3d过渡金属掺杂一维ZnO纳米材料磁光机理研究

【作者】 张富春；

【导师】 张志勇；

【作者基本信息】 中国科学院研究生院（西安光学精密机械研究所） ， 物理电子学， 2009， 博士

【摘要】 氧化锌（ZnO）作为一种具有优良压电、光电特性的Ⅱ-Ⅵ族宽禁带半导体材料,在电学、光学和磁学等方面具有广阔的应用前景。特别是ZnO基稀磁性半导体材料,由于可能具有超过室温的居里温度T_c、大的磁性离子溶解度、在可见光范围内透明等特点,有希望成为新一代信息处理和储存、量子计算和量子通讯等领域的重要材料。本文应用基于密度泛函理论框架下的第一性原理计算方法,系统研究了ZnO纳米材料及其3d过渡金属掺杂的一维ZnO纳米材料的几何结构、电子结构以及磁、光、电属性,分析了一维ZnO基稀磁半导体材料的磁性来源和磁性耦合机理,以及过渡金属掺杂对一维ZnO纳米材料磁学、电学和光学性能的影响,为实验制备高质量、高居里温度的一维ZnO纳米磁性材料提供了理论依据。论文的主要内容和结果如下:1.ZnO体材料电子结构的理论研究。采用基于密度泛函理论的第一性原理计算方法,对比研究了ZnO体材料采用不同的交换关联函数时的基本结构和性质。计算结果显示,采用LDA+U和B3LYP交换关联泛函方法明显优于单独采用GGA和LDA的计算结果,LDA+U和B3LYP密度泛函计算得到的带隙值更接近于实验值,但LDA+U和B3LYP耗时将成倍增加,而GGA所用时间最短。2.ZnO纳米线和纳米管电子结构与属性的理论研究。采用密度泛函理论研究了ZnO纳米线和纳米管的几何结构、电子结构和光学性质。研究发现,随着ZnO纳米线尺寸的增加,结合能逐渐降低,体系逐渐趋于更加稳定的结构。电荷密度计算结果显示整个ZnO纳米线中Zn-O之间主要以共价键为主,同时兼有离子键成分,p-d轨道具有强烈的交叠杂化效应,整个纳米线表面电荷都向外偏聚,电子的离域性增大,Zn-O间共价性减弱,离子性增强。ZnO纳米管理论计算结果显示,所有构型的纳米管都由原来的皱褶型转变成圆柱形管状结构,结合能都为负值,表明单壁ZnO纳米管是可以稳定存在的。电子结构计算显示ZnO纳米管是一直接带隙的宽禁带半导体材料,带隙值都明显大于体材料;随着纳米管管径的增加,整个价带与体材料相比明显展宽,且向低能方向漂移,在价带顶出现了表面效应引起的缺陷态能级;ZnO纳米线和纳米管光学性质计算结果显示,随着ZnO纳米线尺寸和纳米管管径的减小,吸收光谱发生了蓝移现象,且对应于紫外波段。3.3d过渡金属掺杂ZnO纳米线磁学、光学和电学性能的研究。采用基于自旋极化的密度泛函理论计算方法,系统研究了3d过渡金属掺杂ZnO纳米线的几何结构、电子结构以及磁学、电学和光学属性。研究发现,V掺杂体系只具有铁磁性耦合特征,Mn掺杂只具有反铁磁特征,而Cr、Fe、Co和Ni掺杂体系不同的取代位置则对应于不同的磁性耦合特征,因此,3d过渡金属掺杂ZnO纳米线具有丰富的磁学现象,特别是对于Co掺杂,铁磁性耦合形成了半导体磁性材料,理论预测具有优异的磁光性能,而反铁磁性掺杂却形成了半金属磁性材料。光学性能计算结果显示,3d过渡金属掺杂对ZnO纳米线的整体光学性质影响并不大,Mn、Fe、Co、Ni掺杂都发生了蓝移现象,Cr掺杂发生了红移现象。4.3d过渡金属掺杂ZnO纳米管磁学、光学和电学性能的研究。采用基于自旋极化的密度泛函理论框架下的第一性原理计算方法,研究了3d过渡金属掺杂ZnO纳米管的几何结构、电子结构以及磁学、电学和光学属性。研究结果表明,V、Cr和Mn掺杂ZnO纳米管更容易形成铁磁性材料,具有很强的磁性;Fe和Co掺杂ZnO纳米管更容易形成反铁磁材料;而Ni掺杂ZnO纳米管反铁磁性能较铁磁性能稍稳定。电子结构计算结果显示,在费米能级附近3d态分裂为离域的三重态t_2g和局域的二重态e_g,出现了强烈的杂化耦合特征;光学性质计算结果显示,3d过渡金属掺杂ZnO纳米管在紫外区出现了3个明显的吸收峰,近紫外吸收带边发生了红移,400nm处的吸收峰发生了一定的蓝移,且Mn、Fe、Co、Ni吸收峰强度明显增强,而Cr的吸收峰强度减弱。理论计算结果表明3d过渡金属掺杂ZnO纳米管是一种优异的紫外光电子材料。更多还原

