锐钛矿相和金红石相Nb:TiO_2电学性质的GGA(+U)法研究

采用基于密度泛函理论第一性原理GGA和GGA+U相结合的方法研究了不同掺杂浓度下锐钛矿相和金红石相Nb:TiO2的晶体结构、电子结构以及稳定性.结果表明:锐钛矿相Nb:TiO2能带结构与简并半导体类似,呈类金属导电机理.金红石相Nb:TiO2呈半导体导电机理.Nb原子比Ti原子电离产生出更多的电子.锐钛矿相Nb:TiO2中Nb原子的电离率比金红石相Nb:TiO2的大.以上结果说明锐钛矿相Nb:TiO2比金红石相Nb:TiO2更适宜用作TCO材料;掺杂浓度对其杂质能级,费米能级和有效质量都有影响.Nb原子掺杂浓度越高,材料电离率呈降低趋势;形成能计算结果显示:在富钛条件下不利于Nb原子的掺杂,而在富氧条件下有利于Nb原子的掺杂.对于金红石相和锐钛矿相Nb:TiO2,不论是在贫氧或富氧条件下,随着Nb原子掺杂浓度的提高,形成能均增大.     

Optical properties of anatase and rutile titanium dioxide: Ab initio calculations for pure and anion-doped material

The optical properties of rutile and anatase titanium dioxide (TiO₂) are calculated from the imaginary part of the dielectric function using pseudopotential density functional method within its generalized gradient approximation (GGA) and a scissors approximation. The fundamental absorption edges calculated for the unit cell of both rutile and anatase are consistent with experimentally reported results of single crystal rutile and anatase TiO₂ and with previous theoretical calculations. A significant optical anisotropy is observed in the anatase structure which holds promise for investigating the band gap modification with better visible-light response and provides a reliable foundation for addressing the effect of impurities on the fundamental absorption edge/band gap of anatase TiO₂. Further calculations on the electronic structure and the optical properties of C-, N-, and S-doped anatase TiO₂ are performed. The results are analyzed and discussed in terms of optical anisotropy and scissors approximations.     

Elastic,electronic and optical properties of cotunnite TiO2 from first principles calculations

The ultrasoft pseudopotential technique is used to explore the elastic, electronic and optical properties of cotunnite TiO₂ using LDA and GGA proposed by Perdew Wang (PW91), Perdew–Burke–Ernzerhof (PBE) functional as defined by Wu and Cohen (PBEWC) and PBE functional for solids (PBESOL). The calculated elastic constants bulk modulus, shear modulus and Young’s modulus are in agreement with the previous theoretical reports. From our investigated shear anisotropy factors (A₁, A₂, and A₃), we infer that cotunnite TiO₂ is strong anisotropy in case of A₁ and A₂ and less anisotropy in case of A₃. The value of mean sound speed and Debye temperature are calculated using the obtained values of elastic moduli. The calculated structural parameters are in accord with the reported experiment and theoretical results. Our obtained values of direct bandgaps show an improvement over the other previous theoretical reports. The values of the dielectric constant (ε₁(ω)) of cotunnite TiO₂ calculated within LDA and GGA approximations are 7.655 (LDA (CA-PZ)), 7.578 (GGA (PW91)), 7.685 (GGA (WC)) and 7.655 (GGA (PBESOL)), which are slightly higher than the experimental values of rutile (6.69) and anatase (6.55) polymorphs. The obtained values of the refractive index are consistent with rutile TiO₂ and higher than anatase phase. The investigated imaginary part of dielectric constant and absorption spectrum reflect that the cotunnite TiO₂ is a weak photocatalytic material as compared to anatase and similar to rutile phases.     

