Synthesis of Colloidal Semiconductor Quantum Dots: Gradient Alloy Core and Tunable Surface Composition.

摘要

Being versatile nano-materials, quantum dots (QDs) show their value by taking advantage of the unique optical properties and size-tunable emission and are widely applied in many fields. Controllable and reproducible methods as well as reactive precursors are much in demand to render quantum dots better controlled materials with designable properties in potential applications.;The photoluminescence intermittency (blinking) in a single QD is a distinct nature of the material and has become a major limit when applying QDs in biological labeling. One possible reason for blinking phenomena in single nanocrystal is Auger recombination (non-radiative decay with presence of trion). The gradient structure of alloy core (and gradually changed potential wall) may provide a soft confinement to electrons and holes, making Auger recombination inefficient. In this thesis, two synthetic methods were reported to build gradient alloy CdxZn1-xSe QDs cores: alternative hot injection and intra particle diffusion. The hot injection of Zn and Se precursors alternatively to CdSe core was difficult to control and reproduce due to ZnSe nuclei formation at high temperature. Annealing CdSe/ZnSe QDs at high temperature to trigger intra particle diffusion to form CdxZn1-xSe alloy cores was much controllable and reproducible. PL intensity time traces of a Cd xZn1-xSe batch at 280 °C diffusion confirmed to have half blinkers and half nonblinkers. XRD results confirmed the alloy composition of CdxZn1-xSe core and EELS indicated the composition structure of CdxZn1-xSe QDs being Cd rich in the inner core and Zn rich on the outer shell.;Another approach to optimize an easily operated synthetic method is to employ more reactive precursors that allow lower reaction temperature and time. Secondary phosphine sulfide (diphenylphosphine sulfide, DPP-S) was investigated in CdS QD synthesis to replace conventional tertiary phosphine based anion precursors (trioctylphosphine, TOP). Successive ionic adsorption and reaction (SILAR) was applied on CdS cores to make controllable size and surface composition of nanocrystals with Cd or S termination. X-ray photoelectron spectroscopy (XPS) confirmed that there were almost half cadmium and half sulfur on CdS core seeds, 87% sulfur for S-terminated CdS QDs and 91% cadmium for Cd-terminated CdS QDs.;The band edge emission of S-terminated CdS QDs was observed to be quenched completely and then recovered after another layer of Cd added onto the surface. Density of electronic states calculation showed that mid-gap states formed in S-terminated QDs can efficiently provide non-radiative recombination pathways while well-defined band gap remained in Cd-terminated QDs. Geometry models revealed that sulfur atoms on the surface tend to form few S-S bonds and create flaws such as dangling bonds on the surface of QDs while Cd atoms are capable of producing continuous structures even in highly nonstoichiometric clusters.;Also, we reported the investigation of secondary phosphine sulfide as an anion source in SILAR shelling process at lower temperature and less reaction time than conventional SILAR reaction. Repeated trials showed the reproducibility of the shelling method including improved PL of QDs (∼20 fold after shelled) and well-maintained FWHM (25-30 nm).