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The growth morphology of ultrathin (up to 5 ML) vanadium oxide films on TiO2(1 1 0) has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). It has been found that the reactive evaporation technique produces more uniform and better-ordered vanadia layers than the post-oxidation method. At low coverages V-oxide clusters adsorb on top of the fivefold-coordinated Ti rows of the substrate. With increasing coverage the clusters agglomerate and form strands, which are oriented along the [0 0 1] titania direction. For oxide coverage >2 ML the strands cover uniformly the titania substrate, forming a texture along the [0 0 1] direction, and give rise to a (1 × 1) LEED pattern. The latter is consistent with the growth of an epitaxial rutile-type VO2 phase.

The initial stages of the growth of vanadium overlayers on TiO 2 (1 1 0) at room temperature have been investigated with scanning tunneling microscopy. At very low coverages both individual vanadium adatoms and small vanadium clusters have been imaged with good resolution. The V adatoms adsorb preferentially on the so-called “upper threefold hollow” sites, as revealed by atomically resolved STM images: they are thus bonded to two bridging oxygen atoms and one threefold coordinated basal oxygen atom. At higher coverages the vanadium adlayers grow in form of poorly ordered three-dimensional islands. The number of V clusters at low coverages decreases by gentle annealing or with time even at room temperature. This kinetic effect has been interpreted in terms of sub-surface migration of V adatoms.

The epitaxial growth of ultrathin Ni films on the Pd(1 0 0) surface was studied by means of XPS, angle-scanned photoelectron diffraction (XPD) and LEED experiments. The XPD and LEED data indicate that the early stages of deposition are characterised by the formation of a tetragonally strained Ni fcc phase, which subsequently develops into a bulk-like Ni fcc structure as the critical thickness of approximately 12 ML eq is exceeded. The highly strained structure below the critical thickness is associated with subtle electronic structure changes with respect to Ni bulk, as evidenced by the analysis of the satellite structure in the Ni 2p XPS region.

We explore a photochemical approach to achieve an ordered polymeric structure at the sub-monolayer level on a metal substrate. In particular, a tetraphenylporphyrin derivative carrying para-amino-phenyl functional groups is used to obtain extended and highly ordered molecular wires on Ag(110). Scanning tunneling microscopy and density functional theory calculations reveal that porphyrin building blocks are joined through azo bridges, mainly as cis isomers. The observed highly stereoselective growth is the result of adsorbate/surface interactions, as indicated by X-ray photoelectron spectroscopy. At variance with previous studies, we tailor the formation of long-range ordered structures by the separate control of the surface molecular diffusion through sample heating, and of the reaction initiation through light absorption. This previously unreported approach shows that the photo-induced covalent stabilization of self-assembled molecular monolayers to obtain highly ordered surface covalent organic frameworks is viable by a careful choice of the precursors and reaction conditions.

This paper reports on the deposition of a vanadium oxide ultra-thin film on TiO2(1 1 0) by means of e-beam evaporation in an atmosphere of water vapour at room temperature. Photoelectron, X-ray excited Auger and valence band spectra have shown features very similar to those reported in the literature for vanadia ultra-thin films on TiO2(1 1 0) identified as V2O3. An X-ray photoelectron diffraction analysis has been performed as a function of the overlayer thickness and after annealing treatments. It demonstrates that the overlayer grows ordered on the short range, pseudomorphic to the substrate, with a defective rutile crystal structure. The epitaxial relationship is maintained, with an approximately linear decrease in anisotropy, up to at least a coverage of 16 monolayers (ML); short annealing treatments are sufficient to restore the ordered structure, even on 20 ML thick films. Although in the bulk phase V2O3 is characterised by the corundum structure, which is also found when depositing vanadium oxide under similar experimental conditions on other substrates, such as Al2O3(0 0 0 1) and Pd(1 1 1), it appears that TiO2(1 1 0) stabilises the isomorphic rutile lattice, which pertains to bulk VO2 in its metallic phase, as well as to Magneli phases of general formula VnO2n−1. This particular behaviour of vanadia ultra-thin films grown on TiO2 could be a key to understanding the catalytic activity and selectivity of the vanadia/titania systems in several chemical reactions.

