Electron correlations in the chemisorption theory: Self-consistent calculations of the adatom charge

The single hydrogen-like adatom chemisorption on transition metal surfaces is studied by using the generalized model Hamiltonian. This Hamiltonian includes the possibility of influence of the adatom orbital occupancy on the charge transfer between the adatom and the substrate metal. The correlation effects were included up to second order in V (the single particle coupling strength). The numerical calculations of the charge transfer between an adatom and a substrate metal, as well as the comparison with results obtained for the standard Newns-Anderson model indicate that this generalized Hamiltonian can be more efficient in describing the chemisorption process.     

Formation of adatom density gratings on a substrate by channeling of an atomic beam through a pulsed-laser standing wave under a completely off-resonant condition

We show that a transversely uncooled atomic beam can be focused by a microsecond laser standing wave to form adatom density gratings on a substrate with good contrast ratio under a completely off-resonant condition (D?/? ~ 1) .     

Kondo state of Co impurities at noble metal surfaces

We use scanning tunneling microscopy and spectroscopy to study the properties of magnetic Co adatoms on noble metal surfaces at 6 K. Due to spin-flip scattering of the substrate electrons at the impurity the many-body Kondo state forms. This state is characterized by an energy, the Kondo temperature T_K. We measure T_K of adatom systems and a resonant scattering phase shift locally and are thus able to discuss the coupling of the Co adatom to the metal electronic system. From the resonant scattering phase shift of the surface-state electrons scattering off a Co adatom on Ag(111), we find that the coupling to the surface state is rather weak. On the other hand, increasing the number of nearest neighbor substrate atoms increases the coupling of a Co adatom to the host metal and increases T_K. This shows the dominant character of the coupling of the Co atom to the bulk states of the substrate crystal. PACS 72.10.Fk; 68.37,Ef; 72.15.Qm     

Adsorption on amorphous semiconductors: A modified Haldane-Anderson model

The Haldane-Anderson model previously used for describing the adsorption on a crystalline substrate is generalized to the case of an amorphous substrate. It is demonstrated that the main difference between the occupation numbers of the adatom is observed in the case where the atomic level overlaps with the band gap (for a crystalline semiconductor) and the mobility gap (for an amorphous semiconductor). The adatom charge on the amorphous substrate is smaller than that on the crystalline substrate.     

Adsorption and diffusion of a Si adatom on the H/Si(1 1 1) surface: comparison with the H/Si(0 0 1) surface

Using a first-principles method, we investigate the adsorption and diffusion of a Si adatom on the H-terminated Si(1 1 1) substrate, which would be useful in understanding the initial stages of Si homoepitaxy using a H surfactant. The adatom substitutes H atom(s) to form a monohydride structure or a dihydride structure. In forming the monohydride structure, the energy barrier for H substitution is absent. The adatom migrates on the surface with alternating its chemical state between monohydride and dihydride. These behaviors of the adatom are quite similar to those on the H/Si(0 0 1)2 × 1 surface, despite the significant difference in the substrate structure between both orientations. The resulting diffusion barrier is 1.30 eV, which is also comparable to that on the H/Si(0 0 1)2 × 1 surface.     

The influence of entropy effects on the adatom diffusion on a cluster surface

Slow adatom diffusion on the periodic surface of the upper layer of a growing crystal that has not yet completely formed is considered. Island (cluster) motion is investigated as a factor that accelerates adatom diffusion and results in an entropy increase compared to the case of an already formed stationary substrate layer. The diffusion coefficient, entropy, and the effective temperatures that characterize these properties of the substrate have been introduced.     