Calculations of physical properties of pure and doped crystals: Ab initio and semi-empirical methods in application to YAlO3:Ce and TiO2

In this paper we demonstrate that two independent methods of calculations (DFT based ab initio and semi-empirical crystal field theory) can be used to form a complementary picture of the optical and electronic properties of the doped host and impurity ion. The crystals considered in the present paper are: (i) YAlO₃:Ce³⁺ and (ii) two dominant phases of TiO₂—rutile and anatase. As an example, detailed calculations of the band structure and crystal field energy level scheme of YAlO₃:Ce³⁺ are reported. From the analysis of the band structure and density of states, the character of the YAlO₃ energetic bands and positions of the Ce impurity energy levels were established. It was also shown how the ab initio methods can be used for calculations of the structural properties of solids under elevated pressure. Taking the two dominant phases of TiO₂ as an example, it was demonstrated how the elastic properties can be extracted from the calculated unit cell’s volume at different pressures. Particular attention was paid to the microscopic effects of crystal field, which were evidenced by the pressure-induced changes of the structure and shape of distribution of the Ti 3d electrons density of states. It was demonstrated how the difference in crystal structure of the anatase and rutile phases leads to remarkable difference in microscopic crystal field effects, which was explained by different Ti-O distances in both phases. In addition, the pressure dependence of the band gaps for anatase and rutile was investigated. It was shown that the hydrostatic pressure leads to the band gap narrowing in anatase and band gap widening in rutile, with pressure coefficients +0.00681 eV/GPa for rutile and −0.0088 eV/GPa for anatase.     

LiV2O4: electronic,magnetic structure and heavy-fermion behaviour from first principles

First principles studies based on density functional theory (DFT) calculations within the generalized gradient approximations (GGA) and GGA + U approach using the full-potential, augmented plane wave + local orbitals (APW + lo) method, as implemented in the WIEN2k code, have been used to investigate the structural, electronic and magnetic properties of spinel-structure LiV₂O₄, in particular regarding the heavy fermion (HF) behaviour. The calculations were performed for ferromagnetic, anti-ferromagnetic, and ferrimagnetic configurations using two kinds of magnetic structures (tetragonal and rhombohedral). The GGA results showed that the Fermi energy lies in the V 3d (t_2g) bands with 1.5 electrons per V atom occupying this band, and the V 3d bands are separated by a ～1.9 eV energy gap from the O 2p bands and further split into t_2g and e_g bands with a ～1.0 eV energy gap, which are in good agreement with the photoelectron spectra. The GGA + U method indicates that the ground state of LiV₂O₄ is the tetragonal anti-ferromagnetic configuration with metallic character, and ferromagnetic order character at slightly higher energy, which is consistent with experimental result. The geometric frustration and hybridization between 3d (V) and 2p (O) could induce spin fluctuation and help to explain the instability of specific heat, susceptibility and HF behaviour.     

First-principles study on electronic,magnetic properties and optical absorption of vanadium doped rutile TiO2

The electronic, magnetic properties and optical absorption of vanadium (V) doped rutile TiO₂ have been studied by the generalized gradient approximation GGA and GGA+U (Hubbard coefficient) approach respectively. On the one hand, we consider the influence of vanadium with different doping concentration on the electronic structure. On the other hand, we study double V atoms doped TiO₂, mainly study four V-doped TiO₂ configurations, and find the magnetic ground states are ferromagnetic state. For the TiO₂@V-V1, TiO₂@V-V3 and TiO₂@V-V4 configurations without O ion as bridge between V-V atoms, there will have a metastable state of antiferromagnetic configurations, while, for the TiO₂@V-V2 configurations with an O ion as bridge between V-V atoms, due to the existence of superexchange between V-O-V, there will only exist the ground state of ferromagnetic state and there are no other metastable configurations. Furthermore, the optical properties of V-doped TiO₂ are calculated. The results show that the V-doped TiO₂ has strong infrared light absorption and visible light absorption.     

Electronic excitation energies in TiO<Subscript>2</Subscript> in the fluorite phase

The ab initio pseudopotential method within the generalized gradient approximation (GGA) and quasiparticle approximation has been used to investigate the electronic properties of titanium dioxide in the rutile, anatase, and fluorite structures, respectively. Here we present the GW approximation for the electronic self-energy, which allows to calculate excited-state properties, especially electronic band structures. For this calculation, good agreement with the experimental results for the minimum band gaps in rutile and anatase phase is obtained. In the fluorite phase we predict that titanium dioxide will be an indirect (Γ to X) wide band-gap semiconductor (2.367 or 2.369 eV) and the properties remain to be confirmed by experiment.     

