Synthesis And Optical Performance Study Of CdSe QDs/Polymers Nanocomposites

Quantum dots?QDs? with unique optical and electronic properties have been drawn strongly attention to the researchers and increasingly employed as building blocks for functional materials. However, due to oxidation and adsorption at quantum dots surface, Ostwald ripening, special surface and volume effect caused particle aggregation, which influenced the structure performance of quantum dots, leading to the loss of photoelectric properties and practical values. Therefore, how to prevent the occurrence of the above is of great importance. The studies found that these problems can be overcome by covalent modification of the QDs surface with polymers. Polymeric nanocomposites consisting of quantum dots and polymers represent a new class of materials that exhibit improved performances compared to their microparticle counterparts. The resulting composites might exhibit improved chemical and optical properties, thermal, mechanical, rheological, electrical, catalytic and fire retardancy. Therefore, it attracted much attention in many fields, such as bioscience, chemistry and chemical engineering, optoelectronic devices, aerospace and catalysis. So far, there have been reported many methods for polymers covalently modification of QDs, but that do not universal, especially for polymers without reactive structures, such as PS, PMMA, etc. Hence, the research emphasis in this paper is to build a general synthesis method, which is practical and easy to implement various types of polymers to covalently binding of QDs and also solve the above problems and difficulties of nanocomposites preparation. In this dissertation, we explored the way of polymers covalently bond to the surface of nanoparticles by a typical free radical polymerization method, where an initiator grafted on the surface of nanoparticles through esterification reaction. Which provides a new design idea for synthesis novel nanocrystalline composite materials, develop to simplify the process, reduce cycle time and lay the foundation of quantum dots/polymers nanocomposites in application and industrialization. The main research contents are as follows:?1? The synthesis and performance study of UV-shielding CdSe/PMMA nanocompositeCdSe/PMMA nanocomposite is built up by a new CdSe-azo initiator through surface initiated free-radical polymerization, which covalently grafting PMMA on CdSe QDs surface. The CdSe-azo initiator is obtained by the reaction of CdSe QDs with 4,4'-azobis?4-cyanovaleric acid??ACVA?. The nanocomposite is characterized by infrared?FT-IR?, ultraviolet?UV-vis?, fluorescence?FL?, transmission electron microscopy?TEM?, X-ray diffraction?XRD? and thermogravimetric?TG? analyses. Optical spectra of nanocomposite indicate that the luminescent properties of the CdSe QDs remained effective after polymerization and CdSe/PMMA nanocomposite films exhibit prominent UV-absorbing capability and high optical transparency in the visible-wavelength region. The average size of CdSe QDs is found to be less than 5 nm and well dispersed in the PMMA matrices without any apparent agglomeration by the TEM measurement. Moreover, the high quality of nanopartical in TG analysis, which promotes the light absorption of QDs and enhances the ultraviolet?UV? shielding property. In conclusion, this method realizes the polymer covalent modification of nanoparticle, promotes monodisperse and stability of nanoparticle and provides experimental and theoretical basis in general synthetic methods for polymers covalent modification of QDs in the next chapter.?2? The building of a general synthetic method for CdSe/CdS/polymers core-shell quantum dots nanocompositesHigh fluorescent and water-soluble core-shell CdSe/CdS quantum dot is firstly synthesized in this part. The new CdSe/CdS-azo initiator is built up by covalently grafting 4,4'-azobis?4-cyanovaleric acid??ACVA? on QDs surface. Then multi-type polymers brushes on quantum dots surface are synthesized by one approach?the surface-initiated radical polymerization technique? and gain CdSe/CdS nanoparticles with polymer brushes?CdSe/CdS-polymers? of different properties, including hydrophilic polyacrylamide, poly?acrylic acid? and poly?vinyl pyrrolidone?, hydrophobic oleophilic poly?vinyl acetate?, poly?methyl acrylate? and poly?methyl methacrylate?, polystyrene with conjugate group and polyacrylonitrile with strong polarity group. The structures and properties of CdSe/CdS-polymers reveal that the CdSe/CdS-azo initiator has excellent ability to initiate various kinds of vinyl monomers and these synthetic CdSe/CdS-polymers exhibit good stability and different fluorescence properties that can emit multiple fluorescences from the emission 405 nm to 570 nm. Furthermore, CdSe/CdS-polymers brushes show characteristic micro-morphologies by TEM measurement. Such strategy provides a platform to achieve nanocomposites with different properties. At the same time, this method can also realize the combination of polymers with QDs which have low activity and complex unstable structure and provides a general synthesis for the preparation of different properties of quantum dots/polymers composite materials.?3? The fluorescence enhanced effects study of cystine and Ag⁺ dual response characteristic for APTES modified core-shell CdSe/CdS QDs?CdSe/CdS-Si-NH2?The CdSe/CdS QDs is modified by APTES through a simple way, which improved the activity of QDs. The CdSe/CdS-Si-NH₂ nanocomposite is characterized by infrared?FT-IR?, ultraviolet?UV-vis?, fluorescence?FL?, elemental analysis?EDS?, transmission electron microscopy?TEM?, X-ray diffraction?XRD? and thermogravimetric?TG? analysis. Optical spectra of nanocomposite indicate that after the addition of amino acids, cystine shows enhanced fluorescence effects of nanocomposite aqueous solution, and the best mass proportion of CdSe/CdS-Si-NH₂ and cystine has also investigated, which is 5:3. In addition, when various metal cations are added to the binary system of CdSe/CdS-Si-NH₂/cystine, only Ag⁺ shows enhanced fluorescence response and the detection limit is 1.65 nmol/L. To sum up, the CdSe/CdS-Si-NH₂ QDs is a new-type of nanocomposite which has dual response characteristic of cystine and Ag⁺.