|Influence of Fluorine on Electronic Structures and Nonlinear Optical Effects of A Series of Borate Crystals|
|关键词||D10-cation Systems Borate Fluorides Fluorooxoborates First-principles Calculations Structure-property Relationship|
以非线性光学（NLO）材料为核心部件的紫外/深紫外（UV/DUV）激光器件是精密加工、半导体产业、信息存储等方面的一个重要组成部分，这也使得紫外/深紫外光学材料具有至关重要的科学研究价值。特别是在深紫外范围内，对潜在的非线性光学晶体具有严苛的要求，它们要有较宽的透过范围（<200nm），大的倍频系数，合适的双折射率，较大的损伤阈值，容易生长而且化学稳定性要好。通过前期研究表明，目前能实现较短波长特别是深紫外激光输出的材料只有含氟的硼酸盐，如KBe2BO3F2和RbBe2BO3F2等。鉴于此，实现合理设计和有效分析的关键科学问题是：氟引入对紫外/深紫外非线性光学材料的性能影响机理是什么？氟对这些材料性能的影响将为设计深紫外非线性材料提供一个重要的基础。引入氟离子、氟硼酸阴离子基团可获得硼酸盐氟化物（borate fluorides）以及氟化硼酸盐（fluorooxoborates），他们均表现出丰富的化学结构及较好的非线性光学响应。硼酸盐中的F ?阴离子如何影响带隙、倍频等宏观性能是本论文主要研究目标。在这篇论文中，我们以硼（碳）酸盐氟化物和氟化硼酸盐为主要研究对象，采用密度泛函平面波赝势方法研究了在不同体系中电子结构，发现氟离子引入对能隙和倍频具有调控作用，通过调控F ?阴离子可提高倍频响应。本论文研究将有助于预测和探索含氟硼酸盐非线性光学材料。主要研究内容如下：我们系统地研究了氟（F）原子对硼酸盐氟化物（Sr3B6O11F2，Ba3B6O11F2和Ba4B11O20F）的晶体结构和非线性光学性质的直接影响。 F原子的吸电子效应导致阳离子（Sr，Ba）中心多面体周围极化率各向异性的增加，这使得在所研究的硼酸盐氟化物中极化率更容易受到入射光的外部电场的影响。Sr3B6O11F2，Ba3B6O11F2和Ba4B11O20F与Ca5（BO3）3F的比较研究表明，F离子对光学性质有更高的直接贡献，因为F离子的2p态位于费米能级（EF）附近。由于在各个研究系统中，与以氟为中心的多面体（FCa4）相比，以氟为中心的多面体（FSr3）有更大程度的变形。因此，观察到F离子对NLO性质的直接贡献是由氟的电子结构引起的变化。以含d10电子组态阳离子(Cd2+, Zn2+)硼（碳）酸盐氟化物为研究体系，分析p-d（O-p和Zn/Cd-d）相互作用对带隙、倍频效应影响机制。通常含d10电子组态阳离子 (Cd2+, Zn2+) 硼酸盐具有较小带隙，这限制了它们在在发光二极管、激光器、晶体管等方面的商业应用。我们研究发现通过F ?阴离子引入将降低（Zn/Cd-O）耦合强度，调控含d10电子构型阳离子的硼（碳）酸盐氟化物如 BaZnBO3F，BaZnCO3F2，KCdCO3F和 RbCdCO3F的电子结构和光学性质。此外，较大的F离子极化使[Zn/CdO3F2]多面体（即次级结构单元（SBU））成为增强倍频效应的有效基因。该项研究为设计透过范围可达紫外/深紫外区的d10硼（碳）酸盐体系提供有效策略。通过研究取代羟基阴离子（OH ?）àF ?阴离子形成氟化硼酸盐（fluorooxoborates）后，紫外截止边的蓝移和倍频效应增益的内在机理。以含孤对电子阳离子、三价稀土离子硼酸盐为研究体系，分析了分别含F ?阴离子、（OH ?）羟基阴离子BiB2O4F 、BiB2O4(OH) 、LaB2O4F、 LaB2O4OH等材料的性能差异及内在原因。理论研究表明，F ?阴离子较OH ?阴离子更易使化合物紫外截止边蓝移。第一性原理计算发现非线性光学效应具有以下规律：BiB2O4F > BiB2O4(OH) > LaB2O4F ≈ LaB2O4(OH），其原因在于F ?阴离子引入增强了孤对电子立体化学活性，同时也使得BO3F基团具有很强的各向异性，从而导致了BiB2O4F具有大的倍频效应。此外， BiB2O4F化合物的F-2p轨道在能量表象下的展宽比LaB2O4F，Sr3B6O11F2，和Ba3B6O11F2更宽，这表明F-2p轨道与周围原子杂化，从而增强体系极化率，证明了F ?阴离子的间接贡献。该项研究为设计性能优异的氟化硼酸盐（fluorooxoborates）提供指导意义，同时也为设计F ?替代OH ?产生光学效应增益的非线性光学材料提供思路。
Nonlinear optical (NLO) materials based ultraviolet/deep-ultraviolet (UV/DUV) devices are important part of the precision machining, semiconductor industry, information storage etc. that make UV/DUV functional optical materials worthy for scientific research. Especially, for DUV region, the requirement is very strict: a potential DUV NLO crystal should demand a wide transparency below 200 nm, large second harmonic generation (SHG) response, appropriate birefringence, large damage threshold, and facile growth with much desired stability. In the previous studies, there are only fluorine containing compounds such as KBe2BO3F2, RbBe2(BO3)F2, etc., that can realize the output of short wavelength, especially deep ultraviolet laser by SHG process. In view of this, the key scientific question for realizing reasonable design and effective synthesis is: what is the mechanism of the effect of fluorine on the performance of UV/DUV NLO materials? The effect of fluorine on the performance of these materials will provide an important basis for designing DUV NLO materials. The introduction of fluorine in borate anionic groups has developed many new borate ?uorides and ?uorooxoborates, which exhibit rich structure and diverse composition with improved NLO response. Since, fluorine based borates exhibit special characteristics of high ionicity and stability, while the combination of fluorine and the rare earth dopants are important for efficient up-conversion systems. However, to search ideal NLO material with improved applications in UV/DUV region, the chemistry of these systems needs to be clear. In this dissertation, the structure-property relationship of different fluorine containing borates (carbonates) and ?uorooxoborates has been studied by using the plane wave pseudopotential density functional theory method. The study will explore the role of fluorine in tuning the electronic structures and energy band gap of different systems, which leads to the high SHG response. In this contribution, very first time the direct effect of the fluorine (F) atoms on the crystal structures and NLO properties of borate fluorides (Sr3B6O11F2, Ba3B6O11F2 and Ba4B11O20F) has been investigated systematically by First-principles