Metal Adatoms Research Articles

Bonding and charge transfer by metal adatom adsorption on graphene

Adsorption of the alkali-, group-III, and 3$d$-transition-metal adatoms (Na, K, Al, In, V, Fe, Co, and Ni) on graphene was studied systematically by first-principles calculations. The bonding character and electron transfer between the metal adatoms and graphene were analyzed using the recently developed quasi-atomic minimal basis set orbitals (QUAMBOs) approach. The calculations showed that the interaction between alkali-metal adatoms and graphene is ionic and has minimal effects on the lattice and electronic states of the graphene layer, in agreement with previous calculations. For group-III metal adatom adsorptions, mixed covalent and ionic bonding is demonstrated. In comparison, 3$d$-transition-metal adsorption on graphene exhibits strong covalent bonding with graphene. The majority of the contributions to the covalent bonds are from strong hybridization between the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{\mathit{yz}}$ orbitals of the 3$d$-transition-metal adatoms and ${p}_{z}$ orbitals of the carbon atoms. The strong covalent bonds cause large in-plane lattice distortions in the graphene layer. Charge redistributions upon adsorptions also induce significant electric dipole moments and affect the magnetic moments.

Hierarchical Assembly and Reticulation of Two-Dimensional Mn- and Ni–TCNQx (x = 1, 2, 4) Coordination Structures on a Metal Surface

We report a scanning tunneling microscopy (STM) investigation of the self-assembly of two-dimensional metal–organic coordination networks comprised of Mn or Ni atoms (M) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on a Ag(100) surface. The growth is dominated by the coupling of the cyano ligands to the substrate, and this interaction is reduced by lateral coordination of the molecules to metal adatoms. Initially mononuclear M–TCNQ4 complexes form at room temperature, which hierarchically assemble into reticulated M–TCNQ2 networks at sufficiently high metal concentrations. M–TCNQ1 networks with fully coordinated molecules can be obtained at elevated substrate temperatures and sufficient metal concentration. A commensurate α and an incommensurate β phase can be distinguished that are unrelated to the antedecent M–TCNQ4 complexes. The coordination of the cyano groups to metal adatoms alters the balance between lateral interactions and coupling of the molecules to the substrate. This effect is accompanied by distinct changes in the apparent heights of metal centers in the STM data indicating changes in their electronic configuration.

Attractive interaction between transition-metal atom impurities and vacancies in graphene: a first-principles study

We present a density functional theory study of transition metal adatoms on a graphene sheet with vacancy-type defects. We calculate the strain fields near the defects and demonstrate that the strain fields around these defects reach far into the unperturbed hexagonal network and that metal atoms have a high affinity to the non-perfect and strained regions of graphene. Metal atoms are therefore attracted by the reconstructed defects. The increased reactivity of the strained graphene makes it possible to attach metal atoms much more firmly than to pristine graphene and supplies a tool for tailoring the electronic structure of graphene. Finally, we analyze the electronic band structure of graphene with defects and show that some defects open a semiconductor gap in graphene, which may be important for carbon-based nanoelectronics.

Strong metal adatom–substrate interaction of Gd and Fe with graphene

Graphene is a unique 2D system of confined electrons with an unusual electronic structureof two inverted Dirac cones touching at a single point, with high electron mobilityand promising microelectronics applications. The clean system has been studiedextensively, but metal adsorption studies in controlled experiments have beenlimited; such experiments are important to grow uniform metallic films, metalcontacts, carrier doping, etc. Two non-free-electron-like metals (rare earth Gd andtransition metal Fe) were grown epitaxially on graphene as a function of temperatureT andcoverage θ. By measuring the nucleated island density and its variation with growth conditions, informationabout the metal–graphene interaction (terrace diffusion, detachment energy) is extracted. Thenucleated island densities at room temperature (RT) are stable and do not coarsen, at least up to400 °C, which shows an unusually strong metal–graphene bond; mostlikely it is a result of C atom rebonding from the pure graphenesp2 C–C configuration to one of lower energy.

