Publications (18)11.28 Total impact
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Article: Tailor-made ultrathin manganese oxide nanostripes: 'magic widths' on Pd(1 1 N) terraces.
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ABSTRACT: The 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.Journal of Physics Condensed Matter 02/2012; 24(4):042001. · 2.55 Impact Factor -
Article: Deformed Surface Oxides: Uncommon Structure of a (6 × 1) NiO Surface Oxide on Rh(111)
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ABSTRACT: We have investigated the formation of a nickel oxide monolayer on Rh(111) by scanning tunneling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations. This monolayer displays a corrugated (6 × 1) superstructure, with a formal Ni5O5 stoichiometry. The interplay between polarity and interface energy leads to the formation of a structure with pronounced troughs, which may be regarded as a building deformation, containing building blocks derived from nonpolar (100) and polar (111) NiO surface terminations. The calculations also show that the (6 × 1) Ni5O5 phase has a higher thermodynamic stability than the related octopolar NiO bulk terminations.Keywords: oxidation; surface oxides; catalysis; density functional theory; STM01/2012; -
Article: Metamorphosis of ultrathin Ni oxide nanostructures on Ag(100)
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ABSTRACT: The evolution of the morphology of Ni-oxide nanostructures on Ag(100) as a function of temperature has been followed experimentally by scanning tunneling microscopy (STM) and theoretically by density-functional theory (DFT) calculations. After reactive deposition at room temperature and annealing at 450 K, the NiO(2 × 1) phase embedded into the Ag substrate is formed due to kinetic stabilization during growth. The (2 × 1) structure is replaced by the thermodynamically more stable NiO(100)(1 × 1) phase, also embedded into the Ag substrate, for annealing at T > 500 K. As shown by DFT modeling and in agreement with experimental STM contrast, the formation of second-layer NiO patches below the NiO(1 × 1) monolayer is an effective means of strain relief in the NiO(1 × 1) monolayer islands. For T > 600 K, the increased possibilities of mass transport lead to an intriguing surface morphology called “labyrinth” phase, with narrow high-aspect ratio embedded NiO(1 × 1)-type islands. The latter are stabilized by the interaction of the NiO borders with the Ag(100) substrate via double-layer boundary lines, which according to the DFT calculations provide the essential elements of this thermodynamic ground state.Phys. Rev. B. 09/2011; 84(11). -
Article: The two-dimensional cobalt oxide (9 × 2) phase on Pd(100).
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ABSTRACT: The two-dimensional (2D) Co oxide monolayer phase with (9 × 2) structure on Pd(100) has been investigated experimentally by scanning tunneling microscopy (STM) and theoretically by density functional theory (DFT). The high-resolution STM images reveal a complex pattern which on the basis of DFT calculations is interpreted in terms of a coincidence lattice, consisting of a CoO(111)-type bilayer with significant symmetry relaxation and height modulations to reduce the polarity in the overlayer. The most stable structure displays an unusual zig-zag type of antiferromagnetic ordering. The (9 × 2) Co oxide monolayer is energetically almost degenerate with the c(4 × 2) monolayer phase, which is derived from a single CoO(100)-type layer with a Co(3)O(4) vacancy structure. Under specific preparation conditions, the (9 × 2) and c(4 × 2) structures can be observed in coexistence on the Pd(100) surface and the two phases are separated by a smooth interfacial boundary line, which has been analyzed at the atomic level by STM and DFT. The here described 2D Co oxide nanolayer systems are characterized by a delicate interplay of chemical, electronic, and interfacial strain interactions and the associated complexities in the theoretical description are emphasized and discussed.The Journal of chemical physics 05/2011; 134(18):184706. · 3.09 Impact Factor -
Article: Strained c (4× 2) CoO (1 0 0)-like monolayer on Pd (1 0 0): Experiment and theory
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ABSTRACT: a b s t r a c t We report on an interface-stabilized strained c(4 Â 2) phase formed by cobalt oxide on Pd(1 0 0). 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 Â 2) periodicity is shown to arise from a rhombic array of Co vacancies, which form in a pseudomorphic CoO(1 0 0) monolayer to partially compensate for the com-pressive strain associated with the large lattice mismatch ($9.5%) between cobalt monoxide and the sub-strate. Deviation from the perfect 1:1 stoichiometry thus appears to offer a common and stable mechanism for strain release in Pd(1 0 0) supported monolayers of transition metal rocksalt monoxides of the first transition series, as very similar metal-deficient c(4 Â 2) structures have been previously found for nickel and manganese oxides on the same substrate.01/2010; -
Article: Interplay between magnetic, electronic, and vibrational effects in monolayer Mn3O4 grown on Pd(100).
