Adsorbate-induced Reconstruction Research Articles

MEIS investigations of surface structure

The early work of the FOM-AMOLF group in Amsterdam clearly demonstrated the potential of medium energy ion scattering (MEIS), typically using 100keV H+ incident ions, to investigate the structure of surfaces, but most current applications of the method are focussed on near-surface compositional studies of non-crystalline films. However, the key strengths of the MEIS technique, notably the use of blocking curves in double-alignment experiments and absolute yield measurements, are extremely effective in providing detailed near-surface structural information for a wide range of crystalline materials. This potential and the underlying methodology, is illustrated through examples of applications to the study of layer-dependent composition and structure in alloy surfaces, in studies of the surface crystallography of an oxide surface (rutile TiO2(110)) and in investigations of complex adsorbate-induced reconstruction of metal surfaces, including the pseudo-(100) reconstruction of Cu(111) induced by adsorption of atomic N and molecular methylthiolate (CH3S–). In addition to the use of calibrated blocking curves, the use of the detailed spectral shape of the surface peak in the scattered ion energy spectra, as a means of providing single-atomic layer resolution of the surface structure, is also discussed.

Structures of Dense Glycine and Alanine Adlayers on Chiral Cu(3,1,17) Surfaces

Density Functional Theory calculations have been used to predict the structures of dense glycine and alanine adlayers on Cu(3,1,17)(S). Facets of this chiral Cu surface result from adsorbate-induced surface reconstruction when glycine or alanine are adsorbed and annealed on Cu(100). We have calculated the surface energy changes associated with this surface reconstruction. Our results allow the enantiospecificity of this reconstruction following adsorption of enantiopure or racemic alanine on Cu(100) to be discussed. The overall stability of glycine and alanine adlayers on Cu(3,1,17)(S) arises from an interplay between the formation of chemical bonds with the Cu surface, deformations in the adsorbed molecules during adsorption, and intermolecular hydrogen bonds within the adlayer; none of these factors individually dominates.

Pattern organization on Cu(111) in perchlorate solutions

Abstract The first in situ scanning tunneling microscopy (STM) study of Pb underpotential deposition (upd) on Cu(1 1 1) in perchlorate solutions is presented. High resolution STM shows a (4 × 4) Moire structure of a full density close-packed lead monolayer that is identical to that observed in ultrahigh vacuum. Interestingly, the STM experiments revealed nano-organization morphologies of the topmost Cu(1 1 1) layer following stripping of the lead monolayer in naturally aerated solutions. The length-scale and nature of the ordered structures was found to depend on solution pH. A Moire structure indicative of an anion adsorption-induced reconstruction was observed in pH 2 and a star-pattern type of dislocation network structure was observed in pH

Dissociative adsorption and adsorbate-induced reconstruction on Fe{2 1 1}

The products of CO, NO, O 2 and N 2 dissociation on Fe{2 1 1} have been studied by means of first-principles density functional theory. Preferred adsorption sites for adatoms C, N and O are identified, and trends in charge transfer and surface magnetism described. An experimentally observed (2 × 1) reconstruction induced by O is confirmed to be energetically stable, and a similar reconstruction induced by N is tentatively predicted. It is argued that these reconstructions may be important not only in the context of the catalytic reactivity of the Fe{2 1 1} surface, but also for the initial stages of surface nitridation and oxidation.

Effect of S Arrangement on Fe(110) Properties at 1/3 Monolayer Coverage: A DFT Study

Using density functional theory calculations, we compare the relative stabilities and properties of different arrangements of S on Fe(110) at a 1/3 monolayer coverage, including two observed experimentally. For all studied arrangements, S is adsorbed in the three high-symmetry adsorption sites: 4-fold hollow, 3-fold hollow, bridge, and atop sites. The binding energy, work function change, adsorption geometry, charge density distribution, magnetic properties, and density of states are determined and compared. The most stable overlayer arrangement corresponds to the overlayer seen by experiment after dissociative adsorption of H2S and has S adsorbed in 4-fold hollow sites. In the other arrangements, the S atoms are located closer to each other on the surface reducing the stability of the overlayer. S causes a minor adsorbate-induced reconstruction of the Fe surface and quenches the magnetic moment of the Fe atoms it bonds to directly. It adsorbs as an electropositive species, causing a positive work function change and forms polar covalent bonds to the surface.

