Preparation Of CdSe Quantum Dots Doped Silica Composite Gels By Sol-gel Method

Fluorescent quantum dots(QDs)are nanomaterials with three-dimensional quantum confinement.Based on their own quantum size characteristics,they show advantages of high fluorescence efficiency,narrow absorption spectrum,and size-dependent luminous tunability,which have a good application prospect in optical devices,fluorescent labels,and solar cells.However,the extremely small size of QDs leads to their alarge interface area,extremely high interfacial energy,and a large number of surface dangling bonds,which drive the accumulationof QDs,and the luminous properties,would be greatly weakened due to the existence of the defect traps.In traditional QDs synthesis methods,the dispersion of QDs and the passivation of the surface are often realized by adding ligands,which partly reduces the surface defects.However,in the harsh application conditions,such as high temperature and high-intensity light radiation in the field of optoelectronics,the coating of the ligand alone is not enough efficient to maintain the stability of the QDs.The dispersion of QDs in silica gel can not only effectively fix the QDs,but alsoeffectivelyprotect the stability and luminous efficiency of the QDs at high temperature due to the good mechanical properties and heat resistance of silica gel.However,how to achieve the uniform dispersion of the QDs in the gel and avoid the crack of the composite gel material are the key challengesneed to be solved urgently in the preparation of QDs-doped silica composite gels(QDs-Gel).In view of the above problems,this paper explores the preparation method of CdSe QDs-Gel.Water-soluble CdSe QDs were prepared through modification of oil-soluble QDs or direct water phase synthesis approach,for the purpose to achieve uniform dispersion of CdSe QDs in the silica gel precursor solution.The surface passivated and functionalized CdSe QDs were introduced into the silica gel by modifiedsol-gel method to realize the chemical bond between the surface of QDs and the silica gel network.X-ray powder diffractometer,fluorescence spectrometer,and thermogravimetric analyzer were used to characterize the performance of the composite material.It was found that not only the fluorescence efficiency of the original CdSe QDs in the composite gel was maintained,but also the mechanical properties of silica gel were fused.The stability of the QDs structure effectively improves the luminescent stability of the QDs under long-time laser irradiation.The research content of this thesis is mainly divided into the following three aspects:(1)Synthesis and modification of aqueous CdSe QDs.CdSe QDs were prepared by aqueous phase synthesis approach.Three different ligands,cysteine(Cys),mercaptopropionic acid(MPA),and thioglycolic acid(TGA),were used as capping agents.By exploring the concentration and temperature of ligands and precursors,it was found that when TGA was used as an end-capping agent and the reflux temperature was 80 ? for 4 h,the properties of CdSe QDs formed were the best;(2)Exploring the modification of oil-soluble QDs.It was found that the N atom of the pyridine ligand can effectively coordinate with the surface of the QDs.Therefore,the intermediate ligand was selected and the QDs of the oleic acid ligand were exchanged in two steps.It was converted into PEG-coordinated water-soluble QDs.The ultraviolet lightand FT-IRspectrum confirmed that the QDs were uniformly dispersed in the surfactant,and pyridine was distributed in dichloromethane.(3)Synthesis and characterization of CdSe QDs-Gel.A silica gel with optically clear and good mechanical properties was prepared by the sol-gel method.Tetraethyl orthosilicate was used as the precursor.APTES was used as the alkaline hydrolysis reagent.The two-step catalytic process,acid first and then alkaliwere used.During the drying process,additives were added to explorethe conditions to avoidgel cracking.The experimental conditions such as sol composition,hydrolysis reagents,drying assistant were determined,and silica gel with excellent properties was obtained.The passivated and functionalized CdSeQDs were introduced to efficiently synthesize CdSe QDs-Gel with stable fluorescent properties.Compared with the original CdSeQDs solution,the composite material can effectively protect the long-term stability of the QDs in the environment of long-term laser irradiation.In summary,the CdSeQDs-Gel prepared by the optimized sol-gel method takes into account maintaining both the fluorescence properties of the QDs and the excellent mechanical properties of gels,which improves the performance of the QDs under harsh conditions.The experimental research results of this paper will provide theoretical basis and technical support for the research of CdSe QDs-Gel materials.