Observation of sputtering damage on Au(111)

The morphology of a Au(111) surface has been observed with the STM (scanning tunneling microscope) after ion bombardment with 2.5 keV Ne⁺ ions at about 400 K. Mostly triangular and hexagonal shaped vacancy islands are seen in the STM topographs. They are bounded by monatomic steps, oriented along the closed packed 110 directions. The general morphology confirms the conclusions inferred from TEAS (thermal energy atom scattering) measurements on ion bombarded Pt(111) surfaces. The observation of a propensity for the formation of {100} microfacetted 110 ledges is discussed.     

Atomistic structure of stepped surfaces

Atomistic computer simulation with embedded atom method (EAM) interatomic forces was used to study the structure of surface steps on the {111} unreconstructed surface in fcc metallic materials. The energetics and local atomic relaxation behavior of ledges parallel to the 110 direction were studied using a potential describing lattice properties of Au. The vacancy formation energies in the stepped surfaces was also studied, and it was found that the energy of formation of a vacancy in a terrace is the same as that in the perfect unstepped surface. This value is 30% lower than that of the bulk. The vacancy formation energy in the ledge is reduced by a factor of two with respect to that of the terraces. The structure of the “up ledge” (A step) is different from the “down ledge” (B step). These differences do not significantly affect the energy of the ledges, although they do affect the vacancy formation energies in sites in the second surface layer near the ledge. The implications of the results for the formation of kinks and the general structure of high index surfaces are discussed.     

Step patterns on vicinal reconstructed surfaces

Step patterns on vicinal (2 × 1) 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.     

A LEED study of UO2(∼100) vicinal surfaces

The ordering and faceting properties of UO₂(～100) vicinal surfaces have been studied via LEED and Auger measurements. The measurements have demonstrated a reduced tendency for step ordering on UO₂(～100) vicinal surfaces when compared to step ordering on UO₂(～111) vicinal surfaces. The UO₂(～100) vicinal surfaces were observed to decompose irreversibly into low-index facets, including prominent (100) facets, at temperatures below those needed for creation of lowest index faceting on UO₂(～111) vicinal surfaces. These properties suggest that (100) terraces, in contrast to (111) terraces, act as surface diffusion barriers that limit longrange surface communication while growing at the expense of intermediate faceting stages.     

Surface-melting induced faceting of aluminium

Medium-energy ion scattering measurements have been used to study the temperature dependent behaviour of aluminium surfaces with surface orientations in the (11̄0) zone. The ion-scattering measurements show that surface-melting induced faceting occurs between the melting (110) and the non-melting (111) orientations. The dry and melted facet orientations Θ_d and Θ_d have been determined up to 0.3 K from the bulk melting point. Close to the melting point these orientations are almost independent of temperature, and amount to Θ_d = 7 ± 2° and Θ_m = 29 ± 2° with respect to the (111) plane.     

Confining barriers for surface state electrons tailored by monatomic Fe rows on vicinal Au111 surfaces

We fabricated monatomic Fe wires on vicinal Au(111) surfaces and found that decoration of step edges with Fe adatoms has a significant influence on the behavior of surface state electrons confined between regularly arranged steps. On a surface with Fe monatomic rows, angle-resolved photoemission spectra measured in the direction perpendicular to the steps shows parabolic dispersion, in contrast to one-dimensional quantum-well levels observed on a clean surface. Simple analysis using a one-dimensional Kronig-Penney model reveals potential barrier reduction from 20 to 4.6 eV A, suggesting an attractive nature of the Fe adatoms as scatterers.     

A LEED study of the deposition of carbon on platinum crystal surfaces

The deposition of carbon is studied by LEED on four platinum crystal faces: two low-index surfaces Pt(100), Pt(111)and two stepped surfaces Pt(S) ? [5(100) × (111)], a vicinal of (100), Pt(S)?[6(111) × (100)], a vicinal of (111). Carbon, generated by flowing ethylene onto the hot platinum, causes the formation of a graphitic overlayer and surface rearrangements of the substrate. The threshold temperature for graphitization is the lowest on Pt(100). The overlayer exhibits a single preferred orientation on Pt(100), several orientations on Pt(111) and its vicinal. Ordered carbon structures can be detected on the vicinal of (111) for low carbon doses. The orientations found in spot patterns (perfect registry) or ring-like patterns (imperfect alignment) can be associated with a coincidence-site lattice condition at the Pt/C interface. Faceting is observed except on Pt(111); the vicinal of (100) is particularly unstable. The stepped array on the vicinal of (111) starts to disorder at 350°C and can be converted into a hill and valley configuration at higher temperatures and carbon doses. Implications for catalytic studies are discussed.     

