Step-edge Atoms Research Articles

Characterization of CO Adsorbed to Clean and Partially Oxidized Cu(211) and Cu(111).

Copper-based catalysts gain activity through the presence of poorly coordinated Cu atoms and incomplete oxidation at the surface. The catalytic mechanisms can in principle be observed by controlled dosing of reactants to single-crystal substrates. However, the interconnected influences of surface defects, partial oxidation, and adsorbate coverage present a large matrix of conditions that have not been fully explored in the literature. We recently characterized oxygen and carbon monoxide coadsorption on Cu(111), a nominally defect-free surface, and now extend our study to the stepped surface Cu(211). Temperature-programmed desorption of CO adsorbed to bare metal surfaces confirms that two sites dominate desorption from a saturated layer: atop terrace atoms of local (111) character and atop step edge atoms with CO bound more strongly to the latter. At low coverage, discrete CO resonances in reflection adsorption infrared spectra can be assigned to these sites: 2077 cm-1 for extended (111) terraces, 2093 cm-1 for step sites, and additional kink-adsorbed molecules at 2110 cm-1. With increasing coverage, in contrast to Cu(111), the infrared spectral features on Cu(211) evolve and shift as a consequence of dipole-dipole coupling between differentially occupied types of sites. Auger electron spectroscopy shows that exposure to background O2 oxidizes the (211) surface at a rate nearly 1 order of magnitude greater than (111); we argue that the resulting surface is stoichiometric Cu2O, as previously found for Cu(111). This oxide binds CO less strongly than the bare metal and the underlying crystal cut continues to influence the adsorption sites available to CO. On oxidized (111) terraces, broad absorption peaks at 2115-2120 cm-1; on oxidized Cu(211), CO adsorbed to step sites appears as a resolved secondary peak at 2144 cm-1. This suite of spectroscopic signatures, obtained under carefully controlled conditions, will help to determine the origin and fate of adsorbed species in future studies of reaction mechanisms on copper.

Direct Observation of Atomic Step-Assisted Stabilization of Polar Surface.

Polar surface is intrinsically unstable and thus highly reactive due to the uncompensated surface charges. The charge compensation is accompanied by various surface reconstructions, establishing novel functionality for its applications. Our in-situ atomic-scale electron microscopy study directly shows that the atomic step and step-assisted reconstruction play central roles in the charge compensation of polar oxide surface. The flat (LaO)+-terminated LaAlO3 (001) polar surface, when annealed at high temperature in vacuum, transits to the (015) vicinal surface via the dynamic motion and interaction of atomic steps. While the (015) vicinal surface possesses zero polarization along the surface normal, the thermodynamic ground state is achieved when the in-plane polarization is fully compensated via the reconstruction of step edge atoms; the step edge La atoms are displaced from their ordinary atomic sites toward the adjacent Al step edge site, resulting in the formation of negatively charged La vacancy at the corresponding step edges. As confirmed by first-principles calculations, the observed step reconstruction of (015) vicinal surface can completely cancel both out-of-plane and in-plane electric fields. This hitherto unknown mechanism reveals the central role of step reconstruction in stabilizing polar surfaces, providing valuable insights for understanding the novel charge compensation mechanism accompanied by the step reconstruction. This article is protected by copyright. All rights reserved.

Comparative Studies on Local Barrier Field Variations Above Field-Adsorbed Helium and Neon with a Micro-Probe Hole Field Ion Microscope.

The local field ion emission properties of helium and neon around a step edge atom of W(112) were examined at liquid nitrogen temperature using a micro-probe hole field ion microscope combined with a pulse-counting analysis. We have analyzed the mapped field ion densities obtained for both imaging gas atoms at their respective best local image voltages based on the formula for tunneling barrier strength and have evaluated the dipole moment of polarized adatom as well as the local field enhancement factor at the adatom site. We found that the dipole moments of helium and neon adatoms showed the same value, although the best local image field acting on the helium adatom is much higher than that on the neon adatom. We also found the same magnitude of local field enhancement factors for both noble gas field adsorptions. These results imply that the key to the best local image condition is the tunneling barrier field variations above the adatom. The vital role of the imaging gas atoms is to form an optimum dipole moment to create an ideal electric field distribution for the best local image appearance at each atom site depending on the different chemical nature of adatom species.

