Aqueous Phase Synthesis Of CdTe And CdZnTe Quantum Dots Based On Electrochemical Method

At present，most of the studies on the synthesis of semiconductor quantum dots（QDs） mainly focused on two aspects, organic and aqueous phase. In the organicsynthesis of high quality quantum dots, the most widely used hot-injection methodrequires high temperature to promote the growth of QDs, furthermore, the rapidinjection of precursors also needs high temperature. Meanwhile, the nucleation andthe quality of the products depend on the injection speed and stirring intensity, andthe amount of products is minor. These factors limit the widespread application ofhot-injection method in the large-scale production of QDs. Compared tohot-injection method, one-pot method has obvious advantages such as theunnecessary of rapid injection of precursors, slow reaction rate, more controllableand repeatable, user-friendly and suitable for large-scale commercial production.However, the as-prepared QDs in organic phase have poor biocompatibility, and themetal organic is highly toxic, explosive and not stable. And thus, the reactionequipments and conditions are rigour normally. In view of these, the successfulsynthesis of water-soluble QDs overcame these drawbacks. Water-soluble QDspossess unique luminescence properties and good biocompatibilities. Its rapidgrowth rate, high photoluminescence quantum yield and relatively convenientpreparation condition make the synthesis of water-soluble QDs more beneficial.In this dissertation, for the purpose of preparing high fluorescenceperformance water-soluble QDs to satisfy the requirement of practical applicationand large-scale commercial production, we present a green and novel method tosynthesis cubic zinc blende CdTe QDs in normal pressure and low temperature.Based on electrochemical route, we prepared water-soluble TGA-capped CdTe andternary CdZnTe alloyed quantum dots, and the effects of the reaction variables onthe growth and the PL QYs were systematically investigated.The mean size of as-prepared QDs was4.5nm. The QDs exhibited highfluorescence QYs about50%and the best QY of67%without any postpreparativetreatment over a broad spectral range of520-600nm, which was superior to theexisting literature. The method provided a new strategy for the large-scalecommercial production of semiconductor QDs with a stronger cost advantage andzero discharge of telluride and its compounds. Based on the experiments, it playeda key role in the preparation of CdTe QDs to change the concentration of Te and Cd precursors, the Cd-SR/HTe ratio, and the TGA/Cd ratio, the Cd/Te ratio and thestructure of stabilizer.When Cd²⁺concentration is low, less Cd²⁺would participate in the nucleationprocess, and thus, more Cd²⁺could participate in crystal growth and accelerate theQDs growth. In the earlier stage, the insufficient surface modification leads to thelow QYs. In the later period, a large number of monomers remained in the highconcentration system promote the surface modification of QDs, which wouldenhance QYs. The TGA/Cd ratio affects the composition of TGA-Cd complexesand free Cd²⁺monomers concentration in the solution, which would play a key rolein the growth of QDs. The Cd/Te ratio determines the partical size of quantum dotsand further influences the wavelength range of fluorescence emission. The higherTe concentration is, the faster the QDs grow. When the relative concentration of Teis low, the surface modification of as-prepared QDs would become more perfectand a higher QY could be obtained. The synthetic CdTe QDs also perform goodstability. Commonly, the prepared samples can be saved more than half a yearwithout any change in the fluorescence properties in air condition.Generally，tuning the optical properties only by changing the particles’ sizecould cause some problems （such as the instability of very small nanoparticles） inmany applications. One solution to the above problems is to employ alloyed QDs.Since the physical and optical properties of alloyed QDs depend on both size andcomposition, it is possible to tune the spectrum by altering componentstoichiometric ratios in alloyed QDs. For instance, the band gap of CdTe and ZnTeis1.45ev and2.25ev respectively, so we can achieve CdZnTe QDs with the bandgap easily tuned from1.45to2.25ev. However, the as-prepared QDs via currentmethods display distinct fluorescence defects, and the preparation condition isrigorous and costly. In this dissertation, we successfully preparedhighly-fluorescent CdZnTe alloyed QDs based on a one-step electrochemical routein aqueous phase, and the fluorescent emission wavelength of the CdZnTe alloyedQDs can be tuned from460to610nm which could be further extended withprolonged refluxing. The highest PL QY of obtained CdZnTe alloyed QDs is70%.Especially in the wavelength range of510-578nm, the overall PL QYs of theas-prepared CdZnTe QDs were above50%. The effects of the reaction variables onthe growth and the optical properties of CdZnTe QDs were investigated in detail.The absorption peaks and PL peaks shifted to longer wavelengths with increasingCd content in CdZnTe alloyed QDs. The TGA/Cd ratio has an effect on theformation of complexes, and thus controlling the free Cd²⁺monomers concentration and growth rate of CdZnTe QDs.