【作者】 雍永亮；

【导师】 何丕模； 宋斌；

【作者基本信息】 浙江大学 ， 凝聚态物理， 2012， 博士

【摘要】 “每个原子均至关重要”的团簇科学已经成为物理、化学、环境、材料学、和生命科学等交叉领域的研究热点之一。这不仅是因为处于纳米尺度的团簇体系的物理、化学性质极为不同于块体材料,并且对团簇的尺寸、构型和组分具有很强的依赖性,更重要的是,因为团簇是纳米科学的基本单元。以团簇为基本单元形成的团簇组装材料由于可以调控组分的性能和晶格参数,从而被视为是一种可精确调控性能的纳米材料。同时,团簇组装材料发挥了团簇的独特性能,以及作为具有多种优异性能的功能材料具有广泛的应用前景。正因为此,它们已经引起了人们广泛的兴趣。本文利用基于密度泛函理论的第一性原理方法,系统的研究了一些二元半导体团簇以及基于这些二元半导体团簇的组装材料的结构特征、生长规律和电子特性。我们的研究成果为半导体团簇组装材料的实验合成以及它们的应用前景提供了一定的理论指导。论文首先研究了ZnnOn(n=1-13)团簇的结构和电子特性。研究发现,小尺寸(n≤7)团簇以环状结构最稳定,而当n>7时笼状或者管状结构变得更加稳定。其中笼状结构Zn12O12团簇是一种特别稳定的构型,它有着较高的对称性(Th)和较大的HOMO-LUMO能隙,预示着它是一种理想的基本单元来合成团簇组装材料。从能量最低的观点来看,通过单体Zn12O12笼状结构的六圆环方向对接,组装过程极易发生。当每一个单体连接其他单体的八个六圆环时,形成的聚合物更稳定。随着这种聚合的进行,我们得到了一种三维的具有纳米介孔特征的新型ZnO材料。这种材料具有菱面体的晶格结构。每一个单体在此结构中能很好的保持自己的结构,并且单体间的Zn-O键长稍微的大于孤立单体和块体材料中的键长。能带分析说明这种ZnO材料是一种区别于传统ZnO体材料的,具有较大带隙的半导体。由于它具有纳米介孔的特征,从而可以应用于异质催化、分子输运等。其次,我们研究了基于类富勒烯结构的M12N12(M=A1,Ga)团簇的组装材料。我们的研究结果显示类富勒烯结构的M12N12包含六个独立的四圆环和八个六圆环,并且具有Th对称性和较大的HOMO-LUMO能隙,这些结果预示着它们可以作为理想的基本单元来合成团簇组装材料。通过对基本单元M12N12的聚合,我们发现,当六圆环或者四圆环面对面对接时,M12N12团簇可以形成稳定的聚集体。这就保证了聚集体具有菱面体或者简单立方晶格结构所需要的条件。聚集体具有不同尺寸的介孔,从而可以应用于气体储存、异质催化、过滤等方面。菱面体晶格材料要比简单立方结构更稳定。两种结构中的M12N12单体都能保持自己孤立存在时的结构。能带分析说明它们都是宽带隙的半导体材料。接着,利用密度泛函理论方法,我们预测了单壁“盖帽”的具有小直径的硼氮纳米管可以通过稳定的BN团簇的聚合而形成。在室温下,这种聚合过程是自发进行的。尤其是BnNn(n=12,24)富勒烯团簇的聚合分别形成了稳定的armchair型(3,3)和(4,4)的硼氮纳米管。在聚合过程中所形成的有限长度的纳米管是具有宽带隙的半导体材料,从而可以应用于纳米器件的设计。最后,我们研究了SinCn(n=10-15)团簇的几何结构和电子特性、以及基于Si12C12团簇的新型小直径纳米线的结构和电子特性。研究发现SinCn(n=10-15)团簇具有笼状的结构,并且在这些结构中Si原子和C原子呈现明显的分离。C原子组成的部分易形成类富勒烯结构的五圆环或者六圆环,而Si原子组成的部分则尽力的来匹配这个sp2的环境从而没有规律可循,但是仍然显示出笼状的特征。团簇的电荷转移从Si部分转移到C部分。以两种高对称性的Sil2C12团簇结构作为基本单元,我们研究了其组装成新的纳米线的结构和电子特性。研究发现在二聚物中有五种不同的连接方式稳定的。依据标定的连接方式,五种新的纳米线被构造出来。Si和C之间的相互作用主要由C的2p和Si的3p间的杂化所支配。能带计算表明,利用Si/C分离的笼状团簇作为基本单元可以形成窄带隙的纳米线,从而应用于红外光探测以及热电学等。而基于Si和C原子交替排列的团簇,形成的纳米线是宽带隙的半导体,从而可以应用于高性能的场效应晶体管和场发射阴极等。更多还原