We have investigated the room temperature potassium-promoted oxidation of Ni(100) by means of O 1s and K 2p X-ray photoemission spectroscopy (XPS), relative work function and O 1s X-ray absorption spectroscopy (XAS) measurements. Within the potassium coverage range we examine in this study ( θ K =0.10–0.25 ML), coadsorption of about 0.5 ML of oxygen results in the formation of a (3×3) pattern which is peculiar to the coadsorption of alkalis and oxygen on Ni(100). This structure is stable against further oxygen uptake or against adsorption of other species such as CO. However, by means of O 1s XPS and XAS observations we determine an overall potassium-enhanced oxidation rate of about two orders of magnitude. The O 1s XPS data show that oxygen coadsorbed with K on Ni(100) is still bonded primarily to Ni and thus rule out the formation of potassium oxide or other K x O y compounds such as peroxide or superoxide. The line-shape analysis of the K 2p XPS data show that upon oxygen coadsorption the alkali metal layer undergoes a metal–insulator type of transition. The O 1s XPS data rule out the possibility that this transition is due to a large charge transfer between coadsorbed species so other mechanisms are discussed. The work function changes induced by O on the K-precovered surface are consistent with the formation of a bilayered structure in which oxygen is adsorbed underneath the alkali metal layer at K coverages above the work function minimum. However, we find that the alkali-induced minimum of the work function is not related to the occurrence of structural properties, in particular the formation of the (3×3) structure, or electronic properties such as the metal–insulator transition of the K layer. Therefore a coplanar alkali–oxygen coadsorption model cannot be ruled out. The O 1s XAS data show that the presence of potassium on Ni(100) lowers the O coverage for the onset of the formation of cubic NiO.

The analysis by means of multiple scattering cluster (MSC) calculations of full 2π V 2p, Ti 2p and O 1s X-ray photoelectron diffraction (XPD) patterns from an ultrathin film (∼4 ML) of VOx (x≈1) deposited on the (1×1)-TiO2 (110) (rutile) surface leads to a detailed determination of the structure and orientation of the overlayer and of its epitaxial relationship with the substrate. The comparison of XPD experimental data to theoretical simulations confirms the NaCl-like stacking of the overlayer which is suggested by the direct observation of the experimental 2π plots and reveals its (100) orientation. The [001] azimuth of the overlayer is aligned with the [112] direction on the substrate surface and the overlayer experiences an orthorhombic tensile strain (+7%) in order to match the substrate epitaxially, with a consequent 12–16% vertical interlayer spacing contraction. The atomic scale morphology of the (1×1)-TiO2 (110) surface induces a strong buckling in the overlayer along the [110] substrate direction [0.5±0.1 A].

Results are reported on photoemission and resonant photoemission (ResPES) measurements performed on a Ba-deposited TiO 2 (110) using synchrotron radiation. For coverages less than 1 monolayer (ML) of Ba, only oxidized and no metallic Ba states were detected, and in the valence band spectra a new gap state, attributed to Ti 3+ , was seen at 0.8 eV below the Fermi edge and shown by ResPES to give a characteristic 3p→3d resonance of Ti. When the Ba thickness exceeds 1 ML, a resonance of the near-Fermi-edge bandgap state appears, with a maximum found for the Ba 4d→4f resonant transition. The metallic nature of the layers thicker than 1 ML is confirmed by an asymmetry of the Ba 4d peak. Work function measurements showed an abrupt change with Ba thickness when approaching 1 ML, in consistence with the above results.

The structure of ultrathin films of Ni deposited at room temperature on the Pt (111) surface has been determined by low energy electron diffraction (LEED) intensity analysis. The first monolayer of Ni grows pseudomorphically on Pt (111) despite the 11% mismatch between the lattice parameters of nickel and platinum. The results of the analysis also show that the Ni atoms occupy face-centered-cubic sites on the substrate surface. These results in part confirm previous data obtained by X-ray photoelectron diffraction, but also provide more reliable data on the site assignment.

UHV evaporation of Sn on (1 × 2) Pt(110), followed by UHV annealing, provides three different PtSn/Pt(110) surface alloys, each characterized by a specific low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) pattern. When annealed in a controlled oxygen background, the PtSn/Pt(110) surface alloys can be transformed into SnOx/Pt(110) nano-oxides. This paper reports the experimental phase diagram of the metallic and oxidized surface superstructures.

This paper reports experimental evidence for an incipient epitaxial growth of vanadium oxide at the ${\mathrm{TiO}}_{2}$(110) surface. This finding is based on a x-ray photoelectron diffraction and low-energy electron diffraction study of a submonolayer of vanadium deposited at the ${\mathrm{TiO}}_{2}$(110) surface, performed after annealing at 473 K in ultrahigh vacuum conditions. The data unambiguously show that vanadium atoms occupy selected sixfold-coordinated titanium sites which underbridge oxygen atoms of the topmost layer and that the ${\mathrm{TiO}}_{2}$(110) long-range surface structural order is preserved. \textcopyright{} 1996 The American Physical Society.

This Letter reports an X-ray photoelectron diffraction study of ∼5 V monolayers deposited by electron beam evaporation at the TiO 2 (110) surface. Data clearly show that V grows as islands with a bcc structure having the [100] direction normal to the substrate surface. A detailed analysis of the V″2p and Ti″3p azimuthal curves indicates also an alignment of the [001] overlayer azimuth with the [1¯10] direction of the TiO 2 substrate. These results are consistent with an epitaxial growth of bcc V at the TiO 2 (110) surface where a double V surface unit cell is matched to the TiO 2 (110) rectangular cell.

