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1.
A theoretical band structure calculation for lead nitrate hydroxide Pb16(OH)16(NO3)16 single crystal was performed based on the experimental crystallographic data obtained by Chang et al. Calculations exhibit that the conduction band minimum (CBM) is situated at Γ the center of the Brillouin zone (BZ) while the valence band maximum (VBM) is located between Γ and Y points of the BZ, resulting in an indirect energy band gap of about 3.70 eV in close agreement to the measured one (3.78 eV). The angular momentum resolved projected density of states reveals the existence of the strong hybridization between the orbitals and the VBM is originated from Pb-6s/6p and O-2p orbitals while the CBM from N-2p and Pb-6p orbitals. The calculated valence electronic charge density distribution explore the bond characters and the dominancy of the covalent bonding between Pb–O of PbOn ployhedra and N–O of [NO3] triangle. The calculated bond lengths and angles show good agreement with the experimental data.  相似文献   

2.
The band structures, density of states and effective masses of photogenerated carriers for CaZrTi2O7 photocatalyst were performed using first principles method with the virtual crystal approximation. The results indicated that CaZrTi2O7 has an indirect band gap of about 3.25 eV. The upper valence bands of CaZrTi2O7 are formed by O 2p states mixed with Ti 3d states, Zr 4d, 4p and 5s states, while the conduction bands are dominated by Ti 3d states, Zr 4d states and O 2p states. The calculated valence bands maximum (VBM) potential is located at 2.60 V (vs. normal hydrogen electrode (NHE)), while the conduction bands minimum (CBM) potential at ?0.65 V. Therefore, CaZrTi2O7 has the ability to split water to hydrogen and oxygen under UV light irradiation. The calculated minimum effective mass of electron in CBM is about 1.35 m0, and the minimum effective mass of hole in VBM is about 1.23 m0. The lighter effective masses facilitate the migration of photogenerated carriers and improve photocatalytic performance.  相似文献   

3.
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.  相似文献   

4.
First-principles calculation based on density-functional theory in the pseudo-potential approach have been performed for the total energy and crystal structure of BaTaO2N. The calculations indicate a random occupation of the anionic positions by O and N in a cubic structure, in agreement with neutron diffraction measurements and infrared spectra. The local symmetry in the crystal is broken, maintaining a space group Pm3?m, as used in structure refinement, which represents only the statistically averaged result. The calculations also indicate displacive disordering in the crystal. The average Ta-N distance is smaller (2.003 Å), while the average Ta-O distance becomes larger (2.089 Å). The local relaxation of the atoms has an influence on the electronic structure, especially on the energy gap. BaTaO2N is calculated to be a semiconductor with an energy gap of about 0.5 eV. The upper part of the valence band is dominated by N 2p states, while O 2p states are mainly in the lower part. The conduction band is dominated by Ta 5d states.  相似文献   

5.
To deeply understand the effects of Si/N-codoping on the electronic structures of TiO2 and confirm their photocatalytic performance, a comparison theoretical study of their energetic and electronic properties was carried out involving single N-doping, single Si-doping and three models of Si/N-codoping based on first-principles. As for N-doped TiO2, an isolated N 2p state locates above the top of valence band and mixes with O 2p states, resulting in band gap narrowing. However, the unoccupied N 2p state acts as electrons traps to promote the electron-hole recombination. Using Si-doping, the band gap has a decrease of 0.24 eV and the valence band broadens about 0.30 eV. These two factors cause a better performance of photocatalyst. The special Si/N-codoped TiO2 model with one O atom replaced by a N atom and its adjacent Ti atom replaced by a Si atom, has the smallest defect formation energy in three codoping models, suggesting the model is the most energetic favorable. The calculated energy results also indicate that the Si incorporation increases the N concentration in Si/N-codoped TiO2. This model obtains the most narrowed band gap of 1.63 eV in comparison with the other two models. The dopant states hybridize with O 2p states, leading to the valence band broadening and then improving the mobility of photo-generated hole; the N 2p states are occupied simultaneously. The significantly narrowed band gap and the absence of recombination center can give a reasonable explanation for the high photocatalytic activity under visible light.  相似文献   

