Researches On Physical Properties Of ZnS Nanostructures

Low dimensional nanostructures have presented fascinating phenomena in their physical and chemical properties due to the quantum confinement effect and the surface effect. In this thesis, we have systematically studied the stability, electronic, mechanical and doping magnetic properties of an important semiconductor zinc sulfide (ZnS) nanostructures by using density functional theory. The obtained results are summarized as follows:The structure, stability, and electronic properties of the pristine wurtzite (WZ) and zinc-blende (ZB) structural ZnS nanowires (NWs) are comparatively investigated. The emphases are focused on the mechanical properties of wurtzite ZnS NWs. First, the calculated results show that theh WZ-ZnS nanowire is more stable energetically than the ZB-ZnS nanowire at small size. The two kinds of ZnS NWs show different electronic properties whose fundamental origins are theoretically presented. Then, on the basis of geometry optimization, we indicate that the Young's moduli of different surface adsorptions have different behaviors with the size. The Young's moduli of the pristine and water adsorbed NWs are larger than that of bulk ZnS and decrease with the increasing size. In contrast, the Young's modulus of the the hydrogen adsorption NWs is smaller than that of bulk ZnS and increases with the increasing size. The obtained results can be used to explain the contradiction experimental results obtained by different experimental research groups. Further theoretical analysis indicates that the mechanic properties of the three kinds of ZnS NWs are determined by surface relaxations and the core nonlinear effect together.The structue, stability, and magnetic properties of ZnS NWs doped with one or two transition-metal (TM) atoms (Cr, Mn, Fe, Co, and Ni) are comprehensively studied by using the density functional theory. The researches indicate that all of the TM atoms prefer to be at the middle position of the NWs rather than on the surface positions. TM atoms have no tendency to form clusters in the TM-bidoped ZnS NWs. The formation energies of doped NWs are smaller than that of the pristine ones, indicating that the TM-doped is exothermic. The partial density of states (PDOSs) show significant hybridization between the d states of the TM atoms and the p states of the S atoms, which is responsible for the magnetism of doped NWs. More importantly, the ZnS NWs doped with Cr atoms have room temperature ferromagnetism, which indicates that this kind of ZnS NWs has potential for successful implementation into spintronic devices in the future.The structure, electronic and magnetic properties of the ZnS clusters are systematically studied by genetic algorithm incorporated with density functional theory. The results show the structural evolvement trend and magic number rule of small ZnS clusters. On the basis of the result that (ZnS)₁₂ is the smallest cage structure with the highest possible symmetry, it can be taken as a host for the investigation on the structure, electronic and magnetic properties of Mn and Cr doped ZnS clusters. The calculated results indicate that the magnetic coupling between the doped Mn atoms depends on their local environment. The substitutional and exohedral bidoped (ZnS)₁₂ clusters favor the antiferromagnetic (AFM) state, while the endohedral bidoped ones favor the ferromagnetic (FM) state. Theoretical analysis shows the physical origin and applications in future. For Cr doped ones, the magnetic coupling between the Cr atoms is mainly governed by the competition between direct Cr-Cr AFM interaction and the FM interaction between two Cr atoms via S atom due to p-d hybridization. Finally, we indicate that the exohedral bidoped (ZnS)₁₂ clusters favor the FM state, which has potential applications in nanoscale quantum devices.