【作者】 穆跃文；

【导师】 万建国； 王广厚；

【作者基本信息】 南京大学 ， 凝聚态物理， 2011， 博士

【摘要】 纳米材料由于其尺寸接近电子的相干长度而呈现出明显的表面效应、量子尺寸效应和量子隧道效应等,在热力学、磁性、光学和导电性等方面表现出迥异于块体和原子态的性质,因此给医学、制造业、信息通信、材料学和环境保护等领域带来了革命性的变革。近年来,以分子、团簇、纳米线、纳米管和石墨烯等低维结构为基元构建纳米材料的研究蓬勃发展,成为纳米科学技术中最为耀眼的领域之一。这些纳米材料不仅拥有广阔的应用前景,而且有助于从介观的尺度和角度探索微观系统和宏观系统之间的内在联系。团簇作为介于微观原子、分子和宏观凝聚态物质之间的桥梁,是用来构建纳米材料的理想基元。在使用团簇组装各种新型纳米材料之前,首先要弄清楚其性质如何随尺寸和形貌演化,然后弄清楚如何调制其性质以满足特定的需求。混合原子团簇,尤其是二元半导体团簇和合金团簇,由于其性质可以通过改变组分来简单地调制,因而受到人们的广泛关注。此外,随着电子元器件和磁存储器件的飞速发展,元器件和电路的尺寸越来越小,表面和界面的作用变得愈来愈重要。吸附是一种常见的表面现象,它可以对表面吸附物的活性、材料的磁性和能带结构等物理化学性质产生重大影响。在本论文中,我们采用各种经验的和第一性原理的方法对中等尺寸的GanAsm团簇(n+m=17-24,n-m=0,±1)、Mn原子替代掺杂的Pd。团簇(n=3-19)和吸附F、O与N原子的GaN类石墨烯单层的结构和性质进行了研究,具体内容如下：通过使用遗传算法(GA)、Basin-hopping Monte Carlo方法(BHMC)结合Stillinger-Weber经验势和密度泛函理论(DFT)系统地研究了化学计量的和非化学计量的GanAsm团簇(n+m=17-24,n-m=0,±1)的结构、电子性质和极化性质,并重点讨论了决定团簇极化率的因素。我们发现研究范围内的所有最低能量结构都是笼形结构,总的键的数目大约是2n+m。团簇的总极化率对团簇体积表现出近乎与组分无关的线性依赖关系。此外,离化势和组分对极化率也有着重要影响。与化学计量的GanAsm团簇相比,非化学计量的团簇倾向于拥有更大的极化率,但是由于异构体效应这种奇偶震荡行为很难在实验中观察到。由于拥有特殊的构型和特殊的HOMO-LUMO电子分布,Ga10As11表现出反常的电子性质,其HOMO-LUMO能隙和离化势与化学计量的团簇十分相似。通过使用自旋极化的密度泛函方法(DFT)结合遗传算法(GA)和嵌入原子势(EAM)系统地研究了Pdn团簇和双金属Pdn-1Mn团簇(n=3-19)的结构和磁性质,并重点讨论了掺杂锰的钯团簇的磁性随尺寸和结构的演化以及锰元素影响钯团簇磁性的机制。首先我们将所得到的Pdn团簇与前人的结果进行了比较,发现了新的最低能量结构,包括Pd11,Pd12,Pd14,Pd17。通过结构和杂化解释了Pdn团簇的总磁矩的跳跃行为。当n=3-9时,Mn原子替代掺杂几乎不改变Pdn团簇的结构,但是随着团簇尺寸增大,掺杂Mn原子倾向于替代一个内部原子,使团簇发生结构重构,并倾向于形成二十面体基的结构。Mn原子掺杂不仅能增强钯团簇的稳定性,而且可以将绝大多数钯团簇的磁性增强3-5μB。此外,Pdn-1Mn团簇的总磁矩表现出台阶行为基础上的震荡行为,这种行为与Pd-Mn键长密切相关,并且可归因于Pd原子和Mn原子之间的d-d互作用。使用自旋极化的密度泛函理论研究了具有强烈非金属性的F、O和N原子以不同覆盖率(1/8和1/2)吸附在GaN类石墨烯单层上对其磁性和能带结构的影响。通过吸附F原子可以将间接带隙半导体的GaN单层转变为磁性半金属,当F的覆盖率增长到很高时,体系又变回非磁性的半导体。我们使用Ising模型和Monte Carlo模拟估算低覆盖率(1/8)吸附F的GaN单层的的居里温度为260K。不论是高覆盖率(1/2)吸附还是低覆盖率(1/8)吸附N原子,GaN单层都能转变为磁性半金属,相应的居里温度分别估算为220K和50K。上述的吸附诱导磁性主要来自N2p(尤其是N2pz)轨道的自旋劈裂。吸附O原子不会诱导出磁性,但可以减小体系的带隙,且覆盖率越高带隙越小。更多还原