6.
The electronic structure of Sr2Bi2O5 is calculated by the scalar-relativistic full potential linearized augmented plane wave (FLAPW+lo) method using the modified Becke–Johnson potential combined with the local density approximation correlation (MBJ–LDA). Both the valence band maximum (VBM) and conduction band minimum (CBM) exist at the Γ-point, indicating that Sr2Bi2O5 is a direct-band-gap material. The band gap is calculated to be 3.17 eV, which is very close to the experimental value. This result is in great contrast to the underestimation based on the GGA calculation. On the other hand, there is only a small difference in the effective masses of holes and electrons photogenerated near the VBM and CBM for the MBJ–LDA and GGA approaches. The optical properties of Sr2Bi2O5 are calculated from the complex dielectric function ε(ω)=ε1(ω)+2(ω). A highly polarized peak is observed at 3.5 eV in the ε2(ω) function. Furthermore, the absorption coefficient estimated from the MBJ–LDA is very similar to that from the experimental result.  相似文献   

7.
Ni-doped InTaO4 nanocrystallites were synthesized by a reactive pulsed laser ablation process, aiming at visible-light-operating photocatalysts. The third harmonics beam of a Nd:YAG laser was focused onto a sintered In0.9Ni0.1TaO4−δ target in mixture background gases (O2 + He). The deposited species were columnar-structured porous films consisting of primary nanocrystallites. The mean diameter of the primary nanocrystallites was 4 nm. Optical absorption characteristics, especially in low absorbance (sub-band) regions, were evaluated by photoacoustic spectroscopy. Absorption in the sub-band region decreased drastically with increasing O2 partial pressures. It is inferred that oxygen deficiencies are suppressed, because of enough oxygen vapors in the reactive background gases. An absorption band around 420 nm appeared obviously in O2 partial pressures above 5%, in the Ni-doped InTaO4 nanocrystallites. The visible region absorption band is presumably attributed to the Ni 3d-eg orbitals. In contrast, pure InTaO4 nanocrystallites showed a sharp band edge, without the visible absorption band.  相似文献   

8.
Be掺杂纤锌矿ZnO电子结构的第一性原理研究   总被引:1,自引:0,他引:1       下载免费PDF全文
唐鑫  吕海峰  马春雨  赵纪军  张庆瑜 《物理学报》2008,57(12):7806-7813
采用密度泛函理论结合投影缀加波方法,对Be掺杂导致ZnO禁带宽度增加的机理进行了研究.通过对掺杂前后电子能带结构、总态密度以及分态密度的计算和比较,发现导带底(CBM)是由Be 2s电子与Zn 4s电子共同控制;而BexZn1-xO价带顶 (VBM)始终由O 2p电子占据.随着掺杂量的增加,决定带隙宽度的CBM的位置上升,同时VBM的位置下降,从而导致了带隙的变宽,出现了蓝移现象.此外,Be掺杂会使晶胞发生压缩,这种压应变也是导致Be 关键词: 密度泛函理论 电子结构 Be掺杂ZnO  相似文献   

9.
采用密度泛函理论结合投影缀加波方法,对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构,态密度以及分态密度的计算和比较,发现CdxZn1-xO价带顶端(VBM)始终由O-2p占据;而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加,决定带隙宽度的CBM的位置下降,同时VBM的位置上升,从而导致了带隙的变窄,出现了红移现象. 此外,Cd掺杂会使晶胞发生膨胀,这种张应变也是导致Cd  相似文献   

10.
Cd掺杂纤锌矿ZnO电子结构的第一性原理研究   总被引:6,自引:0,他引:6       下载免费PDF全文
采用密度泛函理论结合投影缀加波方法,对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构,态密度以及分态密度的计算和比较,发现CdxZn1-xO价带顶端(VBM)始终由O-2p占据;而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加,决定带隙宽度的CBM的位置下降,同时VBM的位置上升,从而导致了带隙的变窄,出现了红移现象. 此外,Cd掺杂会使晶胞发生膨胀,这种张应变也是导致Cd 关键词: 密度泛函理论 电子结构 Cd掺杂ZnO  相似文献   

