Synthesis Of CdSe QDs Via Aqueous-phase And Their Optical Behaviors

In the past decade, zero dimensional semiconductor nanomaterials have attractedgreat interest owing to the unique optical and electrical properties, and have been usedin solar cells, light-emitting diodes and biological imaging. The synthesized QDs fromorganic phase approach are insoluble in an aqueous solution and therefore are notcompatible with the biological system while the aqueous synthesis is reagent-effective,less toxic and more reproducible which improves the water-stability and biologicalcompatibility. So we synthesize CdSe quantum dots in water phase.First, the CdSe quantum dots (QDs) were synthesized at various temperatures inaqueous solution. The as-synthesized QDs were characterized by X-ray diffraction(XRD) method and transmission electron microscopy (TEM), and their opticalproperties were assessed via ultraviolet-visible (UV-Vis) absorption spectrum andfluorescence spectrum. The results showed that with the increase of the temperaturefrom room temperature to90oC, the band gap of the QDs linearly decreased,corresponding to an essential increase of particle size as well as variable spectralbehaviors. In particular, the starting temperature had an important effect on the QDssynthesis and their optical properties. From a viewpoint of wide controls offluorescence color and intensity, a reaction temperature range of17~90oC wasappropriate for the synthesis of the CdSe QDs.Second, the nucleation and growth kinetics in water-soluble CdSe quantum dots(QDs) formation was characterized by measurements of their optical properties ofultraviolet-visible (UV-Vis) and fluorescence spectra. Two kinetic parameters ofreaction temperature and time were practiced over wide ranges of20~90oC and0~12h. Our investigations in this study showed that with a rather low reactiontemperature, for instance20oC, the nucleation process could be well proceeded,resulting relatively small particle size with narrow size distribution. With a reactiontemperature range of20~90oC, the growing up of the nanocrystals was mainly completed within1h. The reaction temperature virtually affected the kinetic featuresof the CdSe QDs. The fluorescence measurements gave largest variations of thefluorescence features for the70oC samples, endowing the QDs with widest range offluorescence properties. Thus, from a viewpoint of controls of fluorescence color andintensity over a wide range, a reaction temperature of70oC was appropriate for thesynthesis of the CdSe QDs.Third, comparative study was carried out on the CdSe QDs synthesized fromwater and ethanol. In comparison with those from water, the CdSe QDs from ethanolshowed super PL with high brightness. Detailed characterization gave the onlydifference of large agglomerates presented in the QDs from ethanol. The TEM andHRTEM observations revealed a tri-level microstructure for the QDs from ethanolwhile in the case of those from water it was bi-level. The CdSe QDs from ethanolshowed super PL with high brightness because its surface coated more stabilizersTGA.Last, we studied the core/shell structure of the CdSe QDs through the use ofdifferent amount of Zn and S for cladding. It was found that small amount of Zn and Swas not very suitable for cladding, and the fluorescence intensity of the QDs was notobviously improved. When the amount was large, too much cladding could also affectthe fluorescence emission. A molar ratio of Cd:Se:Zn:S=2:1:5:5was appropriate. Wealso studied the different ratio of Cd to Se. The results showed that excess Cd wasgood for the synthesis of the CdSe QDs. When the ratio of Cd to Se was6:1, thefluorescence performance was best. Also, a molar ratio of TGA to Cd of1:1wasappropriate. The use of L-cysteine as ligand instead of TGA could also improve thefluorescence performance of the QDs.