The site (fcc or hcp) occupied by the atoms of a Ni ML deposited on Pt (111) is determined in this paper using angle-scanned photoelectron diffraction (PD). A full 2π MgK α-excited Ni-2p 3/2 PD pattern from 1-ML Ni deposited on Pt(111) is compared to single scattering cluster-spherical wave (SSC-SW) simulations and the agreement between experimental and theoretical data is quantified by R-factor analysis. From the present investigation it turns out that Ni atoms occupy hcp sites. In addition, the Ni-Pt distance has been estimated to be 2.5±0.1 Å.

X-ray photoelectron diffraction (PD) based on a forward scattering approach (FS-PD) has been used to study the growth mode of the first few Ni monolayers deposited on the Pt(111) surface, with a particular attention to the initial stages of epitaxy, i.e. the formation of the first atomic layer. Strong evidences for a layer-by-layer (or Frank-Van der Merwe) growth mode are reported, substantiated also by theoretical simulations carried out with the single scattering cluster-spherical wave (SSC-SW) framework. The first Ni monolayer grows strained in-plane to match the substrate pseudomorphically even if there is a 10% mismatch between the lattice parameters of Ni and Pt. The multilayer (up at least to six monolayers) maintains the horizontal strain and consequently shows a vertical spacing contraction (tetragonal distorsion). It retains the overall threefold symmetry and azimuthal orientation of the substrate, indicative of a single-domain epitaxial fcc stacking. There is also some evidence (even if it is not conclusive) for the fact that the Ni atoms of the first monolayer occupy hcp sites of the substrate surface.

Experimental and theoretical results pertaining to [Zn-10(mu(4)-S)(mu(3)-S)(6)(Py)(9)(SO4)(3)], a possible molecular model of ZnS S-terminated polar surfaces, as well as a potential source of strictly monodispersed ZnS quantum dots, are presented and discussed. The results of density functional theory (DFT) calculations provided a rationale for the peculiar arrangement of [Zn-10(mu(4)-S)(mu(3)-S)(6)(Py)(9)(SO4)(3)] clusters in the solid state, contemporarily indicating the unsuitability of the isolated species to mimic whatever (polar or non-polar) ZnS surface. Despite the fact that such a failure is further confirmed by time-dependent DFT and UV-Vis diffuse reflectance spectroscopy, the combined use of theoretical outcomes, DRIFT measurements, and literature data pertaining to the surface chemical properties of ZnS (Hertl in Langmuir 4:594, 1988) ultimately testifies that [Zn-10(mu(4)-S)(mu(3)-S)(6)(Py)(9)(SO4)(3)] is perfectly suited to model the interaction of pyridine molecules with ZnS surface Lewis acid sites. The herein reported theoretical results are expected to be a useful reference for the interpretation of chemisorption experiments of Py-based Lewis bases on single crystal ZnS surfaces.

A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule-surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.

A heteroepitaxial monolayer deposited on a suitable single-crystal substrate is exploited as the source of the primary photoelectron wave in the determination of chemisorption site geometries using an angle-scanned forward scattering approach. The method is applied to the study of c(2 × 2)-O on Ni (1 monolayer) deposited on Cu(100). In agreement with literature data, oxygen atoms occupy fourfold hollow sites, with a Ni-O distance of 1.92 ± 0.04 A and with an angle between the Ni-O bond and the surface of 20 ± 1°. The major point is that such information is deduced directly from the experimental raw data without any theoretical simulation.

An angle-scanned X-ray photoelectron diffraction (XPD) study of the polar Zn-terminated ZnO(0001) surface has been carried out in order to look for a hypothetical inward relaxation of the outermost Zn layer. From the comparison of the O 1s polar scan with single-scattering-cluster (SSC) simulations it is clearly evident that the surface is bulk terminated and any relaxation is ruled out. Moreover, the capability of the XPD technique in determining the surface polarity (atomic termination) by inspection of θ/φ 2D plots without the aid of theoretical simulations has been demonstrated.

Pt–Sn alloys play an important role as oxygen reduction reaction (ORR) catalysts in fuel cell technology both for their role in rationalizing the mechanism responsible for the better performances of bimetallic catalysts and for their practical applications. Here we present a complete experimental and theoretical study on the geometric and electronic structure of a (4×1) termination over a Pt3Sn near-surface ordered alloy obtained by depositing Sn on the Pt(110) and subsequent annealing. LEED and STM measurements combined with DFT simulations allow us to determine the atomistic structure of this phase, and to rationalize the peculiar dependence of its STM pattern on the applied bias and the compositional order in deep layers. Both photoemission experiments and density functional calculations indicate that in this phase the Sn–Pt alloying process determines important changes in the electronic properties, and in particular a relevant shift of the Pt 5d band centroid with respect to clean Pt.

