Atomic Structure and Special Reactivity Toward Methanol Oxidation of Vanadia Nanoclusters on TiO₂(110)

We have grown highly controlled VO_<i>x</i> nanoclusters on rutile TiO₂(110). The combination of photoemission and photoelectron diffraction techniques based on synchrotron radiation with DFT calculations has allowed identifying these nanostructures as exotic V₄O₆ nanoclusters, which hold vanadyl groups, even if vanadium oxidation state is formally +3. Our theoretical investigation also indicates that on the surface of titania, vanadia mononuclear species, with oxidation states ranging from +2 to +4, can be strongly stabilized by aggregation into tetramers that are characterized by a charge transfer to the titania substrate and a consequent decrease of the electron density in the vanadium 3d levels. We then performed temperature programmed desorption experiments using methanol as probe molecule to understand the impact of these unusual electronic and structural properties on the chemical reactivity, obtaining that the V₄O₆ nanoclusters can selectively convert methanol to formaldehyde at an unprecedented low temperature (300 K)

TiO₂(110) Charge Donation to an Extended π‑Conjugated Molecule

The surface reduction of rutile TiO₂(110) generates a state in the band gap whose excess electrons are spread among multiple sites, making the surface conductive and reactive. The charge extraction, hence the surface catalytic properties, depends critically on the spatial extent of the charge redistribution, which has been hitherto probed by small molecules that recombine at oxygen vacancy (O_vac) sites. We demonstrate by valence band resonant photoemission (RESPES) a very general charge extraction mechanism from a reduced TiO₂(110) surface to an extended electron-acceptor organic molecule. Perylene-tetra-carboxylic-diimide (PTCDI) is not trapped at O_vac sites and forms a closely packed, planar layer on TiO₂(110). In this configuration, the perylene core spills out the substrate excess electrons, filling the lowest unoccupied molecular orbital (LUMO). The charge transfer from the reduced surface to an extended π-conjugated system demonstrates the universality of the injection/extraction mechanism, opening new perspectives for the coupling of reducible oxides to organic semiconductors and supported catalysts

Lattice Mismatch Drives Spatial Modulation of Corannulene Tilt on Ag(111)

We investigated the adsorption of corannulene (C₂₀H₁₀) on the Ag(111) surface by experimental and simulated scanning tunneling microscopy (STM), X-ray photoemission (XPS), and near-edge X-ray absorption fine structure (NEXAFS). Structural optimizations of the adsorbed molecules were performed by density functional theory (DFT) and the core excited spectra evaluated within the transition-potential approach. Corannulene is physisorbed in a bowl-up orientation displaying a very high mobility (diffusing) and dynamics (tilting and spinning) at room temperature. At the monolayer saturation coverage, molecules order into a close-compact phase with an average intermolecular spacing of ∼10.5 ± 0.3 Å. The lattice mismatch drives a long wavelength structural modulation of the molecular rows, which, however, could not be identified with a specific superlattice periodicity. DFT calculations indicate that the structural and spectroscopic properties are intermediate between those predicted for the limiting cases of an on-hexagon geometry (with a 3-fold, ∼8.6 Å unit mesh) and an on-pentagon geometry (with a 4-fold, ∼11.5 Å unit mesh). We suggest that molecules smoothly change their equilibrium configuration along the observed long wavelength modulation of the molecular rows by varying their tilt and azimuth in between the geometric constraints calculated for molecules in the 3-fold and 4-fold phases

Lattice Mismatch Drives Spatial Modulation of Corannulene Tilt on Ag(111)

We investigated the adsorption of corannulene (C₂₀H₁₀) on the Ag(111) surface by experimental and simulated scanning tunneling microscopy (STM), X-ray photoemission (XPS), and near-edge X-ray absorption fine structure (NEXAFS). Structural optimizations of the adsorbed molecules were performed by density functional theory (DFT) and the core excited spectra evaluated within the transition-potential approach. Corannulene is physisorbed in a bowl-up orientation displaying a very high mobility (diffusing) and dynamics (tilting and spinning) at room temperature. At the monolayer saturation coverage, molecules order into a close-compact phase with an average intermolecular spacing of ∼10.5 ± 0.3 Å. The lattice mismatch drives a long wavelength structural modulation of the molecular rows, which, however, could not be identified with a specific superlattice periodicity. DFT calculations indicate that the structural and spectroscopic properties are intermediate between those predicted for the limiting cases of an on-hexagon geometry (with a 3-fold, ∼8.6 Å unit mesh) and an on-pentagon geometry (with a 4-fold, ∼11.5 Å unit mesh). We suggest that molecules smoothly change their equilibrium configuration along the observed long wavelength modulation of the molecular rows by varying their tilt and azimuth in between the geometric constraints calculated for molecules in the 3-fold and 4-fold phases

Symmetry, Shape, and Energy Variations in Frontier Molecular Orbitals at Organic/Metal Interfaces: The Case of F₄TCNQ

