Valence Band X-ray Photoemission Research Articles

Indium Gallium Oxide Alloys: Electronic Structure, Optical Gap, Surface Space Charge, and Chemical Trends within Common-Cation Semiconductors.

The electronic and optical properties of (InxGa1-x)2O3 alloys are highly tunable, giving rise to a myriad of applications including transparent conductors, transparent electronics, and solar-blind ultraviolet photodetectors. Here, we investigate these properties for a high quality pulsed laser deposited film which possesses a lateral cation composition gradient (0.01 ≤ x ≤ 0.82) and three crystallographic phases (monoclinic, hexagonal, and bixbyite). The optical gaps over this composition range are determined, and only a weak optical gap bowing is found (b = 0.36 eV). The valence band edge evolution along with the change in the fundamental band gap over the composition gradient enables the surface space-charge properties to be probed. This is an important property when considering metal contact formation and heterojunctions for devices. A transition from surface electron accumulation to depletion occurs at x ∼ 0.35 as the film goes from the bixbyite In2O3 phase to the monoclinic β-Ga2O3 phase. The electronic structure of the different phases is investigated by using density functional theory calculations and compared to the valence band X-ray photoemission spectra. Finally, the properties of these alloys, such as the n-type dopability of In2O3 and use of Ga2O3 as a solar-blind UV detector, are understood with respect to other common-cation compound semiconductors in terms of simple chemical trends of the band edge positions and the hydrostatic volume deformation potential.

Ambient aging effects on the effective energy gap of ZnO thin films

Using photoconductance spectroscopy, we have studied the influence of different types of thermal annealing on epitaxial ZnO thin films where band bending effects play a major role. Once the film is exposed to ambient air conditions after a simple thermal annealing in oxygen at 600°C, the effective energy gap is stable with a value of ≃3.15 eV, while after a corresponding annealing in vacuum and subsequent air exposure, it starts at ≃3.24 eV, and then it evolves along the days until it reaches the bulk energy gap value of ZnO. By means of valence band x-ray photoemission spectroscopy (XPS), we have confirmed that these phenomena are related via the Franz–Keldysh effect to a downward band bending in the former case and a time dependent upward band bending in the latter one that slowly tends to a flat band condition, tracking the behavior observed in the effective energy gap. Core level XPS measurements suggest that for each type of thermal annealing, a different adsorption kinetics of water and hydrogen take place.

In situ reorganization of Bi3O4Br nanosheet on the Bi24O31Br10 ribbon structure for superior visible-light photocatalytic capability

In situ synthesized 2D/2D heterojunctions often exhibit a more homogeneous mixture and stronger interface interaction, thereby providing higher interfacial charge transfer and larger interface contact area compared with 0D/2D and 1D/2D heterojunctions. Herein, novel 2D/2D Bi24O31Br10/Bi3O4Br heterojunctions with a “nanosheet-on-ribbon” hierarchical structure were fabricated via in situ reorganization of original elements in a hydrothermal process, and this method was accomplished through the intervention of NH3·H2O. The Bi24O31Br10/Bi3O4Br heterojunctions exhibit outstanding visible light activity than pure Bi3O4Br and Bi24O31Br10, reaching photodegradation efficiency of 85% for sulfamethoxazole in 180 min and removal rate of 32.5% for NO in 30 min. Crystalline phases, optical absorption properties, morphologies, microstructures, and band structures were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, differential reflectance spectroscopy, valence band X-ray photoemission spectroscopy, Mott–Schottky methods, density functional theory, photoluminescence spectroscopy, photocurrent analysis, and electrochemical impedance spectroscopy. The superior photocatalytic activity could be ascribed to the special structure and interface interaction between Bi3O4Br and Bi24O31Br10, thereby enhancing surface-active sites, prolonging light absorption, and promoting charge separation capability during the photocatalytic reaction. The formation mechanism of Bi24O31Br10/Bi3O4Br heterojunctions was proposed, and NH3•H2O was not only a pH adjuster but also a structure-controlling agent. This work provides a reference for in situ synthesis of multifunctional 2D/2D photocatalytic nanocomposites.

