Row Reconstruction Research Articles

Atomic-Scale Structural Evolution of Rh(110) during Catalysis

We report direct observation at the atomic scale of the pressure- and temperature-dependent evolution of a model Rh(110) catalyst surface during transient and steady-state CO oxidation, using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) correlated against density functional theory (DFT) calculations. Rh(110) is susceptible to the well-known missing row (MR) reconstruction. O2 dosing produces a MR structure and an O coverage of 1/2 monolayer (ML), the latter limited by the kinetics of O2 dissociation. In contrast, CO dosing retains the (1 × 1) structure and a CO coverage of 1 ML. We show that CO dosing titrates O from the (2 × 1) structure and that the final surface state is a strong function of temperature. Adsorbed CO accelerates and O inhibits the (2 × 1) to (1 × 1) transition, an effect that can be traced to the influence of the adsorbates on the energy landscape for moving metal atoms from filled to empty rows. During simultaneous ...

Oxygen vacancy induced surface stabilization: (110) terminated magnetite

Scanning tunneling microscopy (STM) measurements of the (110) surface of magnetite showed the coexistence of two reconstructions: the known $(1\ifmmode\times\else\texttimes\fi{}3)$ row reconstruction and a surprising atomic structure of high complexity which occupies a small fraction of the surface. Oxygen vacancies on the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(110) B-terminated surface have previously been determined to be the most energetically favorable surface termination of those considered [Li et al., Surf. Sci. 601, 876 (2007)]. However, this study only investigated oxygen vacancies which were threefold coordinated. Here, first principles calculations indicate that twofold coordinated oxygen represents the most energetically stable oxygen vacancy on the B-terminated (110) surface of magnetite. STM simulations reveal that the structure that occupies a small fraction of the surface corresponds to this energetically favorable B-terminated ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(110) surface. The oxygen vacancies form an ordered array: Along the $[\overline{1}10]$ direction, every second twofold coordinated oxygen atom is vacant, and vacancies are separated by 6 \AA{}. In adjacent twofold coordinated oxygen rows, the vacancies are shifted in the $[\overline{1}10]$ direction by 3 \AA{}. Density functional theory calculations of the spin density distributions indicate that surface and subsurface octahedrally coordinated iron atoms are charge ordered. The charge ordering and existence of oxygen vacancies act to reduce the surface charge. However, other polarity compensation mechanisms may be at play to stabilize the surface.

High-pressure operando STM studies giving insight in CO oxidation and NO reduction over Pt(1 1 0)

Two catalytic systems have been studied at high pressures on the Pt(110) surface on an atomic level. The first system was the oxidation of CO by O2 towards CO2. In the framework of the second reaction, namely NO reduction, the effect of room temperature exposure of the surface to NO and H2 was investigated. To study these reaction systems at relevant pressures, the ReactorSTM has been used. This is a unique system which consists of a compact STM in which a flow reactor is integrated. The combined reactor with STM is housed inside a conventional vacuum system to allow for traditional surface science preparation and analysis techniques. The STM images obtained with the ReactorSTM under reaction conditions show the lifting of the (1x2) missing row reconstruction by high-pressure CO exposure. The lifting is followed by the formation of the (1x1) metallic Pt(110) structure for high CO/O2 ratios and a (1x2) lifted-row type surface oxide structure for more O2-rich conditions. The room temperature exposure of Pt(110) to H2 results in the formation of a (1x4) missing-row structure and deeper, nested missing rows. The exposure to high-pressure NO removes these missing-row structures.

