The growth of strained thin films of gadolinium has been investigated with low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) and compared to the film growth of unstrained gadolinium. Strained thin films of gadolinium are distinct from the unstrained films by a substrate induced preferential domain growth direction, which is also reflected in the electronic structure.

: The experimental band structure of Mo(112) and the effects by temperature and adsorbate are presented. A surface resonance, identified as crossing the Fermi level at about 1/3 from to of surface Brillouin zone, was observed to be very sensitive to both contamination and temperature. We find evidence of adsorbate and temperature induced reconstruction of the Mo(112) surface. Examination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) data provides evidence for an adsorbate induced reconstruction of the Mo(112) surface with periodicities consistent with the Fermi level crossing of the surface resonance. The reconstruction is found to occur at coverages as low as 0.03 Langmuirs of oxygen or carbon. The reconstruction and/or adsorbate affects the density of states and bands near the Fermi level of a 1 symmetry.

In previous work we have shown using surface X-ray diffraction (SXRD) and low-energy electron diffraction (LEED) that the Ni(100)p4g(2×2)-N reconstruction is removed by coadsorption of ≥0.16ML of potassium. Here we report scanning tunnelling microscopy (STM) observations of this coadsorbate system at 300K and 120K. At low coverages (θK≤0.10ML) the potassium atoms decorate step edges where they are imaged as large protrusions. Once the steps are saturated, further adsorption takes place randomly on the terraces. The potassium atoms on the terraces interact strongly with nitrogen atoms, their diffusion is inhibited, and bias-dependent imaging reveals the disruption of the nitrogen overlayer underneath the potassium-induced protrusions. At higher coverages (θK≥0.16ML), where it is known that the p4g reconstruction is removed, the protrusions appear to coalesce and the surface [although exhibiting c(2×2) periodicity] looks disordered. Lowering the temperature of the system after adsorption does not reveal any new behaviour. The results indicate that a local chemical interaction produces the removal of the reconstruction, and provide a plausible explanation for the coverage dependence of the LEED data.

We have prepared ordered thin films of NiO and CoO in (100) orientation by evaporating Ni (Co) in an O2 atmosphere onto Ag(100). The films have been analysed by scanning tunnelling microscopy and low-energy electron diffraction. In the initial stage (coverage up to a few monolayers), growth and structure of the grown films drastically depend on the preparation conditions (in particular, on the temperature of the substrate during deposition and post-annealing). In this case we also observe strong interactions with the substrate. Ag atoms are partially removed from the substrate terraces and form islands or migrate to step edges. No indications for incorporation in the oxide thin films are seen. The oxidic features grow on top of the substrate or in the vacancy islands within the first layer of the substrate left behind by the removed Ag atoms. At low substrate temperatures (near room temperature) an essential part of the oxidic features corresponds to a precursor state rather than to the fully developed (100) oxide film which only develops after post-annealing to higher temperatures (typically around 500 K). I/U characteristics and the sample bias dependency of the contrast of the islands grown have been utilised for identification of whether an oxide reaction had taken place or not. The surfaces of the oxide precursor show a typical defect structure similar to those found on cleaved NiO(100) (M. R. Castell etal., Phys. Rev. B:Condens. Matter, 1997, 55, 7859). This feature shows ‘random walk’ at room temperature.

The structure of the (110) surface of the tungsten bronze Na2/3WO3 has been studied by scanning tunnelling microscopy. Two different types of image with threefold periodicity along the [1̄10] direction are observed. The ×3 periodicity is shown to be inconsistent with a model proposed in earlier work that involved an ordered arrangement of Na ions at a bulk-truncated surface. On the basis of atomistic simulation of the WO3 framework, two new structures are proposed, both of which involve {100} nanofacets.

One of the most successful ways of inducing enantioselectivity in a heterogeneous catalytic system is by the adsorption of chiral "modifier" molecules on the reactive metal surface. However, little is known about the nature of the active sites present on the modified metal surface and how such modifiers bestow chirality to an achiral metal surface. In this paper we report the behavior of R,R-tartaric acid adsorption on a Cu(110) surface using high-resolution surface analytical techniques. R,R-Tartaric acid is known to be an extremely successful modifier molecule for the enantioselective hydrogenation of methyl acetoacetate, the simplest beta-keto ester, to the R-enantiomer of the product molecule methyl S-hydroxybutyrate. A combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and Fourier transform reflection-absorption infrared spectroscopy (FT-RAIRS) techniques has allowed us to demonstrate that a complicated adsorption phase diagram exists for this system. A rich variety of ordered overlayer structures are produced, in which preferred molecular forms, bonding and orientations of the chiral molecules are adopted, dependent on coverage, temperature and time. These different adlayers will clearly play a different role in the enantioselective reaction. Of particular interest is the fact that under certain conditions, the 2-dimensional order of the IZ,R-tartaric acid adlayer destroys all symmetry elements at the surface, leading to the creation of extended chiral surfaces! Such chiral surfaces may be an important factor in defining the active site in heterogeneous enantioselective reactions.

