Hybridized Valence Band Research Articles

Single-domain perpendicular magnetization induced by the coherent O 2p -Ru 4d hybridized state in an ultra-high-quality SrRuO3 film

We investigated the Ru $4d$ and O $2p$ electronic structure and magnetic properties of an ultrahigh-quality $\mathrm{SrRu}{\mathrm{O}}_{3}$ film on $\mathrm{SrTi}{\mathrm{O}}_{3}$ grown by machine-learning assisted molecular-beam epitaxy. The high itinerancy and long quantum lifetimes of the quasiparticles in the Ru $4d$ ${t}_{2g}$-O $2p$ hybridized valence band are confirmed by observing the prominent well-screened peak in the Ru $3d$ core-level photoemission spectrum, the coherent peak near the Fermi energy in the valence-band spectrum, and quantum oscillations in the resistivity. The element-specific magnetic properties and the hybridization between the Ru $4d$ and O $2p$ orbitals were characterized by Ru ${M}_{2,3}$-edge and O $K$-edge soft x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements. The ultrahigh-quality $\mathrm{SrRu}{\mathrm{O}}_{3}$ film with the residual resistivity ratio of 86 shows the large orbital magnetic moment of oxygen ions induced by the strong orbital hybridization of the O $2p$ states with the spin-polarized Ru $4d$ ${t}_{2g}$ states. The film also shows single-domain perpendicular magnetization with an almost ideal remanent magnetization ratio of 0.97. These results provide detailed insights into the relevance between orbital hybridization and the perpendicular magnetic anisotropy in $\mathrm{SrRu}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ systems.

Single crystalline Sn2Nb2O7 films with Ti-doping fabricated by pulsed laser deposition

Sn2Nb2O7, a pyrochlore oxide, is expected to have a peculiar flat band structure that can be a source of intriguing physical properties. We have prepared Sn2Nb2O7 thin films by pulsed laser deposition on Y-stabilized ZrO2(111) substrates by finding a growth window at low growth temperature (∼450 °C) and higher laser fluence, possibly due to the rather high volatility of stannous oxide. By Ti-doping, the lattice constant shrinks due to the smaller Ti4+ ion compared with Nb5+. Optical measurements capture an absorption edge at ∼2.6 eV originating from the transition from the Sn-5s and O-2p hybridized valence band to the Nb-4d conduction band, which is not observed in LiNbO3. Opposed to the expectation from the Burstein–Moss shift, Ti-doping results in reduction of the bandgap, indicating that the doping results in the formation of in-gap states and does not contribute to injecting mobile carriers.

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.

(Invited) Bismuth-Based Ternary Oxide Thin Film for Solar Water Oxidation

Bismuth based ternary oxide such as BiVO4, Bi2WO6 and Bi2MoO6 have been reported to oxidise water into oxygen under visible light. These materials have sufficient absorption within the solar spectrum (UV and visible region) and stability against photocorrosion. The hybridized valence band of these ternary oxides made them active towards water oxidation. In this presentation, we investigate the performance of these bismuth based ternary oxides in photoelectrochemical water oxidation. A few aspects will be discussed to highlight the differences when they were employed in photocatalytic water splitting or through photoelectrochemical means. It is generally acknowledged that surface area and crystallinity of photocatalysts are critical factors regulating performances in powder type suspension reactions. When they were made into thin film, quality of the contact between bismuth based ternary oxides and the charge collecting substrate becomes another crucial factor. BiVO4, Bi2WO6 and Bi2MoO6 thin films are synthesised in this work. BiVO4 with controlled exposed facet is engineered to study the dynamic of charge interactions between their respective facets. It is revealed that a balance of charge transfer and charge trapping are occurring to govern the overall performance of BiVO4. From that point of view, specific interface is engineered to boost the higher charge transfer efficiency, for example, by using graphene as electron mediator. Bi2WO6 and Bi2MoO6 thin films are synthesised by a direct conversion of anodised WO3 and MoO3 films. The adhesion of the resultant Bi2WO6 and Bi2MoO6 on the substrate is highly stable. With the control of synthesis parameter, a concentration gradient can be introduced to form the composite of Bi2WO6/WO3 and Bi2MoO6/MoO3 with desired composition. With the favourable band alignment in this composite material, an improved photoelectrochemical performance is observed.

Highly Localized Charge Transfer Excitons in Metal Oxide Semiconductors.

