Synthesis Of Semiconductor Fluorescent Quantum Dots And Regulation Of Blinking Behavior

Semiconductor quantum dots(QDs) are very important optical nanomaterials due to their unique optical properties such as broad excitation spectra, narrow and size-dependent emission profiles, high photoluminescence quantum yield(PL QY), good photostability, and long fluorescence lifetime, etc. So far, QDs have been used in single-particle light source, solar cell, LED light and display, bio-imaging, etc. However, at single particle(molecule) level, QDs show severe fluorescence intermittency(also referred to as blinking) on a wide time scale from millseconds to minutes. QD blinking is an intrinsic drawback for some biological and photoelectric applications that rely on single-particle emission. Currently, some theoretical and experimental studies demonstrate that the blinking behavior of QDs is mainly attributed to nonradiative recombination processes associated with traps at the nanocrystal surface.In order to solve this scientific problem mentioned above, we synthesized fluorescent QDs and developed three new methods in this dissertation for regulating QDs blinking. In detail, alkylthiols were used as surface trap modifiers; the polyphosphazene polymer was introduced as surface passivation layer, and the in situ interfacial alloying approach for obtaining the graded alloying QDs. The main contributions are as follows:(1) We used a step-by-step method to synthesis thiol-modified Cd Se/Cd S QDs by combining inorganic shell and organic alkylthiols as surface ligands. First, Cd Se QDs were prepared via the organometallic-based thermal decomposition method, and Cd Se/Cd S core/shell QDs were synthesized according to the well-established successive ion layer adsorption and reaction(SILAR) method. Then, we systematically investigated the effects of alkylthiols on the blinking of Cd Se/Cd S QDs, and observed that the blinking of thiol-modified Cd Se/Cd S QDs were significantly dependent on the annealing time, the structure and concentration of alkylthiols. The mechanism of the blinking suppression was mainly attributed to the decomposition of alkylthiol and slowly released S2- under high temperature, and then the activated S2- can bind to surface traps of QDs, which induced a secondary growth of the core/shell QDs coupled with surface reconstruction and the efficient suppression of the blinking. In the optimal conditions, we prepared thiol-modified Cd Se/Cd S QDs with ―nonblinking‖ fraction up to 83%(the ―on-time‖ fraction > 99% of the overall observation time). Thiol-modified Cd Se/Cd S QDs possessed high PL QY, narrow and symmetric emission spectra, and excellent optical photostability. The method described here can be used to suppress the blinking of other QDs.(2) We in situ introduced a polyphosphazene shell into Cd Se/Cd S QDs to obtain Cd Se/Cd S/polymer core/shell/shell QDs via the solvothermal method. Firstly, we precisely controlled the growth process of Cd S shell by using a syringe pump, and obtained Cd Se/Cd S QDs with uniform particle size and good monodispersion. And then, three copolymer monomers(dithiothreitol(DTT), phenylenediamine(PDA), and hexamethylenediamine(HDA)) were respectively used to polymerize with hexachlorocyclotriphosphazene(HCCP) and then three polyphosphazene polymers were obtained with cyclotriphosphazene as basic macromolecular backbone. These polyphosphazene polymers were adsorbed to the surface of the nanocrystals with thickness of 0.2-0.5 nm. By controlling the reaction ratios of the activated monomers, we can control the blinking behavior of Cd Se/Cd S/polymer QDs. In the optimal conditions, the percentage of ―non-blinking‖ Cd Se/Cd S/polymer QDs(the ―on-time‖ fraction > 99% of the overall observation time) was up to 78%. The suppression mechanism was attributed to the efficient passivation of QDs surface traps by the sulfydryl and phenyl groups of the polyphosphazene polymers. Our work also proved that QDs surface trap sites can be repaired by the active groups of polymers.(3) We reported a three steps ―one-pot‖ strategy to synthesize(Zn)Cu In S/Zn S core/shell QDs with high PL QY and long lifetime. This new strategy included three steps: firstly, Cu In S QDs with different stoichiometric ratios of cationic precursors were made. Secondly, in situ ion exchange to obtain(Zn)Cu In S alloyed QDs by adding zinc precursors. Finally, Zn S shell were introduced to form(Zn)Cu In S/Zn S core/shell QDs by using alkylthiols as surface ligands and sulfur source. We systematically investigated the PL spectra, Ultraviolet-visible(UV-vis) spectra, PL decay curves and nanocrystal growth rate during the nucleation, alloying and shell growth of QDs. We found a continuous blue shift of the PL and UV-vis spectra during the alloying and shell growth process, and the PL emission maxima reached equilibrium of 530-550 nm. Meantime, the particle size decreased first in the alloying process, and then gradually increased with the introduction of Zn S shell. Most importantly,(Zn)Cu In S/Zn S QDs possessed high PL QY(74%), significantly long fluorescence lifetimes(755 ns), and excellent photostability. We believed that our synthetic strategy could provide methodological guidelines for obtaining other all-solution-processed and multi-component QDs with high luminescent and high uniformity.(4) We systematically investigated the blinking behavior of(Zn)Cu In S/Zn S QDs. To the best of our knowledge, the current studies on blinking behavior mostly focused on Cd Se and Cd Te QDs, and there were very few reports on blinking behavior of Cu In S QDs. We investigated the blinking behavior of(Zn)Cu In S alloying QDs with different stoichiometric ratios of cationic precursors, and(Zn)Cu In S/Zn S QDs with different Zn S shell growth time. We found that the stoichiometric ratios of cationic precursors were very crucial for controlling the single-dot PL emission behavior.(Zn)Cu In S alloyed QDs(Cu:In:Zn stoichiometric ratios of 1:2:3 and 1:4:3) exhibited almost non-blinking behavior.(Zn)Cu In S/Zn S QDs(Cu:In:Zn stoichiometric ratio of 1:2:3) exhibited non-blinking behavior during the growth process of 20 h. A transition from non-blinking to blinking was observed in(Zn)Cu In S/Zn S QDs(Cu:In:Zn stoichiometric ratio of 1:4:3) over the reaction time of 20 h. The suppressed blinking mechanism was mainly attributed to precisely controlling the alloying process of(Zn)Cu In S QDs, and modifying QDs traps from interior to exterior via a step-by-step modification. Non-blinking QDs will enable applications from biology to optoelectronics that were previously hindered by blinking behavior of traditional QDs.