Multicomponent supramolecular self-assemblies of exceptional long-range order and low defectivity are obtained if C(60) and 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (TPP-NH2) are assembled on Ag(110) by sequential evaporation in the submonolayer range of TPP-NH2 and fullerene on the substrate surface and subsequent annealing. A (+/-2 -3, 6 +/- 3) array consisting of supramolecular stripes of a 1:1 C(60)/TPP-NH2 2D adduct develops at 410 K (the low temperature, LT, phase). If the LT phase is annealed at 470 K, then a 3:1 fullerene/TPP-NH2 (+/-3 -5, 5 +/- 5) nanoporous array (the HT phase) forms, with each pore containing a single porphyrin molecule. Phase separation occurs by annealing the HT phase at 520 K. Structural models are proposed and discussed on the basis of the experimental scanning tunneling microscopy results.

The structure of two ordered stoichiometric TiO(2) nanophases supported on Pt(111) and (1x2)-Pt(110) substrates, prepared by reactive evaporation of Ti in a high-oxygen background, is compared by discussing experimental data (i.e. low-energy electron diffraction, scanning tunneling microscopy) and density functional theory calculations. Two rectangular phases, called rect-TiO(2) and rect'-TiO(2) were obtained on both the hexagonal Pt(111) and the rectangular (1x2)-Pt(110) substrates, generally suggesting that they are weakly interacting with the substrates. The rect-TiO(2) phase is actually confined to a TiO(2) double layer, while the rect'-TiO(2) can extend up to a thickness of several layers and is obtained when higher Ti doses are evaporated. While the rect-TiO(2) is best described as a thickness-limited lepidocrocite-like nanosheet, growing as a single-domain-commensurate (14x4) phase on (1x2)-Pt(110) and as a six-domains-incommensurate phase on Pt(111), the thicker rect'-TiO(2) phase can be best described as a TiO(2)(B) supported nanolayer (NL). This represents the first example of the TiO(2)(B) phase in the form of a supported NL, whose properties are still largely unexplored. The important point is that, because of the weak interaction between the oxide NLs and the Pt surfaces, the substrate does not play a role in stabilizing the 2D nanostructures. Rather, it acts as a sort of lab bench where sub-nanosized titania crystallites self-assemble, so that the final NLs are representative of 2D confined titania at the bottom of the nanoscale.

The preparation and characterization of fully oxidized TiO2 ultrathin films obtained by reactive deposition of Ti in an O-2 background on the (1x2)-Pt(110) reconstructed surface is described in details. The structure, the electronic properties, and the morphology of the epitaxial films giving rise to a (14x4) coincidence superstructure are discussed on the basis of low-energy electron diffraction, photoemission (both core and valence band), angle-scanned X-ray photoelectron diffraction, scanning tunneling microscopy data, and density functional theory calculations. We show that the oxide overlayer is a stoichiometric lepidocrocite-like single-domain nanosheet. This can be thought of as originating from a (100) oriented anatase bilayer which spontaneously restructure by a uniaxial relative sliding of one single layer with respect to the other by half a unit cell. According to the results of theoretical calculations, the process is self-driven by the spatial confinement, whereas a minor role is played by the interaction with the substrate. The occurrence of the (14x4) coincidence between the overlayer and the substrate is fully rationalized on the basis of the reported data.

The growth, morphology and epitaxial relationship of Ag nanostructures deposited onto the c(4 x 2)-NiOx/Pd(1 0 0) surface have been investigated by photoemission (both core and valence levels), scanning tunneling microscopy and angle-scanned photoelectron diffraction. Small Ag nanoparticles are obtained on the terraces, whereas the tendency of Ag to decorate the step edges can lead to the formation of extended nanowire-like features. The Ag nanoparticles adopt a fcc structure epitaxially related to the substrate according to the following relationship: Ag(1 0 0)[0 0 1]/Pd(1 0 0)[0 0 1], while by means of STM we have found that the shape of the islands is typically rectangular with the edges running along the [0 1 1] directions. According to the reported data, the NiOx ML is locally disrupted in the vicinity of the Ag nanoparticles so that the c(4 x 2)-NiOx/Pd(1 0 0) surface cannot be considered as an efficient template to stabilize metal nanoparticles against Ostwald ripening phenomena. (C) 2007 Elsevier B.V. All rights reserved.

