Preparation And Fluorescence Properties Of CdTe Quantum Dots Composite Materials

Semiconductor quantum dot (QD) is a type of nanocrystal with size less than lOnm, shows more excellent optical properties than conventional organic dye because of its significant surface effect, quantum size effect etc, and has a wide application in biomarker, ion-detecting agent, photovoltaic devices etc. The synthesis of QDs in water is simple, low toxic, and more economical. QDs can be used as a fluorescent probe to detect metal ions in aqueous solution based on its sensitive fluorescence quenching response to metal ions. However, pure QDs powder is intrinsically instable to thermal environment, which greatly limits its application in solid materials. In this thesis, the CdTe QDs were incorporated into a layered material, magadiite (MD), to prepare CdTe/MD composite fluorescent powders via an electrostatic assembly method and an ion exchange in situ synthesis method, the composites could be further applied to polyethylene (PE) and polypropylene (PP) by a hot melt blending method to obtain polymer composite materials with excellent fluorescence properties. Besides, the as-prepared CdTe QDs were incorporated into poly (methyl methacrylate) (PMMA) matrix to prepare CdTe/PMMA composites via three different methods including electrospinning,in situ polymerization and solvent thermal evaporation methods, the fluorescence properties of the obtained CdTe/PMMA composites were also studied. The results are listed as follows:1. L-cysteine-modified CdTe QDs with size of 2-4 nm, which were synthesized by an aqueous phase refluxing method, could be further used as fluorescent probes for detection of Cu2+in the aqueous solution with the linear range of 400-4000 nmol/L and detection limit of 85nmol/L. The measured values were basically consistent with the actual values during the detection of Cu2+ in the simulated water.2. CdTe QDs could be successfully introduced into the MD laminates to obtain the CdTe/MD composite fluorescent powders by two methods including electrostatic assembly and ion exchange in situ synthesis method. The agglomeration of CdTe QDs was effectively inhibited due to the confinement protective effect of MD laminates, which enabled CdTe QDs to show excellent fluorescence properties and good thermal stability in the powder state:(1) Using the CdTe QDs with λem(max) at about 550nm as the raw solution and setting the material ratio V(CdTe)/m(MD) as 50mL/0.5g, the CdTe/MD composite which was prepared by the electrostatic assembly method, showed the strongest PL intensity and the best heat resistance. HRTEM result showed that CdTe QDs were well dispersed in the MD matrix without agglomeration,XRD results indicated that the presence of QDs did not cause damage to the laminate structure of MD.(2) Changing the refluxing time from 5min to 24h, the λem(max) of CdTe/MD composite which was prepared by ion exchange in situ synthesis method could be tuned from 501nm to 557nm, and when the refluxing time was 30min, the obtained composite showed the strongest PL intensity.(3) CdTe/MD composite powders could be further added to the thermoplastic processes of PE and PP to obtain CdTe/MD-PE, CdTe/MD-PP composite materials with tunable fluorescent colors between yellow, orange and orange-red.3. CdTe/PMMA composite fibers with diameter of 2～3μm could be prepared by electrostatic spinning method, the pores located on the surface of the fibers were about 100nm in diameter; the composite fibers could well maintain the λem(max) of original QDs solution. Transparent fluorescent CdTe/PMMA films with the CdTe contents in the range of 2.12-6.10wt% could be prepared via solvent thermal evaporation method, the 6.10 wt% CdTe/PMMA composite showed the strongest PL intensity. TEM result showed that CdTe QDs were uniformly dispersed in the PMMA matrix. Fluorescence spectra and fluorescence lifetime tests results indicated that the fluorescence properties and optical stability of CdTe QDs in PMMA matrix were significantly enhanced compared with the pure CdTe QDs powders. Thermal gravimetric analysis (TGA) showed that the thermal stability of PMMA was improved in the presense of CdTe QDs. By incorporating the CdTe QDs with λem(max) of 539 nm,555 nm,566 nm and 588 nm into PMMA matrix, the obtained composite films showed the λem(max) at 538,553, 565 and 584 nm, respectively, the fluorescent colors could be tuned in green, green-yellow, yellow, and orange.