Core-level Photoemission Spectra Research Articles

XANES and X-ray photoemission of the La 0.67(Ca,Pb) 0.33(Mn 1− xFe x)O 3 compounds

High energy resolution fluorescence detected XANES and the core-level X-ray electron photoemission spectra of lead and iron substituted manganese perovskites La 0.67(Ca,Pb) 0.33(Mn 1− x Fe x )O 3 with x=0, 0.01, 0.03, 0.06, 0.10 and 0.15 were measured and analyzed. The evolution of the average valence of manganese ions was derived from the energy shift of the Mn K-edge as a function of iron content, which changes by ∼0.25 eV from x=0.0 to x=0.15. This agrees well with the expected average valence evolution from Mn 3.33+ to Mn 3.38+. The relative ratio of Mn 3+/Mn 4+ was calculated as a function of x by means of multiplet analysis of the Mn 2 p 3/2 photoemission lines. They are in fairly good agreement with the expected values.

Charge transfer dynamics of model charge transfer centers of a multicenter water splitting dye complex on rutile TiO2(110)

Charge transfer dynamics between an adsorbed molecule and a rutile TiO(2)(110) surface have been investigated in three organometallic dyes related to multicenter water splitting dye complexes: Ru 535 (cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)), Ru 455 (cis-bis(2,2'-bipyridyl)-(2,2'-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(II)), and Ru 470 (tris(2,2'-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(II)). The adsorption of the dye molecules on the rutile TiO(2)(110) surface has been studied using core-level and valence photoemission. Dye molecules were deposited in situ using ultrahigh vacuum electrospray deposition. Core-level photoemission spectra reveal that each complex bonds to the surface via deprotonation of two carboxylic groups. All three dye complexes show evidence of ultrafast charge transfer to the TiO(2) substrate using the core-hole clock implementation of resonant photoemission spectroscopy.

Nano-Scale Characterization of Poly-Si Gate on High-k Gate Stack Structures by Scanning Photoemission Microscopy

We have investigated poly-Si gate pattern on HfO2/SiO2/Si stack structures by scanning photoemission microscopy using synchrotron radiation. Gate-pattern images have been successfully obtained by scanning the sample position during acquisitions of core-level photoemission spectra. Si 2p, Hf 4f, and O 1s core-level spectra systematically change for different sample positions, which suggests that light source focused by the zone plate is effectively utilized for nano-scale characterization of chemical states in device-pattern structures. We have also demonstrated that etching velocities of HfO2 films on a Si substrate and shallow trench isolation vary when exposing the poly-Si gate pattern to dry etching processes. [DOI: 10.1380/ejssnt.2011.224]

Investigation of surface and non-local screening effects in the Ni 2p core level photoemission spectra of NiO

We studied the Ni 2p core-level spectra of NiO using cluster model calculations and X-ray photoemission spectra. The calculations were performed for a NiO6, a NiO5, and a NiO6:NiO5 cluster, which allows studying the bulk, the surface, and the non-local effects in the spectra. The photoemission spectra were taken with soft (or hard) X-rays to enhance the surface (or bulk) contribution. The calculation shows that both surface and non-local-effects contribute to the shoulder in the experimental data. The photoemission spectra show that the relative intensity of the surface contribution decreases for Hard X-rays.

Tautomerism in 4-Hydroxypyrimidine, S-Methyl-2-thiouracil, and 2-Thiouracil

The keto-enol tautomerism of 4-hydroxypyrimidine and of the related molecules S-methyl-2-thiouracil and 2-thiouracil has been investigated using synchrotron-based techniques. The populations of the constituent tautomers and thermodynamic parameters have been obtained by analysis of core-level photoemission spectra. The effect of substituents on the stability of tautomers has been revealed. Attaching additional OH (or SH) groups to the aromatic ring stabilizes the dioxo (or oxo-thione) forms. However, substitution of hydrogen in position 2 by an S-CH(3) group (that is, in going from 4-hydroxypyrimidine to S-methyl-2-thiouracil) does not significantly affect the tautomeric equilibrium.

