the capacity of the amphoteric surface sites is exceeded. Furthermore, a species with spectral characteristics anticipated for an outer-sphere surface complex is observed for wet paste samples at low pH (3.7–4.4) and both low (∼2 μmol/g) and moderate (∼40 μmol/g) coverage. There are only subtle differences in the nature of sorption complexes formed at different pH values but similar coverages, despite markedly different uranyl speciation in solution. These results indicate that the speciation in the solution has minimal influence on the nature of the sorption complex under these experimental conditions. The primary control on the nature and abundance of the different uranyl sorption complexes appears to be the relative abundance and reactivity of the different sorption sites.

Evidence of repulsive interactions between NO, O, and N on Ir(110). A fast x-ray photoelectron spectroscopy study

The adsorption of NO on Ir(110) was studied in the temperature range from 200 to 500 K with fast x-ray photoelectron spectroscopy (XPS) using synchrotron radiation. Only three surface species were observed: molecular NO, atomic O, and atomic N. NO dissociation starts at temperatures higher than 300 K and molecular NO can coadsorb with Oads in the whole temperature region studied. By collecting O 1s and N 1s spectra within a few seconds we obtained information on the interaction between Nads, Oads, and NOads with increasing coadsorbate coverage. The Nads–Oads interaction has been found to be strongly repulsive. As a consequence, the Nads is destabilized already at moderate coverages and temperatures above 350 K leading to the removal of Nads from the surface in the form of N2 desorption.

Stress-induced anisotropy of phosphorous islands on gallium arsenide

The initial growth of (2×4) phosphorous islands on (4×2) terraces of gallium arsenide (001) has been studied. The islands grow anisotropically in the [110] direction with an aspect ratio of approximately 8 to 1 at moderate coverages. The distribution of island widths in the [1̄10] direction follows a Gaussian function. The mean width increases from 24±6 to 47±11 Å as the phosphorous coverage increases from 0.10 to 0.85 monolayers. Evidently, the island anisotropy is caused by stress imposed on the underlying gallium layer by the smaller, more tightly bound phosphorous dimers.

Effect of adsorption of anions on the kinetics of the copper adatom layer formation at polycrystalline platinum

The formation of copper adatom layers at polycrystalline platinum in sulfate and perchlorate solutions is studied by cyclic voltammetry on a ring-disk electrode. Specific adsorption of anions accelerates the copper underpotential deposition by decreasing the positive potential of the dense part of EDL. Of importance is the anion composition of co-adsorption lattices that form at moderate coverages of platinum by copper. Supermonolayer coatings form under the conditions of strong specific adsorption of anions. In a weakly acid sodium sulfate solution, the surface coverage by copper adatoms reaches 1.7 monolayers.

Trapping-Mediated and Multilayer-Induced Dissociative Chemisorption of Cyclobutane on Ru(001)

The dissociative chemisorption of cyclobutane on the Ru(001) surface has been studied using thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS). Two distinct methodologies have been used to study this reaction. First, we investigated the trapping-mediated reaction by exposing the surface to cyclobutane at 180 K. Second, although an adsorbed monolayer of cyclobutane undergoes nearly complete desorption during subsequent heating, we find that the presence of the condensed multilayer induces significant reaction. Using HREELS we find that both reaction channels yield a common intermediate which we identify to be a C4H8 metallacycle. At moderate and high surface coverages, a small amount of butene from the isomerization of the metallacycle desorbs between 185 and 225 K. Following the desorption of butene, we observe a hydrocarbon fragment on the surface which is tentatively identified to be a C4H4 metallacycle.

Co-adsorption of cesium and oxygen on GaAs(001) surfaces studied by metastable de-excitation spectroscopy

Metastable de-excitation spectroscopy (MDS) has been employed for monitoring the electronic structure variation in Cs- and oxygen-adsorption processes at a Ga rich p-GaAs(001)-(4×2) surface. MDS spectra at low Cs coverages showed a peak at 2.6 eV below E F due to filling of Ga dangling bonds by charges from Cs 6s. At moderate coverages a Cs 6s-induced peak (6s̃) appeared just below E F with an abrupt increase at around θ*∼0.5, whereas contribution from substrate valence states remained on the MDS spectrum. This suggests that the Cs-induced electronic states are not fully delocalized. Upon admission of oxygen onto the surface, O 2p-induced states (2p̃) appeared with multiple peak structures, implying direct bonding of oxygen with substrate atoms even at the initial oxygenation stage. Variations in the Cs 6s̃ and O 2p̃ emissions with oxygen supply showed enhanced oxygen uptake induced by the charge transfer from the Cs 6s̃ states.

Molecular-dynamics simulation of structures and dynamics of n-butane adlayers on Pt(111)

We have used molecular-dynamics simulations to study the structure and mobility of n-butane adlayers on Pt(111). Simulations were performed with a united-atom model of n-butane for adlayer coverages ranging from the submonolayer to the multilayer regime. At submonolayer coverages, n-butane molecules adsorb with their molecular plane parallel to the surface. Upon increasing coverage close to a monolayer, some of the molecules form a tilted structure with their long axes oriented away from the surface. The in-plane ordering of the molecules has been studied. At coverages near monolayer saturation, the molecules exhibit temperature-dependent ordering similar to that observed in experimental low-energy electron diffraction studies. At moderate submonolayer coverages, the diffusion activation energy is coverage-independent and the diffusion coefficients can be accurately described using a site-blocking model. The site blocking model is not accurate at low coverages where correlated flights become important, and at coverages close to a monolayer. In the multilayer regime, molecules in the top layer are an order of magnitude more mobile than those in the layer adjacent to the surface. Finally, we discuss the vibrational characteristics of the adsorbed butane molecules by analyzing their velocity autocorrelation functions.

