ultrafast electron injection into photo excited organic molecules

State-of-the-art X-ray spectroscopy allows femtosecond gating of energy levels of photo-excited molecules on a metal substrate enabling ultrafast and bi-directional charge transfer across the interface with controllable dependence on the molecular adsorption geometry

Making ARPES Measurements on Corrugated Monolayer Crystals: Suspended Exfoliated Single-Crystal Graphene

Free-standing exfoliated monolayer graphene is an ultra-thin flexible membrane, which exhibits out of plane deformation or corrugation. In this paper, a technique is described to measure the band structure of such free-standing graphene by angle-resolved photoemission. Our results show that photoelectron coherence is limited by the crystal corrugation. However, by combining surface morphology measurements of the graphene roughness with angle-resolved photoemission, energy dependent quasiparticle lifetime and bandstructure measurements can be extracted. Our measurements rely on our development of an analytical formulation for relating the crystal corrugation to the photoemission linewidth. Our ARPES measurements show that, despite significant deviation from planarity of the crystal, the electronic structure of exfoliated suspended graphene is nearly that of ideal, undoped graphene; we measure the Dirac point to be within 25 meV of $E_F$ . Further, we show that suspended graphene behaves as a marginal Fermi-liquid, with a quasiparticle lifetime which scales as $(E - E_F)^{-1}$; comparison with other graphene and graphite data is discussed

Length-Independent Charge Transport in Chimeric Molecular Wires

Advanced molecular electronic components remain vital for the next generation of miniaturized integrated circuits. Thus, much research effort has been devoted to the discovery of lossless molecular wires, for which the charge transport rate or conductivity is not attenuated with length in the tunneling regime. Herein, we report the synthesis and electrochemical interrogation of DNA-like molecular wires. We determine that the rate of electron transfer through these constructs is independent of their length and propose a plausible mechanism to explain our findings. The reported approach holds relevance for the development of high-performance molecular electronic components and the fundamental study of charge transport phenomena in organic semiconductors

Electronic properties of the boroxine–gold interface: evidence of ultra-fast charge delocalization

We performed a combined experimental and theoretical study of the assembly of phenylboronic acid on the Au(111) surface, which is found to lead to the formation of triphenylboroxines by spontaneous condensation of trimers of molecules. The interface between the boroxine group and the gold surface has been characterized in terms of its electronic properties, revealing the existence of an ultra-fast charge delocalization channel in the proximity of the oxygen atoms of the heterocyclic group. More specifically, the DFT calculations show the presence of an unoccupied electronic state localized on both the oxygen atoms of the adsorbed triphenylboroxine and the gold atoms of the topmost layer. By means of resonant Auger electron spectroscopy, we demonstrate that this interface state represents an ultra-fast charge delocalization channel. Boroxine groups are among the most widely adopted building blocks in the synthesis of covalent organic frameworks on surfaces. Our findings indicate that such systems, typically employed as templates for the growth of organic films, can also act as active interlayers that provide an efficient electronic transport channel bridging the inorganic substrate and organic overlayer

Determination of the valence band edge of Fe oxide nanoparticles dispersed in aqueous solution through resonant photoelectron spectroscopy from a liquid microjet

We use X-ray photoemission and a near ambient pressure with a liquid microjet setup to investigate the electronic structure of FeOOH nanoparticles dispersed in aqueous solution. In particular, we show that by using X-ray resonant photoemission in dilute solutions, we can overcome the limits of conventional photoemission such as low nanoparticle-to-solvent signal ratio, and local nanoparticle charging and measure the valence band structure of FeOOH nanoparticles in aqueous solution with chemical specificity. The resonant photoemission signal across the Fe 2p3/2 absorption edge is measured for 2 wt% aqueous solutions of FeOOH nanoparticles (NPs) and the valence band maximum (VBM) of the hydrated FeOOH nanoparticles is determined. We compare the obtained VBM value in aqueous solution to that of FeOOH NPs in the dry phase. We show that the valence band edge position of NPs in the liquid phase can be accurately predicted from the values obtained in the dry phase provided that a simple potential shift due to solution chemistry is applied. Our results demonstrate the suitability of resonant photoemission in measuring the electronic structure of strongly diluted nanosystems where the conventional non-resonant photoemission technique fails.ISSN:2516-023