Effects of Processing Conditions on the Work Function and Energy-Level Alignment of NiO Thin Films

We have investigated NiO thin films prepared by in situ and ex situ ozone oxidation, as well as air-exposed and vacuum-annealed NiO films. The core-level and valence-level photoemission spectra, as well as the work function and energy-level alignment with a common hole-injection material, have been measured using X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. We found that in situ oxidation results in the formation of a purely NiO film, while ex situ oxidation and air exposure result in a hydroxide-terminated NiO film. Work functions as high as 6.7 eV can be achieved for in situ-oxidized NiO; however, the work function decreases rapidly with time due to adsorption of residual gases in vacuum. The work functions of ex situ and air-exposed NiO were significantly lower, between 5.2 and 5.6 eV, due to hydroxylation of the oxide surface. We have examined the rate at which the work function decreases with air exposure and found there to be a very rapid initial decrease in work func...

Electronic structure of the two-dimensionally ordered Mn/Cu(110) magnetic surface alloy

We report a systematic investigation of the electronic properties and correlation effects in a two-dimensional magnetic surface alloy Mn/Cu(110) using a combination of spectroscopic techniques ranging from the angle-resolved photoemission, x-ray photoelectron, and x-ray absorption spectroscopy. Our results show that increasing Mn concentration surprisingly induces a progressive charge redistribution on the Cu(110) substrate, increasing the electron occupancy of the Cu Shockley surface state. Furthermore, new Mn-induced surface bands are identified at the $\overline{X}$ symmetry point that are backfolded with respect to the Brillouin-zone boundary and expose a band gap in the occupied states. Distinct satellite features in both the core-level photoemission and absorption spectra for a wide range of Mn concentrations suggest the presence of strong electronic correlations and increased ordering of this alloy with respect to its other known counterparts, such as Mn/Cu(100) and Mn/Ni(110).

Local order and hybridization effects for Mn ions probed by resonant soft x-ray spectroscopies: The Mn:CdTe(110) interface revisited

The electronic structure and the local ordering of Mn:CdTe(110) interface, produced by evaporating at room temperature (RT) 2 ML of Mn on a CdTe(110) single-crystal surface, have been probed by soft x-ray spectroscopies. Strong Mn-Te hybridization effects are found both in core level and valence-band photoemission spectra. The analysis of resonant photoelectron-diffraction patterns indicates that, already at RT, Mn atoms diffuse into CdTe down to the fourth layer, replacing the isovalent Cd atoms in the lattice.

Resonant Photoemission Spectroscopy of Layered Triangular Lattices Ag2MO2 (M = Ni and Mn): Evidence for M 3d States at Fermi Level

We report on the soft X-ray photoemission and absorption spectroscopy of Ag 2 NiO 2 and Ag 2 MnO 2 , which consist of alternate stacks of (Ag 2 ) + and ( M O 2 ) - layers. The core-level photoemission and absorption spectra indicate mainly trivalent Ni 3+ in Ag 2 NiO 2 , and trivalent Mn 3+ with an admixture of Mn 2+ states in Ag 2 MnO 2 . The M 2 p –3 d resonant photoemission clearly shows that the density of states at the Fermi level includes the M 3 d character in both compounds. The results indicate that the Ni 3 d and Mn 3 d states at the Fermi level are responsible for the significant mass enhancement of carriers, as seen in the transport properties of Ag 2 NiO 2 and Ag 2 MnO 2 .

Electronic Structure of La(Fe0.88Si0.12)13

AbstractLa(Fe0.88Si0.12)13 shows peculiar magnetic properties such as the first order paramagnetic-ferromagnetic transition and magnetic-field induced metamagnetic transition accompanied by the lattice expansion. The practical application using the magnetic transition temperature controlled by hydrogen absorption is expected in this compound. Here, the electronic structure of La(Fe0.88Si0.12)13 has been investigated by photoemission spectroscopy using synchrotron soft x-rays. The Fe 3s core-level photoemission spectra below and above the Curie temperature TC exhibit a satellite structure at ~ 4.3 eV higher binding energy than the main peak, which is attributed to the exchange splitting due to the local moment of Fe. The exchange splitting of the Fe 3s photoemission spectrum with the asymmetric line shape shows that the magnetization of La(Fe0.88Si0.12)13 is derived by the exchange split Fe 3d bands like the itinerant ferromagnetism in Fe metal, while the magnetic transition of La(Fe0.88Si0.12)13 is the first order. The valence band photoemission spectrum shows temperature dependence across the TC. The temperature dependence of the photoemission spectra is discussed based on the difference between the electronic structure in the ferromagnetic phase and that in the paramagnetic phase.

