Showing papers on "Photoemission spectroscopy published in 2001"

Lithium doping of semiconducting organic charge transport materials

Abstract: We study the effects of lithium (Li) incorporation in the cathodes of organic light-emitting devices. A thermally evaporated surface layer of metallic Li is found to diffuse through, and subsequently dope, the electron transporting organic semiconducting thin films immediately below the cathode, forming an Ohmic contact. A diffusion length of ∼700 A is inferred from analyses of the current–voltage and secondary ion mass spectrometry data. The conductivity of the Li-doped organic films is ∼3×10−5 S/cm. Photoemission spectroscopy suggests that Li lowers the barrier to injection at the organic/cathode interface, introduces gap states in the bulk of the organic semiconductor, and dopes the bulk to facilitate efficient charge transport.

Controlled p-doping of zinc phthalocyanine by coevaporation with tetrafluorotetracyanoquinodimethane: A direct and inverse photoemission study

Abstract: P-doping of zinc phthalocyanine (ZnPc) with tetrafluorotetracyanoquinodimethane (F4-TCNQ) is investigated with ultraviolet and x-ray photoemission spectroscopy, inverse photoemission spectroscopy, and in situ current–voltage (I–V) measurements. The electron affinity of F4-TCNQ (5.24 eV) is found to be equal, within experimental error, to the ionization energy of ZnPc (5.28 eV), consistent with efficient host-to-dopant electron transfer. As a result, the Fermi level in doped ZnPc drops from near midgap to 0.18 eV above the leading edge of the highest occupied molecular orbital and a narrow space-charge layer (<32 A) is formed at the interface with the Au substrate. In situ I–V measurements show a seven orders of magnitude doping-induced increase in hole current.

Evidence for a multiple superconducting gap in MgB(2) from high-resolution photoemission spectroscopy.

Abstract: We study the new binary intermetallic superconductor ${\mathrm{MgB}}_{2}$ using high-resolution photoemission spectroscopy. The superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a simple isotropic-gap opening. The spectrum can be well reproduced using the weighted sum of two Dynes functions with the gap sizes of 1.7 and 5.6 meV. Temperature-dependent study shows that both gaps close at the bulk transition temperature. These results provide spectroscopic evidence for a multiple gap of ${\mathrm{MgB}}_{2}$.

Electron affinity of AlxGa1−xN(0001) surfaces

Abstract: The electronic properties and the electron affinities of AlxGa1−xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy (UPS) over the whole composition range. The samples were prepared by N-ion sputtering and annealing. Surface cleanliness and stoichiometry were monitored with x-ray photoemission spectroscopy. Samples with high aluminum content showed traces of oxygen which could not be removed by further cleaning cycles. However, we have evidence that the oxygen is located in the bulk and not at the surface. From the UP spectra the ionization energies and electron affinities as a function of composition x were determined. A decrease in electron affinity with increasing aluminum content was found, but the electron affinity remains positive for all x. Thus, earlier predictions of negative electron affinity for high aluminum content were not confirmed.

Red organic light emitting device made from triphenylene hexaester and perylene tetraester

Abstract: Saturated red light emission from organic light emitting diodes is less common than emission in the green or the blue. Most organic red light emitting devices are based on rare earth complexes, mainly europium, which are known to exhibit stability problems. The present article describes new diodes made of indium tin oxide-coated glass/triphenylene hexaether/perylene tetraester/aluminum. The band diagram was determined by ultraviolet photoemission spectroscopy, cyclic voltammetry, scanning tunneling microscopy, and absorbance measurements. The interfaces between electrodes and organic layers were investigated by x-ray photoelectron spectroscopy. The current–voltage and luminance–voltage characteristics are very reproducible from device to device, with an emission peak at 620 nm and a full width at half maximum of 80 nm, a current rectification ratio of about 30, I∼V2 at low voltages and I∼Lum∼V6 at higher voltages.

