Florian Mittendorfer

Accurate surface and adsorption energies from many-body perturbation theory

Kohn-Sham density functional theory is the workhorse computational method in materials and surface science. Unfortunately, most semilocal density functionals predict surfaces to be more stable than they are experimentally. Naively, we would expect that consequently adsorption energies on surfaces are too small as well, but the contrary is often found: chemisorption energies are usually overestimated. Modifying the functional improves either the adsorption energy or the surface energy but always worsens the other aspect. This suggests that semilocal density functionals possess a fundamental flaw that is difficult to cure, and alternative methods are urgently needed. Here we show that a computationally fairly efficient many-electron approach, the random phase approximation to the correlation energy, resolves this dilemma and yields at the same time excellent lattice constants, surface energies and adsorption energies for carbon monoxide and benzene on transition-metal surfaces.

Structural, electronic and magnetic properties of nickel surfaces

Abstract The structural, electronic and magnetic properties of the low-index surfaces of Nickel have been investigated via fully self-consistent ab-initio local-spin-density-functional (LSDF) calculations. Our technique is based on ultrasoft pseudopotentials, residuum minimization techniques for the calculation of the electronic ground-state and of the Hellmann–Feynman forces and stresses, and on a conjugate-gradient technique for the optimization of the atomic structure. The calculations were performed for nine-layer symmetric slabs, allowing for the relaxation of the upper three layers. We also present a detailed analysis of electronic surface states.

Nickel carbide as a source of grain rotation in epitaxial graphene.

Graphene has a close lattice match to the Ni(111) surface, resulting in a preference for 1 × 1 configurations. We have investigated graphene grown by chemical vapor deposition (CVD) on the nickel carbide (Ni(2)C) reconstruction of Ni(111) with scanning tunneling microscopy (STM). The presence of excess carbon, in the form of Ni(2)C, prevents graphene from adopting the preferred 1 × 1 configuration and leads to grain rotation. STM measurements show that residual Ni(2)C domains are present under rotated graphene. Nickel vacancy islands are observed at the periphery of rotated grains and indicate Ni(2)C dissolution after graphene growth. Density functional theory (DFT) calculations predict a very weak (van der Waals type) interaction of graphene with the underlying Ni(2)C, which should facilitate a phase separation of the carbide into metal-supported graphene. These results demonstrate that surface phases such as Ni(2)C can play a major role in the quality of epitaxial graphene.

