Investigation On The Synthesis And Properties Of Zn-based Nanomaterials

Semiconductor heterostructures with modulated compositions can integrate several different functions into one integrated nanostructure on the requirement of some certain applications, which are essential for developing potential nanoelectronic and optoelectronic devices. The main points for this thesis are not only successfully realize the synthesis of the ZnS:Mn²⁺ nanoparticles and nanorods with cubic and hexagonal structure、ZnO quantum dots、ZnS:Mn²⁺ nanowires /ZnO quantum dots@SiO2 nanocomposites, and also investigate the structure and properties in detail by the methods of X-ray diffraction （XRD）, transmission electron microscopy（TEM）, X-ray-absorption fine structure technique （XAFS）, X-ray photoelectron spectrum （XPS）, photoluminescence （PL） and vibrating sample magnetometer （VSM）. The major achievement obtained is as follow:1. Nanocrystalline samples of ZnS and Zn1?xMnxS were synthesized by the solvothermal method without any surface-active. The results showed that the Mn²⁺ ions were substitutionally incorporated into the ZnS host, a slight increase is obtained in the lattice constant, and the size of the particles was decreased as the Mn doped-ratio increased. The yellow-orange emission from the Mn²⁺ 4T1-6A1 transition was observed in the PL spectra, the peak intensity ratio of which to that of sulfur vacancies increased with the increase of the Mn²⁺ concentration, and showed a maximum when the concentration of Mn²⁺ was kept at 3%. A red shift about 8 nm of Mn²⁺ 4T1-6A1 emission is observed by comparing ZnS:Mn²⁺（1%） with ZnS:Mn²⁺（11%）. Three mechanisms can be used to explain the red shift:①the nonradiation energy transfer in Mn clusters②the high density of surface states or strong electron–phonon coupling effects③when the Mn²⁺ concentration in ZnS nanoparticles exceeds a critical limit, a MnS phase tends to form.2. Wurtzite-type ZnS:Mn²⁺ nanowires were prepared by a hydrothermal method at 180°C. An ethylenediamine-mediated template was observed and employed to explain the growth mechanism in detail. A strong yellow–orange emission from the Mn²⁺ 4T1–6A1 transition was observed in the photoluminescence spectra, which exhibited blue shift as the Mn²⁺ doped ratio increased. There is an energy transfer from the defect states to the Mn²⁺ states in terms of the blue shift.3. Investigated and compared the structure and luminescent properties of ZnS:Mn²⁺ nanoparticles and nanorods. The nanoparticles were more disordered than nanorods in the first neighbor shell. The yellow-orange emission from the Mn²⁺ 4T1-6A1 transition was observed, its intensity relative to the blue-green emission increased from nanoparticles to nanorods. The fluorescence lifetime of the Mn²⁺ emission for the NPs and NRs was 0.662 and 0.224 ms. The possible explanation may originate in the small change of the crystal field due to the distinct crystal lattice of the NCs. Strains in the lattice or the breakdown of the regular Coulomb potentials on the surface of the NCs may also be the possible reasons.4. The wurtzite-type ZnS:Mn²⁺Cu²⁺ nanowires were prepared by a simple hydrothermal method at 180°C. Both the Mn²⁺ and Cu²⁺ ions substituted for the Zn²⁺ sites in the host ZnS. The color-tunable emission can be obtained by adjusting the concentrations of Mn²⁺ and Cu²⁺ ions. The ferromagnetism was observed around room temperature, which may be caused by the SE process occured between the S2-, Mn²⁺and Cu²⁺ ions.5. We demonstrated the encapsulation of ZnS:Mn²⁺ nanowires and ZnO quantum dots with a layer of mesoporous SiO2 shell for the purpose of integrating dual emission property into one common nanostructure. The intensity of the yellow-orange emission contributed by ZnS:Mn²⁺ NWs and the UV emission contributed by ZnO QDs was three and ten times higher than their individual components, respectively. The fluorescence intensity ratio of the dual emission can be tuned by adjusting the hydrolysis time of tetraethyl orthosilicate. The anomalous enhancement of the integrated intensity for the UV emission with the temperature indicated that the high surface state density existing in ZnO QDs can overrun the influence of temperature quenching and even alter the photoluminescent properties.