Vicinal Au Research Articles

Electronic states at vicinal surfaces

Vicinal noble metal surfaces with regular arrays of steps and terraces are very convenientmodel systems to test the electronic properties of lateral nanostructures. Usingangle-resolved photoemission with synchrotron radiation we thoroughly characterizeelectronic states and wavefunctions in a variety of vicinal Cu(111) and Au(111)surfaces. By tuning the terrace width, we can observe the fundamental transitionfrom arrays of non-interacting nano-objects (terraces), where electron states areconfined, to lateral coupling between terraces, which leads to superlattice states.

Reflection high-energy electron diffraction study of surface morphology of vicinal Au(1 1 0) throughout the (2 × 1) to (1 × 1) phase transition

The vicinal Au(1 1 0) surface morphology throughout the (2 × 1) to (1 × 1) phase transition is studied by quantitative reflection high-energy electron diffraction. As the surface is heated from 300 K, the average terrace width remains unchanged up to 635 ± 10 K. Above that temperature, the average terrace width increases and at 683 K has a value (34 ± 10)% more than at 300 K. At higher temperatures, the average terrace width decreases. The average string length at step terraces on Au(1 1 0) remains basically unchanged up to ∼650 K and decreases sharply at higher temperatures indicative of a step-induced roughening transition. Thermal-induced adatom/vacancy generation on terraces is shown to increase significantly above ∼680 K. At room temperature, the spacing between the topmost two layers of the Au(1 1 0) surface is contracted by 0.31 ± 0.03 Å or ∼22% of the bulk-terminated structure.

Fabrication of 1D metal nanostructures on a vicinal Au(111) surface

The growth of Mn and Gd on a vicinal Au(111) surface is observed by scanning tunneling microscopy at room temperature. One-dimensional nanostructures have been fabricated with Mn adatoms. Dosed Mn atoms are adsorbed at lower corners of {111} steps, and Mn adlayers grow in a step-flow growth mode on (111) terraces. On the other hand, there is no tendency for Gd to form one-dimensional structures. Dosed Gd atoms nucleate not only at lower corners of the steps but also on the terraces. As the coverage is increased, Gd adislands isotropically grow in size around the nucleation sites on the terraces. The mechanism of these different growth manners is qualitatively discussed. [DOI: 10.1380/ejssnt.2003.142]

Lateral quantum wells at vicinal Au(111) studied with angle-resolved photoemission

Electrons at noble metal surfaces can be confined within terraces leading to one-dimensional surface states. These can be studied with angle-resolved photoemission from vicinal surfaces with regular arrays of (111)-oriented terraces. Here we show the case of Au(23 23 21), which is vicinal to Au(111) and displays $L=56\mathrm{\AA{}}$ wide terraces. The surface state band appears broken up into three quantum well levels that match to those of the infinite quantum well of the same width L. Their parallel momentum dependent photoemission intensity allows mapping the probability density of the confined wave function in reciprocal space using angle-resolved photoemission. By Fourier transformation, their respective experimental wave functions in real space are obtained and compared to the case of the infinite quantum-well, showing excellent agreement. Final state step superlattice diffraction effects have also been observed. Finally, we observe the quenching of the characteristic spin-orbit coupling of Au(111) in the confinement direction. This is another indication of the one-dimensional character of the surface state, as confirmed with first order perturbation theory.

Influence of steps on the electrochemical oxidation of CO adlayers on Pd(111) and on Pd films electrodeposited onto Au(111)

Electrochemically deposited palladium monolayers on well-ordered and on vicinal Au(111) electrodes have been characterised in sulphuric acid solution by cyclic voltammetry. The electrochemical behaviour of these pseudomorphically grown palladium overlayers is compared with that of a well-ordered and a previously oxidised Pd(111) electrode for hydrogen adsorption, sulphate adsorption as well as for surface oxide formation. A strong impact of the surface defect density was observed for the latter reaction. The electrooxidation of irreversibly adsorbed carbon monoxide monolayers on the various palladium surfaces was studied by cyclic voltammetry and chronoamperometry. The oxidation kinetics are strongly dependent on the number of monoatomic high steps, that act as nucleation centres for adsorbing oxygen-containing species and allow for an anodic stripping of carbon monoxide at more negative potentials than oxidation on terraces. A tailing in the chronoamperometric transients at 0.65 V vs. SCE for the palladium surfaces suggests that diffusion of the reacting species on the terraces is rate-determining.

