Synthesis And Characterization Of Doped ZnS Quantum Dots

As a new kind of fluorescent nanomaterials, quantum dots have found their potential application in photoelectric devices, photocatalysis and biochemical analysis field. At present, the commonly used quantum dots in biological analysis are mainly the quantum dots containing cadmium. But the Intrinsic toxicity of quantum dots containing cadmium limit their widespread applications, especially for the applications related to biological systems. By contrast, the toxicity of Zn-based quantum dots can be significantly reduced. ZnS has unique fluorescence properties. Doped with metal ions and coated with inorganic shell on core quantum dots can improve the fluorescence quantum yield and chemical stability. The appropriate surface modification of quantum dots can not only inhibit the agglomeration of the quantum dots but also renders the quantum dots water-soluble, biocompatible and the ability to couple with biological macromolecules. Therefore, the synthesis of surface modified ZnS doped quantum dots has important significance. We expect to get the fluorescent quantum dots with low toxicity and high fluorescence quantum yield which can be directly applied in the field of biology. The main research work and results in this thesis are as following:1) ZnS:Ni and ZnS:Ni/ZnS quantum dots were successfully synthesized in aqueous medium via chemical co-precipitation method using L-cysteine as a modifier. We study the effect of the doping concentration, shell thickness and the illumination time of indoor natural light on the fluorescence quantum yield of the quantum dots prepared. X-ray power diffraction (XRD), energy dispersive X-ray analysis (EDX), transmission electron microscope (TEM), photoluminescence spectrum (PL), UV-Vis absorption spectrum (UV-Vis) and infrared spectrum (IR) were used to characterize the structure, composition, size, morphology and the spectrum properties of the quantum dots. The results show that ZnS:Ni and ZnS:Ni/ZnS quantum dots are spherical, own good monodispersity and belong to the cubic blende structure. The quantum dots prepared exhibit excellent fluorescence property. The fluorescence quantum yield of the quantum dots increased from the original3.1%to22.3%after doped with Ni2+, coated with ZnS shell and illuminated with indoor natural light. The L-cysteine modified on the surface of ZnS:Ni and ZnS:Ni/ZnS quantum dots render the quantum dots water-soluble, biocompatible and the ability to couple with biological macromolecules.2) ZnS:Fe and ZnS:Fe/ZnS quantum dots were successfully synthesized in aqueous medium via chemical co-precipitation method using L-cysteine as a modifier. We study the effect of the doping concentration, shell thickness and the illumination time of indoor natural light on the fluorescence quantum yield of the quantum dots prepared. XRD, EDX, TEM, PL, UV-Vis and IR were used to characterize the structure, composition, size, morphology and the spectrum properties of the quantum dots. The results show that ZnS:Fe and ZnS:Fe/ZnS quantum dots are spherical, own good monodispersity and belong to the cubic blende structure. The quantum dots prepared exhibit excellent fluorescence property. The fluorescence quantum yield of the quantum dots increased from the original2.8%to18.6%after doped with Fe2+, coated with ZnS shell and illuminated with indoor natural light. The L-cysteine modified on the surface of ZnS:Fe and ZnS:Fe/ZnS quantum dots render the quantum dots water-soluble, biocompatible and the ability to couple with biological macromolecules.3) ZnS:Fe quantum dots were successfully synthesized in aqueous medium via chemical co-precipitation method using chitosan as a stabilizer and coating reagent. XRD, EDX, TEM, PL, UV-Vis and IR were used to characterize the structure, composition, size, morphology and the spectrum properties of the quantum dots. The results show that ZnS:Fe quantum dots are spherical, own good monodispersity and belong to the cubic blende structure.The quantum dots prepared exhibit excellent fluorescence property, The fluorescence quantum yield of quantum dots increased from1.7%to6.8%after doped with Fe2+. The chitosan modified on the surface of ZnS:Fe quantum dots render the quantum dots biocompatible and the ability to couple with biological macromolecules.