First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

Valence and conduction band offset measurements in Ni0.07Zn0.93O/ZnO heterostructure

We report valence and conduction band offset measurements in a pulsed laser deposited Ni_0.07Zn_0.93O/ZnO heterostructure using X-ray photoelectron spectroscopy, valence band spectroscopy and ultraviolet visible spectroscopy. Neglecting the strain effect, the valence band offset was estimated to be 0.32 eV and the conduction band offset comes out to be −0.23 eV. Ratio between conduction band and valence band offset is 0.72. Core level shifting due to Ni doping has also been explained. Magnetotransport study of Ni_0.07Zn_0.93O film reveals that the charge carriers might be spin polarized at the interface of the heterojunction.     

Cross section and resonance effects in photoemission from Sn-doped In2O3(111)

Photoemission spectra of Sn-doped In₂O₃(111) have been measured using a range of photon energies between 40 and 1300 eV. The intensity of structure at the bottom of the valence band associated with states of mixed Sn 5s/O 2p character increases with increasing photon energy relative to that of states of more dominantly O 2p character at the top of the valence band, as expected from one electron ionisation cross sections. In addition a pronounced resonance in the intensity of a weak conduction band feature is observed around the In 4p core threshold.     

Electronic structure of non-stoichiometric zirconium nitrides ZrNx

The electronic structures of zirconium nitrides, ZrN_x, have been investigated by X-ray photoelectron spectroscopy over the entire composition range of the rocksalt structure (0.5 ? x ? 1). The valence region spectrum of nearly stoichiometric ZrN is consistent with APW band structure calculation. When removing a nitrogen atom the valence band spectra exhibit an intensity increase of the d conduction band, due in part to the filling of a new created defect state at around 2 eV binding energy.     

Energy band structure of I–III–VI2 semiconductors

We review recent studies of the energy band structure of I-III-VI₂ semiconductors. The structure of the uppermost valence bands of a I-III-VI₂ compound is profoundly influenced by the proximity of noble metal d levels in the valence band. The direct energy gaps observed in I-III-VI₂ compounds are low relative to the energy gaps in the II–VI analogs by amounts up to 1.6eV, and the spin-orbit splittings observed in the ternaries are low relative to the values observed in the binary analogs, owing to a partial cancellation of the positive spin-orbit parameter for p levels and the negative spin-orbit parameter for d levels. The presence of the noble metal d levels in the valence band has been confirmed directly by the observation of electroreflectance structure due to transitions from the d levels themselves to the lowest conduction band minimum.     

Valence band offset of ZnO/BaTiO3 heterojunction measured by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset of the ZnO/BaTiO₃ heterojunction grown by metal-organic chemical vapor deposition. The valence band offset (VBO) is determined to be 0.48±0.09 eV, and the conduction band offset (CBO) is deduced to be about 0.75 eV using the band gap of 3.1 eV for bulk BaTiO₃. It indicates that a type-II band alignment forms at the interface, in which the valence and conduction bands of ZnO are concomitantly higher than those of BaTiO₃. The accurate determination of VBO and CBO is important for use of semiconductor/ferroelectric heterojunction multifunctional devices.     

Electronic structure of the Sr2MgSi2O7:Eu persistent luminescence material

The electronic structures of the distrontium magnesium disilicate (Sr₂MgSi₂O₇(:Eu²⁺)) materials were studied by a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu²⁺ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu²⁺ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.     

Electronic structure,optical dielectric constant and born effective charge of EuAlO3

EuAlO₃ (EAO) is synthesized by the sol–gel process. The Rietveld refinement of the X-ray diffraction data shows that the material has orthorhombic structure with Pbnm space group. The density functional theory calculations are initiated with the experimental lattice parameters. The full potential linearized augmented plane wave method and projector augmented wave method are used to investigate the ground state properties of EAO. An indirect band gap of 1.8 eV is observed with the valence band maximum at the Γ point and the conduction band minimum at the R point. The X-ray photoemission spectroscopy (XPS) spectra of EAO are obtained in the energy window of 0–1000 eV. Using the electronic density of states, the valence band (VB) spectrum of EAO is generated and compared with the observed VB-XPS spectrum. The optical dielectric constant and the refractive index of the material are calculated for the photon energy radiation. The optical properties show a considerable anisotropy in the material. The Born effective charge of various elements and the dielectric tensor of EAO have been calculated.     

Electronic structure of Al2O3 from electron energy loss spectroscopy

The electronic structure of Al₂O₃ has been studied by electron energy loss spectroscopy (ELS), and an energy level model of both filled and empty states has been constructed from the ELS and available optical data. For the high temperature pyrolytic α-polycrystalline Al₂O₃ films, the transitions are assumed to originate at the two principal peaks in the valence band density of states and the O(2s) core state, and to terminate on two peaks within the conduction band density of states. We also report energy loss spectra due to excitations out of the deeper Al(2p), Al(2s), Al(1s), and O(1s) core levels. The excitations originating at the Al(2p), Al(2s), and Al(1s) core levels terminate on levels in the conduction band and on an exciton lying about 1 eV below the conduction-band edge.     

