Full Monolayer Coverage Research Articles

Effect of hydrogenation on the adsorption ofGeonSi(001)

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the effect of hydrogenation on the atomic geometries and energetics of half-monolayer and full-monolayer coverages of Ge on the Si(001)-$(2\ifmmode\times\else\texttimes\fi{}1)$ surface. For the half-monolayer Ge coverage without hydrogenation, we find that the Ge-nondiffused case (i.e., the mixed Ge--Si dimer structure) is 0.25 eV/dimer energetically more favourable than the Ge-interdiffused case (i.e., intermixed Ge--Si bond structure), while for the hydrogenated surface both structures become almost equally favorable. For the full-monolayer coverage of Ge without surface hydrogenation the formation of a pure Ge--Ge dimer is energetically more favourable by $0.19\ifmmode\pm\else\textpm\fi{}0.03 \mathrm{e}\mathrm{V}/\mathrm{d}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{r}$ compared with the interdiffusion of Ge into any of the second, third, and fourth substrate layers. The process of surface hydrogenation makes both the nondiffused and interdiffused structures almost equally favorable. In all cases the effect of hydrogenation is to make the surface dimer symmetric and elongated.

Dye sensitization of nanocrystalline titanium dioxide with square planar platinum(II) diimine dithiolate complexes.

A series of platinum-based sensitizers of the general type Pt(NN)(SS), where NN is 4,4'-dicarboxy-2,2'-bipyridine (dcbpy) or 4,7-dicarboxy-1,10-phenanthroline (dcphen) and SS is ethyl-2-cyano-3,3-dimercaptoacrylate (ecda), quinoxaline-2,3-dithiolate (qdt), 1,2-benzenedithiolate (bdt), or 3,4-toluenedithiolate (tdt), that have various ground-state oxidation potentials has been synthesized and anchored to nanocrystalline titanium dioxide electrodes for light-to-electricity conversion in regenerative photoelectrochemical cells with an I(-)/I(-)(3) acetonitrile electrolyte. The intense mixed-Pt/dithiolate-to-diimine charge-transfer absorption bands in this series could be tuned from 440 to 580 nm by choosing appropriate dithiolate ligands, and the highest occupied molecular orbitals varied by more than 500 mV. Spectrophotometric titration of the Pt(dcphen)(bdt) complex exhibits a ground-state pK(a) value of 3.2 +/- 0.1, which can be assigned to the protonation of the carboxylate group of the dcphen ligand. Binding of Pt(dcbpy)(qdt) to porous nanostructured TiO(2) films was analyzed using the Langmuir adsorption isotherm model, yielding an adsorption equilibrium constant of 4 x 10(5) M(-1). The amount of dye adsorbed at the surface of TiO(2) films was 9.5 x 10(-8) mol/cm(2), which is ca. 50% lower than the full monolayer coverage. The resulting complexes efficiently sensitized TiO(2) over a notably broad spectral range and showed an open-circuit potential of ca. 600 mV with an impressive fill factor of > 0.70, making them attractive candidates for solar energy conversion applications. The visible spectra of the 3,4-toluenedithiol-based sensitizers showed an enhanced red response, but the lower photocurrent efficiency observed for these sensitizers stems in part from a sluggish halide oxidation rate and a fast recombination of injected electrons with the oxidized dye.

Adsorption of 1,4-Benzenedithiol on Gold and Silver Surfaces: Surface-Enhanced Raman Scattering Study

The adsorption behavior of 1,4-benzenedithiol (1,4-BDT) on colloidal gold and silver surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). 1,4-BDT chemisorbed dissociatively on both gold and silver surfaces but as mono- and dithiolate, respectively. Regardless of the bulk concentration of 1,4-BDT, only a monolayer was assembled on the silver surface with a flat orientation by forming two Ag–S bonds. On the gold surface, the monothiolate species,1,4-BDT−1, appeared to assume a rather flat orientation at a very low surface coverage, but as the surface coverage was increased, the adsorbate took a perpendicular orientation. Furthermore, when the bulk concentration of 1,4-BDT was close to that required for a full-monolayer coverage limit, a band assignable to the S–S stretching vibration appeared at ∼536 cm−1 in the gold sol SERS spectra. A separate ellipsometry measurement performed with vacuum-evaporated gold substrates revealed that up to tetralayers could be assembled on gold in 1 mM n-hexane solution of 1,4-BDT while at best a bilayer formed in either methanol or ethanol solution. The different adsorbate structure of 1,4-BDT on gold and silver was overall quite comparable to that of p-xylene-α,α′-dithiol.