Core-level shifts at the Pt/W(1 1 0) monolayer bimetallic interface

We have measured W and Pt 4 f 7/2 core-level photoemission spectra from interfaces formed by ultrathin Pt layers on W(1 1 0), completing our core-level measurements of W(1 1 0)-based bimetallic interfaces involving the group-10 metals Ni, Pd, and Pt. With increasing Pt coverage the sequence of W spectra can be described using three interfacial core-level peaks with binding-energy (BE) shifts (compared to the bulk) of −0.220 ± 0.015, −0.060 ± 0.015, and +0.110 ± 0.010 eV. We assign these features to 1D, 2D pseudomorphic (ps), and 2D closed-packed (cp) Pt phases, respectively. For ∼1 ps ML the Pt 4 f 7/2 BE is 71.40 ± 0.02 eV, a shift of +0.46 ± 0.09 eV with respect to the BE of bulk Pt metal. The W 4 f 7/2 core-level shifts induced by all three adsorbates are semiquantitatively described by the Born–Haber-cycle based partial-shift model of Nilsson et al. [39]. As with Ni/W(1 1 0), the difference in W 4 f 7/2 binding energies between ps and cp Pt phases has a large structural contribution. The Pt 4 f lineshape is consistent with a small density of states at the Fermi level, reflective of the Pt monolayer having noble-metal-like electronic structure.

Bulk5felectronic states of the uranium monochalcogenide US as seen via soft x-ray photoemission

We have investigated the electronic states of the uranium monochalcogenide US, which is a typical ferromagnetic uranium compound, using soft x-ray photoemission spectroscopy (SX-PES). In early ultraviolet photoemission spectroscopy studies, two peak structures of the $\text{U}\text{ }5f$ states were observed and have been interpreted that one has an itinerant character around the Fermi level $({E}_{\text{F}})$ and the other located below ${E}_{\text{F}}$ has a character of localized $\text{U}\text{ }5f$ electrons. In this study, the intrinsic bulk valence-band spectrum of US was first deduced by estimating the contribution of surface states to the valence-band spectrum using core-level photoemission spectra. We conclude that the electronic structure of US can be basically described by the itinerant nature of the $\text{U}\text{ }5f$ electrons from comparison with theoretical valence-band spectra obtained by band-structure calculation in the local-density approximation.

Effect of hole-doping on the valence state and magnetic property in S = 1/2 spin-chain system LiCuVO 4

The valence state and magnetic properties of hole-doped LiCuVO 4 are investigated. By analyzing the Cu 2 p core-level photoemission spectra, the holes are suggested to be introduced into Cu 2+ site in the formation of Cu 3+ ion. The calculation of effective moment also confirms the presence of nonmagnetic Cu 3+ ion which should be responsible for the decrease of high-temperature susceptibility after hole doping. At low temperature, the antiferromagnetic transition at 26 K and 2.3 K disappears due to the enhancement of Curie term. Magnetic hysteresis at 2 K shows that there exists a small ferromagnetic moment of 0.15 emu/g in Li 0.9CuVO 4.

Theory of phonon effects on photoemission spectra

Some important phonon effects observed in core and valence level photoemission spectra are discussed on the basis of nonequilibrium Green’s function theory. This theoretical framework allows us to incorporate phonon effects, such as Debye–Waller (DW) factors, Franck–Condon (FC) factors and electron–phonon interactions in a natural way. For the description of the electron–phonon interaction the phonon part of the screened Coulomb propagator plays a crucial role. We separately discuss those effects on photoemission spectra from localized core and extended valence levels. In case of core level excitation, taking into account the core-hole effects on phonon spectra, we can relate the Franck–Condon factors to the electron–phonon interaction, which are interpreted as the intrinsic phonon losses. Phonon losses during propagation in solids can be described by loss Keldysh diagrams similar to those for the plasmon losses, which are closely related to extrinsic losses. In case of extended valence excitation we show that the phonon effects destroy the interference between photoelectron waves in high-energy region, which enhances specific features in the X-ray photoelectron diffraction as observed before. In contrast the DW factors play a very minor role in low energy (UPS) region, and we can obtain detailed information on band structures by the use of angle-resolved measurements.