Inactive step-edge Pt atoms boost oxygen reduction reaction by activating adsorbed hydrogen atoms

Identifying the influence of the step defects is crucial for the development of high-performance Pt-based catalysts towards oxygen reduction reaction (ORR). To this end, intense research based on the oxygen adsorption energy (Eads(O)) criterion has been carried out for Pt single crystals, yet hardly can these studies be applied to the alloy system, considering the strain and ligand effects. Here we demonstrated that for a typical alloy (that is Pt1.2Cu octahedron), the activity is in positive correlation with the number of steps which, however, are detrimental to ORR as determined by the Eads(O) results. Using a combination of kinetics and electron density differences calculations, a novel “hydrogen activation” pathway is put forward. It is shown that the step-edge atoms can apply a repulsive force on the adsorbed hydrogen atoms, which reduces the energy barrier of the rate-determining oxygen hydrogenation process by 0.1 eV compared with the corresponding smooth surface and eventually accelerates the ORR kinetics.

Quantum dynamics studies of the dissociative chemisorption of CH4 on the steps and terraces of Ni(211).

The dissociative chemisorption of CH4 on the stepped Ni(211) surface is explored. The H and CH3 fragments preferentially bind to the surface along the step edge, and the barriers to dissociation are lowest over the step edge atoms, with activation energies of 0.57 and 0.69 eV, depending upon the orientation of the dissociating bond. The activation energy for dissociation over a terrace atom is much larger, 0.99 eV. Quantum scattering calculations show that dissociation over the lowest barrier step edge site dominates the reactive scattering, except at very high collision energies, where the second step edge transition state becomes important. Even at incident energies of 1.0 eV, reaction over the terrace atoms contributes only about 3% to the total dissociative sticking. The symmetric stretch excitation has the largest efficacy for promoting reaction, with the other modes having somewhat smaller efficacies. While the vibrational properties of the step edge atoms are different from those on the terrace, reactions at both sites exhibit a similar variation with temperature. We find good agreement with experimental measurements of the thermally averaged sticking probability on a Ni step edge.

Increased Back-Bonding Explains Step-Edge Reactivity and Particle Size Effect for CO Activation on Ru Nanoparticles.

Carbon monoxide is a ubiquitous molecule, a key feedstock and intermediate in chemical processes. Its adsorption and activation, typically carried out on metallic nanoparticles (NPs), are strongly dependent on the particle size. In particular, small NPs, which in principle contain more corner and step-edge atoms, are surprisingly less reactive than larger ones. Hereby, first-principles calculations on explicit Ru NP models (1-2 nm) show that both small and large NPs can present step-edge sites (e.g., B5 and B6 sites). However, such sites display strong particle-size-dependent reactivity because of very subtle differences in local chemical bonding. State-of-the-art crystal orbital Hamilton population analysis allows a detailed molecular orbital picture of adsorbed CO on step-edges, which can be classified as flat (η1 coordination) and concave (η2 coordination) sites. Our analysis shows that the CO π-metal dπ hybrid band responsible for the electron back-donation is better represented by an oxygen lone pair on flat sites, whereas it is delocalized on both C and O atoms on concave sites, increasing the back-bonding on these sites compared to flat step-edges or low-index surface sites. The bonding analysis also rationalizes why CO cleavage is easier on step-edge sites of large NPs compared to small ones irrespective of the site geometry. The lower reactivity of small NPs is due to the smaller extent of the Ru-O interaction in the η2 adsorption mode, which destabilizes the η2 transition-state structure for CO direct cleavage. Our findings provide a molecular understanding of the reactivity of CO on NPs, which is consistent with the observed particle size effect.

Two-dimensional interaction of spin chains in the Si(553)-Au nanowire system

Adsorption of Au on Si(553) results in the self-assembly of highly ordered step arrays of one-dimensional (1D) Au atomic wires along the step direction. Charge transfer from the terrace to the step edge causes every third Si atom at the step edge to exhibit a partially filled dangling bond hosting a single fully spin-polarized electron which forms in an ordered 1D spin chain along the step. The interstep correlation of this threefold periodicity in neighboring Si step edges and the geometry of the unit cell has been determined by means of high-resolution spot profile analysis low-energy electron diffraction, scanning tunneling microscopy, and density functional theory. While the twofold periodicity of the Au wires exhibits a weak interwire interaction, leading to streaks in the diffraction pattern, the correlation of the Si step edge atoms is by far a stronger interaction, resulting in clear spots. The corresponding unit cell spanned by threefold ordered step edge atoms can be described as a centered structure which is magnetically frustrated and may stabilize a (two-dimensional) quantum spin liquid.

