Core-level Photoelectron Spectra Research Articles

Electronic structure of Fe-Pd alloys studied by using photoemission spectroscopy

We investigated the electronic structure of Fe x Pd1−x (x = 0.25, 0.5, and 0.75) alloys by measuring valence-band and core-level photoelectron spectra. The Fe 3d and Pd 4d partial spectral weights were determined by using the Cooper minimum phenomenon of the Pd 4d photoionization cross section. We found that the experimentally determined Fe partial spectral weight of Fe50Pd50 alloy differ much from the band calculation results, and we could not observe a spectral structure due to the Pd 4d states mixed with the Fe 3d majority states at the binding energy of 0.9 eV. We suggest that a plausible explanation for these discrepancies should be the spin-dependent lifetime of the Fe 3d states.

Communication: Direct evidence for sequential dissociation of gas-phase Fe(CO)5 via a singlet pathway upon excitation at 266 nm.

We prove the hitherto hypothesized sequential dissociation of Fe(CO)5 in the gas phase upon photoexcitation at 266 nm via a singlet pathway with time-resolved valence and core-level photoelectron spectroscopy with an x-ray free-electron laser. Valence photoelectron spectra are used to identify free CO molecules and to determine the time constants of stepwise dissociation to Fe(CO)4 within the temporal resolution of the experiment and further to Fe(CO)3 within 3 ps. Fe 3p core-level photoelectron spectra directly reflect the singlet spin state of the Fe center in Fe(CO)5, Fe(CO)4, and Fe(CO)3 showing that the dissociation exclusively occurs along a singlet pathway without triplet-state contribution. Our results are important for assessing intra- and intermolecular relaxation processes in the photodissociation dynamics of the prototypical Fe(CO)5 complex in the gas phase and in solution, and they establish time-resolved core-level photoelectron spectroscopy as a powerful tool for determining the multiplicity of transition metals in photochemical reactions of coordination complexes.

Growth of Bi2Se3 topological insulator thin film on Ge(1 1 1) substrate

Atomically smooth, single crystalline Bi2Se3 thin films were grown on a Ge(111) substrate using molecular beam epitaxy. Crystallinities of both the surface and the bulk as well as the stoichiometry of the grown film were characterized by using low-energy electron diffraction, scanning tunneling microscopy, X-ray diffraction, and photoelectron spectroscopies. Hexagonal atomic structures, quintuple layer steps observed in STM images confirmed that the Bi2Se3 film with a (0001) surface was grown. Diffraction peak positions as well as the chemical composition determined from the core-level photoelectron spectra coincide well with those expected for the Bi2Se3 crystal. The surface state with a Dirac cone was observed in the valence photoelectron spectra, which also support that a high-quality Bi2Se3 film was grown on the Ge(111) substrate. The interface between Ge(111) and Bi2Se3(0001) is expected to be abrupt due to the small lattice between them.

The effects of core-level broadening in determining band alignment at the epitaxial SrTiO3(001)/p-Ge(001) heterojunction

Chemical effects at the surface and interface can broaden core-level spectra in X-ray photoemission for thin-film heterojunctions, as can electronic charge redistributions. We explore these effects and their influence on the measurement of valence and conduction band offsets at the epitaxial SrTiO3(001)/p-Ge(001) heterojunction. We observe a clear broadening in Ge 3d and Sr 3d core-level X-ray photoelectron spectra relative to those of clean, bulk Ge(001), and homoepitaxial SrTiO3(001), respectively. Angle-resolved measurements indicate that this broadening is driven primarily by chemical shifts associated with surface hydroxylation, with built-in potentials playing only a minor role. The impact of these two interpretations on the valence band offset is significant on the scale of transport energetics, amounting to a difference of 0.2 eV.

Temperature dependence of Yb valence in the sub-surface of YbB12(001)

The typical Kondo insulator, YbB12, is a candidate of topological Kondo insulators. To clarify the surface electronic structure of not only bulk but also surface by photoelectron measurements, we developed a method to obtain a clean surface of YbB12(001) by annealing at 1650 K in an ultra-high vacuum. By surface-sensitive photoelectron measurements, it was known that the surface consists of only B atoms without Yb. In the sub-surface region, Yb atoms exist and the temperature dependence of the valence agrees with those of previous HAXPES results. Here we report the cleaning method to obtain well-defined YbB12(001) surfaces and the temperature-dependent Yb valence obtained from Yb 3d core-level and valence photoelectron spectra.