(Invited) Si Ultrathin Films on Silver Surfaces: An Intriguing Epitaxial System

The large interest in graphene layers and their applications has motivated scientists to investigate other group-IV two-dimensional (2D) layers that could be graphene's analogous, i.e. with honeycomb structure and sp2 bonding. Among these, silicene, a single sheet of silicon, has attracted much attention in the last past five years with respect to fundamental interest and potential use in electronics. Pioneering works concern the Si/Ag system and it has been shown that 2D epitaxial layers can be grown on Ag(110) [1] and Ag(111) [2]. A graphene-like signature in photoemission spectra has been reported on such films, attributed to the formation of silicene with sp2-sp3hybridized Si atoms arranged in a two-dimensional honeycomb lattice [2-3]. However, recent experimental and theoretical studies have shown that these conclusions have to be reconsidered and challenge the current understanding of the Si growth on silver substrates, interpreted within the framework of silicene models. After an introduction devoted to a review of the results obtained on the Si/Ag system, I will focus, in a first part, on our recent results concerning ultrathin Si films grown on the anisotropic Ag(110) surface. Upon submonolayer condensation of Si at room temperature (RT), long, parallel and thin lying Si single nanoribbons develop spontaneously (see Figure 1a). At 460 K, these nanoribbons self-assemble to form a one-dimensional (1D) array of double nanoribbons with a pitch of ~ 2 nm (see Figure 1b) . At completion, this 1D grating covers uniformly the entire substrate with a remarkably high degree of structural order. By following in situthe Si growth on Ag(110) with STM and grazing incidence X-ray diffraction using synchrotron radiation, we provide compelling evidence that nanoribbon growth induces a surface reconstruction associated with the release of Ag atoms [4]. Our results are in agreement with an unexpected effect, a missing row reconstruction of the Ag layer underneath the Si nanoribbons. Our findings call for a reconsideration of the proposed honeycomb structure of the Si nanoribbons based on a non-reconstructed substrate. In a second part, I will focus on the Si/Ag(110) system used as a novel Si template for the growth of 1D nanostructures. Recent works have shown that Si nanoribbons grown on Ag(110) can be used for the growth of identical 1D nanostructures such as Co nanolines [5], Mn nanolines [6] and C60chains [7]. Under Co adsorption in the temperature range 220 K-300 K, the thermally activated process of Co incorporation into the Si matrix is partially hindered. Co atoms are adsorbed on top of the Si nanoribbons and give rise to the growth of identical Co nanostructures with a high lateral order. These nanolines are composed of Co dimers oriented perpendicularly to the axis of nanolines. Processes involved in the Co nanostructure growth at the atomic scale have been studied by high-resolution STM. Interestingly, the Co nanoline growth proceeds in a nearly layer-by-layer growth, reproducing the 1D pattern of the Si template up to five monolayers. Recent results concerning the magnetic properties of the Co nanostructures studied with X-ray magnetic circular dichroism will also be presented.

Salvage of a post-traumatic arthritic wrist using the scaphoid as an osteochondral graft

The authors describe a case of post-traumatic wrist arthritis with an osteochondral defect in the scaphoid fossa of the radius. The patient was treated with proximal row carpectomy, radial styloidectomy and reconstruction of the defect using the proximal half of the scaphoid as an autologous osteochondral graft. Pain relief was achieved while wrist motion and strength were improved. The carpal bones are a source of osteochondral grafts and can be used to expand the indications of motion-preserving wrist salvage procedures.

Understanding Scanning Tunneling Microscopy Contrast Mechanisms on Metal Oxides: A Case Study

A comprehensive analysis of contrast formation mechanisms in scanning tunneling microscopy (STM) experiments on a metal oxide surface is presented with the oxygen-induced (2√2×√2)R45° missing row reconstruction of the Cu(100) surface as a model system. Density functional theory and electronic transport calculations were combined to simulate the STM imaging behavior of pure and oxygen-contaminated metal tips with structurally and chemically different apexes while systematically varying bias voltage and tip-sample distance. The resulting multiparameter database of computed images was used to conduct an extensive comparison with experimental data. Excellent agreement was attained for a large number of cases, suggesting that the assumed model tips reproduce most of the commonly encountered contrast-determining effects. Specifically, we find that depending on the bias voltage polarity, copper-terminated tips allow selective imaging of two structurally distinct surface Cu sites, while oxygen-terminated tips show complex contrasts with pronounced asymmetry and tip-sample distance dependence. Considering the structural and chemical stability of the tips reveals that the copper-terminated apexes tend to react with surface oxygen at small tip-sample distances. In contrast, oxygen-terminated tips are considerably more stable, allowing exclusive surface oxygen imaging at small tip-sample distances. Our results provide a conclusive understanding of fundamental STM imaging mechanisms, thereby providing guidelines for experimentalists to achieve chemically selective imaging by properly selecting imaging parameters.

