Showing papers by "Bjørk Hammer published in 2012"
TL;DR: An optimized genetic algorithm used in conjunction with density-functional theory in the search for stable gold clusters and O2 adsorption ensembles in F centers at MgO(100) recovers known structures and identifies several more stable ones.
Abstract: We present an optimized genetic algorithm used in conjunction with density-functional theory in the search for stable gold clusters and O2 adsorption ensembles in F centers at MgO(100). For Au8 the method recovers known structures and identifies several more stable ones. When O2 adsorption is investigated, the genetic algorithm is used to imitate structural fluxionality, increasing the O2 bond strength by up to 1 eV. Extending the method to Au(6,10,12), strong O2 adsorption configurations are found for all sizes. However, the effect of fluxionality appears to wear off with increasing cluster size.
117 citations
TL;DR: Annealing experiments and density functional theory calculations demonstrate that the basal plane of the graphene stays intact and point to a graphene-mediated mechanism for the H-induced lifting of the reconstruction.
Abstract: The limitations of graphene as an effective corrosion-inhibiting coating on metal surfaces, here exemplified by the hex-reconstructed Pt(100) surface, are probed by scanning tunneling microscopy measurements and density functional theory calculations. While exposure of small molecules directly onto the Pt(100) surface will lift the reconstruction, a single graphene layer is observed to act as an effective coating, protecting the reactive surface from O2 exposure and thus preserving the reconstruction underneath the graphene layer in O2 pressures as high as 10–4 mbar. A similar protective effect against CO is observed at CO pressures below 10–6 mbar. However, at higher pressures CO is observed to intercalate under the graphene coating layer, thus lifting the reconstruction. The limitations of the coating effect are further tested by exposure to hot atomic hydrogen. While the coating can withstand these extreme conditions for a limited amount of time, after substantial exposure, the Pt(100) reconstruction i...
94 citations
TL;DR: In this paper, mass spectrometer measurements show the formation of highly super-hydrogenated derivatives of the polycyclic aromatic hydrocarbon molecule coronene through H atom addition reactions.
Abstract: Mass spectrometry measurements show the formation of highly superhydrogenated derivatives of the polycyclic aromatic hydrocarbon molecule coronene through H atom addition reactions. The observed product mass distribution provides evidence also for abstraction reactions resulting in H2 formation, in agreement with recent IR measurements. Complementary density functional theory calculations confirm the stability of the observed superhydrogenated species toward spontaneous H and H2 loss indicating that abstraction reactions may be the dominant route to H2 formation involving neutral polycyclic aromatic hydrocarbons (PAHs). The results indicate that highly superhydrogenated PAHs could well be formed and could act as efficient catalysts for H2 formation in the interstellar medium in low UV flux regions.
79 citations
TL;DR: In this paper, the interaction of graphene with various metal surfaces is investigated using density functional theory and the meta-generalized gradient approximation (MGGA) M06-L functional.
Abstract: The interaction of graphene with various metal surfaces is investigated using density functional theory and the meta-generalized gradient approximation (MGGA) M06-L functional. We demonstrate that this method is of comparable accuracy to the random-phase approximation (RPA). With M06-L we study large systems inaccessible to RPA with H adsorbed on graphene on a selected strongly (Ni) and a selected weakly (Pt) interacting substrate. Very stable graphane-like clusters, where every other C atom binds to a H atom above and every other to a metal atom below, are found on both substrates. Such graphane-like clusters have been proposed to be responsible for opening a band gap in graphene. On Ni we find that the binding energies of the H clusters are almost constant with the cluster size, whereas on Pt the binding energies increase with the cluster size. Comparing the Perdew-Burke-Ernzerhof and M06-L functionals we demonstrate the importance of accounting for dispersive interactions.
62 citations
TL;DR: In this article, a density functional theory study of the structure and stability of self-assembled monolayers (SAMs) of alkylthiolate on Au(111) was presented.
Abstract: We present a density functional theory study of the structure and stability of self-assembled monolayers (SAMs) of alkylthiolate on Au(111) as a function of the alkyl chain length. The most favorable structure of the SAMs involves an RS−Au−SR complex (S being sulfur, R being an alkyl chain) formed through sandwiching one Au adatom by two alkylthiolates (RSs). Comparing a generalized gradient (GGA-PBE) and a meta-GGA (MGGA-M06-L) exchange-correlation functional we find that only the meta-GGA functional predicts the experimentally observed attractive intermolecular interactions within the SAMs. In particular, the use of M06-L yields an increased stability of the SAMs with increasing alkyl chain length and an increased attractive interaction between RS−Au−SR complexes at shorter distances.