Quantitative ionization energies and work functions of aqueous solutions

Despite the ubiquitous nature of aqueous solutions across the chemical, biological and environmental sciences our experimental understanding of their electronic structure is rudimentary—qualitative at best. One of the most basic and seemingly straightforward properties of aqueous solutions—ionization energies—are (qualitatively) tabulated at the water–air interface for a mere handful of solutes, and the manner in which these results are obtained assume the aqueous solutions behave like a gas in the photoelectron experiment (where the vacuum levels of the aqueous solution and of the photoelectron analyzer are equilibrated). Here we report the experimental measure of a sizeable offset (ca. 0.6 eV) between the vacuum levels of an aqueous solution (0.05 M NaCl) and that of our photoelectron analyzer, indicating a breakdown of the gas-like vacuum level alignment assumption for the aqueous solution. By quantifying the vacuum level offset as a function of solution chemical composition our measurements enable, for the first time, quantitative determination of ionization energies in liquid solutions. These results reveal that the ionization energy of liquid water is not independent of the chemical composition of the solution as is usually inferred in the literature, a finding that has important ramifications as measured ionization energies are frequently used to validate theoretical models that posses the ability to provide microscopic insight not directly available by experiment. Finally, we derive the work function, or the electrochemical potential of the aqueous solution and show that it too varies with the chemical composition of the solution.ISSN:1463-9084ISSN:1463-907

Planar growth of pentacene on the dielectric TiO2(110) surface

We have studied the growth of pentacene molecules on the unreconstructed and stoichiometric surface of TiO2(110). At variance with its characteristic homeotropic growth mode, pentacene is found to be physisorbed on this dielectric substrate with its long molecular axis oriented parallel to the surface and aligned along the [001] direction. Pentacene molecules couple side-by-side into long stripes running along the [11̅0] direction, where the overlayer preserves the substrate lattice periodicity (6.5 Å). In the opposite direction, head-to-head pentacene repulsion drives the ordering of the stripes, whose spacing simply depends on the surface coverage. By near-edge X-ray absorption, NEXAFS, we have determined the pentacene molecules to be tilted by ∼25° off the surface around their long axis. At the monolayer coverage, the pentacene orientation and spacing are very close to that of the (010) bulk planes (also called a−c planes) of pentacene crystals. We have observed that at least two additional layers can be grown on top of the monolayer following a planar configuration. Both the strong side-by-side intermolecular attraction and the full development of the bulklike electronic states, as probed by NEXAFS, suggest an optimal charge transport along the monolayer stripes of lying-down moleculesFinancial support from the Spanish CYCIT (MAT2008-1497) and the Ministry of Science and Innovation (CSD2007-41 NANOSELECT) is greatly acknowledged. C.S.-S. is grateful to Ministerio de Educación for the AP2005-0433 FPU grant. Funding from the European Community Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 226716 is acknowledged.Peer Reviewe

Strong Chemical Interaction and Self-Demetalation of Zinc-Phthalocyanine on Al(100)

We investigate the early stages of the growth of zinc-phthalocyanine on Al(100) using X-ray photoemission spectroscopy (XPS) and low-energy electron diffraction (LEED). Diffraction patterns show a (5 7 5) reconstruction, characteristic of flat-lying molecules forming a long-range-ordered structure with a square unit cell. The degree of ordering (i.e., the average domain size) is increased when the substrate is kept above 100 \ub0C during the deposition. At low coverage ( 641 ML), a sizeable charge transfer from the substrate to the molecules is observed, indicating a strong interaction at the organic\u2013inorganic interface. As a consequence of charge filling of ZnPc LUMO, a self-demetalation of the molecule occurs while the structure of the ligand remains mostly unaffected

Ultrafast Bidirectional Charge Transport and Electron Decoherence at Molecule/Surface Interfaces: A Comparison of Gold, Graphene, and Graphene Nanoribbon Surfaces

We investigate bidirectional femtosecond charge transfer dynamics using the core-hole clock implementation of resonant photoemission spectroscopy from 4,4'-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core-hole creation in all systems, opening an additional decay channel for the core-hole, involving electron donation from substrate to the molecule. Furthermore, using the core-hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the core-excited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface