Controllable Synthesis And Water-Solublization Of CdSe-Based Semiconductor Quantum Dots

As a result of the quantum confinement at nano-sized scale, semiconductor quantum dots (QDs) have size-dependent optical properties, which endow them with potential for application in electronic devise such as light-emitting device, solar sells, and in biological science such as bio-imaging and bio-probing. It is very important that QDs should have a good optical property and stability, and requirement of water-solubility is also acclaimed in the bio-related and chemical analysis applications. Colloidal chemistry conducted in water or organic solvent is common used method to synthesize semiconductor QDs. Being carried out at relatively low temperature, the samples synthesized in aqueous solution normally have both poor crystal structure and size distribution, and thus their properties are not desirable. With the great advances in synthesis of colloidal nanoctystals (NCs), QDs synthesized at a high reaction temperature in organic solvent which have a higher boiling point exhibit a narrow distribution of particle size, high crystallinity, and wide range of fluorescence emission. However, due to their hydrophobic surface capping agent such as oleic acid (OA), these high quality QDs has no intrinsic solubility in water media, which greatly limits their applications conducted in aqueous phase. This work is emerging to fabricate water-soluble CdSe-based QDs with prompt optical property and robust water-solubility and colloidal stability. Firstly organic QDs with higher quality were synthesized at higher temperature by optimizing the reacting condition and simplifying the synthesis process. And then, the resulting QDs were water-solublized using ligand exchange process. Comparing to previous works in the related field, the advantage of this work is that an efficient and simplified synthesis and water-phase transfer process for higher quality products is provided. Following are the specification of the works and results:Firstly, the approach for synthesizing QDs was investigated. QDs capped with various and sole agents were synthesized with hot-injecting and heating-up method. And then an inorganic shell of ZnS was coated to the CdSe to enhance the optical property and stability of the core. Meanwhile, using OA, oleyamine and mixed of them as the reacting solvent, CdSe NCs was fabricated under control. The influence of reacting temperature and time, dosage ratio and concentration of the precursors and the ration of these two solvent on the growth process, properties, final morphology and assemble behavior of the products was investigated.Secondly, by simplifying the synthesis process, "one pot" method was used to synthesis CdSe based QDs with hybrid core shell structures which have a interface with gradient chemical distribution. The emitting wavelength of the resulting samples can be adjusted from green to red by change the dosage ratio of the precursors and the times of injecting action. Thanks to the interface gradient structure, the problem of the mismatch between the core and shell was solved and thus the resulting QDs has higher quality with little crystal defects.Thirdly, the above mentioned QDs were transferred to water via ligand exchange method. Mercaptopropionic acid (MPA) and mercaptoethylamine (MEA) have been used respectively as the surface substitutes to obtain water-soluble quantum dots with negative and positive surface charge. The method achieved highly efficient phase transfer (nearly 100%) of quantum dots from non-polar media to water. The resulting water soluble QDs exhibit a remarkably colloidal stability and have a relatively small hydrodynamic diameter with a maximum quantum yield up to 30%, which could benefit the their applications carried out in aqueous media, such as bio-imaging, analysis sensors and photo-catalysis.