| Cluster science is the intersection of physics and chemistry, as well as a new growing point of material science recently. Clusters present a variety of unique properties such as magic stabilities, quantum size effects and magnetism, attracting great attentions of many basic and applied sciences. In order to study the properties of a cluster, one should first get an insight into its geometry. However, experimental characterization show large-scale morphology, but it is very difficult to determine the atomic structure directly. Meanwhile, to find the ground-state structure of a cluster theoretically is also extremely complicated since this is a global optimization problem. Thus, it becomes a great challenge currently. In this work, genetic algorithm combined with density functional theory is adopted to investigate three typical alloy clusters, i.e. platinum-tin alloy clusters, vanadium multiply doped silicon clusters and A@B12@A20 icosahedral matryoshka clusters, respectively. Structures, growth patterns and electronic properties of these clusters are systematically studied, as well as their promising applications in catalytic fields and magnetic materials.Platinum (Pt) is an important industrial catalyst. However, in realistic reactions, Pt catalysts can be poisoned easily due to blocking of active sites, which further degrades the activity significantly. One effective way to solve this problem is using Pt-based binary alloys. Especially, platinum-tin (Pt-Sn) alloy clusters can greatly improve the performance of Pt clusters as catalyst. But little is known about their structures. In Chapter 3, the low-energy structures of PtnSnn (n=1-10) and Pt3mSnm (m=1-5) clusters are determined using genetic algorithm incorporated with density functional theory. Pt and Sn atoms tend to mix with each other due to the energetically favorable Pt-Sn bonds. However, because of larger atomic radius of the Sn atom, segregation of Sn atoms on the surface of PtnSnn clusters is preferable. This will make one or two Pt atoms available for reaction. For Pt3mSnm clusters, Sn atoms are well separated in the clusters and prefer to form sharp vertices leaving triangular faces of three Pt atoms available for reaction. These theoretical results provide general trends for the structural and bonding features of the Pt-Sn alloy clusters and help understand their catalytic behaviors.It is well-known that silicon (Si), the most important elementary semiconductor, is the backbone of modern microelectronics industry. Generally, Si clusters prefer to form compact three-dimensional geometries via sp3 hybridization. However, due to the dangling bonds, elemental silicon cages are not stable. In the last decade, it has been shown both theoretically and experimentally that incorporation of transition metal atoms into a silicon cluster not only stabilizes the cluster but also brings into peculiar physical properties such as high magnetic moments. There are plenty of studies on the single atom doped silicon clusters. However, considering the complexity of structures, little is known about the silicon clusters doped with multiple transition metal atoms. In Chapter 4, multi-vanadium-doped silicon cluster anions, V3Sin-(n=3-14), have been systematically explored by genetic algorithm combined with density functional theory. The calculated photoelectron spectra, vertical detachment energies and adiabatic detachment energies agree well with the experimental data. Among the V3Sin-clusters, the V3Si5-, V3Si9- and V3Si12- are relatively more stable. Generally speaking, three V atoms prefer to stay close with others and form strong V-V bonds. Starting from the V3Si11-, cage configurations with one interior V atom emerge. Most interestingly, the V3Si12- exhibits a ferrimagnetic bicapped hexagonal antiprism wheel-like structure with a total spin of 4 μB. We have further investigated the VxSi12-(x=1-3) clusters and designed a kind of ferrimagnetic nanowires.There is a kind of onion type multi-layer core-shell clusters which have high symmetry, such as [As@Ni12@As2o]3- and [Sn@Cu12@Sn20]12- clusters. They also have been verified in experiments. Based on the experimental progress, we propose a series of icosahedral matryoshka clusters of A@B12@A2o (A=Sn, Pb; B=Mg, Zn, Cd, Mn), which possess large HOMO-LUMO gaps and low formation energies. A global minimum search confirms that such onion-like three-shell structures are indeed ground states of the A21B12 binary clusters. All of these icosahedral matryoshka clusters, including two previously reported ones, i.e., [As@Ni12@As2o]3- and [Sn@Cu12@Sn2o]12-, follow a 108-electron rule, which originates from the high h symmetry and consequently the splitting of superatom orbitals of high angular momentum. Particularly, two magnetic matryoshka clusters, i.e., Sn@Mn12@Sn2o and Pb@Mn12@Pb20, are designed, both of which present a large magnetic moment of 28 μB, a moderate HOMO-LUMO gap, and a weak inter-cluster interaction, making them ideal building blocks for novel magnetic materials and devices.The present thesis provides an important theoretical guidance for deep understanding the ground state structures of different types of alloy clusters. It is beneficial to explore the growth patterns and electronic properties of alloy clusters, explain the experimental data and design new magnetic nanoparticles. |
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