Angle-scanned Photoelectron Diffraction Research Articles

Graphene/Ni(111) surface structure probed by low-energy electron diffraction, photoelectron diffraction, and first-principles calculations

The structural properties of graphene/Ni(111) are investigated by a combination of low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), angle-scanned photoelectron diffraction (PED), and first-principles calculations. XPS data indicate that graphene interacts strongly with the topmost Ni layer. Diffraction data show that graphene is deposited commensurably with the underlying Ni surface atoms. Twelve different graphene preparations were analyzed by LEED and one by PED. Considering the relative position between the carbon and Ni atoms, five experimental sets indicate a top-fcc structure, three show a bridge-top structure, and four have a mixed structure. The analysis of the PED experiment suggests the top-fcc termination. Our first-principles calculations show that the total energies of the top-fcc and bridge-top structures are nearly degenerate, which corroborates the observed bistability of those phases. Moreover, by comparing the structural parameters obtained by the three methods (LEED, PED, and ab initio calculations), excellent agreement is achieved.

Silver nanostructures on a c(4 × 2)-NiO x/Pd(1 0 0) monolayer

The growth, morphology and epitaxial relationship of Ag nanostructures deposited onto the c(4 × 2)-NiO x /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 NiO x ML is locally disrupted in the vicinity of the Ag nanoparticles so that the c(4 × 2)-NiO x /Pd(1 0 0) surface cannot be considered as an efficient template to stabilize metal nanoparticles against Ostwald ripening phenomena.

Ultrathin SiO 2-films on 4H-SiC(0 0 0 1) studied by angle-scanned photoelectron diffraction

X-ray photoelectron spectroscopy and angle-scanned photoelectron diffraction were used to investigate thermally grown ultrathin SiO 2 films on 4H-SiC(0 0 0 1). Due to the chemical sensitivity of the applied methods it was possible to study different aspects of the films. A chemically shifted component in the Si 2p photoemission spectrum was ascribed to Si emitter atoms at the interface and allowed a determination of the interface structure between the ultrathin SiO 2 film and the SiC substrate. The photoelectron diffraction pattern of another chemically shifted Si 2p component contained information about the local environment of near-interface Si emitters in the oxide film. This information was made accessible by means of an extensive comparison between experimental and simulated data. In the simulations the use of the cluster radius as a fitting parameter turned out to provide information about the range of local order in the oxide film. A chemically shifted component in the C 1s photoemission spectrum was used to study the structural properties of carbon contaminations, which were detected in the SiO 2 layer after special preparation procedures.

Experimental and theoretical evidence for substitutional molybdenum atoms in theTiO2(110)subsurface

Molybdenum was deposited at room temperature on the ${\mathrm{TiO}}_{2}(110)$ surface in the 0--1.3 equivalent monolayer (eqML) range and was then annealed at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in order to reach a kind of equilibrium state. A threshold was found in the behavior of the deposit: below 0.2 eqML, substitutional molybdenum occurs in titanium sites located under the bridging oxygen atoms of the ${\mathrm{TiO}}_{2}(110)$ surface. In this position, molybdenum atoms are in a structural and chemical ${\mathrm{MoO}}_{2}$-like environment. Density-functional theory calculations show that this molybdenum site is actually the most stable one in the case of isolated molybdenum atoms. Angle-scanned photoelectron diffraction data are in perfect agreement with such a hypothesis. For deposits higher than 0.2 eqML, the increased amount of molybdenum atoms raises the probability of $\mathrm{Mo}\text{\ensuremath{-}}\mathrm{Mo}$ interactions during the annealing at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, taking to a reduction process of the deposit. However, such annealing does not allow the deposit to become fully metallic: molybdenum clusters formed during the annealing are in strong interaction with the substrate, and metallic molybdenum can be obtained only depositing a film thicker than $1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$.

Structural analysis of the SiO2/Si100 interface by means of photoelectron diffraction.