Probing wave functions at step superlattices: confined versus propagating electrons

Electron wave functions at lateral nanostructures can be readily explored using photoemission with tunable synchrotron radiation. Such systems are ideal for applications in nanotechnology because they are easily accessed by read–write devices that scan across a surface. As model system here we use vicinal Au(111) surfaces, with an array of terraces separated by straight, monatomic steps, which exhibit atomically accurate periodicity due to an intrinsic surface reconstruction. We analyze two extreme cases where electrons are either localized into lateral quantum-well states or propagating across the step superlattice. We are able to probe the electron probability density at the lateral quantum-wells as well as the Fourier spectrum of the two-dimensional (2D) superlattice states.

Non-monotonic step-induced magnetic anisotropy of Fe films prepared on vicinalAu(001) surfaces with different step orientations

We report on investigations of the crystallographic structure and the magneticanisotropies of epitaxial Fe films deposited on vicinal Au(001) surfaces for twomiscut directions and various miscut angles. The stepped surface generates auniaxial in-plane anisotropy contribution in addition to the fourfold anisotropycharacteristic for Fe(001). With the step edges perpendicular to [100]Fethe uniaxial anisotropy constant is a non-monotonic function of the vicinal angle.At low vicinal angles the easy magnetization axis is parallel to the step edges. Theeffective uniaxial anisotropy constant first increases and then decreases for increasingvicinal angle, crossing zero at about 1.3°± 0.2°. For higher angles the easy axis isperpendicular to the step edges. For the step edges perpendicular to [110]Fethe step-induced in-plane uniaxial anisotropy increases approximately linearlywith the vicinal angle.

One-dimensional versus two-dimensional surface states on steppedAu(111)

Surface states at vicinal Au(788) and Au(322) have been investigated with angle-resolved photoemission and synchrotron radiation. Both surfaces are characterized by highly regular one-dimensional step arrays with relatively wide (\ensuremath{\sim}3.9 \AA{}) or narrow (\ensuremath{\sim}1.3 \AA{}) terraces in Au(788) and Au(322), respectively. Depending on the terrace size we observe that surface electrons behave in a completely different way. In Au(788) terraces become one-dimensional, lateral quantum wells that confine surface electrons between adjacent steps. In Au(322) surface electrons propagate across the step array forming two-dimensional superlattice bands. By tuning photon energy and angle we probe fundamental properties of the electron wave functions in both cases.

Electron Confinement in Surface States on a Stepped Gold Surface Revealed by Angle-Resolved Photoemission

STM images show that vicinal Au(788) surfaces are made up of a uniform array of (111)-oriented terraces of similar width ( approximately 3.8 nm). This uniformity makes it possible to study the electronic structure of the resulting step superlattice by angle-resolved photoemission. We show that for this terrace array the surface state appears to be broken up into one-dimensional quantum-well levels, indicating total electron confinement within the terraces. The angular resolution allows the probability density of the terrace quantum well state to be mapped in reciprocal space, complementing nicely the wave function measured in real space by STM.

Continuous and discontinuous transitions on 3D equilibrium crystal shapes: a new look at Pb and Au

Equilibrium crystal shapes exhibit flat facets and rough vicinal surfaces, with transitions between them being either continuous or discontinuous, the latter recognizable by a sharp edge. In general, mixed repulsive/attractive step–step interactions may lead to continuous or discontinuous facet-to-vicinal transitions. In can be shown that the contact angle at the facet for a discontinuous transition is directly related to the ratio of the step interaction strengths. Alternatively, surface reconstruction of facets can also be responsible for sharp edges at the facet boundary. In this case the contact angle is related to the difference between surface free energies of the reconstructed and unreconstructed facet as well as the corresponding difference of step interaction energies. Fitting the experimental shapes by theoretical expressions can be used to extract the relevant surface and step free energies and also step interaction energies. Experimental examples of Pb and Au equilibrium shapes are evaluated and discussed. Step free energies of vicinal Au(1 1 1) and Au(1 0 0) surfaces, evaluated by both models, are 30 and 10 meV/Å 2, respectively.