We discuss the results of an extensive investigation on NiO ultrathin films grown on Pd(100), carried out by means of high-resolution X-ray photoelectron spectroscopy (core levels) and ultraviolet photoelectron spectroscopy (valence band levels). The peculiarity of the investigated system is the rather high lattice mismatch between NiO and the Pd substrate (7.3%). Detailed information on the electronic structure of NiO ultrathin films is obtained as a function of thickness, with a particular emphasis put on the fully wetting single c(4 x 2)-Ni 3 O 4 defective monolayer, up to the bulklike relaxed films, through the intermediate three-dimensional strained islands resulting from a Stranski-Krastanov growth behavior.

International audience; The interaction of single C60 molecules with the (1 × 2)-Pt(110) surface has been studied by scanning tunneling microscopy and density functional theory (DFT) calculations on slab models. Molecules are observed to be frozen at room temperature and are found to be almost exclusively in the same configuration. Extensive DFT calculations show that this configuration is the global energy minimum, suggesting that adsorbed molecules have enough rototranslational freedom to escape from the numerous local minima. The adsorption energy (3.81 eV) is the strongest ever found for C60, and it is roughly proportional to the number of the Pt and C atoms at contact distance. Analysis of DFT results shows that the surface−adsorbate interaction is covalent in nature. A minority fraction of C60 molecules appear to be adsorbed on surface defects. A careful investigation of their registry and height with respect to the regularly adsorbed units leads to an indirect structural characterization of the nanopits which act as their adsorption sites.

A bottom-up route towards the synthesis of titania nanosheets is explored, alternative to the exfoliation of layered titanates. Nanosheets are assembled from the constituent elements and epitaxially matched to a suitable substrate: $(1\ifmmode\times\else\texttimes\fi{}2)\mathrm{\text{\ensuremath{-}}}\mathrm{Pt}(110)$. Their basic lepidocrocite structure is modulated at the nanoscale due to coincidence with the substrate. Density functional calculations reveal the structure details of the nanosheet, which is also shown to be in close relationship with a (001)-oriented anatase bilayer.

A bottom-up route towards the synthesis of titania nanosheets is explored, alternative to the exfoliation of layered titanates. Nanosheets are assembled from the constituent elements and epitaxially matched to a suitable substrate: (1 x 2)-Pt(110). Their basic lepidocrocite structure is modulated at the nanoscale due to coincidence with the substrate. Density functional calculations reveal the structure details of the nanosheet, which is also shown to be in close relationship with a (001)-oriented anatase bilayer.

International audience; We have studied the initial stages of adsorption of C60 on the Pt (1 1 0)-(1 2) surface by means of STM. At room temperature, fullerene molecules adsorb in the troughs between two adjacent Pt rows of the missing row reconstruction. Mobility over the terraces is negligible, denoting strong bonding with the surface, also testified by a well-defined orientation of fullerene monomers with respect to the substrate. Upon annealing at 750 K, molecular migration towards kinks and step edges occurs, where small islands nucleation begins. A commensurate registry with the substrate is maintained by small (5–10 molecules) C60 aggregates, leading to expanded nearest-neighbour distances with respect to those found in hexagonal close packed fullerene ad-islands grown on other metallic substrates.

Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.

A surface stabilized monolayer phase of nickel oxide, c(4 x 2)-Ni(3)O(4), has been found to grow epitaxially under reactive deposition conditions on Pd(100), in the presence of other adsorbed phases and in competition with them. High-quality scanning tunneling microscopy data are reported and discussed, including a detailed analysis of the defects and of the border morphology of this new phase. The data are discussed in the light of ab initio simulations of the electronic, energetic, and geometric properties of such a phase. A hybrid-exchange density functional theory approach has been used, and a slab model is adopted where palladium is simulated by a thin film covered on both sides by regular epilayers. A growth model has been developed that explains both the unusual stoichiometry of the phase and the observed defects.

The structure of the interface-stabilized c(4 × 2) Ni 3 O 4 /Pd(1 0 0) monolayer has been determined by using Low Energy Electron Diffraction (LEED) I – V experiments. Tensor LEED simulations converged to a Rp factor of 0.22 with a structural model based on the NiO(1 0 0) surface, where all the atoms of the oxide phase are coplanar. In order to obtain the correct c(4 × 2) periodicity of the phase, a Ni vacancy array has been introduced into the model. In details, oxygen atoms sit on top of palladium atoms, while nickel atoms occupy the fourfold hollow sites. The lattice constant of NiO was contracted in order to match the substrate periodicity, and only a very small buckling of the Ni atoms (0.04 A) was introduced. The most relevant change with respect to a rigid NiO(1 0 0) structure is the in-plane displacement of oxygen atoms by about 0.2 A along directions almost coinciding with the 〈0 1 0〉 substrate azimuths, away from the vacant Ni site. The proposed structure is perfectly compatible with the STM images preliminarly reported in the literature.

