Zinc(II) Tetraphenylporphyrin on Ag(100) and Ag(111): Multilayer Desorption and Dehydrogenation

The interactions between zinc­(II) tetraphenylporphyrin (ZnTPP) molecules and the Ag(100) and Ag(111) surfaces were investigated using a combination of scanning tunneling microscopy as a local probe of the molecular adsorption configuration and X-ray, ultraviolet, and inverse photoemission spectroscopies as probes of the electronic structure. For each surface, a monolayer of ZnTPP, formed by multilayer desorption, exhibits a highly ordered structure in registry with the underlying surface lattice. Subsequent annealing leads to a transition from intact molecular adsorption to dehydrogenation and subsequent rehybridization. This rehybridization is both intramolecular, with a flattening of the molecules and a measurable alteration of the electronic structure, and intermolecular, leading to two-dimensional growth of extended covalently bound structures

Conversion Reaction of CoO Polycrystalline Thin Films Exposed to Atomic Lithium

We have studied the reaction of 5 nm thick polycrystalline CoO films with atomic lithium as a model for the discharge of lithium-ion conversion battery electrodes. The electronic structure has been investigated with X-ray photoemission, ultraviolet photoemission, and inverse photoemission, while the morphology, crystal structure, and unoccupied states of the films have been examined with transmission electron microscopy. It is found that exposure to atomic lithium leads, at room temperature, to partial conversion with formation of Co and Li₂O, but also of a Li₂O₂ or LiOH overlayer at the surface of the sample. As full conversion was obtained at 150 °C, a comparison with room-temperature measurements enables the understanding of the kinetic limitations during lithiation

Chemical Interaction, Space-Charge Layer, and Molecule Charging Energy for a TiO₂/TCNQ Interface

Three driving forces control the energy level alignment between transition-metal oxides and organic materials: the chemical interaction between the two materials, the organic electronegativity, and the possible space charge layer formed in the oxide. This is illustrated in this study by analyzing experimentally and theoretically a paradigmatic case, the TiO₂(110)/TCNQ interface; due to the chemical interaction between the two materials, the organic electron affinity level is located below the Fermi energy of the <i>n</i>-doped TiO₂. Then, one electron is transferred from the oxide to this level and a space charge layer is developed in the oxide, inducing an important increase in the interface dipole and in the oxide work function

Tuning Energy Level Alignment At Organic/Semiconductor Interfaces Using a Built-In Dipole in Chromophore–Bridge–Anchor Compounds

A chromophore–bridge–anchor molecular architecture is used to manipulate the molecular level energy position, with respect to the band edges of the substrate, of a chromophore bound to a surface via an anchor group. An energy shift of the chromophore’s frontier orbitals is induced by the addition of an oriented molecular dipole into the bridge part of the compound. This principle has been tested using three Zinc Tetraphenylporphyrin derivatives of comparable structure: two of which possess a dipole, but pointing in opposite directions and, for comparison, a compound without a dipole. UV–vis absorption and emission spectroscopies have been used to probe the electronic structure of the compounds in solution, while UV photoemission spectroscopy has been used to measure the relative position of the molecular levels of the chromophore with respect to the band edges of a ZnO(11–20) single crystal substrate. It is shown that the introduction of a molecular dipole does not alter the chromophore’s HOMO–LUMO gap, and that the molecular level alignment of the compounds bound to the ZnO surface follows the behavior predicted by a simple parallel-plate capacitor model

Increasing Photocurrents in Dye Sensitized Solar Cells with Tantalum-Doped Titanium Oxide Photoanodes Obtained by Laser Ablation

Laser ablation is employed to produce vertically aligned nanostructured films of undoped and tantalum-doped TiO₂ nanoparticles. Dye-sensitized solar cells using the two different materials are compared. Tantalum-doped TiO₂ photoanode show 65% increase in photocurrents and around 39% improvement in overall cell efficiency compared to undoped TiO₂. Electrochemical impedance spectroscopy, Mott–Schottky analysis and open circuit voltage decay is used to investigate the cause of this improved performance. The enhanced performance is attributed to a combination of increased electron concentration in the semiconductor and a reduced electron recombination rate

