Valence Band Width Research Articles

Low temperature preparation of oxygen-deficient tin dioxide nanocrystals and a role of oxygen vacancy in photocatalytic activity improvement

The introduction of oxygen vacancies (Vos) into tin dioxide crystal structure has been found as an effective method to improve its photocatalytic performance. Herein, oxygen-deficient tin dioxide (SnO2−x) nanocrystals were successfully prepared via a facile, one-step hydrothermal method at the temperature lower than those reported previously. The effect of hydrothermal temperature on phase composition and Vos content was also firstly investigated. Due to its high oxygen vacancy concentration, the SnO2−x prepared at 80 °C provides the best photocatalytic degradation of methyl orange under UV–visible light. Scavenger trapping and nitroblue tetrazolium experiments also show that the Vos act as electron trapped sites and molecular oxygen adsorption sites, therefore increasing the production of active O2− radical which is the main species governing the photocatalytic activity of SnO2−x nanocrystals. Raman spectroscopy, X-ray photoelectron spectroscopy, photoluminescence measurement and electron spin resonance investigation clearly indicate that increasing the hydrothermal temperature results in the coexistence of SnO2−x and Sn3O4 phases and the reduction of Vos concentration which are detrimental to the photocatalytic performance. Density functional theory calculations also reveal that the presence of Vos is responsible for the upshift of valence band maximum and an extended conduction band minimum, hence a valence band width broadening and band gap narrowing which consequently enhance the photocatalytic performance of the oxygen-deficient SnO2−x.

Electronic properties of achiral and chiral gold nanotubes

The band structure of ten single-walled gold nanotubes of different radius and chirality angle have been calculated by the linearized augmented cylindrical wave method. For all tubes, the Fermi level crosses the half-filled band; therefore, the tubes are characterized by a metallic electronic structure. The band structure of the nanotubes changes relatively weakly with a change in nanotube structure. The valence band width for all the tubes is 9.1 eV. The density of states at the Fermi level remains unaltered with a change in chirality angle and decreases by 30% with an increase in radius from 3 to 12 A.

Oxygen vacancies confined in SnO2 nanoparticles for desirable electronic structure and enhanced visible light photocatalytic activity

Electronic structure in principle determines the light absorbance, charge transfer and separation, and consequently, photocatalytic property of a photocatalyst. Herein, we report rutile SnO2 with a desirable electronic structure that exhibits a narrowed bandgap and an increased valence band width resulted from the introduction of homogeneous oxygen vacancies. XPS, Raman, ESR and PL spectra demonstrate the homogeneous oxygen vacancies confined in SnO2 nanoparticles. Moreover, the first principle calculations theoretically reveal the desirable electronic structure. The narrowed bandgap further contributes to extended light absorption range and the increased valence band width leads to efficient charge transfer and separation, hence facilitating the visible light photoreactivity. As a result, the defected SnO2 exhibits a superior visible light photocatalytic activity. More strikingly, the photodegration of methyl orange (MO) is completely accomplished within only 20min under λ≥420nm. Briefly, this work both experimentally and theoretically indicates that homogeneous oxygen vacancies confined in SnO2 nanoparticles lead to the optimized electronic structure and, consequently, the remarkable visible light photocatalytic activity. This could open up an innovative strategy for designing potentially efficient photocatalysts.

Role of orbital filling on nonlinear ionic Raman scattering in perovskite titanates

The linear and nonlinear phononic interactions between an optically excited infrared (IR) or hyper-Raman mode and a driven Raman mode are computed for the ${d}^{0} ({\mathrm{CaTiO}}_{3})$ and ${d}^{1} ({\mathrm{LaTiO}}_{3})$ titanates within a first-principles density functional framework. We calculate the potential energy surface expanded in terms of the ${A}_{g}$ or ${B}_{1g}$ mode amplitudes coupled to the ${A}_{u}$ or the ${B}_{3u}$ mode and determine the coupling coefficients for these multimode interactions. We find that the linear-quadratic coupling dominates the anharmonicities over the quadratic-quadratic interaction in the perovskite titanates. The IR and Raman modes both modify the electronic structure with the former being more significant but occurring on a different time scale; furthermore, the coupled-mode interactions lead to sizable perturbations to the valence bandwidth ($\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\text{meV}$) and band gap ($\ensuremath{\sim}50$ meV). By comparing the coupling coefficients of undoped ${\mathrm{CaTiO}}_{3}$ and ${\mathrm{LaTiO}}_{3}$ to those for electron-doped $({\mathrm{CaTiO}}_{3})$ and hole-doped $({\mathrm{LaTiO}}_{3})$ titanates, we isolate the role of orbital filling in the nonlinear coupling process. We find that with increasing occupancy of the $d$ manifold, the linear-quadratic interaction decreases by approximately 30% with minor changes induced by the cation chemistry (that mainly affect the phonon mode frequencies) or by electron correlation. We identify the importance of the Ti-O bond stiffness, which depends on the orbital filling, in governing the lattice anharmonicitiy. This microscopic understanding can be used to increase the nonlinear coupling coefficient to facilitate more facile access of nonequilibrium structures and properties through ionic Raman scattering processes.