Superheating in metal nanoparticles with non-melting surfaces

We construct microcanonical caloric curves for aluminium nanoparticles with non-melting surface facets and diameters of up to 11 nm using molecular dynamics simulations. We find that fcc aluminium particles can be superheated above the bulk melting temperature, but only for a finite range of particle sizes i.e. diameters between 5–9 nm. We also observe a critical particle size where solid-liquid phase coexistence becomes stable, and a second larger critical size where premelted (100) facets can coexist with solid (111) facets. Ultimately, it is the premelting of the (100) facets that appears to limit the superheating effect in these particles.     

LEED studies of UO2(∼111) vicinal surfaces

The ordering/disordering of terraces, ledges and kinks on three UO₂(~ 111) vicinal surfaces were examined by LEED. Optical transforms were used as an indication of the types of diffraction displays expected. On surfaces with high kink and ledge densities and unheated above 600 °C, the ledges on the average are equally spaced and parallel, but the kink-site positions are random. At higher temperatures of 700 < T < 900 °C, the ledges rotate and decompose irreversibly into {553} and {311} microfacets. By transform simulations it was inferred that ledges on the UO₂{553} surfaces are highly ordered and contain less than 7% kink-sites.     

Optical recognition of atomic steps on surfaces

Visible and UV light spectra display striking differences in optical reflectivity for the two types of monatomic steps on copper (111) surfaces. Electronic structure calculations trace these differences to the specific contributions of p(axially) and p(radially) local densities of states, parallel and perpendicular to the steps, of the distinctly coordinated corner atoms. The local origin of the spectral reflectance anisotropy is further corroborated by experiments on Cu(111) surfaces with varying step densities. Site specificity of the electronic structure of atoms in low coordinated sites on Cu(111) vicinals is thus revealed by reflectance anisotropy spectroscopy which can thereby detect step atom densities down to 10(13) atoms/cm(2).     

Surface Diffusion Induced Growth of Solid Solution over a Free Crystal Surface

It has been shown that interdiffusion along a free vicinal crystal surface can lead, at comparably low temperatures, to layered growth of solid solution over the surface without the involvement of bulk diffusion. The alloy concentration distribution along the surface, as well as the normal rate of solution growth has been calculated. The formation of a layer of the solid solution has been experimentally observed on a vicinal (111) surface of Ni single crystal as a result of surface interdiffusion between Ni and a thin film of Au deposited on a part of the Ni surface. The surface diffusion coefficients, as well as other parameters responsible for the exchange rate between the adatoms and kinks of elementary steps have been measured in the temperature range 550–700°C.     

Admetal-induced substrate surface restructuring during metal-on-metal electrochemical deposition studied by in situ scanning tunneling microscopy

Based on time-dependent in situ scanning tunneling microscopy (STM) studies, we demonstrate that for Ni on Ag(111) and Ru on Au(111), electrochemical metal-on-metal deposition can result in pronounced substrate surface restructuring. For Ni/Ag(111), we observe that at low deposition flux and low coverage, Ni submonolayer islands at steps are partly embedded in the Ag terraces, whereas at higher deposition flux and higher coverage, substrate restructuring results in the formation of monolayer bays in the Ag terraces. We suggest that this restructuring process proceeds predominantly via step edge diffusion of Ag atoms. For Ru/Au(111), the formation of fjords and monolayer holes in the Au terraces is observed at low and high Ru coverage, respectively. The importance of the Au surface mobility for the restructuring process is demonstrated by comparing experiments in H₂SO₄ and HCl solutions, in which Au exhibits strongly different surface mobilities. For this system, restructuring involves Au diffusion along Au steps, Au atom detachment from the Au steps, and upward exchange diffusion. According to these observations and their comparison with similar findings for vacuum deposition, we conclude that this restructuring requires (i) a high substrate surface mobility and (ii) a stronger bonding of substrate atoms to deposit islands than to the substrate.     

Interplay between atomic and mesoscopic order on gold vicinal surfaces

Self-organization on Au(1,1,1) vicinal surfaces provides a unique opportunity to study the interplay between atomic and mesoscopic order. First, experimental results demonstrate the different interactions between steps and surface reconstruction on Au(1,1,1) vicinal surfaces. Depending on the step atomic structure, lines of discommensurations are found to be either parallel or perpendicular to the step edges. This leads to a complete understanding of the mesoscopic self-organization on theses surfaces, which drastically depends on the step structure. This points out the crucial role played by the edge energy cost which can monitor the faceting periodicity in a wide range of values.     

Stability of metal vicinal surfaces revisited

The stability of metal vicinal surfaces with respect to faceting is investigated using empirical potentials as well as electronic structure calculations. It is proven that for a wide class of empirical potentials all vicinal surfaces between (100) and (111) are unstable at 0 K when the role of third and farther nearest neighbors is negligible. However, electronic structure calculations reveal that the answer concerning the stability of vicinal surfaces is not so clear-cut. Finally, it is shown that surface vibrations at finite temperatures have little effect on the stability of vicinal surfaces.     