Metal Adatoms Induced Stability, Electronic and Magnetic Behaviors on Graphene

The formation energy, geometry, magnetic properties and electronic structure of metal adatoms (Cu and Pt) on graphene have been studied using first-principles method based on density-functional theory with the generalized gradient approximation. It is found that the bridge site is the most stable adsorption site of metal adatom on graphene, and the adatoms caused the distortion of graphene layer. The formation energy of Pt adsorption on graphene is larger than that of Cu on graphene. An isolated Pt atom appears to be usually magnetic, however, its magnetic behavior vanishes when it is absorbed on graphene. The spin-resolved density of states of C atoms in graphene are symmetric exhibiting no magnetism. In contrast, the Cu-graphene exhibits magnetic behaviors, and the Cu adatom induces magnetism on the C atoms in the graphene. The magnetic properties of graphene can be tuned by the different metal atoms adsorption.

Adsorbate-enhanced transport of metals on metal surfaces: Oxygen and sulfur on coinage metals

Coarsening (i.e., ripening) of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals (Cu, Ag, Au), both clean and with an adsorbed chalcogen (O, S) present. For the clean surfaces, three aspects are summarized: (1) the balance between the two major mechanisms—Ostwald ripening (the most commonly anticipated mechanism) and Smoluchowski ripening—and how that balance depends on island size; (2) the nature of the mass transport agents, which are metal adatoms in almost all known cases; and (3) the dependence of the ripening kinetics on surface crystallography. Ripening rates are in the order (110)>(111)>(100), a feature that can be rationalized in terms of the energetics of key processes. This discussion of behavior on the clean surfaces establishes a background for understanding why coarsening can be accelerated by adsorbates. Evidence that O and S accelerate mass transport on Ag, Cu, and Au surfaces is then reviewed. The most detailed information is available for two specific systems, S/Ag (111) and S/Cu(111). Here, metal-chalcogen clusters are clearly responsible for accelerated coarsening. This conclusion rests partly on deductive reasoning, partly on calculations of key energetic quantities for the clusters (compared with quantities for the clean surfaces), and partly on direct experimental observations. In these two systems, it appears that the adsorbate, S, must first decorate—and, in fact, saturate—the edges of metal islands and steps, and then build up at least slightly in coverage on the terraces before acceleration begins. Acceleration can occur at coverages as low as a few thousandths to a few hundredths of a monolayer. Despite the significant recent advances in our understanding of these systems, many open questions remain. Among them is the identification of the agents of mass transport on crystallographically different surfaces e.g., 111, 110, and 100.

Metal adatoms on graphene and hexagonal boron nitride: Towards rational design of self-assembly templates

Periodically corrugated epitaxial graphene and hexagonal boron nitride $(h\text{-BN})$ on metallic substrates are considered as perspective templates for the self-assembly of nanoparticles arrays. By using first-principles calculations, we determine binding energies and diffusion activation barriers of metal adatoms on graphene and $h\text{-BN}$. The observed chemical trends can be understood in terms of the interplay between charge transfer and covalent bonding involving the adatom $d$ electrons. We further investigate the electronic effects of the metallic substrate and find that periodically corrugated templates based on graphene in combination with strong interactions at the metal/graphene interface are the most suitable for the self-assembly of highly regular nanoparticle arrays.

Transition metal adatom and dimer adsorbed on graphene: Induced magnetization and electronic structures

