Theoretical Studies On Nano-system And Semiconductor Energy Material

This thesis is devoted to the study of thermal properties of small clusters, formation mechanism of twined nano-wire and structural, electronic, optical, intrinsic defect properties of copper-based semiconductors.In the first chapter, we propose and use enhanced sampling methods, which combine the replica exchange method and metadynamics method, to study the thermal properties of small water clusters (H2O)9。We find that the configura-tional entropy plays a key role in this system and it will turn the second relevant structure to the most stable structure at finite temperature.In the second chapter, we propose a growth model to explain the formation mechanism of periodical twinned zinc-blende nano-wire. We find that compe-tition between the electrostatic energy and staking fault energy results in the periodical stacking fault of the zinc-blende nano-wire. Furthermore, the model "an explain the linear relationship between radius and segment thickness of the periodical twinned zinc-blende nano-wire.In the third chapter, using first-principles calculation, we investigate the structural, electronic and optical properties of C112S11S3and find that:(i) the diverse range of crystal structures such as the monoclinic, cubic and tetragonal phases can all be derived from the zinc-blende structure with tetrahedral coor-dination,(ii) The energy stability of different structures is determined primarily by the local cation coordination around anions, which can be explained by a generalized valence octet rule. Structures with only Cu3Sn and Cu2Sn2cluster-s around the anions have low and nearly degenerate energies, which makes Cu and Sn partially disordered in the cation sublattice.(iii) The direct band gaps of the low energy compounds Cu2SnS3and Cu2SnSe3should be in the range of0.8-0.9eV and0.4eV respectively, and are weakly dependent on the long-range structural order.In the forth chapter, we use the first-principles calculation to investigate the intrinsic defect properties of wurtzite kesterite Cu2ZnXS(X=Si, Ge, Sn). We find that,(i) Wurtzite kesterite Cu2ZnSiS4have the largest chemical po-tential region that it can form stoichiometrically. So the high-quality single-crystal can be synthesized easily.(ii)The defect properties of wurtzite kesterite Cu2ZnXS4(X=Si, Ge,Sn) are similar to the properties of zinc-blende struc-ture. VCu and CuZn are strong candidates for the dominant acceptor,(ii-i)Compared with zinc-blende kesterite structure, the transition energy of the acceptor VCu(0/-) is deeper and the transition energy of the acceptors VZn(0/-), VZn(0/-2), CuZn(0/-) are more shallow in wurtizte kesterite structure, which results from the difference of p-d coupling between these two kind of structures.