Electronic structures and optical properties of TiO₂:Improved density-functional-theory investigation

TiO2 has been recently used to realize high-temperature ferromagnetic semiconductors.In fact,it has been widely used for a long time as white pigment and sunscreen because of its whiteness,high refractive index,and excellent optical properties.However,its electronic structures and the related properties have not been satisfactorily understood.Here,we use Tran and Blaha’s modified Becke-Johnson(TB-mBJ) exchange potential(plus a local density approximation correlation potential) within the density functional theory to investigate electronic structures and optical properties of rutile and anatase TiO2.Our comparative calculations show that the energy gaps obtained from mBJ method agree better with the experimental results than that obtained from local density approximation(LDA) and generalized gradient approximation(GGA),in contrast with substantially overestimated values from many-body perturbation(GW) calculations.As for optical dielectric functions(both real and imaginary parts),refractive index,and extinction coefficients as functions of photon energy,our mBJ calculated results are in excellent agreement with the experimental curves.Our further analysis reveals that these excellent improvements are achieved because mBJ potential describes accurately the energy levels of Ti 3d states.These results should be helpful to understand the high temperature ferromagnetism in doped TiO2.This approach can be used as a standard to understand electronic structures and the related properties of such materials as TiO2.     

Structural, electronic and optical properties of SrCl2 under hydrostatic stress

The results of first-principles theoretical study of the structural, electronic and optical properties of SrCl₂ in its cubic structure, have been performed using the full-potential linear augmented plane-wave method plus local orbitals (FP-APW+lo) as implemented in the WIEN2k code. In this approach both the local density approximation (LDA) and the generalized gradient approximation (GGA) are used for the exchange-correlation (XC) potential. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. We performed these calculations with and without spin-orbit interactions. Including spin-orbit coupling cause to lifts the triple degeneracy at Γ point and a double degeneracy at X point. Results are given for structural properties. The pressure dependence of elastic constants and band gaps are investigated. The dielectric function, reflectivity spectra and refractive index are calculated up to 30 eV. Also we calculated the pressure and volume dependence of the static optical dielectric constant.     

Structural,electronic,and magnetic properties of vanadium atom-adsorbed MoSe_2 monolayer

Using the first-principles calculations, we study the structural, electronic, and magnetic properties of vanadium adsorbed MoSe_2 monolayer, and the magnetic couplings between the V adatoms at different adsorption concentrations. The calculations show that the V atom is chemically adsorbed on the MoSe_2 monolayer and prefers the location on the top of an Mo atom surrounded by three nearest-neighbor Se atoms. The interatomic electron transfer from the V to the nearestneighbor Se results in the polarized covalent bond with weak covalency, associated with the hybridizations of V with Se and Mo. The V adatom induces local impurity states in the middle of the band gap of pristine MoSe_2, and the peak of density of states right below the Fermi energy is associated with the V- dz~2 orbital. A single V adatom induces a magnetic moment of 5 μBthat mainly distributes on the V-3d and Mo-4d orbitals. The V adatom is in high-spin state, and its local magnetic moment is associated with the mid-gap impurity states that are mainly from the V-3d orbitals. In addition,the crystal field squashes a part of the V-4s electrons into the V-3d orbitals, which enhances the local magnetic moment.The magnetic ground states at different adsorption concentrations are calculated by generalized gradient approximations(GGA) and GGA+U with enhanced electron localization. In addition, the exchange integrals between the nearest-neighbor V adatoms at different adsorption concentrations are calculated by fitting the first-principle total energies of ferromagnetic(FM) and antiferromagnetic(AFM) states to the Heisenberg model. The calculations with GGA show that there is a transition from ferromagnetic to antiferromagnetic ground state with increasing the distance between the V adatoms. We propose an exchange mechanism based on the on-site exchange on Mo and the hybridization between Mo and V, to explain the strong ferromagnetic coupling at a short distance between the V adatoms. However, the ferromagnetic exchange mechanism is sensitive to both the increased inter-adatom distance at low concentration and the enhanced electron localization by GGA+U, which leads to antiferromagnetic ground state, where the antiferromagnetic superexchange is dominant.     