Thermoelectric power and ac conductivity of A-type Nd2O3

Measurement of thermoelectric power Θ of pressed pellets of A-type Nd₂O₃ from 550 to 1180K and electrical conductivity (σ) at dc, 50 Hz, 1.542 kHz and 3 kHz at different temperatures is reported. It is concluded that electrical conduction at high temperature (T>600K) in this solid is due to positive large polarons in O²⁻ : 2p (valence) band and negative intermediate polarons in Nd³⁺ : 5d (conduction band). The energy band gap of the solid has been found to be 2.44 eV. At low temperatures, conduction by hopping of charge carriers from one impurity centre to another has been predicted.     

The electronic structures of the core and valence ion states of metal oxides

SCF-Xα SW MO calculations on metal core ion hole states and X-ray emission (XES) and X-ray photoelectron (XPS) transition states of the non- transition metal oxidic clusters MgO₆^10?, AlO₄^5? and SiO₄^4? show relative valence orbital energies to be virtually unaffected by the creation of valence orbital or metal core orbital holes. Accordingly, valence orbital energies derived from XPS and XES are directly comparable and may be correlated to generate empirical MO diagrams. In addition, charge relaxation about the metal core hole is small and valence orbital compositions are little changed in the core hole state. On the other hand, for the transition metal oxidic clusters FeO₆^10?, CrO₆^9? and TiO₆^8? relative valence orbital energies are sharply changed by a metal core orbital or crystal field orbital hole, the energy lowering of an orbital increasing with its degree of metal character. Consequently O 2p nonbonding → M 3d-O 2p antibonding (crystal field) energies are reduced, while M 3d bonding → O 2p nonbonding and M 3d-O 2p antibonding → M 4s,p-O 2p antibonding (conduction band) energies increase. Charge relaxation about the core hole is virtually complete in the transition metal oxides and substantial changes are observed in the composition of those valence orbitals with appreciable M 3d character. This change in composition is greater for e _g than for t_2g orbitals and increases as the separation of the e_g crystal field (CF) orbitals and the O 2p nonbonding orbital set decreases. Based on the hole state MO diagrams the higher energy XPS satellite in TiO₂ (at about 13 eV) is assigned to a valence → conduction band transition. The UV PES satellites at 8.2 eV in Cr₂O₃ and 9.3 eV in FeO are tentatively assigned to similar transitions to conduction band orbitals, although the closeness in energy of the crystal field and O 2p nonbonding orbitals in the valence orbital hole state prevents a definite assignment on energy criteria alone. However the calculations do clearly show that charge transfer transitions of the e_g bonding → e_g crystal field orbital type would generally occur at lower energy than is consistent with observed satellite structure.A core electron hole has little effect upon relative orbital energies and is only slightly neutralized by valence electron redistribution for MgO and SiO₂. For the transition metal oxides a core hole lowers the relative energies of M3d containing orbitals by large amounts, reducing O → M charge transfer and increasing M 3d crystal field → conduction band energies. Large and sometimes overcomplete neutralization of the core hole is observed, increasing from CrO₆^9? to FeO₆^10? to TiO₆^8?. as the O → M charge transfer energy declines.High energy XPS satellites in TiO₂ may be assigned to O 2p nonbonding → conduction band transitions while lower energy UV PES satellites in FeO and Cr₂O₃ arise from crystal field or O 2p nonbonding → conduction band excitations. Our “shake-up” assignment for FeO₆^10?, CrO₆^9? and TiO₆^8? are less than definitive because no procedure has yet been developed to calculate “shake-up” intensities resulting from transitions of the type described. However the results do allow a critical evaluation of earlier qualitative predictions of core and valence hole effects. First, we find that the comparison of hole or valence state ionic systems with equilibrium distance systems of higher nuclear and/or cation charge (e.g. the comparison of the FeO₆^10? Fe 2p core hole state to Co₃O₄) is dangerous. For example, larger MO distances in the ion states substantially reduce crystal field splittings. Second, core and CF orbital holes sharply reduce O → M charge transfer energies, giving 2e_g → 3e_g energy separations which are generally too small to match observed satellite energies. Third, highest occupied CF-conduction band energies are only about 4–5 eV in the ground states, but increase to about 7–11 eV in the core and valence hole states of the transition metal oxides studied. The energetic arguments presented thus support the idea of CF and/or O 2p nonbonding → conduction band excitations as assignments for “shake-up” satellites, at least in oxides of metals near the beginning of the transition series.     