Geometries, electronic structures, and magnetic properties of transition metal $M$ adatom and dimer adsorbed graphene have been studied ($M=\text{Fe}$, Co, Ni, and Cu). With adatom adsorption, we confirm the previously reported stable adsorption site, and the adatoms are chemically bonded with graphene except the copper adatom. With dimer adsorption, we observed that the stable configurations are dependent on the exchange-correlation functional for the iron dimer and nickel dimer; while the stable configurations do not depend on the functional for the cobalt and copper dimer. The adsorption energy indicates the copper dimer can barrierlessly diffuse along the graphene C-C bonds. With iron or cobalt adatom adsorption, graphene becomes a half-metal which can be used as a spin-filtering material. The iron dimer adsorbed graphene is also a half-metal, but the cobalt dimer adsorbed graphene is not. Both local-density approximation and generalized gradient approximation yield consistent results for the nickel adatom adsorbed graphene, which is the system is semiconducting and nonmagnetic due to the strong binding between nickel and graphene. However, different exchange-correlation functionals lead to controversial results for the magnetic and transport properties of nickel dimer adsorbed graphene. The copper adatom adsorbed on graphene exhibits $1{\ensuremath{\mu}}_{B}$ local magnetic moment, but the dimer does not show any magnetism. These results show that graphene properties can be effectively modulated by transition metal adsorption and that the transition metal adsorbed graphene can serve as potential materials in nanoelectronics, spintronics, or electrochemistry.

Density functional calculation of transition metal adatom adsorption on graphene

The adsorption of 15 different transition metal adatoms on graphene is studied using first-principles density-functional theory with the generalized gradient approximation. The adsorption energy, stable geometry, density of state, and magnetic moment of each adatom–graphene system are calculated. For the adatoms studied from Sc to Zn of the Periodic Table, and noble metals, the distortion of the graphene layer on B of T sites is quite significant in some cases, and the adsorption is characterized by strong hybridization between adatom and graphene electronic states. The favored adsorption site indicates the main chemical bond between adsorbate and graphene. Half filled d shell TM atoms and Au, Ag, Zn have small adsorption energy. The reduction in magnetic moment from the isolated to the adsorbed atom is explained by the perspective of charge transfer, and electron shift between different orbit states of the adatom.

Hydrogen storage in Al and Ti dispersed on graphene with boron substitution: First-principles calculations

The characteristics of hydrogen adsorption on Al and Ti metal atoms dispersed on graphene with boron substitution is investigated including metal adatom clustering and electronic structure of H 2/metal-adsorbed graphene using density functional theory calculations. It is found that Al and Ti atoms are well dispersed on boron-substituted graphene and can form a (2 × 2) pattern because clustering of metal atoms is hindered by the repulsive Coulomb interaction between metal adatoms and strong bonding force between dispersed metal atom and boron-substituted graphene. In addition, Al and Ti can bind up to eight H 2 molecules on the double side of the boron-substituted graphene. This allows for a storage capacity of a 9.9 wt.% and 7.9 wt.% hydrogen for Al and Ti adatom, respectively.

Imaging of individual adatoms on oxide surfaces by dynamic force microscopy

An experimental study of the adsorption sites of metal adatoms on an oxide surface is presented. The adsorption sites of gold on the ultrathin aluminum oxide grown on NiAl110 are determined. For the study low-temperature frequency modulation dynamic force microscopy is employed to identify the adsorption sites in real space. Results show that wave trough-like depressions in the alumina surface are preferred locations for gold adspecies.

Aggregation and Contingent Metal/Surface Reactivity of 1,3,8,10‐Tetraazaperopyrene (TAPP) on Cu(111)

The structural chemistry and reactivity of 1,3,8,10-tetraazaperopyrene (TAPP) on Cu(111) under ultra-high-vacuum (UHV) conditions has been studied by a combination of experimental techniques (scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy, XPS) and DFT calculations. Depending on the deposition conditions, TAPP forms three main assemblies, which result from initial submonolayer coverages based on different intermolecular interactions: a close-packed assembly similar to a projection of the bulk structure of TAPP, in which the molecules interact mainly through van der Waals (vDW) forces and weak hydrogen bonds; a porous copper surface coordination network; and covalently linked molecular chains. The Cu substrate is of crucial importance in determining the structures of the aggregates and available reaction channels on the surface, both in the formation of the porous network for which it provides the Cu atoms for surface metal coordination and in the covalent coupling of the TAPP molecules at elevated temperature. Apart from their role in the kinetics of surface transformations, the available metal adatoms may also profoundly influence the thermodynamics of transformations by coordination to the reaction product, as shown in this work for the case of the Cu-decorated covalent poly(TAPP-Cu) chains.

