N-Mo-W共掺杂金红石相TiO_2的第一性原理研究

基于密度泛函理论第一性原理与平面波超软赝势法是目前对物质光学性质计算的成熟手段,本文利用MS软件采取该方法对金红石相TiO_2进行了不同掺杂情况下的模拟计算.内容包括未掺杂与单掺杂Mo、单掺杂N、共掺杂Mo-N以及共掺杂N-Mo-W这五种不同情况下TiO_2的能带结构、电子态密度与光学性质分析,通过对计算得出的数据分析有以下结论:单掺杂能改变TiO_2禁带宽度,但相对于共掺杂Mo-N和W-N以及N-Mo-W来说效果欠佳.其中,掺杂W时由于在导带底中出现新的杂质能级,并出现了导带下降幅度大于价带下降幅度的情况,禁带宽度变窄,使得在单掺杂情况中效果明显.而共掺杂中N-Mo-W的价带出现清晰的杂质能级,并且由于该能级介于费米能级附近的关系使得价电子跃迁至导带更为容易,并且此时能级密度较大也是掺杂效果明显的一个重要原因.     

Al-N共掺杂金红石相TiO_2的第一性原理研究

本文采用了基于密度泛函理论的第一性原理平面波超软赝势法对金红石相TiO_2进行了计算,其中内容包括未掺杂与单掺杂Al、单掺杂N以及共掺杂Al-N这四种不同情况下TiO_2的能带结构与态密度和光吸收系数的研究.计算结果表明:单掺杂Al和N时,均不同程度地改变了其能带结构,光吸收能力均有提高但效果不佳.在共掺杂Al-N时,TiO_2晶格常数产生了改变,并出现了新的杂质能级.由于杂质能级存在于TiO_2禁带范围内,减小了电子跃迁至导带所需能量,从而提高了其光吸收能力,其效果相对于单掺杂来说更有明显提高.     

金红石型TiO2点缺陷性质的第一性原理研究

本文运用基于局域密度泛函和赝势的第一性原理方法研究了金红石相TiO2点缺陷的电子性质,结果表明氧空位缺陷使晶体的费米能量升高,在能隙中没有产生杂质能级.钛空位缺陷使晶体的费米能量降低,并在价带顶部产生了一个杂质能级,与价带顶能量相差约0.4 eV.本文还计算了金红石相TiO2在具有氧空位和钛空位点缺陷情况下的键长变化、态密度和电荷布居状况.     

TiO2混晶光催化性能的第一性原理研究

本文采用第一性原理平面波超软赝势方法,通过搭建混晶界面的方式计算并分析了TiO2混晶的光催化性能.从光的吸收能力、光生电子-空穴对的分离效率以及载流子的转移效率三方面,与锐铁矿相和金红石相TiO2单晶进行对比分析.结果显示TiO2混晶的电子-空穴对分离率高于锐钛矿相TiO2、金红石相TiO2单晶,TiO2混晶中吸收峰明...     

N-S共掺杂金红石相TiO2电子结构与光学性质的第一性原理研究

采用基于第一性原理的平面波超软赝势方法研究N和S单掺杂以及N和S共掺杂金红石相TiO2的能带结构,态密度和光学性质.结果表明:N掺杂导致禁带宽度减小为1.43 eV,并且在价带上方形成了一条杂质能带;S掺杂导致费米能级上移靠近导带,直接带隙减小为0.32 eV;N和S共掺杂导致能带结构中出现了两条杂质能带,靠近导带的一条杂质能级距离导带底约0.35 eV,靠近价带的一条杂质能级距离价带顶约0.85 eV,杂质能级主要由N原子的2p轨道和S原子的3p轨道组成.N和S掺杂后不但使TiO2的吸收带产生红移,而且在可见光区具有较大的吸收系数,光催化活性增强.     

