| 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...
|
PDF Full Text Request