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Estimating soft-mode frequencies of surface overlayers by means of photoelectron diffraction: The (2x2) surface-V2O3/Pd(111)
No full textThe (2x2) surface-V2O3 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 tunneling 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 quasi rotation around the V emitter of the three nearest-neighbor O atoms, and the estimated DFT frequency amounts to 15 cm-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±25 cm-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
Two-dimensional manganese oxide nanolayers on Pd(100): the surface phase diagram
No full textTwo-dimensional manganese oxide layers have been grown on Pd(100) and have been characterized by means of scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy (XPS). The complex surface phase diagram of MnO(x) on Pd(100) is reported, where nine different novel Mn oxide phases have been detected as a function of the chemical potential of oxygen mu(O). Three regions of the chemical potential of oxygen can be identified, in which structurally related oxide phases are formed, often in coexistence at the surface. The different regions of mu(O) are reflected in the oxidation states of the respective Mn oxide nanolayers as revealed by the Mn 2p and O 1s XPS binding energies. The MnO(x) nanolayers form two-dimensional wetting layers and it is speculated that they mediate the epitaxial growth of MnO on Pd(100) by providing structurally graded interfaces
Tailor-made ultrathin manganese oxide nanostripes: 'magic widths' on Pd(1 1 N) terraces
No full textThe growth of ultrathin two-dimensional manganese oxide nanostripes on vicinal Pd(1 1 N) surfaces leads to particular stable configurations for certain combinations of oxide stripe and substrate terrace widths. Scanning tunneling microscopy and high-resolution low-energy electron diffraction measurements reveal highly ordered nanostructured surfaces with excellent local and long-range order. Density functional theory calculations provide the physical origin of the stabilization mechanism of 'magic width' stripes in terms of a finite-size effect, caused by the significant relaxations observed at the stripe boundaries
Reactive growth of NiO ultrathin films on Pd(100): a multitechnique approach
No full textReactivegrowth 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
Strain relaxation and surface morphology of nickel oxide nanolayers
No full textThe surface morphology and the lattice constants of NiO overlayers in the thickness range of 1–20 monolayers (NiO nanolayers) on Pd(1 0 0) have been investigated by high-resolution spot profile low-energy electron diffraction (SPA-LEED) and scanning tunneling microscopy (STM). NiO islands grow epitaxially on Pd(1 0 0) on top of a c(4 × 2) Ni3O4 monolayer with a compressed strained lattice, which relaxes gradually attaining the bulk lattice constant at 10–12 monolayers. The strain relaxation is accompanied by the formation of small angle mosaic defect regions at the surface, which have been characterised quantitatively by following the behaviour of the satellites to the main Bragg diffraction rods. The analysis of the diffuse scattering intensity around the (0 0) diffraction spot reveals anisotropic NiO island shapes, whose orientation depends on the growth conditions. An incommensurate superlattice in LEED and STM at intermediate NiO coverages (∼2–6 monolayers) is observed and its origin is discussed
Vanadium on TiO2(110): adsorption site and sub-surface migration
No full textThe initial stages of the growth of vanadium overlayers on TiO2(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 adatom
The growth of ultrathin films of vanadium oxide on TiO2(110)
Full text linkThe 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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