An Experimental and Theoretical Core-Level Study of Tautomerism in Guanine

The core level photoemission and near edge X-ray photoabsorption spectra of guanine in the gas phase have been measured and the results interpreted with the aid of high level ab initio calculations. Tautomers are clearly identified spectroscopically, and their relative free energies and Boltzmann populations at the temperature of the experiment (600 K) have been calculated and compared with the experimental results and with previous calculations. We obtain good agreement between experiment and the Boltzmann weighted theoretical photoemission spectra, which allows a quantitative determination of the ratio of oxo to hydroxy tautomer populations. For the photoabsorption spectra, good agreement is found for the C 1s and O 1s spectra but only fair agreement for the N 1s edge.

Hybridization between the conduction band and3dorbitals in the oxide-based diluted magnetic semiconductorIn2−xVxO3

The electronic structure of In2-xVxO3 (x=0.08) has been investigated by photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The V 2p core-level photoemission and XAS spectra reveale ...

Tautomerism in Cytosine and Uracil: An Experimental and Theoretical Core Level Spectroscopic Study

The O, N, and C 1s core level photoemission spectra of the nucleobases cytosine and uracil have been measured in the vapor phase, and the results have been interpreted via theoretical calculations. Our calculations accurately predict the relative binding energies of the core level features observed in the experimental photoemission results and provide a full assignment. In agreement with previous work, a single tautomer of uracil is populated at 405 K, giving rise to relatively simple spectra. At 450 K, three tautomers of cytosine, one of which may consist of two rotamers, are identified, and their populations are determined. This resolves inconsistencies between recent laser studies of this molecule in which the rare imino-oxo tautomer was not observed and older microwave spectra in which it was reported.

Surface core-level shifts at vicinal tungsten surfaces

Core-level $4{f}_{7/2}$ photoemission spectra are measured for W(110) and for two stepped-surface planes along the {100} zone: W(540) and W(320). The vicinal-surface spectra are fit using a four-component Doniach-Sunjic--Gaussian function with variable parameters that allow independent variation in the peak locations and intensities based on the different nearest-neighbor coordination of surface atoms. The new experimental results differ significantly from all prior experimental studies of surface core-level shifts associated with vicinal tungsten surfaces. Surface step-atom core-level binding energies obtained from the photoemission spectra are found to be in excellent agreement with simple local-coordination models and initial-state shifts from ab initio calculations of surface core-level shifts and confirm general trends associated with application of coordination models to both low- and high-index tungsten surfaces. No compelling evidence of coordination-dependent final-state effects is observed. Effects on core-level spectra and surface core-level shifts associated with surface hydrogen dose are investigated. Vicinal surface spectra of tungsten are shown to be more sensitive to hydrogen doses than parent low-index surfaces, possibly accounting for prior difficulty in obtaining consistent core-level photoemission spectra for vicinal tungsten surfaces.

Core-level spectroscopy of the Pd/W(1 1 0) interface: Evidence of long-range Pd-island–W interactions at submonolayer Pd coverages

We have measured W 4 f 7/2 core-level photoemission spectra from W(1 1 0) in the presence of Pd overlayers for coverages up to ∼1 pseudomorphic monolayer (ML). At coverages close to 0.05 ML a striking change in the W core-level spectrum is observed, which we interpret as indicating a long-range lateral effect of 2D Pd islands upon the W electronic structure in both the first and second W layers. As the coverage increases the long-range effect weakens and finally vanishes near 0.85 ML. Above this coverage the W spectra are typical for a W-based bimetallic interface, with the first-layer W atoms exhibiting a small interfacial core-level shift (−95 ± 5 meV) compared to the bulk atoms.

Core-level and valence-band electron states in Zr-based bulk metallic glass

A Zr-based bulk metallic glass of Zr70Ni20Al10 has an excellent glass-forming ability (GFA), and can form a massive bulk glass by simple water-quenching. In order to clarify the role of electronic structure in this excellent GFA of Zr70Ni20Al10 glass by comparing to the Zr70Ni30 reference glass, having a worse GFA, valence-band photoemission spectra were measured with changing the incident photon energy using synchrotron radiation at BL7 of HiSOR. We have also measured Ni 3p, Zr 3d and Zr 4p core-level photoemission spectra of the Zr70Ni20Al10 glass, and compared them to those of the Zr70Ni30 reference glass. The shifts of the binding energies in every core-level are observed, indicating changes of the chemical nature by replacing the Ni atoms with Al atoms.