Theory of chemisorption

We determine the electronic structure of a semiinfinite crystal with a general coverage by adatoms, if there exists a partial order described by the long-range order parameter5. The tight-binding formalism is used, the presence of the surface is included via the surface Green function method, and the effect of disorder is described by a version of the coherent potential approximation. The local densities of states at/near the surface as well as the interaction energy of adatoms are calculated as functions of the coverage, and the long-range order parameterS. The numerical examples modelling the cases of a gas adsorption on the transition metal surfaces as well as the transition metal atoms on the transition metal surfaces, are investigated.     

Effects of water and electric field on atomic oxygen adsorption on Pt–Co alloys

Density functional theory is used to study the effect of atomic oxygen adsorption at various coverages with and without the presence of water on ordered and Pt-segregated PtCo surfaces. The strength of O adsorption, as well as surface reconstruction effects due to the adsorbate are strongly influenced by the presence of the oxygen-philic transition metal on the surface or subsurface. At high O coverage, buckling of the Co atom on PtCo surfaces is much smaller than that of Pt on Pt(1 1 1) surfaces, and buckling of Pt atoms on Pt-skin surfaces is negligible. Also, the effect of an electric field perpendicular to the surface on adsorbed water and atomic oxygen is investigated. Spontaneous water dissociation is not found on the ordered and segregated alloy surfaces within the entire applied electric field range (−0.51 to 0.51 V/Å). Water changes orientation under strong negative fields, switching from a metal–O to a metal–H interaction, and the effect is much more pronounced in the low-coordination sites of cluster models.     

Chemisorption on a model insulator

The adsorption both of a single atom and a monolayer of atoms on the (001) surface of a model two-band crystal with the CsCI structure is investigated using the Green's function formalism and the phase shift technique. The electronic structure of the surface is described within the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight Binding (TB) approximation. Each adatom is represented by a single non-degenerate energy level. The adatoms are placed on the surface in both the on-site and the centered fourfold-site configuration. The change in the density of electronic states upon chemisorption is found, and comparison is made with similar results on a metal surface. It is shown that many, but not all, of the qualitative features in chemisorption on metallic surfaces can be transferred to the case of an insulating surface. In addition, it is shown that there are systematic variations in the density of states with adatom coverage which depend upon the absorption site.     

A theoretical study of Fe adsorption along Bi-nanolines on the H/Si(0 0 1) surface

We have investigated the energetic stability and equilibrium geometry of the adsorption of transition metal Fe atoms near the self-organized Bi lines on hydrogen passivated Si(0 0 1) surface. Our total energy results show that there is an attractive interaction between Fe adatoms along the Bi-nanolines. For the energetically most stable configuration, the Fe adatoms are seven-fold coordinated, occupying the subsurface interstitial sites aside the Bi-nanolines. With increased coverage, Fe atoms are predicted to form two parallel lines, symmetrically on both sides of the Bi line. Within our local spin-density functional calculations, we find that for the most stable geometries the Fe adatoms exhibit an antiferromagnetic coupling.     

Simple models of hydrogen adsorption on germanium

The dependence of work function Δφ on degree of coverage Θ for the Ge(100) and Ge(111) surfaces determined in terms of simple models that include the dipole-dipole interaction of hydrogen adatoms. It is found that experimental dependence Δφ(Θ) for the Ge(111) surface can be explained by taking into account an increase in the adsorption bond length with Θ. The charge of the adatoms as a function of Θ is calculated, and the variation of the surface conductivity of the substrate is estimated.     

