【作者】 王静；

【导师】 刘英；

【作者基本信息】 河北师范大学 ， 凝聚态物理， 2009， 博士

【摘要】 C60和碳纳米管以其独特的结构和新颖的特性引起了人们广泛的关注。它们在场发射、传感器、复合增强材料、储氢材料等众多领域展现出的广泛的应用前景,激发了人们对纳米笼和纳米管的研究热情。本文基于密度泛函理论,对3d过渡金属原子和贵金属Au原子掺杂的硅团簇的结构演化和电子性质进行了分析;预测了新的金纳米管、硼纳米管的结构稳定性和电子结构。纯硅团簇由于其化学键的特性,不能形成碳团簇那样稳定的笼状结构。众多的实验和理论研究都表明,掺杂一些寄宿原子可以稳定硅的笼状结构。本文系统研究了3d过渡金属原子对硅笼状结构稳定性和电子特性的影响,发现了笼状Si15和Si16团簇的理想宿主----Ti元素;计算结果表明,Ti@Si15和Ti@Si16团簇均具有较高的结构稳定性和较好的化学稳定性。基于密度泛函理论对内包金属Si20富勒烯的稳定结构进行了广泛搜索,发现了一个迄今为止最为稳定、Ih对称性的中性Eu@Si20富勒烯;在Eu@Si20中,中心Eu原子具有较大的自旋磁矩;以Ih-Eu@Si20为结构单元构造了一类能稳定存在的项链状准一维链,计算结果表明,该链具有半导体特性,中心Eu原子仍具有较大的自旋磁矩。对过渡金属Ni原子和贵金属Au原子掺杂的硅小团簇,NiSin (n=1-14)和AuSin (n=1-16)的基态结构随尺寸的演化进行了计算。计算结果显示:当团簇原子数目较小时,不论Ni原子还是Au原子都倾向于占据表面格点的位置;随着团簇原子数目的增加,当n=9时Ni原子开始占据中心格点的位置,而当n=12时,形成内包Au原子的硅笼状结构Au@Si12;进一步分析表明,Ni原子的加入增强了硅团簇的稳定性,而Au原子的加入没有明显地改善硅团簇的稳定性;键能的二阶差分和HOMO-LUMO能隙表明,Au@Si12是幻数团簇,其基态结构具有较高的结构稳定性和较好的化学活性;以Au@Si12团簇为结构单元得到了一类稳定的Au-Cored硅纳米管,计算结果显示,这类内包金线硅纳米管具有较小的能隙和良好的电子传输特性。论文基于稳定的二十面体Au32富勒烯和由Au (1 1 1)面卷曲而成的(5, 5)金纳米管,构建了一类新的中空金纳米管状结构Aun (n=37, 42, 47, 52,…, ?n =5);采用密度泛函理论,对该类中空金纳米管的结构稳定性和电子性质进行了广泛的探索,研究发现Au37、Au42、Au47纳米管状结构具有较高的结构稳定性;首次预言了42个金原子组成的大的、中空纳米管结构是具有比密积结构更强稳定性的基态结构。基于Yakobson等人对B80富勒烯笼稳定结构的理论预测,构建了两类新的、稳定的金属性的硼片结构,进而得到两类新的稳定的硼纳米管,计算结果表明:在我们的研究范围内除第一类(4, 0)超细硼纳米管表现出半导体特性并伴随有0.441 eV的直接带隙以外,其它硼纳米管均表现出金属性。硼掺杂过渡金属晶态和非晶态合金表现出优异的铁磁性能和良好的抗氧化、耐腐蚀、耐磨损等性能,成为理想的磁记录介质和薄膜磁头候选材料。本文对硼掺杂过渡金属铁、钴、镍混合团簇的电子结构和性能进行了系统的分析研究,找到了FenB、ConB、NinB (n=1-12, 14, 18)体系的最稳定几何构型,计算结果表明B原子的掺入大大提高了过渡金属团簇的稳定性,改变了团簇的磁特性。在我们的研究范围内,B原子的加入未能明显地改善过渡金属团簇的自旋磁矩;NinB混合团簇的自旋磁矩在n=5-13的范围内存在一个由量子限制效应引起的平台。更多还原

