Structural and vibrational properties of two-dimensional MnxOy\rm Mn_xO_y nanolayers on Pd(100)

Using different experimental techniques combined with density functional based theoretical methods we have explored the formation of interface-stabilized manganese oxide structures grown on Pd(100) at (sub)monolayer coverage. Amongst the multitude of phases experimentally observed we focus our attention on four structures which can be classified into two distinct regimes, characterized by different building blocks. Two oxygen-rich phases are described in terms of MnO(111)-like O-Mn-O trilayers, whereas the other two have a lower oxygen content and are based on a MnO(100)-like monolayer structure. The excellent agreement between calculated and experimental scanning tunneling microscopy images and vibrational electron energy loss spectra allows for a detailed atomic description of the explored models.Comment: 14 pages, 11 figure

Density functional study of the polar MnO(111) surface

By 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)

Density functional study of the polar MnO(111) surface

By 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)

Strained c(4 x 2) CoO(100)-like monolayer on Pd(100): Experiment and theory

We report on an interface-stabilized strained c(4 x 2) phase formed by cobalt oxide on Pd(100). The structural details and electronic properties of this oxide monolayer are elucidated by combination of scanning tunneling microscopy data, high resolution electron energy loss spectroscopy measurements and density functional theory. The c(4 x 2) periodicity is shown to arise from a rhombic array of Co vacancies, which form in a pseudomorphic CoO(100) monolayer to partially compensate for the compressive strain associated with the large lattice mismatch (similar to 9.5%) between cobalt monoxide and the substrate. Deviation from the perfect 1:1 stoichiometry thus appears to offer a common and stable mechanism for strain release in Pd(100) supported monolayers of transition metal rocksalt monoxides of the first transition series, as very similar metal-deficient c(4 x 2) structures have been previously found for nickel and manganese oxides on the same substrate. (C) 2009 Elsevier B.V. All rights reserved

Two-dimensional manganese oxide nanolayers on Pd(100): the surface phase diagram

Two-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

Silver nanostructures on a c(4x2)-NiOx/Pd(100) monolayer

The growth, morphology and epitaxial relationship of Ag nanostructures deposited onto the c(4 × 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 × 2)-NiOx/Pd(1 0 0) surface cannot be considered as an efficient template to stabilize metal nanoparticles against Ostwald ripening phenomena