Band structure and optical constants of GaAs1-xNx

The composition dependence of direct and indirect band gap energies, anti-symmetric gap, valence band width, refractive index and high-frequency and static dielectric constants has been investigated for GaAs1-xNx ternary semiconductor alloys with the zinc-blende crystal structure over the whole nitrogen concentration range (x from 0 to 1). The calculations are mainly based on the pseudopotential approach within the virtual crystal approximation. The variation of band-gaps versus nitrogen content show important bowing parameters. Trends in ionicity have been discussed in terms of the anti-symmetric gap. The alloy concentration dependence of the optical parameters of interest is found to be highly nonlinear.

Pressure-dependent structural, electronic and optical properties of ZnO with native defect: A first-principles study

Defects are usually unavoidable in lattices and have great impacts on the electronic structures, which can also be adjusted by pressure. Here, we report a systemic first-principles investigation on the pressure-dependent electronic and optical properties of wurtzite ZnO containing O vacancy or Zn interstitial. The pressure is loaded in the range of 0–12 GPa. The calculated result shows that the top valence bandwidth of ZnO materials varies with the pressure loaded. In particular, the top valence bandwidth of ZnO with O vacancy under about 5 GPa gets an extreme value. Meanwhile, it is also found that there are different energy shifts in the optical spectrums with the increase of pressure. The influence of increasing pressure on the optical properties of ZnO containing Zn interstitial is found to be notable, especially in the energy range of 3.0–4.7 eV. So the electronic and optical properties of ZnO with native defect may be tuned through changing the pressure. Our research results may provide important references to the choice and production of ZnO-based ultraviolet photoelectric materials.

The phase transformation and electrochemical properties of TiNi alloys with Cu substitution: Experiments and first-principle calculations

Cu substituted TiNi alloys have been investigated as hydrogen storage material for Ni-MH batteries by experiments and first principle calculations. The amount of Cu (x in TiNi1−xCux) is varied from 0.1 to 0.3. All of samples were prepared by mechanical alloying using a planetary high-energy ball mill and subsequent heat treatment at 750 °C for 0.5 h. The structural transformation was characterized by X-ray diffraction method (XRD) and scanning electron microscopy (SEM). It indicated that mechanical alloyed TiNi1−xCux alloys possessed broad diffraction peaks related to BCC structure. After heat treatment, studied TiNiCu materials consisted of Ti(Ni,Cu) B2 phase as main phase. The cell parameter of B2 phase increased linearly with increasing Cu content. The influence of Cu substitution for Ni in TiNi cubic phases was investigated by first principle calculation. It is found that TiNi0.7Cu0.3 possessed the most negative enthalpy of formation. The stability of TiNi1−xCux phase increased with Cu content. The width of valence bands was enlarged by substituting Cu atom. The discharge capacities of annealed samples were tested by electrochemical measurements at galvanostatic conditions. The results showed that the substitution of Cu for Ni seemed to deteriorate the activation properties of TiNi based alloys. Among all of studied materials, unmodified TiNi showed the highest discharge capacity which is equal to 154 mAh/g. Accompanying the substitution of Cu for Ni, the discharge capacity and cycling abilities of Cu substituted TiNi alloys declined and increased, respectively. All of the substituted samples exhibited at least 97% retaining rate after 20 cycles.