【Abstract】 Since the discovery of C60 and carbon nanotubes, these molecules have have attracted wide attention due to their novel electronic properties and great potential of applications. Especilly, the unique structures and novel mechanical, themal, electronic and chemical properties have revealed potential applicantions on field emission, nanoelectronics, sensing probe, superconductivity, and chemistry. Recently, great attentions have addressed on the studies of nanocages and nanotubes.In this project, the density functional theory is employed to investigate the stabilities, properties and performances of the transition metal-encapsulated silicon fullerenes, gold nanotubes, and boron nanotubes, as well as the growth behaviors and electronic properties of Ni-doped and Au-doped silicon clusters.Unlike fullerene cages, a hollow Si cage is unstable because sp2 hybridization is highly unfavorable in silicon. By introducing metal atoms, the silicon clusters can be dramatically stabilized, and the choice of the central metal atom becomes a key point in the design of cage clusters and in their resultant chemical behavior. In this work, we have focused on Si15 and Si16 cage clusters with encapsulated 3d TM atoms and systematically investigated the effects of the encapsulated metal atoms on the configurations, stability, and magnetic properties of the clusters at two different all-electron DFT levels. The results show that Ti@Si16 and Ti@Si15 have the largest embedding energies and relative large HOMO-LUMO gaps in this series. It suggests that titanium atom is an ideal guest for Sin (n=15, 16) cages as far as stability is concerned.Furthermore, a metal-encapsulated silicon fullerene, Eu@Si20, has been predicted by density function theory to be by far the most stable fullerene-like silicon structure. The Eu@Si20 structure is a regular dodecahedron with Ih symmetry in which the europium atom occupies the center site. The calculated results show that the europium atom has a large magnetic moment of nearly 7.0 Bohr magnetons. In addition, it was found that a stable“pearl necklace”nanowire, constructed by concatenating a series of Ih-Eu@Si20 units, each with a central europium atom retains the high spin moment.The configurations, stability, and electronic structure of NiSin (n=1-14) and AuSin (n=1-16) clusters have been investigated within the framework of the density functional theory. The calculated results of NiSin clusters reveal that the Ni atom prefers to occupy the surface site when n < 9 and for the clusters with n≥9, the Ni atom starts to encapsulate in the cage. The results of AuSin clusters show that the Au atom begins to occupy the interior site from Si11 and for Si12 the Au atom completely falls into the interior site forming Au@Si12 cage. Furthermore, the doping of the Ni atom enhances the stability of silicon clusters while the doping of the Au atom does not enhance the stability of pure Sin clusters, similar to the case of Ag-doped silicon clusters. Relatively large embedding energy and small HOMO-LUMO gap are also found for this Au@Si12 structure indicating the enhanced chemical activity and good electronic transfer property. In addition, stable Au-cored Si nanotube is also found to be formed by Au@Si12 unit and it has relatively small energy gap making it attractive for electron transport device.A series of Aun (n=37, 42, 47, 52,…, ?n =5) hollow tubelike structures have been constructed by capping the nanotube with halves of the icosahedral Au32 cage at both ends of the (5,5) single-wall gold nanotube, rolled from the (1 1 1) Au crystal plane, repeated unit cell. Based on the scalar relativistic density functional theory, the stablity and electronic properties of tubelike Aun structures have been investigated. The results indicate that tubelike structures of Au37、Au42、Au47 are relatively stable. At the same time, a large hollow tubelike ground state Au42 is predicted as a new ground-state configuration.The recent prediction of the boron buckyball B80 cage has stimulated intensive studies on boron sheets and boron nanotubes. Here, two new classes of flat stable boron sheets have been constructed. Rolled from them, two series of boron nanotubes are obtained. Within the framework of density functional theory, the configurations, stability and electronic structures of new classes of boron sheet and related boron nanotubes have been predicted. The theoretic results show that within the scope of our research, the nanotubes rolled from the stable sheets with various diameters and chiral vectors are stable, and they are all metallic except for the thin (4, 0) tube with a band gap of 0.441 eV.Boron is known to be an important additive to magnetic materials. It can affect the magnetic properties substantially and improve the magnetostrictive characteristics of transition-metal thin films. Within the framework of all-electron density functional theory, the configurations, electronic structure and magnetic properties of MnB (M = Fe, Co, Ni; n=1-12, 14, 18) clusters have been calculated. The ground-state structures of the MnB clusters were determined. The results reveal that the doping of a B atom enhances the stabilities significantly and has a slight effect on the spin magnetic moments at the scope of our study. In addition, the size dependence of the spin moments in NinB clusters exists one terrace from n = 5 to n = 13, which is mostly due to the quantum confinement.更多还原