First-principles calculations of the adsorption of S on theSi(001)c(4×2)surface

The adsorption of S on the Si(001)c(4 x 2) surface is studied by first-principles total-energy calculations. We started with the adsorption of a single atom up to a full-monolayer coverage. The first S atom occupies a bridge site, on top of a Si dimer that becomes completely symmetric. At half-monolayer coverage, all bridge sites are occupied, and all Si dimers become symmetric. The overall periodicity is (2 x 1). The adsorption of an additional S atom results in the breaking of two Si dimers. From this point and up to one monolayer, it is energetically more favorable for the S atoms to be adsorbed along the [{bar 1}10] direction. At full monolayer coverage, all Si dimers are broken, and the Si surface is dereconstructed with all Si atoms near bulk ideal positions.

Acetylene on Si(100) from first principles: adsorption geometries, equilibrium coverages, and thermal decomposition

Adsorption of acetylene on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is a “bridge” configuration, where the C2H2 molecule is bonded to two Si atoms. Instead, “pedestal” configurations, recently proposed as the lowest-energy structures, are found to be much higher in energy and, therefore, can represent only metastable adsorption sites. We have calculated the surface formation energies for two different saturation coverages, namely 0.5 and 1 monolayer, both observed in experiments. We find that although, in general, the full monolayer coverage is favored, a narrow range of temperatures exists in which the 0.5 monolayer coverage is the most stable one, where the acetylene molecules are adsorbed in a 2×2 structure. This result disagrees with the conclusions of a recent study and represents a possible explanation of apparently controversial experimental findings. The crucial role played by the use of a gradient-corrected density functional is discussed. Finally, we study thermal decomposition of acetylene adsorbed on Si(100) by means of finite-temperature molecular dynamics, and we observe an unexpected behavior of dehydrogenated acetylene molecules.

Relativistic density functional study of gold coated magnetic nickel clusters

Coating of magnetic clusters by gold atoms is becoming an experimental technique of increasing interest for passivation and stabilization of these small metal particles. To computationally investigate the effect of gold coating, we have studied the magnetic clusters Ni6 and Ni13 employing an all-electron scalar-relativistic density functional method. We examine two series of octahedral clusters with increasing gold coverage of up to a monolayer: Ni6Aun (n=0,8,32) and Ni13Aun (n=0,6,8,14,24,30,42). Structural features, binding energies, and gold adsorption energies are determined and discussed. The different atomic radii of Au and Ni lead to rather short Au–Au and relatively long Ni–Ni distances in these clusters. The Au–Ni contacts are found to be the longest nearest-neighbor distances; a detailed analysis indicates these bonds to be the strongest in these Au-covered Ni clusters. The atomization energies change only slightly with increasing Au coverage. Also, the effect of gold adsorption on the magnetic properties of the Ni cores is analyzed. For the Ni6Aun series the magnetism decreases with n, while for Ni13Aun a maximum cluster magnetization is calculated for incomplete gold coverage. This different behavior of the two cluster series can be traced to differing numbers of unpaired electrons per atom in the pure Ni clusters and to an increased magnetic moment due to the adsorption of isolated Au atoms. Both series exhibit a residual magnetism at full monolayer coverage of the Ni cores.

Adsorption and decomposition of NO on Pt (112)

Adsorption and decomposition of NO on Pt (1 1 2) have been studied by temperature programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). NO adsorbs molecularly on Pt (1 1 2) at 95 K. About half amount of NO molecules adsorbs at the terrace sites and remaining half amount adsorbs at the step sites at a full monolayer coverage. Then about half of NO molecules adsorbed at step sites decomposes at around 483 K desorbing N 2, promptly.

