Showing papers by "Bjørk Hammer published in 2015"

In Situ Detection of Active Edge Sites in Single-Layer MoS2 Catalysts.

Abstract: MoS2 nanoparticles are proven catalysts for processes such as hydrodesulfurization and hydrogen evolution, but unravelling their atomic-scale structure under catalytic working conditions has remained significantly challenging. Ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS) allows us to follow in situ the formation of the catalytically relevant MoS2 edge sites in their active state. The XPS fingerprint is described by independent contributions to the Mo 3d core level spectrum whose relative intensity is sensitive to the thermodynamic conditions. Density Functional Theory (DFT) is used to model the triangular MoS2 particles on Au(111) and identify the particular sulphidation state of the edge sites. A consistent picture emerges in which the core level shifts for the edge Mo atoms evolve counterintuitively toward higher binding energies when the active edges are reduced. The shift is explained by a surprising alteration in the metallic character of the edge sites, which is a distinct spectroscopi...

Growth and electronic structure of epitaxial single-layer WS 2 on Au(111)

Abstract: Large-area single-layer ${\text{WS}}_{2}$ is grown epitaxially on Au(111) using evaporation of W atoms in a low pressure ${\text{H}}_{2}\text{S}$ atmosphere. It is characterized by means of scanning tunneling microscopy, low-energy electron diffraction, and core level spectroscopy. Its electronic band structure is determined by angle-resolved photoemission spectroscopy. The valence-band maximum at $\overline{K}$ is found to be significantly higher than at $\overline{\mathrm{\ensuremath{\Gamma}}}$. The observed dispersion around $\overline{K}$ is in good agreement with density functional theory calculations for a free-standing monolayer, whereas the bands at $\overline{\mathrm{\ensuremath{\Gamma}}}$ are found to be hybridized with states originating from the Au substrate. Strong spin-orbit coupling leads to a large spin-splitting of the bands in the neighborhood of the $\overline{K}$ points, with a maximum splitting of 419(11) meV. The valence-band dispersion around $\overline{K}$ is found to be highly anisotropic with spin-branch dependent effective hole masses of $0.40(02){m}_{e}$ and $0.57(09){m}_{e}$ for the upper and lower split valence band, respectively. The large size of the spin splitting and the low effective mass of the valence-band maximum make single-layer ${\text{WS}}_{2}$ a promising alternative to the widely studied ${\text{MoS}}_{2}$ for applications in electronics, spintronics, and valleytronics.

Isolating a Reaction Intermediate in the Hydrogenation of 2,2,2-Trifluoroacetophenone on Pt(111)

Abstract: The isolation and identification of surface intermediates is of the utmost importance for the elucidation of mechanisms and selectivity patterns in heterogeneous catalysis. However, the metastable nature of reaction intermediates makes their detection and differentiation from other species challenging. This work reports a combined variable temperature scanning tunneling microscopy (VT-STM) and van der Waals-corrected density functional theory (opt88-vdW DFT) study showing that a hydroxy intermediate (hy-TFAP) formed in the hydrogenation of 2,2,2-trifluoroacetophenone (TFAP) is trapped by parent TFAP to form a H-bonded bimolecular TFAP/hy-TFAP structure. The facile formation of the hydroxy intermediate, by residual hydrogen present in the ultrahigh vacuum chamber, was predicted based on a previous DFT study of the hydrogenation pathway for TFAP on Pt(111). The prediction is confirmed by comparison of calculated TFAP/TFAP and TFAP/hy-TFAP structures with STM images of bimolecular structures formed through T...

Selection of conformational states in self-assembled surface structures formed from an oligo(naphthylene–ethynylene) 3-bit binary switch

Abstract: Supra-molecular self-assembly on surfaces often involves molecular conformational flexibility which may act to enrich the variation and complexity of the structures formed. However, systematic and explicit investigations of how molecular conformational states are selected in surface self-assembly processes are relatively scarce. Here, we use a combination of high-resolution scanning tunneling microscopy and Density Functional Theory (DFT) calculations to investigate self-assembly for a custom-designed molecule capable of assuming eight distinct surface conformations (four enantiomeric pairs). The conformations result from binary positions of n = 3 naphtalene units on a linear oligo(naphthylene-ethynylene) backbone. On Au(111), inter-molecular interactions involving carboxyl and bulky tert-butyl-phenyl functional groups induce the molecules to form two ordered phases with brick-wall and lamella structure, respectively. These structures each involve molecules in two conformational states, and there is a clear separation between the conformers involved in the two types of structures. On Cu(111), individual molecules isolated by carboxylate-substrate binding show a distribution involving all possible conformational states. Together these observations imply selection and adaptation of conformational states upon molecular self-assembly. From DFT modeling and statistical analysis of the molecular conformations, the observed selection of conformational states is attributed to steric interaction between the naphthalene units. The present study enhances our understanding of how ordering and selection of molecular conformations is controlled by intermolecular interactions in a complex situation with many distinct conformational states for the participating molecules.