Self-organized epitaxial growth on spontaneously nano-patterned templates

Self-ordering at crystal surfaces has been the subject of intense efforts during the last ten years, since it has been recognized as a promising way for growing uniform nanostructures with regular sizes and spacings in the 1–100 nm range. In this article we give an overview of the self-organized nanostructures growth on spontaneously nano-patterned templates. A great variety of surfaces exhibits a nano-scale order at thermal equilibrium, including adsorbate-induced reconstruction, surface dislocations networks, vicinal surfaces or more complex systems. Continuum models have been proposed where long-range elastic interactions are responsible for spontaneous periodic domain formation. Today the comparison between experiments such as Grazing Incidence X-Ray Diffraction experiments and calculations has lead to a great improvement of our fundamental understanding of the physics of self-ordering at crystal surfaces. Then, epitaxial growth on self-ordered surfaces leads to nanostructures organized growth. The present knowledge of modelization of such an heterogeneous growth using multi-scaled calculations is discussed. Such a high quality of both long-range and local ordered growth opens up the possibility of making measurements of physical properties of such nanostructures by macroscopic integration techniques. To cite this article: S. Rousset et al., C. R. Physique 6 (2005).

Characterization and control of molecular ordering on adsorbate-induced reconstructed surfaces

Understanding molecular ordering is a critical step in achieving molecular self-assembly for the fabrication of nanomaterials, and molecular ordering in the adsorption of large molecules on atomically flat surfaces can be characterized with precision by scanning tunneling microscopy (STM). Complications arise therein from the expanded possibility of various adsorption structures, conformations and surface reconstructions. Here we present two cases of C 60 adsorbed on Ag(1 0 0) and Cu(1 1 1) that illustrate the importance of competitive interactions in the presence of adsorbate-induced reconstruction. In both studies, strong STM contrasts derive from topographic features arising from reconstructed substrates. C 60/Ag(1 0 0) presents a unique uniaxially incommensurate molecular packing. We have also found that one can control molecular ordering with a stepped surface to produce a single-domain film. In C 60/Cu(1 1 1), we are able to obtain metastable, yet well-defined, molecular ordering with precise annealing procedures. These metastable states exhibit a clear correlation between molecular contrast patterns and the adlayer rotation angle, as a consequence of competitive interactions between optimizing C 60 bonding at preferred reconstructive sites and C 60–C 60 repulsion. Finer control of selective preparation of these metastable structures offers a plausible way of fabricating nanostructures by design.

Phase transitions in adsorption-induced reconstruction model

In this work we analyze a simple model of adsorption-induced surface reconstruction, which is developed in the framework of the lattice gas model. The critical behavior of the substrate atoms is obtained by monitoring the specific heat and the fluctuations in the fraction of unreconstructed surfaces, where the critical temperature is calculated by using finite size scaling techniques. The phase diagram (critical temperature as a function of surface coverage) of the system is obtained for different conditions. Two different behaviors are obtained depending on the structures of the adsorbed gas. In fact, if the adsorption process favors the island formation, two critical regions appear, characterized by two critical temperatures, however, if the adsorbed particles do not form island there is only one critical region with one critical temperature. The mechanism of island formation is also discussed.

Reconstruction of a Cu[formula omitted] electrode in the presence of thiocyanate anions

In situ electrochemical scanning tunneling microscopy has been used to examine the surface phase behavior of specifically adsorbed thiocyanate on Cu(100) within the potential range from −950 to −600 mV vs. Ag/AgSCN.In contact with the thiocyanate-containing electrolyte a highly ordered adsorbate layer is formed at negative potentials which can be described by a rectangular c(2×6)-4SCN unit-mesh corresponding to a coverage of Θ=0.33 ML.By sweeping the electrode potential into positive direction a phase transition takes place, which involves not only the adsorbate layer but also the substrate structure and thereby the entire surface morphology. As a consequence square and rectangular adislands and vacancy islands arise which lead to an enormous roughening of the copper substrate. Both, the adislands and the vacancy islands are preferentially aligned parallel to the substrate 〈001〉 directions. Ultimately the phase transition leads to a chequerboard-like arrangement of adislands stabilized by the thiocyanate adsorbate. A plausible model for the appearance of this latter phase is based on an adsorbate-induced reconstruction of the topmost copper layer with adsorbate molecules residing in “pseudo-fourfold-hollow sites” of an expanded copper layer. This phase can be described as a “pseudo-c(2×2)-reconstruction”.

