Showing papers on "Photoemission spectroscopy published in 2007"

Surface transfer p-type doping of epitaxial graphene.

Abstract: Epitaxial graphene thermally grown on 6H-SiC(0001) can be p-type doped via a novel surface transfer doping scheme by modifying the surface with the electron acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Synchrotron-based high-resolution photoemission spectroscopy reveals that electron transfer from graphene to adsorbed F4-TCNQ is responsible for the p-type doping of graphene. This novel surface transfer doping scheme by surface modification with appropriate molecular acceptors represents a simple and effective method to nondestructively dope epitaxial graphene for future nanoelectronics applications.

Inelastic light scattering from correlated electrons

Abstract: Inelastic light scattering is an intensively used tool in the study of electronic properties of solids. Triggered by the discovery of high-temperature superconductivity in the cuprates and by new developments in instrumentation, light scattering in both the visible (Raman effect) and x-ray part of the electromagnetic spectrum has become a method complementary to optical (infrared) spectroscopy while providing additional and relevant information. The main purpose of the review is to position Raman scattering with regard to single-particle methods like angle-resolved photoemission spectroscopy, and other transport and thermodynamic measurements in correlated materials. Particular focus will be placed on photon polarizations and the role of symmetry to elucidate the dynamics of electrons in different regions of the Brillouin zone. This advantage over conventional transport (usually measuring averaged properties) provides new insights into anisotropic and complex many-body behavior of electrons in various systems. Recent developments in the theory of electronic Raman scattering in correlated systems and experimental results in paradigmatic materials such as the A15 superconductors, magnetic and paramagnetic insulators, compounds with competing orders, as well as the cuprates with high superconducting transition temperatures are reviewed. An overview of the manifestations of complexity in the Raman response due to the impact of correlations and developing competing orders is presented. In a variety of materials, observations which may be understood and a summary of important open questions that pave the way to a detailed understanding of correlated electron systems, are discussed.

Synthesis and characterization of atomically-thin graphite filmson a silicon carbide substrate,

Abstract: This paper reports the synthesis and detailed characterization of graphite thin films produced by thermal decomposition of the (0001) face of a 6H-SiC wafer, demonstrating the successful growth of single crystalline films down to approximately one graphene layer. The growth and characterization were carried out in ultrahigh vacuum (UHV) conditions. The growth process and sample quality were monitored by low-energy electron diffraction, and the thickness of the sample was determined by core level x-ray photoelectron spectroscopy. High-resolution angle-resolved photoemission spectroscopy shows constant energy map patterns, which are very sharp and fully momentum-resolved, but nonetheless not resolution limited. We discuss the implications of this observation in connection with scanning electron microscopy data, as well as with previous studies.

Internal photoemission at interfaces of high-κ insulators with semiconductors and metals

Abstract: Internal photoemission spectroscopy provides the most straightforward way to characterize the relative energies of electron states at interfaces of insulators with metals and semiconductors by measuring the spectral onset of electron/hole photoemission from one solid into another. The article reviews the application of this technique for characterization of advanced nanometer-thin insulators prospected to be used in microelectronic devices. Fundamental aspects and technical features of the internal photoemission experiments are discussed together with basic electronic properties of a number of investigated high-permittivity insulating films and their interfaces in semiconductor heterostructures. Significant differences are found in the electronic properties of nanometer-thin amorphous insulating layers as compared to the known bulk phase characteristics. The band alignment at the interfaces of these insulators with metals is found to be highly sensitive to the surface preparation procedures. By contrast, ...

Intra- and intermolecular band dispersion in an organic crystal.

Abstract: The high crystallinity of many inorganic materials allows their band structures to be determined through angle-resolved photoemission spectroscopy (ARPES). Similar studies of conjugated organic molecules of interest in optoelectronics are often hampered by difficulties in growing well-ordered and well-oriented crystals or films. We have grown crystalline films of uniaxially oriented sexiphenyl molecules and obtained ARPES data. Supported by density-functional calculations, we show that, in the direction parallel to the principal molecular axis, a quasi-one-dimensional band structure of a system of well-defined finite size develops out of individual molecular orbitals. In contrast, perpendicular to the molecules, the band structure reflects the periodicity of the molecular crystal, and continuous bands with a large dispersion were observed.

