The Synthetic Control Of Optical Monodisperse CdSe/CdS Core/shell Quantum Dots

In the past 20 years,synthetic chemistry of colloidal quantum dots(QDs)have advanced drastically.Specifically,control of size,shape,and size/shape distribution of QDs is mostly a solved problem.However,as potential emissive materials,the key parameter for QDs should be their emissive properties.On top of control of size and shape,monodispered emissive properties should be the ultimate goal for synthetic chemistry of colloidal QDs,which includes 100%photoluminescence(PL)quantum yield,ensemble PL peak width close to the intrinsic value,mono-exponential PL decay dynamics,and non-blinking PL of single dot.At present,emissive monodisperse QDs have not yet been reported in literature.This thesis aims to reinvent synthetic chemistry of CdSe/CdS core/shell QDs to promote them to be emissive monodisperse.The research is divided into two consecutive steps.In the first step,a new synthetic strategy for CdSe core nanocrystals is studied in order to control the surface configuration of the nanocrystals,which is the essential for the final goal.In the second step,synthetic techniques established in the first step are extended to synthesis of the core/shell nanocrystals,which finally realize the goal.The new synthetic strategy is called layer-by-layer growth borrowed from growth of bulk crystals.This strategy would result in atomically flat surfaces of crystals.Because of their extremely large surface to volume ratio,layer-by-layer growth has never been realized for synthesis of colloidal nanocrystals.Key parameters for the growth strategy are explored systematically for synthesis of CdSe nanocrystals,which results in successful realization of this new strategy for synthesis of monodisperse CdSe nanocrystals in hexahedral morphology.Growth is confirmed to occur in layer-by-layer mode.Detailed characterization reveals that all hexahedrons are terminated with three(100),one(110),and two(111)facets.The full-width-of-half-maximum(fwhm)of the ensemble PL for the hexahedral CdSe QDs at room temperature is reproducibly in the range between 50 and 70 meV,which is significantly narrower than those reported in literature and comparable to the intrinsic values for the CdSe QDs in the size range.Consistent with the layer-by-layer growth,the hexahedral CdSe nanocrystals can be fully converted to monodisperse spherical ones through intra-particle ripening by activating ligand dynamics.The activation energy of the shape conversion is identified to be 160±5 kJ/mol regardless of the nature of ligands,concentration of the ligands,the chain length of the ligands,and the particle concentration.In the second step,the new synthetic strategy are extended for synthesis of monodisperse CdSe/CdS core/shell QDs with either hexahedral or spherical shape.In addition to the size/shape control and narrow PL peak width,core/shell structures allow us to reach the other requirements for monodisperse emissive properties.PL quantum yield of the resulting CdSe/CdS core/shell QDs in either shape is determined to be near unity for the QDs 1-8 monolayers of the CdS shells.Transient PL spectroscopy reveals that the PL decay dynamics of the core/shell QDs is mono-exponential and depends on both the core size and shell thickness.Single molecular spectroscopy confirms that the CdSe/CdS core/shell QDs with the shell thickness beyond four monolayers of the CdS shells are nearly non-blinking.The peak width of ensemble PL and single-dot PL of the core/shell QDs is found to be nearly the same(±5 meV).The new synthetic strategy allows synthesis of monodisperse CdSe/CdS core/shell QDs with 3-7.5 nm core sizes and shell thickness between 1-8 monolayers of CdS shells.This offers opportunity to study both core size and shell thickness dependence of optical properties of the core/shell QDs.Furthermore,the new strategy yields the core/shell nanocrystals with either carboxylate or amine surface ligands.Single molecular spectroscopy measurements reveal that,though both types of ligands do not affect PL quantum yields,they do affect other emissive properties and stability at single-do level.