Study of band structure, transport and magnetic properties of BiFeO3–TbMnO3 composite

Charge transfer across the interface of two materials in a composite can create reconstruction of bands near the interface which in turn brings multiple changes in physical properties of the materials. Thus, investigation of band structure experimentally is of immense importance in studying composite materials to understand their physical properties. Here, we have studied magnetoelectric multiferroic composite of two types of multiferroic (types I and II) consisting of BiFeO3 and TbMnO3 for enhanced magnetic and transport properties. The band structure was investigated with the help of UV–visible absorption spectrum, the valence band X-ray photoemission spectra (XPS), and ultraviolet photoemission spectra. The band structure thus obtained can successfully explain the magnetic and transport properties of the composite. The insulating behavior of the system is understood from the reconstruction of the energy bands at the interface and subsequent decrease in the band gap which happens due to lattice mismatch of the two materials. The large coercivity and the increase in the magnetization value are understood to be due to superexchange interaction between different Mn ions (Mn2+, Mn3+, and Mn4+). From the composition study of EDXA and core-level XPS, oxygen vacancy was found which in turn creates the mixed valence state of Mn to maintain the charge neutrality.

The role of surface depletion layer effects on the enhancement of the UV emission in ZnO induced by a nanostructured Al surface coating

The UV enhancement of Al-coated ZnO single crystals with a wide range of carrier densities is systematically studied using depth-resolved cathodoluminescence and photoluminescence as well as valence band X-ray photoemission spectroscopy (VB-XPS). An up to 17-fold enhanced PL UV emission for Al-coated ZnO with the highest carrier density was measured, which falls to a 12-fold increase for the lowest carrier density. Depth-resolved cathodoluminescence measurements confirm that the enhancement is strongest near the Al-ZnO interface consistent with an increased UV emission due to an exciton-localized surface plasmon coupling mechanism. Correlative cathodoluminescence, photoluminescence and VB-XPS studies reveal that a number of additional effects related to the presence of the Al surface coating also contribute to the UV enhancement factor. These include increased UV enhancement due to the formation of a surface depletion layer induced by the Al coating, which also passivates competitive non-radiative surface recombination channels found in uncoated ZnO. Significantly, it was established that the magnitude of the emission enhancement factor can be raised in a controlled way by reducing the thickness of the depletion layer by increasing the carrier density. The contribution of these effects collectively provides an explanation for the large span of enhancement factors reported in the literature.

Role of Ti-Ru interaction in SrTi0.5Ru0.5O3 : Physical properties, x-ray spectroscopy, and cluster model calculations

We have performed a systematic study of the physical properties and electronic structure of ${\mathrm{SrTiO}}_{3}, {\mathrm{SrTi}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{O}}_{3}$, and ${\mathrm{SrRuO}}_{3}$. For the mixed compound, the temperature dependence of the magnetization is consistent with the occurrence of $\mathrm{Ru}Ti$ and $\mathrm{Ti}Ru$ defects. Despite being a semiconductor, the behavior of the electrical resistivity as a function of temperature is compatible with the emergence of small metallic regions richer in Ru concentration, a feature supported by the finite spectral weight at the Fermi level observed in the valence-band x-ray photoemission spectroscopy spectrum. The x-ray photoemission and absorption spectra of the ${\mathrm{SrTi}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{O}}_{3}$ compound were simulated by double cluster model calculations, which include the ${\mathrm{TiO}}_{6}\ensuremath{-}{\mathrm{RuO}}_{6}$ interaction, and also by the linear combination of single cluster calculations for ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{SrRuO}}_{3}$. The results indicate that the interaction between different octahedra may give rise to distinct peak characters, depending on the experimental spectrum being calculated. We argue that these effects are only captured with the explicit inclusion of the Ti-Ru interaction.

Electronic Structure of Lanthanide-Doped Bismuth Vanadates: A Systematic Study by X-ray Photoelectron and Optical Spectroscopies

