【作者】 朱爱玲；

【导师】 杜勇；

【作者基本信息】 中南大学 ， 材料学， 2012， 博士

【摘要】 稀土发光材料因其独特的发光性能在颜色显示、照明和防伪等领域有着广泛的应用,始终是材料学和物理学的一个研究热点。Pr掺杂碱土钛酸盐在紫外光激发时具有理想红光发射、微弱余辉及化学稳定性好等优良性能。近十年来,通过各种实验方法拓宽这种红色发光材料的激发范围、延长余辉等提高其发光性能的方法是发光材料重点研究领域之一。为了获得发光性能较好的产品,往往需要反复实验直到获得最佳结果为止。单凭实验研发高性能发光材料不仅时间冗长而且效率低。材料性能是材料中各微观性质的宏观表现,实验方法很难对影响宏观性质的各微观因素进行系统详细地研究。而第一性原理计算模拟方法能很好地描述材料的微观性质,进而揭示影响宏观性能的微观机理。同一实验方法合成的产品有时发光性能差异较大,实验上无法得出具体原因。据此,本论文采用基于密度泛函理论的第一性原理计算方法,以Pr掺杂CaTiO3和SrTiO3为研究对象,并结合Zn2+离子对Pr掺杂CaTiO3发光和余辉有益以及Al3+离子对Pr掺杂SrTiO3发光有益的两个实验现象,计算了Pr掺杂的CaTiO3、Pr和A1掺杂的SrTiO3的缺陷形成能和电子结构,进而预测了(Zn,Pr)掺杂CaTiO3和(Al,Pr)掺杂SrTiO3的光学性质。本工作取得的主要研究结果如下：(1)(Zn,Pr)掺杂的CaTiO3的缺陷、能带结构和光学性质计算了状态A (CaTiO3. CaO和O2平衡)和状态B (CaTiO3.TiO2和O2平衡)下CaTiO3中各本征缺陷的缺陷形成能以及电子结构。计算表明当费米能级靠近价带时,VTi4、CaTi2-和VCa2-是主要受主缺陷。各本征缺陷体系的禁带中没有缺陷能级。VTi4、CaTi2-和VCa2-本征缺陷体系能隙比完整CaTiO3能隙分别少0.21eV、0.12eV和0.06eV, Vo2+和TiCa2+体系带隙基本保持不变。浓度最大的本征缺陷体系决定着基质中电子跃迁的最低能量。带隙变小的本征缺陷体系有利于基质中电子从价带顶跃迁至导带底。因此,调整本征缺陷的组成能够降低电子吸收能量,从而使激发光谱范围变宽,对实验掺杂改性具有参考价值。·计算了CaTiO3中Pr缺陷的形成能和电子结构。计算表明：Pr掺杂的格位与体系中平衡状态和电子费米位置相关。PrCa0、PrCa1+缺陷体系的禁带中没有出现缺陷能级。PrCa2+的缺陷体系在禁带中出现了缺陷能态。这缺陷能态与现有Pr作为发光中心的发光模型基本类似。·计算了(Zn,Pr)掺杂的CaTiO3能带结构、电子结构和光学性质。Pr掺杂的CaTiO3在价带顶～1.30eV和～2.06eV处出现以Pr4f、O2p和Ti3d的杂化能态。(Zn,Pr)掺杂后体系仍保持Pr掺杂体系特征,但在价带顶～0.18eV出现了新的缺陷能级。(Zn,Pr)掺杂后的吸收光谱在300nm、372nm和457nm处出现了宽带吸收。这一计算结果与实验观察的吸收光谱特征330nm、375nm和458-495nm基本相符。(2)(Al,Pr)掺杂的SrTiO3的缺陷、能带结构和光学性质·当SrTiO3、TiO2和O2三相平衡时,计算了SrTiO3中本征缺陷的形成能以及其电子结构。结果表明体系主要受主和施主缺陷分别为VSr2-和VO2+。各含本征缺陷体系禁带中没有缺陷能级。与完整SrTiO3相比,TiSr1+体系能隙增大0.05eV, VTi4-、Vo2+、SrTi2-和VSr2-带隙基本保持不变。没有明显减少的本征缺陷的带隙不利于电子从价带从导带的跃迁。·当SrTiO3. TiO2和O2三相平衡时,Pr优先占据Sr格位,Al占据Ti格位。Prsr0、PrSr1+缺陷体系的禁带中没有出现缺陷能级,只有PrSr2+的缺陷体系在高于价带顶～0.73eV处出现可作为发光中心的带间态。在1573K时,Al掺杂前后SrTiO3:Pr体系电子费米能级由0.645eV下降至0.601eV,从而(Al,Pr)代替VSr2-缺陷成为主要的受主缺陷参与电中性平衡。·计算了SrTiO3掺杂的(Al,Pr)(?)带结构、电子结构和光学性质。当两个Al原子与Pr原子距离最近时,Pr原子轨道发生劈裂,缺陷体系在禁带中有较多能带。SrTiO3掺杂的(287nm.308nm吸收光谱在260-400nm区间的的特征吸收峰(270nm.300nm和350nm)结果与文献实验激发峰和360nm)相接近。本论文从电子层面上计算了Pr和Zn/Al掺杂的钛酸盐的缺陷形成能、能带结构和光学性质,得出了与实验接近的结果。在发光材料和光催化材料等进行改性的实验研究时,本论文的相关研究成果将对掺杂种类、掺杂方法和掺杂浓度等有积极的参考价值,也将有助于提高这些材料的研发效率。更多还原