CO adsorption on alkali-metal covered Ni(100)

The effect of preadsorbed alkali metal atoms Na, K and Cs on CO adsorption on Ni(100) has been studied using Auger spectroscopy and thermal desorption. It was found that the presence of alkali metals causes an appearance of several more tightly bound states in the CO thermal desorption spectra. The observed difference in carbon and oxygen Auger peak line shape on a bare and alkali modified Ni(100) is indicative that the presence of alkali adatoms induces CO decomposition on the Ni(100) surface. The fraction of dissociated CO increases with the amount of alkali adatoms present. At the same overlayer coverage the dissociation probability increases in the sequence Na, K, Cs. A comparison of the strength of the promoting effect on CO dissociation with the changes in the surface electron density in the presence of alkali adatoms has shown that at low overlayer coverages the electronic factor plays a major role in explaining the action of the surface modificators.     

Hydrogen, sulphur and chlorine coadsorption on Pd(111): a theoretical study of poisoning and promotion

First principles calculations for the coadsorption of hydrogen with sulphur and chlorine on Pd(111) are presented. Individually, both sulphur and chlorine poison hydrogen adsorption, sulphur being the more efficient poison. The observed sulphur poisoning is a structural effect. The adsorption energy decreases and the diffusion barrier increases for hydrogen atoms in the vicinity of sulphur adatoms. A sulphur coverage of 0.33 ML is sufficient to completely poison hydrogen adsorption on Pd(111). The presence of chlorine adatoms on the sulphur-poisoned surface is found to stabilise localised hydrogen adsorption. The possible promotional effects of chlorine on sulphur-poisoned catalysts are discussed.     

Adatom-adatom interaction mediated by an underlying surface phase transition

Low temperature scanning tunneling microscopy measurements on the adsorption of single Pb adatoms on Si(111)-(square root 3 x square root 3)-Pb surfaces reveal the vertical displacement patterns induced on the substrate by these Pb adatoms as well as a novel adatom-adatom interaction. The origin of both can be traced back to the (square root 3 x square root 3)<-->(3 x 3) phase transition taking place at lower temperatures. A Landau-like approach explains the displacement patterns as due to the corresponding order parameter and shows that the vicinity of a surface phase transition gives rise to a nonmonotonic adatom-adatom interaction.     

Comparative study of adsorption characteristics of Cs on the GaN （0001） and GaN （0001） surfaces

The adsorption characteristics of Cs on GaN （0001） and GaN （0001） surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave uttrasoft pseudopotential method based on first-principles calculations. The results show that the most stable position of the Cs adatom on the GaN （0001） surface is at the N-bridge site for 1/4 monolayer coverage. As the coverage of Cs atoms at the N-bridge site is increased, the adsorption energy reduces. As the Cs atoms achieve saturation, the adsorption is no longer stable when the coverage is 3/4 monolayer. The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer, and then rises with Cs atomic coverage. The most stable position of Cs adatoms on the GaN （000i） surface is at H3 site for 1/4 monolayer coverage. As the Cs atomic coverage at H3 site is increased, the adsorption energy reduces, and the adsorption is still stable when the Cs adatom coverage is 1 monolayer. The work function reduces persistently, and does not rise with the increase of Cs coverage.     

Comparative study of adsorption characteristics of Cs on the GaN(0001) and GaN(000) surfaces

The adsorption characteristics of Cs on GaN(0001) and GaN(000) surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave ultrasoft pseudopotential method based on first-principles calculations.The results show that the most stable position of the Cs adatom on the GaN(0001) surface is at the N-bridge site for 1/4 monolayer coverage.As the coverage of Cs atoms at the N-bridge site is increased,the adsorption energy reduces.As the Cs atoms achieve saturation,the adsorption is no longer stable when the coverage is 3/4 monolayer.The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer,and then rises with Cs atomic coverage.The most stable position of Cs adatoms on the GaN(000) surface is at H3 site for 1/4 monolayer coverage.As the Cs atomic coverage at H3 site is increased,the adsorption energy reduces,and the adsorption is still stable when the Cs adatom coverage is 1 monolayer.The work function reduces persistently,and does not rise with the increase of Cs coverage.     