Reactive growth of NiO ultrathin films on Pd(1 0 0) has been performed by evaporating metallic Ni in an oxygen atmosphere. The evolution of the ultrathin film is followed by means of low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD) and scanning tunnelling microscopy (STM). The first monolayer (ML) of the deposited oxide develops as a completely wetting 2D overlayer with a c(4 × 2) periodicity, as shown by sharp LEED patterns and atomically resolved STM images. Further NiO deposition results in 3D NiO(1 0 0) islands formation on-top the c(4 × 2) superstructure, as shown by STM images and XPD data. Two layers thick islands are shown to be pseudomorphic to the substrate, i.e. characterised by in-plane compressive strain and interlayer expansion. LEED, XPD and STM give independent yet converging evidence that partial strain relaxation occurs within the third monolayer.

NiO ultrathin films have been grown on Pd(1 0 0) following a reactive deposition procedure. Ni has been dosed at room temperature on the substrate surface in an oxygen partial pressure of 4 × 10 −6 mbar. The electronic and structural evolution of the resulting NiO(1 0 0) ultrathin films has been followed by means of X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and scanning tunnelling microscopy (STM). XPS, XPD and STM data indicate a 2D growth of the first NiO monolayer, while further growth leads to the nucleation of 3D islands, in a Stranski–Krastanov growth scheme. Combined XPD and LEED data indicate an initially pseudomorphic growth, characterised by in-plane compressive tetragonal strain of the NiO film, with a consequent out-of-plane interlayer expansion. Partial strain relaxation occurs abruptly, very likely between the second and the third atomic layer of the 3D islands, while a completely bulk-like cubic environment is reached only gradually as a function of thickness. NiO(1 0 0) films even ∼50 equivalent monolayers thick can be grown with good long-range order, as shown by (1 × 1) LEED images.

The $(2\ifmmode\times\else\texttimes\fi{}2)$ surface-${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ layer, an interface-mediated vanadium oxide phase observed on Pd(111) in the submonolayer coverage range, has been investigated by means of angle scanned x-ray photoelectron diffraction (XPD), which gives a direct experimental confirmation of the model derived by scanning tunnelling microscopy (STM) and density functional calculations (DFT), with a quantitative determination of the V-O interlayer spacing. In addition, XPD measurements compared to single scattering cluster--spherical wave (SSC-SW) simulations revealed a peculiar broadening of V-O forward scattering (FS) maxima that is limited to azimuthal scans and that cannot be accounted for by isotropic Debye-Waller attenuation of the diffraction features. However, the existence of a soft phonon mode in the overlayer, associated with substantial in-plane displacements from equilibrium of O scatterers with respect to V emitters, could explain the experimental observation. The existence of such a soft mode has been confirmed by DFT calculations. It consists of an in-plane quasirotation around the V emitter of the three nearest-neighbor O atoms, and the estimated DFT frequency amounts to $15{\mathrm{cm}}^{\ensuremath{-}1}.$ The XPD data have been analyzed by means of SSC-SW simulations wherein a harmonic oscillator model has been employed to approximate the effect of the soft phonon mode on XPD curves. As a result, an experimental determination of the frequency of the mode has been obtained $(40\ifmmode\pm\else\textpm\fi{}25{\mathrm{cm}}^{\ensuremath{-}1}),$ which is of the same order of magnitude as the DFT predicted frequency. Moreover, the sensitivity of XPD scans to the correlation of soft-mode atomic displacements has been studied, leading to the estimate of a ``soft-mode XPD coherence length'' for the system under investigation. This work therefore explores an application of XPD as a surface spectroscopy sensitive to vibrational soft modes.

Molybdenum was deposited on blue (i.e. non-stoichiometric) TiO2(1 1 0) surface using a very low deposition rate (less than 0.05 eqML min−1). The resulting deposit was investigated by means of X-ray photoelectron diffraction (XPD), LEED and XPS. Just after deposition, the film is mainly constituted of metallic molybdenum, contains oxygen homogeneously dispersed through the whole deposit and the broad features detected in XPD scans are interpreted as a coarse epitaxy between TiO2(1 1 0) surface and the (0 0 1) face of bcc molybdenum. The orientation relationship is: Mo(1 0 0)[0 0 1]//TiO2(1 1 0)[0 0 1]. After annealing the deposit at 673 K, XPD scans become sharper and epitaxy is achieved even if no LEED pattern is observed. Moreover, the annealing induces a noticeable decrease of the oxygen content inside the film.

The epitaxial growth of ultrathin Ni films on the Pd(1 0 0) surface was studied by means of X-ray photoelectron diffraction (XPD) and LEED experiments. In excellent numerical agreement with the predictions of elasticity theory, the data indicate the formation of tetragonally strained Ni epitaxial layers, which subsequently turns into a bulk-like Ni structure as the thickness of approximately 12 MLE is exceeded. This study demonstrates that LEED and XPD methodologies are rather complementary in order to have a detailed picture of the evolution of the overlayer structure in different thickness regimes.