Noble and alkali adatoms on a –Ag surface: a first-principles study

Using first-principles calculations, we present a comprehensive study on the atomicand electronic structures of metal adatoms (noble metals Ag, Au, Cu and alkalimetals Li, Na, K) adsorbed on a –Ag (hereafter -Ag) surface. We found that adsorption of noble and alkali adatoms can inducesignificant structural changes in the topmost Ag layer. The most striking andinteresting results are the immersion of the noble and Li adatoms into the substrateAg layer and the finding of the most stable configurations with three adatomsincorporating into or being adsorbed on the surface dependent on their atomic radii.We also found that the almost empty two-dimensional free-electron-like bands1 and its band foldings1* of the originalsurface band s1 of the -Ag surface split into a gap at the surface Brillouin zone (SBZ) boundary withadsorption of an adatom. The two surface bands gradually move downwards and thes1 band is gradually filled with an increase of coverage. Thes1 band is fully occupied with the largest band gap ∼ 0.25 eV betweenthe s1 and s1* bands at the critical coverage of 0.14 monolayers (ML) [three adatoms in a –Ag (hereafter -Ag) unit cell], which corresponds to the most stable adsorption phase. Although theadsorption configurations are different, both the noble and alkali adatom adsorptions giverise to similar electronic structures at low coverages, indicating a free-electron-likecharacter of the adsorption surfaces.

Adatoms-Induced Local Bond Contraction, Quantum Trap Depression, and Charge Polarization at Pt and Rh Surfaces

The extremely high catalytic efficiency of undercoordinated noble metal adatoms is indeed fascinating, but its chemical and electronic origin remains yet puzzling. Incorporating the BOLS correlation theory [Sun, C. Q. Prog. Solid State Chem. 2007, 35, 1] into the high-resolution XPS measurements [Baraldi, A.; et al. New J. Phys. 2007, 9, 143; Bianchettin, L.; et al. J. Chem. Phys. 2008, 128, 114706] has affirmed the BOLS expectations that the broken bonds induce local strain and quantum trapping in addition to polarization of the otherwise conductive half-filled s-shell charge by the tightly- and densely-trapped inner electrons of the adatoms. Both the trapped and polarized states would be detectable from the density-of-states evolution of the valence and the core bands. The trapped states have been discovered at the bottom edges of Pt(5d106s0) 4f7/2 and Rh 3d5/2 bands, and the polarized states only present at the upper edge of Rh(4d85s1) 3d5/2. It is suggested that the quantum trapping increases the electroaffinity and the polarization does oppositely. Therefore, the Rh adatom serves as a donor and the Pt adatom as an acceptor in the process of catalytic reaction.

A comparison of the effects of chemisorbed sulfide ions on the electrochemical behavior of gold and silver in thiourea electrolytes

It is shown that, in the case of gold, the catalytic activity of sulfur adatoms is significantly higher in the anodic dissolution of metal than in its deposition. In both processes, the catalytic activity increases with increasing overvoltage. The catalytic activity of sulfur adatoms is considerably lower for silver than for gold both in the metal deposition and dissolution, and it is virtually independent of overvoltage. In the case of silver, the catalytic activity of sulfur adatoms in the electrodeposition is slightly higher than in the metal dissolution. There results are compared with the action of heavy metal adatoms on the dissolution and deposition of gold and silver in the cyanide solutions. It is shown that the above regularities are qualitatively true for both systems. The main distinction is that the catalytic activity of sulfide ions in the thiourea solutions reaches a plateau with increasing surface coverage with sulfur adatoms, whereas the activity of heavy metals passes through a maximum with increasing surface coverage. The results are explained in view of earlier determined regularities of kinetics of electrode processes in these systems and the effect of electrocatalysis on these regularities.