Electronic band structure trends of perovskite halides: Beyond Pb and Sn to Ge and Si

The trends in electronic band structure are studied in the cubic $AB{X}_{3}$ halide perovskites for $A=\text{Cs}$; $B=\text{Pb}$, Sn, Ge, Si; and $X=\text{I}$, Br, Cl. The gaps are found to decrease from Pb to Sn and from Ge to Si, but increase from Sn to Ge. The trend is explained in terms of the atom $s$ levels of the group-IV element and the atomic sizes which changes the amount of hybridization with $X\text{\ensuremath{-}}p$ and hence the valence bandwidth. Along the same series spin-orbit coupling also decreases and this tends to increase the gap because of the smaller splitting of the conduction band minimum. Both effects compensate each other to a certain degree. The trend with halogens is to reduce the gap from Cl to I, i.e., with decreasing electronegativity. The role of the tolerance factor in avoiding octahedron rotations and octahedron edge sharing is discussed. The Ge containing compounds have tolerance factor $tg1$ and hence do not show the series of octahedral rotation distortions and the existence of edge-sharing octahedral phases known for Pb and Sn-based compounds, but rather a rhombohedral distortion. ${\mathrm{CsGeI}}_{3}$ is found to have a suitable gap for photovoltaics both in its cubic (high-temperature) and rhombohedral (low-temperature) phases. The structural stability of the materials in the different phases is also discussed. We find the rhombohedral phase to have lower total energy and slightly larger gaps but to present a less significant distortion of the band structure than the edge-sharing octahedral phases, such as the yellow phase in ${\mathrm{CsSnI}}_{3}$. The corresponding silicon based compounds have not yet been synthesized and therefore our estimates are less certain but indicate a small gap for cubic ${\mathrm{CsSiI}}_{3}$ and ${\mathrm{CsSiBr}}_{3}$ of about $0.2\ifmmode\pm\else\textpm\fi{}0.2$ eV and $0.8\ifmmode\pm\else\textpm\fi{}0.6$ eV for ${\mathrm{CsSiCl}}_{3}$. The intrinsic stability of the Si compounds is discussed.

Effect Of Non-metal Elements (C, N, S) As Anionic Dopants On Electronic Structure Of Tio2-Anatase By Density-Functional Theory Approach

This article is a theoritical approach to calculate the electronic structure of undoped- and non-metal anions doped-TiO2-anatase. The objective of the research is to calculate abinitio the band structure and the density of states (DOS) of undoped-, C-, N-, and S-doped TiO2-anatase. Kohn-Sham equations are performed with the density functional theory (DFT) using the local density approximation (LDA) for exchange-correlation functional. The first-principle calculations were done using supercell (2x2x1) methods as implemented within Amsterdam Density Functional (ADF)-BAND version 2014.10. The ab-initio calculation of the band structures show that all samples are direct- and indirect-gap type semiconductor. The band gap of TiO2-anatase with DFT using LDA is 2.43 eV. The addition of C atom at 0.943% in 48 atoms produces width intermediate band about 0.76 eV, which is 0.38 eV above the valence band (VB) and 1.38 eV below the conduction band (CB). The addition of N atom at 1.103% and S atom at 2.478% in the lattice structure of TiO2-anatase resulted in the addition of the VB width to 0.47 eV and 0.11 eV, while the resulting gap between the VB and the CB to 1.97 eV and 2.33 eV, respectively.

Electronic structure and optical properties of α-RDX crystal under pressure from first-principles calculations

ABSTRACTIn this study, we investigate the electronic structure and optical properties of α-RDX crystal under pressure from 0 to 4 GPa using first-principles calculations based on the density functional theory. When pressure increases, the calculations figure out that the band gap linearly decreases, the peaks of the total density of states are lowered, the valence bandwidths broadened and the conduction bandwidths compressed. The results also show that the optical properties including dielectric function, absorption spectrum, refractive index, reflectivity and energy-loss function respond to compression, that is, the intensity of the maximal peak of absorption coefficient increases from 7.07 × 104 to 7.98 × 104 cm−1 with corresponding photon energy from 5.10 to 5.15 eV, the static refractive index increases from 1.23 to 1.30, the intensity of the maximal peak of reflectivity function increases from 0.24 to 0.32 accompanied by its photon energy from 5.74 to 5.97 eV, and the plasma resonance energy and its intensity increases from 5.92 to 6.20 eV and 3.81 to 5.13, respectively.

On the applicability of hybrid functionals for predicting fundamental properties of metals

Abstract The repercussions of an inaccurate account of electronic states near the Fermi level by hybrid functionals in predicting several important metallic properties are investigated. The difficulties include a vanishing or severely suppressed density of states (DOS) at EF, significantly widened valence bandwidth, greatly enhanced electron–phonon (el–ph) deformation potentials, and an overestimate of magnetic moment in transition metals. The erroneously enhanced el–ph coupling calculated by hybrid functionals may lead to a false prediction of lattice instability. The main culprit of the problem comes from the simplistic treatment of the exchange functional rooted in the original Fock exchange energy. The use of a short-ranged Coulomb interaction alleviates some of the drawbacks but the fundamental issues remain unchanged.