H在Al(111)面上吸附的多重散射Xα自洽计算

利用多重散射X_α自洽场方法研究了H在Al(111)表面吸附的桥位和顶位模型。结果表明顶位吸附的结合能优于桥位。顶位吸附中H的1s电子与Al相互作用形成不同形式的σ键,本文分析了这些成键细节,并给出了顶位吸附的态密度。 关键词：     

On the theory of the adsorption of atoms onto a metal surface in the presence of an external quantizing magnetic field

The Anderson-Newns model for the adsorption of atoms onto metals is generalized to the case of the presence of an external transverse quantizing magnetic field. Analytical expressions for the density of metal states in the external magnetic field are obtained. Analytical expressions for the density of adatom states are obtained in the approximation taking only the change in the density of substrate states into account. The charge exchange between the adatom and the metal is studied. The dependences of the density of adatom and transition-charge states on the magnetic field are studied for different values of the interaction constant. An analytical expression for the density of substrate states perturbed by adsorption is obtained.     

Exchange interaction between single magnetic adatoms

The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy. The Kondo resonance, which is due to magnetic correlation effects between the spin of a magnetic adatom and the conduction electrons of the substrate, is modified in a characteristic way by the coupling of the neighboring adatom spins. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 A we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.     

Adsorptions de H2 et de O2 sur des alliages NiCu divisés supportés sur SiO2, etudiées par mesure d'aimantation á saturation

The linear combination of surface atom orbitals to which an adatom orbital will couple plays the important role of being the essential link between the adsorbate and the semi-infinite substrate. The group orbital (as it is called) is a function of the substrate character, the local adsorption geometry, and the adatom orbital. We present the spectral densities of states appropriate to the adsorption of a hydrogen or transition metal atom on a transition metal substrate for simple geometries, and comment upon some of the implications of the results for both past and future assumptions in chemisorption theory.     

Simulation of island aggregation influenced by substrate temperature, incidence kinetic energy and intensity in pulsed laser deposition

Using kinetic Monte Carlo method, we have simulated a pulsed energetic growth process in pulsed laser deposition. During the growth of film, substrate temperature mainly influences upon film morphology by directly enhancing the adatom mobility through the temperature-dependent thermal vibration. By contrast, the effect of incidence kinetic energy on film growth is complex resulting from the collisions between the incident particles and the adatoms. The results show that improving incident kinetic energy cannot significantly accelerate the migration rate of adatom but change surface microstructure and promote single adatom formation resulting in more island aggregation density. Moreover, since pulse-influx characterizes pulsed laser deposition, the intensity per pulse contributes to the evolvement of nucleation density and the results illustrate that a general scaling law different from ordinary power law still exists in energetic growth of pulsed laser deposition.     

Adsorption of barium and rare-earth metals on silicon

The change in the work function of the Si(111) surface caused by deposition of a submonolayer film of barium has been calculated with due account of dipole-dipole adatom repulsion and metallization effects. The results of the calculations agree satisfactorily with experimental data. The character of the variation in the model parameters in the Ba-Sm-Eu-Yb series is discussed. The electron tunneling probability from an adatom quasi-level to the substrate conduction band and the energy barrier for interatomic transitions of electrons in an adlayer were estimated.     

Influence of the conduction bandwidth of the substrate on the electronic state of an adatom

The positions of the local and quasilocal levels of an adsorption system and their contributions to the occupation number of an adatom are found within the Anderson Hamiltonian using a simple model of the density of states. The dependence of the occupancy of an adatom on the conduction bandwidth of a metallic substrate is analyzed. Zh. Tekh. Fiz. 68, 15–19 (April 1998)     

Elastic interactions between surface adatoms and between surface clusters

One major term is omitted in most conventional treatments of interactions between surface adatoms or groups of adatoms. This is the elastic interaction, in which adatoms interact through mutual distortion of the substrate. The indirect elastic interaction explains a range of observed surface phenomena in a consistent quantitative way. These phenomena include static and dynamic effects ranging from ordered structures to correlated motions of adatom clusters. It is likely also that substrate distortion is important in clean surface reconstruction. The elastic interaction is typically comparable with or larger than the indirect electronic interactions usually presumed. It follows that detailed calculations which ignore the substrate distortions produced will be of limited value only.     