Electronic,optical and bonding properties of MgCO3

The electronic, optical and bonding properties of MgCO₃ (magnesite, rhombohedral calcite-type structure) are calculated using a first-principles density-functional theory (DFT) method considering the exchange-correlation function within the local density approximation (LDA) and the generalized gradient approximation (GGA). The indirect band gap of magnesite is estimated to be 5.0 eV, which is underestimated by ～1.0 eV. The fundamental absorption edge, which indicates the exact optical transitions from occupied valence bands to the unoccupied conduction band, is estimated by calculating the photon energy dependent imaginary part of the dielectric function using scissors approximations (rigid shift of unoccupied bands). The optical properties show consistent results with the experimental calcite-type structure and also show a considerable optical anisotropy of the magnesite structure. The density of states and Mulliken population analyses reveal the bonding nature between the atoms.     

Thermoreflectance of CuI

The thermoreflectance spectrum of CuI in the 4–7 eV region is reported. New structure in the $\text{E}\text{O}\text{′}$ region has been assigned to the spin-orbit split triplet between the first valence band and the second conduction band at k=0. The magnitude of the splitting indicates an inverted order in the second conduction band and a large d-state mixture in the valence band.     

Thin layers of grey arsenic: A molecular orbital study

A thin layer of grey arsenic has been manufactured by molecular beam epitaxy and its He(I) photoelectron spectrum has been recorded. The quasi-relativistic CNDO/1 method has been used to investigate the band structure of {As}₁₁₄ and {As}₂₂₈ clusters: the DOS profiles and their projections are under question. These data were correlated with the periodic crystal orbitals of the EHT quality. The first excitation energy serves as a better estimate of the energy gap between the filled valence band and the empty conduction band.     

Preparation and electronic structure of phase pure K3C60

Phase pure K₃C₆₀ films have been grown using vacuum distillation. The structure of such films could be shown to be face centered cubic consistent with X-ray diffraction studies. The electronic structure of the films has been studied using electron energy-loss spectroscopy in transmission. From C1s core excitation measurements the unoccupied density of states has been determined. Performing the dielectric function has been derived in a wide energy range (0–45 eV). It is shown that the low energy part of the optical conductivity cannot be understood within a simple free electron model but that interband transitions between the three conduction bands have to be taken into account. The spectral weight of interband transitions between valence and conduction bands shows strong momentum dependence due to optical selection rules demonstrating the molecular-like nature of the electronic states.     

Valence band offset of MgO/TiO2 (rutile) heterojunction measured by X-ray photoelectron spectroscopy

The valence band offset (VBO) of MgO/TiO₂ (rutile) heterojunction has been directly measured by X-ray photoelectron spectroscopy. The VBO of the heterojunction is determined to be 1.6 ± 0.3 eV and the conduction band offset (CBO) is deduced to be 3.2 ± 0.3 eV, indicating that the heterojunction exhibits a type-I band alignment. These large values are sufficient for MgO to act as tunneling barriers in TiO₂ based devices. The accurate determination of the valence and conduction band offsets is important for use of MgO as a buffer layer in TiO₂ based field-effect transistors and dye-sensitized solar cells.     

Energy band alignment of PbTe/CdTe(111) interface determined by ultraviolet photoelectron spectra using synchrotron radiation

The energy band structure with type-I alignment at the PbTe/CdTe(111) heterojunction interface is determined by the ultraviolet photoelectron spectrum using synchrotron radiation.The valence band and conduction band offsets are obtained to be 0.09±0.12 and 1.19±0.12 eV,respectively.These results are in agreement with theoretically predicted ones.The accurate determination of the valence band and conduction band offsets is useful for the fundamental understanding of the mid-infrared light emission from the PbTe/CdTe heterostructures and its application in devices.     

Photoemission studies of single crystals of titanium carbide

The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h?ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3d and carbon 2p states at ca. 3–4-eV binding energy, and a single carbon 2s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2p band at ? 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented.The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV.By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3d and C 2p states. The C 2p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW^5–9, MAPW¹⁰ and EPM¹¹ band structure calculations predict a flat band of p-character between the symmetry points X′₄ and K₃, most likely responsible for the majority of C 2p excitations observed. The C 2s states, on the other hand, form a single band centered around ?10.4 eV.The results obtained are consistent with several recent energy band structure calculations^{5–11, 13} that predict a combined bonding of covalent, ionic and metallic nature.     

A DFT study on the interaction between europium,uranium and SWCNT

We investigate the electronic and band structure for the (8; 0) single-wall carbon nanotube (SWCNT) with a europium (Eu) and a uranium (U) atom outside by using the first-principles method with the density functional theory (DFT). The calculated band structure (BS), total density of state (TDOS), and projected density of state (PDOS) can elucidate the differences between the pure (8; 0) SWCNT and the nuclei outside the SWCNT. The indirect band gaps are obtained when Eu and U atom are put outside the (8; 0) CNT; they are 0.037 eV and 0.036 eV, respectively, which is much smaller than 0.851 eV for pure CNT. Compared with pure (8; 0) SWCNT, the bottom of the conduction band moves down by 0.383 eV and 0.451 eV with the Eu and U outside, and the top of valence band moves up by 0.127 eV and 0.162 eV, respectively. More significantly, the top of the valence band has exceeded the fermi-level. So, a single nucleus changes the semiconductor character of pure nanotube to semi-metal.