Aqueous Synthesis And Surface Modification Of ?-VI Quantum Dots Based On Ligand Control

Quantum dots (QDs) are semiconductor nanomaterials smaller than their Bohr radius with excitons confined in three dimensions. QDs are also called semiconductor nanocrystals. Compared to their bulk materials, QDs exhibit size-dependent optoelectronic properties. There are many advantages for QDs, such as tunable photoluminescence emission, narrow full width at half maximum (FWHM), high photoluminescence quantum yield (PLQY), excellent photostability. QDs can be used to build light emitting diodes (LED) and novel type photovoltaic cells. Also, QDs can be utilized as bright fluorescent probes and have wide applications in bio-analysis and bio-imaging.As the fundamental of various applications, the material preparation of QDs has been studied as a focus. Generally, there are two types of method to synthesize QDs including organic phase and aqueous phase synthesis. Although high quality QDs can be prepared by using the former, the reaction conditions are critical and the precursors are often very toxic. As a greener alternative, aqueous phase method uses water soluble precursors instead and the reaction conditions are mild, which is easy to manipulate. Though aqueous method has a large numbers of virtues and has gained much success, plenty of issues still exist compared with the well developed organic phase method. For example, the commonly used ligands in aqueous phase synthesis are only thiols, which limit the further understanding of the growth mechanism; the quality of the as-prepared QDs by aqueous method, especially the PLQY and the stability should be improved.The significant differences between nanomaterials and their bulk state are the size-dependent properties. Due to the tiny geometric size, considerable amount of atoms located on the surface of the QDs makes the surface effect cannot be neglected. Since the bonding of the surface atoms are unsaturated, ligands are needed to bond with these atoms and made of the essential parts of the QDs. Ligands significantly influence the material preparation and optoelectronic properties by controlling the exchange of matter and energy between the surface and environment. So the ligand is a key factor for QDs research. First, the absorption-deabsorption dynamics on the surface of the QDs directly influences the growth of the QDs; Second, the electronic structures of the QDs are strongly influenced by the ligands coordinated to the surface atoms; Third, the solubility and stability of the QDs are provided by the ligands absorbed on the surface; Fourth, thiols on the surface can supply conjugation sites for biomolecules and have impact on the biocompatibility and toxicity.Due to the crucial effect of the ligands, in this thesis novel ligands were designed and synthesized to render the growth of the QDs, new surface modification techniques were developed to improve the quality of the QDs synthesized in aqueous solutions. The main contents are as follows:1. Using dithiocarbamate derivatives (DTCs) as novel ligands to tune the synthesis of CdTe and other II-VI QDsBy reacting relative amines with carbon disulfide, DTC derivatives such as proline dithiocarbamate (ProDTC) were synthesized. CdTe QDs grew fast when ProDTC was used as ligand even under low temperature(30-50?). Although the PLQY of the as prepared QDs was low, after adding mercaptopropionic acid to exchange the original ProDTC ligands the PLQY was improved up to 50%. Because the stability constant of DTC-Cd is much smaller than that of MPA-Cd, the activation energy of DTC-Cd is lower than the later, which causes larger growth rate. The DTC ligands were also used to synthesize CdSe and CdS nanocrystals under the similar conditions. Using DTC as ligands to synthesize?-?QDs is a facile, green, and energy-saving route, which is especially useful for large-scale industrial production.2. Using thiol functionalized poly(acrylic acid) as multidentate ligand to tune the growth and photoluminescence of CdTe QDsBy reacting cysteamine with poly(acrylic acid), multidentate ligand PAA-SH was synthesized. With the synergistic effect of multiple thiols of ligand, the affinity of PAA-SH to QDs and the activation energy to grow CdTe QDs became much stronger for PAA-SH than those for the single thiol ligands. Using PAA-SH as ligands CdTe QDs were grown in aqueous solution with microwave irradiation. Without any post-treatment, the PLQY of the QDs reached as high as 75%, which was higher than those grown by traditional aqueous synthesis. Also, the hydrodynamic diameter of the PAA-SH coated CdTe QDs was around 10 nm, which was about half of the those synthesized via organic phase method and encapsulated within polymers or SiO2. The CdTe QDs capped with PAA-SH showed much higher photostability than the ones stabilized with mono-thiol ligands. As novel multidentate ligands, PAA-SH with synergistic effect had been used to successfully grow highly bright and stable CdTe QDs. Since the unique functionality of multidentate ligand to tune the growth and property of QDs originating from their molecular structure but not the components of the materials synthesized, the multidentate ligands may also be used to synthesize other nanomaterials in aqueous solution.3. Developping novel surface modification techniques by combing ligand exchange and amphiphilic polymer encapsulation for QDs synthesized in aqueous phaseSo far the surface modification techniques for QDs synthesized in organic phase have been developed well, however, in aqueous phase only SiO2 encapsulation can be used which often causes decrease of photoluminescence and low yield. In order to improve the quality of the aqueous phase synthesized QDs, we designed one-pot and in situ polymer encapsulating technique based on ligand exchange. In the encapsulation process, amphiphilic polymer PAA-ODA was first synthesized by reacting octadecylamine with poly(acrylic acid). The polymer was used as the form of micelles in aqueous solution. The hydrophilic ligands of CdTe/CdS QDs prepared in aqueous phase were exchanged with hydrophobic 1-dodecanethiol and the hydrophobic QDs were captured by polymer micelles to form the polymer protected QDs. By decreasing the amount of the PAA-ODA, the morphology of polymer coated QDs changed from single QD containing micelles to multiple QDs containing nanobeads. After the polymer modification the PLQY of the QDs could be enhanced by 50% because the existence of rich thiol ligands in the micelles reduced the surface defects. Owing to the thick hydrophobic polymer layer, both the diffusion of molecules or ions to QDs and the diffusion of the oxidized product out of QDs were constrained, which improved the photostability significantly. In situ amphiphilic polymer encapsulation is a novel and effective surface modification technique for QDs prepared in aqueous phase, which not only significantly increases the PLQY but also improves the photostability of the QDs. Moreover this new technique also has potential use for other nanomaterials prepared in aqueous solution.