Hydrogen bonds in liquid water studied by photoelectron spectroscopy

Abstract: The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.

Symmetry breaking in few layer graphene films

Abstract: Recently, it was demonstrated that the quasiparticle dynamics, the layer-dependent charge and potential, and the c-axis screening coefficient could be extracted from measurements of the spectral function of few layer graphene films grown epitaxially on SiC using angle-resolved photoemission spectroscopy (ARPES). In this paper we review these findings, and present detailed methodology for extracting such parameters from ARPES. We also present detailed arguments against the possibility of an energy gap at the Dirac crossing ED.

ZnIr2O4, a p-type transparent oxide semiconductor in the class of spinel zinc-d6-transition metal oxide

Abstract: The authors report on the growth of spinel ZnMd62O4 M=Co, Rh, and Ir, a ρ-type wide band gap semiconductor by pulsed laser deposition. The band gap of these compounds is determined by the ligand field splitting in the subbands of the metallic d6 cation. Photoemission spectroscopy revealed that the valence band maximum is composed of occupied t2g 6 states. The observed band gap is increasing for higher quantum numbers, being as large as 3 eV for ZnIr2O4, which is expected from theoretical predictions. Grown in polycrystalline phase, films of these materials display high conductivity, well above 2 S cm−1.

Low-energy electronic structure of the high-Tc cuprates La2−xSrxCuO4 studied by angle-resolved photoemission spectroscopy

Abstract: We have performed a systematic angle-resolved photoemission spectroscopy (ARPES) study of the high-Tc cuprates La2−xSrxCuO4, ranging from the underdoped insulator to the superconductor to the overdoped metal. We have revealed a systematic doping evolution of the band dispersions and (underlying) Fermi surfaces, pseudogap and quasi-particle features under the influence of strong electron–electron interaction and electron–phonon interaction. The unusual transport and thermodynamic properties are explained by taking into account the pseudogap opening and the Fermi arc formation, due to which the carrier number decreases as the doped hole concentration decreases.

NiO: correlated band structure of a charge-transfer insulator.

Abstract: The band structure of the prototypical charge-transfer insulator NiO is computed by using a combination of an ab initio band structure method and the dynamical mean-field theory with a quantum Monte-Carlo impurity solver. Employing a Hamiltonian which includes both Ni d and O p orbitals we find excellent agreement with the energy bands determined from angle-resolved photoemission spectroscopy. This brings an important progress in a long-standing problem of solid-state theory. Most notably we obtain the low-energy Zhang-Rice bands with strongly k-dependent orbital character discussed previously in the context of low-energy model theories.

Investigation of the amino acids glycine, proline, and methionine by photoemission spectroscopy.

Abstract: The valence and core level photoelectron spectra of glycine, proline, and methionine in the gas phase have been investigated by VUV and soft X-ray radiation. The outer valence band photoemission spectra are similar to previously reported He I spectra, although relative peak intensities are different due to the different photon energy. We extended the spectral range to include the inner valence region. The carbon, nitrogen, and oxygen 1s as well as the sulfur 2p core level spectra of these amino acids have been measured and the states identified. Valence band spectra of proline have been recorded as a function of temperature, and they provide information about the relative populations of the lowest energy conformers.

Dependence of the electronic structure of SrRuO3 and its degree of correlation on cation off-stoichiometry

Abstract: We have grown and studied high quality SrRuO3 films grown by molecular beam epitaxy as well as pulsed laser deposition. By changing the oxygen activity during deposition, we were able to make SrRuO3 samples that were stoichiometric (low oxygen activity) or with ruthenium vacancies (high oxygen activity). This ability to control the ruthenium stoichiometry has permitted us to make a systematic study of the dependence of the degree of electron correlation in SrRuO3 on stoichiometry using transport and photoemission experiments. We have compared the measured ultraviolet photoemission spectroscopy spectra with calculated density of states spectra and offer explanations for the large observed differences between the two.