X-ray photoelectron diffraction (XPD) has been applied to the study of the surface-(s)-V 2 O 3 (2 × 2) layer on Pd(1 1 1), which is a novel interface-stabilised vanadium oxide phase with no bulk oxide counterpart. It has been detected by scanning tunnelling microscopy (STM) during the growth of ultrathin films of vanadium oxide on Pd(1 1 1). XPD confirms the general features of the model for s-V 2 O 3 /Pd(1 1 1), which has been proposed previously on the basis of STM measurements and ab initio density-functional-theory (DFT) calculations. In addition, quantitative agreement is found between the DFT model and the XPD experiment in the estimate of the average V–O interlayer spacing: the experimental result is 0.72±0.07 A, while the DFT-derived value is 0.723 A.

Selected examples of the application of angle-scanned x-ray photoelectron diffraction (XPD) to structural studies of epitaxial oxide ultrathin films and nanoclusters deposited either on metal or on oxide single-crystalline substrates are briefly reviewed. A short introduction discusses the preparative strategies adopted in order to grow the desired oxide systems, as well as the basic features of the XPD technique which are relevant to the field of oxide epitaxy. Synthesis routes include both e-beam metal evaporation and oxidation and a modification of metal-organic chemical vapour deposition to suit ultra-high-vacuum conditions. These introductory remarks are followed by an overview of some systems that have been investigated in our laboratory. The discussion is particularly aimed at highlighting the peculiar capabilities and strengths of photoelectron diffraction applied to - even short-range-ordered - oxide epitaxial systems. Oxide overlayers obtained by means of reactive deposition or post-oxidation comprise various vanadium oxide phases grown on rutile TiO2(110), while metalcarbonyl precursor decomposition in an oxygen atmosphere as a source of oxide ultrathin films is illustrated through the cases of RuO2/TiO2(110) from Ru3(CO)12 and MnO/Pt(111) from Mn2(CO)10. Finally, some remarks are made on recent developments and future perspectives in the field of synchrotron-radiation-based implementations of photoelectron diffraction and holography.

The research activity of our group in the last few years has mainly been devoted to the study of ultrathin vanadium oxide films deposited on a (110)-oriented TiO2 single crystal, in order to prepare systems which may be largely thought of as simplified models for the investigation of the structure/properties relationships in real world catalysts, sensing and optical devices. The main objective of our work consists of setting up reproducible synthesis routes for the deposition of vanadium oxide ultrathin films on TiO2 (110), through strict control of the reaction parameters. The films obtained are then characterised from a chemical, electronic and structural point of view, and their properties are compared to those of their bulk-related phases. Results are presented concerning growth procedures and structural and electronic properties of vanadium oxide ultrathin films on titania, with a stoichiometry ranging from VO2, down to approximately VO. In particular, it will be shown that the oxidation product of metallic vanadium in an oxygen or water atmosphere (in the 10−6 mbar range) retains the rutile lattice structure typical of stoichiometric VO2, despite the increasing degree of oxygen defectiveness and the electronic properties, very similar to those pertaining to bulk V2O3. The peculiar behaviour of vanadium oxide on titania demonstrates how important the epitaxial influence of the substrate is on the nature of the overlayer. These results could represent a good starting point to understand why vanadium oxides on TiO2 show an enhanced catalytic activity and selectivity in many industrially relevant reactions.

An RuO 2 /TiO 2 (110) ultrathin film, epitaxially grown in ultrahigh vacuum by reactive deposition of Ru 3 (CO) 12 in an oxygen atmosphere (2 × 10 -6 mbar) at 300°C, has been characterised by means of X-ray photoelectron diffraction (XPD) measurements coupled to multiple-scattering (MS) simulations. Comparison of the overlayer Ru 3d 5/2 and the clean substrate Ti 2p 3/2 2π plots gives direct evidence for an RuO 2 overlayer well ordered in the short range, with the rutile structure (typical of both the underlying substrate and bulk RuO 2 ) which continues the (110) stacking of the substrate in a single orientational domain. The presence of a sharp low-energy electron diffraction (LEED) pattern, identical to that of the clean substrate, proves the extension of the translational order to the long range and suggests a pseudomorphic growth. Multiple-scattering calculations of the diffraction pattern lead to a detailed quantification of the thin film relaxation.