Phase stability,electronic, magnetic and elastic properties of Ni2CoZ(Z = Ga,Sn): A first principles study with GGA method and GGA+U approach

First principles calculations based on density functional theory are used to investigate the phase stability, electronic, magnetic and elastic properties of ferromagnetic metallic full-Heusler Ni₂CoZ(Z = Ga, Sn) alloys via the FP-LAPW method by the generalized gradient GGA and GGA+U approximations for the exchange and correlation energy, within the Perdew–Burke–Ernzerhof (PBE 96) parameterization. The results of calculating electronic structures and magnetic properties reveal that the both Ni₂CoGa and Ni₂CoSn crystallize in L2₁ phase with regular cubic structure. The two investigated compounds exhibit metallic ferromagnetic behaviors for the GGA+U calculation. The computation of elastic constants with GGA+U approach shows that our compounds are mechanically stable.     

Ferromagnetic Half-Semiconductor (HSC) gaps in co-doped CdS: Ab-initio study

In this work we have used the density functional theory (DFT) to study structural, magnetic and electronic properties of Co doped CdS in the zinc-blende (ZB) phase. Three approximations have been used to treat the exchange-correlation potential: the (PBE) GGA, (PBE) GGA+U and the model of Tran–Blaha modified Becke–Johnson potential (TB-mBJ). The on–site Coulomb interaction correction given by the Hubbard U has been calculated by the local density approximation constraint for the Co electronic orbitals. The theoretical results show that all the properties under study are affected by the doping of Co atom. Co doped CdS becomes a Ferromagnetic Half-Semiconductors (HSC). The total magnetic moment increases with increasing Co concentration; reaching the value of 9 (in the units of Bohr magneton) at high concentration (x = 18.75 %). The total magnetic moment value is an integer in the GGA+U and TB-mBJ approximations. Furthermore, to validate the effects resulting from the exchange splitting process, we have calculated the values of the spin-exchange constants N_0α and N_0β, respectively. All these changes make CdS:Co extremely interesting system for the spintronic applications.     

Electronic structures and optical properties of TiO2:Improved density-functional-theory investigation

TiO2 has been recently used to realize high-temperature ferromagnetic semiconductors.In fact,it has been widely used for a long time as white pigment and sunscreen because of its whiteness,high refractive index,and excellent optical properties.However,its electronic structures and the related properties have not been satisfactorily understood.Here,we use Tran and Blaha＇s modified Becke-Johnson（TB-mBJ） exchange potential（plus a local density approximation correlation potential） within the density functional theory to investigate electronic structures and optical properties of rutile and anatase TiO2.Our comparative calculations show that the energy gaps obtained from mBJ method agree better with the experimental results than that obtained from local density approximation（LDA） and generalized gradient approximation（GGA）,in contrast with substantially overestimated values from many-body perturbation（GW） calculations.As for optical dielectric functions（both real and imaginary parts）,refractive index,and extinction coefficients as functions of photon energy,our mBJ calculated results are in excellent agreement with the experimental curves.Our further analysis reveals that these excellent improvements are achieved because mBJ potential describes accurately the energy levels of Ti 3d states.These results should be helpful to understand the high temperature ferromagnetism in doped TiO2.This approach can be used as a standard to understand electronic structures and the related properties of such materials as TiO2.     

爆轰法制备纳米TiO2及其参数控制

 采用偏钛酸与炸药混合进行爆轰合成，制备出晶粒尺寸在10 nm到60 nm之间的纳米TiO₂粉末。结合XRD、TEM、BET等表征手段，研究了纳米TiO₂的制备参数与其晶粒尺寸和晶相结构之间的关系。发现当炸药含量增加时，产物晶粒尺寸增大，同时晶相结构随炸药量的增加逐渐由低温的锐钛矿型和板钛矿型向高温的金红石型转变，爆轰温度在这种转变过程中起着极为重要的作用。     

Ferromagnetism in phosphorus-doped ZnO: First-principles calculation

The electronic structure and magnetic properties of nonmagnetic phosphorus doped ZnO are investigated using first-principles calculation. Both generalized gradient approximation (GGA) and GGA + U calculations show that each substitutional P atom in ZnO induces a magnetic moment of about 1.0 μ_B, which come mainly from the partially filled p orbitals of the substitutional P and its 12 second neighboring O atoms. The magnetic coupling between the moments induced by P doping is ferromagnetic. The calculated electronic structures indicate that the ferromagnetic coupling can be explained in terms of the two band coupling model.     