Effect of relaxation on the energetics and electronic structure of clean Ag3PO4(111) surface**Project supported by the National Natural Science Foundation of China (Nos. 51472081, 51102150, 61106046), the Development Funds of Hubei Collaborative Innovation Center (Nos. HBSKFMS2014003, HBSKFMS2014011), and the Foundation for High-Level Talents (No. GCRC13014).

The effect of relaxation on the energetics and electronic structure of clean Ag3PO4(111) surface has been studied, carried out using first-principles density functional theory (DFT) incorporating the GGA+U formalism. After atomic relaxation of the Ag3PO4(111) surface, it is found that O atoms are exposed to the outermost surface, due to an inward displacement of more than 0.06 nm for the two threefold-coordinated Ag atoms and an outward displacement of about 0.004 nm for three O atoms in the sublayer. The atomic relaxations result in a large transfer of surface charges from the outermost layer to the inner layer, and the surface bonds have a rehybridization, which makes the covalence increase and thus causes the surface bonds to shorten. The calculated energy band structures and density of states of the Ag3PO4(111) surface present that the atomic relaxation narrows the valence band width 0.15 eV and increases the band gap width 0.26 eV. Meantime, the two surface peaks for the unrelaxed structure disappear at the top of the valence band and the bottom of the conduction band after the relaxed structure, which induces the transformation from a metallic to a semi-conducting characteristic.

Fabrication of Wide-Range-Visible Photocatalyst Bi2WO6-x nanoplates via Surface Oxygen Vacancies.

Bi2WO6 as a high visible-light-driven catalyst has been aroused broad interest. However, it can only be excitated by the light with λ < 450 nm and the solar energy utilization need to be improved. Here, the wide–range–visible photoresponse Bi2WO6−x nanoplates were fabricated by introducing surface oxygen vacancies through the controllable hydrogen reduction method. The visible photoresponse wavelength range is extended from 450 nm to more than 600 nm. In addition, the photocatalytic activity of Bi2WO6−x is also increased and is 2.1 times as high as that of pristine Bi2WO6. The extending of the photoresponse range and the enhancement of the photoactivity both can be attributed to the surface-oxygen-vacancy states. This is because surface-oxygen–vacancy states generated above and partly overlapping of with the valence band (VB) will result in the rising of valence band maximum (VBM), thus broadening the VB width. This approach is proposed to develop many types of wide–range–visible optical materials and to be applicable to many narrow and wide bandgap materials.

Comparison of the electron work function, hole concentration and exciton diffusion length for P3HT and PT prepared by thermal or acid cleavage

The electron work function, hole concentration and diffusion length were compared for poly(3-hexylthiophene) polymer (P3HT) that is commonly used for construction of solar cells, and two types of native polythiophene (PT) samples which are prospective candidates for this purpose. The polythiophene samples were prepared from 2 different precursors by thermal or chemical treatment at room temperature. Cyclic voltammetry and work function measurements were used for estimating the concentration of holes. The measured data were evaluated assuming the validity of band theory based on the tight-binding model. Published data on the valence bandwidth were used for calculating the value of the overlap integral which is related to the hole effective mass. Energy band diagrams were constructed for all 3 materials. Finally, the exciton diffusion length, which is a critical parameter for the application of conjugated polymer materials in solar cells, was measured by a modified surface photovoltage method. The approach allowed us to identify the differences in the material properties related to the processing method. Morphology of the samples determined by AFM was another tool showing these differences. It is stated that a native polythiophene prepared by treatment with acids is a prospective material for solar cells and shows a similar quality as that produced by a thermal process.

Calculated hybrid and semilocal functionals andGWelectronic structure of the metal trifluoridesMF3(M=Sc, Y, Al)

A thorough investigation of the effect of exchange and correlation on the electronic structure of wide-band-gap insulators ${\mathrm{ScF}}_{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{YF}}_{3}$, and ${\mathrm{AlF}}_{3}$ is carried out using local, semilocal, and hybrid functionals in the density functional theory framework and the $GW$ approximation with four current plasmon-pole models. It is shown that the hybrid functionals, which attribute more weight to electron exchange, lead to a decent agreement with the state of the art $GW$ results, whereas the Tran-Blaha semilocal functional does not improve significantly the local density approximation results of the insulating transition-metal trifluorides, because of the high localization of the conduction band minimum states which are mainly of $d$ character, and underestimate considerably the valence-band width of the studied materials.

