| Luminescent semiconductor nanocrystals, or quantum dots (QDs) have become one of the most attractive areas of current research due to their potential applications light-emitting devices (LEDs), lasers, sensors, solar cells and biomedical labeling. Among the various semiconductor materials, CdSe and ZnSe have been the most extensively studied system. High quality CdSe or ZnSe QDs could be achieved via organometallic routes, the as-prepared QDs were high luminescent, while this method usually needed high temperature and the QDs possessed poor water-solubility due to the hydrophobic surface, which may not be suitable for biological applications. In contrast, aqueous synthesis route has attracted more attention in recent years due the moderate reaction temperature,"green" synthesis procedure and water-soluble product, which is a promising alternative route to prepare QDs. Yet, the QDs synthesized in aqueous system suffered from low photoluminescence quantum yield, instability of luminescent properties and numerous surface defects. Forming alloyed or core-shell structure is an effective method to overcome these drawbacks. Moreover, many water-soluble foreign ions from the reagents are usually introduced into the system in the aqueous synthesis procedure, which will affect the properties of QDs. Based on these problem, we did two part of the work as followed:1. Characteration of water-soluble CdZnSe alloyed quantum dots synthesized via cation exchange reactionThe water-soluble CdZnSe alloyed QDs have been successfully prepared via cation exchange reaction in aqueous system. The as-prepared alloyed QDs were characterized by X-ray powder diffraction, HRTEM, UV-Vis absorption spectroscopy, photoluminescent spectroscopy. The results show that these nanocrystals have high crystallinity and small particle size of4-5nm, and it is demonstrated that the heterogeneous nucleation was avoided while growing compared to conventional synthesis of alloyed QDs. Moreover, the CdZnSe alloyed QDs exhibit good luminescence properties such as favorable photoluminescent tunability (from430to480nm), and high PL quantum yield (13%), which is supposed to be caused by the formation of quasi-core-shell structure and the elimination of surface defects.2. Effects of Na+ions on Fluorescence properties of ZnSe Quantum Dots synthesized in Aqueous SystemIn this work, the effects of Na+ions on the properties of ZnSe QDs were studied. The ZnSe QDs were synthesized in aqueous system by introducing excess Na+ions, which were characterized by Photoluminescence (PL), PL decay curves, UV-vis absorption spectrum (ABs), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Results indicate that the particle size of ZnSe is about4nm when Na+ions are rich in the synthesis system, which is bigger than that of ZnSe (about2nm) synthesized in the solution without Na+ions; a special emission peak around590nm was observed when the concentration of Na+ions is high enough in the reaction solution, such emission can be attributed to the selenium vacancies (SVs). Furthermore, we also studied the effects of K+, Cl", OAc-, F-on the properties of ZnSe QDs.3. Synthesis of Water-soluble Fluorescent Carbon Dots Derived from Paper Ash and its application in biomedicineHighly photoluminescent carbon dots with a PL quantum yield of9.3%have been prepared via simple one-step synthesis route using paper as a novel carbon source. The as-prepared carbon dots have good water solubility, small particle size and acceptable fluorescence lifetime. Besides, due to high photostability and biocompatibility, the obtained carbon dots are demonstrated as excellent probes in cellular imaging.4. Green Synthesis of Water-soluble Fluorescent Carbon dots using Renewable Carbon Source and their applicationsFluorescent carbon dots (CDs) have been prepared via a simple hydrothermal method using green, eco-friendly wheat, rice and corn powder as carbon sources. HRTEM, SAED and XRD indicate that the obtained CDs with average diameters of20,16and3nm have poor crystalline nature. The resulting CDs exhibit excellent water-solubility due to the presence of large amount of oxygenous functional groups, which were confirmed by FTIR and XPS. The CDs show a typical excitation wavelength dependent PL behavior, and the PLQYs of these CDs are7.3%,4.8%and6.7%, respectively. Fairly low toxicity indicates the potential applications of biolabelling. Furthermore, the CDs using corn as carbon source are successfully used as fluorescent sensor for detecting Cu2+... |
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