In the present communication, we discuss the results of an angle resolved photoemission extended fine structure (ARPEFS) study of a VO 2 monolayer (ML) grown on the TiO 2 (110) surface by successive cycles of sub-ML vanadium metal deposition followed by annealing at 473 K in 2×10 −6 mbar O 2 . The V 3p photoemission peak shows two distinct components chemically shifted by 1.3 eV. While the higher binding energy (BE) component produces a rather flat ARPEFS curve, the lower BE signal, associated with the VO 2 phase, shows well defined intensity modulations whose main features are similar to the ARPEFS scan on the Ti 3p signal of the substrate. This observation demonstrates that the ordered VO 2 phase grows epitaxially to the substrate, with a rutile type structure. However, some oxide is present in a more highly oxidized and less-ordered phase. In order to investigate the actual arrangement of the ML with respect to the question related to the possible formation of an intermixed VO 2 /TiO 2 layer, the ARPEFS data have been interpreted by means of single-scattering spherical wave (SSC-SW) simulations. They are compatible with the hypothesis that the deposited ML evolves toward an intermixed VO 2 /TiO 2 double layer where the vanadium atoms occupy the six-fold oxygen-coordinated sites. In addition, our data are in good agreement with a surface relaxation similar to that found by surface XRD on the stoichiometric TiO 2 (110) surface.

Metallic vanadium ultrathin films deposited on the TiO 2 (110) surface are easily oxidised to VO 2 when they react with O 2 in the high vacuum pressure range. We have found that the oxidation can be stopped at VO x ( x ≈1) when annealing of the metal film is carried out in UHV, by exploiting the reaction with bulk-to-surface diffusing oxygen. By adopting carefully optimised experimental conditions (e.g. metal film thickness and annealing temperature), locally ordered VO x films with a thickness of up to 5 ML can be prepared. X-ray photoelectron spectroscopy and angle-scanned photoelectron diffraction, as an in situ structural characterisation tool, have proven to be extremely useful in order to find optimised conditions for the whole process.

A detailed structural study of V growth on Pt (111) has been performed mainly by means of angle resolved X-ray photoelectron spectroscopy and diffraction, with the aid of multiple scattering cluster-spherical wave simulations of the diffraction curves. Vanadium on Pt (111) grows with a two-domain bulk-like bcc (111) stacking largely incoherent with the substrate, at least up to a thickness of eight equivalent monolayers. For thicker layers, an orientational transition is observed, leading to a six-domain bulk-like bcc (110) structure. Although bcc (111)-oriented V is preserved in the thicker layer, we cannot exclude the fact that some of the initially (111) ultrathin film has been partially restructured to the (110) orientation when the critical thickness associated with the transition has been exceeded. Strong three-dimensional clustering of the overlayer is observed for any investigated thickness which supports a Volmer–Weber growth of the V film. Our findings are compared to literature data concerning the growth of Cr on the same substrate.

Electrons emitted from the core levels of a photon-irradiated crystalline sample undergo scattering by atoms in the vicinity of the emitting species. Subsequent interference phenomena between the electron wave portions propagating to the detector produce intensity modulations as a function of the direction of detection. This process constitutes the physical basis of the angle-scanned X-ray Photoelectron Diffraction (XPD) technique. The resulting modulations, properly interpreted, are rich in structural information concerning the near-surface atomic layers.

Angle-scanned photoelectron diffraction measurements have been performed prior to and after CO exposure onto a Ni monolayer deposited on Pt(111). Ni is known to grow epitaxially on Pt(111), but there is evidence for an adsorbate-induced restructuring such that the initial atomically flat Ni layer gives place to at least two-layer-high Ni islands. As far as the driving force for the restructuring is concerned, some hypotheses can be put forward considering the Ni–Ni nearest neighbor distances in the Ni monolayer and in the restructured layer, along with some thermodynamical arguments.

Chemisorption site geometries of formate and acetate species on Cu(100) have been studied by means of angle-scanned X-ray photoelectron diffraction (XPD) and first-principle quantum-mechanical calculations on a Cu60-formate cluster in the framework of local density functional theory (LDF). According to our LDF calculations, the short bridge site results to be more stable than the cross bridge one by about 20kcalmol. The reasons for the more effective interaction of the adsorbate in the short bridge site have been clearly outlined on the basis of the analysis of the LDP wavefunctions. LDF-optimized structural parameters have been used as a starting input in a series of single-scattering cluster spherical wave (SSC-SW) simulations of the O 1s XPD curves, which have experimentally confirmed the occupation of the short bridge site proposed by LDF calculations. The SSC-SW simulations have also outlined the role of the low-frequency-hindered rotational modes of the adsorbate (librations) and have furnished an estimate of the frequency (∼ 70 cm−1) of the hindered rotation on the axis perpendicular to the surface. Furthermore, as regards the formate species, the experimental data give evidence of a Cu-O distance of 1.95 A, in agreement with the LDF calculations and with angle-resolved photoemission fine structure (ARPEFS) literature results. An estimate of the OCO angle (129° ± 5°) has been obtained from polar scans for both formate and acetate species.