Substrate steps/adsorbed layer interaction; selective effect on the possible epitaxial relationships: Case of the S/Cu ∼ (111) system

The interaction between monoatomic steps on a vicinal ～ (111) copper surface and the adsorbed sulphur monolayer has been investigated by a method involving the existence of two possible structures of the monolayer. The parts of the surface occupied by each structure, which are equal on a perfect (111) crystal plane, and differ on a vicinal surface (selective effect of the steps), have been deduced from LEED intensity measurements. Experiments on a wide range of surfaces have revealed a very strong selective effect of the steps, even for misorientations as small as 4° off the (111) plane. On surfaces with $\text{〈3}\text{2}\text{1}\text{〉}$ steps, only one monolayer structure exists, which appears to fit perfectly the steps. The behaviour of the other surfaces having the same misorientation angle allowed us to devise a model of the step/monolayer interaction, which includes a partial decomposition of a step into the two nearest $\text{〈3}\text{2}\text{1}\text{〉}$ steps (2D “faceting”). A general framework for exploiting experiments of the kind described above, on other adsorption systems, is also outlined.     

Tailoring confining barriers for surface states by step decoration: CO /vicinal Cu(111)

The influence of CO adsorption on the Shockley type surface state on vicinal Cu(111) surfaces is investigated using angle resolved photoemission. As the steps are decorated with CO the surface state shifts to higher binding energies, which is opposite to the known behavior on flat Cu(111). This is described within a one-dimensional potential model in which clean steps represent repulsive barriers and decorated steps become attractive wells. From the coverage dependence the integrated CO well potential can be quantified. It is U(CO)a = -2.9 eV A on both Cu(332) and Cu(221) surfaces. Density functional calculations reveal that this attractive potential is due to the very local charge transfer from the Cu step atom to the adsorbed molecule.     

Steps, ledges and kinks on the surfaces of platinum nanoparticles of different shapes

Platinum nanoparticles with a high percentage of cubic-, tetrahedral- and octahedral-like shapes, respectively, have been synthesized by a shape-controlling technique that we developed recently [Ahmadi et al., Science 272 (June 1996) 1924]. High resolution transmission electron microscopy (HRTEM) is used here to directly image the atomic scale structures of the surfaces of these particles with different shapes. The truncated shapes of these particles are mainly defined by the {100}, {111}, and {110} facets, on which numerous atom-high surface steps, ledges and kinds have been observed. This atomic-scale fine structure of the surfaces of these particles is expected to play a critical role in their catalytic activity and selectivity.     

Atomic and electronic structure of steps and kinks on MgO(100) and MgO(110)

The electronic and atomic structure of vicinal MgO surfaces are studied using a quantum self-consistent method associated with a geometry optimization code. 10n, (n + 1)0n and n1n surfaces, with periodic monoatomic steps separating {001} or {101} terraces, are considered. Diatomic steps along the {10n} orientation and periodic kinks on the {3 1 10} surface are also modelled. We assign most electronic peculiarities of stepped surfaces to the values of the Madelung potential acting on the under-coordinated atoms, which is a function of their first and second coordination numbers. An analytical model is proposed to explain the bond contractions around these atoms. Finally the microscopic contributions to the step energy are discussed, together with the strength of the step interaction as a function of their separation.     

Uncommon dislocation processes at the incipient plasticity of stepped gold surfaces

Gold vicinal surfaces (788), with a high density of steps, along with (111) flat surfaces taken as a reference, have been nanoindented and their resulting penetration curves and related defect structure comparatively analyzed by AFM and atomistic simulations. Stepped surfaces are shown to yield at smaller loads than (111) ones in agreement with calculations of the critical resolved shear stress needed to nucleate a dislocation. In the stepped surfaces, a novel intermediate state is identified in which the penetration curves depart from a Hertzian behavior prior to the appearance of pop-ins. This state is shown to result from heterogeneous nucleation at preexisting surface steps of dislocation loops, most of which retract and vanish when the indenter load is removed.     

Phase transitions on clean and nickel containing vicinal Si(111) surfaces

Structure and reversible structural phase transitions on clean vicinal Si surfaces inclined from the (111) plane towards [2 ] and [ 11] poles and the effect of nickel on the structure and the structural transitions on these surfaces have been studied by LEED. The structures of clean surfaces inclined towards [2 ] and [ 11] are different. Phase transitions take place at about 870°C and 800° C, respectively. Above the transition temperatures these two surfaces consist of regular steps with heights equal to one interplanar distance d₁₁₁ (3.135 Å). The terrace width between steps is determined by the angle of inclination to the (111) plane. Below the transition temperatures the surfaces inclined to [2 ] contain steps with height 3d₁₁₁, those inclined towards the [ 11] pole consist of combinations of (111) facets and some other, apparently (133). The effect of nickel leads to the formation of regular steps with height 2d₁₁₁ on the surfaces inclined towards [2 ] and with height 2d₁₁₁ or 1d₁₁₁, depending on the nickel concentration, on the inclined towards [ 11]. On nickel-containing surfaces there also take place reversible structural transitions with varying temperature.