Monoclinic BiVO 4 has emerged in recent years as one of the most promising materials for photocatalytic evolution of oxygen under solar irradiation. However, it is in itself unable to phototcatalyze reduction of water to hydrogen due to the placement of the conduction band edge below the potential required for H 2 O/H 2 reduction. As a consequence, BiVO 4 only finds application in a hybrid system. Very recently, tetragonal lanthanide-doped BiVO 4 powders have been shown to be able to both reduce and to oxidize water under solar irradiation, but to date there has been no comprehensive study of the electronic properties of lanthanide-doped bismuth vanadates aimed at establishing the systematic trends in the electronic structure in traversing the lanthanide series. Here, the accessible family of lanthanide-doped BiVO 4 quaternary oxides of stoichiometry Bi 0.5 Ln 0.5 VO 4 (Ln = La to Lu, excluding Pm) has been studied by X-ray powder diffraction, X-ray photoemission spectroscopy, and diffuse reflectance optical spectroscopy. The compounds all adopt the tetragonal zircon structure, and lattice parameters decrease monotonically in traversing the lanthanide series. At the same time, there is an increased peak broadening in the diffraction patterns as the mismatch in ionic radius between Bi 3+ and the Ln 3+ ions increases across the series. Valence band X-ray photoemission spectra show that the final state 4f n-1 structure associated with ionization of lanthanide 4f n states is superimposed on the valence band structure of BiVO 4 in the quaternary materials: in the case of the Ce-, Pr- and Tb-doped BiVO 4 , 4f-related states appear above the top of the main valence band of BiVO 4 and account for the small bandgap in the Ce compound. In all cases, the 4f structure is characteristic of the lanthanide element in the Ln(III) oxidation state. Vanadium 2p and lanthanide 3d or 4d core level photoelectron spectra of those compounds where the lanthanide may in principle adopt a higher (Ln = Ce, Pr, Tb) or lower (Ln = Eu, Yb) oxidation state further confirm the prevalence of the Ln(III) valence state throughout. The visible region optical properties of all samples were studied by diffuse reflectance spectroscopy, with a particular focus on the optical bandgap and the details of transitions associated with localized 4f states. Taken together, the results demonstrate the remarkable tunability of optical and electronic properties for these quaternary materials.

Hole-induced electronic and optical transitions in La1−xSrxFeO3 epitaxial thin films

We have investigated the electronic and optical properties of epitaxial La1-xSrxFeO3 for x from 0 to 1 prepared by molecular beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward to the valence band maximum. Both Fe 2p and O 1s spectra broaden to higher binding energy with increasing x, consistent with delocalization of Sr-induced holes in the Fe 3d/O 2p hybridized valence band. Combining X-ray valence band photoemission and O K-edge x-ray absorption data, we map the evolution of the occupied and unoccupied bands and observe a narrowing of the gap, along with a transfer of state density from just below to just above the Fermi level, resulting from hole doping. In-plane transport measurements confirm that the material becomes a p-type semiconductor at lower doping levels and exhibits a insulator-to-metal transition at x equal to 1. Sub-gap optical transitions revealed by spectroscopic ellipsometry are explained based on insight from theoretical densities of states and first-principles calculations of optical absorption spectra.

Insights into the electronic structure of OsO2 using soft and hard x-ray photoelectron spectroscopy in combination with density functional theory

Theory and experiment are combined to gain an understanding of the electronic properties of OsO2, a poorly studied metallic oxide that crystallizes in the rutile structure. Hard and soft valence-band x-ray photoemission spectra of OsO2 single crystals are in broad agreement with the results of density-functional-theory calculations, aside from a feature shifted to high binding energy of the conduction band. The energy shift corresponds to the conduction electron plasmon energy measured by reflection electron energy loss spectroscopy. The plasmon satellite is reproduced by many-body perturbation theory.

X-ray photoelectron spectra for single-crystalTi2O3: Experiment and theory

We have measured high-resolution core-level and valence-band x-ray photoemission spectra for single-crystal $\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$ cleaved anoxically. The Ti(III) spectra for this lattice are considerably more complex than those measured for Ti(IV)-based oxides due to the presence of a single unpaired electron in the conduction band. This open-shell electron configuration leads to ligand-field split and frequently unresolved multiplets. The Ti $2p$ and $3p$ spectra have been calculated using relativistic Dirac-Hartree-Fock (DHF) theory with the sudden approximation for the intensities. Agreement between theory and experiment is excellent for the $3p$ spectrum, and very good for the $2p$ spectrum, the primary deficiency being a pair of features not captured by theory for the latter. The spectral line shapes are driven by final-state effects associated with angular momentum coupling of the unpaired valence electron with the core hole, one- and two-electron ligand-to-metal charge-transfer (shake) processes accompanying core photoionization, and core-hole screening by conduction-band electrons. The first two of these are accurately predicted by DHF theory with a small embedded cluster containing a single Ti cation and six oxygen ligands. The third effect is not predicted using this cluster in which screening of the core hole from electrons associated with more distant atoms is not possible.

Phase quantification by X-ray photoemission valence band analysis applied to mixed phase TiO2 powders

A method of quantitative phase analysis using valence band X-ray photoelectron spectra is presented and applied to the analysis of TiO2 anatase-rutile mixtures. The valence band spectra of pure TiO2 polymorphs were measured, and these spectral shapes used to fit valence band spectra from mixed phase samples. Given the surface sensitive nature of the technique, this yields a surface phase fraction. Mixed phase samples were prepared from high and low surface area anatase and rutile powders. In the samples studied here, the surface phase fraction of anatase was found to be linearly correlated with photocatalytic activity of the mixed phase samples, even for samples with very different anatase and rutile surface areas. We apply this method to determine the surface phase fraction of P25 powder. This method may be applied to other systems where a surface phase fraction is an important characteristic.