Surface Transfer Doping of Diamond (100) by Tetrafluoro-tetracyanoquinodimethane

Abstract: Surface transfer doping is demonstrated on hydrogenated diamond (100) by the electron acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Synchrotron-based photoemission spectroscopy reveals that electrons are transferred from diamond to adsorbed F4-TCNQ, leaving a diamond surface p-type doped with a high areal density of holes of about 1.6 × 1013 cm-2.

Surface studies of nitrogen implanted TiO2

Abstract: Rutile TiO2(1 1 0) single crystals have been doped by nitrogen-ion implantation. The change in the valence band and in the core level peak shapes are characterized by photoemission spectroscopy. Surface morphologies are characterized by scanning tunneling microscopy. N-dopants are observed to be in a 3� charge state and to substitute for O-anions in the TiO2 lattice for N-concentrations up to � 5% of the anions. The higher valency of the N-dopants compared to the host O-anions is proposed to be compensated by the formation of O-vacancies and/or Ti-interstitials. Two chemically shifted components arise in the Ti-2p core level upon N-doping. These components, shifted by 0.9 eV and 2.1 eV, are assigned to Ti-bound to N-ligands and possibly due to O-vacancies in the lattice. The Ti-3d band gap state observed in UPS is initially suppressed upon room temperature N-implantation and recovers a similar intensity as for undoped TiO2 samples upon annealing. This indicates that electrons left behind upon creation of O-vacancies are filling the N2p level rather than Ti-3d states. The filled N-2p state is found at the top of the TiO2 valence band and is believed to be responsible for the band gap narrowing of N-doped TiO2 that shifts the photoactivity of TiO2 into the visible spectrum. � 2007 Elsevier B.V. All rights reserved.

Photoelectron spectrum of isolated ion-pairs in ionic liquid vapor.

Abstract: The gas-phase valence binding energy spectrum of isolated ion-pairs of the commonly used [1-ethyl-3-methylimidazolium][bis(trifluoromethylsulfonyl)imide)] room-temperature ionic liquid is obtained by photoionization of a molecular beam of ionic liquid vapor by extreme ultraviolet light. The isolated ion-pair nature of the ionic liquid vapor is corroborated by single photon ionization mass spectroscopy, complemented by computed vaporization energetics of ion-pairs and clusters of ion-pairs. The valence binding energy spectrum of the isolated ion-pairs is discussed in comparison with available liquid-phase data and theoretical density functional theory calculations.

Very High Resolution Photoelectron Spectroscopy

Abstract: Many-Body Effects- Photoemission Spectroscopy with Very High Energy Resolution: Studying the Influence of Electronic Correlations on the Millielectronvolt Scale- Photoemission as a Probe of the Collective Excitations in Condensed Matter Systems- High-resolution Photoemission Spectroscopy of Solids Using Synchrotron Radiation- Low-Dimensional Systems- Photoemission on Quasi-One-Dimensional Solids: Peierls, Luttinger & Co- Atomic Chains at Surfaces- Ultimate Resolution- High-Resolution Photoemission Spectroscopy of Low-T c Superconductors- Molecules- Very-High-Resolution Laser Photoelectron Spectroscopy of Molecules- High-Temperature Superconductors and Transition-Metal Oxides- Doping Evolution of the Cuprate Superconductors from High-Resolution ARPES- Many-Body Interaction in Holeand Electron-Doped High-T c Cuprate Superconductors- Dressing of the Charge Carriers in High-T c Superconductors- High-Resolution Photoemission Spectroscopy of Perovskite-Type Transition-Metal Oxides- High Energy and High Resolution- High-Resolution High-Energy Photoemission Study of Rare-Earth Heavy Fermion Systems- Hard X-Ray Photoemission Spectroscopy

One-Dimensional Phase-Change Nanostructure: Germanium Telluride Nanowire

Abstract: High-quality nanowires of germanium telluride (GeTe), a one-dimensional chalcogenide phase-change nanostructure, were synthesized via thermal evaporation method under vapor−liquid−solid mechanism. The physical morphology, chemical composition, and crystal structure of the as-synthesized GeTe nanowires were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and X-ray photoemission spectroscopy. Through real-time TEM imaging of nanowire sample heated in an incrementally controllable heating system, the melting point of a single crystalline GeTe nanowire (∼40−80 nm diameter) is found to be significantly lower than that of its bulk counterpart (46% reduction, from 725 to 390 °C). The significant reduction in melting point makes one-dimensional phase-change chalcogenide nanowire a potential material for application in low-power high-density resistive switching nonvolatile data storage in which the thermal energy for mat...