Growth of Si ultrathin films on silver surfaces: Evidence of an Ag(110) reconstruction induced by Si

We report here in situ measurements of the evolution of the Ag(110) surface during Si growth, using scanning tunneling microscopy and grazing incidence x-ray diffraction. We provide compelling evidence of an Ag(110) surface reconstruction associated with the release of Ag atoms induced by the growth of Si nanoribbons. Our results are in agreement with a missing row reconstruction of the Ag layer underneath the nanoribbons. This challenges the current understanding of the Si growth on nonreconstructed Ag(110), interpreted within the framework of silicene models.

Tuning and probing interfacial bonding channels for a functionalized organic molecule by surface modification

The potassium-induced missing row reconstruction of Ag(110) is used to selectively modify the local chemical interaction between the functional anhydride groups of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and Ag(110). We find a significant upward shift of the anhydride groups, while the adsorption height of the perylene core is essentially preserved. This demonstrates an attractive perylene/substrate interaction for PTCDA/K:Ag(110), elucidating also the bonding situation for the potassium-free system.

Formation of a missing row reconstruction on a Cu(100) surface: An atom scale density functional theory based study

The oxygen-induced reconstruction of Cu(100) surfaces has been studied in great detail. Experimentally, missing row reconstruction occurs with an O coverage of 0.5 monolayer (ML). Several DFT-based calculations of vacancy-surface Cu pairs were performed at O coverage between 0 and 0.5ML. No structure was identified in these calculations as being energetically favoured over the ideal Cu(100) structure. When a Cu atom (0.125ML) was added to the system, a new energetically favourable missing row and added Cu-O-Cu row structure was found. It has been suggested that this structure is an important intermediate step towards the missing row formation. The transition from the missing and added row structure to the (2√2×√2)R45o-O reconstruction has the energetic requirement of very good binding sites for the Cu atoms, which can be found on the step boundaries. These results are in excellent agreement with the step edge growth and the formation of strongly anisotropic islands observed by Lahtonen et al. (J. Chem. Phys. 129 (2008) 124703).

Thermal Faceting of Clean and Oxygen-Covered Ir Nanocrystals

Thermal faceting of a clean and oxygen-covered iridium nanosized crystal was studied by field ion microscopy for oxygen exposures up to 6 langmuirs (1 langmuir = 1.3 × 10−4 Pa·s) and annealing temperatures of 300−1100 K. The clean Ir specimen (the “tip”), originally hemispherically shaped, started to facet upon annealing to at least 850 K, exposing only {111} and {001} facets in the final cuboctahedral form. Even small amounts of adsorbed oxygen caused changes in the Ir tip morphology. An oxygen exposure of 0.3 langmuir caused {113} facets to develop at 700 K. For larger (1 to 6 langmuirs) exposures, well characterized {111}, {001}, {113}, and {011} facets were observed to develop when annealing to at least 650 K. The recorded micrographs suggest (1 × 3) or, to a lesser extent, (1 × 4) missing row reconstructions of the {011} facets, while other facets do not exhibit any clear evidence of reconstruction.

O2 dissociation before the onset of added row nucleation on Ag(110): an atomistic scanningtunnelling microscopy view

We present here a scanning tunnelling microscopy (STM) study onO2 adsorption atAg(110) at T = 175 K, i.e. in the temperature range between the onset ofO2 dissociation and the formation of the added row reconstruction. In agreement with previousstudies at lower surface coverage, we observe several structures forming uponO2 dissociation, including Ag–O complexes randomly distributed on the surface.We suggest that the presence of the latter structures, characterized by a largecross section for low energy electrons, can account for the marked decrease of thesurface electron reflectivity and for the corresponding increase of the diffuse elasticintensity previously reported in a HREELS investigation of the same system.

