Preparation And Optical Characterization Of ZnO Quantum Dots

ZnO quantum dots are new semiconductor nanomaterials with the band gap of Eg=3.37eV at room temperature.Because of the excellent biocompatibility and photoelectric properties,ZnO quantum dots have been extensively studied and applied in many fields such as cellular imaging,semiconductor diodes and solar cells.Due to the toxicity of CdX fluorescent quantum dots,the applications of these quantum dots in bioimaging are limited.Therefore,other luminescent materials,like ZnO,with superior nontoxity and biocompatibility,can effectively solve this contradiction.In order to improve the size uniformity,photoelectric properties and structural stability of ZnO quantum dots,in this paper,ultrasonic chemical method is adopted to improve the preparation process of ZnO quantum dots and the optical properties of the prepared ZnO quantum dots are studied and analyzed.The main work of this study is listed as follows.(1)In this paper,we use zinc acetate dihydrate as zinc source to synthesize the ZnO quantum dots by ultrasonic chemical synthesis in an anhydrous ethanol system at room temperature.Through the improvement of the preparation process,some important parameters of the reaction system,such as reaction time,temperature,OH~-content,have been researched and analyzed.We have systematically discussed the effects of these parameters on the morphology and optical properties of ZnO quantum dots.Results show that when the temperature of 40?,reaction time for 30 min,ZnO quantum dots have uniform size around 4.2nm.ZnO quantum dots monodisperse degree and fluorescence intensity were high,and the quantum yield increased to 82.2%.The surface modification of ZnO quantum dots with oleic acid(OA)and polyethylene glycol(PEG400)in ethanol system can enhance the structural stability and optical properties,and can increase dispersion.We analyzed the stability of ZnO quantum dots stored at room temperature based on the attenuation of fluorescence intensity.It was found that ZnO quantum dots fluorescence intensity decays a lot at the initial stage of preservation.Because ZnO quantum dots surface have a lot defect,and free radical,ZnO quantum dots appear apparent agglomerate phenomenon.Through the analysis of 365nm ultraviolet irradiation and CIE chromaticity diagram,we found that ZnO quantum dots emit soft white light.This property can be well applied in the field of lighting and cell imaging,and have good application value.(2)On the basis of the preparation of ZnO quantum dots with uniform size,we have used ultrasonic chemistry method to coat ZnS as a shell on the surface of ZnO quantum dot to obtain ZnO/ZnS core-shell quantum dots.Through analysis by X-ray diffractometer,high-resolution transmission electron microscope and selective electron diffraction,we found that the ZnO core and ZnS shell are both of hexagonal structure,and average particle size is 10 nm.The fluorescence spectrophotometer analysis shows that ZnS shell effectively inhibits the ZnO quantum dots visible region luminescence,and enhances the ultraviolet band fluorescence.