Angle-resolved photoemission spectroscopy with a femtosecond high harmonic light source using a two-dimensional imaging electron analyzer.

Abstract: An experimental setup for time- and angle-resolved photoemission spectroscopy using a femtosecond 1 kHz high harmonic light source and a two-dimensional electron analyzer for parallel energy and momentum detection is presented. A selection of the 27th harmonic (41.85 eV) from the harmonic spectrum of the light source is achieved with a multilayer MoSi double mirror monochromator. The extinction efficiency of the monochromator in selecting this harmonic is shown to be better than 7:1, while the transmitted bandwidth of the selected harmonic is capable of supporting temporal pulse widths as short as 3 fs. The recorded E(k) photoelectron spectrum from a Cu(111) surface demonstrates an angular resolution of better than 0.6 degrees (=0.03 A(-1) at E(kin,e)=36 eV). Used in a pump-probe configuration, the described experimental setup represents a powerful experimental tool for studying the femtosecond dynamics of ultrafast surface processes in real time.

X-ray photoemission study of CoFeB /MgO thin film bilayers

Abstract: The authors present results from an x-ray photoemission spectroscopy study of CoFeB∕MgO bilayers where they observe process-dependent formation of B, Fe, and Co oxides at the CoFeB∕MgO interface due to oxidation of CoFeB during MgO deposition. Vacuum annealing reduces the Co and Fe oxides but further incorporates B into the MgO forming a composite MgBxOy layer. Inserting an Mg layer between CoFeB and MgO introduces an oxygen sink, providing increased control over B content in the barrier.

Electronic structure at highly ordered organic/metal interfaces : Pentacene on Cu(110)

Abstract: The electronic structure at highly ordered pentacene monolayer prepared on Cu(110) substrate was studied by angle-resolved ultraviolet photoemission spectroscopy The valence-level photoemission line shape showed the evidences of (i) formation of the interface states and (ii) two-dimensional energy-band dispersion of the resultant interface states The lattice constant deduced from the observed energy-band dispersion is consistent with the reported one based on the low-energy electron diffraction experiments Thus, the observed energy-band dispersion can be ascribed to the in-plane intermolecular energy-band dispersion in the pentacene monolayer on Cu(110) These phenomena may originate from the hybridization between the molecular orbital and the wave function of the substrate surface Furthermore, work-function change of about $\ensuremath{-}09\phantom{\rule{03em}{0ex}}\mathrm{eV}$ by adsorption of pentacene was observed from the shift of the secondary-electron cutoff Such a decrease of the work function indicates the formation of a dipole layer at the interface with the molecule positively charged This direction is opposite to the naive expectation from the electron transfer from the substrate to the molecule, which was suggested from the previous work of core-level photoemission spectroscopy [McDonald et al, Surf Sci 600, 1909 (2006)] This unexpected result may originate from the charge redistribution at the interface due to the induced image charge in the metal and the push back of electrons spilled out from the metal surface by the adsorbed molecules, which may overwhelm the effect of electron transfer

X-ray absorption spectroscopy of vanadium dioxide thin films across the phase-transition boundary

Abstract: X-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy of the V L edge and O K edge were performed on VO{sub 2} thin films rf sputtered at various conditions. The spectra give evidence of the changes in the electronic structure depending on the film quality. XAS of the O K edge shows a decrease of the spacing between 3d{sub {pi}} and 3d{sub {sigma}} bands by 0.8 eV with concurrent broadening of both bands for the sample sputtered at lower substrate temperature and consequently having more polycrystalline and disordered character. 3d{sub {sigma}} band position appears to be more sensitive to the sample quality, indicating that the cation-ligand interaction is mostly affected likely due to the distortion of the local O coordination surrounding a V ion. The observed variation of the spectra in films of different morphologies may reflect the changes of the density of states responsible for the considerable variation of the metal-insulator transition (MIT) properties reported for VO{sub 2} thin films synthesized at different conditions. The study of the temperature dependence of the XAS spectra including repeated measurements across the MIT revealed both reversible and irreversible V L-edge and O K-edge changes. The thermal cycling of the VO{sub 2} filmsmore » through the MIT shows irreversible shifts of the conduction bands toward lower photon energies apparently caused by the sample deterioration due to the lattice transformations at the MIT. The signature of a phase transition in a VO{sub 2} film at MIT temperature (T{sub MIT}) is clearly seen in the XAS O K-edge spectra which show reversible switches of the 3d{sub {pi}} and 3d{sub {sigma}} bandwidths by approximately 20% depending on the sample being above or below T{sub MIT}.« less

