Synthesis And Optical Properties Of CdSe/CdS Core/Shell Quantum Dots Coated With Thiols

Surface ligands are critical to the optical properties of quantum dots (QDs). The understanding of the role that surface ligand plays is still very limit so far, especially on the relationship between the photoluminescence (PL) properties of QDs and their surface modification. In this thesis, we used high quality CdSe/CdS core/shell QDs with 1-10 monolayers of CdS shell as the model system to study the thickness-dependent effects of thiol ligands on the optical properties of QDs. The QDs with original ligands possessed near unity PL quantum yield and single-exponential PL decay dynamics. Comparing the PL spectra before and after ligand exchange, we systematically and quantitatively studied the mechanism of thiol effect at both ensemble and single dot levels.By optimizing the ligand concentration, pH value, and reaction time, water-soluble CdSe/CdS core/shell QDs were obtained through ligand exchange with mercaptopropionic acid. The CdSe/CdS-MPA QDs possessed high PL quantum yield and high stability in aqueous phase, expanding the application potential of QDs in biomedicine.The surface ligands of CdSe/CdS core/shell QDs were exchanged to 1-dodecanethiol in oil phase under an optimized experimental conditions including ligand concentration, temperature, reaction time, atmosphere and so on. The atmosphere was found to be the most critical factor to the thiol ligand exchange experiments. Oxygen could quench the PL of QDs both in the process of or after the thiol ligand exchange. The optical properties of QDs with thiol ligands were influenced by the thickness of CdS shell. This thickness-dependent effects include differences of PL quantum yield, PL lifetime, PL blinking behavior and the degree of PL quenching by oxygen. The PL of the QDs with thin CdS shells, i.e., less than 6 monolayers of CdS shell, was quenched after thiol ligand exchange. For QDs with medium shell thickness, namely 6-9 monolayers of CdS shell, the PL properties remained after ligand exchange. For the QDs with 10 or more monolayers of CdSe shell, thiol ligand exchange improved the dispersibility and surface modification of the QDs, hence improving their solubility and PL properties.At single-dot level, both original and thiol-coated CdSe/CdS core/shell QDs with 6-10 monolayers of CdS shell were nonblinking under low power excitation. The thiol effects on the optical properties of CdSe/CdS QDs emerged with excitation power increased. The QDs with thiol ligands blinked more seriously than the original QDs did. The PL decay dynamics of both "on" and "dim" states remained after ligand exchange. Systematic studies on thiol effect of QDs with thiol ligand would help to uncover the PL blinking mechanism of QDs.