Theoretical Investigation Of The Topologic Structures Of The Medium Sized Silicon Clusters And Their Properties

In this thesis, we investigated the structures and properties of medium-sized silicon clusters Si_n (n=31-70). We proposed a new strategy named as "compressing-liquid" to search for candidates of clusters. Combining this strategy with the "stuffing-fullerene" scheme, we have extensively searched the structures of silicon clusters in the range of n=31-70, and found out the lowest-energy isomers of Si₃₁-Si₅₀ for most sizes and low-lying isomers of Si₅₁-Si₇₀. By examining the structural features of these low-lying isomers, we explained the structural evolutions and relative reactivities of these sized Si clusters. Furthermore, the deposition and diffusion of Si₂, Si₃ and Si₄ on Si(111) 7×7 surface were studied by using the tight-binding model and the deposition, growth and diffusion patterns of silicon clusters on silicon surfaces were revealed.The thesis is composed of five chapters. In the first chapter, we introduce the main progress in the research of silicon clusters. Silicon clusters are particular states of silicon material. The evolution of their structures and properties with cluster sizes is the basis of understanding the growth process of silicon. As cluster sizes increase, the structures of silicon clusters transform from highly compact spherical structures to prolate structures constructed based on particular building blocks, and then to spherical structures with interior atoms. The main structural motifs of prolate silicon clusters feature tricapped-trigonal-prism, puckered-hexagonal-ring Si₆ and fused-puckered-hexagonal-ring Si₉. For the spherical silicon clusters, endohedral silicon fullerene structures are the main structural pattern. Their binding energies and physical and chemical properties exhibit strong size-dependence.In the second chapter, we introduce the most extensively used geometry search algorithms in silicon clusters, and the computational methods used in this thesis. We introduce the "stuffing-fullerene" method and the "compressing-liquid" method we propose in details. Comparison of the features between the different DFT packages used in this thesis is addressed.In the third chapter, our low-lying isomers of neutral and ionic silicon clusters of n=31-70 are presented. Structurally, silicon clusters with n=31-60 feature endohedral fullerene structures, while endohedral fullerene structures are not the most favored for silicon clusters of n>60. The structures of the core atoms in silicon clusters also evolve as increasing cluster size. For n≤52, the core atoms usually adhere to the inner surfaces of the outer cages and form loose structures with empty spaces in their centers. For n≥57, the core structures are more compact where inclusive atoms enclosed by other core atoms exist. In the range of n=53-56, the core structures of silicon clusters are evolved from loose structures to more compact structures. The transition from loose structures to compact structures of the cores in silicon clusters can be considered as the transition state of the evolution to bulk silicon.In the fourth chapter, our calculated HOMO-LUMO gaps, ionization potentials and affinity energies, dissociation energies, mobilities and polarizabilities of the low-lying isomers of Si₃₁-Si₆₀ are summarized. We discussed the chemical reactivities, the movement of silicon clusters under an external electric field, the electronic response to the external electric fields and the electron diffraction spectra of them. The results indicate that the obtained isomers with n=31-50 are very close to the isomers produced in experiments. The agreement of the calculated and measured mobilities shows that the fullerene-based structures are the most favored structural patterns for the silicon clusters in the size range of n=31-60. In addition, we found that silicon clusters with different structures always have different features between their electron diffraction spectra. This implies that the electron diffraction spectra are regarded as an important mean to identify the structures of silicon clusters.In the fifth chapter, we investigated the deposition and diffusion behavior of Si₂, Si₃ and Si₄ on Si(111) 7×7 surface. For these clusters, the most favored adsorptive sites locate in the trigonal area around the center restatom. The system is stabilized through saturating the dangling bonds existing on the restatom and adatoms. The stabilities of the systems are related with the number of dangling bonds that are saturated. The more the number of the saturated dangling bonds is, the more stable the system is. The adsorbed Si₂ can diffuse in one halfcell by rotating around the restatom. However it is hard to diffuse across the dimer wall into the other halfcell. Si₃ and Si₄ can diffuse not only in one of the halfcells, but also from one halfcell to the other. Si₂, Si₃ and Si₄ can all form because of the diffusion and congregating of isolated silicon atoms in the silicon surface.