Self-Assembly of Protein Nanofibrils Orchestrates Calcite Step Movement through Selective Nonchiral Interactions.

The recognition of atomically distinct surface features by adsorbed biomolecules is central to the formation of surface-templated peptide or protein nanostructures. On mineral surfaces such as calcite, biomolecular recognition of, and self-assembly on, distinct atomic kinks and steps could additionally orchestrate changes to the overall shape and symmetry of a bulk crystal. In this work, we show through in situ atomic force microscopy (AFM) experiments that an acidic 20 kDa cement protein from the barnacle Megabalanus rosa (MRCP20) binds specifically to step edge atoms on {101̅4} calcite surfaces, remains bound and further assembles over time to form one-dimensional nanofibrils. Protein nanofibrils are continuous and organized at the nanoscale, exhibiting striations with a period of ca. 45 nm. These fibrils, templated by surface steps of a preferred geometry, in turn selectively dissolve underlying calcite features displaying the same atomic arrangement. To demonstrate this, we expose the protein solution to bare and fibril-associated rhombohedral etch pits to reveal that nanofibrils accelerate only the movement of fibril-forming steps when compared to undecorated steps exposed to the same solution conditions. Calcite mineralized in the presence of MRCP20 results in asymmetric crystals defined by frustrated faces with shared mirror symmetry, suggesting a similar step-selective behavior by MRCP20 in crystal growth. As shown here, selective surface interactions with step edge atoms lead to a cooperative regime of calcite modification, where templated long-range protein nanostructures shape crystals.

Exothermic Mechanism for the Abstraction of Hydrogen from Methane on Li-Doped MgO

Despite its importance, it is still not known by which mechanism Li-doped MgO catalyzes the oxidative coupling of methane to ethane. Nevertheless, it is commonly assumed that the mechanism goes through catalytic H abstraction from methane via a Li+O– surface defect. In this paper we use first-principles density functional theory calculations to show that the reaction is significantly more exothermic when the Li+O– defect is situated on a step edge instead of on the flat surface. We find that the reaction on the step is exothermic by 0.25 eV, whereas it is endothermic by 0.3 eV on the flat surface. The presence of the Li dopant in the step edge is crucial for the exothermicity of the reaction. These findings suggest that surface steps which include lithium defects could be responsible for the catalytic behavior of Li/MgO. Following the binding of hydrogen to the Li+O– defect on the step edge the methyl radical can either depart to the gas phase or bind to an adjacent step-edge oxygen atom, increasing the e...

Temperature-dependent scanning tunneling spectroscopy on the Si(557)-Au surface

Room-temperature and low-temperature (77 K) scanning tunneling spectroscopy (STS) and voltage-dependent scanning tunneling microscopy (STM) data are used to study the local electronic properties of the quasi-one-dimensional Si(557)-Au surface in real space. A gapped local electron density of states near the Gamma-point is observed at different positions of the surface, i.e., at protrusions arising from Si adatoms and step-edge atoms. Within the gap region, two distinct peaks are observed on the chain of localized protrusions attributed to Si adatoms. The energy gap widens on both types of protrusions after cooling from room temperature to T = 77 K. The temperature dependence of the local electronic properties can therefore not be attributed to a Peierls transition occurring for the step edge only. We suggest that more attention should be paid to finite-size effects on the one-dimensional segments.

Kinetic Monte Carlo simulation of electrodeposition using the embedded-atom method

A kinetic Monte Carlo (KMC) method for deposition is presented and applied to the simulation of electrodeposition of a metal on a single crystal surface of the same metal under galvanostatic conditions. This method utilizes the multi-body embedded-atom method (EAM) potential to characterize the interactions of metal atoms and adatoms. The method accounts for collective surface diffusion processes, in addition to nearest-neighbor hopping, including atom exchange and step-edge atom exchange. Steady-state deposition configurations obtained using the KMC method are validated by comparison with the structures obtained through the use of molecular dynamics (MD) simulations to relax KMC constraints. The results of this work support the use of the proposed KMC method to simulate electrodeposition processes at length (microns) and time (seconds) scales that are not feasible using other methods.

