Effects of Thermal Fluctuations on the Structure, Level Alignment, and Absorption Spectrum of Dye-Sensitized TiO₂: A Comparative Study of Catechol and Isonicotinic Acid on the Anatase (101) and Rutile (110) Surfaces

The adsorption of catechol and isonicotinic acid on the TiO₂ anatase (101) and rutile (110) surfaces has been studied by means of first-principles molecular dynamics simulations and time-dependent density functional calculations. Our results show that thermal fluctuations induce changes in the position of the molecular levels around the TiO₂ valence band edge. For the anatase (101) surface, the alignment of the molecular levels with the TiO₂ valence band edge has a significant effect on the absorption spectrum. For rutile (110), instead, the adsorption of catechol and isonicotinic acid induces only a minor sensitization. The sensitization of anatase (101) by catechol and isonicotinic acid can be enhanced by increasing the hybridization between the adsorbed dye and TiO₂ states

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

Electronic States of Silicene Allotropes on Ag(111)

Silicene, a honeycomb lattice of silicon, presents a particular case of allotropism on Ag(111). Silicene forms multiple structures with alike in-plane geometry but different out-of-plane atomic buckling and registry to the substrate. Angle-resolved photoemission and first-principles calculations show that these silicene structures, with (4×4), (√13×√13)<i>R</i>13.9°, and (2√3×2√3)<i>R</i>30° lattice periodicity, display similar electronic bands despite the structural differences. In all cases the interaction with the substrate modifies the electronic states, which significantly differ from those of free-standing silicene. Complex photoemission patterns arise from surface umklapp processes, varying according to the periodicity of the silicene allotropes

Electronic States of Silicene Allotropes on Ag(111)

Silicene, a honeycomb lattice of silicon, presents a particular case of allotropism on Ag(111). Silicene forms multiple structures with alike in-plane geometry but different out-of-plane atomic buckling and registry to the substrate. Angle-resolved photoemission and first-principles calculations show that these silicene structures, with (4×4), (√13×√13)<i>R</i>13.9°, and (2√3×2√3)<i>R</i>30° lattice periodicity, display similar electronic bands despite the structural differences. In all cases the interaction with the substrate modifies the electronic states, which significantly differ from those of free-standing silicene. Complex photoemission patterns arise from surface umklapp processes, varying according to the periodicity of the silicene allotropes

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

Programming Hierarchical Supramolecular Nanostructures by Molecular Design

Supramolecular nanostructures with tunable dimensionalities are fabricated by deposition of benzene–carboxylic acids on the Cu(110) surface. By tailoring the number and position of the functional moieties, the structure of the final molecular assemblies can be rationally modified ranging from isolated one-dimensional chains to compact two-dimensional islands. Molecular units are chosen that can assemble through metal–organic and electrostatic interactions. The hierarchy between these intermolecular forces guarantees that a primary organization level, constituted by metal–organic polymeric chains, is developed by all molecular units while the secondary interchain interactions can be arbitrarily adjusted. Scanning tunneling microscopy, density functional theory calculations, and kinetic Monte Carlo simulations are used to characterize and rationalize the experimental findings

Complex Stoichiometry-Dependent Reordering of 3,4,9,10-Perylenetetracarboxylic Dianhydride on Ag(111) upon K Intercalation

Alkali metal atoms are frequently used for simple yet efficient n-type doping of organic semiconductors and as an ingredient of the recently discovered polycyclic aromatic hydrocarbon superconductors. However, the incorporation of dopants from the gas phase into molecular crystal structures needs to be controlled and well understood in order to optimize the electronic properties (charge carrier density and mobility) of the target material. Here, we report that potassium intercalation into the pristine 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) monolayer domains on a Ag(111) substrate induces distinct stoichiometry-dependent structural reordering processes, resulting in highly ordered and large K_<i>x</i>PTCDA domains. The emerging structures are analyzed by low-temperature scanning tunneling microscopy, scanning tunneling hydrogen microscopy (ST­[H]­M), and low-energy electron diffraction as a function of the stoichiometry. The analysis of the measurements is corroborated by density functional theory calculations. These turn out to be essential for a correct interpretation of the experimental ST­[H]­M data. The epitaxy types for all intercalated stages are determined as point-on-line. The K atoms adsorb in the vicinity of the oxygen atoms of the PTCDA molecules, and their positions are determined with sub-Ångström precision. This is a crucial prerequisite for the prospective assessment of the electronic properties of such composite films, as they depend rather sensitively on the mutual alignment between donor atoms and acceptor molecules. Our results demonstrate that only the combination of experimental and theoretical approaches allows for an unambiguous explanation of the pronounced reordering of K_<i>x</i>PTCDA/Ag­(111) upon changing the K content

Fully Atomistic Understanding of the Electronic and Optical Properties of a Prototypical Doped Charge-Transfer Interface

The current study generates profound atomistic insights into doping-induced changes of the optical and electronic properties of the prototypical PTCDA/Ag(111) interface. For doping K atoms are used, as K_<i>x</i>PTCDA/Ag­(111) has the distinct advantage of forming well-defined stoichiometric phases. To arrive at a conclusive, unambiguous, and fully atomistic understanding of the interface properties, we combine state-of-the-art density-functional theory calculations with optical differential reflectance data, photoelectron spectra, and X-ray standing wave measurements. In combination with the full structural characterization of the K_<i>x</i>PTCDA/Ag­(111) interface by low-energy electron diffraction and scanning tunneling microscopy experiments (<i>ACS Nano</i> <b>2016</b>, <i>10</i>, 2365–2374), the present comprehensive study provides access to a fully characterized reference system for a well-defined metal–organic interface in the presence of dopant atoms, which can serve as an ideal benchmark for future research and applications. The combination of the employed complementary techniques allows us to understand the peculiarities of the optical spectra of K₂PTCDA/Ag­(111) and their counterintuitive similarity to those of neutral PTCDA layers. They also clearly describe the transition from a metallic character of the (pristine) adsorbed PTCDA layer on Ag(111) to a semiconducting state upon doping, which is the opposite of the effect (degenerate) doping usually has on semiconducting materials. All experimental and theoretical efforts also unanimously reveal a reduced electronic coupling between the adsorbate and the substrate, which goes hand in hand with an increasing adsorption distance of the PTCDA molecules caused by a bending of their carboxylic oxygens away from the substrate and toward the potassium atoms