Sulphur adsorption on Au{1 1 0}: DFT and LEED study

The adsorption of sulphur on clean reconstructed Au{1 1 0}-(1 × 2) surface was studied using density functional theory (DFT) and quantitative low energy electron diffraction (LEED) calculations. The results show that the sulphur atoms form a (4 × 2) ordered structure which preserves the missing row reconstruction of the clean surface. The sulphur atom is found to adsorb on threefold hollow sites, on the {1 1 1} microfacets that border the trenches of the missing rows.

Geometry of the Au(1 1 0)-(1 × 2) missing-row clean surface: A New LEED and DFT study

In this work we report on a very accurate low-energy electron diffraction (LEED) and a DFT study of Au(1 1 0)-(1 × 2) in order to determine its surface geometry. The LEED results indicate a surface missing row reconstruction with a large contraction of ( - 0.29 ± 0.02 ) Å , a pairing in the second layer ( 0.06 ± 0.05 ) Å and in the fourth layer ( 0.07 ± 0.04 ) Å , and a buckling in the third layer of 0.30 Å and in the fifth layer of 0.01 Å. These LEED results are in very good agreement with those found by the DFT calculations.

Missing row reconstruction on three low index surfaces of ten FCC metals

AbstractWith modified embedded‐atom method (MEAM), the surface energies of the perfect and (1×2) missing row structure on (100), (110) and (111) surfaces, and (1×3), (1×4) and (1×5) missing row structure on (110) surface have been calculated for ten FCC metals Pb, Au, Pt, Ir, Ag, Pd, Rh, Cu, Ni and Al. According to the energy minimization, we know that the (1×2) missing row reconstruction cannot be formed spontaneously on (100) and (111) surfaces of all ten FCC metals. However, not only (1×2) but also (1×3) missing row reconstruction can be formed on (110) surface of Au, Pt and Al spontaneously. These are consistent with the experimental results except Al for which the experiment has not been reported. The results are also analyzed in terms of the surface configurations and valence electron structures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Giant missing row reconstruction of Au on Ge(001)

We report on a giant missing row reconstruction emerging upon the adsorption and subsequent annealing of (sub)monolayer amounts of Au on Ge(001). The emerging microfacets are of (111) type and reminiscent of those in the well-known $(2\ifmmode\times\else\texttimes\fi{}1)$ missing row reconstruction of the clean (110) surfaces of the fcc transition metals Au, Ir, and Pt. The (111) microfacets are aligned along the [110] directions and decorated with Au atoms. The periodicity perpendicular to the ridges amounts to 1.6 nm. Surprisingly the corrugation of these Au-induced facets is found to be not less than 0.6 nm. The facets exhibit a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction similar to the Au-induced reconstruction of Ge(111). Scanning tunneling microscopy and spectroscopy measurements reveal that the top ridges consist of buckled Ge-dimer rows.

Adsorption of O and CO on Ir(100) from first principles

The adsorption of O and CO on Iridium (100) surface with different coverages (Θ = 1.0, 0.5, 0.25 monolayer (ML)) is studied using density functional theory (DFT). The most energetically preferred site of adsorption for O is found to be the bridge site. However, the top site is the preferred one for CO at coverages of 0.25 ML and 0.5 ML. Oxygen adsorbed on the bridge site at 0.25 ML and 0.5 ML coverages causes a row pairing. A missing row reconstruction appears in the case of 0.25 ML coverage. We find that the adsorption of O (CO) on Ir(100) surface causes disruptions of Ir–Ir bonds in the metal, which reduces (increases) the Ir–Ir bond length.

