Showing papers on "Photoemission spectroscopy published in 2009"
TL;DR: In this article, the UHV multichamber photoelectron gun was used to study the removal of surface pollutants and the degraded (Cs,O)-activation layer during the cleaning procedure.
Abstract: Atomic hydrogen, produced by thermal dissociation of H2 molecules inside a hot tungsten capillary, is shown to be an efficient tool for multiple recleaning of degraded surfaces of high quantum efficiency transmission-mode GaAs photocathodes within an ultrahigh vacuum (UHV) multichamber photoelectron gun. Ultraviolet quantum yield photoemission spectroscopy has been used to study the removal of surface pollutants and the degraded (Cs,O)-activation layer during the cleaning procedure. For photocathodes grown by the liquid-phase epitaxy technique, the quantum efficiency is found to be stable at about 20% over a large number of atomic hydrogen cleaning cycles. A slow degradation of the quantum efficiency is observed for photocathodes grown by metal-organic chemical vapor deposition, although they reached a higher initial quantum efficiency of about 30%–35%. Study of the spatial distributions of photoluminescence intensity on these photocathodes proved that this overall degradation is likely due to insertion o...
388 citations
TL;DR: In this paper, the authors demonstrate that the excess negative charge can be fully compensated by non-covalently functionalizing graphene with the strong electron acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ).
Abstract: Paul Scherrer Institut, CH-5232 Villigen-PSI, Switzerland(Dated: March 16, 2010)Epitaxial graphene on SiC(0001) suffers from strong intrinsic n-type doping. We demonstrate thatthe excess negative charge can be fully compensated by non-covalently functionalizing graphene withthe strong electron acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ). Charge neutralitycan be reached in monolayer graphene as shown in electron dispersion spectra from angular re-solved photoemission spectroscopy (ARPES). In bilayer graphene the band gap that originates fromthe SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a con-sequence of the molecular doping, the Fermi level is shifted into the band gap. The reduction ofthe charge carrier density upon molecular deposition is quantified using electronic Fermi surfacesand Raman spectroscopy. The structural and electronic characteristics of the graphene/F4-TCNQcharge transfer complex are investigated by X-ray photoelectron spectroscopy (XPS) and ultravioletphotoelectron spectroscopy (UPS). The doping effect on graphene is preserved in air and is temper-ature resistant up to 200
342 citations
TL;DR: It is demonstrated that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data, in good agreement with density functional calculations.
Abstract: Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this experimental technique to characterize discrete orbitals of large π-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations.
254 citations
TL;DR: In this paper, the initial oxidation stages of perfect and defective graphitic surfaces exposed to atomic oxygen have been studied with a combined high-resolution photoemission spectroscopy (HR-PES) and density functional theory (DFT) computational approach.
Abstract: The initial oxidation stages of perfect and defective graphitic surfaces exposed to atomic oxygen have been studied with a combined high-resolution photoemission spectroscopy (HR-PES) and density functional theory (DFT) computational approach. The resulting oxygen-containing surface functional groups are identified by analyzing the multicomponent C 1s and O 1s core level spectra that are then interpreted on the basis of DFT calculations. In the initial oxidation stage, epoxy groups are formed on perfect graphene, whereas the preferential adsorption of the O atoms on the vacancies of the defective surfaces results in structures containing pairs of oxygen atoms in ether and carbonyl (semiquinone) configurations. The formation of these functional groups is preceded by metastable structures consisting of single O atoms occupying single C vacancies.
230 citations
21 Mar 2009-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this paper, the authors describe the development and applications of novel instrumentation for photoemission spectroscopy of solid or liquid surfaces in the presence of gases under ambient conditions of pressure and temperature.
Abstract: We describe the development and applications of novel instrumentation for photoemission spectroscopy of solid or liquid surfaces in the presence of gases under ambient conditions of pressure and temperature. The new instrument overcomes the strong scattering of electrons in gases by the use of an aperture close to the surface followed by a differentially-pumped electrostatic lens system. In addition to the scattering problem, experiments in the presence of condensed water or other liquids require the development of special sample holders to provide localized cooling. We discuss the first two generations of Ambient Pressure PhotoEmission Spectroscopy (APPES) instruments developed at synchrotron light sources (ALS in Berkeley and BESSY in Berlin), with special focus on the Berkeley instruments. Applications to environmental science and catalytic chemical research are illustrated in two examples.
224 citations
TL;DR: It is argued that the high atomic polarizability of Se plays an important role to stabilize the excitonic state of Ta2NiSe5.