Electronic structure and imaging contrast of graphene moiré on metals

Realization of graphene moiré superstructures on the surface of 4d and 5d transition metals offers templates with periodically modulated electron density, which is responsible for a number of fascinating effects, including the formation of quantum dots and the site selective adsorption of organic molecules or metal clusters on graphene. Here, applying the combination of scanning probe microscopy/spectroscopy and the density functional theory calculations, we gain a profound insight into the electronic and topographic contributions to the imaging contrast of the epitaxial graphene/Ir(111) system. We show directly that in STM imaging the electronic contribution is prevailing compared to the topographic one. In the force microscopy and spectroscopy experiments we observe a variation of the interaction strength between the tip and high-symmetry places within the graphene moiré supercell, which determine the adsorption sites for molecules or metal clusters on graphene/Ir(111).Realization of graphene moiré superstructures on the surface of 4d and 5d transition metals offers templates with periodically modulated electron density, which is responsible for a number of fascinating effects, including the formation of quantum dots and the site selective adsorption of organic molecules or metal clusters on graphene. Here, applying the combination of scanning probe microscopy/spectroscopy and the density functional theory calculations, we gain a profound insight into the electronic and topographic contributions to the imaging contrast of the epitaxial graphene/Ir(111) system. We show directly that in STM imaging the electronic contribution is prevailing compared to the topographic one. In the force microscopy and spectroscopy experiments we observe a variation of the interaction strength between the tip and high-symmetry places within the graphene moiré supercell, which determine the adsorption cites for molecules or metal clusters on graphene/Ir(111). ∗ Corresponding author. E-mail: Yuriy.Dedkov@specs.com 1 ar X iv :1 21 0. 16 02 v1 [ co nd -m at .m tr lsc i] 4 O ct 2 01 2 Graphene layers on metal surfaces have been attracting the attention of scientists since several decades, starting from middle of the 60s, when the catalytic properties of the closepacked surfaces of transition metals were in the focus of the surface science research [1–4]. The demonstration of the fascinating electronic properties of the free-standing graphene [5, 6], renewed the interest in the graphene/metal systems, which are considered as the main and the most perspective way for the large-scale preparation of high-quality graphene layers with controllable properties [7–10]. For this purpose single-crystalline as well as polycrystalline substrates of 3d− 5d metals can be used. One of the particularly exciting questions concerning the graphene/metal interface is the origin of the bonding mechanism in such systems [2–4, 11]. This graphene-metal puzzle is valid for both cases: graphene adsorption on metallic surfaces as well as for the opposite situation of the metal deposition on the free-standing or substrate-supported graphene. In the latter case the close-packed surfaces of 4d and 5d metals are often used as substrates [12, 13]. A graphene layer prepared on such surfaces, i. e. Ru(0001) [14–16], Rh(111) [13, 17, 18], Ir(111) [19, 20], or Pt(111) [21, 22], forms so-called moiré structures due to the relatively large lattice mismatch between graphene and metal substrates. As a consequence of the lattice mismatch the interaction strength between graphene and the metallic substrate is spatially modulated leading to the spatially periodic electronic structure. Such lateral graphene superlattices are known to exhibit selective absorption for organic molecules [23] or metal clusters [24]. Especially, the adsorption of different metals Ir, Ru, Au, or Pt on graphene/Ir(111) has been intensively studied showing a preferential nucleation around the so-called FCC or HCP high-symmetry positions within the moiré unit cell [12, 25]. In the subsequent works [25, 26] this site-selective adsorption was explained via local sp to sp rehybridization of carbon atoms with the bond formation between graphene and the cluster. However, a fully consistent description of the local electronic structure of graphene/Ir(111), the observed imaging contrast in scanning probe experiments and the bonding mechanism of molecules or clusters on it is still lacking, motivating the present research. Here we present the systematic studies of the graphene/Ir(111) system by means of density functional theory (DFT) calculations and scanning tunnelling and atomic force microscopy (STM and AFM) performed in constant current / constant frequency shift (CC / CFS) and constant height (CH) modes. The obtained results for the graphene/Ir(111) system allow to separate the topographic and electronic contributions in the imaging contrast in

The adsorption of aromatics on sp-metals: benzene on Al 111

Abstract We have studied the adsorption of benzene on Al(111) using angle-resolved ultraviolet photoelectron, high-resolution electron energy loss, and thermal desorption spectroscopies (ARUPS, HREELS, and TDS, respectively), work function measurements, and by density functional theory (DFT) calculations using the ab-initio vasp code. The analysis of ARUPS and HREELS spectra of a benzene monolayer unambiguously indicate C 6v symmetry and a weak benzene–Al interaction in an adsorption geometry with the ring plane parallel to the surface. The weak interaction is confirmed by TDS. The DFT calculations indicate an electrostatic bond and yield an average benzene–Al(111) distance of 3.7 A. A weak minimum of the potential energy is observed at the hollow adsorption position.

Platinum-group and noble metals under oxidizing conditions

Platinum-group metals and noble metals play an important role in catalysis, for total oxidation as well as for partial oxidation reactions. Only in recent years have advances in microscopic, spectroscopic and computer simulation techniques made it possible to investigate the interaction of oxygen with metallic substrates at an atomistic level. We present an overview on the formation of adsorption structures and surface oxides on Rh, Pd, Ag, Cu and Pt surfaces, with particular focus on the phase diagrams calculated from first-principles thermodynamics. The low-index (111), (100) and (110) surfaces as well as selected high-index surfaces have been considered. We predict the stability of novel structures such as the c(4 × 6) on Cu(100) and the α-PtO2 trilayer on Pt(100). The knowledge of the Gibbs free surface energies allows us to predict the adsorbate-induced changes in the thermodynamic equilibrium shape of metal nanoparticles. At low oxygen chemical potential, corresponding to clean surfaces, the (111) facets dominate the particle shape, with a significant contribution from (100) facets. But even under these conditions a small fraction of the overall surface corresponds to more open facets. As oxygen adsorption sets in, their contribution becomes larger. At high oxygen partial pressures, surface oxides form on the platinum-group metals. They do not only display different chemical properties than the metal, but also determine the exposed surface orientations of the particles. The latter effect might play an important role for the catalytic activity of transition metal nanoparticles.

