Synthesis And Properties Of ZnS Fluorescent Quantum Dots By Oil-Water Interface

Fluorescent quantum dots (QDs) with a size less than 10 nm are the important part of luminescent material. ZnS with wide band gaps is an excellent semiconductor compound and has a wide range of applications in electroluminescent devices, laser diode and photovoltaic device. Meanwhile, doping-ZnS with excellent photoluminescence property has a broad application prospect in protein markers, cell biology and genomics and so on.Nowadays, preparation technologies of ZnS fluorescent quantum dots include solid-phase method, gas-phase, organic solvents, sol-gel, hydrothermal synthesis, microemulsion method and so on. Hydrothermal and microemulsion method are in common use. But hydrothermal method is not convenient to operate and the obtained particles are easy to coagulate. The quantum dots prepared by microemulsion have many excellent properties, but the yield is relative low. In this paper, we prepared ZnS quantum dots which were dispersed in oil phase by oil-water interface method. This method has the superiorities such as mild reaction conditions and simple operation in comparision to traditional methods. We investigated the influence factors such as the concentration of reactants, additives, organic reagents and other conditions on preparing ZnS nano materials. We got doping-ZnS fluorescent quantum dots by adopting different metal ions to ZnS dispersing in oil phase, and discussed the influence of doping ratio on the photoluminescent property.The following is the main research results:(1) Synthesizing the oil-soluble ZnS QDs by oil-water interface method successfully. TEM images showed that the average diameter of ZnS QDs is nearly 3.3 nm and the particles are monodispersed. Effects of reaction conditions were also discussed by several means. When the ratio of [S2-]/[Zn2+] was taken as 1.2:1 with [Zn2+]=0.02 M at 80℃for 2 h, the strongest absorption of as-prepared ZnS QDs was found. We also analysed the TEM images of ZnS after annealing for 24 h at 100℃and 150℃,respectively. From these images, the average dimension of ZnS nanoparticles after annealing at 100℃for 24 h with no aggregation was 4.3 nm. This suggests that the ZnS QDs prepared at 100℃has some stability in size even after annealing at the same temperature. However, the average size of single ZnS particles after annealing for 24h at 150℃was nearly 460 nm,indicating that the dimension of ZnS QDs was easy to increase to submicron scale under such high temperature. Fortunately, the change in morphology and monodispersion was tiny.(2) The possible formation mechanism of monodispersed particles prepared in oil-water interface was deduced in this paper. The probable process was:reactants dispersing in two phase diffused onto the oil-water interface, after the nucleation occurred, the adsorption of surfactants to the nuclei exhibited two ways as (a)fast absorption and (b)slow absorption. (a) the fast one showed that the nuclei generated from the oil-water interface were capped by surfactants swiftly and then entered into oil phase, therefore the monodispersed QDs'were obtained.easily. The better solubility the solvent had, the faster diffusion to the interface the reactant showed. Meanwhile, higher the reaction temperature was, easier the surfactants reached the interface to cap the nuclei, and the adsorption mainly displayed in the fast way in this situation. (b) the slow way showed that the lower the concentration of surfactants was, the slower adsorption to the interface of the nuclei the surfactants showed. The particles didn't enter into the oil phase untill the adsorption of surfactants to the nucleus increased to sufficient quantity, and the particles continuned growing as the new reactants deposited onto the nuclei, therefore the final particles might has various dimensions and the polydispersed QDs would be obtained. Therefore, the absorption velocity of surfactants to the nuclei was the key factor which determined the dispersion of QDs. In this work, we adopted the proper concentration of reactants and surfactant at the high temperature of 80℃in preparing ZnS QDs, therefore the adsorption mainly displayed in the fast way under these conditions and the products had the excellent monodispersion.(3) ZnS:Mn and ZnS:Eu fluorescent quantum dots were successfully prepared by doping ZnS with Mn2+ and Eu3+ respectively. TEM images showed that the average size of as-prepared ZnS:Mn and ZnS:Eu fluorescent quantum dots was nearly 3.3 nm. And the influence on the fluorescence property of doping-ZnS by doping Mn2+ and Eu3+ with different molar doping ratio was also studied. The results showed that the strongest fluorescence emission peaks were found at 586 nm and 616 nm, corresponding the doping ratio of 4.0%(Mn2+) and 5.0%(Eu3+), respectively.(4) In addition, ZnS:Mn, Eu co-doped composite fluorescent quantum dots were obtained by doping Mn2+ and Eu3+ together. And the influence on fluorescence property of ZnS:Mn, Eu by doping Mn2+ and Eu3+with different molar doping ratio was also discussed. Fluorescent patterns indicated that two high peaks were found at 586 nm and 616 nm when the excitation wavelengths were chosen as 323 nm and 396 nm, respectively.