1,720,970 research outputs found
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
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
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
Density functional study of the polar MnO(111) surface
No full textBy application of a density functional approach within the PBE and PBE+U approximations we investigate the ground state terminations of the polar MnO(111) surface being in thermodynamic equilibrium with an oxygen reservoir. In the allowed range of the oxygen chemical potential and for realistic oxygen partial pressures the surface is found to undergo different structural transitions. In the oxygen-poor regime the most stable phases are the O- and Mn-terminated octopolar structures, which are almost degenerate in energy. For oxygen-rich conditions we observe a competition between the O-terminated unreconstructed bulk face and a stripes structure. We show that the stabilization of the polar surface in the thermodynamic equilibrium with the oxygen environment is due to remarkable changes of the geometrical structure (i.e., reconstruction and relaxation) and of the electronic structure (i.e., metallization)
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
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
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
Cobalt oxide nanolayers on Pd(100): The thickness-dependent structural evolution
No full textThe growth of interface-stabilized cobalt oxide (CoOx) nanolayers on Pd(100) has been investigated and their structures are reported as a function of coverage. Several different phases have been observed by LEED and STM experiments, and they have been characterized spectroscopically by photoemission and X-ray absorption. The data indicate that in the low coverage regime (up to ΘCo ≈ 2–3 ML) rock-salt CoO type phases are formed (defective in the single layer regime, and stoichiometric in multilayers) with (100) or (111) termination. At higher coverage (ΘCo ≈ 10–20 ML) spinel Co3O4(111) and CoO(100) layers have been detected, in ratios dependent on the preparation conditions. The observed structures are discussed in relation to similar structures reported recently for CoOx films on Ir(100) [W. Meyer et al., J. Phys.: Condens. Matter 20 (2008) 265011
A photoelectron diffraction study of the surface-V2O3 (2 x 2) layer on Pd(111)
No full textX-ray photoelectron diffraction (XPD) has been applied to the study of the surface-(s)-V2O3 (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-V2O3/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 Å, while the DFT-derived value is 0.723 Å
- …