Adsorption of metal adatoms on FeO(111) and MgO(111) monolayers: Effects of charge state of adsorbate on rumpling of supported oxide film

We present a theoretical density-functional theory study on the deposition of metal atoms (Ir, Pd, Pt, Ag, and Au) on FeO(111) and MgO(111) monolayers supported on Pt(111). We show the existence of a strong coupling between the charge state of the adsorbed adatom and the local polaroniclike distortion of the oxide film, and we identify two qualitatively different adsorption modes in which the distortion either reinforces the rumpling of the supported oxide film (positively charged adsorbates) or reduces or even reverses the cation-anion stacking (negatively charged adsorbates). Thus, the adsorption mode is a response to the charge state of the adsorbate and is driven mainly by the capacity of adatoms to exchange electrons with the support.

Destruction of graphene by metal adatoms

The formation energies for mono- and bivacancies in graphene in the presence of adatoms of various metals and small metallic clusters have been calculated. It is shown that transition metal impurities such as iron, nickel, and, especially, cobalt reduce dramatically the vacancy formation energies whereas gold impurities have almost no effect on characteristics of the vacancies. These results highlight that special measures are required in order to protect graphene from damage by transition metal leads.

Controlling electron transfer processes on insulating surfaces with the non-contact atomicforce microscope

We present the results of theoretical modelling that predicts how a process of transfer ofsingle electrons between two defects on an insulating surface can be induced using ascanning force microscope tip. A model but realistic system is employed which consists of aneutral oxygen vacancy and a noble metal (Pt or Pd) adatom on the MgO(001) surface. Weshow that the ionization potential of the vacancy and the electron affinity of the metaladatom can be significantly modified by the electric field produced by an ionic tip apex atclose approach to the surface. The relative energies of the two states are also afunction of the separation of the two defects. Therefore the transfer of an electronfrom the vacancy to the metal adatom can be induced either by the field effectof the tip or by manipulating the position of the metal adatom on the surface.

Step decoration of chiral metal surfaces

Highly stepped metal surfaces can define intrinsically chiral structures and these chiral surfaces can potentially be used to separate chiral molecules. The decoration of steps on these surfaces with additional metal atoms is one potential avenue for improving the enantiospecificity of these surfaces. For a successful step decoration, the additional metal atoms should ideally remain at the kinked step sites on the surface. We performed density functional theory (DFT) calculations to identify pairs of metal adatoms and metal surfaces where this kind of step decoration could be thermodynamically stable. These calculations have identified multiple stable examples of step decoration. Using our DFT results, we developed a model to predict surface segregation on a wide range of stepped metal surfaces. With this model, we have estimated the stability of step decoration without further DFT calculations for surface segregation for all combinations of the 3d, 4d, and 5d metals.

Positional and Orientational Templating of C60 Molecules on the Ag/Pt(111) Strain-Relief Pattern

We report on the site-specific nucleation of small C60 molecular clusters on the strain-relief pattern formed by two monolayers of Ag on Pt(111) as well as on the orientational ordering within a molecular monolayer formed on this same template surface. Small triangular patches, which are characteristic of the strain-relief pattern, are found to be the most favorable sites for trapping single molecules. The reason for this site specificity seems to reside in a higher adsorption energy of the C60 molecules in these patches. This is different from the previously reported case of metal (Ag) deposition where diffusion barriers and therefore kinetic reasons play the major role in the confinement of the metal adatoms in hexagonal patches of the strain-relief pattern, where they form nanoislands [Brune, H., et al. Nature 1998, 394, 451]. Here, for organic molecules, the small triangular patches are definitely the nucleation centers of two-dimensional islands covering the surface up to the full monolayer coverage. The particular pinning of the molecular monolayer in these patches imposes specific orientations for the molecules, whereas anywhere else on the surface the orientational order is strongly perturbed due to intrinsic irregularities of the strain-relief pattern.