Rapid Self-Assembly of Alkanethiol Monolayers on Sputter-Grown Au(111)

Atomically flat, sputter-grown Au(111) films allowed well-ordered alkanethiol (exemplified by octanethiol) monolayers to be self-assembled from solution markedly faster and in larger domain sizes than previously reported. An X-ray photoelectron spectroscopy analysis showed that complete monolayer coverage was reached by 0.2−60 min of incubation in 0.1−0.001 mM ethanolic solution at room temperature (∼17 °C), with single-step (0.1 and 0.01 mM) or two-step (0.001 mM) adsorption kinetics. Increasing the temperature to 35 °C was enough to cause a single-step, diffusion-controlled adsorption also from the 0.001 mM solution, yielding the full monolayer coverage in approximately 10 min. Scanning tunneling microscopy (STM) imaging proved that well-ordered islands, with the (√3 × √3)R30° structure more-or-less strongly modulated by the c(4 × 2) superlattice, begin to form at 0.6−0.7 monolayer coverage most likely by homogeneous nucleation and grow rapidly thereafter. This kinetics of ordering requiring the conside...

Adsorption Characteristics of p-Xylene-α,α‘-dithiol on Gold and Silver Surfaces: Surface-Enhanced Raman Scattering and Ellipsometry Study

The adsorption behavior of p-xylene-α,α‘-dithiol (p-XDT) on colloidal gold and silver surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). p-XDT chemisorbed dissociatively on both the gold and silver surfaces but as mono- and dithiolate, respectively. Regardless of the bulk concentration of p-XDT, only a monolayer was assembled on the silver surface with a flat orientation by forming two Ag−S bonds. On the gold surface, the monothiolate species, p-XDT1-, appeared to assume a rather flat orientation at a very low surface coverage, but as the surface coverage was increased, the adsorbate took a perpendicular orientation. Furthermore, when the bulk concentration of p-XDT was close to that required for a full-monolayer coverage limit, a band assignable to the S−S stretching vibration appeared at ∼509 cm-1 in the gold sol SER spectra. A separate ellipsometry measurement performed with vacuum-evaporated gold substrates revealed that up to trilayers could be assembled on gold in 1...

Gauche-Trans Conformational Changes in Ethyl Formate Chemisorbed on Ni(111): Coverage-Dependent RAIRS Spectra

Infrared reflectance spectra of ethyl formate on Ni(111) at 105 K were recorded as a function of surface coverage. The RAIRS activities of several bands are found to vary in a correlated manner as a function of coverage. The activity of both the asymmetric stretching vibration and the twisting vibration of the CH2 group at low coverages conclusively show that adsorption occurs initially in the s-cis,gauche conformation. These two bands fade out on increasing the coverage and are completely removed at full monolayer coverage. The coverage dependence is interpreted in terms of rotational isomerization to a planar conformation as the packing density in the adlayer is increased. The results are consistent with an attribution of the full coverage molecular structure to the s-trans,trans conformation.

Pd–Ni nanoparticles: segregation and reactivity for the 1,3-butadiene hydrogenation reaction

A Monte-Carlo (MC) simulation has been employed to study the coordination of surface atoms in hydrogen-covered 2406-atom Pd 1Ni 99 and Pd 5Ni 95 nanoparticles. For both systems, Ni is found to segregate to the surface. It has been found that, for a full monolayer coverage of hydrogen, the average number of surface Pd neighbour atoms per surface Ni atom, 〈 n Ni–Pd〉, for Pd 5Ni 95 alloy surface is 6 times larger than the corresponding number 〈 n Ni–Pd〉 for the Pd 1Ni 99 alloy. The surface coordination in the alloys has been shown to explain qualitatively the experimentally observed increase in activity of Pd 5Ni 95 alloy over Pd 1Ni 99 alloy for the 1,3-butadiene hydrogenation reaction.