Thiolate adsorption on Au(${\bm {hkl}}$hkl) and equilibrium shape of large thiolate-covered gold nanoparticles

The adsorption of thiolates on Au surfaces employing density-functional-theory calculations has been studied. The dissociative chemisorption of dimethyl disulfide (CH(3)S-SCH(3)) on 14 different Au(hkl) is used as a model system. We discuss trends on adsorption energies, bond lengths, and bond angles as the surface structure changes, considering every possible Au(hkl) with h, k, l ≤ 3 plus the kinked Au(421). Methanethiolate (CH(3)S-) prefers adsorption on bridge sites on all surfaces considered; hollow and on top sites are highly unfavourable. The interface tensions for Au(hkl)-thiolate interfaces is determined at low coverage. Using the interface tensions in a Wulff construction method, we construct atomistic models for the equilibrium shape of large thiolate-covered gold nanoparticles. Gold atoms in a nanoparticle change their equilibrium positions upon adsorption of thiolates towards shapes of higher sphericity and higher concentration of step-edge atoms.

Spectroscopic evidence for spin-polarized edge states in graphitic Si nanowires

The step edges on the Si(553)–Au surface undergo a 1 × 3 reconstruction at low temperature which has recently been interpreted theoretically as the ×3 ordering of spin-polarized silicon atoms at the edges of the graphitic Si nanowires on this vicinal surface. This predicted magnetic ground state has a clear spectroscopic signature—a silicon step-edge state at 0.5 eV above the Fermi level—that arises from strong exchange splitting and hence would not occur without spin polarization. Here we report spatially resolved scanning tunneling spectroscopy data for these nanowires. At low temperature we find an unoccupied state at 0.5 eV above every third step-edge silicon atom, in excellent agreement with the spin-polarized ground state predicted theoretically. This spin-polarized state survives up to room temperature where the position of the spins rapidly fluctuates among all Si step-edge sites.

Spin-split silicon states at step edges of Si(553)-Au

The quasi-one-dimensional Si(553)-Au surface is investigated with time-resolved two-photon photoemission and laser-based photoemission. Several occupied and unoccupied states inside and outside the bulk band gap of silicon were found near the center of the surface Brillouin zone. A nondispersing unoccupied state 0.62 eV above the Fermi level with a lifetime of 125 fs matches the spin-split silicon step-edge state predicted by density functional theory calculations. Two occupied bands can be associated with the bands calculated for nonpolarized step-edge atoms.

Hydrogen adsorption on polar ZnO(0001)-Zn: Extending equilibrium surface phase diagrams to kinetically stabilized structures

Combining density-functional-theory calculations and atomistic thermodynamics we study hydrogen adsorption on the Zn-terminated polar ZnO(0001) surface. We extend the concept of equilibrium surface phase diagrams to include kinetically stabilized surface reconstructions by constructing metastable phase diagrams and going beyond the thermodynamic limit. Under these conditions we find a monolayer H coverage to be extremely favorable on triangular-shaped reconstructions due to a simultaneous protonation of step-edge oxygen atoms and surface terminating Zn atoms. Experimental situations, which realize hydrogen chemical potentials outside the stability range of the ${\text{H}}_{2}$ molecule are discussed.

First-principles investigation of the surface structures of Cu( n, n−1, 0) ( n=2, 3 and 4) stepped surfaces

Multilayer relaxation at high-index Cu( n, n−1, 0) ( n=2, 3 and 4) stepped surfaces was determined by the first-principles pseudopotential plane wave (PPPW) method. For those surfaces that have n and n−1 atom-rows in terrace and subterrace, respectively, the topmost 2 n−2 interlayer spacings contract, while the 2 n−1st interlayer spacing expands. There is no similar rule found for the relaxations parallel to the surfaces. Compared with the bulk terminated structure, a thin compact layer, which consisted of the topmost 2 n−1 atom layers and separated slightly from the underneath atom layers, makes the surface more flat after relaxation. The bond-lengths between the step edge (first layer) atom and its nearest-neighbors do not depend on the surface termination, but only on the local coordination.