The ability to observe charge localization in photocatalytic materials on the ultrafast time scale promises to reveal important correlations between excited state electronic structure and photochemical energy conversion. Of particular interest is the ability to determine hole localization in the hybridized valence band of transition metal oxide semiconductors. Using femtosecond extreme ultraviolet reflection absorption (XUV-RA) spectroscopy we directly observe the formation of photoexcited electrons and holes in Fe2O3, Co3O4, and NiO occurring within the 100 fs instrument response. In each material, holes localize to the O 2p valence band states as probed at the O L1-edge, while electrons localize to metal 3d conduction band states on this same time scale as probed at the metal M2,3-edge. Chemical shifts at the O L1-edge enable unambiguous comparison of metal-oxygen (M-O) bond covalency. Pump flux dependent measurements show that the exciton radius is on the order of a single M-O bond length, revealing a highly localized nature of exciton in each metal oxide studied.

Identifying the acceptor state in NiO hole collection layers: direct observation of exciton dissociation and interfacial hole transfer across a Fe2O3/NiO heterojunction.

NiO is widely utilized as a hole transport layer in solar energy devices where light absorption in a photoactive layer is followed by charge separation and hole injection into a NiO collection layer. Due to the complex electronic structure of the hybridized valence band in NiO, the chemical nature of the hole acceptor state has remained an open question, despite the fact that hole localization in this material significantly influences device efficiency. To comment on this, we present results of ultrafast charge carrier dynamics in a NiO based model heterojunction (Fe2O3/NiO) using extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. Element specific XUV-RA spectroscopy demonstrates the formation of transient Ni3+ within 10 ps following selective photoexcitation of the underlying Fe2O3 substrate. This indicates that hole transfer in this system occurs to NiO valence band states composed of significant Ni 3d character. Additionally, we show that this hole injection process proceeds via a two-step sequential mechanism where fast, field-driven exciton dissociation occurs in Fe2O3 in 680 ± 60 fs, followed by subsequent hole injection to NiO in 9.2 ± 2.9 ps. These results reveal the chemical nature of the hole acceptor state in widely used NiO hole transport layers and provides a direct observation of exciton dissociation and interfacial hole transfer in this model system.

Mediation of Valence Band Maximum of BiOI by Cl Incorporation for Improved Oxidation Power in Photocatalysis

Cl-incorporated BiOI nanostructures with different Cl/I molar ratios have been successfully fabricated via a facile room-temperature method in H2O/EG mixture solvent. Combined with FT-IR, Raman spectrum, XRD Rietveld refinements, and DFT calculation, the incorporated Cl ions are found mainly occupied on I sites, resulting in a hybrid valence band consisting of I 5p, O 2p, and Cl 3p orbitals, which deepens the valence band location of the materials and induces photogenerated holes with higher oxidation power. The incorporated Cl ions into the lattice also change the evolution of photogenerated electrons and holes, inducing the broadening of the bandgap but efficient production of photogenerated carriers. The enhanced oxidation power of Cl-incorporated BiOI products was evaluated by photocatalysis degradation of methylene blue, an organic dye for which pure BiOI shows negligible direct oxidation efficiency. Ascribed to the relatively low valence band maximum (VBM) and efficient generation of photoinduced el...

Optical constants and band structure of trigonal NiO

Using spectroscopic ellipsometry and transmission measurements, the authors determined the optical constants (absorption coefficient, complex refractive index, and dielectric function) of bulk trigonal NiO from 0.08 to 6.5 eV. By careful discussion of the data, elastic scattering by oxygen bubbles was ruled out and the effects of surface roughness were removed numerically to obtain an accurate dielectric function of NiO. A direct band gap of 0.85 eV was found from transmission and assigned to direct interband transitions from the Ni-O hybrid valence band states to the Ni 4s conduction band at the center of the Brillouin zone. At 4 eV, the authors find the well-known charge transfer gap from the lower to the upper Hubbard band. Several intermediate sharp peaks were also found. The temperature dependence of the NiO charge transfer gap is similar to the E1 gap of Si between 100 and 700 K. At higher temperatures, heating NiO in vacuum leads to sublimation, which has drastic irreversible consequences for the pseudodielectric function of the sample, including a strong Ni nanoparticle plasmon peak at 2 eV.