Laser induced Bi diffusion in As40S60 thin films and the optical properties change probed by FTIR and XPS

Amorphous chalcogenide semiconducting materials are playing a pivotal role in modern technology. Such type of materials are very sensitive to electromagnetic radiations which is useful for infrared optics. In the present report, Bi doped in As40S60 thin films (As40S60, Bi06As40S54) of 800 nm thickness were prepared by thermal evaporation method. The Bi06As40S54 thin film is subjected to laser irradiation for photo induced study. The X-ray diffraction study reveals no structural change due to laser irradiation. The optical parameters are affected by both Bi addition and laser irradiation which brings a change in the transmitivity and absorption coefficient. The indirect optical band gap is found to be increased by 0.08 eV with laser irradiation with the decrease in disorderness. The Tauc parameter and Urbach energy which measures the degree of disorderness changes with Bi doping and irradiation. The refractive index is modified by the illumination process which is useful for optical applications. The optical property change is well supported by the X-ray photoelectron core level spectra.

Effect of Al and Fe doping in ZnO on magnetic and magneto-transport properties

The structural, magnetic and magneto-transport of undoped ZnO, Zn0.97Al0.03O, Zn0.95Fe0.05O and Zn0.92Al0.03Fe0.05O thin films grown on Si(100) substrate using pulsed laser deposition were investigated. The single phase nature of the films is confirmed by X-ray diffraction and Raman spectroscopy measurements. The possibility of Fe metal cluster in Fe doped/co-doped films is ruled out by Fe 2p core level photoelectron spectra. From O 1s core level spectra it is observed that oxygen vacancy is present in all the films. The undoped ZnO film shows magnetic ordering below ∼175K, whereas Fe doped/codoped samples show magnetic ordering even at 300K. The Al doped sample reveals paramagnetic behavior. The magneto-transport measurements suggest that the mobile carriers undergo exchange interaction with local magnetic moments.

Bi atoms mobility-driven circular domains at the Bi/InAs(111) interface

Bi films deposited on InAs(111) A and B sides have been studied by photoemission electron microscopy. A series of snapshots acquired during sequential annealing of the interfaces at temperatures below and above the melting temperature of Bi allowed obtaining a comprehensive image of the topographic and chemical evolutions of the Bi films that are found to be InAs side dependent. On the A side, a morphology of circular patterns controlled by Bi atoms mobility is observed. The patterns are formed on the pristine In-terminated InAs(111) surface covered by a weakly bonded Bi bilayer. On the B side, no particular morphology is observed due to a stronger chemical interaction between Bi and As atoms as evidenced by the spatially-resolved core-level photoelectron spectra.

Atomistic aspects of carrier concentration variation in post-annealed indium tin oxide films

Post-annealing environment-dependent optical and electrical properties of indium tin oxide films grown on glass were examined. X-ray diffraction measurements revealed that all of the films exhibited poly-crystallinity after annealing at 400 °C for 10 min O2, in-air and N2. The optical property measurements yielded >80% transmittances for all the films except for the as-grown and O2-annealed films, even though there were no significant optical band-gap energy differences. In the Hall measurements, all of the films exhibited n-type characteristics. However, the film annealed under the N2 environment showed the best electrical properties (highest carrier concentration and conductivity). The physical origin of electrical property variations due to annealing environment differences was explained by examining the core-level x-ray photoelectron spectra.

Multi-reference approach to the calculation of photoelectron spectra including spin-orbit coupling.

X-ray photoelectron spectra provide a wealth of information on the electronic structure. The extraction of molecular details requires adequate theoretical methods, which in case of transition metal complexes has to account for effects due to the multi-configurational and spin-mixed nature of the many-electron wave function. Here, the restricted active space self-consistent field method including spin-orbit coupling is used to cope with this challenge and to calculate valence- and core-level photoelectron spectra. The intensities are estimated within the frameworks of the Dyson orbital formalism and the sudden approximation. Thereby, we utilize an efficient computational algorithm that is based on a biorthonormal basis transformation. The approach is applied to the valence photoionization of the gas phase water molecule and to the core ionization spectrum of the [Fe(H2O)6](2+) complex. The results show good agreement with the experimental data obtained in this work, whereas the sudden approximation demonstrates distinct deviations from experiments.

Defect-induced optical and electrical property modification in amorphous InGaZnO4 films

We report the physical characteristics of InGaZnO4 films deposited at various temperatures. The films were deposited on Al2O3(0001) substrates using pulsed laser deposition technique. Based on X-ray diffraction and field emission scanning electron microscopy measurements, the crystal structure changed from amorphous to polycrystalline as deposition temperature increased to 550°C. Furthermore, UV–vis measurements revealed a decrease in tail state i.e. improvement of local ordering, resulting in an increase of optical band-gap energy as deposition temperature increased. The core-level X-ray photoelectron spectra also showed an increase (decrease) in metal-oxide (oxygen deficiency) bond as the deposition temperature increased. The carrier concentration, Hall mobility and conductivity variation with deposition temperature are related to the competition between oxygen deficiency and grain boundary formation.