金红石结构TiO2结构和热力学性质的第一性原理研究

利用基于密度泛函理论的第一性原理计算方法以及准谐德拜模型研究了金红石TiO2的结构和热力学性质。常温常压下所计算的晶格常数、体弹模量及其对压强的一阶导数与实验值和其他理论计算结果相符的较好。另外,我们还计算了体弹模量、热膨胀系数、热容与温度和压强的关系。     

P掺杂锐钛矿相TiO2的第一性原理计算

利用基于密度泛函理论的第一性原理对不同P掺杂形式(P替位Ti, P替位O, 间隙P)的锐钛矿相TiO₂的晶格常数、电荷布居、能带结构、分态密度和吸收光谱进行了计算. 结果表明, P替位Ti时, TiO₂体积减小, P替位O和间隙P的存在使TiO₂的体积膨胀; 替位Ti的P和间隙P均有不同程度的氧化, 而替位O的P带有负电荷. 三种P掺杂形式均导致锐钛矿相TiO₂禁带宽度的增大, 并在TiO₂禁带之内引入了掺杂局域能级. P掺杂导致TiO₂禁带宽度增大的程度依次为: 间隙P>P替位Ti>P替位O. 吸收光谱的计算结果表明, P替位Ti并不能增强TiO₂的可见光吸收能力, 但间隙P的存在大幅提高了TiO₂的可见光光吸收能力, 间隙P有可能是造成实验上P掺杂增强锐钛矿相TiO₂光催化活性的重要原因. 关键词： P掺杂 2')" href="#">锐钛矿相TiO₂ 第一性原理     

共掺杂金红石TiO2的电子结构和红外光谱研究

本文对大别山双河、碧溪岭地区硬玉石英岩中的金红石进行了Fourier变换红外光谱(FTIR)分析, 结果显示所有金红石颗粒分别在3280 cm^-1和3295 cm^-1 出现强的吸收峰. 基于前人提出H在金红石结构中以孔道中心(CC)和八面体共边(BOE)方式存在的两种模型, 本文采用第一性原理计算方法研究了掺杂(Al, H)和(Fe, H)时金红石相TiO₂的晶体结构和电子结构. 根据O—H键的振动频率和O—H…O键中O—O之间距离的经验关 关键词： 2')" href="#">金红石相TiO₂ 孔道中心 电子结构 第一性原理     

I掺杂金红石TiO$lt;sub$gt;2$lt;/sub$gt;(110)面的第一性原理研究

利用基于密度泛函理论的第一性原理研究了I掺杂金红石TiO₂（110）表面的形成能和电子结构,分析了不同掺杂位置的结构对TiO₂光催化性能的影响. 计算表明,氧化环境下I最容易替代掺杂表面五配位的Ti,而还原环境下最容易替代掺杂表面的桥位氧. I替位Ti或I替位O都能降低禁带宽度,可能使TiO₂吸收带出现红移现象或产生在可见光区的吸收,其中I替位桥位氧的禁带宽度最小. 吸收光谱表明,I掺杂不仅能提高TiO₂可见光响应,同时可增加紫外光的吸收能量,提高其可见光及紫外光下的光催化性能. 关键词： 第一性原理 I掺杂 2(110)')" href="#">金红石相TiO₂(110) 光催化     

金红石TiO_2中本征缺陷扩散性质的第一性原理计算

基于密度泛函理论的爬坡弹性带方法,对金红石相二氧化钛晶体中钛间隙、钛空位、氧间隙、氧空位4种本征缺陷的扩散特征进行了研究.对比4种本征缺陷在晶格内部沿不同扩散路径的过渡态势垒后发现,缺陷扩散过程呈现出明显的各向异性.其中,钛间隙和氧间隙沿[001]方向具有最小的扩散势垒路径,激活能分别为0.505 eV和0.859 eV;氧空位和钛空位的势垒最小的扩散路径分别沿[110]方向和[111]方向,激活能分别为0.735 eV和2.375 eV.     

