Study On Synthesis And Properties Of Sulfide Semiconductor Nanomaterials

As a wide-gap semiconductor nanostructure material, sulfides have drawn a lot of attention more than ten years owing their exceptional optical, electrical and magnetic properties. In this thesis, we focus our attention on two kinds of sulfides, ZnS and CdS semiconductive nanostructure materials. The ZnS nanocrystals, aqueous CdS quantunm dots and film were prepared by sol-gel method and chemical bath deposition, respectively, and the optical properties have also been explored systemetically, especially the optical and magnetic properties of rare-earth ion Eu3+ and transition metal ions Cu2+, Mn2+ in ZnS nanocrystals. Moreover, the crystal-field of ZnS:Eu3+ nanocrystal has been analyzed theoretically. In order to obtain the uniform and periodic sulfides nano-array, the nano-scale porous anodic alumina (PAA) template has been synthesized preliminarilly.(1) The sol-gel method has been used for the synthesis of ZnS and ZnS:Eu3+ nanocrystals. The crystal structure, morphology, optical and magnetic properties were investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), UV absorption spetroscopy, pholtoluminescence spectroscopy and Quantum Design physical properties measurement system (PPMS-9). The crystal-field effect on the emission of Eu3+ ions in ZnS nanocrystals has been analyzed in detail by the characteristic emission spectra of Eu3+ ions dependent on temperature. The results show that the ZnS nanocrystals have a zinc blende structure, exhibiting obvious quantum size effect. It is the first time that all the transitions 5D0→7FJ(J=0,1,2,3,4,5,6)of Eu3+ ions have been observed in ZnS nanocrystals host. According to the temperature dependent photoluminescence of 5D0→7F1 transition and the crystal-field theoretical analysis, it is confirmed that the crystal field has no effect on the multiplet 7F1 of Eu3+ ions in the cubic symmetry ZnS nanocrystals. The crystal-field parameters B04 and B06 of the ZnS:Eu3+ nanocrystals were determined to be-59.8 meV and 41.6 meV for the first time, respectively. At both 300 K and 5 K, at low doping concentration ZnS:Eu3+ nanocrystals exhibit obvious ferromagnetism and magnetization saturation.(2) The ZnS:Cu2+, ZnS:Mn2+ and co-doped ZnS:Cu2+, Mn2+ nanocrystals have been prepared by sol-gel method. The crystal morphology, chemical composition, optical and magnetic properties were investigated by high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV absorption spetroscopy, pholtoluminescence spectroscopy and Quantum Design physical properties measurement system (PPMS-9). The results show that for ZnS: Cu2+ nanocrystals in water-ethanol solution, the S/Zn=1:2 sample has a main green emission band relative with the Cu t2 level, while the S/Zn=1:1 sample has a main blue emission band arised from zinc vacancy. The S/Zn ratio has no effect on the photoluminescence properties of ZnS:Cu2+ film. The film samples have a green emission and a blue emission shoulder. Compared with the samples in water-ethanol solution, the two emission centers exhibit a certain blue shift. The XPS results show that the Cu 3d level has a splitting in ZnS:Cu2+ nanocrystals. For ZnS:Mn2+ nanocrystals, the influences of substrate and temperature on the luminescence properties have been studied. With the increase of temperature, the characteristic emission of Mn2+ ions exhibited a blue shift. The ZnS:Mn2+ nanocrystals showed obvious ferromagnetism at 300 K at low doping concentration. The average size of co-doped ZnS:Cu2+, Mn2+ nanocrystals is 2.5 nm. When the mole ratio of Cu2+ and Mn2+ is 0.1% and 0.5%, respectively, the emission of ZnS:Cu2+, Mn2+ nanocrystals could cover the region of red, green and blue primary colors. Moreover, at high doping concentration a great number of dopant ion pairs formed, which could increase the probability of non-radiative recombinations, inducing the quenching of the characterictic emission of dopant ions and the defect states emission of ZnS.(3) The aqueous CdS quantum dots and CdS film were synthesized by chemical bath deposition. The results show that the surface modification of aqueous CdS quantum dots by CdCl2 solution could enhance the band-edge emission intensity of CdS quantum dots by decreasing the number of surface defects. According to the results of excitation spectroscopy, it is confirmed that there is an energy transfer mechanism from thiourea to CdS devoting to the luminescence of CdS quantum dots. Nanometer scale CdS film has a wurtzite structure, while after annealling at 400℃the crystallinity was enhanced, and the position of emission peak had a red shift.(4) The nano-scale porous anodic alumina (PAA) template has been synthesized by anodic oxidation under constant voltage.In conclusion, sulfides semiconductor nanostructure materials have become to the hot research topic due to their excellent optical and magnetic properties. Our work includes the synthesis, optical and magnetic properties of ZnS nanocrystals, aqueous CdS quantum dots and CdS film. The new type low dimensional luminescent materials were prepared by doping, investigating their luminescence, ferromagnetism properties and luminescence mechanism, and analyzing the crystal-field characteristic of ZnS:Eu3+ nanocrystals.