Missing-row Reconstruction Research Articles

Pt25Rh75(111), (110), and (100) studied by scanning tunnelling microscopy with chemical contrast

Scanning tunnelling microscopy images with chemical contrast allowed the direct determination of the composition and short-range order behaviour of the clean Pt25Rh75(111), (110), and (100) alloy surfaces. All measurements were performed at room temperature and showed a strong platinum enrichment depending on the preparation temperature. In the top layers of both Pt25Rh75(111) and Pt25Rh75(110)-(1×2) we find a preference for unlike nearest neighbours. Pt25Rh75(100) exhibits after a preparation temperature of 900°C a preference for clustering whereas after a preparation temperature of 600°C, comparable with Pt25Rh75(111) and (110), ordering tendencies appear. Additional investigations of the chemical identity of atoms surrounding hollow positions show mostly small deviations from a random distribution. However, the number of hollow sites surrounded by Rh atoms only can be significantly affected by the short-range order.Pt25Rh75(110) exhibits a (1×2) missing-row reconstruction after annealing above 700°C. After the first annealing of the sputtered surface it is accompanied by mesoscopic long-range ‘waves’ with a height of approximately 2nm and a wavelength up to 200nm depending on the preparation temperature.

PHASE DIAGRAM OF ADSORBATE-INDUCED ROW-TYPE ALIGNMENTS

The phase diagram of adsorbate-induced row-type alignments, such as missing-row reconstructions induced by adsorbate atoms on the FCC(110) surface, is calculated by the Blume–Emmery–Griffiths (BEG) model. In the model, we introduce adatom–adatom and dipole–dipole interactions between nearest-neighbor (NN) and next-nearest-neighbor (NNN) rows. The calculation of the temperature versus adatom chemical potential phase diagram is performed using mean-field approximation. It is indicated that when NN and NNN interactions are competitive, there appear either dipole or coverage modulated (incommensurate) phases at high temperatures for a wide regime of the interactions.

Lattice vibrations and stability of reconstructed (110) noble metal surfaces

The interplay between the surface phonons and different missing-row reconstructions of (110) noble-metal surfaces is investigated. The phonon dispersion curves and densities of states are calculated for a slab model with 24 layers of atoms, using the Sutton–Chen many-body potential. The phonon contribution to the surface free energy is evaluated for Au, Pt and Ag surfaces. The results show that surface phonons play an important role in stabilizing a surface reconstruction when the energy difference between different reconstructions is of the order 1meV/(surface unit cell) or smaller, and in particular in the case of Au(110) when the (1×2) and (1×3) reconstructions are close in energy. We also observe that the Sutton–Chen potential reproduces the phonon dispersion curves better for Ag (4d metal) than for Au and Pt (5d metal).

Atomically resolved p(3×1) reconstruction on the W(1 0 0) surface imaged with magnetic tips

We studied the surface of W(1 0 0) in an ultrahigh vacuum at room temperature using STM and LEED. We employed a novel STM tip of equiatomic MnNi alloy. This material with a CuAu-I-type crystalline structure is an antiferromagnet with a high Néel temperature of 1140 K. We have observed a bulk termination with a pattern of orthogonal `domains’ of p(3×1) missing row reconstructions. We have noted an anomalous feature in image acquisition at the atomic scale, namely that different images are obtained on reversal of scanning direction. This effect may result from the particular tip material used.

K adsorption on Ag(110): effect on surface structure and surface electronic excitations

