Geometric And Electronic Structures Of Noble-metal-based Clusters

Alloy clusters showing us with novel structures and properties which are largely different from those of clusters formed from single element have accepted much attention. They provide us with the possibility of controlling the structure and chemical composition of particles at the nanoscale level. In this thesis, using the first-principles (ab initio) calculation methods based on Density-Functional Theory (DFT), we investigate the structures, electronic properties and stability of noble metal-doped aluminum clusters and noble metal-based clusters. This dissertation which mainly concentrates on structures and electronic properties of noble-metal-based clusters is organized as follows:In Chapter 1, the history, background, structures, properties and application perspective are introduced.In Chapter 2, we first briefly introduce some basic principles and related current progresses of DFT. Then we discuss the theoretical foundations and key characteristics of DMol3 package used in this thesis in detail.Chapter 3, the growth behavior and stability of Au_nTi (n=2-16) are studied. A structural transition of Au_nTi clusters from two-dimensional to three-dimensional geometry occurs at n=8, while the Au_nTi (n=12-16) prefer gold cage structure with Ti atom locating at the center. The size-dependence of cluster properties such as binding energy, second-order energy, HOMO–LUMO gaps, ionization potentials, and electron affinities have been calculated and analyzed. The Au₁₄Ti cluster is found to have special stability, which may be due to the electron shell effects. Further calculations are performed to study cluster-cluster interaction between two Au₁₄Ti clusters. Whether the clusters retain their individual identities depends on the relative orientation of two clusters.Chapter 4, structures and stability of Au_nTi²⁺ (n≦7) are investigated to examine the interplay between geometric shell and electronic shell. The two titanium atoms form a dimer in the gold clusters. The second-order energy differences and HOMO-LUMO gap provide a clear explanation of the abundance peaks and odd-even staggering observed recently in photofragmentation experiments. The magnetism of Au_nTi²⁺ cluster shows an odd-even effect when n increases from 1 to 4 and drops to zero at n=5 and 7. The local magnetic moment and charge partition of Ti 4s, 3d orbital are discussed. The peculiar magnetic properties are relate to the structures and the hybridization between the Au 5d, 6s states and Ti 3d, 4s states.Chapter 5 is devoted to the doping effect of noble metal atom on the geometric and electronic properties of Al₇ clusters. Structures for Al₇Ag and Al₇Au are significantly different from those of Al₇Li and Al₇Cu. For Al₇Ag, the ground structure is similar to that of Al₇H, in which impurity atom binds to a single Al atom and occupies the top sit, while Al₇Au can be view as the combination of an Al atom and Al6Au. Both Al₇Ag and Al₇Au are found to have a large HOMO-LUMO gap, which imply they are chemically inert. It is found that relativistic effect plays an important role in the structures for Al₇Au, because relativistic calculations and non-relativistic calculations give different results for the most stable structure. In the most stable structure of Al₇Au, an Al atom which occupied the top sit behaves like a monovalent atom. Moreover, we investigate the structures, stability and bonding nature of Al₇H_n clusters. In the most stable structures Al₇ cluster remains its gross geometry and H atoms prefer to top sits of Al₇. Though Al₇H, Al₇H₃ and Al₇H₇ show noticeable stability, the binding nature between H and Al are different from that in Al₇I_n and those in noble metal-doped Al₇ clusters.In the last chapter, summarizations of our works as well as some prospects are given.