Chemical and electrical properties of interfaces between magnesium and aluminum and tris-(8-hydroxy quinoline) aluminum

Abstract: The chemistry, electronic structure, and electron injection characteristics at interfaces formed between tris(8-hydroxy quinoline) aluminum (Alq3) and magnesium (Mg) and aluminum (Al) are studied via x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and current–voltage (I–V) measurements. Both metal-on-Alq3 and Alq3-on-metal interfaces are investigated. All interfaces are fabricated and tested in ultrahigh vacuum in order to eliminate extrinsic effects related to interface contamination. The propensity for Mg and Al to form covalent metal–carbon bonds leads to broad and heavily reacted interfaces when the metal is deposited on the organic film. For this deposition sequence, we propose the formation of an organometallic structure where a single metal atom attaches to the pyridyl side of the quinolate ligand of the molecule and coordinates with an oxygen atom of another ligand or of a neighboring molecule. The other deposition sequence leads to significantly more abrupt interfaces wh...

Naphthyl Radical: Negative Ion Photoelectron Spectroscopy, Franck−Condon Simulation, and Thermochemistry

Abstract: The naphthyl anion (C10H7 - , naphthalenide) is prepared in a flow tube reactor by proton transfer between NH2 - and naphthalene (C10H8). The photoelectron spectrum of this anion is measured at a fixed laser wavelength of 364 nm. A single electronic band is observed, corresponding to the neutral naphthyl radical (C 10H7, naphthalenyl). The Franck-Condon profiles for both 1-naphthyl (R-naphthyl) and 2-naphthyl (‚-naphthyl) are simulated on the basis of density functional theory calculations of the vibrational frequencies and normal coordinates. Issues involving Franck-Condon simulations for large polyatomic molecules and Duschinsky rotation are discussed, and improved Franck-Condon simulation algorithms are presented. Comparison of the Franck-Condon simulations with the photoelectron spectrum shows that the observed band is predominantly due to the 1-naphthyl isomer, consistent with previous measurements showing the 1-naphthyl anion as more stable than the 2-naphthyl anion. The observed electron affinity of the 1-naphthyl radical is EA 0(1-C10H7) ) 1.403 ( 0.015 eV. On the basis of an evaluation of literature data, the recommended gas-phase acidity of naphthalene is ¢acidH298(1-C10H7-H) ) 1649 ( 14 kJ/mol and the recommended bond dissociation energy of naphthalene is DH298(1-C10H7-H) ) 472 ( 14 kJ/mol.

Systematics of 4f electron energies relative to host bands by resonant photoemission of rare earth ions in aluminum garnets

Abstract: The energies of trivalent rare earth ions relative to the host valence band were measured for a series of rare-earth-doped yttrium aluminum garnets, RxY3-xAl5O12 (R=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and 0

Anisotropy of quasiparticle lifetimes and the role of disorder in graphite from ultrafast time-resolved photoemission spectroscopy

Abstract: Femtosecond time-resolved photoemission of photoexcited electrons in highly oriented pyrolytic graphite (HOPG) provides strong evidence for anisotropies of quasiparticle (QP) lifetimes. Indicative of such anisotropies is a pronounced anomaly in the energy dependence of QP lifetimes between 1.1 and 1.5 eV---the vicinity of a saddle point in the graphite band structure. This is supported by recent ab initio calculations and a comparison with experiments on defect-enriched HOPG which reveal that disorder, e.g., defects or phonons, increases electron energy relaxation rates.

Quantum electronic stability of atomically uniform films.

Abstract: We have studied the structural stability of thin silver films with thicknesses of N = 1 to 15 monolayers, deposited on an Fe(100) substrate. Photoemission spectroscopy results show that films of N = 1, 2, and 5 monolayer thicknesses are structurally stable for temperatures above 800 kelvin, whereas films of other thicknesses are unstable and bifurcate into a film with N ± 1 monolayer thicknesses at temperatures around 400 kelvin. The results are in agreement with theoretical predictions that consider the electronic energy of the quantum well associated with a particular film thickness as a significant contribution to the film stability.

Electronic structure, diffusion, and p-doping at the Au/F16CuPc interface

Abstract: Interfaces formed by evaporating Au on copper hexadecafluorophthalocyanine (F16CuPc) and F16CuPc on Au are studied via ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy. The energy position of the molecular levels is found to depend on the deposition sequence. The Au-on-F16CuPc interface exhibits a lower hole injection barrier than the F16CuPc-on-Au interface. This behavior is attributed to the diffusion of Au into the organic matrix at the Au-on-top interface, in which they behave as acceptors. Band bending consistent with p doping, i.e., upward away from the metal interface, is observed in Au-doped F16CuPc films. Current–voltage measurements on Au/F16CuPc/Au sandwich structures show a higher electron injection barrier at the top Au electrode contact than at the bottom electrode contact, also consistent with p-type doping F16CuPc by Au.