The growth of K adlayers on Ag(110) was studied by energy loss spectroscopy–low energy electron diffraction (ELS–LEED) which allows the analysis of both elastically and inelastically scattered electrons. Ordered (1× n) missing row reconstructions and disordered adsorption were investigated. We find that extended surface reconstruction takes place already at 3% K coverage generating a (1×3) missing row structure. Elastic spot profile analysis shows that a long range correlation between K adatoms is present along the 〈1 1 ̄ 0〉 direction on the reconstructed surface, implying the stabilization of the reconstruction also at long distances from the K adatoms. With increasing coverage [(1×2) and (1×3) structures] the distance between the K adatoms along 〈1 1 ̄ 0〉 decreases and the adsorption sites become correlated also along 〈001〉 in contrast with the behaviour of K on Ni, Cu and Pd. For disordered K adsorption we observe that a weak correlation between the adatoms along both directions is already present in the background. The inelastic intensity shows only one marked loss at the surface plasmon frequency, indicating that in the investigated coverage range both K-induced s and p levels are empty. Ag surface plasmon dispersion and damping are affected by the presence of K in accord with the electronic mechanism generating the reconstruction. For the ordered (1× n) reconstructions the structural anisotropy is enhanced, while the anisotropy of Ag surface plasmon dispersion is reduced. K adsorption strongly affects in particular the quadratic term of the dispersion along 〈001〉, which drops by 60% already at 3% K coverage while the linear term remains initially unaffected independently of surface reconstruction. Above 30% K coverage also the linear term decreases and the anisotropy present for the surface plasmon dispersion of the bare Ag(110) is removed.

Temperature-dependent segregation and (1×2) missing-row reconstruction of Pt25Rh75(110)

The surface structure and composition of the clean Pt25Rh75(110) surface is investigated by low energy electron diffraction (LEED) and low energy ion scattering (LEIS). For the equilibrated Pt25Rh75(110) surface we observe a (1×2) missing-row reconstruction in analogy to the pure Pt(110) surface, and a significant Pt enrichment of the topmost atomic layer (up to 80at.% Pt). As the same strong surface enrichment in Pt was found in a previous study on the (100) and (111) surface of the same bulk composition, this means that in contrast to Pt–Ni and Pt–Co alloys, for Pt25Rh75 alloys the segregation behavior is not influenced extensively by the surface orientation. In addition to the structure analysis by LEED we performed LEIS experiments to determine the temperature-induced changes of the surface composition and structure. Since the Pt segregation is less pronounced at elevated temperature, the surface reveals a temperature-induced deconstruction of the (1×2) structure around 750°C, resulting in an fcc(110) (1×1) surface at high temperature. Temperature-dependent measurements further show a hysteresis-like behavior of the top-layer composition, which is attributed to an enhanced Pt segregation on the (1×2) reconstructed surface.

Oxygen-Induced Reconstruction and Surface Oxidation of Rhodium

The reaction of oxygen with Rh single-crystal tips was investigated using field ion microscopy (FIM). Emphasis was laid on revealing the atomic structure of individual tip surface planes along with their influence on the early stages of the reaction, i.e., chemisorption and nucleation. Under field-free conditions, the interaction of oxygen with a (001) oriented Rh tip was found to lead to major reconstructions at temperatures above 400 K. While the original shape of the tip was nearly hemispherical before the reaction, it was polyhedral thereafter. In particular, the {011} and {113} planes were seen to adopt a “missing row” type reconstruction in the presence of oxygen adsorbate. On the Rh{113} plane, a (1 × 3) reconstruction prevailed for oxygen exposures larger than 60 L (1 L = 1.3 × 10-4 Pa·s) and temperatures T ≥ 550 K. In this type of missing-row reconstruction two adjacent dense-packed chains of atoms are absent. Surface oxidation was found to be promoted by the presence of an external electric field of ∼15 V/nm. Studies as a function of the surface temperature were performed in real time by video−FIM leading to the observation of a strong local variation in the oxidation activity. While the {113} planes and the vicinals of the (111) pole turned out to undergo rapid oxidation, the flat planes with {001} and {111} symmetry remained rather inactive at temperatures between 350 and 483 K. The formation of surface granules with sizes of ∼5−10 nm in areas of high oxidation activity was interpreted as being due to a nucleation process forming RhxOy precipitates. Surface granules could be easily removed by reaction with CO gas, meaning that only the topmost layers of the Rh tip were involved in surface oxidation.

Structure determination of Cu(210)–(1×2)-O by use of SEXAFS measurements

Polarisation-dependent surface-extended X-ray-absorption fine-structure experiments have been performed on Cu(210)–(1×2)-O. The analysis shows that oxygen chemisorbs in long-bridge sites along the [001] direction with nearest-neighbour and next-nearest-neighbour O–Cu distances of 1.81±0.02 Å and 1.91±0.03 Å respectively. A missing-row reconstruction with missing [001] Cu rows is suggested.