Abstract: We report on a photoemission study of ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ that has a quasi-one-dimensional structure and an insulating ground state Ni $2p$ core-level spectra show that the Ni $3d$ subshell is partially occupied and the Ni $3d$ states are heavily hybridized with the Se $4p$ states In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni $3d$--Se $4p$ hole and the Ta $5d$ electron We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state
206 citations
TL;DR: An imaging photoelectron photoion coincidence spectrometer at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source is presented and a few initial measurements are reported.
Abstract: An imaging photoelectron photoion coincidence spectrometer at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source is presented and a few initial measurements are reported. Monochromatic synchrotron VUV radiation ionizes the cooled or thermal gas-phase sample. Photoelectrons are velocity focused, with better than 1 meV resolution for threshold electrons, and also act as start signal for the ion time-of-flight analysis. The ions are accelerated in a relatively low, 40–80 V cm−1 field, which enables the direct measurement of rate constants in the 103–107 s−1 range. All electron and ion events are recorded in a triggerless multiple-start/multiple-stop setup, which makes it possible to carry out coincidence experiments at >100 kHz event frequencies. As examples, the threshold photoelectron spectrum of the argon dimer and the breakdown diagrams for hydrogen atom loss in room temperature methane and the chlorine atom loss in cold chlorobenzene are shown and discussed.
187 citations
TL;DR: The theory for time-resolved, pump-Probe, photoemission spectroscopy and other pump-probe experiments is developed, incorporating all of the nonequilibrium effects of the pump pulse and the finite time width of the probe pulse and including possibilities for taking into account band structure and matrix element effects, surface states, and the interaction of the photoexcited electrons with the system leading to corrections to the sudden approximation.
Abstract: The theory for time-resolved, pump-probe, photoemission spectroscopy and other pump-probe experiments is developed. The formal development is completely general, incorporating all of the nonequilibrium effects of the pump pulse and the finite time width of the probe pulse, and including possibilities for taking into account band structure and matrix element effects, surface states, and the interaction of the photoexcited electrons with the system leading to corrections to the sudden approximation. We also illustrate the effects of windowing that arise from the finite width of the probe pulse in a simple model system by assuming the quasiequilibrium approximation.
184 citations
TL;DR: A giant Rashba-type spin splitting on a semiconducting substrate by means of a Bi-trimer adlayer on a Si(111) wafer with an energy much larger than what has previously been reported for any semiconductor heterostructure is demonstrated.
Abstract: We demonstrate a giant Rashba-type spin splitting on a semiconducting substrate by means of a Bi-trimer adlayer on a Si(111) wafer. The in-plane inversion symmetry is broken inducing a giant spin splitting with a Rashba energy of about 140 meV, much larger than what has previously been reported for any semiconductor heterostructure. The separation of the electronic states is larger than their lifetime broadening, which has been directly observed with angular resolved photoemission spectroscopy. The experimental results are confirmed by relativistic first-principles calculations.
170 citations
21 Mar 2009-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: A review of the history of X-ray photoemission spectroscopy using high-brilliance high-flux X-rays from third generation synchrotron radiation facilities is presented in this paper.
Abstract: Except in the very early stage of the development of X-ray photoemission spectroscopy (XPS) by Kai Siegbahn and his coworkers, the excitation sources for XPS studies have predominantly been the Al Kα and Mg Kα emission lines. The advent of synchrotron radiation sources opened up the possibility of tuning the excitation photon energy with much higher throughputs for photoemission spectroscopy, however the excitation energy range was limited to the vacuum ultra violet and soft X-ray regions. Over the past 5–6 years, bulk-sensitive hard X-ray photoemission spectroscopy using high-brilliance high-flux X-rays from third generation synchrotron radiation facilities has been developed. This article reviews the history of HXPES covering the period from Kai Siegbahn and his coworkers’ pioneering works to the present, and describes the fundamental aspects, instrumentation, applications to solid state physics, applied physics, materials science, and industrial applications of HXPES. Finally, several challenging new developments which have been conducted at SPring-8 by collaborations among several groups are introduced.
165 citations
TL;DR: The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/ Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.
Abstract: LixCoO2 and LixNiO2 (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of LixCoO2 and LixNiO2 as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Lix(Co,Ni)O2 (0 < x⩽ 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.
TL;DR: The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using X-ray spectroscopy and theoretical calculations show excellent agreement.