Carbon in palladium catalysts: A metastable carbide

The catalytic activity of palladium toward selective hydrogenation of hydrocarbons depends on the partial pressure of hydrogen. It has been suggested that the reaction proceeds selectively toward partial hydrogenation only when a carbon-rich film is present at the metal surface. On the basis of first-principles simulations, we show that carbon can dissolve into the metal because graphite formation is delayed by the large critical nucleus necessary for graphite nucleation. A bulk carbide Pd(6)C with a hexagonal six-layer fcc-like supercell forms. The structure is characterized by core level shifts of 0.66-0.70 eV in the core states of Pd, in agreement with experimental x-ray photoemission spectra. Moreover, this phase traps bulk-dissolved hydrogen, suppressing the total hydrogenation reaction channel and fostering partial hydrogenation.

A first-principles study of bulk oxide formation on Pd(100)

The catalytic oxidation activity of palladium is influenced by the oxidation state of the metal. Under technologically relevant conditions, bulk and surface oxides may form and decompose. By employing first-principles calculations based on density functional theory, we have investigated the transition from the surface oxide to the bulk oxide on Pd(100). We show that the most stable orientation of the oxide film is PdO(101)@Pd(100) at any film thickness. The monolayer has unique electronic, chemical, and thermodynamic properties in comparison to thicker oxide films. In particular, carbon monoxide adsorbs by approximately 0.3 eV more strongly on thicker oxides than on the surface oxide, a fact that should influence the catalytical activity. Finally, we show that a simple model employing density functional theory energies predicts a Stranski-Krastanov growth mode for the oxide film, with a critical thickness of 1 ML. Our results give a framework for the interpretation of experiments of Pd oxide growth.

Artificially lattice-mismatched graphene/metal interface: Graphene/Ni/Ir(111)

We report the structural and electronic properties of an artificial graphene/Ni(111) system obtained by the intercalation of a monoatomic layer of Ni in graphene/Ir(111). Upon intercalation, Ni grows epitaxially on Ir(111), resulting in a lattice mismatched graphene/Ni system. By performing Scanning Tunneling Microscopy (STM) measurements and Density Functional Theory (DFT) calculations, we show that the intercalated Ni layer leads to a pronounced buckling of the graphene film. At the same time an enhanced interaction is measured by Angle-Resolved Photo-Emission Spectroscopy (ARPES), showing a clear transition from a nearly-undisturbed to a strongly-hybridized graphene -band. A comparison of the intercalation-like graphene system with flat graphene on bulk Ni(111), and mildly corrugated graphene on Ir(111), allows to disentangle the two key properties which lead to the observed increased interaction, namely lattice matching and electronic interaction. Although the latter determines the strength of the hybridization, we find an important influence of the local carbon configuration resulting from the lattice mismatch.

An experimental and theoretical investigation of the thiophene/aluminum interface

The adsorption of thiophene and 2, 2′-bithiophene on Al(111) has been studied using thermal desorption spectroscopy (TDS), angle-resolved UV photoemission (ARUPS), and work function measurements. Ab initio density functional theory calculations have been performed for thiophene on Al(111). Both thiophene and bithiophene bond only very weakly to Al(111), as indicated by TDS and calculations of the thiophene absorption energy, which is found to be only 0.54 eV. There is no indication of π-bonding in either the ARUPS data or the calculations. The calculated S–Al distance, 3.7 A, is much greater than either measured or calculated S–metal distances for covalent bonding. The bonding is shown to be almost entirely electrostatic, with a small contribution from the sulfur lone pair. This is in direct contrast to calculations for Al–thiophene complexes which show covalent bonds between the Al atoms and the thiophene α carbons. The calculations show the molecule to be essentially flat, with a tilt angle of only 2° o...