Single Adatom Research Articles

Surface self-diffusion behavior of individual tungsten adatoms on rhombohedral clusters

The diffusion of single tungsten adatoms on the surfaces of rhombohedral clusters is studied by means of molecular dynamics and the embedded atom method. The energy barriers for the adatom diffusing across and along the step edge between a {110} facet and a neighboring {110} facet are calculated using the nudged elastic band method. We notice that the tungsten adatom diffusion across the step edge has a much higher barrier than that for face-centered cubic metal clusters. The result shows that diffusion from the {110} facet to a neighboring {110} facet could not take place at low temperatures. In addition, the calculated energy barrier for an adatom diffusing along the step edge is lower than that for an adatom on the flat (110) surface. The results show that the adatom could diffuse easily along the step edge, and could be trapped by the facet corner. Taking all of this evidence together, we infer that the {110} facet starts to grow from the facet corner, and then along the step edge, and finally toward the {110} facet center. So the tungsten rhombohedron can grow epitaxially along the {110} facet one facet at a time and the rhombohedron should be the stable structure for both large and small tungsten clusters.

Cotunneling theory of atomic spin inelastic electron tunneling spectroscopy

We propose cotunneling as the microscopic mechanism that makes possible inelastic electron spectroscopy of magnetic atoms in surfaces for a wide range of systems, including single magnetic adatoms, molecules and molecular stacks. We describe electronic transport between the scanning tip and the conducting surface through the magnetic system (MS) with a generalized Anderson model, without making use of effective spin models. Transport and spin dynamics are described with an effective cotunneling Hamiltonian in which the correlations in the magnetic system are calculated exactly and the coupling to the electrodes is included up to second order in the tip-MS and MS-substrate. In the adequate limit our approach is equivalent to the phenomenological Kondo exchange model that successfully describe the experiments . We apply our method to study in detail inelastic transport in two systems, stacks of Cobalt Phthalocyanines and a single Mn atom on Cu$_2$N. Our method accounts both, for the large contribution of the inelastic spin exchange events to the conductance and the observed conductance asymmetry.

Monoatomic and dimer Mn adsorption on the Au(111) surface from first principles

A theoretical study based on the density functional theory of the adsorption of Mn monomers and dimers on a Au-(111) surface is presented. As necessary preliminary steps, the bulk and clean surface electronic structure are calculated, which agree well with previous reports. Then, the electronic structure of the Mn adatom, chemisorbed on four different surface geometries, is analyzed. It is found that the most stable geometry is when the Mn atom is chemisorbed on threefold coordinated sites. Using this geometry for a single adatom a second Mn atom is chemisorbed and the most stable dimer geometrical structure is calculated. The lowest-energy configuration corresponds to the molecule lying parallel to the surface, adsorbed on two topological equivalent threefold coordinated sites. It is also found that the lowest-energy magnetic configuration corresponds to the antiferromagnetic arrangement with individual magnetic moments of $4.64{\ensuremath{\mu}}_{B}$. Finally, it is concluded that the dimer is not stable and should fragment at the surface.

Structural stabilities and diffusion of small Fe clusters on Fe (1 1 0) surface: A molecular dynamics study

Using Embedded-atom-method (EAM) potential of iron, structural stabilities of small Fe clusters on a Fe (1 1 0) surface have been investigated by molecular dynamics studies. It is presented that a tetramer and heptamer clusters are more stable than other sizes. These two clusters have high transition energies. They can be a critical nucleus at low and high temperature, respectively. A dimer diffuses more easily with lower energy barrier than single adatom. The trimer's rotation and dimer shearing mechanisms have been investigated in this paper.

Nucleation and growth of Si on Pb monolayer covered Si(111) surfaces

With a scanning tunneling microscope (STM), we study the initial stage of nucleation and growth of Si on Pb monolayer covered Si(111) surfaces. The Pb monolayer can work as a good surfactant for growth of smooth Si thin films on the Si(111) substrate. We have found that nucleation of two-dimensional (2D) Pb-covered Si islands occurs only when the substrate temperature is high enough and the Si deposition coverage is above a certain coverage. At low deposition coverages or low substrate temperatures, deposited Si atoms tend to self-assemble into a certain type of Si atomic wires, which are immobile and stable against annealing to ~200°C. The Si atomic wires always appear as a double bright-line structure with a separation of ~9Å between the two lines. After annealing to ~200°C for a period of time, some sections of Si atomic wires may decompose, meanwhile the existing 2D Pb-covered Si islands grow laterally in size. The self-assembly of Si atomic wires indicate that single Si adatoms are mobile at the Pb-covered Si(111) surface even at room temperature. Further study of this system may reveal the detailed atomic mechanism in surfactant-mediated epitaxy.