47 citations
TL;DR: The diffusion of ethanol molecules perpendicular to the row of surface Ti atoms was found to be mediated by H adatoms in the rows of bridge-bonded O (Obr) atoms similarly to previous results obtained for water monomers.
Abstract: We have studied the diffusion of ethanol on rutile TiO2(110)–(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. Time-lapsed STM images recorded at ∼200 K revealed the diffusion of ethanol molecules both parallel and perpendicular to the rows of surface Ti atoms. The diffusion of ethanol molecules perpendicular to the rows of surface Ti atoms was found to be mediated by H adatoms in the rows of bridge-bonded O (Obr) atoms similarly to previous results obtained for water monomers. In contrast, the diffusion of H adatoms across the Ti rows, mediated by ethanol molecules, was observed only very rarely and exclusively on fully hydrogenated TiO2(110) surfaces. Possible reasons why the diffusion of H adatoms across the Ti rows mediated by ethanol molecules occurs less frequently than the cross-row diffusion of ethanol molecules mediated by H adatoms are discussed.
33 citations
TL;DR: In this article, it was shown that a layer of graphene can be grown on the hex-reconstructed Pt(100) surface and that the reconstruction is preserved after growth.
23 citations
TL;DR: It is argued that strands can form particularly easy on stepped surfaces because building material is available at step sites and these adstructures are shown to be more stable than an equivalent amount of bulk defects such as Ti interstitials.
Abstract: We have studied vicinal ${\mathrm{TiO}}_{2}(110)$ surfaces by high-resolution scanning tunneling microscopy and density functional theory calculations. On ${\mathrm{TiO}}_{2}$ surfaces characterized by a high density of $⟨1\overline{1}1⟩$ steps, scanning tunneling microscopy reveals a high density of oxygen-deficient strandlike adstructures. With the help of density functional theory calculations we develop a complete structural model for the entire strand and demonstrate these adstructures to be more stable than an equivalent amount of bulk defects such as Ti interstitials. We argue that strands can form particularly easy on stepped surfaces because building material is available at step sites. The strands on ${\mathrm{TiO}}_{2}(110)$ represent point defects that are densely packed into ordered adstructures.
22 citations
TL;DR: The adsorption of benzaldehyde is investigated using density functional theory, and Benzaldehyde is found to chemisorb preferentially with its aromatic ring in the flat-lying bridge geometry that is also preferred for benzene.
Abstract: Building on results for the adsorption of benzene on Pt(111), the adsorption of benzaldehyde is investigated using density functional theory. Benzaldehyde is found to chemisorb preferentially with its aromatic ring in the flat-lying bridge geometry that is also preferred for benzene. Across the investigated geometries, adsorption is homogeneously weakened compared to corresponding benzene geometries. This is found to be true for very different adsorption modes, namely, η6 and η8 modes, the latter having metal atoms inserted in the carbonyl bond. Reorientation and diffusion of benzaldehyde is found to have low energy barriers. Aggregation of molecules in dimers bound by aryl C–H⋯O hydrogen bonds is investigated, and specific configurations are found to be up to 0.15 eV more favorable than optimally configured, separated adsorbates. The binding is significantly stronger than what is found for gas phase dimers, suggesting an enhancing effect of the metal interaction.
20 citations
TL;DR: In this article, a method to induce linear structures of hydrogen adsorbates on graphite by covering the surface with a self-assembled molecular monolayer of cyanuric acid and exposing it to atomic hydrogen is presented.
11 citations
TL;DR: The hydrogenation of a prochiral substrate, 2,2,2-trifluoroacetophenone (TFAP), on Pt(111) was studied using room-temperature scanning tunneling microscopy (STM) measurements and some monomers display an extra protrusion compared to TFAP in dimer structures.
Abstract: The hydrogenation of a prochiral substrate, 2,2,2-trifluoroacetophenone (TFAP), on Pt(111) was studied using room-temperature scanning tunneling microscopy (STM) measurements. The experiments were carried out both on a clean surface and on a chirally modified surface, using chemisorbed (R)-(+)-1-(1-naphthyl)ethylamine, ((R)-NEA), as the modifier. On the nonmodified surface, introduction of H2 at a background pressure of ∼1 × 10–6 mbar leads to the rapid break-up of TFAP dimer structures followed by the gradual removal of all TFAP-related images. During the latter step, some monomers display an extra protrusion compared to TFAP in dimer structures. They are attributed to a half-hydrogenated intermediate. The introduction of H2 to a mixture of (R)-NEA and TFAP on Pt(111) leads to the removal of TFAP without any change in the population of the modifier, as required for an efficient chirally modified catalyst.