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ABSTRACT: The surface stabilized MnO(100)-like monolayer, characterized by a regular c(4 x 2) distribution of Mn vacancies, is studied by hybrid functionals and discussed in the light of available scanning tunneling microscopy and high-resolution electron energy loss spectroscopy data. We show that the use of hybrid functionals is crucial to account for the intermingled nature of magnetic interactions, electron localization, structural distortions, and surface phonons. The proposed Pd(100) supported Mn(3)O(4) structure is excellently compatible with the experiments previously reported in literature.The Journal of chemical physics 04/2009; 130(12):124707. · 3.09 Impact Factor -
Article: Two-dimensional manganese oxide nanolayers on Pd(100): the surface phase diagram.
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ABSTRACT: 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 μ(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 μ(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.Journal of Physics Condensed Matter 04/2009; 21(13):134008. · 2.55 Impact Factor -
Article: Interplay between magnetic, electronic and vibrational effects in monolayer Mn_3O_4 grown on Pd(100)
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ABSTRACT: The surface stabilized MnO(100)-like monolayer, characterised by a regular c(4x2) distribution of Mn vacancies, is studied by hybrid functionals and discussed in the light of available scanning tunneling microscopy and high-resolution electron energy loss spectroscopy data. We show that the use of hybrid functionals is crucial to account for the intermingled nature of magnetic ineractions, electron localization, structural distortions and surface phonons. The proposed Pd(100) supported Mn3O4 structure is excellently compatible with the experiments previously reported in literature. Comment: 16 pages, 5 figures02/2009; -
Article: Adsorption and Dissociation of CO on Bare and Ni-Decorated Stepped Rh (553) Surfaces
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ABSTRACT: The adsorption and dissociation of carbon monoxide were studied with plane-wave density functional theory on flat Rh(111), stepped and kinked Rh(553), and Ni-decorated Rh(553) surfaces. The theoretical results were compared to high-resolution X-ray photoelectron spectroscopy (HR-XPS) experiments. The most favorable CO adsorption sites for low coverages were identified by a systematic calculation of the adsorption energies, and their sequence of occupation as a function of CO exposure was determined experimentally in C 1s HR-XPS spectra via their characteristic surface core-level shifts. On the clean, stepped (553) surface, molecular CO is adsorbed more strongly on low-coordinated top sites at the step edge, but on the Ni-decorated surface, the binding is stronger at the terrace sites. The barrier for dissociation with respect to the gas phase is about 1 eV lower on the stepped Rh(553) surface than on the flat Rh(111) surface, implying a substantially higher reaction rate. The presence of kinks at the clean Rh(553) surface does not lead to a significant additional decrease of the dissociation barriers, resulting in dissociation energies just above the desorption threshold for both stepped and kinked surfaces, whereas the barrier can be additionally lowered by about 0.1 eV by decorating the step edges with Ni stripes. Whereas no dissociation of CO was observed by HR-XPS on the clean Rh(553) surface, a minor amount of CO dissociation was found on the Ni-decorated Rh surface, in agreement with the theoretical predictions.J. Phys. Chem. C. 01/2009; 113:942-949. -
Chapter: The (100)→(111) Transition in Epitaxial Manganese Oxide Nanolayers
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ABSTRACT: The growth and structure of epitaxial MnO(100) and MnO(111) nanolayers on Pd(100) surfaces have been investigated. We found that despite the large lattice mismatch to the Pd(100) substrate MnO(100) layers can be kinetically stabilised at low temperatures (≤350°C) and at oxygen pressures between 2x10−7 mbar and 5x10−7 mbar. Annealing in ultra-high vacuum to 650°C or, alternatively, deposition of manganese metal in oxygen pressure < 1x10−7 mbar causes the transformation of the MnO(100) to a polar MnO(111) surface. It is suggested that the growth of MnO(111) layers is energetically preferred over MnO(100) due to the epitaxial stabilisation at the metal-oxide interface.12/2008: pages 163-170; -
Article: Structural and vibrational properties of two-dimensional $\rm Mn_xO_y$ nanolayers on Pd(100)
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ABSTRACT: 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 figures11/2008; -
Article: Growth of cobalt on a VO (1 1 1) surface: Template, surfactant or encapsulant role of the oxide nanolayer?