X-ray photoelectron spectra of some Ni mono- and polynuclear complexes

The core-level photoelectron spectra for Ni(bpy)2(OOCCMe3)2, Ni2(H2O)bpy2(OOCCMe3)4, Nibpy(OOCCMe3)2, Ni(C10H4(CH3)2N(SO2C6H4(CH3), Ni2(2,4-Lut)2(OOCCMe3)4, and (2,4-Lut is 2,4-lutidine) are reported and discussed, and the role of ligands in their appearance is determined. X-ray irradiation of some complexes was found to induce removal of some pyridyl rings and oligomerization both at room and liquid nitrogen temperature. The dependence of composition on temperature was used to find correlation between ligand surrounding and the spectra.

XPS and DFT studies of the electronic structures of AgAuS and Ag3AuS2

X-ray photoelectron spectra of core levels and the valence bands were acquired, and the density-of-states (DOS) were calculated using periodic density functional theory (DFT) for synthetic ternary compounds AgAuS and Ag3AuS2, known as minerals petrovskaite uytenbogaardtite, respectively, for the first time. The chemical shift of the Au 4f binding energy as large as 0.65eV relative to bulk elemental gold was found for Ag3AuS2, in contrast to only about 0.3eV for AgAuS. The DFT analysis showed, in particular, that the S 3p states are largely pushed out from the Ag 4d and Au 5d bands affording two sub-bands in the valence region of both substances, but the overlapping of the S 3p and Au 5d states is larger for Ag3AuS2. It was demonstrated that the energy position of Au 4f levels is mainly influenced by Au–Ag interaction, and increasing the number of Ag atoms in the first coordination shell of S–Au–S units on going from AgAuS to Ag3AuS2 results in the positive shift of the Au 4f bands consistent with the experimental data.

Reducibility of Co at the La0.8Sr0.2CoO3/(La0.5Sr0.5)2CoO4hetero-interface at elevated temperatures

The fast kinetics of oxygen reduction reaction (ORR) at oxide hetero-structures made of La0.8Sr0.2CoO3 and (La0.5Sr0.5)2CoO4 (LSC113/214) attracted great interest to enable high performance cathodes for solid oxide fuel cells. The aim of this work is to uncover the underlying mechanism of fast ORR kinetics at the LSC113/214 system from a defect chemistry and electronic structure perspective. X-ray photoelectron spectroscopy with depth profiling was used to compare the reducibility of the Co cation and the valence band offset in the LSC113/214 multilayer (ML) and in single phase LSC113 and LSC214 films. At 250 °C, the Co 2p core-level photoelectron spectra showed the presence of Co2+ across the ML interfaces in both the LSC113 and LSC214 layers. While this is similar to the Co valence state of LSC113 single phase films, it is contrary to the single phase LSC214 films which had only the higher oxidation state of cobalt, Co3+. The greater reducibility of Co in LSC214 in the ML structure compared to that of Co in the LSC214 single phase film was attributed to electron donation and transfer of oxygen vacancies from LSC113 across the interfaces, and it is one mechanism to enhance the oxygen reduction activity at the LSC113/214 hetero-structure.

Coherent metallic screening in core-level photoelectron spectra for the strongly correlated oxides La1−xBaxMnO3and V1−xWxO2

Coherent metallic screening structures shown in core-level photoemission spectra for strongly correlated oxide materials were studied by hard x-ray photoemission spectroscopy (HAXPES) and the configuration interaction (CI) theory based on the cluster model, including the coherent metallic screening process. For the La${}_{1\ensuremath{-}x}$Ba${}_{x}$MnO${}_{3}$ thin film, the normalized intensity of the coherent metallic screening structure (${I}_{S}$) seen in the Mn 2$p$ core-level spectra was proportional to the square of the hybridization strength, ($V$*), between the transition metal 3$d$ and coherent metallic states. In contrast, the normalized ${I}_{S}$ seen in the V 2$p$ core-level spectra for the V${}_{1\ensuremath{-}x}$W${}_{x}$O${}_{2}$ thin film was not proportional to ($V$*). The different behaviors of the normalized ${I}_{S}$ for the La${}_{1\ensuremath{-}x}$Ba${}_{x}$MnO${}_{3}$ and V${}_{1\ensuremath{-}x}$W${}_{x}$O${}_{2}$ thin films as a function of $V$* were understood by the series of the CI cluster model calculation. From the CI cluster model calculation, we found that the charge transfer energy (\ensuremath{\Delta}*) between the transition metal 3$d$ states and the coherent metallic states strongly affect the normalized ${I}_{S}$ in the 2$p$ core-level photoemission final states. Therefore, the behavior of the normalized ${I}_{S}$ in the 2$p$ core-level HAXPES is thus not simply explained by the change of $V$*. The electronic structure parameters such as \ensuremath{\Delta}* and $V$* relating to the coherent metallic states strongly contribute to the variation of the normalized ${I}_{S}$ in the photoemission final states. In contrast, we found that the detailed ${I}_{S}$ evaluation in the core-level HAXPES experiments for materials, in which the coherent metallic screening structures appear, allows us to evaluate the value of $V$* when we have the experimental core-level spectra and the electronic structure parameters set for a reference material. We also found that the intensity at the Fermi level is proportional to ($V$*), as expected from the Anderson impurity model.