Adsorption of Single Oxygen Atoms on Vicinal Si(001) Surfaces: Lattice Relaxation and Si Oxidation ¡¤¡¤¡¤¡¤¡¤¡¤ ¡¤¡¤¡¤¡¤¡¤¡¤

By employing ab initio electronic structure calculations, we investigated the adsorption of single oxygen atoms on vicinal Si(001) surfaces with single-layer SA and SB steps. The calculations in this study showed that the oxygen adatom preferetially adsorbs not only on the rebond sites of the rebonded step-edge Si atoms (RSAs) at the SB step but also on the backbond site of the down Si atom of the step-edge Si dimer on the upper terrace of the SB step. For more detailed information, the electronic properties of the adsorption structures of the oxygen adatoms were also examined. All these calculations showed that the lattice relaxation of the RSAs near the rebonded SB step played a signi cant role in the adsorption of the single oxygen atoms on the vicianl Si(001) surfaces with the single-layer steps.

Ab initiostudy of the oxidation on vicinal Si(001) surfaces: The step-selective oxidation

Initial oxygen adsorption structures and oxidation reaction on vicinal Si(001) surfaces with single-layer ${S}_{A}$ and ${S}_{B}$ steps were studied using ab initio total energy and electronic structure calculations. At the very early stage of the oxygen adsorption on Si(001), the calculations in this study show that oxygen atoms are preferentially incorporated into sites at the rebonded ${S}_{B}$ step edge. Various stable oxidation structures at the rebonded ${S}_{B}$ step were identified based on total energy calculations. Their stabilities are concerned with the structural attribute of the rebonded step-edge Si atoms (RSAs) with dangling bonds and weak rebonds. In addition, the proximity of the RSAs helps form the stable oxidation structures in high oxygen coverage regions that have in common a one-dimensional (1D) -Si-O- chain structure along the step edge as a basic 1D oxide building element. By forming this 1D chain structure, it is possible to effectively reduce the number of dangling bonds (DBs) at the step edge. This indicates that the reduction of the number of step-edge DBs plays a crucial role in the formation of oxidation complexes at the steps with local strain, as seen on flat semiconducting surfaces. For more detailed information, the electronic properties of the oxidation complexes were also calculated. The calculated site-projected density of states and scanning tunneling microscopy images of the oxidation structures exhibit characteristic features distinct from those of the clean vicinal Si surface. All these results clearly show that the ${S}_{B}$ step acts as a strong sink for the oxidation of Si as suggested in experiments.

Interplay between electronic and atomic structures in the Si(557)-Au reconstruction from first principles

The quasi-one-dimensional Si(557)-Au reconstruction has attracted a lot of attention in recent years. We study here the interplay between the electronic and structural degrees of freedom in this system. Our calculations are in good agreement with recent experimental data obtained using scanning tunneling microscopy and spectroscopy both at room and low temperatures. Together with the quite successful description of the experimental band structure, these results give further support to the current structural model of the Si(557)-Au surface. We consider in detail the energetics and variation of the band structure as a function of the buckling of the step edge and its implications to explain the observed metal-insulator transition. Finally, we present the results of a first-principles molecular dynamics simulation of several picoseconds performed at room temperature. As expected, we find a strong oscillation of the step-edge atoms. The dynamics associated with other vibrational modes is also observed. Particularly apparent are the oscillations of the height of the restatoms and adatoms and the associated fluctuation of the Si--Au--Si bond angles along the gold chain. This mode, together with step-edge buckling, has a strong influence on the insulating and/or metallic character of the surface.

Extraordinary Atomic Mobility of Au{111} at 80 Kelvin: Effect of Styrene Adsorption

We describe how the presence of styrene, a weakly adsorbed molecule, dramatically restructures the Au{111} surface at temperatures as low as 80 K. The restructuring manifests itself both in mobility of step-edge atoms, as well as changes in the position of the herringbone reconstruction over time. These effects are explained in terms of the preferential adsorption sites of styrene allowing it to assist in atom detachment from step edges, as well as lowering of the energetic barrier for movement of the herringbone reconstruction. This work has important consequences for studies in which Au is used as a support for or as an electrical contact to molecules.