【Abstract】 The realm of cluster science, where "one atom make a difference", is one of current researching hotspots in the cross-field of physics, chemistry, environment, material science, and biology. This is not only because the physical and chemical properties of cluster system at the nanoscale are often found to differ from those of the bulk, but also they have a strong dependence on cluster size, geometry, and composition. More important, this is because clusters are the building blocks of nanoscience. Cluster-assembled materials, where clusters sever as building blocks, offer the ability to tune component properties, lattice parameters, and thus are viewed as new nanomaterials with precise control over properties. Meanwhile, they also exploit the uniqueness of clusters and are functional materials with manifold excellent performances for a variety of applications. For these reasons, they have attracted much attention. In this thesis, using first-principles calculations based on density functional theory, the structural features, growth pattern, and electronic properties of several binary semiconductor clusters, and corresponding cluster-assembled materials based on binary semiconductor clusters have been systematically investigated. Our research results will encourage experimental efforts toward the synthesis and characterization of such cluster-assembled materialsFirst, the structural and electronic properties of ZnnOn (n=1-13) clusters have been studied using spin-polarized density functional theory. For ZnnOn (n=1-13) clusters, ring structures are most stable for cluster size n≤7, while cage or tube structures become energetically favorable for n>7. Calculated results show that the Zn12O12 cluster possesses relatively higher stability. It has a cage structure with high symmetry (Th) and a large HOMO-LUMO gap, indicating the Zn12O12 cluster would be ideal building blocks for the synthesis of cluster-assembled materials. From the viewpoint of lowest-energy, our results show that assembly can form by attaching Zn12O12 cage on hexagonal site. A Zn12O12 cage should combine with eight hexagons in adjacent eight Zn12O12 cages respectively, forming more stable assemblies. As assembly process continues, we find that the Zn12O12 cages form a new three-dimensional nanoporous ZnO phase with a rhombohedral lattice framework. The Zn12O12 cage structure in the phase is preserved, and the Zn-O bond lengths between Zn12O12 monomers are slightly larger than that in isolated Zn12O12 cage and the bulk wurtzite ZnO phase. The band analysis reveals that this new phase is a semiconductor with large gap value. Because of the nanoporous character of this new phase, it could be used for heterogeneous catalysis, molecular transport, and so on.Second, we report the results of density functional theory calculations on cluster-assembled materials based on M12N12 (M=A1, Ga) fullerenelike clusters. Our results show that the M112N12 fullerenelike structure with six isolated four-numbered rings (4NRs) and eight six-numbered rings (6NRs) has a Th symmetry and a large HOMO-LUMO gap, indicating that the M12N12 cluster would be ideal building blocks for the synthesis of cluster-assembled materials. Via the coalescence of M12N12 building blocks, we find that the M12N12 clusters can bind into stable assemblies by either 6NR or 4NR face coalescence, which enables the construction of rhombohedral or cubic nanoporous framework of varying porosity. The rhombohedral-MN phase is energetically more favorable than the cubic-MN phase. The M12N12 fullerenelike structures in both phases are maintained and the M-N bond lengths between M12N12 monomers are slightly larger than that in isolated M12N12 clusters and the bulk wurtzite phases. The band analysis of both phases reveals that they are all wide-gap semiconductors. Because of the nanoporous character of these phases, they could be used for gas storage, heterogeneous catalysis, filtration and so on.Then, using density functional theory calculations, we predict that single-walled hemispherical-caped boron nitride (BN) nanotubes with small diameters can be produced via the coalescence of stable nanoclusters. Specifically, the assembly of BnNn (n=12,24) clusters exhibiting particularly high stability and leading to armchair (3,3) and (4,4) BN nanotubes, respectively, are considered. The formed finite-length BN nanotubes have semiconducting properties with wide band gaps attractive to nano-device applications.At last, the structural and electronic properties of SinCn (n=10-15) clusters and Si12C12-assembled nanowires with small diameter have been investigated using density functional theory calculations. SinCn (n=10-15) clusters are found to prefer cagelike structures, and in which the silicon atoms and the carbon atoms form two distinct subunits. It is found that the carbon atoms favor to form fullerene-like structure (five-membered ring and six-membered ring). The silicon atoms are trying to cope with an unfavorable sp2 environment, but distorted tetrahedra still show up somewhere of the cagelike structures. An electronic charge transfer from the Si-populated to the C-populated regions is observed. Two Si12C12 structures with high symmetries are suitable for assembling other nanostructures. Five different interactions in the dimers are found to be more stable than others, which opens possibilities of new systems. With the characterized dimers, five new nanowires with small diameter have been characterized. The hybridization between Si 3p and C 2p states is responsible for the interaction between C and Si atoms. The band analysis of these nanowires reveals that they are all semiconductors. The nanowires with narrow band gaps, which are formed via the coalescence of cagelike structures, may be used as infrared detectors or thermoelectrics, however, the other nanowires with wide-band gap, which are formed via the coalescence of fullerenelike structures, are suitable for high-performance field-effect transistors and field-emission cathodes, and so on更多还原