Preparation And Characterization Of Quantum Dots And Magnetic-Fluorescent Bifunctional Nanocomposites

Quantum dots （QDs）, known as semiconductor nanocrystals, have received greatattentions from the physicists, chemists and biologists, due to their unique electricaland optical properties. With a nanostructure of bifunctional sections, there’s greatpotential for magnetic-fluorescent nanocomposites （MFNCs） to possess magnetic andfluorescent properties, simultaneously. As a result, MFNCs have been attractingimmense interests for a wide range of potential applications in various fields, such asbiology, chemistry, medicine and so on.（1） The CdSe/ZnS and ZnCdSe QDs were synthesized by single-step syntheticmethod, respectively. The influences of the reaction time, temperature, and rawmaterials ratio on morphology and optical properties were investigated. Theas-prepared QDs were characterized by ultraviolet visible spectra （UV-vis）,photoluminescence （PL）, high resolution transmission electron microscopy （HRTEM）and X-raydiffraction patterns （XRD）. The results revealed that:For the CdSe/ZnS QDs, the PL quantum yields （QY） could reach 80-90% byadjusting the raw materials ratio and concentration of Cd and Se precursors. When thereaction temperature was higher than 250°C, 180 seconds proved to be the idealreaction time. And 5-10min was needed in the case of reaction temperature below250°C.For the ZnCdSe QDS, the inter-diffusion of the Zn and Se atoms was triggered bythe further alloying progress at the elevated temperature, resulting in the alloyedZnCdSe QDs with a small size distribution. The alloying process started to occurslowly at ²70°C and speeded up considerably at further elevated temperatures. Byadjusting the reaction time, the full wavelength at half maximum （FWHM） could becontrolled in the range of 26.5-32.4nm, and the PLQYcould reach 70-75%.（2） The new Fe₃O₄-ZnCdSe MFNCs were prepared by the seed-mediatednucleation and growth method at high temperatures. The influences of the rawmaterials ratio and temperature on structure and properties of the as-preparedFe₃O₄-ZnCdSe MFNCs were discussed. The resulted MFNCs were characterized byUV-vis, PL, HRTEM, XRD and VSM （Vibrating Sample Magnetometer）. It revealedthat the as-prepared Fe₃O₄-ZnCdSe MFNCs possessed heterostructures, which resembled dumbbell. With the increase of the used amounts of Zn precursors, thesizes of Fe₃O₄-ZnCdSe MFNCs first increased up to crest value then decreased. Withthe molar ratio of Cd/Zn ranging from 1/10 to 1/20, the Fe₃O₄-ZnCdSe MFNCsexhibited relatively perfect structure and morphology. When the molar ratio of Cd/Znwas fixed at 1/5, the PL QY could reach the maximum value of 21%. In contrast toFe₃O₄-ZnCdSe MFNCs, the higher PL QY and narrower FWHM were resulted byfurther passivation with ZnS shells.This study will provide important experimental basis for the application of the QDsand MFNCs in the diagnosis of diseases.