Ab initio study of hydrogen adsorption on the ZnO [formula omitted] surface

Chemisorption of hydrogen at half and full monolayer coverage on the nonpolar ZnO (101̄0) surface has been studied by means of the ab initio Hartree–Fock method in the two-dimensionally periodic slab model. Both surface and hydrogen geometry were optimized for hydrogen bound at surface zinc and oxygen atoms. We find adsorption energies to be substantial for heterolytic dissociation of hydrogen molecules. ZnO surface derelaxation plays an important role in the chemistry of the adsorption process. A posteriori correlation corrections to the total energies have been calculated, and the dependence of adsorption properties on surface coverage is discussed.

SIMULATION STUDIES OF ADSORPTION-INDUCED SURFACE SEGREGATION IN BIMETALLIC Pd–Cu NANOSTRUCTURES

We use the Monte-Carlo (MC) simulation technique to study the role of various coverages of (CO) adsorption on the spatial distribution of atoms on the surface of fcc cubo-octahedral Pd–Cu nanoparticles. The coordination-dependent pair interaction energy, required for the MC simulation, has been computed by an empirical formula obtained from the experimental values of dimer energy, (100) surface energy and the energy for monovacancy creation for constituent metals. It has been found that for clean particles of all sizes the surface is enriched in Cu. However, with increase in (CO) adsorption the extent of Cu segregation gradually reduces and at higher (CO) coverage there is segregation reversal leading to Pd segregation. At low coverage, usually the six- and seven-coordinated sites are occupied by Cu atoms. But at higher coverage (θ>0.25) they are also gradually occupied by Pd atoms along with the fcc(100)- and fcc(111)-like face sites. For a full monolayer coverage surface sites of all coordinates are occupied by Pd atoms. The significance of these results in the context of catalytic activity of alloys is discussed.

Complex terraced spreading of perfluoropolyalkylether films on carbon surfaces

Complex molecular layering of OH-terminated perfluoropolyalkylether films over amorphous carbon surfaces has been observed using spatially resolved microellipsometry. The first layer is diffusive in nature, and the subsequent layers exhibit sharp steps of about twice the thickness of the first layer. This behavior, characteristic of a coexisting two-dimensional gas, with cohesive, liquidlike multilayers, is in clear contrast with that of the ${\mathrm{CF}}_{3}$-terminated analog, which shows a smooth diffusive profile. In this particular case, spreading, for thickness greater than one monolayer, can be exactly described by a Poiseuille flow in a disjoining pressure gradient originating solely from van der Waals interactions. For both types of polymers, diffusion rate reaches a maximum at full monolayer coverage, and decreases in the submonolayer regime. Spreading for both types of polymers unambiguously follows a ${t}^{1/2}$ time dependence.

Orientational Ordering in Monolayer Films of Molecular Hydrogen on Boron Nitride

NMR studies of monolayer hydrogen films for both the expected \(\sqrt {\text{3}} \times \sqrt {\text{3}} \) commensurate solid and the incommensurate solid (for full monolayer coverage) on hexagonal boron nitride (BN) are reported for ortho-H2concentrations 0.30≤X≤0.74 and temperatures 0.08 Xcritfor commensurate films.

Icelike Water Monolayer Adsorbed on Mica at Room Temperature

The structure of a water film formed on mica at room temperature, in equilibrium with water vapor at various relative humidities (RH), was studied using sum-frequency-generation (SFG) vibrational spectroscopy and scanning polarization force microscopy (SPFM). Analysis of the O-D stretch modes in the SFG spectra of D{sub 2}O on mica indicates that as RH increases, the submonolayer water structure evolves into a more ordered hydrogen-bonding network. At full monolayer coverage ({approximately} 90{percent} RH) , the SFG spectrum suggests an icelike film with no dangling O-D groups, in agreement with a recent molecular dynamics simulation. {copyright} {ital 1998} {ital The American Physical Society}

Preparation of Novel Core‐Shell Nanocomposite Particles by Controlled Polymer Bridging