First-principles study on ferromagnetism in C-doped AlN

First-principles calculations are performed to study the electronic structures and magnetic properties of C-doped AlN. Both generalized gradient approximation (GGA) and GGA+U calculations show that a substitutional C atom introduces magnetic moment of about 1.0 μB, which comes from the partially occupied 2p orbitals of the C, its first neighboring Al and first neighboring N atoms (GGA) or out-of-plane first and fifth neighboring N atoms (GGA+U), among which the atomic moment of the C is the biggest. The U correction for the anion-2p states obviously changes the magnetic moment distribution of Al and N atoms and transforms the ground state of C-doped AlN to insulating from half-metallic. The C atoms can induce ferromagnetic ground state with long-range couplings between the moments in C-doped AlN. The ferromagnetic coupling can be explained in terms of the two band coupling model.     

Synthesis of Different TiO_2 Nanostructures and Their Physical Properties

Titanium dioxide(TiO_2) nanosheet, nanorod and nanotubes are synthesized using chemical vapor deposition(CVD) and anodizing processes. TiO_2 nanosheets are grown on Ti foil which is coated with Au catalyst in CVD,TiO_2 nanorods are synthesized on treated Ti foil with HCI by CVD, and TiO_2 nanotubes are prepared by the three-step anodization method. Scanning electron microscopy shows the final TiO_2 structures prepared using three processes with three different morphologies of nanosheet, nanorod and nanotube. X-ray diffraction verifies the presence of TiO_2. TiO_2 sheets and rods are crystalized in rutile phase, and TiO_2 tubes after annealing turn into the anatase crystal phase. The optical investigations carried out by diffuse reflection spectroscopy reveal that the morphology of TiO_2 nanostructures influencing their optical response and band gap energy of TiO_2 is changed for different TiO_2 nanostructures.     

锐钛矿相和金红石相TiO2:Nb的光电性能研究

采用基于密度泛函理论的第一性原理计算了锐钛矿相和金红石相TiO₂:Nb的晶体结构、电子结构和光学性质. 结果表明, 在相等的摩尔掺杂浓度下(6.25%), 锐钛矿相TiO₂:Nb的导带底电子有效质量小于金红石相TiO₂:Nb, 且前者室温载流子浓度是后者的两倍左右, 即具有更大的施主杂质电离率, 从而解释了锐钛矿相TiO₂:Nb比金红石相TiO₂:Nb具有更优异电学性能的实验现象. 光学计算也表明锐钛矿相在可见光区有更大的透过率, 从而在理论上解释了锐钛矿相TiO₂:Nb比金红石相TiO₂:Nb更适于做透明导电材料的原因. 计算结果与实验数据能较好符合. 关键词： 2:Nb')" href="#">TiO₂:Nb 第一性原理 电子结构 光学性能     

First-principles study of the influences of oxygen defects upon the electronic properties of Nb-doped TiO_2 by GGA + U methods

The influence of oxygen defects upon the electronic properties of Nb-doped TiO2has been studied by using the general gradient approximation(GGA)+U method. Four independent models(i.e., an undoped anatase cell, an anatase cell with a Nb dopant at Ti site(NbTi), an anatase cell with a Nb-dopant and an oxygen vacancy(NbTi+VO), and an anatase cell with a Nb-dopant and an interstitial oxygen(NbTi+Oi)) were considered. The density of states, effective mass, Bader charge, charge density, and electron localization function were calculated. The results show that in the NbTi+VOcell both eg and t2glevels of Ti 3d orbits make contributions to the electronic conductivity, and the oxygen vacancies(VO) collaborate with Nb-dopants to favor the high electrical conductivity by inducing the Nb-dopants to release more excess charges. In NbTi+Oi, an unoccupied impurity level appears in the band gap, which served as an acceptor level and suppressed the electronic conductivity. The results qualitatively coincide with experimental results and possibly provide insights into the preparation of TCOs with desirable conductivity.