Impact of Mg concentration on energy-band-depth profile of Mg-doped InN epilayers analyzed by hard X-ray photoelectron spectroscopy

The electronic structures of Mg-doped InN (Mg-InN) epilayers with the Mg concentration, [Mg], ranging from 1 × 1019 to 5 × 1019 cm−3 were systematically investigated by soft and hard X-ray photoelectron spectroscopies. The angle-resolved results on the core-level and valence band photoelectron spectra as a function of [Mg] revealed that the energy band of Mg-InN showed downward bending due to the n+ surface electron accumulation and p type layers formed in the bulk. With an increase in [Mg], the energy-band changed from monotonic to two-step n+p homojunction structures. The oxygen concentration rapidly increased at the middle-bulk region (∼4.5 to ∼7.5 nm) from the surface, which was one of the reasons of the transformation of two-step energy band.

Tl1−xIn1−xSnxSe2 (x = 0, 0.1, 0.2, 0.25) single-crystalline alloys as promising non-linear optical materials

Novel materials for the infrared two-photon absorption—Tl1−xIn1−xSnxSe2 single crystals (x = 0, 0.1, 0.2, 0.25) were grown. Their optoelectronic properties including two-photon absorption at wavelength equal to 9.4 μm at different temperatures were studied. From the spectral data it was established that band gap energy increases with increasing SnSe2 content in the solid solutions reaching its maximum for the largest content (x = 0.25). The infrared two photon absorption has achieved its maximal value at x = 0.1. The temperature dependence of the optical band gap at various compositions in the range of 77–300 К is practically linear. The band gap value decreases with the increase of T causing the spectral shift of the absorption edge to the low-energy region. Additionally X-ray photoelectron core-level spectra for pristine and Ar+-ion irradiated surfaces of Tl1−xIn1−xSnxSe2 single crystals have been measured.

Strain induced intermixing of Ge atoms in Si epitaxial layer on Ge(111)

The relationship between strain and effective mass of the √3 × √3-Ag structure surface metallic state, when formed on strained Si(111) layers on Ge(111), has been investigated by angle resolved photoelectron spectroscopy at various film thicknesses. Si layer lattice spacing expands and effective mass decreases at film thickness greater than 2 bilayers. This result is inconsistent with a previous study showing increasing effective mass with tensile strain for the √3 × √3-Ag structure. Ge-3d core level photoelectron spectra confirm that this disparity is caused by the intermixing of Ge atoms in the Si layer. The relationship between effective mass and strain is useful for gauging intermixing, and the effective mass of surface metallic states is useful for gauging nanoscale strain.

5f Electron correlations and core level photoelectron spectra of Uranium compounds

AbstractMicroscopic model calculations for core‐level spectra of U compounds are presented. The consequences of strong intra‐atomic correlations within the 5f shell are analyzed. The formation of atomic multiplets leads to a re‐distribution of spectral weight. The results suggest that the orbital‐selective localization resulting from strongly anisotropic renormalization of the hybridization between 5f‐ and conduction electrons should be reflected in the line shapes.

Molecular beam epitaxial growth of graphene using cracked ethylene

We investigated the growth of graphene by molecular beam epitaxy (MBE) using cracked ethylene as a growth source gas under various growth conditions. The analyses of the C 1s core-level photoelectron spectra and the D-, G-, and 2D-band peaks in Raman spectra revealed that graphene was successfully grown by MBE when cracked ethylene was used as a source gas. Analyses of surface morphology by atomic force microscopy and the substrate temperature dependence of the grown graphene thicknesses indicated that the graphene growth process is essentially the same as that for ethanol. Furthermore, when graphene was grown under a high vacuum, the flow-rate dependence of the deposition rate of graphitic materials demonstrated the expected trend that the deposition rate increased with increasing flow rate. In addition, we found that use of the ethylene improves the domain size of the MBE graphene and that the high-vacuum growth further enhances it, suggesting that the migration enhancement of the growth materials is the key to improving the quality of the graphene in MBE growth.