Kinetics of Abstraction of D and O on Cu(110) Surfaces by Gaseous H Atoms

The kinetics of HD and water formation during admission of H atoms to D- and O-covered Cu(110) surfaces was measured and compared to the kinetics of the analogous reactions on Cu(100), Cu(111), and other metal surfaces. The phenomenology of the HD abstraction kinetics on Cu(100) is according to an Eley-Rideal (ER) mechanism with a cross-section of 2.4 Å2 and only a small contribution of D2 products provides evidence for the operation of a hot-atom mechanism. Similar features were observed on the other Cu surfaces. As origin of the ER- phenomenology on Cu surfaces the inefficiency of electron−hole excitation for hot-atom sticking due to the small density of states at the Fermi level of Cu is proposed. Unlike at Cu(100), the H adsorption-induced reconstruction of the surface does not affect the abstraction kinetics. The interaction of gaseous H with oxygen adsorbed on Cu(100) leads to adsorbed and gaseous water products at 100 K. The occurrence of a considerable fraction of gaseous water at reaction temperatures 60 K below the water desorption temperature indicates that the substantial exothermicity of the reaction leads to energetically ejected product upon their formation. In accordance with other water formation reactions between gaseous H and adsorbed oxygen the mechanism is identified as two sequential hydrogenation reactions, O to adsorbed OH and OH to gaseous and adsorbed water, of which the latter is rate-determining with a cross-section of 0.6 Å2. At reaction temperatures well above the water desorption temperature, isothermal desorption is absent in the water kinetics and only gaseous water was monitored. The coverage of oxygen has no effect on the water kinetics.

Low energy electron diffraction structure determination of the Ni [formula omitted]c(2×2)–CN surface phase

The structure of the Ni(1 1 0)c(2×2)–CN surface phase has been determined by quantitative low energy electron diffraction (LEED). Contrary to earlier suggestions that relatively intense fractional order LEED beams imply that adsorbate-induced Ni surface reconstruction occurs, the results confirm previous findings by scanned-energy mode photoelectron diffraction and medium energy ion scattering that no significant reconstruction occurs. The C–N axis is oriented along an open-packed 〈1 0 0〉 azimuth, with the molecule lying above a second layer Ni atom; the intramolecular axis is tilted by 23.7±4.5° relative to the surface plane. The adsorption does lead to a substantial (9±4%) outward relaxation of the outermost Ni layer spacing and small distortions of the outermost layers of the Ni substrate.

Properly interpreting scanning tunneling microscopy images: the Cu( [formula omitted])-c(2×2)N surface revisited

Recent scanning tunneling microscopy (STM) and photoelectron diffraction studies of the Cu(1 0 0)-c(2×2)N system have led to the proposal of a “new type of adsorbate-induced surface reconstruction” involving the movement of Cu atoms perpendicular to the surface. Claims were made that images from prior STM studies were misinterpreted. We present STM images that clearly contradict these claims. We discuss the role of tip structure in the imaging process and demonstrate how tips with asymmetric apexes can result in STM images that can easily be misinterpreted.

Steps, facets and nanostructures: investigations of Cu (11 n ) surfaces

Step structure and dynamics and adsorbate-induced reconstruction of stepped surfaces are useful features for investigating fundamental surface phenomena and their practical consequences. In this respect, vicinal Cu (11n) surfaces with n=3, 5 and 9 were studied by scanning tunneling microscopy (STM) at 300 K. While the regular monoatomic steps fluctuate for Cu (119), they are apparently stabilized for n≤5 by their strong repulsive interaction and this leads to a very low kink activity. In addition to the regular steps, the formation of double steps was observed as a particular phenomenon on those surfaces. These double steps determine the dynamic behavior at room temperature. By applying existing models for the fluctuations of the step positions in space and time, the formation energy for kinks at these double steps was determined to be 0.16 eV for Cu (115). According to this evaluation, diffusion along step edges is the dominating mass-transport mechanism. A model for the structure of the double steps and for the atomic displacement processes necessary for kink migration at these steps is presented. Complete faceting is observed for these surfaces upon oxygen adsorption at elevated temperatures (around 500 K) and was studied in detail for n=5 and 9. Both surfaces reconstruct into two types of {104} facets and a third facet, the orientation of which is determined by the macroscopic crystal orientation. The facet size is governed by the formation kinetics and can be controlled by varying the crystal temperature or the oxygen partial pressure. The formation kinetics is discussed as a nucleation and growth process and the relevant parameters are given. Facets within a range of size between 5 nm and 100 nm could thus be produced. They remained stable at ambient atmosphere, up to about 620 K, and also if covered by additional metal layers such as Ni.