The growth mode of domains of the hexagonal lattice of strained gadolinium deposited on Mo(112) has been investigated with low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The molybdenum substrate corrugations and the expansive strain within the gadolinium films dominate the growth of the thin Gd films, which is characterized by a preferential domain growth direction of the hexagonal Gd crystal structure, unlike the more uniform, epitaxial growth of ‘unstrained’ gadolinium, grown on W(110).

We show for the first time that, with two coadsorbed periodic structures it is possible to observe in a diffraction condition unique to one structure a relative enhancement of the vibrational losses characteristic of the species contained in the respective periodic structure. A recently developed SPA-LEED instrument equipped with an electron monochromator and analyser was used to distinguish true elastic diffraction from inelastic diffraction in the vibrational losses of coadsorbed (2×3)N/Cu(110) and the α-phase benzoate43−15structure on Cu(110). Vibrational enhancements by factors of 4–20 were found in energy loss spectra and momentum-resolved spot profiles. Inelastic spot profiles are qualitatively consistent with a kinematic description of the energy–momentum conservation induced broadening from both loss-before-diffraction (L-D) and diffraction-before-loss (D-L) events.

We have determined the azimuthal orientation of an adsorbate on a metal surface from an intramolecular-transition-derived feature in reflectance anisotropy spectroscopy (RAS). Adsorption of 9-anthracene carboxylic acid onto p(2 x 1)O/Cu(110) led to an ordered structure with a strong (2%), derivativelike feature at 4.5 eV. Fresnel theory predicts the measured intensity, functional behavior, and sense of the RAS signal for the molecule aligned along [110]. IR measurements confirm that the molecular plane is perpendicular to the surface and STM measurements support the azimuthal orientation. We reassign the sense of the clean Cu(110) surface RA spectrum.

Thermal N atoms from an atomic beam adsorb on Cu(111) at 300K to produce a disordered surface with a N coverage of up to 2ML. Annealing above 500K produced an ordered surface showing three domains of a Cu(100)-c(2×2)N overlayer, with a structure similar to that of bulk Cu3N. Parallel to the Cu(111) and the Cu(100) close packed directions, the overlayer is expanded by ∼3% compared to the Cu close packed distance, but by less than 1% perpendicular to this. STM images show the overlayer has an irregular corrugation with rows running approximately parallel to the close packed direction. This corrugation is caused by buckling of the Cu(100) overlayer to obtain local registry with the Cu(111) close packed rows and relieve stress in the Cu3N overlayer. N from the ordered Cu(100)-c(2×2)N overlayer, formed by atom dosing, desorbed in a zero order peak near 700K with an activation barrier of 143kJmol−1. For N coverages θN>0.42ML a broad desorption feature appeared above 500K with an activation energy ≥88kJmol−1. This peak is associated with desorption from a disordered N/Cu(111) surface, which can accommodate in excess of 2ML of N with considerable penetration into the Cu surface. Desorption from surfaces with θN>0.42ML forms the stable Cu(100)-c(2×2)N overlayer and also populates a new desorption peak, near 780K, which is not seen for initial coverages less than 0.42ML. This peak is intense for N+/N+2 sputtered surfaces and is attributed to a subsurface site. At high coverage and heating rates the presence of excess N stabilises desorption from the Cu(100)-c(2×2)N overlayer and N2 desorption becomes explosive. The desorption behaviour can be modelled by assuming desorption occurs preferentially from a dilute phase on Cu(111) terraces, with Cu(100)-c(2×2)N islands acting as a reservoir for N. We discuss evidence for this and other possible models using information from STM images of the surface and speculate on the N2 desorption site.

We have extended the displaced harmonic oscillator model for negative ion resonance (NIR) scattering in high-resolution electron energy loss spectroscopy (HREELS), to interpret the energy transfer processes leading to disordering in STM and LEED, by allowing the curvature of the negative ion PES to be different to that of the ground PES. For the benzoate-Cu(110) system, the results of the extended model are significantly better if the ground and negative ion PESs have frequencies hω = 0.40 and 0.48 eV, respectively, rather than the same frequency. The large resonance width and asymmetry, but not the small overtone to fundamental excitation probability ratio, observed in HREELS can be accounted for with values of the coupling constant, β = 22.2 and inverse lifetime of Г = 0.6 eV (1.1 fs). These parameters, with nD = 7 are simultaneously able to account for the ratio of the STM:LEED disordering cross-sections and, most significantly, predict the large slope observed in the STM disordering cross-section as a function of bias. The properties of the probability distribution, Pn(ϵi, for the NIR process, involving two Franck-Condon transitions, is contrasted with the DIET process and some non-classical behaviour for the case ω0 ≉ ω1 is noted.

We report the first I–V measurements of a negative ion mediated, molecular manipulation process, propose a model involving field-induced shifting of the resonance which predicts a disordering rate symmetric about zero sample bias, and then extend the displaced harmonic oscillator model to account for the larg slope in the I–V curve found for c(8 × 2) benzoate/Cu(110).