Ab initio calculation of diffusion barriers for Cu adatom hopping on Cu(1 0 0) surface and evolution of atomic configurations

The self-diffusion dynamics of Cu adatoms on Cu(1 0 0) surface has been studied based on the calculation of the energy barriers for various hopping events using lattice-gas based approach and a modified model. To simplify the description of the interactions and the calculation of the energy barrier, a three-tier hierarchy of description of atomic configurations was conceived in which the active adatom and its nearest atoms were chosen to constitute basic configuration and taken as a whole to study many-body interactions of the atoms in various atomic configurations, whereas the impacts of the next nearest atoms on the diffusion of the active adatom were considered as multi-site interactions. Besides the simple hopping of single adatoms, the movements of dimers and trimers as the results of multiple hopping events have also been examined. Taking into account the hopping events of all adatoms, the stability of atomic configurations has been examined and the evolution of atomic configurations has also been analyzed.

Diffusion of Pt dimers on a Wulff polyhedral surface

The surface diffusion of Pt dimers on Wulff polyhedral clusters with 586-12934 atoms was studied by the embedded atom method. The minimum energy diffusion path and the corresponding energy barrier for dimer diffusion on cluster surfaces were determined through a combination of the quenched Molecular Dynamics and the Nudged Elastic Band method. It was found that the diffusion of a dimer across the step-edge by dissociation and consecutive single-atom exchange with the edge atoms, rather than diffusion over the edge as a unit. Therefore, the step can enhance the dissociation of the dimer. For small Wulff polyhedral clusters containing up to 2000 atoms, the energy barrier for dimer diffusion from the {111} to the {100} facet is almost equal to that for the single adatom diffusion on the flat {111} surface. From the calculated cluster size dependence of the energy barrier, we conclude that the Wulff polyhedral cluster is stable, as the cluster has more than about 2000 atoms.

MONTE CARLO SIMULATION OF THE KINETICS IN THE GROWTH OF SEMICONDUCTOR QUANTUM DOTS

By means of kinetic Monte Carlo simulation, which is based on the random selection of the surface hops of single adatom, we investigate the atoms' kinetics during the growth of the semiconductor quantum dots in a molecular beam epitaxy system, the deposition, diffusion and nucleation are considered as the main relevant processes during the growth of the quantum dots, taking into account the contribution of the dangling bond of the adatoms in the simulation. The dependence of the quantum dot size on the temperature and flux as well as the atomic kinetic effects are discussed in detail. The simulation results are in good qualitative agreement with those of the experiment.

Tungsten cluster migration on nanoparticles: minimum energy pathway and migration mechanism

Saddle point searches have been employed to investigate the migration mechanisms of W clusters on W nanoparticles, and to determine the corresponding migration energies for the possible migration paths of these clusters. The tungsten clusters containing up to four adatoms are found to prefer 2D-compact structures with relatively low binding energies. The effect of interface and vertex regions on the migration behavior of the clusters is significantly strong, as compared to that of nanoparticle size. The migration mechanisms are quite different when the clusters are located at the center of the nanoparticle and near the interface or vertex areas. Near the interfaces and vertex areas, the substrate atoms tend to participate in the migration processes of the clusters, and can join the adatoms to form a larger cluster or lead to the dissociation of a cluster via the exchange mechanism, which results in the adatom crossing the facets. The lowest energy paths are used to be determined the energy barriers for W cluster migrations (from 1- to 4-atoms) on the facets, edges and vertex regions. The calculated energy barriers for the trimers suggest that the concerted migration is more probable than the successive jumping of a single adatom in the clusters. In addition, it of interest to note that the dimer shearing is a dominant migration mechanism for the tetramer, but needs to overcome a relatively higher migration energy than other clusters.

