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Synthesis and Characterization of MSexS1-x（M=Cd, Zn） Ternary Compound and InP Quantum Dots

Author: WangBinBin
Tutor: JiangYang
School: Hefei University of Technology
Course: Materials Physics and Chemistry
Keywords: CdSexS1-x InP Ternary alloyed quantum dots Photoluminescentquantum yield (PL QYs)
CLC: O611.4
Type: Master's thesis
Year: 2012
Downloads: 34
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Abstract


In the past several decades, many research efforts have focused on thesynthesis and characterization of colloidal luminescent semiconductor nanocrystals（also called quantum dots, QDs）. In particular, binary II-VI group semiconductorQDs are of tremendous interest to researchers due to their unique optoelectronicproperties which could be applied in light-emitting devices （LEDs）, lasers, solarcells, biological fluorescence labeling, etc. In general, the optoelectronic propertiesmainly depend on two main factors, namely, the size and the composition.Changing the size of semiconductor QDs is a useful method commonly used foradjusting their band gap energy through quantum size effect. However, thesize-dependent optoelectronic properties of semiconductor QDs may limit theirapplication in some cases. For example, very small QDs are desirable for in-vivoimaging, yet multiplexing experiments require a range of fluorescence colors thatcannot be achieved only with small-size QDs. One solution to the above problemsis to employ alloyed QDs. Since the physical and optical properties of alloyed QDsdepend on both size and composition, it is possible to tune the spectrum by alteringcomponent stoichiometric ratios in alloyed QDs while maintaining a small size.It’s important to prepare high-quality alloyed quantum dots by developingchemical synthesis methods, thus it is possible to obtain various nanodevices.Mainly, hot-injection based approaches are documented to produce II-VI groupternary alloyed quantum dots. This synthetic route involves toxic, costly phosphineserving as solvent and ligand. And it is necessary to effectively separate thenucleation and growth processes to obtain a narrow size distribution forhigh-quality QDs. It is, therefore, strongly required that the hot-injection ofprecursor solution is performed swiftly. In addition, strong stirring is mandatory forefficient mass transfer. The realization of these operations is not easy, sohot-injection approaches have poor synthetic reproducibility and are not suitablefor large-scale production.Wide band-gap Ⅲ-Ⅴ group compounds have attracted many researchers’interest. Especially, the InP quantum dots have more significant applicant value inthe field of solar cells, due to the bigger Bohr exciton radius and the more distinct quantum confinement effect than Ⅱ-Ⅵ group compounds. However, thedisadvantages lie in the poor chemical stability, hard manipulation and dangerousphosphine/phosphine alkyl during the synthesis of InP quantum dots. Looking fornew sources of phosphorus to replace the existing （TMS）3P and PH3, meanwhile,optimizing synthetic parameters will make InP QDs to be in large-scale productionand wide application.In this dissertation, we have obtained the results as listed as following:（1） Composition-tunable ternary CdSexS1-xquantum dots (shortened form, CdSexS1-xQDs) with different emission colors were prepared via a non-injection, one-pot approach ina green N-oleoylmorpholine solvent for the first time, with air-stable compounds cadmiumoxide （CdO）, selenium （Se） powder, and sulfur （S） powder as Cd, Se and S sources,respectively. The synthesis of ternary alloyed CdSexS1-xQDs with homogeneouscomposition were performed by mixing cadmium oleate solution obtained from the reactionbetween CdO and oleic acid, and the premixed solution containing Se and S inN-oleoylmorpholine solvent. By controlling the reaction temperature （pre-mixture with1hat120°C, growth at230°C） and the heating rate （from120°C to230°C under10°C/min）,we can prepare CdSexS1-x（0xS1-xQDs were characterized through UV-visible absorption spectrophotometer andfluorescence spectrophotometer, and they exhibited well-resolved absorption feature anddominant band-edge emission with a moderate size distribution （the full width at halfmaximum （FWHM） was30-50nm）. Keeping the size at a steady value, thephotoluminescence emission range can cover the460-650nm visible region, and themaximum photoluminescent （PL） quantum yield （QY） was as high as60%. Thehigh-resolution transmission electron microscopy （HRTEM） images and powder X-raydiffraction （XRD） patterns suggested that the as-prepared ternary QDs possessed highcrystalline, a cubic zinc-blende structure with about5.2nm size.（2） This work intended to develop synthetic approaches for ZnSexS1-xnanocrystalsusing greener chemicals at elevated temperatures by the introduction of “activation ofprecursors” and “identical growth temperatures”. This synthetic method based on thermaldecomposition has the advantage lies in easily obtaining nanometer-sized particle and fastreaction rate. That is conducive to synthesize ZnSexS1-xQDs with small exciton Bohr radius.Monodisperse zinc-blende-structure ZnSexS1-xQDs were prepared in octadecene solvent, inwhich the zinc fatty acid salt was used as the zinc precursor. The resulting ternary QDshave the absorption range of240nm-430nm and the fluorescence emission range of280-470nm with the full width at half maximum （FWHM） of30nm. Through changing theproportion of two anionic, we could get the composition-dependent ZnSexS1-xternary quantum dots emitting in the blue/UV range, meanwhile the nanocrystals maintaining at asmall size.（3） The InP quantum dots （QDs） were synthesized via a colloidal chemical methodwith white phosphorus （P4）, indium acetate （In（Ac）3）, stearic acid and octadecene （ODE） asphosphorus source, indium source, surfactant and solvent, respectively. The structure, sizeand shape of the quantum dots were analyzed by XRD and TEM. The resulting InP QDswere also characterized by UV-Visible absorption and fluorescence spectroscopy. The titlematerial exhibits well-resolved absorption and emission properties. Meanwhile, the InPQDs emit at415～517nm in the electromagnetic spectrum showing obvious quantum sizeeffect.

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CLC: > Mathematical sciences and chemical > Chemistry > Inorganic Chemistry > Chemical elements and inorganic compounds > Inorganic synthetic chemistry
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