Abstract: The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using X-ray spectroscopy. Using X-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e., without an exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.
TL;DR: In this article, high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe 2, which is one of the parent compounds of the Fe-pnictide superconductors, were performed at 20 and 300 K.
Abstract: We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of ${\text{BaFe}}_{2}{\text{As}}_{2}$, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the $\ensuremath{\Gamma}$ point and an electron pocket at each of the $X$ points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.
TL;DR: In this paper, the authors performed high-resolution angle-resolved photoemission spectroscopy on the Ba0.6K0.4Fe2As2 compound and determined the accurate momentum dependence of the superconducting gap in four Fermi-surface sheets including a newly discovered outer electron pocket at the M-point.
Abstract: We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally doped Ba0.6K0.4Fe2As2 compound and determined the accurate momentum dependence of the superconducting (SC) gap in four Fermi-surface sheets including a newly discovered outer electron pocket at the M-point. The SC gap on this pocket is nearly isotropic and its magnitude is comparable (?~11?meV) to that of the inner electron and hole pockets (~12?meV), although it is substantially larger than that of the outer hole pocket (~6?meV). The Fermi-surface dependence of the SC gap value is basically consistent with the ?(k)=?0?cos?kx?cos?ky formula expected for the extended s-wave symmetry. The observed finite deviation from the simple formula suggests the importance of multi-orbital effects.
TL;DR: In this paper, the propagation of the photoelectron cloud to the detector is studied with a full $N$-body numerical simulation and the influence of various parameters, in particular number of electrons per pulse, source size, pulse duration, kinetic energy and emission-angle distributions as well as presence of mirror charges in the sample, is investigated in detail.
Abstract: Solid-state photoemission spectroscopy relies to a large part on pulsed photon sources: third-generation synchrotron-radiation sources and ultrafast laser systems in particular. Especially when the photon pulses are intense, Coulombic repulsion between the emitted electrons will be a limiting factor for photoemission experiments aiming at highest energy and angle resolutions. In the present work, the propagation of the photoelectron cloud to the detector is studied with a full $N$-body numerical simulation. The influence of various parameters, in particular number of electrons per pulse, source size, pulse duration, kinetic-energy and emission-angle distributions as well as presence of mirror charges in the sample, is investigated in detail. Previous experimental results obtained with various picosecond and femtosecond light sources are successfully reproduced and the general resolution limits of solid-state photoemission using pulsed photon sources are explored. The results are potentially important for the design and interpretation of photoemission experiments with next-generation light sources, such as free-electron lasers and high-harmonic generation sources.
TL;DR: In this paper, the authors investigate the doping behavior of the strongly electron accepting molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane coevaporated with the host molecule N,N,N′,N −tetrakis(4-methoxyphenyl)-benzidine by photoemission spectroscopy and conductivity measurements.
Abstract: We investigate the doping behavior of the strongly electron accepting molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane coevaporated with the host molecule N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine by photoemission spectroscopy and conductivity measurements. Using interface resolved measurements, we compare the alignment on different substrates and investigate the effects of varying doping concentrations on the Fermi level position. We find that at high doping concentrations the Fermi level gets pinned at the exponentially decaying tail of the highest occupied molecular orbital and compare these results with different dopants and host molecules. The measurement of the doping dependent space charge layer thickness yields information on the amount of free charge carriers and thereby the efficiency of the doping.
TL;DR: Accurate aqueous-phase vertical ionization potentials provide a significant improvement to the corrected gas-phase values used in the literature and represent important information in assessing the threshold energies for photooxidation and oxidation free energies of solvent-exposed DNA components.
Abstract: Vertical ionization energies of the nucleosides cytidine and deoxythymidine in water, the lowest ones amounting in both cases to 8.3 eV, are obtained from photoelectron spectroscopy measurements in aqueous microjets. Ab initio calculations employing a nonequilibrium polarizable continuum model quantitatively reproduce the experimental spectra and provide molecular interpretation of the individual peaks of the photoelectron spectrum, showing also that lowest ionization originates from the base. Comparison of calculated vertical ionization potentials of pyrimidine bases, nucleosides, and nucleotides in water and in the gas phase underlines the dramatic effect of bulk hydration on the electronic structure. In the gas phase, the presence of sugar and, in particular, of phosphate has a strong effect on the energetics of ionization of the base. Upon bulk hydration, the ionization potential of the base in contrast becomes rather insensitive to the presence of the sugar and phosphate, which indicates a remarkable screening ability of the aqueous solvent. Accurate aqueous-phase vertical ionization potentials provide a significant improvement to the corrected gas-phase values used in the literature and represent important information in assessing the threshold energies for photooxidation and oxidation free energies of solvent-exposed DNA components. Likewise, such energetic data should allow improved assessment of delocalization and charge-hopping mechanisms in DNA ionized by radiation.