Surface self-diffusion of a Pt adatom on cuboctahedral and truncated decahedral clusters, size dependence

The self-diffusion of single Pt adatom on the surface of cuboctahedral and truncated decahedral clusters with 561–10 179 atoms are studied within the context of the many-body potentials obtained via the embedded atom method. The minimum energy diffusion path and the corresponding energy barrier for adatom diffusion on the cuboctahedral and truncated decahedral clusters surfaces are determined through a combination of the quenched molecular dynamics and the nudged elastic band method. The calculated energy barriers are consistent with the available experimental data. The dependence of energy barrier for adatom diffusion across the step edge on the cluster size is found. For the larger cuboctahedral and truncated decahedral clusters, the simulations show that the movement of the adatom is confined to a central region, and the adatom may escape from the center region only at elevated temperatures. In addition, we also find that the truncated decahedral structure is more favorable over the cuboctahedral structure for smaller clusters. The cluster growth experiments support our results.

Diffusion of 1,4-butanedithiol onAu(100)−(1×1): A DFT-based master-equation approach

The functionalization of metal surfaces via thiol-bonded molecules and the assembly of nanodevices on the surfaces should profit from a detailed atomistic understanding of the binding and diffusion properties. These differ substantially from the in-depth investigated situation of single adatoms. We report density-functional calculations for the elementary diffusion steps of 1,4-butanedithiol radicals (BDTRs) adsorbed on a $\text{Au}(100)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surface. The elementary diffusion steps are then combined into a description of the diffusion mechanism on long-time scales by integrating a master equation. The two S-Au bonds cause a multivalley potential-energy surface, which implies a complex diffusion mechanism. We identify the effect of the geometry constraints imposed by the ${({\text{CH}}_{2})}_{4}$ backbone on binding and diffusion. To this purpose we compare to the diffusion of a single ${\text{SCH}}_{3}$ radical on the same $\text{Au}(100)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surface. Altogether, the motion of the BDTR is walkinglike with the S-atoms crossing bridge sites of the Au surface one after the other. The lowest density-functional theory-Perdew and Wang 91 energy barrier for translation is 0.35 eV while the energy barrier for rotation comes out larger, 0.43 eV. As a result of this difference there will be correlations between subsequent diffusive displacements of the molecule. The isotropic diffusion constant on long-time scales is computed and the numerical results follow an Arrhenius law.

Movement of a tungsten adatom on the W(112) surface

Investigation of a single adatom as well as cross–channel infinite chain movement on the W(112) surface has been performed using ab initio methods. The energetics of the isolated adatom has been derived and is in very good agreement with experimental data. The asymmetry of the potential barrier associated with a lateral displacement of the W(112) surface is clearly identified. Surprisingly, the movement of an adatom in a cross–channel chain is only slightly more demanding than the movement of a single adatom, showing the importance of compensation effects for diffusion. The differences in the minimum energy paths of adatoms are discussed.

Spin dynamics of current-driven single magnetic adatoms and molecules

A scanning tunneling microscope can probe the inelastic spin excitations of a single magnetic atom in a surface via spin-flip assisted tunneling in which transport electrons exchange spin and energy with the atomic spin. If the inelastic transport time, defined as the average time elapsed between two inelastic spin flip events, is shorter than the atom spin relaxation time, the STM current can drive the spin out of equilibrium. Here we model this process using rate equations and a model Hamiltonian that describes successfully spin flip assisted tunneling experiments, including a single Mn atom, a Mn dimer and Fe Phthalocyanine molecules. When the STM current is not spin polarized, the non-equilibrium spin dynamics of the magnetic atom results in non-monotonic $dI/dV$ curves. In the case of spin polarized STM current, the spin orientation of the magnetic atom can be controlled parallel or anti-parallel to the magnetic moment of the tip. Thus, spin polarized STM tips can be used both to probe and to control the magnetic moment of a single atom.

Alkali metal adsorption on graphite: Calculation of a work function variation in the Anderson–Newns–Muscat model

Within the scope of the Anderson–Newns model of adsorption the Na, K, and Cs submonolayer films on graphite are considered. The adatoms dipole–dipole repulsion is taken into account with the use of the Muscat approach. The calculated work function variations are in a reasonable agreement with the experimental data. We have found that the charges of K, Na, and Cs adatoms are Z 0 = 0.22, 0.38, and 0.41 for the zero coverage limit and Z ML = 0.09, 0.15, and 0.16 for the monolayer coverage correspondingly. The crude estimations of the single adatom binding energies got E b = 0.76, 0.67, and 0.37 eV for Na, K, and Cs accordingly.

Orbital selective and tunable Kondo effect of magnetic adatoms on graphene: Correlated electronic structure calculations

We have studied the effect of dynamical correlations on the electronic structure of single Co adatoms on graphene monolayers with a recently developed novel method for nanoscopic materials that combines density functional calculations with a fully dynamical treatment of the strongly interacting 3d-electrons. The coupling of the Co 3d-shell to the graphene substrate and hence the dynamic correlations are strongly dependent on the orbital symmetry and the system parameters (temperature, distance of the adatom from the graphene sheet, gate voltage). When the Kondo effect takes place, we find that the dynamical correlations give rise to strongly temperature-dependent peaks in the Co 3d-spectra near the Fermi level. Moreover, we find that the Kondo effect can be tuned by the application of a gate voltage. It turns out that the position of the Kondo peaks is pinned to the Dirac points of graphene rather than to the chemical potential.

Quantum conductance of a single magnetic atom: Anab initiostudy

Our ab initio study explains recent puzzling experiments on the conductance of a single Co adatom deposited either on Cu(111) or on ferromagnetic Co islands and contacted with both magnetic and nonmagnetic electrodes [N. N\'eel, J. Kr\"oger, and R. Berndt, Phys. Rev. Lett. 102, 086805 (2009)]. We provide clear evidence that the conductance of a single atomic junction in the contact regime is close to ${G}_{0}/2$ (${G}_{0}$ is the quantum of conductance) for ferromagnetic electrodes and to ${G}_{0}$ for nonmagnetic ones. Spin-dependent calculations reveal that a conductance of ${G}_{0}/2$ originates from a combination of partially open majority and minority channels. The bonding between the Co adatom and the Co island reduces significantly the contribution of the minority $d$ electrons to the conductance, leading to the observation of half-integer conductance.

Inversion of spin polarization above individual magnetic adatoms

Using a cryogenic scanning tunneling microscope, the spin-resolved differential conductance through a magnetic tunnel junction consisting of a Cr tip, a vacuum barrier, and a single Co adatom on a Co monolayer is measured. It is proved, that the spin polarization at the Fermi level above this atomic protrusion is reversed with respect to the atomically flat Co layer, and thus dominated by majority electrons. Experiments with the same Cr-tip electrode and Co adatoms or dimers on Pt(111) as the second electrode suggest that the inversion might be unique for single atomic protrusions due to the reduced coordination.

Kondo peak splitting on a single adatom coupled to a magnetic cluster

The Kondo resonance induced by a single cobalt adatom interacting with a ferromagnetic iron nanocluster is measured using low-temperature scanning tunneling microscopy. The persistence of the Kondo resonance is evidenced along with the predicted splitting of the spectral density peak for a Kondo impurity surrounded by a spin-polarized electron bath. Reversible quenching of the split feature is observed using atom manipulation between adjacent adsorption sites. Using a Green's-function formalism, we model a double Fano resonance leading to a quantitative insight of our observations.

Effect of RKKY Interaction on the System of Two Magnetic Atoms on a Metal Surface at Finite Temperatures

For a system of two magnetic adatoms on a metal surface at finite temperatures, we calculate the spectral density at adatom sites. At sufficiently low temperatures, the peak structure at the Fermi level, which originates from the coupling between the local moments of adatoms and conduction electrons, corresponds to the Yosida–Kondo resonance. In addition, we also consider the indirect Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between the adatoms, which is mediated by the conduction electrons of the metal surface, and depends strongly on the adatom separation. Under strong ferromagnetic (FM) RKKY interaction, the peak structure disappears, while under antiferromagnetic (AF) RKKY interaction, the width of the peak structure increases in comparison to that of the single adatom case. Our conclusion of the spectral density variation in the two dimensional system can be confirmed by experiments on the surface, for example by the scanning tunneling spectroscopy.

Mapping antibonding electron states of a Pb adatom on Pb(111)

We have used low-temperature scanning tunneling spectroscopy to spatially map the energy-resolved electron density of a single Pb adatom on a Pb(111) surface. We observe a clear spatial inhomogeneity of the spectral density of the Pb adatom. This spatial inhomogeneity is rotated with bias voltage, reflecting the modification of the structure and symmetry of the adatom orbitals by the substrate. The simulated spectral maps of the Pb adatom based on density-functional-theory calculations agree well with experimental observations. This spatial inhomogeneity originates from the local extension of antibonding Pb states and the coupling to the substrate.