Half-metallic behaviour in doped TiO2 (rutile) with double impurities: ab initio calculation

Dilute magnetic oxides are without doubt among the most interesting classes of magnetic materials. However, the nature of their electronic structure and magnetic exchange is far from understood. Here, we apply the ab initio augmented spherical wave (ASW) method, with corrected generalised gradient approximation to study the electronic structure and magnetic properties of doped TiO₂ rutile with double impurities. The study reveals a half-metallic ferromagnetic behaviour for Ti_1?2x Cr _x Mo _x O₂, and the local magnetic moments of the impurities and their oxidation states agree with the charge transfer between Cr and Mo, which would lead to the ferromagnetic state through the double-exchange mechanism in transition metal oxides.     

Ab initio modelling of band states in doped diamond

Presented in this study is an analysis of the electronic properties of doped diamond calculated using the Vienna ab initio simulation package, employing density functional theory within the generalized-gradient approximation. The dopants studied here have been inserted substitutionally into a 64-atom diamond supercell and include the single-electron acceptors boron and aluminium, the single-electron donors nitrogen and phosphorus and the double-electron donors oxygen and sulphur. Co-doping of diamond with sulphur and boron has also been briefly examined. The doped supercells have been relaxed, followed by calculation of electronic properties from the electronic density of states such as the indirect bandgap E _g, the valence bandwidth and an examination of the acceptor and donor states in the bandgap. It is anticipated that this study will provide a useful comparison of the third- and fourth-row donors and acceptors in diamond.     

Role of dopant Ga in tuning the band gap of rutile TiO2 from first principles

The influences of different doped Ga amount on the band gaps of rutile TiO₂ have been investigated systematically using density functional theory calculations. The calculation results show that the substitutions of Ga can make the configurations more stable, and the lower doped amount can induce the red shift of the absorption edge by reducing their band gaps, such as Ga₁Ti₁₅O₃₂ and Ga₃Ti₁₃O₃₂ samples, whereas the band gaps slightly increase with the higher doping amount. The narrower band gaps are due to the valence band edges shifting upwards with energy levels of impurities produced by lower Ga doping amount, however, the wider band gaps are caused by the dominant rises of conduction band bottoms when Ga doping contents are larger. As a result, proper doping content of Ga can reduce their band gaps and improve the light absorption performances of TiO₂ gradually.     

Titanium atoms dimerization phenomenon and magnetic properties of titanium-antisite (TiO) and chromium doped rutile TiO2, ab-initio calculation

The ab-initio calculations based on the Korringa Kohn Rostoker approximation approach combined with coherent potential approximation (KKR-CPA), were used to study the magnetic properties of the titanium anti-site (Ti_O) and chromium (Cr) doped TiO₂. In the considered systems, we used different concentrations for Ti_O defect and Cr doping. In TiO_2(0.98)(Ti_O)_0.02, the obtained results indicate that Ti_O is a donor having half-metal behavior. Ti_O[3d] band is located at the Fermi level, although isn’t 100% polarized, the ferromagnetic (FM) state is verified as being more stable than disordered local moment (DLM) state. For Ti_0.98Cr_0.02O₂ the Cr doping introduced new states which give the material half-metallic feature. The majority spin of Cr impurities are located at the Fermi level and the conduction electrons around the Fermi level are 100% spin polarized. This indicates the stability of (FM) state. Moreover, in Ti_0.98Cr_0.02O_2(0.98)(Ti_O)_0.02, the top of the valence band is shifted to lower energy compared to pure TiO₂, and the n-type of TiO₂ is verified. The majority spin of Cr[3d] are located at 0.025 Ry close to the Fermi level. The predicted Curie temperatures (Tc) were calculated using the mean field approximation (MFA) and we predicted that Ti_O defect in Cr doped TiO₂ makes Tc higher. This kind of defect makes the material useful for spinotronics's applications and devices.     