In-situ Observation of Nitriding Processes of Deposited-Ti Thin Films due to Ion Implantation in an Analytical Transmission Electron Microscope

Both hcp-Ti and CaF2-type TiHx (x ≅ 1.5), and only hcp-Ti, with preferred orientations have been deposited on NaCl (001) substrates held at room temperature (RT) and 250°C, respectively. In order to clarify atomistic growth processes of TiNy films due to the implantation of nitrogen ions (N2+) with 62 keV into as-deposited Ti films, changes of the concentration of H and/or N atoms in Ti films, and of the crystallographic and electronic structures of the films by heating and by nitriding have been investigated by in-situ transmission electron microscope (TEM) equipped with electron energy loss spectroscopy (EELS), with the aid of molecular orbital calculations. Hydrogen atoms which constitute TiHx in the Ti films deposited at RT are released with heating, and are completely released at 350°C. The H-released unstable fcc-Ti sublattice is then transformed into hcp-Ti. The energy loss peak due to the plasmon excitation observed by EELS for TiHx shifts to the lower energy side with the decrease in the electron density in the hybridized valence band, which means the release of H atoms from TiHx during heating. On the other hand, the TiNy is epitaxially formed by the N-implantation into the hcp-Ti, through the epitaxial transformation of the hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti, and accompanying the occupation of O-sites of the transformed fcc-Ti by N atoms. The energy loss peak due to the plasmon excitation during N-implantation into the hcp-Ti films gradually shifts to the higher energy side with the increase in dose of N, which means the increase in the electron density in the hybridized valence band. By comparing the results of EELS with those of Rutherford Backscattering Spectrometry, it is clarified that the maximum concentration of N in Ti films during the N-implantation depends on the implantation temperature. The comparison between the results of EELS and those of the molecular orbital calculations elucidates that the changes of electronic structures near the Fermi level depend on the ratio of N/Ti in the films. Furthermore, taking into account the bonding interaction of Ti sublattices with ligand N atoms, the transformation mechanisms between hcp-Ti and TiNy due to the implantation of N atoms are proposed. [DOI: 10.1380/ejssnt.2011.191]

Effect of size asymmetry of QDs on splitting of hybrid valence band states

The off-diagonal element of the Luttinger Hamiltonian responsible for the coupling of heavy hole–light hole states has been calculated for symmetric/asymmetric semiconductor quantum dots. It has been found that the splitting arising due to finite off-diagonal elements of the Luttinger Hamiltonian and the size asymmetry of the quantum dot is counterbalanced. The numerical estimates made for CdSe/ZnSe quantum dot manifest minimal energy splitting when the size asymmetry parameter is around 0.3.

Microscopic Analysis of the Valence Band and Impurity Band Theories of (Ga,Mn)As

We analyze microscopically the valence and impurity band models of ferromagnetic (Ga,Mn)As. We find that the tight-binding Anderson approach with conventional parametrization and the full potential local-density approximation+U calculations give a very similar band structure whose microscopic spectral character is consistent with the physical premise of the k·p kinetic-exchange model. On the other hand, the various models with a band structure comprising an impurity band detached from the valence band assume mutually incompatible microscopic spectral character. By adapting the tight-binding Anderson calculations individually to each of the impurity band pictures in the single Mn impurity limit and then by exploring the entire doping range, we find that a detached impurity band does not persist in any of these models in ferromagnetic (Ga,Mn)As.

Polarization rotation in asymmetric semiconductor quantum dots

Rotation of input polarization arising due to the recombination of electrons from the 1s-excitonic state to the hybrid valence band states have been theoretically examined in asymmetric semiconductor quantum dots. The Jones matrix calculations suggest that the polarization rotation directly depends on the asymmetry of the quantum dot and strength of hh–lh coupling. The results advocate the suitability of quantum dots as polarizing devices.

Visible-light photocatalytic activity of the metastable Bi 20TiO 32 synthesized by a high-temperature quenching method

Metastable Bi 20TiO 32 samples were synthesized by a high-temperature quenching method using α-Bi 2O 3 and anatase TiO 2 as raw materials. The photocatalytic activity of the as-prepared samples was measured with the photodegradation of methyl orange at room temperature under visible light irradiation. The Bi 20TiO 32 samples exhibited good absorption in the visible light region with a band gap of about 2.38 eV and the band structure of Bi 20TiO 32 was studied. Photodegradation against methyl orange was much better than α-Bi 2O 3 prepared by the same way. The photocatalytic activity of Bi 20TiO 32 samples is supposed to be associated with the hybridized Bi 6 s and O 2 p orbitals. In addition, the dispersive characteristic of Bi 6 s orbital in the hybridized valence band facilitates the mobility of the photogenerated carriers and hampers their recombination.

