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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 layers have been grown on Pd(100) and have been characterized by scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The complex surface phase diagram of MnOx on Pd(100) is reported, where nine different novel Mn oxide phases have been detected as a function of the chemical potential of oxygen mO. 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 mO 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 MnOx 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., 24 pages, 12 figures

The growth of epitaxial MnO(100) and MnO(111) layers on Pd(100) surface has been investigated by spot-profile analysis low-energy electron diffraction, dynamic atomic force microscopy, photoemission and high-resolution electron energy loss spectroscopy, and density functional theory. We have found that despite the large lattice mismatch to the Pd(100) substrate, the MnO(100) layers are kinetically stabilized at low temperatures $(\ensuremath{\leqslant}350\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and at oxygen pressures between $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ and $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}\mathrm{mbar}$. Annealing in ultrahigh vacuum at $650\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ or, alternatively, deposition of manganese metal in oxygen pressure $l1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}\mathrm{mbar}$ causes the transformation of the MnO(100) to a polar MnO(111) surface, which is decorated by triangular pyramids with (100) side facets. It is suggested that the growth of MnO(111) layers is energetically preferred over MnO(100) due to the epitaxial stabilization at the metal-oxide interface.

By application of a density functional approach within the PBE and $\mathrm{PBE}+\mathrm{U}$ approximations we investigate the ground state terminations of the polar $\mathrm{MnO}(111)$ 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).