TL;DR: In this paper, the valence-band density of states of single-crystalline rock-salt CdO(001), wurtzite $c$-plane ZnO, and rock- salt MgO (001) are investigated by high-resolution x-ray photoemission spectroscopy.
Abstract: The valence-band density of states of single-crystalline rock-salt CdO(001), wurtzite $c$-plane ZnO, and rock- salt MgO(001) are investigated by high-resolution x-ray photoemission spectroscopy. A classic two-peak structure is observed in the VB-DOS due to the anion $2p$-dominated valence bands. Good agreement is found between the experimental results and quasi-particle-corrected density-functional theory calculations. Occupied shallow semicore $d$ levels are observed in CdO and ZnO. While these exhibit similar spectral features to the calculations, they occur at slightly higher binding energies, determined as 8.8 eV and 7.3 eV below the valence band maximum in CdO and ZnO, respectively. The implications of these on the electronic structure are discussed.
TL;DR: The results indicate that the geometric factors play only a minor role in the enhancement of the electrocatalytic activity of GN for ORR, and demonstrates that the activity of the catalysts arises from the electronic states near Fermi level developed as a result of the incorporation of hetero-atoms, N and (or) Fe, especially N, into the graphite lattice.
Abstract: Graphite nitride (GN) was prepared from graphite oxide (GO) by reacting with ammonia at high temperature. Its electrochemical properties as catalyst for oxygen reduction reaction (ORR) were evaluated by cyclic voltammetry (CV) and steady state polarization (SP) measurements. In order to explore the origin of the activity of the catalysts, the lattice symmetry, the component and the band structure of the catalyst surface were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), respectively. The results indicate that the geometric factors, e.g. the surface area and the disorder degree, play only a minor role in the enhancement of the electrocatalytic activity of GN for ORR. The correlation between the electrocatalytic activity and the density of state (DOS) at 6.5 eV below Fermi level of GN demonstrates that the activity of the catalysts arises from the electronic states near Fermi level developed as a result of the incorporation of hetero-atoms, N and (or) Fe, especially N, into the graphite lattice.
TL;DR: In this article, a combined study of the electronic structure of the superconducting skutterudite derivative SrPt4Ge12 by means of x-ray photoelectron spectroscopy and full-potential band structure calculations including an analysis of the chemical bonding is presented.
Abstract: We present a combined study of the electronic structure of the superconducting skutterudite derivative SrPt4Ge12 by means of x-ray photoelectron spectroscopy and full-potential band structure calculations including an analysis of the chemical bonding. We establish that the states at the Fermi level originate predominantly from the Ge 4p electrons and that the Pt 5d shell is effectively full. We find excellent agreement between the measured and the calculated valence-band spectra, thereby validating that band structure calculations in combination with photoelectron spectroscopy can provide a solid basis for the modeling of superconductivity in the compound series MPt4Ge12 M=Sr,Ba,La,Pr.
TL;DR: This work suggests that x-ray two-photon photoelectron spectroscopy using x-rays free-electron lasers will provide access to electronic-structure information not currently available.
Abstract: The inner-shell single and double ionization spectra of the organic molecule para-aminophenol are calculated using many-body Green's function methods. The inner-shell double ionization spectrum displays more pronounced sensitivity to the chemical environment and to electronic many-body effects than does the inner-shell single ionization spectrum. A kinetic model is employed to determine the probability of inner-shell double hole formation in para-aminophenol exposed to an intense, 1 fs x-ray pulse. The resulting photoelectron spectrum at a photon energy of 1 keV is calculated. This work suggests that x-ray two-photon photoelectron spectroscopy using x-ray free-electron lasers will provide access to electronic-structure information not currently available.
TL;DR: Comparison of experimental results and theoretical calculations indicates that the present state of the art many-body calculations, although improving the description of correlation effects in Fe, give too small mass renormalizations and scattering rates thus demanding more refined many- body theories including nonlocal fluctuations.
