Adatom Interactions Research Articles

Motion of in-channel dimers on W(112): A DFT study

The adsorption and the mechanism of motion for in-channel tungsten dimers on a W(112) surface were investigated using DFT ab-initio calculations. The dimers turn out to diffuse as one object with an activation energy of 1.2eV, and motion is accompanied by a response of the lattice. Additionally, we explored the dimer dissociation–association process and the length of adatom–adatom interactions. It was also found that the long-range interaction between tugsten adatoms in the same surface channel influences the height and shape of the activation barrier for the diffusion of adatoms.

The Alexander-Anderson problem for two atoms adsorbed on graphene

The Green’s functions for the Alexander-Anderson problem have been obtained using the previously proposed model density of states for graphene. Both the ferromagnetic and antiferromagnetic dimers have been considered. It has been shown that, in order to describe the density of states of the dimer adatom, the density of states of the isolated adatom with two positions of the gravity center of the quasi-level shifted in opposite directions can be used. It has been demonstrated that the approximate method of obtaining the Green’s function of the dimer proposed by us previously and consisting in that the Green’s function of the adatom rather than that of the atom is taken as the seed function gives the same result as the Alexander-Anderson approach. The dependences of the indirect interaction of dimer adatoms on the problem parameters have been evaluated in the limit of low energies.

Strain induced adatom correlations

A Born–Green–Yvon type model for adatom density correlations is combined with a model for adatom interactions mediated by the strain in elastic anisotropic substrates. The resulting nonlinear integral equation is solved numerically for coverages from zero to a limit given by stability constraints. W, Nb, Ta and Au surfaces are taken as examples to show the effects of different elastic anisotropy regions.Results of the calculation are shown by appropriate plots and discussed. A mapping to superstructures is tried. Corresponding adatom configurations from Monte Carlo simulations are shown.

Spin-Polarized Rb2 Interacting with Bosonic He Atoms: Potential Energy Surface and Quantum Structures of Small Clusters

A new full-dimension potential energy surface of the three-body He-Rb₂(³Σ(u)(+)) complex and a quantum study of small (⁴He)(N)-Rb₂(³Σ(u)(+)) clusters, 1 ≤ N ≤ 4, are presented. We have accurately fitted the ab initio points of the interaction to an analytical form and addressed the dopant's vibration, which is found to be negligible. A Variational approach and a Diffusion Monte Carlo technique have been applied to yield energy and geometric properties of the selected species. Our quantum structure calculations show a transition in the arrangements of the helium atoms from N = 2, where they tend to be separated across the diatomic bond, to N = 4, in which a closer packing of the rare gas particles is reached, guided by the dominance of the He-He potential over the weaker interaction of the latter adatoms with the doping dimer. The deepest well of the He-Rb₂ interaction is placed at the T-shape configuration, a feature which causes the dopant to be located as parallel to the helium "minidroplet". Our results are shown to agree with previous findings on this and on similar systems.

Strong single-ion anisotropy and anisotropic interactions of magnetic adatoms induced by topological surface states

The nature of the magnetism brought about by Fe adatoms on the surface of the topological insulator Bi2Se3 was examined in terms of density functional calculations. The Fe adatoms exhibit strong easy-axis magnetic anisotropy in the dilute adsorption limit due to the topological surface states (TSS). The spin exchange J between the Fe adatoms follows a Ruderman-Kittel-Kasuya-Yosida (RKKY) behavior with substantial anisotropy, and the Dzyaloshinskii-Moriya (DM) interaction between them is quite strong with |D/J|~0.3 under the mediation by the TSS, and can be further raised to ~0.6 by an external electric field. The apparent single-ion anisotropy of a Fe adatom is indispensable in determining the spin orientation.

Fe-Fe adatom interaction and growth morphology on graphene

The nucleation and growth of Fe on graphene is highly unusual. A constantly increasing island density indicates the presence of strong, predominantly repulsive, adatom interactions. We study these interactions by first-principles calculations to identify their origin. We find that the interactions consist of a short-range attraction and longer-range repulsion. We show that electric dipole-dipole interaction can contribute only part of the repulsive force (\ensuremath{\sim}30$%$) between Fe adatoms, and the repulsion due to the elastic interaction is small. We suggest that the dominant contribution to the repulsive energy would originate from the indirect (or RKKY) interactions mediated through the delocalized electrons on graphene.

DFT study of the coverage effects for Al adsorption on Si(1 1 1) surfaces

Density functional theory (DFT) calculations have been carried out to investigate the interactions between the Si(1 1 1) surface and the Al adatoms. Different adsorption sites and coverage effects have been considered. For low Al coverage, the threefold-filled adsorption site is the most energy favored site. With the increase of Al coverage, adatom–adatom interactions become increasingly important and Al atomic chains or clusters are formed. With the clean Si(1 1 1) surface of metallic feature, we found that 1/3 ML Al adsorption leads to a semiconducting surface. The results for the electronic behavior suggest the formation of the polarized covalent bonding between the Al adatom and the Si(1 1 1) surface.

An interplay between the processes of condensation and ordering in open submonolayers of adsorbed atoms

Abstract We study the coupled processes of condensation and ordering in open atomic submonolayers adsorbed on the (001) crystal plane of a bcc alloy. The system of interstitial adsorption sites is viewed as consisting of two equivalent square sublattices. It is established that at sufficiently low substrate temperatures in a certain range of pressures of the surrounding vapor and under the repulsive character of lateral adatom – adatom interaction the adlayer may exhibit positional long-range order. A P–T diagram is constructed showing the stability region of the ordered surface phase. A system of kinetic equations is derived, which permits to analyze numerically the simultaneous evolution of the long-range order parameter and the adsorbate film coverage in the course of establishment of thermodynamic equilibrium. The processes of adsorption and surface ordering prove to be substantially interdependent. Evolution of the long-range order parameter is shown to bear a distinct threshold character. The equilibrium ordered state is attained via formation of a kinetically-slowed disordered surface phase. In evolutionary curves this is manifested by the two-stage variation of coverage. Reasons for such type of the coverage temporal evolution are suggested.

Geometric rearrangement of adsorbate driven by the charge transfer

AbstractAdsorption of alkali atoms induces a significant charge redistribution in the region around the adatom. Such charge displacement is associated with a large dipole moment responsible for the interaction of adatoms and a reduction of the surface work function. In addition to these well‐known effects our first principles simulations for the ${\rm Na}_9^ +$ cluster on the Cu(001) surface demonstrate how the charge transfer (CT) from the adsorbate to the substrate can drastically change the geometric structure of the cluster. We report on a detailed study of the adsorption process using quantum chemistry. A representation of the substrate by a cluster of 54 Cu atoms allows us to treat quantum mechanically the electronic structure of both systems, the adsorbate and the surface, on equal footing. Subsequently, we analyze the charge distribution in the composite system. Convergence of the results is verified by considering a much larger substrate cluster containing 126 Cu atoms. The role of the CT is further elucidated by the geometry optimization of the bare cluster with and without an electron deficit. It is shown that the CT drives the system to a meta‐stable state which thereafter relaxes to a new configuration.

Different magic number behaviors in supported metal clusters

Two different magic number behaviors in supported metal clusters, which contain several to hundreds of atoms, are revealed on a series of fcc(001) metal surfaces based on the calculations with the tight-binding potential. The magic number sequence persists on some surfaces while gradually disappears on the others with the increasing cluster size. A theory is proposed to explain these behaviors in terms of atomic interactions. We find in surprise that the different magic number behaviors are triggered by the relatively weak adatom–adatom interactions between next nearest-neighbor (NNN) atoms, although the closed shell of the magic cluster is enhanced by nearest-neighbor interactions. For an attractive NNN interaction, the closed shell of the magic cluster is gradually destabilized and eventually broken, leading to the disappearance of the magic number sequence with increasing cluster size. For a repulsive one, the closed shell and magic number sequence persists. Besides, our theory also allows a good understanding of the equilibrium shape of Cu islands on the Cu(001) surface.

Structural and electronic properties of Na/Cu(111) at different coverages by first principles

Nanostructures are presently enjoying an increasing interest in the field of materials science. In particular, importance is given to ordered monolayers prepared by deposition of atoms on a crystalline surface. The growth of these superlattices can be controlled so as to obtain an ordered structure by means of the lateral interaction of adatoms lying on the metal surface. The objective of our study is to investigate the structural and electronic properties using DFT total-energy calculations; we employ a jellium-like model to describe the substrate but we also take into account the presence of discrete surface states that are known to affect the lateral interaction. Our treatment of the substrate is based on the model proposed by E.V. Chulkov et al. [Surf. Sci. 437, 330 (1999)]; in this model one constructs a mono-dimensional potential so as to reproduce some important electronic properties of the metal surface, such as i) the energy gap in the projected bulk band-structure and ii) the energy position of surface states. We put into practice Chulkov potential implementing into an existing plane-waves code (ABINIT, URL http://www.abinit.org) an ionic potential, so as to obtain a self-consistent Kohn-Sham effective potential which corresponds to the Chulkov one. Using this effective potential in a fully three-dimensional code we are able to study the adsorption process and the interaction between adsorbates. We illustrate some details of our implementation of the Chulkov model and we present our results about the simple system of Na adatoms on a Cu(111) surface for different coverages. In particular, we compare electronic properties and adsorption energies with those obtained within a standard jellium model substrate and with those obtained for Na adsorption on a realistic Cu(111) surface.

Atomic superlattice formation mechanism revealed by scanning tunneling microscopy and kinetic Monte Carlo simulations

We study the interaction of single Fe atoms on Cu(111) and Ag(111) substrates with low-temperature scanning tunneling microscopy and kinetic Monte Carlo simulations. In Fe/Cu(111), a self-assembled hexagonal quasisuperlattice with perturbation of around 20% dimers/clusters is obtained. In Fe/Ag(111), however, a disorderlike structure is found even though long-range interactions among atoms are observed. In combination with kinetic Monte Carlo simulations, possible mechanisms of the superstructure formation are discussed. We find that two parameters, i.e., the ratio of adatom interaction energy (the depth of the first energy minimum) to diffusion barrier and the square of the repulsive ring radius versus the superstructure lattice constant, play important roles for superstructure formation.

Lateral manipulation of small clusters on the Cu and Ag(1 1 1) surfaces with the single-atom and trimer-apex tips: Reliability study

We study the reliability of the lateral manipulation of small Cu clusters (dimer and trimer) on the flat Cu(1 1 1) surface with both the single-atom and trimer-apex tips and that for the Ag/Ag(1 1 1) system, and compare the results between the two systems as well as with the single-atom manipulation on these surfaces. Manipulations are simulated using molecular statics method with semi-empirical potentials. The dependence of the manipulation reliability on the tip height and tip orientation are investigated. Overall, the manipulation reliability increases with decreasing tip height although it depends obviously on the tip orientation. For the Cu/Cu(1 1 1) system, the manipulation of the dimmer and trimer can be successful with both tips. The manipulation reliability can be improved by the trimer-apex tip, and the tip-height range for the successful manipulation is also broader, as compared to the single-atom apex tip. Differently from the single-atom manipulation, the tip orientation has a noticeable influence on the manipulation reliability even for the single-atom tip due to the stronger tip–cluster and surface–adatom interactions in cluster manipulation. For the Ag/Ag(1 1 1) system, successful manipulations only be achieved with the trimer-apex tip, and the manipulation reliability is worse than that of the Cu/Cu(1 1 1) system, indicating the difference in mechanic properties between the two surfaces at the atomic level.

STM fingerprint of molecule–adatom interactions in a self-assembled metal–organic surface coordination network on Cu(111)

A novel approach of identifying metal atoms within a metal-organic surface coordination network using scanning tunnelling microscopy (STM) is presented. The Cu adatoms coordinated in the porous surface network of 1,3,8,10-tetraazaperopyrene (TAPP) molecules on a Cu(111) surface give rise to a characteristic electronic resonance in STM experiments. Using density functional theory calculations, we provide strong evidence that this resonance is a fingerprint of the interaction between the molecules and the Cu adatoms. We also show that the bonding of the Cu adatoms to the organic exodentate ligands is characterised by both the mixing of the nitrogen lone-pair orbitals of TAPP with states on the Cu adatoms and the partial filling of the lowest unoccupied molecular orbital (LUMO) of the TAPP molecule. Furthermore, the key interactions determining the surface unit cell of the network are discussed.

Reliable lateral manipulation of a single Ag adatom on a Ag(1 1 1) surface with a trimer-apex tip

We study the reliability of the lateral manipulation of a single Ag adatom on a Ag(1 1 1) surface with the single-atom and trimer-apex tips based on molecular statics simulations using surface embedded-atom-method potential. The dependence of the manipulation reliability on tip height and orientation is investigated. For the single-atom tip the manipulation reliability increases monotonically with decreasing tip height, which is owing to the strengthened lateral tip–adatom interaction as the tip height lowers. For the trimer-apex tip, the manipulation reliability is sensitive to the tip orientation in the lower tip-height range, while in the higher tip-height range the manipulation reliability is independent of the tip orientation and moreover can be greatly improved due to the strong vertical attraction of the tip on the adatom as compared to the single-atom tip. We also compare these results to those for manipulating single Cu adatoms on the Cu(1 1 1) surface, reveal the underlying physics, and propose the method to improve the manipulation reliability for different systems.