The adsorption and decomposition of methanol on the Rh(100) surface

The adsorption and decomposition of methanol on the Rh(100) surface have been studied using high-resolution electron energy loss spectroscopy and thermal desorption mass spectrometry. Below 200 K, methanol is molecularly adsorbed and bonds to the surface via the oxygen atom. At 200–220 K, a saturated methanol layer undergoes two competing reactions: desorption and OH bond cleavage to form an O-bonded methoxy species. The methoxy species is stable to approximately 250 K. Between 250 and 320 K, a fraction of the methoxy species decomposes to form coadsorbed CO and hydrogen adatoms while the remainder recombines with hydrogen adatoms to desorb as molecular methanol. The hydrogen adatoms remaining on the surface desorb as H₂ between 270 and 400 K, and the CO desorbs between 450 and 550 K. Following a saturation exposure, approximately 0.2 monolayers of methanol decompose to eventually yield CO and H₂ as desorption products. These results are compared to the chemistry of methanol on other metal surfaces.     

Chemisorption of a monolayer of atoms on a bcc metal surface

The change in the density of states due to the adsorption of a monolayer of atoms on the (001) surface of a bcc metal is presented. The substrate is described by the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight-Binding (TB) approximation, and both the Green's function formalism and the phase shift technique are employed. Each adatom is represented by a single nondegenerate energy level. Two binding sites for the commensurate monolayer are considered: the on-site and the centered fourfold-site. By assuming that screening of the charges on the adatoms is complete within the surface layer of atoms, the selfconsistency condition of satisfying Friedel's sum rule can be met by varying the orbital energies of the adatoms and the surface plane of atoms of the substrate. The changes in the density of states show strongly skewed bonding and antibonding resonances which occur at different energies for the two binding sites even though equal binding strengths are assumed. A comparison with previous single adatom results shows that the shape and position of the bonding resonance are dependent upon adatom coverage.     

Surface atom displacement processes

Progress in field ion microscope studies of adatom displacements on metal surfaces is reviewed. It is concluded that of the displacement processes that contribute to surface diffusion only displacements between low-coordination (terrace) sites are well characterised. Procedures and preliminary results of FIM studies of adatom displacement over steps are described. Activation energies measured for passage of Ta, W, Re, Ir and Pt adatoms across (110) W steps are found to equal activation energies for diffusion over (110) W, despite the highly reflecting character of the step for all the adsorbates except Pt. Displacements of adatoms interacting with other adatoms are discussed. Results presented show that interaction of transition metal adatoms forming close-packed dimers on (110) W is rather weak, with a minimum interaction energy [?U(r) < 4kJ/mol] for Re₂ corresponding to a very weak attraction for Re adatoms 0.27 nm apart.     

Field emission and field ion microscope study of Ga,In and Sn on W: Structure,work function,diffusion and binding energy

Gallium, indium and tin were deposited on a tungsten tip by making a contact between the tip and these metals in the liquid state. The activation energies of diffusion of the adsorbates on tungsten were found to be 0.29 eV for Ga, 0.35 eV for In and 0.71 eV for Sn. The adsorbates were field-evaporated by gradually increasing a positive tip voltage by a small increment each time and the variation of the work function with the decreasing coverage was examined for each evaporation stage. The result indicates that the adatoms assume one of two different adsorption states. The adatoms bound as strongly as in a bulk crystal were field-evaporated at a low evaporation field. The remaining adatoms form a more strongly bound covering layer which maximizes the average work function of the covered surfaces, 4.75 eV for Ga, 4.63 eV for In and 5.10 eV for Sn, and are field-evaporated at a significantly higher field. The covering layer of the strongly bound adatoms were observed on the areas from the {001} to {114} planes and were hardly noticed on the {011} and 112 areas. The arrangement of the strongly bound adatoms, particularly on the {114} planes, is found to be a precise replica of the substrate arrangement. Thus, the surface density of the adatoms is exactly the surface density of the substrate atoms. The observed results suggest that an adatom occupying a tungsten lattice site and contacting four substrate tungsten atoms can establish unusually strong bonding with the substrate.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