【Abstract】 The size of nanomaterials is comparable to the electron coherent length, which leads to obvious surface effect, quantum size effect, quantum tunnel effect, and so on. Accordingly, they exhibit many unusual properties in the fields of thermodynamics, magnetism, optics and conductivity, etc, which are much different from those of corresponding bulk and atomic state. In the past few decades, nanomaterials based on molecules, clusters, nanowires, nanotubes or graphene developed rapidly, which bring about a revolutionary change in medical science, manufacturing industry, information communication, material science, and so on. The nanomaterials have not only wide application prospect but also a great fundamental importance of exploring the internal relations between macroscopic and microscopic systems.Clusters, as the bridge between microscopic atoms or molecules and macroscopic condensed matter, are ideal building blocks to design new nanomaterials. Before assembling new materials from clusters, we need to make it clear how the properties of clusters vary with their shapes and sizes, and how to modulate their properties for different purpose as well. Mixed atomic clusters, especially binary semiconductor clusters and alloy clusters, have drawn much attention, due to that their properties can be modulated easily by changing the composition. In addition, with the rapid development of electronic components and magnetic storage devices, the size of the components and electric circuit becomes smaller and smaller, and as a result the surface and interface get more and more important. The adsorption is a common surface phenomenon, which can exert a great influence on the activity of adsorbate, the magnetism and band structure of the materials, and so on. In this thesis, we use various methods from empirical to first-principles to study the structures and properties of medium-sized GanAsm clusters （n+m=17-24, n-m=0,±1）, Pdn clusters （n=3-19） doped by Mn atoms and graphene-like GaN monolayer adsorbing F, O and N adatoms.By using genetic algorithm（GA）, basin-hopping Monte Carlo method （BHMC） combined with Stillinger-Weber empirical potential and density functional theory （DFT） calculations, the geometries, electronic properties and polarizabilities of stoichiometric and nonstoichiometric GanAsm clusters （n+m=17-24, n-m=0,±） are investigated systematically, with emphasis on what determines the polarizabilities. All the lowest energy structures are hollow cages with a total number of bonds of about2n+m. The total polarizabilities of GanAsm clusters exhibit a linear dependence on their volumes despite of different composition. The polarizabilities are also strongly correlated to ionization potentials and composition. The nonstoichiometric GanAsm clusters tend to possess higher polarizabilities than stoichiometric ones, but this behavior is hard to be observed from experiment due to isomerization. Ga10As11, seen as a fragment of Ga12As12, has special HOMO-LUMO electron density distribution which accounts for its unusual electronic properties like stoichiometric clusters.The geometries and magnetic properties of both Pdn and Pdn-1Mn （n=3-19） clusters are calculated by using spin-polarized density functional theory approach combined with genetic algorithm and embedded atom method （EAM）.We emphatically discussed the magnetism evolution of the Mn-doped palladium clusters with their structures and size, and how the Mn element influences the magnetic behavior of palladium clusters as well. By comparing our geometries with early results, several new lowest energy structures of Pdn clusters （n=11,12,14,17） are found. The jump of the total magnetic moments of Pdn clusters is explained in terms of structures and hybridization. For large cluster size （n>9）, when a Mn atom is doped into the Pdn cluster, it tends to occupy an interior site of the cluster and the geometry reconstruction generally occurs, and the cluster is apt to form an icosahedron based structure for n>13. The doping of Mn atoms not only enhances the stability of Pdn clusters, but also increases their total magnetic moments by a magnitude of3-5μB. Moreover, the doping of Mn atoms causes the Pdn₁Mn clusters to give rise to an evident oscillation of step-like magnetic behavior, which is closely related to the Pd-Mn bond length and can be attributed to the d-d interaction between Pd and Mn atoms.The effect of adatoms （F, N and O） on the magnetism and band structure of graphenelike GaN monolayer （ML） at different coverages （1/2and1/8） is studied by spin-polarized density functional theory calculations. Adsorbing F adatoms at low coverage causes the GaN monolayer to be transformed from a nonmagnetic indirect-band-gap semiconductor into a magnetic half metal, whereas the monolayer turns back to a nonmagnetic semiconductor at high coverage. Using the Ising model and Monte Carlo simulation, we get an estimation of the Curie temperature of low coverage F-adsorbed GaN monolayer of260K. The GaN monolayer becomes a magnetic half metal after adsorbing N adatoms, no matter whether the coverage is high （1/2） or low （1/8）, and their Curie temperatures are estimated to be220K and50K. The magnetism mentioned above mainly comes from the spin splitting of N2p （especially N2pz） orbits. On the other hand, the adsorption of O does not induce magnetism but reduces the band gap of the GaN monolayer and the higher the coverage is the smaller the gap is.更多还原