Periodic Arrays of Cu-Phthalocyanine Chains on Au(110)

The structure of ultrathin Cu-phthalocyanine (Cu-Pc) films on the (1 × 2)-Au(110) surface has been studied. The overlayer deposition has been monitored in real time by helium atom scattering (HAS) and low energy electron diffraction (LEED). Throughout the monolayer regime the Cu-Pc molecules are systematically observed to line-up edge-to-edge along the [11̅0] direction of the Au substrate, yielding a commensurate 5-fold periodicity (14.4 Å). Cu-Pc chains deconstruct the 2-fold Au missing row order in the early stage of deposition. A set of higher order periodicities (5-, 7-, and 3-fold) are progressively observed along [001] with increasing Cu-Pc deposition, the 3-fold phase appearing at the monolayer saturation coverage. The corresponding molecular orientation has been studied by variable polarization absorption spectroscopy (XAS), whereas the Au substrate structure has been determined by out-of-plane surface X-ray diffraction. The (5 × 5) phase is found to be rather corrugated, and it exhibits a high degree of long-range order yielding the most prominent diffraction pattern. In the (5 × 5) phase, the Cu-Pc chains are found to lift the underneath missing row reconstruction, being separated by residual Au rows. Similarly, in the more compressed 3-fold monolayer phase, the Cu-Pc molecules were formerly found to lie within a shallow (1 × 3) Au reconstruction [Cossaro, A.; et al. J. Phys. Chem. B 2004, 108, 14671]. From comparison of the different deposition stages, as measured in real time by HAS, we can draw a comprehensive picture of the system evolution. In fact, the observed periodicities at different coverage are always formed by an array of Cu-Pc chains in shallow troughs that are equally spaced by a number of uncovered Au rows, as dictated by the Cu-Pc coverage. The growth of Cu-Pc arrays in the submonolayer range is thus driven by an interchain repulsion mechanism.

In Situ X-Ray Diffraction Study of Ag(100) at Ambient Oxygen Pressures

We have investigated the interaction of oxygen with the (100) surface of the noble metal Ag in a wide O2 pressure range, in situ, by surface X-ray diffraction. At 370 K and 10-2 mbar oxygen pressure, we observe a strong change in the diffraction signal, which is in line with the formation of oxygen induced Ag vacancies, up to a concentration of 25% in the topmost Ag layer. We argue that these vacancies are associated with local fluctuations of the p(2√2 × √2)R45° missing row reconstruction in a highly dynamical oxygen adsorption−desorption process. A detailed structural model is derived from the X-ray data and the observed stability of the structure as a function of temperature and oxygen pressure is compared to recent ab initio thermodynamic calculations. Finally, at 440 K and 1 mbar oxygen pressure, we did not observe noticeable subsurface oxygen formation (<0.2 at. % within the experimental resolution).

Missing row reconstructed (110), (211) and (311) surfaces for FCC transition metals

AbstractModified embedded atom method (MEAM) has been used to ascertain the change in the surface energy density of (1 × 2) missing row (MR) reconstruction from initial (1 × 1) ideal (110), (211) and (311) surfaces for seven FCC transition metals Au, Pt, Ag, Pd, Rh, Cu and Ni. The results show that the MR reconstruction can be formed naturally on the Au (110), Pt (110) and Pt (311) surfaces and are better than calculated results of the embedded atom method (EAM) while comparing with experimental results. In addition to the surface energy explanation, the results are also explained in terms of the valence electron structure and surface topography. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Effect of oxygen adsorption on the energy losses in grazing scattering of hydrogen ions on Ag(1 1 0)

In this work we present a study of the effect of oxygen adsorption on the energy loss of 4keV hydrogen ions, which are scattered off an Ag (110) single-crystal surface with varying coverages of oxygen. In this case oxygen adsorption leads to an added row reconstruction of the surface. We performed measurements for grazing angles as a function of crystal azimuthal orientation, which show large differences in energy losses. Experimental results are discussed in the light of trajectory calculations of protons scattered under grazing incidence conditions on the surface. Using non-linear models for stopping power, ab initio crystal structure calculations of the electronic density and semi-classical simulations, we obtain data that is in very good agreement with experimental results. These simulations in particular allow us to properly take into account the variations of the surface electronic density and hence obtain an accurate description of the energy loss processes for ion scattering along various azimuthal orientations of the target.