Comparisons between adsorption and diffusion of alkali,alkaline earth metal atoms on silicene and those on silicane:Insight from first-principles calculations

The adsorption and diffusion behaviors of alkali and alkaline-earth metal atoms on silicane and silicene are both investigated by using a first-principles method within the frame of density functional theory.Silicane is staler against the metal adatoms than silicene.Hydrogenation makes the adsorption energies of various metal atoms considered in our calculations on silicane significantly lower than those on silicene.Similar diffusion energy barriers of alkali metal atoms on silicane and silicene could be observed.However,the diffusion energy barriers of alkali-earth metal atoms on silicane are essentially lower than those on silicene due to the small structural distortion and weak interaction between metal atoms and silicane substrate.Combining the adsorption energy with the diffusion energy barriers,it is found that the clustering would occur when depositing metal atoms on perfect hydrogenated silicene with relative high coverage.In order to avoid forming a metal cluster,we need to remove the hydrogen atoms from the silicane substrate to achieve the defective silicane.Our results are helpful for understanding the interaction between metal atoms and silicene-based two-dimensional materials.     

Adsorption of transition metal monolayers on tungsten surfaces

In a simple self-consistent band model, we calculate the work function change when one adsorbs monolayers of transition metals on tungsten. The work function change with the coverage is in agreement with the experimental results. It shows a maximum for one monolayer and is nearly equal to the difference between the work functions of W and the adsorbed element for a coverage equal to a few monolayers. In all the cases, the charge transfer between the adatoms and the tungsten surface atoms are small. This point allows us to simply calculate the binding energies of the monolayer. The same model is also applied to discuss the diffusion of a monolayer into a bulk transition metal.     

Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method

Individual Si and C adatoms, as well as SiC clusters, on a Si surface are simulated by the molecular dynamics method in the course of investigation of the initial stages of formation of a SiC layer on silicon with the help of molecular beam epitaxy. The potential energy surfaces for Si and C adatoms on the (2 × 1) reconstructed Si(001) surface and on the nonreconstructed Si(111) surface, as well as on the Si(111) surface with a SiC cluster, are calculated and analyzed. The values of migration barriers for adatoms on these surfaces are calculated. The effect of the SiC cluster on deformation of the surface region of Si(111) and on the migration of adatoms is investigated. The deep minima observed on the potential energy surfaces immediately above a cluster and at its boundaries can trap diffusing adatoms. The distributions of stresses and strains in the silicon lattice under a cluster on the surface are studied and described.     

Ab initio investigation of the clustering of carbon adatoms on Fe(001) and Fe(111) surfaces

A theoretical investigation of the interaction between carbon adatoms on the Fe(001) and Fe( 111 ) surfaces is performed using ab initio calculations in terms of density functional theory. Calc ulations of the adsorption energy demonstrate the existence of a strong bonding between single carbon adatoms and the iron surface. An analysis of the calculated energies of the interaction between carbon adatoms reveals for the first time that the repulsion between the carbon adatoms located at the nearest neighbor sites on the Fe(001) surface occurs and that clusters with a looser packing are formed on the surface.     

The role of repulsive interactions in molecular bromine adsorption and patterning of Si(100)-2×1

Scanning tunneling microscopy (STM) was used to investigate the role of repulsive interactions in the adsorption and patterning of molecular bromine on the Si(100) surface. At room temperature and low coverage, chemisorption of bromine occurs dissociatively on the same side of adjacent dimers of the same row. Using the STM tip as a probe, we demonstrate the existence of repulsive interactions at adjacent sites on the Si(100)-2×1 surface. These repulsive interactions also contribute to the arrangement of adatoms on the surface. In particular, we report the presence of a stable c(4×2) surface phase that results after exposing the Si(100) surface to bromine under certain conditions. This phase involves adsorption on non-neighboring dimers and is stabilized by repulsive interactions that force bromine adatoms to occupy alternating dimers within rows with an out-of-phase occupancy between adjacent rows.