Ab-initio calculations of interactions between Cu adatoms on Cu(1 1 0): Sensitivity of strong multi-site interactions to adatom relaxations

We have parameterized the various interactions between Cu adatoms on Cu(1 1 0) using density-functional theory based ab-initio calculations. Our results indicate that in addition to pair interactions, 3-adatom and 4-adatom interactions of significant strengths are present in this system. This further stresses the importance of multi-site interactions in constructing a complete lattice–gas picture. Even though adding these multi-site interactions leads to good convergence in interaction energies, we find that some multi-site interactions are very sensitive to adatom relaxations. This makes the application of a simple lattice–gas picture inadequate for such surfaces. We also parameterize adatom interactions on this surface using the recently developed connector model. The connector model parameterization is as efficient as the parameterization using lattice–gas model. Further, we present diffusion barriers for nearest-neighbor (NN) and next-nearest-neighbor (NNN) hops on this surface.

Study of the island morphology at the early stages of Fe/Mo(110) MBE growth

Abstract We present theoretical study of morphology of Fe islands grown at Mo(110) surface in submonolayer MBE mode. We utilize atomistic SOS model with bond counting, and interactions of Fe adatom up to third nearest neighbors. We performed KMC simulations for different values of adatom interactions and varying temperatures. We have found that, while for the low temperature islands are fat fractals, for the temperature 500 K islands have faceted rhombic-like shape. For the higher temperature, islands acquire a rounded shape. In order to evaluate qualitatively morphological changes, we measured average aspect ratio of islands. We calculated dependence of the average aspect ratio on the temperature, and on the strength of interactions of an adatom with neighbors.

Interaction of Gold with Cerium Oxide Supports: CeO2(111) Thin Films vs CeOx Nanoparticles

Morphology, electronic structure, and CO adsorption of gold supported on well-ordered CeO2(111) thin films and CeOx nanoparticles were studied by scanning tunneling microscopy (STM), photoelectron spectroscopy (XPS), and infrared reflection absorption spectroscopy (IRAS). Ceria nanoparticles grown on crystalline thin silica films possess Ce in both the 3+ and 4+ oxidation states, with the former dominating in smaller particles. Upon deposition on CeO2(111) films, most of the Au particles are formed at the step edges. The particles on terraces grow presumably through the decoration of point defects, which are more numerous on reduced surfaces. Combined XPS and IRAS data show that partially charged Auδ+ species are formed by deposition at low temperatures (∼100 K) and low coverages on both ceria supports. Formation of Auδ+ on CeO2(111) films is kinetically limited and is attributed to the interaction of the gold ad-atoms with defects. In variance to extended ceria surfaces, where only metallic Au nanoparticles are observed at 300 K, the “cationic” gold species are formed in abundance on nano-CeOx and exhibit enhanced thermal stability. It is shown that nanoceria stabilizes small Au clusters, which may even be incorporated into the ceria nanoparticles at elevated temperatures.

Connector model for describing many-body interactions at surfaces

First-principles calculations based on density-functional theory indicate that high-order many-body interactions are significant in Al clusters on Al(110) and Al(100). The large number of many-body interactions renders a full-lattice-gas approach ineffective for such systems. To simplify the description of adsorbate interactions, we utilize two different schemes. First, we find effective parameters for Al adatom interactions using the leave-one-out cross-validation method. Second, we propose the connector model, which is based on additive single-atom connector units. The central idea of the connector model is to combine groups of many-body interactions into important structural units (e.g., step edges) that have a single interaction energy. We find that the connector model is more accurate and efficient in representing high-order many-body interactions than the traditional lattice-gas approach and it may be suitable for describing a variety of surface phenomena such as thin-film and crystal growth, adsorption, phase transitions, and catalysis at surfaces.

Reliable lateral and vertical manipulations of a single Cu adatom on a Cu(111) surfacewith multi-atom apex tip: semiempirical and first-principles simulations

We study the reliability of the lateral manipulation of a single Cu adatom on a Cu(111)surface with single-atom, dimer and trimer apex tips using both semiempirical andfirst-principles simulations. The dependence of the manipulation reliability on tip height isinvestigated. For the single-atom apex tip the manipulation reliability increasesmonotonically with decreasing tip height. For the dimer and trimer apex tips themanipulation reliability is greatly improved compared to that for the single-atom apex tipover a certain tip-height range. Two kinds of mechanism are found responsible for thisimprovement. One is the so-called enhanced interaction mechanism in which the lateraltip–adatom interaction in the manipulation direction is improved. The other is thesuspended atom mechanism in which the relative lateral trapping ability of the tip isimproved due to the strong vertical attraction of the tip on the adatom. Both mechanismsoccur in the manipulations with the trimer apex tip, while in those with the dimerapex tip only the former is effective. Moreover, we present a method to realizereversible vertical manipulation of a single atom on a Cu(111) surface with thetrimer apex tip, based on its strong vertical and lateral attraction on the adatom.