First-Principles Investigation On Pd_n,Pd_n-1S Clusters And The Phenomena About Blue-shift In The GaN By Al-doping

In this paper, we investigate the geometric and electronic properties of Pdn(n≤15) and Pdn-1S (n=2-10) clusters and the phenomena about blue-shift which happened in the GaN by Al-doping using first-principles quantum chemistry methods based density functional theory (DFT). The main contents are presented as the followings:In Chapter 1, we introduce prieftly the basic concept of density functional theory. Theorem of Hohenberg-Kohn is the fundament of DFT and is developed to Kohn-Sham equation, which can be used to perform real calculations. New corrections and extensions, together with developed exchange-correlation functionals, have made DFT more accurate and suitable for more systems. At the end of this chapter, we introduce some widely used simulation packages, in which we pay more attention to the Doml3 Software and the Castep Software.In Chapter 2, at first, we give a brief introduction to clusters. Then we systematically investigated the geometries, stabilities and electronic properties of Pdn (n≤15) and Pdn-1S (n=2-10) clusters by using density-functional theory (DFT). The all-electron spin-polarized generalized gradient approximation (GGA) has been used. For Pdn, we have found that 3D structures were more stable than 1D and 2D isomers. The binding energies, fragmentation energies, and the second-order difference of energies show that the magic number is 4, 6, 8, 10, 13 for Pdn. According to optimized Pdn-1S geometries, growth patterns of the Pdn-1S clusters are S-capped or S-substituted Pdn clusters. On the basis of the optimized geometries, various energetic properties are calculated for the most stable isomers of Pdn and Pdn-1S clusters, including the average binding energies, the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) gaps, fragmentation energies and the second-order difference of energies (△2E). The investigation on atomic average binding energies, fragmentation energies, and the second-order difference of energies show that the Pdn-1S (n=4, 6, 8) clusters have highly stability. Furthermore, comparing with the pure Pdn clusters, a universal narrowing of the HOMO-LUMO gaps in Pdn-1S clusters is found. According to the Mulliken charge population analysis, charges always transfer from Pd atoms to S atom.In Chapter 3, we investigated the electronic structure of pure and Al-doped wurtzite GaN by means of first-principles ultrasoft pseudopotential in the gengralized gradient approximation at Castep software. We use a 2×2×1 supercell and the radio of Al doped GaN is 1/8, 2/8, 3/8, 4/8, 5/8, 6/8, 7/8, and 8/8. The calculation of the constant of crystal lattice, total energy, energy band structure, total electronic density of states and partial density of states of Al-doped wurtzite GaN were carried out. Our results show that N electronic density shifting to Al results in the Ga 4s electron states shifting to a higher energy. This is why the band gap increasing with the concentration of Aluminum increasing.