Study On Preparation And Characterization Of ZnO Quantum Dots

ZnO is a wide-gap semiconductor with the direct gap of3.3eV and an exciton binding energy of60meV, which make it a good candidate for practical applications in the field of optics, electronics and catalysts. Preparing ZnO quantum dots (QDs) with uniform size and stable structure is of great importance for their practical applications. Surface modification is one of significant methods to obtain ZnO QDs with the uniform size and stable structure. Furthermore, the significant change of ZnO properties can be achieved by suitable surface modifications. Studying the surface modification mechanism of ZnO QDs is beneficial to choose proper ligands and to enhance the knowledge about the surface chemistry of QDs. This thesis focuses on these problems and selects solution chemistry route to ZnO QDs.ZnO QDs were synthesized in alkaline absolute-ethanol solution at mild conditions. At the early stage of ZnO QDs growth, there is a bimodal size distribution of ZnO QDs in the ethanol solvent caused by Ostwald ripening. Prolonging the reaction time or increasing the reaction temperature, the bimodal size distribution of these ZnO QDs can be disappeared owning to the uniform size of ZnO QDs. From the viewpoint of chemical reaction kinetics, the particle size of ZnO QDs can be well designed upon their reaction-controlled crystallization processes in solution. The visible emission of these ZnO QDs can be tunable from blue to yellow. Some key factors such as the reaction time and temperature, the content of OH" and exotic ion Mg2+were comprehensively studied, which finely control the particle size of ZnO QDs and can be easily monitored by UV-vis spectral variations.A two-phase methanol-hexane system is developed to modify the surface of these colloidal ZnO QDs. ZnO QDs surface modification with oleic acid (OA) can be done via the two-phase route with highly stable structure and optical properties as well as dispersity. This two-phase route successfully inhibits ZnO QDs from turning to acetate anion intercalated Zn based hydroxyl double salts. The visible emission intensity of these ZnO QDs keeps unchanged after surface modification. The dispersity of these ZnO QDs after surface modification is improved greatly. These ZnO QDs can have long stability in non-polar solvents after surface modification.With oleylamine and acetic acid as ligands can realize a facile and reversible phase-transfer for luminescent ZnO QDs between methanol and hexane, during which the structure and optical properties of the ZnO QDs are well protected. The Pearson’s hard and soft (Lewis) acid and base principle, together with the principle that similar substances are more likely to be dissolved by each other, describes the current reversible phase-transfer process. One major advantage of this phase-transfer process is that it can be recycled. In addition, the surfaces of the ZnO QDs dispersed in polar solvents can be further modified with poly(vinyl pyrrolidone)(PVP) to achieve aqueous stability. When these ZnO QDs are dispersed in nonpolar solvents they can self-assemble into hydrophobic and transparent luminescent semiconductor films upon evaporation of solvent.