In-plane magnetic anisotropies in Fe films on vicinal Ag(001) and Au(001) surfaces

In-plane fourfold and uniaxial magnetic anisotropies were studied in Fe films epitaxially grown on vicinal Ag(001) and Au(001) surfaces, which were prepared by molecular beam epitaxy on miscut GaAs(001) substrates. The effective fourfold and uniaxial anisotropy constants, K1eff and Kueff, which are determined from magnetisation curves measured with the magneto-optic Kerr effect, are linear functions of the inverse Fe layer thickness. The fourfold anisotropy shows a rotation of the easy and hard axes by 45° below a critical thickness of 6–7 ML. The uniaxial term is mainly an interface contribution. We find that the orientation of the uniaxial easy axis depends on the Fe thickness. In thinner films (tFe⩽20 ML) it is oriented perpendicular to the step edges, i.e., parallel to Fe[100]. This excludes shape anisotropy as the main mechanism. Above a critical thickness the uniaxial easy axis is aligned parallel to the step edges, i.e., along Fe[010]. This step-induced uniaxial anisotropy may be due to modified electronic states and to strain from the large vertical misfit at the steps.

Scanning Tunneling Microscopy Study of Fe Submonolayer Film on Vicinal Au(001)

The initial stage of the growth process of Fe submonolayer films on a vicinal Au(001) surface was studied by means of a scanning tunneling microscope. The vicinal Au(001) surface was characterized by an average terrace width of 20 nm, separated by straight, monoatomic steps aligned along the [110] or [110] direction. The correlation between the formation of steps and the reconstructed structure is discussed. One-dimension-like Fe structures with 4 nm width and 0.5 nm height have been grown by decoration of monoatomic steps of Au(100) surface.

Orientational phase diagram of the stepped Au(001) surface

We report an X-ray scattering study of the structure and morphology of a stepped Au(001) surface between room temperature and bulk melting. At high temperatures, between 1170 and 1220 K, the vicinal Au(001) surface appears rough. Below T 0 = 1170 K, there is a facetting transition: hexagonally reconstructed (001) facets appear in coexistence with unreconstructed, stepped domains. The tilt angle of the stepped domains increases with decreasing temperature, exhibiting a power-law behavior versus reduced temperature. The phase behavior of the hexagonally reconstructed domains follows that observed on nominally unstepped Au(001) surfaces. At T 1 = 992 K, there is a second facetting transition at which the stepped domains reconstruct to form a rotated and corrugated, quasi-hexagonal structure. The corrugation is threefold commensurate with the average terrace width, consistent with a 2° magic vicinal phase. The orientational phase diagram of vicinal Au(001) is deduced and found to be qualitatively similar to that of vicinal Pt(001), although with intriguing differences.

Surface morphologies—Equilibria and transient states

The relationship between the orientation dependent surface free energy and the surface morphology of crystalline materials is the main subject of this study. Small particles under equilibrium conditions as well as periodic profiles in transient states are considered. A second relationship of interest is that between step energy, step interaction energy and the orientation dependent surface free energy. Periodic surface profiles as transient states offer the possibility of measuring kinetic changes that provide information on the products mobility × energy while the shape alone may also be used to extract energy parameters. The rate of kinetic changes of profiles on vicinal surfaces depends sensitively on the orientation of the profile relative to the miscut direction. This has recently been demonstrated for vicinal Au crystals.

Anisotropic profile decay on perturbed Au(111) vicinal surfaces

The anisotropy of the surface free energy has a large influence on the decay rate of periodic surface profiles at temperatures below the roughening temperature. This has been demonstrated by a study of the periodic surface profiles that were etched into vicinal Au(111) crystals in two orientations: with their wavevector parallel ( ψ = 0) and perpendicular ( ψ = π 2 ) to the intrinsic step direction of the vicinal surface. Due to this modulation, steps become either oscillatory in shape or remain straight, with their average separation modulated. Accordingly, profile decay is driven primarily by step self-energy ( ψ = 0) or step interaction energy ( ψ = π 2 ). The rate of decay was measured under ultra-high vacuum conditions at 750°C using a scanning tunneling microscope for imaging. A large anisotropy of decay was observed on two vicinal crystals of 1.5 and 5° miscut, with the ψ = 0 decay being much faster than with ψ = π 2 ). The results are evaluated and discussed in the framework of recent theory.

