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Synthesis and Characterization of High-quality Near Infrared-emitting Quantum Dots in an Organic Phase or in Water.

Zhao, Haiguang (2011). Synthesis and Characterization of High-quality Near Infrared-emitting Quantum Dots in an Organic Phase or in Water. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 179 p.

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Résumé

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Near infrared (NIR) emitting PbS quantum dots (QDs) have attracted much attention due to their unique size-dependent photophysical properties that are distinctly different from the corresponding bulk material. They are currently exploited for various applications, such as optoelectronics and biological sensors. To have significant impact in practical applications, they are required to show high quantum efficiency and good stability, such as photostability and colloidal stability, depending on targeted applications. It is still a big challenge to synthesize high-quality NIR-emitting QDs stable in an organic phase or in aqueous environments. I firstly developed a simple and greener approach to synthesize high-quality PbS QDs in the organic phase. I investigated the effects of the reaction time, molar ratios of reactants, reaction temperature, capping ligands and purity of ligands and precursors on the growth of PbS QDs and how these synthetic conditions influence their structure and optical properties. Finally, the optimal synthesis conditions for high-quality PbS QDs emitting at varying NIR wavelengths were identified. Such synthesized PbS QDs show a quantum yield (QY) as high as 40% in the organic phase. Aiming to enhance the QY of PbS QDs, the cation exchange approach was used to form PbS/CdS core/shell QDs. With this strategy, I was able to obtain a QY of 67% at optimal shell thickness of 0.7 nm. The shell composition was investigated in detail. As the experimental identification of the shell composition of thin-shell QDs is difficult, experimental data and calculations were combined to give clues. Based on the comparison of band gap versus core size plots of different compositional models, it was found that the thin shell is primarily made of CdS. Furthermore, by developing a two-step cation exchange approach, the previous synthesis barrier was overcome and I achieved PbS/CdS QDs with various shell thickness. Owing to the preparation of a relatively thick shell, not only can the core/shell structure be easily observed by transmission electron microscopy (TEM), the characteristic absorption and emission of CdS can also be observed when shell thickness reaches 1.8 nm. Furthermore, X-ray diffraction (XRD) shows the overall diffraction pattern is basically the same as that of the CdS standard when the thickness of shell reaches 3.6 nm. The thick-shell QDs were further analyzed by performing energy dispersive X-ray spectrometry (EDX) in core and shell regions, respectively. It was found that Pb is absent in the shell region. All of these results consistently suggest that, in thick-shell PbS/CdS QDs, the shell is also made of CdS, instead of ternary PbxCd1-xS alloy. In addition to exhibiting significantly improved QY, importantly, these core/shell structured PbS/CdS QDs also show better photo- and thermal stability than the shell-free PbS QDs. As-synthesized PbS QDs are insoluble in water. In order to disperse them into water, amphiphilic polymers of poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)) (PMAO-PEG) were used as phase transferring agents. I firstly transferred the PbS QDs capped by oleylamine (OLA) ligands from chloroform into water via PMAO-PEG. The PbS QDs lose their photoluminescence (PL) in 5 minutes and change the size distribution from mono to double, attributed to ligand etching together with Ostwald ripening. These QDs further show self-selected size-dependent recovery of the PL with time, which was not reported before. After thorough investigations, it was found that the decrease in the percentage of unpassivated surface atoms during aging explains the PL recovery behavior of the subset of smaller QDs stored in water, which is distinctly different from that of the subset of larger QDs. Realizing a significant role of ligands in the water transfer process, I further investigated the effect of different types of surface ligands on the structure and optical property of water-soluble PbS QDs encapsulated by amphiphilic polymers. Among all the samples, PbS QDs capped with oleic acid (OA)/trioctylphosphine show the highest QY (20% in water) and those capped by OA show the least spectral shift. Overcoating PbS with a robust inorganic shell before water transfer may further improve the properties of PbS nanocrystals in aqueous solutions and minimize their dependence on the original capping ligands. I therefore applied a two-step strategy to synthesize water soluble PbS/CdS QDs. In the first step, I overcoated PbS QDs with CdS in an organic phase as aforementioned and in the second step, I transferred them into water via amphiphilic polymers. The CdS shell around the PbS core maintains the structural integrity of PbS nanocrystals and leads to a significantly higher QY in buffer as compared to that can be achieved with “bare” (without an inorganic shell) PbS QDs. Further improvement may be made by optimizing core/shell structures. By carefully varying the initial size of PbS QDs and finely tuning cation exchange experimental conditions, I am able to synthesize PbS/CdS core/shell QDs with a similar PbS core size of 4.4 ~ 4.5 nm yet different CdS shell thickness from 0.2 to 2.3 nm via a cation exchange approach. This enables me to study the effect of the shell thickness on the optical properties of these NIR emitting PbS/CdS core/shell QDs after their transfer from an organic solvent into water via PMAO-PEG. It was found that the QY of PbS core QDs (~4.5 nm in diameter) dispersed in water firstly increases with the increase of the shell thickness up to ~0.7 nm, reaching the maximum of 33%, due to better surface passivation and then decreases to 1.7% when the shell thickness reaches 2.3 nm. The decline in the QY is due to the formation of new defects with shell deposition. In contrast, as the CdS shell thickness increases, the amplitude of variation of QY, due to water transfer, decreases monotonically from 58% to 42%, because a thicker shell can endow the PbS core better protection from their environments. For the same reason, the photostability of PbS core QDs is steadily enhanced with increasing CdS shell thickness. It is clear that although the defects introduced during relatively thick shell deposition play a fundamental role in the absolute QY, they do not show any overwhelmingly negative effects on the variation of QY with environments and QD photostability. On the other hand, the colloidal stability of QDs in buffers containing different salt concentrations seems not affected by the shell thickness, quite possibly due to the same steric stabilization effect of the amphiphilic polymer in all the samples. Further investigation on a series of core/shell samples with different core size and different shell thickness confirms that ~ 0.7 nm is an optimal shell thickness for the various core sizes investigated herein, consistently yielding the maximum QY and reasonably good photostability.

Type de document: Thèse
Directeur de mémoire/thèse: Ma, Dongling
Co-directeurs de mémoire/thèse: Chaker, Mohamed
Informations complémentaires: Résumé avec symboles
Mots-clés libres: PbS/CdS; quantum dots
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 06 août 2014 20:52
Dernière modification: 16 mars 2016 14:34
URI: http://espace.inrs.ca/id/eprint/2170

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