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ABSTRACT: a b s t r a c t Cobalt overlayers, from submonolayer coverages to several monolayers thick, have been grown on to an oxygen-terminated VO(1 1 1) bilayer supported on a Rh(1 1 1) surface. Scanning tunnelling microscopy (STM) images show that the Co deposits exhibit a rough morphology at room temperature, but that annealing in ultrahigh vacuum (UHV) up to 670 K results in a remarkable ordering of the VO–Co layer system, producing flat structures which display the same lattice parameters as the bare VO(1 1 1) surface. X-ray photoelectron spectra (XPS) and scanning tunnelling spectroscopy (STS) differential conductance spectra reveal that the VO bilayer segregates on top of the Co layer already during the Co deposition at room temperature, encapsulating the Co film. It is proposed that the segregation of the VO layer is dri-ven by the low surface free energy of the oxygen-terminated VO(1 1 1) phase.08/2008; -
Article: Growth and oxidation of Ni nanostructures on stepped Rh surfaces
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ABSTRACT: The growth and oxidation of Ni nanostructures on stepped Rh(15 15 13) and (553) surfaces, both vicinals of Rh(111), have been studied by scanning tunneling microscopy (STM), LEED, core level spectroscopies with use of synchrotron radiation (XPS and NEXAFS), and work function measurements. At low coverages, quasi-one-dimensional Ni rows decorate the Rh step edges. With increasing coverages, the Ni adlayers grow layer-by-layer in a restricted step-flow growth mode, pseudomorphically up to ∼2 monolayer (ML). Relaxation to the Ni bulk lattice occurs in the range 2-10 ML. The oxidation of the low-coverage Ni nanowires creates ordered quasi-one-dimensional Ni-O structures at the Rh step edges. In the Ni coverage range up to 2 ML, uniaxially ordered (2 × 1) and (6 × 1) oxide structures are observed. On thicker films, local NiO (100)-type and (111)-type reconstructions are both observed in the STM. The NEXAFS measurements indicate that the low-dimensional Ni-O nanophases are distinctly different from bulk-type NiO, and three chemically different oxygen species are identified in the O 1s XPS core level spectra. The chemical reactivity of the Ni-Rh surfaces toward oxygen seems to be highest for the one-dimensional Ni-Rh nanowires, where the Ni atoms are coupled directly with the Rh step atoms.J. Phys. Chem. C. 01/2008; 112:19272-19278. -
Article: Epitaxial stabilization of MnO(111) overlayers on a Pd(100) surface
Phys. Rev. B. 01/2007; 75(22):224120. -
Article: Structural and vibrational properties of two-dimensional Mn_ {x} O_ {y} layers on Pd (100): Experiments and density functional theory calculations
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ABSTRACT: Using different experimental techniques combined with density functional based theoretical methods we have explored the formation of interface-stabilized manganese oxide structures grown on Pd100 at sub-monolayer coverage. Among 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 MnO111-like O-Mn-O trilayers, whereas the other two have a lower oxygen content and are based on a MnO100-like monolayer structure. The excellent agreement be-tween calculated and experimental scanning tunneling microscopy images and vibrational electron-energy-loss spectra allows for a detailed atomic description of the explored models.43. -
Article: Structural and vibrational properties of two-dimensional Mn_ {x} O_ {y} layers on Pd (100): Experiments and density functional theory calculations
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ABSTRACT: 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. Among 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.Phys. Rev. B. 79(3). -
Article: Cobalt oxide nanolayers on Pd(100): The thickness-dependent structural evolution
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ABSTRACT: The 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].Surface Science. -
Article: Oxygen adsorption on stepped Pd(100) surfaces
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ABSTRACT: We use scanning tunneling microscopy (STM) and high-resolution core-level spectroscopy (XPS) measurements to study the initial oxidation of vicinal Pd(100) surfaces exhibiting close-packed (111) steps. The XPS data analysis is supported by detailed surface-core level shift calculations based on densityfunctional theory. Both STM images and the XPS spectra are found to be perfectly consistent with a characteristic zigzag O decoration of the Pd steps predicted by a preceding cluster-expansion based theoretical study [Y. Zhang and K. Reuter, Chem. Phys. Lett. 465, 303 (2008)]. Continued oxygen uptake leads to the additional stabilization of a p(2×2)-O overlayer on the Pd(100) terraces, and ultimately to step bunching with the resulting large Pd(100) terraces covered by the PdO(101) surface oxide.Surface Science, v.604, 1813-1819 (2010).
Top co-authors
Top Journals
Institutions
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2009–2012
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University of Vienna
- Fakultät für Physik
Vienna, Vienna, Austria
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2007–2012
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Karl-Franzens-Universität Graz
- Institute of Physics
Graz, Styria, Austria
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