Ceramics that are composed of nanosized granules show enhanced properties in various applications. A layer‐by‐layer deposition process is developed in this study to prepare core—shell nanocomposite particles, using controlled polymer adsorption. Submicrometer‐sized alumina particles that have been chosen as the starting core particles are first dispersed by pH control and then modified by adsorption of anionic poly(acrylic acid) to facilitate the subsequent deposition of nanoparticles on them. A key step in the processing involves the removal of excess polymer in the supernatant after the adsorption by controlled washing, so that the added nanoparticles do not self‐flocculate among themselves. A coating of nanosized particles is indicated by the charge reversal of the micrometer‐sized composite particles. The isoelectric point of the composite particle is similar to that of nanosized particles under both low and high coverage conditions. Scanning electron microscopic observation of coated particles shows full monolayer coverage of the nanoparticles on core particles, which confirms the efficiency of the coating scheme that is discussed here. The present study reveals the feasibility of coating nanoparticles onto micrometer‐sized core particles, using controlled polymer bridging, for the preparation of the multilayer nanocomposite powders.

IQENS from hydrated lysozyme

We present quasi-elastic incoherent neutron scattering data on wet lysozyme powders at water contents (grams of water/grams of protein) of h = 0.20 and 0.45, corresponding to hydration levels below and above the full monolayer coverage of the protein surface. The data were collected under H 2O and D 2O hydration condition; using the IRIS spectrometer with a resolution of 11 μeV (full-width). From spectra for the D 2O hydrated sample we obtain information on the protein dynamics. These spectra are used to separate from the H 2O hydrated sample the contribution due to the hydration water. We show that at low hydration the dynamics of water is very slow and on the same time scale as that of the protein dynamics, while at higher hydration the water molecules show a faster dynamics. The results support the idea that there are clusters of hydration water.

Calculation of atomic geometry, electronic states, and bonding for the S-deposited InP(110) surface

Ab initio calculations, based on pseudopotentials and the density functional theory, have been made to investigate the equilibrium atomic geometry, electronic states, and bonding of S adsorbates on the InP(110) surface with $(1\ifmmode\times\else\texttimes\fi{}1)$ periodicity. For nonreacted half-monolayer coverage of S, we find that the interchain bridging geometry has the lowest total energy. For nonreacted full monolayer coverage the epitaxially continued layer structure is found to be the most energetically favorable, and exhibits a good semiconducting nature. We have also studied four geometrical models which involve a reaction of S atoms with the substrate. For an ordered reacted model with the adsorbate S atoms exchanged with their neighboring P atoms, the average vertical distance between the top two layers is 1.98 \AA{}, which is in agreement with a recent result obtained from x-ray standing wave analysis, but is characterized by a small band gap.

Repulsive interactions of potassium on Re(001)

Repulsive interactions of potassium on Re(001) were investigated using temperature programed desorption (TPD), work-function measurements, and optical second-harmonic generation (SHG). The TPD and work-function results were used for the first time to evaluate critically the validity of available electrostatic models for explaining simultaneously both sets of data. The activation energy for desorption in the limit of zero coverage was determined to be 68.4±0.1 kcal/mol, decreasing monotonically with increasing potassium coverage to 22.1±0.1 kcal/mol at a full monolayer (ML) coverage. The TPD data were fit well by a depolarization model which includes the adsorbate–surface distance. The work function decreases monotonically upon potassium adsorption until reaching a minimum value of 4.35 eV below the work function of the bare rhenium surface at a coverage of approximately 0.5 ML, after which the work function increased to an ultimate value of 3.00 eV below the work function of Re(001). The work function of the bare Re(001) surface was determined to be 5.26±0.05 eV. The work-function data were fit using a model which considers the change in work function assuming local contributions by the substrate and by the adsorbate, using the same parameters obtained from fitting the TPD data. SHG measurements were consistent with previous alkali–transition metal systems, including a characteristic resonantlike signal enhancement at a potassium coverage of 0.3–0.4 ML. The SHG data taken during potassium adsorption and desorption complement information obtained by TPD and work-function measurements.