The Ã2E/B̃2B2 Photoelectron Bands of Allene beyond the Linear Coupling Scheme: An ab Initio Dynamical Study Including All Fifteen Vibrational Modes†

Abstract: In an earlier publication [J. Chem. Phys. 1999, 111, 10452] we theoretically investigated the photoelectron spectrum of allene (C3H4+) pertinent to the A2E/B2B2 interacting electronic manifold of its radical cation (C3H4+). Employing a linear vibronic coupling scheme it was demonstrated that in addition to the E⊗B Jahn−Teller activity within the 2e electronic manifold, there is a strong (E⊗B) + E pseudo-Jahn−Teller interaction with the B2B2 electronic state, which causes the diffuse structures observed at high energies. Here, the same photoelectron spectrum is reinvestigated including all fifteen vibrational degrees of freedom of the system and a higher order coupling scheme. The coupling parameters of the Hamiltonian are calculated by ab initio methods. The photoelectron band is calculated by the wave packet propagation method within the multiconfiguration time-dependent Hartree (MCTDH) scheme and compared with the experimental results of Baltzer et al. [Chem. Phys. 1995, 196, 551]. The progressions at...

Direct observation of Fermi-level pinning in Cs-doped CuPc film

Abstract: The electronic structures of pristine and Cs-doped CuPc films are investigated using photoemission spectroscopy and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital can be directly observed by IPES and ultraviolet photoemission spectroscopy simultaneously. We found that the Fermi-level position in organic film can be modified by Cs doping. The observed onset of the LUMO of the CuPc film is shifted by Cs doping to less than 0.2 eV above the Fermi level. The result indicates that the energy alignment and charge injection properties of the organic materials can be modified by a simple doping process.

Laser-based apparatus for extended ultraviolet femtosecond time-resolved photoemission spectroscopy

Abstract: A novel laser-based apparatus is presented utilizing high harmonic radiation for visible pump–EUV probe experiments on ultrafast processes. True femtosecond temporal resolution is achieved by a monochromator making use of dedicated narrowband multilayer mirrors rather than gratings for selection of single harmonic orders in the photon energy range between 66 and 73 eV. First applications of this new light source for electron spectroscopy on gas phase helium and xenon demonstrate the selection of a single high harmonic order with the intensity ratio between the selected and its adjacent harmonic not exceeding 10:1. A pump–probe study of hot electron production on a solid Pt(110) surface yields a cross-correlation corresponding to a temporal system resolution of 100 fs.

Electronic structure of Ca1 − xSrxVO3: A tale of two energy scales

Abstract: We investigate the electronic structure of Ca1 − xSrxVO3 using photoemission spectroscopy. Core level spectra establish an electronic phase separation at the surface, leading to a distinctly different surface electronic structure compared to the bulk. Analysis of the photoemission spectra of this system allowed us to separate the surface and bulk contributions. These results help us to understand properties related to two vastly differing energy scales, namely the low-energy scale of thermal excitations ( ~ kBT) and the high-energy scale related to Coulomb and other electronic interactions.

Observation of dx2-y2-Like Superconducting Gap in an Electron-Doped High-Temperature Superconductor

Abstract: High-resolution angle-resolved photoemission spectroscopy of the electron-doped high-temperature superconductor Nd2-xCexCuO4 (x = 0.15, transition temperature Tc = 22 K) has found the quasiparticle signature as well as the anisotropicdx2-y2–like superconducting gap. The spectral line shape at the superconducting state shows a strong anisotropic nature of the many-body interaction. The result suggests that the electron-hole symmetry is present in the high-temperature superconductors.