11.
The vacuum ultraviolet photoemission spectra of quasi-one-dimensional charge density wave ( CDW ) system, (TaSe4)2I, were measured for photon energies between 32 and 100 eV at room temperature ( in the normal phase ) and at about 100 K ( in the CDW phase ). The spectrum of Ta 4f core-levels has shown no additional splitting due to the two different Ta sites. The spectra of the valence and conduction bands have revealed the resonant enhancement for the excitation of the Ta 5p core states, which demonstrates the remarkable hybridization of Ta 5d orbitals with Se 4p orbitals with binding energies smaller than 4 eV. In the CDW phase, the partial cross section decreases for both Ta 5d bands and Se 4p bands with Ta 5d components.  相似文献   

12.
The nitrogen concentration effects on electronic band structures and photocatalytic performance of N-doped sodium niobate (NaNbO3) have been investigated by first-principles calculations based on density functional theory (DFT). At lower nitrogen doping levels, some localized N 2p states are formed above the valence band (O 2p) in N-doped NaNbO3, leading to the reduction of the photon transition energy in comparison to that of undoped compound. Under higher doping levels, the N 2p states mix with O 2p states and then move the top of valence band upward. Two possible mechanisms for increasing visible light absorbance in N-doped NaNbO3 are tentatively put forward according to the doping levels, which would be of importance in understanding and developing the visible-light-sensitive nitrogen-doped multimetal oxide.  相似文献   

13.
Ab initio quantum chemistry calculations of the structural and electronic properties of monoclinic wolframite-type ZnWO4 crystal have been performed within the periodic linear combination of atomic orbitals (LCAO) method using six different Hamiltonians, based on density functional theory (DFT) and hybrid Hartree-Fock-DFT theory. The obtained results for optimized structural parameters, band gap and partial density of states are compared with available experimental data, and the best agreement is observed for hybrid Hamiltonians. The calculations show that zinc tungstate is a wide band gap material, with the direct gap about 4.6 eV, whose valence band has largely O 2p character, whereas the bottom of conduction band is dominated by W 5d states.  相似文献   

14.
High resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of the non-conductor pyroxene minerals, bronzite ((Mg0.8,Fe0.2)2Si2O6) and diopside (Ca(Mg0.8Fe0.2)Si2O6) have been obtained with the Kratos magnetic confinement charge compensation which minimizes differential charge broadening. Observed Si 2p, O 1s, Mg 2p and Ca 2p total linewidths are all about 1.3 eV, very similar to those observed previously with the same instrument for SiO2 and olivines ((Mg,Fe)2SiO4); and we consider that these widths are within 0.05 eV of the minimum room temperature linewidths for these samples with the experimental resolution of this instrument of 0.35 eV. These linewidths are all determined by vibrational broadening due to the M-O symmetric stretch in the ion states. The Si 2p binding energies (BE) are intermediate between the quartz and olivine Si 2p binding energies; but the O 1s spectra resolve the bridging oxygen (BO) and non-bridging oxygen (NBO) in the unit, with the NBO O 1s very close in BE to the O in olivine, and the BO very close to the BO in SiO2. Indeed in both diopside and bronzite, it is possible to separate the three structurally inequivalent O atoms in the O 1s spectra: the BO at a BE of about 532.6 eV, a NBO peak from the MgOSi moiety (Mg in the M1 site) at about 531.3 eV, and a NBO peak at 531 eV from the CaOSi or the FeOSi moieties (Ca and Fe in the M2 site). The O 1s BE increases with the increase in the electronegativity Ca < Mg < Fe < Si. Moreover, the linewidths of these peaks increase when Fe and Mg are both present in either M1 (diopside) or M2 (bronzite) sites.The valence band spectra for the two pyroxenes are rather similar, and quite different from the VB spectra of quartz and olivines. The dispersion of the pyroxene VB spectra is intermediate between the VB spectra of quartz and olivines; the valence band spectrum of pyroxenes are more dispersed than in olivines by about 1.5 eV but less dispersed than quartz by about 1.5 eV. These VB spectra can be assigned using the previous olivine VB spectra and high quality pseudopotential density functional theoretical calculations in the generalized gradient (GGA) approximation. As for the olivine VB spectra, the Fe 3d t2g and eg orbitals in M1 and M2 sites of the pyroxene are located at the top of the pyroxene valence band, and the BE of the Fe 3d peaks from M1 are about 0.7 eV smaller than the Fe 3d peaks in M2. The theoretical XPS valence band spectra using the theoretical density of states and the Gelius intensity approximation are is in good semi-quantitative agreement with the experimental spectra. This intermediate dispersion of pyroxenes is due to the partial polymerization of the Si-O units in pyroxenes, and the intermediate charge on the Si atoms as indicated by the Si 2p BE.  相似文献   