Nanoscale Internal Fields in a Biased Graphene–​Insulator–​Semiconductor Structure

Measuring and understanding electric fields in multilayered materials at the nanoscale remains a challenging problem impeding the development of novel devices. At this scale, it is far from obvious that materials can be accurately described by their intrinsic bulk properties, and considerations of the interfaces between layered materials become unavoidable for a complete description of the system’s electronic properties. Here, a general approach to the direct measurement of nanoscale internal fields is proposed. Small spot X-ray photoemission was performed on a biased graphene/SiO₂/Si structure in order to experimentally determine the potential profile across the system, including discontinuities at the interfaces. Core levels provide a measure of the local potential and are used to reconstruct the potential profile as a function of the depth through the stack. It is found that each interface plays a critical role in establishing the potential across the dielectric, and the origin of the potential discontinuities at each interface is discussed

Concentration and Surface Chemistry Dependent Analyte Orientation on Nanoparticle Surfaces

The development of surface-enhanced Raman spectroscopy (SERS)-based sensors necessitates a deeper understanding of the analyte–nanoparticle interaction. For optimal reliability, factors that may affect the resulting spectra need to be understood. First and foremost, the signal enhancement (and hence the improved sensitivity) offered by these systems highly relies on the localization of molecules or moieties in molecules as close as possible to the nanoparticle surface and decreases the farther a molecule is from the surface. Furthermore, the relative peak intensity, and thus the possibility to rely on a specific peak (or set of peaks) to build a calibration curve, depends on the orientation of the molecule with respect to the metallic surface due to the tensorial nature of the Raman polarizability. As a consequence, a change in analyte orientation on a nanoparticle surface impacts the resulting spectral pattern. Herein, factors that affect analyte orientation on a nanoparticle surface and their effect on the resulting SERS spectra are investigated. To do so, two unique nanostar morphologies and three analytes were selected. SERS spectra were acquired at varying analyte concentrations, and deconvoluted. X-ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations were conducted to confirm the hypothesized adsorbate/nanostars environment. Our study reveals three factors theorized to impact the molecular orientations: (1) analyte concentration, (2) nanoparticle surface properties, and (3) analyte–nanoparticle bond nature. Results herein suggest that when the analyte concentration is sufficiently high, the molecules reorient from parallel to perpendicular or remain perpendicular relative to the nanoparticle surface compared to the situation at low concentration. The way in which the analyte and nanoparticle interact (e.g., physisorb or chemisorb) will determine the preferred analyte orientation at low concentration. If covalently bound, this preliminary orientation is believed to be dictated by the preferred bond angle between surface and bound moiety. If physisorbed, the analyte will be parallel relative to the nanostar surface at low concentrations and then reorient perpendicular at increased concentrations. The work presented here, explaining in detail the concentration-dependent nature of the analyte orientation, will aid in the development of more reliable SERS sensors

Symmetry-Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics

Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis­(dodecachloro-5-mesityldipyrrinato)­zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C₆₀-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C₆₀ forms a CT state of 1.45 ± 0.05 eV in a DBP/C₆₀-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between <i>E</i>_CT and <i>qV</i>_OC of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices

A Sensitized Nb₂O₅ Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell

Orthorhombic Nb₂O₅ nanocrystalline films functionalized with [Ru­(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺ were used as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. A set of experiments to establish key propertiesconduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiencywere undertaken to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors. We have found that, for a T-phase orthorhombic Nb₂O₅ nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO₂. Anatase TiO₂ has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb₂O₅ is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage (<i>V</i>_oc) for orthorhombic Nb₂O₅, compared to anatase TiO₂. In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA^4–) added as a reductive scavenger, H₂ quantum yield and photostability measurements show that Nb₂O₅ is comparable, but not superior, to TiO₂