Ab initio study of pressure-induced phase transition, band gaps and X-ray photoemission valence band spectra of YVO4

High-pressure induced structural transition from zircon-type phase into scheelite-type phase in YVO4 is studied using ab initio calculations. Several structures with compressed volumes are evaluated, where for every considered volume the c/a ratio and atomic positions are optimised. The transition pressure and transition volume change are calculated. The reports on YVO4 electronic structure and electronic band gap behaviour are followed by results of X-ray photoemission spectra XPS calculations. Most of our theoretical predictions are compared with experimental results taken from literature.

Electronic and magnetic properties of epitaxial perovskite SrCrO3(0 0 1)

We have investigated the intrinsic properties of SrCrO3 epitaxial thin films synthesized by molecular beam epitaxy. We find compelling evidence that SrCrO3 is a correlated metal. X-ray photoemission valence band and O K-edge x-ray absorption spectra indicate a strongly hybridized Cr3d–O2p state crossing the Fermi level, leading to metallic behavior. Comparison between valence band spectra near the Fermi level and the densities of states calculated using density functional theory (DFT) suggests the presence of coherent and incoherent states and points to strong electron correlation effects. The magnetic susceptibility can be described by Pauli paramagnetism at temperatures above 100 K, but reveals antiferromagnetic behavior at lower temperatures, possibly resulting from orbital ordering.

Charge transfer between photosynthetic proteins and hematite in bio-hybrid photoelectrodes for solar water splitting cells

Abstract Functionalization of the hematite photoanode with the photosynthetic light antenna protein C-phycocyanin (PC) can yield substantial enhancement of the photocurrent density. Photoelectrochemical cells with bio-hybrid electrodes from photosynthetic proteins and inorganic semiconductors have thus potential for the use in artificial photosynthesis. We investigate here processing routes for the functionalization of hematite photoanodes with PC, including in situ co-polymerization of PC with enzymatically-produced melanin, and using a recombinant PC genetically engineered to carry a hexa-histidine tag (αHisPC). First, the effect of the immobilisation of PC on the electrode morphology and photocurrent production is evaluated. Then, the electronic charge transfer in dark and light conditions is assessed with electrochemical impedance spectroscopy and valence band (VB) X-ray photoemission spectroscopy. The relative shift of the VB spectrum towards the Fermi energy EF upon illumination is smaller for the more complex processed coating and virtually disappears for αHisPC immobilised with a melanin film. Optimal conditions for protein immobilisation are determined and the dark currents benefit most from the most advanced protein coating processes.

Electronic and surface properties of Ga-doped In2O3 ceramics

The limit of solubility of Ga2O3 in the cubic bixbyite In2O3 phase was established by X-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepared at 1250°C. Density functional theory calculations suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga2O3 as compared to In2O3. However both diffuse reflectance spectroscopy and valence band X-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga+ surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addition photoemission reveals a broadening of the valence band edge. Core X-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chemically shifted structure in Ga 2p core XPS associated with Ga+. However experiments on ion bombarded Ga2O3, where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga+ at the surface, show that the chemical shift between Ga+ and Ga3+ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chemically shifted structure associated with Ga+ is not inconsistent with the proposal that band gap narrowing is associated with lone pair states at surfaces and interfaces.

Hole-induced insulator-to-metal transition inLa1−xSrxCrO3epitaxial films

We have investigated the evolution of the electronic properties of La1-xSrxCrO3 (for the full range of x) epitaxial films deposited by molecular beam epitaxy (MBE) using x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectrometry, x-ray absorption spectroscopy, electrical transport, and ab initio modeling. LaCrO3 is an antiferromagnetic insulator whereas SrCrO3 is a metal. Substituting Sr2+ for La3+ in LaCrO3 effectively dopes holes into the top of valence band, leading to Cr4+ (3d2) local electron configurations. Core-level and valence-band features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward the valence band maximum. The material becomes a p-type semiconductor at lower doping levels and an insulator-to-metal transition is observed at x greater than or equal to 0.65, but only when the films are deposited with in-plane compression via lattice-mismatched heteroepitaxy. Valence band x-ray photoemission spectroscopy reveals diminution of electronic state density at the Cr 3d t2g-derived top of the valence band while O K-edge x-ray absorption spectroscopy shows the development of a new unoccupied state above the Fermi level as holes are doped into LaCrO3. The evolution of these bands with Sr concentration is accurately captured using density functional theory with a Hubbard U correction of 3.0 eV (DFT + U). Resistivity data in the semiconducting regime (x less than or equal to 0.50) do not fit perfectly well to either a polaron hopping or band conduction model, but are best interpreted in terms of a hybrid model. The activation energies extracted from these fits are well reproduced by DFT + U.