Strain dependence of the Mn anisotropy in ferromagnetic semiconductors observed by x-ray magnetic circular dichroism

Strain dependence of the Mn anisotropy in ferromagnetic semiconductors observed by x-ray magnetic circular dichroism K. W. Edmonds, 1 G. van der Laan, 2 N. R. S. Farley, 1 E. Arenholz, 3 R. P. Campion, 1 C. T. Foxon, 1 and B. L. Gallagher 1 School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom Daresbury Laboratory, Warrington WA4 4AD, United Kingdom Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (Dated: March 24, 2008) Abstract We demonstrate a sensitivity to strain of the Mn 3d valence states in the ferromagnetic semi- conductors (Ga,Mn)As and (Al,Ga,Mn)As, using x-ray magnetic circular dichroism (XMCD). The spectral shape of the Mn L 2,3 XMCD is dependent on the orientation of the magnetization, and features with cubic and uniaxial dependence are distinguished. Reversing the strain gives an oppo- site sign of the uniaxial anisotropy of the Mn L 3 pre-peak which is ascribed to transitions from the Mn 2p core level to p-d hybridized valence band hole states. With increasing carrier localization, the L 3 pre-peak intensity increases, indicating an increasing 3d character of the hybridized holes. PACS numbers: 75.50.Pp, 75.25.+z, 75.30.Gw, 78.70.Dm

Nitriding transformation of titanium thin films by nitrogen implantation

Changes of electronic structure during nitrogen implantation into heated Ti thin films, which were accompanied by crystallographic structural transformation, were studied by applying in situ electron energy loss spectroscopy (EELS) combined with self-consistent charge discrete variational (DV)-Xα molecular orbital (MO) calculations. Hydrogen atoms constituting TiH x were released from the films with heating at temperatures more than about 100 °C. The H-released unstable fcc-Ti was transformed into hcp-Ti. The energy loss peak observed by EELS for TiH x shifted to lower energies with the release of H atoms during heating. Nitrogen ions of N 2 + with 62 keV were implanted into the hcp-Ti films held at 350 °C. The TiN y films were “epitaxially” formed by the transformation of hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of octahedral sites of the fcc-Ti sublattice by N atoms. The energy loss peak for hcp-Ti films observed by EELS during N implantation gradually shifted to higher energies with increasing the dose. The shift meant increase in the density of electrons occupying the hybridized valence band. The formation mechanism of TiN y was discussed with the results of the calculated overlap populations of Ti–Ti and Ti–N bonds.

Electronic structure and Chemical bonding in Sr4Nb17O26

The electronic structure of oxoniobate Sr4Nb17O26 is studied by the linear muffintin orbital (LMTO) method. It is shown that the highenergy conduction band consists of the Nb4d states and the hybridized valence band is formed by the Nb4d and O2p states. The band structure of this compound is characterized by superposition of the bands of the 2p states of perovskite oxygen atoms and the 4d states of monoxide niobium atoms. The degree of oxidation of the perovskite and monoxide niobium atoms is +5 and +2.56, respectively. Chemical bonding is analyzed using the electron localization function and model Huckel calculations. The niobiumoxygen bond is shown to be the strongest. The Fermi level is localized in the vicinity of the bottom of the niobium antibonding state band, which explains the existence of Sr4−xNb17O26 in the homogeneity region corresponding to 0 < x < 0.3.

Occupied electronic states of single-crystal Bi2Ca1+xSr2+xCu2O8+y.

X-ray photoemission results for ${\mathrm{Bi}}_{2}{\mathrm{Ca}}_{1+x}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ reveal hybrid valence-band states within \ensuremath{\sim}8 eV of the Fermi level with a centroid that is shifted \ensuremath{\sim}1.5 eV to a higher binding energy relative to the independent-particle density of states. Core-level studies reveal a single chemical environment for Bi, inequivalent O sites in the lattice, and Ca-Sr disorder. Comparison with results for Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.9}$ shows nearly identical valence-band emission with a more pronounced Fermi level cutoff for the Bi compound.