Near edge X-ray absorption, valence and core-level photoemission, and density functional theory calculations are used to study molecular levels of tetracyano-2,3,5,6-tetrafluoroquinodimethane (F₄TCNQ) deposited on Ag(111) and BiAg₂/Ag­(111). The high electron affinity of F₄TCNQ triggers a large static charge transfer from the substrate, and, more interestingly, hybridization with the substrate leads to a radical change of symmetry, shape, and energy of frontier molecular orbitals. The lowest unoccupied molecular orbital (LUMO) shifts below the Fermi energy, becoming the new highest occupied molecular orbital (<i>n</i>-HOMO), whereas the <i>n</i>-LUMO is defined by a hybrid band with mixed π* and σ* symmetries, localized at quinone rings and cyano groups, respectively. The presence of Bi influences the way the molecule contacts the substrate with the cyano group. The molecule/surface distance is closer and the bond more extended over substrate atoms in F₄TCNQ/Ag­(111), whereas in F₄TCNQ/BiAg₂/Ag­(111) the distance is larger and the contact more localized on top of Bi. This does not significantly alter molecular levels, but it causes the respective absence or presence of optical excitations in F₄TCNQ core-level spectra

Intermolecular Hydrogen Bonding and Molecular Orbital Distortion in 4‑Hydroxycyanobenzene Investigated by X‑ray Spectroscopy

Electronic structure of 4-hydroxycyanobenzene in the gas phase, thick films, and single crystals has been investigated by X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). We have used resonant photoemission spectroscopy (RESPES) to identify the symmetry and atomic localization of the occupied and unoccupied molecular orbitals for the free molecule. Upon condensation into a thick film, we find XPS energy shifts in opposite directions for the oxygen and nitrogen core levels, consistent with the formation of an intermolecular hydrogen bond. This interaction is also accompanied by a significant spatial distortion of the lowest unoccupied molecular orbital that is displaced from the nitrogen atom, as indicated by the RESPES measurements. Thick films and single crystals display the same dichroism in polarization dependent NEXAFS, indicating that the intermolecular hydrogen bonding also steers the molecular assembly into a preferred molecular orientation

Chemistry of the Methylamine Termination at a Gold Surface: From Autorecognition to Condensation

13The self-assembly of the naphthylmethylamine molecules (NMA) on the Au(111) surface is investigated by a combined experimental and theoretical approach. Three well-defined phases are observed upon different thermal treatments at the monolayer stage. The role played by the methylamine termination is evidenced in both the molecule–molecule and molecule–substrate interactions. The autorecognition process of the amino groups is identified as the driving factor for the formation of a complex hydrogen bonding scheme in small molecular clusters, possibly acting also as a precursor of a denitrogenation condensation process induced by thermal annealing.reservedmixedDri, Carlo; Fronzoni, Giovanna; Balducci, Gabriele; Furlan, Sara; Stener, Mauro; Feng, Zhijing; Comelli, Giovanni; Castellarin-Cudia, Carla; Cvetko, Dean; Kladnik, Gregor; Verdini, Alberto; Floreano, Luca; Cossaro, AlbanoDri, Carlo; Fronzoni, Giovanna; Balducci, Gabriele; Furlan, Sara; Stener, Mauro; Feng, Zhijing; Comelli, Giovanni; Castellarin Cudia, Carla; Cvetko, Dean; Kladnik, Gregor; Verdini, Alberto; Floreano, Luca; Cossaro, Alban

Noncontact Layer Stabilization of Azafullerene Radicals: Route toward High-Spin-Density Surfaces

We deposit azafullerene C59N• radicals in a vacuum on the Au(111) surface for layer thicknesses between 0.35 and 2.1 monolayers (ML). The layers are characterized using X-ray photoemission (XPS) and X-ray absorption fine structure (NEXAFS) spectroscopy, low-temperature scanning tunneling microscopy (STM), and by density functional calculations (DFT). The singly unoccupied C59N orbital (SUMO) has been identified in the N 1s NEXAFS/XPS spectra of C59N layers as a spectroscopic fingerprint of the molecular radical state. At low molecular coverages (up to 1 ML), films of monomeric C59N are stabilized with the nonbonded carbon orbital neighboring the nitrogen oriented toward the Au substrate, whereas in-plane intermolecular coupling into diamagnetic (C59N)2 dimers takes over toward the completion of the second layer. By following the C59N• SUMO peak intensity with increasing molecular coverage, we identify an intermediate high-spin-density phase between 1 and 2 ML, where uncoupled C59N• monomers in the second layer with pronounced radical character are formed. We argue that the C59N• radical stabilization of this supramonolayer phase of monomers is achieved by suppressed coupling to the substrate. This results from molecular isolation on top of the passivating azafullerene contact layer, which can be explored for molecular radical state stabilization and positioning on solid substrates

Understanding Energy-Level Alignment in Donor–Acceptor/Metal Interfaces from Core-Level Shifts

The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donor–acceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donor–acceptor stoichiometry in the contact blend