【Abstract】 Luminescent materials doped with rare earth, which are widely applied in the fields of color display, lighting and anti-counterfeiting and so on, are one of hot research topics among materials and physics. Pr doped CaTiO3and SrTiO3luminescent materials can be excited in the ultra violet and show a unique red emission as well as weak afterglow and high chemical stability at room temperature. Better red luminescent materials have been intensively investigated by broadening excitation area and improving the long decay properties during the recent years. It takes inevitably researchers long periods to obtain a better luminescent material through iterative experiments in the laboratory. Generally speaking, macroscopical properties of materials are the statistical results of all microcosmic exhibitions. Sometimes the macroscopical property of one sample is different from that of the other which is obtained by the same treatment. It is difficult to investigate all the relative microcosmic aspects for the macroscopical properties by using only experimental methods. Meanwhile, first-principles calculations are specialized in illustrating microcosmic exhibitions and inner mechanisms. Therefore, first-principles calculations are used to obtain defects, electronic structures of Pr doped CaTiO3, Pr and Al doped SrTiO3. Based on the two experimental findings that Zn2+ions are beneficial to the afterglow and red luminescent intensities of Pr doped CaTiO3and Al3+ions are helpful to red luminescent intensities of Pr doped SrTiO3, the optical properties of (Zn,Pr) doped CaTiO3and (Al,Pr) doped SrTiO3are investigated by first-principles calculations. Main conclusions are as follows:(I) Defects, band structures and optical propertis Pr and (Zn,Pr) doped CaTiO3· Formation energies and electronic structures of native defects in orthorhombic CaTiO3are explored using the first-principles calculations under A condition in which CaTiO3is in equilibrium with CaO and O2and under condition B (TiO2, CaTiO3and O2are in equilibrium). The Ca vacancy (VCa2), Ti vacancy (VTi4-) and Ca antisite (CaTi2-) are the acceptors energetically preferable. There is no defective gap state in electronic structures of decfective CaTiO3containing either of native defects. The bandgaps of VTi4-, CaTi2-and VCa2defective systems are reduced by0.21eV,0.12eV and0.06eV in comparasion with perfect system. Those of Vo2+and TiCa2+keep unchangeable. The shinking band gaps will low the absorption energy of the transition of excited phonon from the valence band to the conduction band. Therefore, it is possible to broaden the excitation spectra by modulating the compoition of native defects.· Formation energies and electronic structures of Pr impurities in orthorhombic CaTiO3are explored. In Pr-doped CaTiO3, Pr substituting for Ca (Prca) is likely to form under A condition. Under condition B, Pr substituting for Ti (PrTi) defect is energetically preferable depending on the Fermi levels. No gap state appears within the band gap of Prca0and PrCa1+defective system. For PrCa2+, the gap states within the band gap are suitable for luminescent centers, which agree with the proposed luminescent models of CaTiO3:Pr.· Band structures, electronic and optical properties are calculated for (Zn,Pr) codoped CaTiO3. There are gap states consisting of Pr4f, O2p and Ti3d hybrid orbitals above the top of valence bands of～1.30and2.06eV in Pr-doped systems, which are retained in (Zn,Pr)-codoped systems. There are new states above the top of valence bands of～0.18eV in (Zn,Pr) codoped systems. The calculated characteristic absorption peaks of (Zn,Pr) codoped systems are300nm,372nm and457nm, which are close to the experimental results of the corresponding material at330nm,375nm and458-495nm,respectively.(Ⅱ) Defcts, band structures and optical propertis (Al,Pr) doped SrTiO3· Formation energies and electronic structures of native defects in cubic SrTiO3are calculated under the condition that TiO2, SrTiO3and O2are in equilibrium. The Sr vacancies (VSr2-) and O vacancies (Vo2+) are found to be energetically preferable as acceptors and donors for the pure sysytem. There is no defective state in electronic structures of decfective SrTiO3. In comparasion with the perfect system, the band gap of TiSr1+defective system is enlarged by0.05eV while those of Vo2+, SrTi2,VTi4-and VSr2are kept unchangeable. The inflexible band gaps of native defects are not beneficial to the transition of excited phonon from the valence band to the conduction band.· Formation energies and electronic structures of Pr and Al impurities in cubic SrTiO3are calculated under the condition that TiO2, SrTiO3and O2are in equilibrium. Pr prefers to occupy Sr site while Al to Ti lattice site. The gap states of PrSr defects depend on charged states, which are similar to those of PrCa defects. The calculated Fermi-level pinning positions at1573K for Al-free and Al doped SrTiO3:Pr decline from0.645eV to～0.601eV. AlTi-defects substituting with VSr2-are major acceptors in the overall charge neutrality of (Al,Pr)-codoped SrTiO3.· Band structures, electronic and optical properties are calculated for (Al,Pr) doped SrTiO3. The multiple gap states of Pr orbitals within then band gap are deduced by the two Al atoms which are nearest to Pr atom. For (Al,Pr) doped SrTiO3, the calculated absorption peaks at287nm,308nm and350nm are similar to the experimental excitation peaks at270nm,300nm and360nm, respectively.In this paper, formation energies, band structures, electronic and optical properties are calculated for Pr and Zn/Al doped CaTiO3and SrTiO3systems. The calculated results agree reasonably with the experimental findings. The methodology of the dissertation is beneficial to obtain the logical orientation and improve the investigation efficiency for a better luminescent material and a higher efficiency photocatalyst, such as doping method, species and concentration and so on.更多还原