Abstract: The strength of electronic correlation effects in the spin-dependent electronic structure of ferromagnetic bcc Fe(110) has been investigated by means of spin and angle-resolved photoemission spectroscopy. The experimental results are compared to theoretical calculations within the three-body scattering approximation and within the dynamical mean-field theory, together with one-step model calculations of the photoemission process. This comparison indicates that the present state of the art many-body calculations, although improving the description of correlation effects in Fe, give too small mass renormalizations and scattering rates thus demanding more refined many-body theories including nonlocal fluctuations.
TL;DR: In this article, high-resolution angle-resolved photoemission spectroscopy was performed on heavily electron-doped non-superconducting (SC) BaFe1.7Co0.3As2.
Abstract: We have performed high-resolution angle-resolved photoemission spectroscopy on heavily electron-doped non-superconducting (SC) BaFe1.7Co0.3As2. We find that the two hole Fermi surface pockets at the Brillouin zone center observed in the hole-doped superconducting Ba0.6K0.4Fe2As2 are absent or very small in this compound, while the two electron pockets at the zone corner significantly expand due to electron doping by the Co substitution. Comparison of the Fermi surface between non-SC and SC samples indicates that the coexistence of hole and electron pockets connected via the antiferromagnetic wave vector is essential in realizing the mechanism of superconductivity in the iron-based superconductors.
TL;DR: Ab initio simulations of the ARPES spectra of graphene including both electron-electron and electron-phonon interactions on the same footing are reported, reproducing some of the key experimental observations related to many-body effects.
Abstract: Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental technique for directly probing electron dynamics in solids. The energy versus momentum dispersion relations and the associated spectral broadenings measured by ARPES provide a wealth of information on quantum many-body interaction effects. In particular, ARPES allows studies of the Coulomb interaction among electrons (electron-electron interactions) and the interaction between electrons and lattice vibrations (electron-phonon interactions). Here, we report ab initio simulations of the ARPES spectra of graphene including both electron-electron and electron-phonon interactions on the same footing. Our calculations reproduce some of the key experimental observations related to many-body effects, including the indication of a mismatch between the upper and lower halves of the Dirac cone.
TL;DR: The electronic structure of phenylalanine, tyrosine, tryptophan, and 3-methylindole in the gas phase was investigated by x-ray photoemission spectroscopy and NEXAFS, and calculations show that variations of the vertical excitation energies of different conformers are small, and cannot be resolved in the present experiment.
Abstract: The electronic structure of phenylalanine, tyrosine, tryptophan, and 3-methylindole in the gas phase was investigated by x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C, N, and O K-edges. The XPS spectra have been calculated for the four principal conformers of each amino acid, and the spectra weighted by the Boltzmann population ratios calculated from published free energies. Instead of the single peaks expected from the stoichiometry of the compounds, the N 1s core level spectra of phenylalanine and tryptophan show features indicating that more than one conformer is present. The calculations reproduce the experimental features. The C and O 1s spectra do not show evident effects due to conformational isomerism. The calculations predict that such effects are small for carbon, and for oxygen it appears that only broadening occurs. The carbon K-edge NEXAFS spectra of these aromatic amino acids are similar to the published data of the corresponding molecules in the solid state, but show more structure due to the higher resolution in the present study. The N K-edge spectra of tryptophan and 3-methylindole differ from phenylalanine and tyrosine, as the first two both contain a nitrogen atom located in a pyrrole ring. The nitrogen K-edge NEXAFS spectra of aromatic amino acids do not show any measurable effects due to conformational isomerism, in contrast to the photoemission results. Calculations support this result and show that variations of the vertical excitation energies of different conformers are small, and cannot be resolved in the present experiment. The O NEXAFS spectra of these three aromatic compounds are very similar to other, simpler amino acids, which have been studied previously.
TL;DR: The electrical properties of InN nanowires were investigated in four-point probe measurements and the shell conductivity is ascribed to an electron accumulation layer forming at the radial InN/In(2)O(3) interface.
Abstract: The electrical properties of InN nanowires were investigated in four-point probe measurements. The dependence of the conductance on the wire diameter allows distinguishing between "core" bulk (quadratic) and "shell" sheet (linear) contributions. Evidence of the formation of a thin In(2)O(3) layer at the surface of the nanowires is provided by X-ray core level photoemission spectroscopy. The shell conductivity is therefore ascribed to an electron accumulation layer forming at the radial InN/In(2)O(3) interface. Although conductance through the accumulation layer dominates for nanowires below a critical diameter of about 55 nm, the core channel cannot be neglected, even for small nanowires.