The surface composition, structure and oxygen-induced (2×1) reconstruction of Cu 3Pt(110)

The surface composition, structure, and oxygen-induced reconstruction of Cu 3Pt(110) have been investigated by low-energy ion scattering, recoiling spectrometry, and low-energy electron diffraction. The surface composition of the top two layers was determined from Li + single-scattering intensities using selective scattering geometries with calibration measurements on reference standards. Under thermal equilibrium conditions, the measurements consistently showed a surface composition of 82% Cu–18% Pt in the first layer, while the second layer was composed almost entirely of Cu. From computer simulations and analysis of the shadowing critical edges, the first interlayer spacing was found to be contracted by about 6±2%, whereas the surface Pt atoms were buckled outward by 0.1±0.06 Å. Adsorption of oxygen on the clean Cu 3Pt(110) surface at room temperature followed by subsequent annealing leads to a (2×1) reconstruction. By Li + ion scattering and negative O − recoil measurements the oxygen-induced structure was determined to be a missing-row type reconstruction which involves loss of Pt from the first layer in a thermally activated place exchange with Cu atoms. The first interlayer spacing was expanded by 23±10% relative to the ideal spacing, while the O atoms were located 0.1±0.1 Å below the first Cu layer. The structural features found are discussed and compared with previous results on the Cu(110)-(2×1)-O and Cu 3Au(110)-(2×1)-O surfaces.

H-induced reconstructions on Pd(110)

The (110) surface of many transition metals undergoes reconstructions either on the clean substrate or induced by adsorbates. The reconstructions can be induced by a variety of strongly bound adsorbates, e.g., H, O, S, and alkali-metal atoms. In this work, H/Pd(110) is chosen as a prototype of such reconstructions. The pairing-row and missing-row reconstructions are studied in a wide range of coverages, $\ensuremath{\theta}=0.5--1.5\mathrm{ML},$ by using density-functional theory with the local-density approximation and the generalized gradient approximation. The driving force for the reconstructions is also analyzed in detail. The pairing-row reconstruction is driven essentially by the repulsion between the hydrogen atoms adsorbed in the same trough while the driving force for the missing-row reconstruction is the better adsorbate-substrate interaction on the reconstructed surface.

High-temperature scanning tunnelling microscopy studies of oxygen-induced reconstructions of Pd(110)

The adsorbate-induced reconstructions of the Pd(110) surface during dissociative chemisorption of oxygen have been studied at elevated temperatures (between 400 and 520 K) by scanning tunnelling microscopy (STM). For relatively high exposures, a well-ordered c(2×4)-O overlayer is observed which consists of a Pd(1×2) added-row reconstruction decorated by oxygen adatoms. At 520 K this surface displays very little mobility, with defects and step edges remaining stationary over long time periods. For low exposures of oxygen, adsorption onto the clean surface at 430 K leads initially to the formation of a Pd(1×3) missing-row reconstruction which is again decorated by oxygen atoms. The decoration shows considerable disorder between the Pd rows, while along the rows the structure is relatively well ordered into a zig-zag chain. Structural models are proposed which satisfy both LEED and STM data and concur with coverages derived by other techniques.

LOW ADATOM MOBILITY ON THE (1 × 2)-MISSING-ROW RECONSTRUCTED Au(110) SURFACE

Adsorption and mobility of Au adatoms on Au (110)-(1×2) were studied by low temperature scanning tunneling microscopy (STM). Up to 0.4-monolayer coverage the troughs of the missing row reconstruction are preferentially filled. By following the mobility of individual adatoms at 125 K and 170 K the adatom hopping barrier inside the troughs in the [Formula: see text] direction is determined to Ed=0.40–0.44 eV, while for diffusion on locally unreconstructed parts of the surface we obtain a lower limit of 0.38 eV. By comparing the cluster size distribution obtained for themally immobile adatoms at 125 K with that from simulations, it is demonstrated that transient mobility plays no significant role in Au / Au (110) self-adsorption. During Au deposition at 190 K domain boundaries ("faulted troughs") are observed to form in the 1× 2 substrate; a tentative mechanism involving exchange diffusion of Au adatoms is proposed.