Photoemission spectroscopy of size selected zinc sulfide nanocrystallites

Abstract: We report photoemission spectroscopic studies of zinc sulfide nanocrystallites in the quantum size regime. The nanocrystallites studied have average sizes of 1.8, 2.5, and 3.5 nm and narrow size distributions as determined from UV-visible absorption spectroscopy, as well as x-ray diffraction and high-resolution transmission electron microscopy. Analysis of sulfur core levels from the nanocrystallites show the presence of the three types of sulfur species corresponding to the core, the surface, and the capping layer of the nanocrystallites. We show that a quantitative analysis of these different sulfur components can be used to estimate the sizes of the nanocrystallites; thus, the obtained sizes are in good agreement with the sizes determined independently from small angle x-ray diffraction and high-resolution electron microscopy.

Electronic structure of Ca$_{1-x}$Sr$_x$VO$_3$: a tale of two energy-scales

Abstract: We investigate the electronic structure of Ca$_{1-x}$Sr$_x$VO$_3$ using photoemission spectroscopy. Core level spectra establish an electronic phase separation at the surface, leading to distinctly different surface electronic structure compared to the bulk. Analysis of the photoemission spectra of this system allowed us to separate the surface and bulk contributions. These results help us to understand properties related to two vastly differing energy-scales, namely the low energy-scale of thermal excitations (~$k_{B}T$) and the high-energy scale related to Coulomb and other electronic interactions.

Atomic scale oxidation of a complex system: O2/alpha-SiC(0001)-( 3 x 3).

Abstract: The atomic scale oxidation of the alpha-SiC(0001)-(3 x 3) surface is investigated by atom-resolved scanning tunneling microscopy, core level synchrotron radiation based photoemission spectroscopy, and infrared absorption spectroscopy. The results reveal that the initial oxidation takes place through the relaxation of lower layers, away from the surface dangling bond, in sharp contrast to silicon oxidation.

Photoemission spectroscopy in solids

Abstract: The photoelectric effect was discovered and explained in terms of quantum physics about a hundred years ago. Today, photoemission spectroscopy using vacuum ultra- violet radiation and x-rays is an important tool for studying the electronic properties and atomic structure of solids. This paper reviews the development of an understanding of the photoemission process and the basic applications of photoemission spectroscopy in solids, including band structure determination, studies of quasiparticle properties, and core level spectroscopy.

On the photoelectron spectrum of p-benzoquinone

Abstract: A high-resolution photoelectron spectrum of p-benzoquinone in the low energy (9.5–11.5 eV) region is reported and analyzed with the aid of simulations based on high-level ab initio calculations. The results generally support the notion that the two prominent spectral features in this region are each due to a pair of final ion states. The lower energy feature beginning near 10 eV is due to oxygen lone-pair ionizations, while that beginning near 11 eV comes from π electron removal. Contrary to previous interpretations of the spectrum, however, the results of this study indicate that the two π states are nearly degenerate, with the strongest peak in the photoelectron spectrum representing a convolution of the corresponding pair of 0–0 ionizations.

Thermal stability of stacked high-k dielectrics on silicon

Abstract: The thermal stability of ZrO2/ZrSixOy and ZrO2/ZrSixOy/SiNx thin films on silicon was examined by synchrotron radiation ultraviolet photoemission spectroscopy. The ZrO2/ZrSixOy layer deposited by atomic-layer-controlled deposition is stoichiometric, uniform, amorphous, and has an equivalent oxide thickness of ∼1 nm and a dielectric constant of ∼18 with low leakage current. These ZrO2/ZrSixOy samples are thermally stable in vacuum up to 880 °C at which the film decomposed to form ZrSi2, the most thermodynamically stable metal silicide at a per zirconium atom basis, and the desorption of SiO(g) and ZrO(g) accounted for the greatly reduced oxygen and zirconium photoemission intensities. The thermal stability of ZrO2/ZrSixOy is improved to 950 °C when deposited on a 0.5–0.7 nm SiNx film.

X-ray photoemission study of MgB2

Abstract: A c-axis oriented thin film and a high density sintered pellet of MgB2 have been studied by x-ray photoemission spectroscopy, and compared to measurements from MgO and MgF2 single crystals The as-grown surface has a layer which is Mg-rich and oxidized, which is effectively removed by a nonaqueous etchant The subsurface region of the pellet is Mg-deficient This nonideal near-surface region may explain varied scanning tunneling spectroscopy results The MgB2 core level and Auger signals are similar to measurements from metallic Mg and transition metal diborides, and the measured valence band is consistent with the calculated density of states