Ultra-sensitive pressure dependence of bandgap of rutile-GeO2 revealed by many body perturbation theory.

The reported values of bandgap of rutile GeO2 calculated by the standard density functional theory within local-density approximation (LDA)/generalized gradient approximation (GGA) show a wide variation (∼2 eV), whose origin remains unresolved. Here, we investigate the reasons for this variation by studying the electronic structure of rutile-GeO2 using many-body perturbation theory within the GW framework. The bandgap as well as valence bandwidth at Γ-point of rutile phase shows a strong dependence on volume change, which is independent of bandgap underestimation problem of LDA/GGA. This strong dependence originates from a change in hybridization among O-p and Ge-(s and p) orbitals. Furthermore, the parabolic nature of first conduction band along X-Γ-M direction changes towards a linear dispersion with volume expansion.

Optical properties of orthovanadates, and periodates studied from first principles theory

Detailed ab-initio studies on electronic structure and optical properties have been carried out for orthovanadates, and periodate compounds, ScVO4, YVO4, LuVO4, and NaIO4, KIO4, RbIO4, CsIO4 based on the Full potential linearized augmented plane wave method within the frame work of Density Functional Theory using Tran and Blaha modified Becke–Johnson potential (TB-mBJ). We have compared the optical properties of orthovanadates with periodates, and also with its high pressure phase. The main difference observed in moving from orthovanadates to periodates is the increase in band gap, and bands turn out to be less dispersive. By considering all these facts, we predict orthovanadates to be better scintillators than periodates, which is well explained from the band structure, and optical properties calculations. In addition, we also compared the optical properties of orthovanadates at ambient, and high pressure and we observed a decrease in the band gap of orthovanadates, increase in valence band width at high pressure when compared to ambient phase. Tuning the band gap, which is an important criteria for scintillators, can be observed in orthovanadates by decreasing the cation size, and also by moving to the high pressure scheelite phase. High pressure phase of orthovanadates might be more favourable as the zircon to scheelite transition is irreversible, and the transition pressure is also less around 8 GPa.

Electronic structure of a gold nanotube

The electronic structure of a gold nanotube has been studied by quantum-chemical methods. The energy dependences of total and partial densities of states of a nanotube with 16 atoms in a translational unit cell have been calculated by the linearized augmented-cylindrical-wave method. It has been demonstrated that the nanotube has a metal-like band structure. The s(Au) states are located completely in the valence band and are not involved in electron transport. The Fermi level is located at the peak of the total and partial d(Au) densities of states, which should contribute to the high electron tunneling conductance of the system. The valence band width is 11 eV.

Electronic Properties and Stability of Silicon Nanoclusters Passivated by Hydrogen

The total energy, geometry and electronic spectra of nanoclusters \shm{} ($m=0\ldots11$) are calculated using the evolutionary algorithm and density functional theory (DFT). It is shown, that the features of electron spectrum, namely HOMO-LUMO gap and valence band width, correlate with cluster geometry and stability. The HOMO-LUMO gap becomes wider as the number of hydrogen atoms increases whereas the width of valence band gets lower. The widening of the band gap indicates the increasing of cluster stability which is consistent with existing data on reaction energy.

The impact of Mg content on the structural, electrical and optical properties of MgZnO alloys: A first principles study

The structural, electronic and optical properties of wurtzite MgZnO with Mg concentration ranging from 0 to 0.5 are studied using the first principle calculations. It's found that the lattice constants c and specific volume V of the MgZnO alloys decrease and their band gap widens as Mg concentration increases, which are in agreement with our experimental results. A particular Mg concentration is found to exist at around 0.375, equals to which the corresponding MgZnO alloy has the minimum width of the top valence band. This indicates that Mg concentration may be used to tune the electronic properties of MgZnO alloy. Meanwhile, it's also found that the energy response range of the optical spectrums decreases with the increase of Mg concentration. There are different energy shifts toward high energy (blue shift) of the peaks in the optical spectrums with the increase of Mg concentration, which are explained by the variations of the density of states in details. So the electronic and optical properties of MgZnO may be tuned through Mg concentration, and our research results may provide meaningful references to the development and design of photoelectric devices.