Structure of a CH3S monolayer on Au(111) solved by the interplay between molecular dynamics calculations and diffraction measurements

We have investigated the controversy surrounding the (sqrt[3] x sqrt[3]) R30 degrees structure of self-assembled monolayers of methylthiolate on Au(111) by first principles molecular dynamics simulations, energy and angle resolved photoelectron diffraction, and grazing incidence x-ray diffraction. Our simulations find a dynamic equilibrium between bridge site adsorption and a novel structure where 2 CH3S radicals are bound to an Au adatom that has been lifted from the gold substrate. As a result, the interface is characterized by a large atomic roughness with both adatoms and vacancies. This result is confirmed by extensive photoelectron and grazing incidence x-ray diffraction measurements.     

Anomalous X-Ray yields under surface wave resonance during reflection high energy electron diffraction and adatom site determination

In L x-ray emissions from a Si(111)-sqrt[3]xsqrt[3]-In surface induced by electron beam irradiation were measured as functions of the incident glancing angle. Under surface wave resonance conditions, anomalous x-ray intensities were clearly observed. Using dynamical calculations, these intensities are well explained as changes in density of the electron wave field at adatom positions. From these intensities, the adatom site was analyzed, and it was found that the T4 model is better than the H3 model.     

Substrate induced enhancement of atomic layer growth on Al(1 1 1): The effect of the mass anisotropy

The evolution of the thin-film morphology is studied by molecular dynamics simulations and we find a strong tendency of adatom island growth on the (1 1 1) surface of a thin Al overlayer placed on a heavy substrate (Pt(1 1 1)) when the system is subjected to low-energy Xe⁺ irradiation. The large adatom yield of 10²−10³ is found for 5-10 keV rare gas ion impacts. We also find that the mass effect due to the small atomic mass ratio (large mass anisotropy) in the bilayer has a direct effect on the atomic layer growth on the surface. A mass anisotropy induced scattering of the light overlayer atoms from the heavy substrate contributes to the enhancement of adatom production. It has been found that the volume increase (density decrease) of the amorphous intermixed phase keeps the adatoms on the surface. The competition between cratering and atomic layer growth can also be seen: three events out of 10 leads to cratering (erosion) morphology at 6 keV ion energy. The substrate induced enhancement of atomic layer growth might be a promising tool for making arrays of nanodots as nanotemplates for nanofabrication.     

The chemostress coefficient

It is shown that the chemical potential of an adatom interacting with a solid substrate can change significantly when external stresses are applied to the solid. The analysis indicates the need to perform a number of specific experiments and to develop further the theory of adsorption forces at high pressures applied to the solid substrate.     

Threshold nucleation of a nanometer-scale periodic adatom structure with the participation of a static surface acoustic wave

A theory is developed to describe the nucleation of a periodic nanometer-scale adatom structure formed as a result of the instability caused by the interaction of adatoms with a self-consistent quasi-Rayleigh acoustic wave. The periods of the structure are determined as functions of adatom concentration and temperature. On the basis of the theoretical results, a mechanism is proposed that explains the recently observed effect of nanometer-scale structurization in the course of low-temperature molecular beam epitaxy of Ga under laser irradiation.     

Influence of Pb adatom on adsorption of oxygen molecules on Pb(111) surface: a first-principles study

Using first-principles calculations, we systematically study the influence of Pb adatom on the adsorption and the dissociation of oxygen molecules on Pb(111) surface, to explore the effect of a point defect on the oxidation of the Pb(111) surface. We find that when an oxygen molecule is adsorbed near an adatom on the Pb surface, the molecule will be dissociated without any obvious barriers, and the dissociated O atoms bond with both the adatom and the surface Pb atoms. The adsorption energy in this situation is much larger than that on a clean Pb surface. Besides, for an adsorbed oxygen molecule on a clean Pb surface, a diffusing Pb adatom can also change its adsorption state and enlarge the adsorption energy for O, but it does not make the oxygen molecule dissociated. And in this situation, there is a molecule-like PbO2 cluster formed on the Pb surface.