15.
The XPS examinations of the AgNbO3 and NaNbO3 single crystals and ceramics allowed estimate their average composition as Ag1.1Nb0.9O3 and Na1.2Nb0.9O2.9. The valence bands of the AgNbO3 compound, formed mainly of the Nb 4d, Ag 4d and O 2p states, show an energy gap about 3 eV while for the NaNbO3 compound consist of the O 2p states hybridized with the Nb 3d states and show an energy gap about 4 eV. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. The broadening of the core level lines of AgNbO3 suggests a stronger structural disorder in comparison with NaNbO3 compound.  相似文献   

16.
K. Ozawa  Y. Oba 《Surface science》2009,603(13):2163-1659
Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)-Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O2 atmosphere (1.3 × 10−4 Pa) leads to an ordered Cu2O overlayer with (1 1 1) orientation. Good crystallinity of the Cu2O(1 1 1) overlayer is proved by energy dispersion of one of Cu2O valence bands. The Cu2O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu2O(1 1 1)/ZnO(0 0 0 1)-Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.  相似文献   

17.
李敏  张俊英  张跃  王天民 《中国物理 B》2012,21(8):87301-087301
The N-doping effects on the electronic properties of Cu2O crystals are investigated using density functional theory. The calculated results show that N-doped Cu2O with or without oxygen vacancy exhibits different modifications of electronic band structure. In N anion-doped Cu2O, some N 2p states overlap and mix with the O 2p valence band, leading to a slight narrowing of band gap compared with the undoped Cu2O. However, it is found that the coexistence of both N impurity and oxygen vacancy contributes to band gap widening which may account for the experimentally observed optical band gap widening by N doping.  相似文献   

18.
The electronic structure and chemical bonding in HgGa2S4 crystals grown by vapor transport method are investigated with X-ray photoemission spectroscopy. The valence band of HgGa2S4 is found to be formed by splitted S 3p and Hg 6s states at binding energies BE=3-7 eV and the components at BE=7-11 eV generated by the hybridization of S 3s and Ga 4s states with a strong contribution from the Hg 5d states. At higher binding energies the emission lines related to the Hg 4f, Ga 3p, S 2p, S 2s, Hg 4d, Ga LMM, Ga 3p and S LMM states are analyzed in the photoemission spectrum. The measured core level binding energies are compared with those of HgS, GaS, AgGaS2 and SrGa2S4 compounds. The valence band spectrum proves to be independent on the technological conditions of crystal growth. In contrast to the valence band spectrum, the distribution of electron states in the bandgap of HgGa2S4 crystals is found to be strongly dependent upon the technological conditions of crystal growth as demonstrated by the photoluminescence analysis.  相似文献   

19.
Optical transmittance and reflectance on ferroelectric BaTi2O5 glasses prepared recently by a containerless synthesis technique are measured at room temperature in the wavelength range 190-800nm. The fundamental absorption edge located around 340nm demonstrates the colourless and transparent character of the glass. The optical band gap of 3.32eV has been estimated. The tail of the optical absorption near the fundamental absorption edge is found to follow the Urbach rule. Our analysis of the experimental spectra supports an indirect allowed interband transition between the valence band formed by O-2p orbitals and the conduction band formed by Ti-3d orbitals.  相似文献   

20.
濮春英  李洪婧  唐鑫  张庆瑜 《物理学报》2012,61(4):47104-047104
采用射频磁控溅射技术, 在不同温度下制备了N掺杂Cu2O薄膜.透射光谱分析发现, N掺杂导致Cu2O成为允许的带隙直接跃迁半导体, 并使Cu2O的光学禁带宽度增加.不同温度下沉积的薄膜光学禁带宽度Eg=2.52± 0.03 eV.第一性原理计算表明, N掺杂导致Cu2O的禁带宽度增加了约25%, 主要与价带顶下移和导带底上移有关, 与实验报道基本符合.N的2p电子态分布不同于O原子, 在价带顶附近具有较大的态密度是N掺杂Cu2O变成允许的带隙直接跃迁半导体的根本原因.  相似文献   

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