Polarity determination of polar and semipolar (112¯2) InN and GaN layers by valence band photoemission spectroscopy

We demonstrate that the polarity of polar (0001), (0001¯) and semipolar (112¯2) InN and GaN thin layers can be determined by valence band X-ray photoemission spectroscopy (XPS). The polarity of the layers has been confirmed by wet etching and convergent beam electron diffraction. Unlike these two techniques, XPS is a non-destructive method and unaffected by surface oxidation or roughness. Different intensities of the valence band states in spectra recorded by using AlKα X-ray radiation are observed for N-polar and group-III-polar layers. The highest intensity of the valence band state at ≈3.5 eV for InN and ≈5.2 eV for GaN correlates with the group-III polarity, while the highest intensity at ≈6.7 eV for InN and ≈9.5 eV for GaN correlates with the N-polarity. The difference between the peaks for the group-III- and N-polar orientations was found to be statistically significant at the 0.05 significance level. The polarity of semipolar (112¯2) InN and GaN layers can be determined by recording valence band photoelectrons emitted along the [000 ± 1] direction.

Tunable sulfur desorption in exfoliated MoS2 by means of thermal annealing in ultra-high vacuum

The effects of thermal annealing in ultra-high vacuum on the electronic structures of bulk and liquid exfoliated MoS2 have been studied by core level and valence band X-ray photoemission spectroscopy. A quantitative analysis of core level spectra demonstrates, in the case of exfoliated MoS2, that, upon annealing above 200°C, defect formation occurs in the form of sulfur single and double vacancies. Sulfur vacancies introduce surface states in the band gap (determined by the analysis of the valence band spectra). This determines a rigid shift of the core levels to lower binding energies, as a consequence of an upward band bending.

Electronic structure of CuCrO2 thin films grown on Al2O3(001) by oxygen plasma assisted molecular beam epitaxy

Thin films of CuCrO2 have been grown on Al2O3(001) substrates by oxygen plasma assisted molecular beam epitaxy. With a substrate temperature of 700 °C or 750 °C, the films showed an unanticipated (015) orientation but at a higher substrate temperature of 800 °C the expected basal (001) orientation predominates. The optical absorption spectrum of CuCrO2 shows a direct allowed absorption onset at 3.18 eV together with a weak peak at 2.0 eV which is suppressed by Sn doping. This suggests that the low energy peak should be attributed to 3d→3d excitations associated with Cu2+ defect states rather than excitations localised on Cr3+. Valence band X-ray photoemission spectra of (001) and (015) oriented CuCrO2 are compared with those obtained from polycrystalline samples.

Electronic and magnetic properties of CeCoGa

We report the magnetic and electronic transport properties of CeCoGa, using magnetic susceptibility, magnetization, specific heat and electrical resistivity investigations. We also present X-ray photoemission valence band spectra (VB-XPS) as well as ab-initio band structure calculations for the ferromagnetic and antiferromagnetic ordering. The dc and ac magnetic susceptibility data reveal a magnetic phase transition at TN = 4.3 K. For T < TN magnetization M vs. magnetic field B isoterms have two metamagnetic transitions. Above TN susceptibility shows evidence of spin-glass like behavior. The magnetic entropy is strongly reduced at TN due to the on-site Kondo coupling. For CeCoGa the Kondo temperature is ∼20 K, much larger than the temperature of the magnetic ordering. Our calculations show that within bare local spin density approximation (LSDA) the ground state of CeCoGa is weakly magnetic, however, the 4f correlations when they are included within LSDA + U approach lead to stable antiferromagnetic ground state. Calculations for moderate U parameter indicate that magnetic ordering is present not only at the Ce sublattice, but also for the Co sites, moreover small induced magnetic moment is also present at the Ga sites. All sublattices exhibit antiferromagnetic ordering with different values of local magnetic moments what explains the ferrimagnetic behavior of CeCoGa. The magnetization M vs. magnetic field B shows at T = 2 K two metamagnetic features at B = 0.1 T and B = 5 T, which well correlates with the calculated magnetic moments of Co and Ga. The first transition can be attributed to spin–flop within the magnetic moments induced in Ga sublattice, whereas the anomaly at B = 5 T is related to the spin–flop within the Co sublattice.