The local environment of Si atoms at the interface between a thermally grown SiO2 film and Si(100) was studied by angle-scanned photoelectron diffraction. Experimental photoelectron diffraction patterns for each Si oxidation state were obtained from the results of least squares fitting on Si 2p core-level spectra. A comparison of the diffraction patterns with multiple-scattering calculations including an R-factor analysis was performed. An excellent agreement between experimental and simulated data was achieved within the proposed bridge-bonded interface model [Phys. Rev. Lett. 84, 4393 (2000)]].

Investigation of the SiO 2/Si(1 0 0) interface structure by means of angle-scanned photoelectron spectroscopy and diffraction

The local environment of Si at the interface between a thermally grown SiO 2 film and Si(1 0 0) was studied by angle-scanned photoelectron spectroscopy and diffraction. Si 2p core-level spectra containing chemically shifted components were recorded. These components were deconvoluted by least squares fitting and assigned to different Si oxidation states. The angular dependence of the photoelectron intensity was obtained from the least squares fit results. A simple statistical model was developed to describe the population and depth distribution of the different suboxide species. Using a simple electron attenuation scheme we can simulate the photoemission polar angle dependence of the various Si oxidation states within the model. The fitting of these model curves to the experimental data results in parameters which indicate a Si–O–Si bridge-bonded interface structure.

Analysis of the angular distribution of soft-X-ray photoelectrons: a tool for surface structure determination

Selected applications of angle-scanned photoelectron diffraction (PED) to adsorbate-covered surface systems are described, showing the possibility to obtain, from the analysis of the experimental data, quantitative information on the surface structure. Recent case studies performed by our group are presented, where the technique has been successfully applied to different systems, ranging from poorly ordered low-coverage overlayers to environments involving multiple adsorption sites. In particular, the advantages offered by state-specific PED and by the angle-scan approach are highlighted.

Buried interfaces studied by photoelectron diffraction

Angle- and energy-scanned photoelectron diffraction data can be used to investigate structures below surfaces. The modulations in photoelectron intensity result from diffraction of the emitted electron wave at neighbor atoms. In the past, scanned-energy photoelectron diffraction had been mainly used to determine the adsorption site of molecules at surfaces. Recent data show, however, that the technique can also be employed to obtain information about the upper substrate layer(s). At low kinetic energies, backward scattering is strong and in scanned-angle photoelectron diffraction the recorded patterns result from backward- and multiple-scattering effects. For a structural analysis, the intensity modulations have to be compared with the results for simulations performed for model clusters. As an example, recent angle-scanned photoelectron diffraction patterns recorded for the technologically important silicon oxide/silicon interface were compared with simulations. At the Si(001) surface orientation, the interface is extended over a few layers, whereas at the Si(111) surface orientation the transition is rather abrupt and occurs within one or two layers.

Structure of highly strained ultrathin Ni films on Pd( [formula omitted

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.

Structural transition in Fe ultrathin epitaxial films grown on Ni(111)

A structural study of Fe ultrathin epitaxial films, grown at room temperature on Ni(111), has been performed in the 1.5--18 ML coverage range by angle-scanned photoelectron diffraction. Both backscattering and forward-scattering energy regimes have been employed, in order to enhance the structural sensitivity at lower and higher film thicknesses, respectively. Modeling of the experimental data has been performed with multiple scattering calculations. We found indications that Fe atoms in the first layer occupy fcc hollow sites and stack with a pseudomorphic fcc structure up to 2 ML. Concerning the growth mode at these early stages, data suggest that a good substrate wetting and a sharp Fe/Ni interface take place. Between 3 and 6 ML, transition to a bcc(110) phase develops. By quantitative R-factor analysis, we found that Nishiyama-Wassermann (NW) in-plane orientation of the bcc(110) cell $(〈001{〉}_{\mathrm{bcc}}\ensuremath{\parallel}〈11\ifmmode\bar\else\textasciimacron\fi{}0{〉}_{\mathrm{fcc}})$ is favored over the Kurdjumov-Sachs $(〈11\ifmmode\bar\else\textasciimacron\fi{}1{〉}_{\mathrm{bcc}}\ensuremath{\parallel}〈11\ifmmode\bar\else\textasciimacron\fi{}0{〉}_{\mathrm{fcc}})$ orientation. The best-fit vertical interlayer distance between bcc(110) planes is ${d}_{\mathrm{NW}}=2.11\AA{}$ (+3.9% expansion) at 6 ML and relaxes to ${d}_{\mathrm{NW}}=2.05\AA{}$ (+1.0%) at 18 ML, in agreement with the angular shift observed for the forward-focusing features. In the same coverage range, the angle between bcc(110) surface basis vectors changes from 67.7\ifmmode^\circ\else\textdegree\fi{} to 69.0\ifmmode^\circ\else\textdegree\fi{}, corresponding to -1.7% and -1.0% contractions of the surface cell area, respectively.