Nature of the O-fcc(110) Surface-Bond Networking

New insight into the nature and formation of the multiphase ordering of oxygen onto threefold sites of fcc(110) (Rh, Pd and [Formula: see text] analogue) is presented. By including bond-to-band model and the potential-barrier for oxygen-metal chemisorption with theoretical and experimental observations, it is shown that the multiphase ordering, composed of (2×1)pmg, (2×1)p2mg and (2×2)p2mg structures, originates from the hybridized- O -2 forming at different oxygen coverages with a specific bonding environment. Unlike the long-bridge sited O -2 ions on the Cu(Ni, Ag)(110) which give the missing-row type reconstruction, the threefold-coordinated O -2 ions on the Rh(Pd)(110) form a tetrahedron through one bond to the metal atom underneath and two nonbonding lone pairs acting on its two neighbors in the first layer. In the (2×1)p2mg structure, the lack of one metal atom for the tetrahedron is compensated by a virtual bond pulling the electron-cloud of the dipoles that are induced by the lone pairs of other O -2 ions. The tetrahedron in the (2×2)pg structure requires an electron from a metal atom in the next nearest row at the surface. Therefore, the bond network interlocks all the surface atoms; and thereby, no atoms are missing. The zigzag protrusions in the STM images are recognized as metal dipoles deformed by the lone pairs of O -2 ions. The depressions correspond to rows of M + metal ions other than missing-row vacancies as had been expected previously.

Model of adsorbate-induced missing-row reconstructions of the (110) surface of fcc metals

A microscopic model for missing-row reconstructions (MRR's), induced by atoms adsorbed in the threefold fcc hollow sites on the (110) surface of fcc metals, is proposed. The adsorbate subsystem is accounted for in the model by including interactions between dipoles, each formed by an adsorbate atom and its neighboring metal atoms. The phase diagram of the model has been calculated employing the Monte Carlo and cluster variation methods. We find overall agreement between our results and the experimental data on MRR's of fcc(110) surfaces.

Metastable (1×2) missing-row reconstruction of Pd(311)

The formation of ordered oxygen overlayers on Pd(311) as well as the structures obtained after their reduction in hydrogen have been studied by He scattering. Exposure of the clean surface to oxygen at 110 K and subsequent annealing to 500 K leads to the formation of a well-ordered (2×4)pg structure. Oxygen removal from this phase by hydrogen at 320 K leads to the appearance of a (1×2)-reconstructed structure, which is metastable and reverts to the clean (1×1) surface at ∼370 K. The presence of a pronounced rainbow pattern in the (1×2) in-plane spectra signals unambiguously that its structure is of the missing-row type, suggesting that the reconstruction was induced by oxygen in the (2×4)pg phase. He diffraction data from the metastable (1×2) structure also suggest that significant unreconstructed (1×1) patches coexist with the missing-row reconstruction. Possible structural models for the (2×4)pg phase are also discussed.

H-Induced Reconstruction and Faceting of Al Surfaces

First principles calculations show that chemisorbed H causes vacancy reconstructions and faceting of all Al low index surfaces. On Al(111) H-decorated vacancies are stable; on H-covered Al(100) vacancies are easily activated thermally. H-covered Al(110) forms a missing row reconstruction with H-decorated vacancies on the {l_brace}111{r_brace} microfacets. At high H coverages, low index Al surfaces are unstable against faceting. Al(111) and Al(110) form {l_brace}211{r_brace} facets, on Al(100) islands and pits with {l_brace}311{r_brace} and {l_brace}211{r_brace} facets are stable. The H-induced structural changes are caused by the preferential binding of H at low coordinated Al surface atoms and at {l_brace}100{r_brace} microfacets or, more generally, at {open_quotes}surface tetrahedral{close_quotes} sites. {copyright} {ital 1997} {ital The American Physical Society}

Theoretical study of the oxygen-induced missing-row reconstruction on the Rh(110) surface

A microscopic model based on the Blume-Emery-Griffiths model is proposed to describe the missing-row reconstructions on the Rh(110) surface induced by oxygen. The phase diagram of the model at finite temperatures has been calculated employing the Monte Carlo and cluster variation methods. We find overall agreement between our results and the experimental data on the missing-row reconstruction for the O Rh(110) surface as well as for other fcc (110) surfaces covered by atoms adsorbed in the threefold fcc hollow sites.