Theoretical study of thiol-induced reconstructions on the Au(1 1 1) surface

A new suggestion for the structure of the Au substrate underlying self-assembled monolayers (SAM) made of thiols is presented on the basis of density functional theory results for methylthiolate (–SCH 3) adsorption on Au(1 1 1). It is found that by introducing vacancy defects on the substrate, the adsorption of SCH 3 is stabilized by about 0.8 eV with respect to the perfect Au(1 1 1) surface. As this overcomes the vacancy formation energy (≈0.6 eV), a net driving force exists leading to an adsorbate-induced reconstruction, that enhances adsorption at defected Au(1 1 1). A comparison of results at high and low SCH 3 coverage provides further insight into which specific gold vacancy sites enhance the adsorption energy of the SCH 3 molecule.

Nickel clusters: The influence of adsorbates on magnetic moments

Magnetic moments have been measured for bare, isolated nickel clusters Nin and their association complexes with carbon monoxide, oxygen, and hydrogen using a molecular beam deflection method. The moments measured for bare Nin are in general agreement with those previously reported by Apsel et al. [Phys. Rev. Lett. 76, 1441 (1996)], lying slightly lower overall. It is found that adsorbed carbon monoxide and hydrogen decrease magnetic moments of Nin. The reduction in moments can be substantial for smaller clusters: the moment of Ni8 is decreased by approximately 6 bohr magnetons by a single absorbed CO molecule. In general, the adsorbate-induced changes in magnetic moments diminish with increasing cluster size. The present results are in qualitative accord with previous measurements of the magnetization quenching effects of CO and H on nickel nanoparticles and thin nickel films. Atomic oxygen decreases cluster moments of some nickel clusters and increases those of others, an effect attributed to adsorbate-induced reconstruction. The experimental results are compared to detailed electronic structure calculations of nickel cluster–adsorbate complexes and to the predictions of the semiempirical bond order-rigid band model of Fourier and Salahub [Surf. Sci. 238, 330 (1990)].

Sulphur adsorption on the fivefold surface of the i-Al–Pd–Mn quasicrystal

The adsorption of sulphur on the fivefold flat-terraced surface of the icosahedral Al–Pd–Mn quasicrystal ( i-Al–Pd–Mn) has been studied using extended X-ray absorption fine structure (EXAFS), Auger electron spectroscopy and low energy electron diffraction (LEED). Surface EXAFS data collected by Auger electron yield from the S saturated surface give average bond lengths for S–Al and S–Pd of 2.10±0.06 and 2.40±0.06 Å respectively. The data indicate that single-site adsorption on the unreconstructed surface may be ruled out; this is consistent with near-edge EXAFS data and the disappearance of the LEED pattern upon adsorption of sub-monolayer amounts of S. The implication of this finding is that adsorbate-induced reconstruction and/or multiple-site adsorption occurs in this system.

Wettability and surface energy of oxidized and hydrogen plasma-treated diamond films

The wettability of CVD diamond films with liquids of different physico-chemical natures (water, glycerin, tin melt) was investigated by measuring the contact angles using the sessile drop method. The translucent diamond films of 0.5-mm thickness were grown in a microwave plasma CVD using CH4–H2 mixtures as the source gas, and separated from Si substrates. The growth surfaces have been polished to a roughness of Ra≤10 nm, and then exposed to microwave hydrogen plasma or thermal oxidation in air at 500 °C. Raman and Auger/XPS spectroscopies, optical and atomic force microscopies have been used to characterize the diamond films. Based on the wettability data, the variations in surface energy induced by hydrogenation and oxidation of polycrystalline diamond have been evaluated by the Fowkes equation. The hydrogenation of the films in the plasma essentially increases hydrophobic properties (contact angle for water increases up to θ=93°) as compared to oxidation (θ=32°). This is attributable to a reduction in a polar component of the surface energy due to hydrogen adsorption-induced reconstruction of the film surface. For comparison, the wettability of HPHT diamond single crystal, diamond ceramics and pyrolytic graphite have been measured and the differences observed will be discussed in this paper.