Using variable temperature scanning tunneling microscopy vicinal surfaces of Ge(100) have been studied in a temperature range between 80 and 300 K (room temperature). Annealing the sample gives rise to a nonuniform terrace and step distribution. Terraces with dimer rows perpendicular to the step edges cover approximately 3/4 of the entire surface. At room temperature about 1/2 of the surface is covered by buckled dimers. Dimers of straight SA steps are always pinned, i.e., they appear in their buckled form. On top of terraces buckling is also found, associated either with kinks in the step edges or in the formation of a single antisite dimer. At 80 K all dimers appear asymmetric in c(2×4) or p(2×2) geometry. A transition between both structures may occur due to sample–tip interaction which indicates that the energies of both reconstructions are rather similar.

We have prepared thin ordered NiO(100) films by evaporation of Ni in an O2 atmosphere onto Ag(100). The films have been analyzed by using scanning tunneling microscopy and low-energy electron diffraction. For room temperature deposition a c(1×2) Ni/O Ag structure in two orthogonal domains is obtained in the submonolayer coverage range. Annealing such a film produces regular islands with NiO(100) double layers. For higher coverage (five monolayers) a layer-by-layer-like growth mode is obtained.

By using STM, LEED and ISS we have studied the condensation of Pt on clean NiAl(110) and on a thin ordered Al2O3 film grown on NiAl(110). On Al2O3NiAl(110) and up to a coverage of one monolayer Pt forms highly dispersed two-dimensional islands with an average diameter of 10–30 Å whose density increases with the coverage. For higher Pt coverage the formation of three-dimensional Pt clusters is observed. Annealing of the Pt-covered surface leads to a diffusion of Pt into the oxide film. On clean NiAl(110) and up to one monolayer we found two-dimensional growth of Pt the density of the islands remaining approximately constant.

Small metal particles have strongly size-dependent properties, which - in the case of supported particles - are also influenced by the particle substrate interaction. We have investigated such effects for Pd deposited on a thin alumina film grown on NiAl(110) by probing the adsorption behaviour of CO. Structure and size of the metal islands were varied by employing different substrate temperatures during evaporation. We found that at 90 K small, disordered aggregates are formed, whereas deposition at 300 K results in larger crystallites with (111) facets. The CO thermal desorption spectra show a size-dependent behaviour which can be attributed to a higher degree of CO coordination with decreasing particle size. The results are compared with the corresponding behaviour of Pt. In contrast to Pd, Pt leads to a strong interaction with the substrate at 300 K. This gives rise to a desorption feature of CO in a temperature range which is typical for transition metal oxides.

Experiments are described, where by deposition of Ni from a Knudsen-type effusion cell in an O2 atmosphere onto an Au(111) substrate epitaxial and ordered NiO layers are formed. By analyzing the surfaces with scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) it is found for room temperature condensation a three-domain NiO(100) film, while for deposition on a heated (300-degrees-C) substrate NiO(111) is obtained. In the latter case three-dimensional NiO(111) crystallites and a thin smooth NiO(111) structure showing a p(2 x 2) reconstruction is observed. A p(2 x 2) reconstruction is theoretically expected, if the surface is composed of octopolar subunits ((NiO)4), which overcomes the problem of Madelung instability of a polar surface.

Via oxidation a well ordered Al2O3 film may be grown on an ordered NiAl(110) surface. Its structure has been studied with SPA-LEED (spot-profile analysis) as well as with scanning tunneling microscopy (STM). The oxide overlayer grows strictly two-dimensional with a thickness of close to 5 Angstrom. Double diffraction spots have been observed but they are very weak, thus not excluding the existence of an interfacial layer between NiAl(110) and the oxide film. STM provides preliminary evidence for such a film and presents first clues to what the structure of the interface may be.The defect structure of the Al2O3 film has been investigated. In addition to boundaries between two rotational domains constituting the Al2O3 film, we also identify anti-phase domain boundaries through both the SPA-LEED as well as the STM measurements.

By controlled oxidation of a clean NiAl(110) surface an epitaxial thin layer of Al2O3 may be grown. The oxide layer shows a sharp low-energy electron diffraction pattern of two domains of nearly rectangular unit cells. A stable tunneling current was obtained for a wide range of sample bias voltages U (from a few mV up to nearly 10 V). For U in the mV range the images essentially contain contributions of the interface layer, which can be measured with atomic resolution. For large U (e.g., 4 V) the electronic states of the oxide film contributed additionally to the substrate electrons, which allows an accurate analysis of the growth mode of the oxide film.

An overview is given on the use of scanning tunneling microscopy for characterizing metal oxide layers. We describe results on the initial stages of oxidation of Ni(100), on the preparation and measurement of a thin ordered layer of Al2O3 grown on NiAl(110), and on experiments to grow ordered NiO on Au(111) by evaporation of Ni in an O-2 atmosphere. It is demonstrated that at low sample-bias voltages tunneling is accomplished by the substrate electrons while for higher voltages the electronic states of the oxide film contribute to tunneling. This phenomenon can be used as a fingerprint for the identification of oxide species. Finally, we show characteristic results for a condensed metal (Ag) on an oxidized and on a metal surface.