Cross luminescence of BaF2 crystal: Ab initio calculation

Using ab initio methods and taking into account the lattice relaxation and polarization caused by the occurrence of the core hole, we have studied theoretically the cross luminescence in barium fluoride crystals in terms of the embedded-cluster approach. Two schemes of modeling of the core hole have been performed—in the form of an additional point charge and in the form of the 5p state of the barium ion. Calculations have been done both by the Hartree-Fock method and by the density functional method. We have showed that the deformation of the lattice caused by the occurrence of the core hole leads to states localized on fluorine ions of the nearest environment splitting off from the valence band of the BaF₂ crystal. The cross-luminescence bands at 5.7, 6.3, and 7.1 eV are caused by transitions from these localized states. We have also showed that the low-energy edge of the cross luminescence is formed by transitions from states that are localized on ions of the second coordination sphere.     

Ab initio calculation of intrinsic point defects in CuInSe2

Preliminary ab initio calculation of different point defects energy and electronic density of states have been performed on the prototype chalcopyrite semiconductor CuInSe₂. The simulation method used is based on the density functional theory within the framework of pseudo-potentials and plane waves basis. The isolated neutral defects considered are: V_Cu, V_In, V_Se, Cu_i, In_i, In_Cu, Cu_In and the complex defects are 2Cu_i+Cu_In, In_Cu+Cu_In and 2V_Cu+In_Cu, some of which being computed for the first time by advanced ab initio techniques. In agreement with previous results, we show that some point defects (such as V_Cu) and pair defects (2V_Cu+In_Cu) have very low formation energies. Some energies of formation were found significantly lower than previous estimations. The comparison of the formation energies with the exchange correlation (LDA or GGA) is discussed. The perturbation induced by the presence of some of these ideal defects on the density of states is also presented.     

Ab initio calculation of the Hoyle state

The Hoyle state plays a crucial role in the helium burning of stars heavier than our Sun and in the production of carbon and other elements necessary for life. This excited state of the carbon-12 nucleus was postulated by Hoyle as a necessary ingredient for the fusion of three alpha particles to produce carbon at stellar temperatures. Although the Hoyle state was seen experimentally more than a half century ago nuclear theorists have not yet uncovered the nature of this state from first principles. In this Letter we report the first ab initio calculation of the low-lying states of carbon-12 using supercomputer lattice simulations and a theoretical framework known as effective field theory. In addition to the ground state and excited spin-2 state, we find a resonance at -85(3) MeV with all of the properties of the Hoyle state and in agreement with the experimentally observed energy.     

Ab initio study of phonons in the rutile structure of TiO2

The local-density approximation is used to find the phonon dispersion relations, total and partial phonon density of states for TiO₂ crystal of rutile structure. For that the Hellmann-Feynman forces were computed and direct method applied. Some thermodynamic quantities are also presented. Calculated results are analyzed and compared with the experimental neutron scattering and optic data. Good agreement has been achieved. A giant LO/TO splitting is observed for A_2u and E_u modes.     

Ab initio calculation of double ionization of atoms

The Solov’ev-Vinitsky method was used to perform an ab initio calculation of the triple-differential cross section for the double single-photon photoionization of helium for the case of equal emitted-electron energies. A Gaussian width γ describing angular electron-electron correlations at the total electron energy E taking values in range between 0.1 and 100 eV was obtained for this cross section. The results agree with available experimental data, but they raise a doubt as to whether the well-known Wannier law γ ∝ E ^1/4 is applicable at experimentally accessible energies. The Gaussian width γ was investigated as a function of the total emitted-electron energy for targets that have a strongly asymmetric configuration of the initial state—specifically, a negative atomic-hydrogen ion H^? and heliumin the 1s2s ¹ S and 1s3s ¹ S excited states. It was found that this function, γ(E), had a maximum at low energies. It was also shown that, at low energies, the dependence of the double-differential cross section on the angle between the emitted-electron momenta for the targets indicated above differed substantially from the Gaussian dependence, featuring maxima whose number was equal to the number of radial nodes in the initial state. This opens new possibilities for a qualitative analysis of the electron structure of targets.