Wide angle X-ray scattering (WAXS) study of “two-line” ferrihydrite structure: Effect of arsenate sorption and counterion variation and comparison with EXAFS results

Wide angle X-ray scattering (WAXS) measurements have been made on a suite of “two-line” ferrihydrite (FHY2) samples containing varying amounts of coprecipitated arsenate. Samples prepared at pH 8 with counter ions chloride, nitrate, and a mixture of both also were examined. The raw WAXS scattering functions show that “two-line” ferrihydrite actually has a large number of non-Bragg (i.e., diffuse scattering) maxima up to our observation limit of 16 Å −1. The type of counter ion used during synthesis produces no significant change in this function. In unarsenated samples, Radial Distribution Functions (RDFs) produced from the scattering functions show a well-defined Fe-O peak at 2.02 Å in excellent agreement with the mean distance of 2.01 Å from extended X-ray absorption fine structure (EXAFS) analysis. The area under the Fe-O peak is consistent with only octahedral oxygen coordination about iron, and an iron coordination about oxygen of 2.2, in agreement with the EXAFS results, the sample composition, and XANES measurements. The second peak observed in the RDFs is clearly divided into two populations of correlations, at 3.07 and 3.52 Å, respectively. These distances are close to the EXAFS-derived Fe-Fe subshell distances of 3.02–3.05 and 3.43–3.46 Å, respectively, though this is misleading as the RDF peaks also include contributions from O-Fe and O-O correlations. Simulated RDFs of the FeOOH polymorphs indicate how the observed RDF structure relates to the EXAFS pair-correlation function, and allow comparisons with an ordered ferrihydrite structure. The effect of increasing arsenate content is dramatic, as the RDF peaks are progressively smeared out, indicating a wider range of interatomic distances even at moderate surface coverages, and a loss of longer range correlations. At an As/Fe ratio of 0.68, the surface saturation level of arsenate, the RDF shows little order beyond what would be expected from small pieces of dioctahedral Fe oxyhydroxyl chains or small “sheet” units. Analysis of the first RDF peak yields components due to As-O and Fe-O correlations. As the As-O component at 1.67 Å increases in size, the Fe-O component decreases, reflecting a decrease in Fe coordination about the average oxygen. This reduction is consistent with a decrease in mean crystallite size as suggested by EXAFS studies. Analysis of the second RDF peak components shows the progressive decrease in Fe-Fe correlations, and the enhancement of As-Fe correlations, as arsenate level increases. Comparison of the experimental RDF from coprecipitated arsenate-saturated FHY2 with simulated RDFs of model iron oxyhydroxyl structures further constrains possible sizes and geometry for the precipitates, and is consistent with sorbed complexes of the bidentate binuclear (apical oxygen sharing) type.

Dissociation and electronic transitions in low energy grazing collisions of CO +2 ions with W(110) surfaces

Electron energy spectra from slow (50 eV) CO + 2 molecular ions colliding under grazing incidence (5°) with W(110) surfaces are reported. The surface work function was varied by the exposure to alkali atoms. The electron emission is caused by Auger processes, such as Auger capture for clean W(110) and at moderate alkali coverages, and Auger deexcitation of core excited CO∗ 2 states (formed by the resonant capture of a surface electron by the CO + 2 projectile) at larger alkali coverages (> 0.3 monolayers). A comparison is made with the spectra of the electrons ejected in CO + collisions (also reported here). The CO + and CO + 2 induced spectra are very similar for coverages beyond 0.5 monolayers. This can be explained by assuming that in this coverage range efficient dissociation of the core excited CO 2 states is in competition with the Auger process. The dissociation apparently leads to the formation of excited CO∗ fragments whose Auger deexcitation is observed. O − ions are seen in the spectra of the ions scattered into 90 deg with respect to the beam direction. These could originate from oxygen atoms from the dissociation process which have captured a surface electron thus forming O − ions.

Infrared Study of the Adsorption of 1-Aminopropyltrialkoxysilanes on Silica at the Solid/Liquid Interface

Infrared spectra are reported for silica in two states of surface hydroxylation immersed in carbon tetrachloride containing in particular (MeO)3SiCH2CH2CH2NH2. Adsorption involved hydrogen bonding between silanol and OMe or NH2 groups. At moderate coverages the silane was bonded to two or three silanol groups via OMe groups, but at high coverages reorientation in the adsorbed layer accompanied further adsorption, leading to 1:1 silanol:silane hydrogen bonding solely via amino groups. This behavior was unaffected by the degree of overall surface hydroxylation. Adjacent interacting surface silanol were more reluctant to be involved in bi- or tridentate bonding but at high silane concentrations also gave a 1:1 interaction via NH2 groups.