calculations. The electron withdrawing effect of the F atoms leads to the higher values for anisotropy of the polarizability around cations (Sr, Ba)-centered polyhedra which make the dielectric susceptibility more easily affected by the external electric field of the incident light in studied borate fluorides. Comparative studies of Sr3B6O11F2, Ba3B6O11F2 and Ba4B11O20F to Ca5(BO3)3F showed that F ? anions have much higher direct contribution to optical properties because the 2p states of F ? anions are located near the fermi level (EF). As the fluorine-centered polyhedra (FSr3) undergo larger degree of distortion as compared to another fluorine-centered polyhedra (FCa4) in respective studied systems. Thus, direct contribution of F ? anion on the NLO properties is observed as ?uorine-induced variation of the electronic structure.In this study to consolidate the key mechanism about the combine functionality of d10 cations with F ? anions influencing the electronic structure and optical properties, a comparative analysis among d10 borate/carbonate systems with and without F ? anions has been preferred. In d10 systems, the strong p-d interaction between (O-p and Zn/Cd-d) orbitals increases the bandwidth of valence band and shrinks band gap, which has limited their commercial application in numerous devices (e.g., light-emitting diodes, lasers, and transistors). To overcome these limitations, the F ? anions are introduced that decrease the intensity of cation-anion (Zn/Cd-O) bonding, shift the energy levels, and consequently tune the electronic and optical properties of d10 borate/carbonate systems such as in BaZnBO3F, BaZnCO3F2, KCdCO3F, and RbCdCO3F. Moreover, large polarizability of F ? anions makes the [Zn/CdO3F2] polyhedral (secondary building units (SBU)) an active NLO unit for the enhancement of SHG. Thus, the functionality of d10 cations (Zn, Cd) is consolidated with F ? anions to influence the electronic structure and optical properties of d10 borate/carbonate systems by using ab initio approach. In addition, an effective strategy is developed for the choice of appropriate cations, which can make d10 borate/carbonate fluoride systems excellent candidate for laser, window, and lens applications in the UV/DUV spectral region.Replacing hydroxyl anions (OHˉ) by F ? anions in borates, we analyzed the blue shift of UV cut-off edge, the enhancement of NLO and, the mechanism behind it. Here we studied BiB2O4F, BiB2O4(OH) with lone pair cations, and LaB2O4F, LaB2O4(OH) with trivalent rare-earth cations systematically. The theoretical studies reveal that the blue shift in the band gap of borates with F ? anions as compared to those with OHˉ anions can be assumed based on the weaker interaction of the cation-fluoride (La/Bi/B-F) bonds than that of the cation-oxygen and hydroxyl bonds. NLO properties are found to have the order of BiB2O4F > BiB2O4(OH) > LaB2O4F ≈ LaB2O4(OH) in the studied systems, where the large difference is mainly attributed to the stereo-chemical activity of lone pair (SCALP) effect of the Bi cations, and to the special BO3F with strong anisotropy as compared to the BO3 and BO4 groups. The energy spanning of F-2p orbitals is more extended in BiB2O4F as compared to LaB2O4F, Sr3B6O11F2, and Ba3B6O11F2 due to the bonding of Bi/B-F, which indicates F-2p orbitals have more chance to overlap with surrounding atoms and enhance the polarizability in all the studied systems. Moreover, the degree of SCALP of the Bi cations is apparently activated by the introduction of the F ? anions, which causes an obvious enhancement in NLO properties in bismuth borates with F ? anions. These investigations will help us to classify the solid-state chemistry of F ? and OHˉ anions in borate systems with different type of metal cations. We hope that present study will put a clear insight regarding the chemistry of borate (carbonate) ?uorides and ?uorooxoborates, which may help to predict and explore novel fluorine based NLO materials with potential applications in present age and coming future technologies.
|Beenish Bashir. Influence of Fluorine on Electronic Structures and Nonlinear Optical Effects of A Series of Borate Crystals[D]. 北京. 中国科学院大学,2018.|
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