【Abstract】 Zinc Oxide （ZnO）, a typical wide band gap （E_g =3.37eV） II-VI semiconductor material with a good piezoelectric and photoelectric properties, is a promising application in the fields of electrics, optics and magnetics. For ZnO-based dilute magnetic semiconductor material, its Curie temperature （T_c） is higher than the room temperature. In particular, it shows a large solubility of magnetic ions and transparent in the visible light region. These fantastic features make ZnO-based dilute magnetic semiconductor promising in information processing and storage, quantum computing and quantum communication. In this thesis, the first-principles based upon the density-functional theory （DFT） are systematically performed to study the geometric and electronic structures, magnetic, optical and electrical properties of 3d transition metal doped one-dimensional ZnO nanomaterials, and these results may analyse the origin of ferromagnetism and the mechanism of ferromagnetic coupling of one-dimensional ZnO diluted magnetic semiconductor materials. The effects of transition metal doping on the magnetic, optical and electrical properties are investigate, and some helpful instruction can be provided for the preparation of high-quality one-dimensional ZnO magnetic nanomaterials with high Curie temperature.The main contents and results are listed below:1. Theoretical study on electronic structure of bulk ZnO materials. With the adoption of different exchange-correlation potential, the basic structure and properties of bulk ZnO are studied using the first-principles calculation based on the density functional theory. The calculated results by using the methods of LDA + U and B3LYP are better than those calculated by using the individual method of GGA and LDA. Although the calculated band gap by using the LDA + U and B3LYP methods is much closer to the experimental data, the above two methods are multiplied in the time-consuming with respect to the shortest time consuming by the GGA method.2. Study on the structures and properties of ZnO nanowires. The geometric structures, electronic structures and optical properties are studied by using the density functional theory with respect to one-dimensional ZnO nanowires and nanotubes. It is found that the band gap and the binding energy are gradually reduced, and the system is becoming more stable with the increase in ZnO nanowires size. Meanwhile, obvious size effects and surface effects are observed in the ZnO nanowires. The calculated charge density results show that ZnO nanowires look like strong covalent bonds character rather than ionic bonds. The strong p-d orbital hybridization appears in ZnO NWs and surface charges on the whole of nanowires move outward. The electronic delocalization is increased, So ionic bonds in ZnO are stronger than covalent bond. For the ZnO nanotubes, the initial polygon structure transforms into a perfect cylindrical tube and the binding energy of different ZnO nanotubes is negative, which implies that those ZnO nanotubes can exist in principle. Moreover, the calculated electronic structure results show that one-dimensional ZnO nanowires and nanotubes are direct wide gap semiconductor materials, and their band gap values are significantly larger than that of bulk ZnO. With the increase of nanotube diameter, the whole valence band is significantly broadened and moves towards low energy, the defect levels appear in the valence band top due to surface effects. At the same time, the calculated optical results for ZnO nanowires and nanotubes show that there is a significant blue shift in the absorption spectrums with the decrease of NWs size, and the absorption spectrum locate in UV region. This implies that one-dimensional ZnO nanomaterials can be used for the development of UV-electronic devices.3. Study on magnetic, optical and electrical properties of 3d transition metal doped ZnO nanowires. The geometric and electronic structures, together with magnetic, optical and electrical properties are studied by using the spin-polarized density functional theory with regard to 3d transition metal doped ZnO nanowires. The calculated results indicate that V-doped system is only ferromagnetic coupling, and the Mn-doped system is only antiferromagnetic coupling. However, it is interesting that Cr, Fe, Co and Ni-doped systems have different magnetic coupling when the doped atoms replace the different location, which indicates that Cr, Fe, Co and Ni-doped ZnO nanowires possess more rich magnetism phenomenon. Especially, the ferromagnetic coupling forms magnetite semiconductor materials for Co-doped, which exhibits excellent magneto-optical properties from theoretical prediction, but the antiferromagnetic coupling forms half-metal magnetite materials. Meanwhile, the calculate results of optical properties indicate that there is little change in the optical properties, and significant blue shift and red shift are respectively observed in the Mn, Fe, Co, Ni-doped systems and Cr-doped system.4. Study on magnetic, optical and electrical properties of 3d transition metal doped ZnO nanotubes. The geometric and electronic structures, together with magnetic, optical and electrical properties of 3d transition metals doped ZnO nanotubes are investigated by using the first-principles based on spin-polarized density functional theory. It is found that the V, Cr and Mn-doped systems are more easier to form the ferromagnetic coupling and possess a strong magnetic properties, while the Fe and Co-doped system are more easier to form antiferromagnetic coupling. but the Ni-doped system is instable for the magnetic properties. Furthermore, overlook from the electronic structure calculation, it is clear that the 3d states of transition metals split into one triply degenerate t_2g and one doubly degenerate e_g near the fermi level, which shows a strong hybridization between TMs-3d and O-2p states. The calculated optical properties show that three obvious absorption peaks are observed in the UV region, and there is a red shift in the near UV region, meanwhile, the absorption peaks at 400nm is a blue shift and the intensity of absorption peaks in Mn-, Fe-, Co-, Ni-doped system increase obviously,but the intensity of absorption peaks in Cr-doped system decrease. In one word, we can come to a conclusion that 3d transition-metal-doped ZnO nanotubes is a good UV magneto- optical electronic materials.更多还原