Evolution of the pseudogap across the magnet-superconductor phase boundary of Nd2-xCexCuO4

Abstract: The doping dependence of electronic states in an electron-doped high-temperature superconductor (HTSC) Nd2-xCexCuO4 was studied by high-resolution angle-resolved photoemission spectroscopy. We observed that the high-energy pseudogap around the hot spot sh

Hydrogen peroxide treatment induced rectifying behavior of Au∕n-ZnO contact

Abstract: Conversion of the Au∕n-ZnO contact from Ohmic to rectifying with H2O2 pretreatment was studied systematically using I-V measurements, x-ray photoemission spectroscopy, positron annihilation spectroscopy, and deep level transient spectroscopy. H2O2 treatment did not affect the carbon surface contamination or the EC–0.31eV deep level, but it resulted in a significant decrease of the surface OH contamination and the formation of vacancy-type defects (Zn vacancy or vacancy cluster) close to the surface. The formation of a rectifying contact can be attributed to the reduced conductivity of the surface region due to the removal of OH and the formation of vacancy-type defects.

Changes in the electronic structures and optical band gap of Ge2Sb2Te5 and N-doped Ge2Sb2Te5 during phase transition

Abstract: Changes in the electronic structures of Ge2Sb2Te5 (GST) and N-doped Ge2Sb2Te5 film during the phase transition from an amorphous to a crystalline phase were studied using synchrotron radiation high-resolution x-ray photoemission spectroscopy. The changes in tetrahedral and octahedral coordinated Ge 3d peaks are closely related to the changes in the chemical bonding state of GST films. The metallic Sb peak in the Sb 4d spectra of annealed GST films demonstrates that the metallic Sb atoms become segregated during thermal treatment resulting in phase separation. The incorporation of nitrogen into the GST film affects its structure and chemical bonding state, resulting in the suppression of crystallization. The incorporation of nitrogen also increases the optical band gap of the film due to the formation of a nitride.

Evolution of the electronic structure of 1T-Cu(x)TiSe(2).

Abstract: The electronic structure of a new charge-density-wave system or superconductor, 1T-Cu(x)TiSe(2), has been studied by photoemission spectroscopy. A correlated semiconductor band structure is revealed for the undoped case, which resolves a long-standing controversy in the system. With Cu doping, the charge-density wave is suppressed by the raising of the chemical potential, while the superconductivity is enhanced by the enhancement of the density of states, and possibly suppressed at higher doping by the strong scattering.

Effect of Visible and UV Illumination on the Water Contact Angle of TiO2 Thin Films with Incorporated Nitrogen

Abstract: Doping TiO2 with nitrogen is recognized as a procedure to get sensitization of this material with visible light. In the present work, incorporation of nitrogen within the structure of TiO2 thin films has been accomplished by N2+ ion implantation in TiO2 anatase thin films (50 keV ion energy for doses of 3 × 1016, 6 × 1016, and 1.2 × 1017 ions cm-2) and during preparation by metalorganic chemical vapor deposition (MOCVD) using nitrogen as carrier gas. The analysis of the samples by X-ray photoemission spectroscopy (XPS) and for the MOCVD samples also by secondary ion mass spectroscopy (SIMS) has shown that nitrogen, in the form of nitride-like species, (N/Ti ratios of 0.03 and 0.12 for the MOCVD and the implanted samples, respectively) has become effectively incorporated within the structure of TiO2. The water contact angle on the implanted thin films varied from about 80° to around 30° when illuminated with visible light, depending on the ion dose. Similarly, the MOCVD samples showed a sharp decrease in w...