Step patterns on vicinal reconstructed surfaces

Step patterns on vicinal $(2\times1)$ reconstructed surfaces of noble metals Au(110) and Pt(110), miscut towards the (100) orientation, are investigated. The free energy of the reconstructed surface with a network of crossing opposite steps is calculated in the strong chirality regime when the steps cannot make overhangs. It is explained why the steps are not perpendicular to the direction of the miscut but form in equilibrium a network of crossing steps which make the surface to look like a fish skin. The network formation is the consequence of competition between the -- predominantly elastic -- energy loss and entropy gain. It is in agreement with recent scanning-tunnelling-microscopy observations on vicinal Au(110) and Pt(110) surfaces.

Oxygen reduction on Au(111) and vicinal Au(332) faces: A rotating disc and disc-ring study

The kinetics of oxygen reduction on the Au(111) surface in acidic solution has been re-examined by means of a rotating disc electrode technique. Its vicinal Au(332) = Au[6(111) × (111)] surface has been chosen to examine the role of a small density of steps as active sites. The increase of the average surface energy, caused by the presence of steps on the Au(332) face, causes a slight shift of the half-wave potential to a more positive value, compared to the Au(111). The reaction on Au(111) is kinetically hindered in the whole potential region up to hydrogen evolution. The presence of steps on the Au(332) face apparently causes the kinetic limitations to be lifted. The kinetics of oxygen reduction on the Au(332) face in alkaline solution has been studied by means of a rotating disc-ring method. The mechanism of the reaction has been found the simplest among all the faces of Au investigated so far. In the kinetic and mixed control regions the reduction proceeds through the simplest 2e-series pathway. At large overpotentials, a 4e-reduction occurs, whose extent depends on the potential, with a surface diffusion of HO−2a as a rate determining step. No splitting of the OO bond has been found. A map of the reaction pathways for the whole potential range of oxygen reduction has been constructed.

Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.

Oxygen reduction on Au (100) and vicinal Au (910) and Au (11, 1, 1) faces in alkaline solution: A rotating disc-ring study

The kinetics of oxygen reduction have been studied on Au (100) and vicinal Au (910) and Au (11, 1, 1) faces in 0.1 M NaOH by a rotating disc-ring method. A very high activity of these surfaces for O 2 reduction was demonstrated. The reaction involves a four-electron reduction without HO 2 − reaching the ring electrode in the presence of the AuOH −(1-λ 2) layer on the Au (100) surface. At more negative potentials, as θ AuOH → 0, a four-electron reduction takes place in an interactive pathway, followed by a two-electron series pathway and, at very large overpotentials, by a four-electron series pathway. The first electron transfer is the rate-determining step for all three faces. For vicinal faces, the region of the four-electron reduction is reduced due to the smaller region of AuOH formation. Evidence for the surface diffusion of HO 2 − was found from analysis of the disc-ring data. There were also indications for the splitting of the OO bond by AuOH in the region of the four-electron reduction. Maps of the reaction pathways have been constructed.

The influence of steps on the formation of missing row reconstructions: STM and LEED investigations of Au(991)

Au (991) — a vicinal Au(110) surface with (111) steps parallel to the close-packed rows and a terrace width that can accommodate two missing rows — has been investigated with scanning tunneling microscopy and low energy electron diffraction. Irrespective of the annealing temperature (600–800 K) Au(991) stabilizes with small domains of Au(991)1 × 2). Predominant single step height as well as many kinks along [110] confirm that the reconstruction process proceeds mainly by movement in the MR direction. In addition, extended regions of Au(551)(1 × 2) are found indicating that Au(551) may be one of the “magically” stable vicinal surfaces.