The NO-induced (1 × 2)→(1 × 1) structural transition of Pt(110): a quantitative RHEED investigation

The adsorption of NO on Pt(110) and its induced (1 × 2)→(1 × 1) structural transition was studied with quantitative reflection high-energy electron diffraction (RHEED). Time/exposure series of rocking curve sets were recorded during on-going adsorption processes. A direct evaluation focusing on the development of reflection intensities and rocking curve shapes in dependence on the NO exposure allowed dosages to be found for the most rapid structural change and for saturation. 10 sets of rocking curves from room temperature adsorption were quantitatively evaluated using an automated fitting procedure in conjunction with dynamic RHEED theory to determine the values of a comprehensive set of structural parameters for the substrate and the adsorbate. We found considerable changes of the Pt relaxations just prior to the transition from the (1 × 2) missing-row reconstruction into the (disordered) (1 × 1) structure. As long as the substrate is (1 × 2) reconstructed, NO adsorbs on bridge sites on the first Pt layer and on hollow sites on the (111) microfacets while in the case of the (1 × 1) structure first-layer top sites are occupied too. As soon as the transition begins, the rearrangement of the first Pt layer provides for additional vacant bridge and top sites in the second layer. These are then occupied while the hollow sites loose their population.

Oxygen on Pd(110): substrate reconstruction and adsorbate geometry by tensor LEED

The c(2 × 4) and (2 × 3) structures formed by oxygen adsorption on Pd(110) were studied in an LEED I–V experiment by employing the tensor LEED method. Adsorptiondashinduced reconstructions occur in both cases, giving rise to respectively (1 × 2) and (1 × 3) periodicities of the Pd atoms. Oxygen removal by H 2 treatment leads to clean, metastable, reconstructed phases, which have been studied as starting points for the analysis of the oxygendashcovered surfaces. The reconstruction is of the missing-row type in both cases, with the removal of two rows of atoms out of every three for the (1 × 3) surface. In both cases a contraction of the first three interlayer spacings has been observed. The oxygen adatoms are adsorbed on the (111) facets formed as a consequence of the missing-row reconstruction, and form zig-zag chains along the [11̄0] rows. These chains are in antiphase in the c(2 × 4) structure, and with random relative phase in the (2 × 3) structure. Compared to bulk Pd, the c(2 × 4) structure presents a slight increase in the first interlayer distance and a contraction of the second. On the other hand, the (2 × 3) structure shows a relatively large increase in the first interlayer distance and again a reduction in the second. Lateral changes of atomic positions and buckling in the second or third substrate layers have been found in all of the four structures analysed.

Low-coverage sulfur induced reconstruction of Ni(111)

STM, LEED and AES data are reported for S segregated on Ni(111) between 675 and 1175 K. Two chemisorbed phases corresponding to different coverages (√39 for Θ S = 0.22 ML and (5√3 × 2)rect for Θ S = 0.40 ML) have been observed which are identical to phases produced by adsorption and post-annealing of H 2S. STM data obtained for the (5√3 × 2)rect-S phase confirm the missing row reconstruction of the first Ni plane in this phase. For the √39-S phase, STM and LEED data give evidence of reconstruction of the topmost Ni plane to generate 4-fold coordinated S adsorption sites. Two models are proposed, one with a (100) arrangement of the first Ni plane to give an overstructure similar to that of the Ni(100)-(2 × 2)-S phase, the other with a clockwise-anticlockwise reconstruction of the (100) arrangement of the first Ni plane similar to that reported for C and N adsorption on Ni(100). STM data provide direct evidence of a preferential formation of the √39-S phase at substrate monoatomic steps, on the upper terraces. Nucleation takes place preferentially at step edges where the lower coordination of Ni atoms allows an easier reconstruction of the upper terrace to produce the topmost plane of the √39 phase. The growth can proceed then on both the upper and the lower terraces.