TL;DR: The results presented here are compared with recent molecular dynamics simulations of a NaNO(3) solution and are found to agree at high photoelectron kinetic energies and are discussed in relation to the observed increase in photochemical activity of nitrate-containing aerosols in the atmosphere.
Abstract: Depth-resolved ion spatial distributions of nitrate and nitrite anions in aqueous solution have been quantitatively measured using X-ray photoemission spectroscopy on a 15 μm aqueous liquid jet containing 3 M NaNO3, 3 M NaNO2, or an equimolar mixture of the two. The surface region, which extends to photoelectron kinetic energies of 400−500 eV, is partially depleted in anions relative to the bulk 3 M concentration. The nitrate and nitrite solutions exhibit similar depth-dependent anion profiles. The results presented here are compared with recent molecular dynamics simulations of a NaNO3 solution and are found to agree at high photoelectron kinetic energies. At shallower probe depths, the experiment measured a surface anion concentration less than that predicted by theory. Possible origins of the discrepancy are discussed in terms of the confined size of the simulation box and uncertainties that remain in regard to the inelastic mean free path of photoelectrons in aqueous media. The importance of our findi...
TL;DR: In this article, the authors investigate the band bending that occurs at the interface between disordered organic semiconductor and a metal electrode and demonstrate that the observed band bending can be explained equally well by either energetic relaxation of charge carriers on a timescale longer than photoemission (polaronic relaxation) or by a Gaussian density of states representing energetic disorder.
Abstract: We investigate the band bending that occurs at the interface between a disordered organic semiconductor and a metal electrode. Ultraviolet photoemission spectroscopy measurements of thin organic layers on conducting substrates have revealed band bending within a few nanometers of the interface. It has been proposed that this is caused by the transfer of carriers from the substrate into empty states in the organic film. Here we numerically model this process by simulating a film with a given density of states in thermal equilibrium with a metallic substrate. Comparing the model with various published experiments, we demonstrate that the observed band bending can be explained equally well by either energetic relaxation of charge carriers on a timescale longer than photoemission (polaronic relaxation) or by a Gaussian density of states representing energetic disorder. In the former case, our results suggest that the thermal injection of carriers to higher-energy states has led observers to overestimate the relaxation energy by as much as several hundred meV. We also show that band-bending effects due to disorder are expected to significantly reduce the open-circuit voltage in organic photovoltaic devices, and we quantify the relationship between the amount of voltage loss and the degree of disorder.
TL;DR: In this paper, a slot-plane-antenna (SPA) high density radical oxidation was used to grow a metal-oxide-semiconductor (Al2O3) gate stack with a GeO2 interfacial layer.
Abstract: GeO2 was grown by a slot-plane-antenna (SPA) high density radical oxidation, and the oxidation kinetics of radical oxidation GeO2 was examined. By the SPA radical oxidation, no substrate orientation dependence of growth rate attributed to highly reactive oxygen radicals with low oxidation activation energy was demonstrated, which is highly beneficial to three-dimensional structure devices, such as multigate field-effect transistors, to form conformal gate dielectrics. The electrical properties of an aluminum oxide (Al2O3) metal-oxide-semiconductor gate stack with a GeO2 interfacial layer were investigated, showing very low interface state density (Dit), 1.4×1011 cm−2 eV−1. By synchrotron radiation photoemission spectroscopy, the conduction and the valence band offsets of GeO2 with respect to Ge were estimated to be 1.2±0.3 and 3.6±0.1 eV, which are sufficiently high to suppress gate leakage.
TL;DR: In this article, the formation of different interface dipoles at the heterojunction interfaces is strongly influenced by the orientation dependent ionization potentials of the underlying F16CuPc or CuPc thin films.
Abstract: Molecular orientation dependent energy level alignments at organic−organic heterojunction (OOH) interfaces have been investigated with synchrotron based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) measurements. Model systems of the lying-down 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) films on both standing-up and lying-down copper hexadecafluorophthalocyanine (F16CuPc) and copper(II) phthalocyanine (CuPc) thin films have been used to illustrate the molecular orientation dependent interface properties. The formation of different interface dipoles at the heterojunction interfaces is strongly influenced by the orientation dependent ionization potentials of the underlying F16CuPc or CuPc thin films. This is attributed to the intrinsic surface dipoles induced in the standing-up F16CuPc (CuPc) film due to the polar intermolecular C−F (C−H) bonds formed at the interface. In situ NEXAFS measurements reveal that the room-temperature deposition of PT...