Structural studies of epitaxial ultrathin oxidefilms and nanoclusters by means of angle-scanned photoelectrondiffraction (XPD)

Selected examples of the application of angle-scanned x-rayphotoelectron diffraction (XPD) to structural studies ofepitaxial oxide ultrathin films and nanoclusters depositedeither on metal or on oxide single-crystalline substrates arebriefly reviewed. A short introduction discusses the preparative strategies adopted in order to grow the desiredoxide systems, as well as the basic features of the XPDtechnique which are relevant to the field of oxide epitaxy.Synthesis routes include both e-beam metal evaporation andoxidation and a modification of metal-organic chemical vapourdeposition to suit ultra-high-vacuum conditions. These introductory remarks are followed by an overview of some systems that havebeen investigated in our laboratory. The discussion isparticularly aimed at highlighting the peculiar capabilitiesand strengths of photoelectron diffraction applied to - evenshort-range-ordered - oxide epitaxial systems. Oxide overlayersobtained by means of reactive deposition or post-oxidationcomprise various vanadium oxide phases grown on rutile TiO2(110), while metalcarbonyl precursor decomposition inan oxygen atmosphere as a source of oxide ultrathin films isillustrated through the cases of RuO2/TiO2(110) fromRu3(CO)12 and MnO/Pt(111) from Mn2(CO)10.Finally, some remarks are made on recent developments andfuture perspectives in the field of synchrotron-radiation-basedimplementations of photoelectron diffraction and holography.

THE TRANSITION FROM AMORPHOUS SILICON OXIDE TO CRYSTALLINE SILICON STUDIED BY PHOTOELECTRON DIFFRACTION

Angle-scanned photoelectron diffraction patterns of the Si 2p signal of oxidized Si(111) and Si(001) surfaces have been recorded and compared with simulations. The chemically shifted components of the Si 2p signal were deconvoluted by least squares fitting. The different oxidation states exhibit individual diffraction patterns for both surface orientations, indicating a different environment for each suboxide. For the Si(111) surface the results agree with a previously proposed sharp interface, whereas a graded interface is indicated for the Si(001) surface.

The role of the Si-suboxide structure at the interface: an angle-scanned photoelectron diffraction study

Photoelectron diffraction patterns of the different silicon sub-oxides have been recorded and compared with simulated patterns for various model structures. Each silicon sub-oxide is embedded in an ordered environment since individual diffraction patterns and differences among them were exhibited. In particular, the intensity maxima are located at different angles. In the simulation the silicon-oxide/silicon interface was assumed to be abrupt and within one atomic layer. Excellent agreement between experimental and calculated patterns was achieved. At the interface, horizontally compressed SiO 2 was found. Furthermore, the highest oxidation state of silicon, Si 4+, displays a diffraction pattern indicating an ordered structure for this chemical state.

Ultrathin film growth and spectroscopic characterization of VO x (0.8⩽ x⩽1.3) on Pt(1 1 1)

VO x ultrathin epitaxial films (0.8⩽ x⩽1.3) have been grown on Pt(1 1 1) by evaporating vanadium in a low and well-controlled water background (1×10 −7 Pa). X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and X-ray excited auger electron spectroscopy data strongly indicate that vanadium(II) in the predominant oxidation state. Angle-scanned photoelectron diffraction data are compatible with a single domain film, bearing a rock-salt VO structure and exposing the (1 1 1) plane. Such films evolve toward more oxidized species within hours, probably due to oxidation by residual water in the chamber.