Dynamics of surface reactions studied by field emission microscopy and atom-probe mass spectrometry

Recent progress in imaging surface chemical reactions and adsorbate-induced reconstruction by means of field ion microscopy is reviewed. Achievements and prospects of a local chemical analysis during imaging are also presented and discussed. Major shape transformation is reported to occur during the field-free interaction of oxygen with Rh crystal tips. Rather than hemispherical when clean they appear polyhedral after reaction at temperatures between 500 and 550 K. Kinetic non-linearities are found in both the NO and NO 2 reaction with hydrogen on the surface of a pyramidal Pt crystal tip. Oscillatory reaction behavior is observed for both reactions. In the NO 2–H 2 case explosive ignition in {0 1 2} planes is followed by one-dimensional chemical wave propagation along the 〈1 0 0〉 zone lines. Atom-probe time-of-flight mass spectrometry demonstrates that water (field ionized as H 2O + and H 3O +) is the product of the catalytic reaction. During the NO/H 2 reaction, dynamic formation and motion of small islands are observed. These islands appear on the (0 0 1) pole and are interpreted as being due to adsorbed hydroxyl species. Island sizes are confined to the equivalent of 10–30 Pt atoms.

Morphological aspects of sulfate-induced reconstruction of Cu(1 1 1) in sulfuric acid solution: in situ STM study

Using in situ STM the atomic structure and the morphology of a Cu(1 1 1) surface exposed to a dilute sulfuric acid solution have been studied depending on the applied electrode potential. At anodic potentials near the onset of copper dissolution the electrode surface is reconstructed (expanded) caused by the specific adsorption of sulfate anions. The extent of the surface reconstruction strongly depends on the sulfate adsorption rate. Fast sulfate adsorption results in a mainly disordered sulfate adlayer on an unreconstructed copper surface. Conversely, slow sulfate adsorption produces a mainly reconstructed copper surface with a highly ordered sulfate/water coadsorption layer. This adsorbate structure shows an additional long-range Moiré modulation, due to a misfit between the first reconstructed and the second unreconstructed copper layer. This is verified by spectroscopy-like STM experiments, which allow the imaging not only of the adsorbate overlayer, but also of the underlying reconstructed substrate. This type of adsorbate-induced reconstruction is characterized by an expansion of the topmost copper layer. The kinetically slow process of reconstruction can be easily followed by dynamic STM measurements revealing a mass transport out of the topmost copper layer during the slow sulfate adlayer formation. Characteristically, new copper islands nucleate and grow, while the sulfate Moiré adlayer expands over the electrode surface. At cathodic electrode potentials the desorption of the sulfate adlayer is accompanied by the lifting of the surface reconstruction and the massive formation of surface defects, such as small pits and vacancy islands. A continuous cycling of the electrode potential leads to an enormous roughening of the surface morphology.

Methanethiolate structural phases on Cu(1 0 0) and Cu(4 1 0)

Scanning tunnelling microscopy has been used to study the interaction of dimethyl disulphide and methanethiol with Cu(1 0 0) and Cu(4 1 0) surfaces at room temperature, conditions believed to lead to the formation of adsorbed methanethiolate CH 3S–. On Cu(1 1 1) these interactions are known to produce a pseudo-(1 0 0) reconstruction of the surface, but with a periodicity some 14% larger than on the ideal Cu(1 0 0) surface. On Cu(1 0 0) there is no evidence of adsorbate-induced substrate reconstruction, with commensurate (2×2) and (defected) c(2×2) overlayers being formed, although at the highest coverages the surface appears to be characterised by a c(2×6) phase which is sometimes poorly ordered. On Cu(4 1 0) the structure formed on the (1 0 0) terraces appears to be c(2×2), although characteristic patterns of kink formation on the steps may be explained in terms of relief of adsorbate-induced compressive surface stress as may be expected if a larger lattice parameter is preferred. The c(2×6) phase on the extended (1 0 0) surface is explained in terms of a buckled c(2×2) structure, also a consequence of compressive surface stress relief.