Local atomic environment of Si suboxides at the SiO2/Si(111) interface determined by angle-scanned photoelectron diffraction.

Local environments of Si suboxides at the interface between a thermally grown SiO2 film and Si(111) were studied by angle-scanned photoelectron diffraction. Si 2p core-level spectra containing chemically shifted components were recorded. The components were deconvoluted by least squares fitting and assigned to different Si oxidation states. The obtained diffraction patterns of the various suboxides exhibit different features. Comparison of these patterns with multiple scattering calculations including a multipole R-factor analysis shows that a simple chemical abrupt interface model describes well the environment of the suboxides and indicates ordered SiO2 close to the interface.

Investigation of the SiO2/Si(111) interface by means of angle-scanned photoelectron diffraction

Abstract Photoelectron diffraction measurements of the Si 2p core level were carried out in order to investigate the interface structure of a SiO 2 film thermally grown on Si(111). The photoemission spectra show chemically shifted components derived from the individual oxidation states, which exhibit different diffraction patterns. These diffraction patterns are compared with those obtained by multiple scattering calculations for a simple chemical abrupt interface structure model. In this model each oxidation state occurs in various bonding configurations. Thus the experimental data are compared with superpositions of simulated patterns for possible model clusters. As a result of a multipole R -factor analysis we obtain a parameter set that reproduces the experimental data well.

Ultrathin VO x/TiO 2(110) ( x≈1) film preparation by controlled oxidation of metal deposits

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.

Epitaxial growth of bcc Co films on Sb-passivated GaAs(110) substrates

Abstract We have combined angle-scanned photoelectron diffraction (PED), low-energy electron diffraction and synchrotron radiation photoemission to examine the formation of the Co/GaAs(110) and Co/Sb/GaAs(110) interfaces. We find that Co forms a metastable bcc phase on both surfaces, with its principal crystallographic axes parallel to the substrate lattice. For Co films grown on non-passivated substrates, we determine an outward distortion of the lattice constant perpendicular to the surface. By Sb deposition on GaAs(110) surface prior to the Co growth, the epitaxy quality of the metallic layer is improved. The Sb-passivation of the substrate decreases the intermixing reaction and reduces the intralayer outward expansion of the Co films to less than 1%.

High-temperature arsenic adsorption on InP(110)—surface bonding and surface structure

Abstract A study including core level photoelectron spectroscopy and photoelectron diffraction was performed in order to obtain information on the high-temperature arsenic adsorption on the InP(110) surface. Surface bonding, surface structure and interface reactions were investigated in comparison to InP(110). An arsenic–phosphorus exchange reaction replacing the first layer phosphorus atoms by arsenic and forming a monolayer of InAs was observed using photoelectron spectroscopy. A polar angle scanned photoelectron diffraction study was performed in order to obtain structural information on this arsenic-exchanged surface. Comparison of simulations with experimental angle distribution curves applying double scattering calculations supports a bond angle rotation of this surface with a tilt angle of ω=29°.

Polarization effects to enhance surface sensitivity of angle-scanned X-ray photoelectron diffraction in synchrotron-radiation-based experiments

We show that the surface sensitivity of angle-scanned photoelectron diffraction applied to bulk crystals or epitaxial multilayers can be enhanced by exploiting the angular anisotropy of the photoelectron primary wave excited by linearly polarized light, combined with the possibility of performing measurements in two different light polarization/electron detection geometries. The obtained relative photoelectron diffraction patterns of the form I 1( k )/I 2( k ), where 1 and 2 denote the two different geometric set-ups discussed in the present paper, show an enhanced structural surface sensitivity, which should allow the determination of the presence and the extent of surface relaxation by comparing the experimental data to suitable theoretical simulations. In a set of preliminary model calculations, the best results are obtained for emission from an s-initial state. High angular resolution of the electron detectors is an additional requirement.