| White light-emitting diodes (LEDs) is considered as a new generation solid lighting source because of their advantages, such as long service life, high efficiency and environmental protection. At present, the mostly method to realize white LED is the phosphor-converted-LED (pc-LED), which coats the yellow light-emitting phosphor, Y3Al5O12:Ce3+(YAG:Ce) on a blue LED chip. Although this type of white LED has been commercialized, it exhibits low luminous efficiency and low color rendering index (CRI) because of its red spectral deficiency. Therefore, to find a fluorescent material with high luminous efficiency and good stability will become a research hotspot in the white light LED. Quantum Dots (QDs) is a new-style nanometer materials exhibiting distinctive photoluminescence (PL) properties due to the quantum confinement effect. Having many advantages over organic dyes, such as broad excitation, color-tunable, resistance to photobleaching and so on. At present, the excellent properties show great promise for use in the fields of biomedicine, light-emitting diodes (LEDs), solar cells, catalysis and other prospects. Most commercial quantum dots are synthesized by the traditional organic metal method, but the synthesis process with large toxicity. In this study, according to the requirements of applications, our research was mainly focused on the greener synthesis route of quantum dots for light-emitting diodes in an aqueous system. The crystalline structure, component, morphology and size, and optical properties of as-prepared QDs were characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), Energy dispersive spectrometer (EDS), photoluminescence spectrum (PL), and UV-Vis absorption spectrum (UV-Vis), respectively. A part of as-prepared quantum dots were used as phosphor for white LED, and the performance of the fabricated W-LED was evaluated. In addition, CdTe/Y nanocomposites were synthesised in an ion exchange method using zeolites Y as hosts. The photocatalytic activities of the CdTe/Y nanocomposites were also investigated systematically. The main contributions were as follows:(1) Cysteamine (CA)-CdTe QDs were successfully synthesized using a one-step aqueous method. XRD and TEM results show that CdTe QDs with CA modification exhibit a zinc-blended crystal structure in a sphere-like shape. The maximum emission wavelength of CdTe quantum dots could be changed by controlling the reaction time, and the results show that the maximum emission peak exhibited an evident red shift with increasing reflux time due to the quantum size effect. The influences of the reflux temperature, the precursor Cd/Te molar ratios and the pH of the original solution on the luminescence property of the obtained CdTe QDs were investigated systematically. An optimization program of synthesis CdTe QDs was obtained:the precursor Cd/Te molar ratio is1:0.05, the pH of the original solution is5.85, and the reflux temperature is100?, with a maximum photoluminescence QY of10.73%.(2) A series of Mn-doped CdTe nanocrystals (NCs) with different concentration of Mn ion were synthesised through a one-step approach in an aqueous medium using thioglycollic acid (TGA) as stabilizer. The obtained Mn-doped CdTe nanocrystals were consistent with the zinc-blended CdTe crystal structure, with approximately spherical aspect of about3.0nm. A red-shift in the emission peak wavelength was observed by doping Mn2+ions into the CdTe QDs to substitute some Cd2+ions. Therefor, the red emission QDs will be more likely obtained by doping Mn2+ions into the CdTe QDs, and red emission QDs is a promising candidate that can be applied in WLEDs.(3) The obtained CdTe QDs using Na2TeO3as Te source, and three different sulfhydryl compound as ligands, namely tiopronin (TP), L-cysteine (Cys) and mercaptosuccinic acid (MSA). The as-synthesis CdTe with different ligands were zinc-blended structure and approximately spherical. In an aqueous medium, the size-distribution and growth rate of CdTe quantum dots (QDs) are shown to depend on the type of ligands. The growth rate of MSA coated QDs are faster than Cys-and TP-coated QDs. The maximum emission wavelength of MSA-CdTe QDs up to650nm, while the TP-CdTe QDs was586nm at the refluxing time was7h. So larger-size QDs can be synthesized more easily when MSA is chosen as ligand. (4) A one-step of blue light emission cysteine-capped ZnxCd(1-x)Te alloy QDs has been accomplished by reacting a mixture of ZnCl2and CdCl2with TeO2and using cysteine as surface-stabilizing agent. Without changing the particle size, the fluorescence emissions of QDs can be determined by controlling the Zn2+/Cd2+molar ratio. An evident blue shift was exhibited on the maximum emission peak with increasing the Zn molar ration in the mixture. The ZnxCd(1-X)Te alloy QDs had a blue emission when the Zn/Cd molar ratio was fixed at70:2. The ZnxCd(1-x)Te alloy QDs surface were modified by growth a thin CdSe semiconductor shell on the outer surface of the ZnCdTe NCs. The yellow emission ZnCdTe/CdSe core-shell nanocrystals were prepared to use for white LED.(5) The RYB "three-band" white LED was fabricated by combining a blue InGaN chip with YAG:Ce and MPA-CdTe QDs used as hybrid phosphors. The WLED showed improved color rendering of light in red with color rendering index (CRI) was increased from62.2to75under20mA forward bias current. A red-light QDs-LED was obtained by encapsulating the red-emiting Cd(1-x)MnxTe onto a blue LED chip, which proved the Cd(1-x)MnxTe QDs available for LED application. Then the white LED was fabricated by combining a blue InGaN chip with YAG:Ce and Cd(1-x)MnxTe QDs used as hybrid phosphors. Because the enhancement of the red spectrum from Cd(1-x)MnxTe QDs can broaden the white emission band, the resultant CRI increased to78at20mA, which is better than single core CdTe QDs. The other white LED consisted of ZnCdTe/CdSe QDs as the phosphor with a blue InGaN chip as the excitation source and showed a low color rendering index (CRI) of51.1caused by the lack of green emissions under a working current of20mA. White LED with a high CRI was obtained with the addition of Ca8Mg(SiO4)4Cl2:Eu2+phosphor, and the CRI of the hybridphosphor white LED was improved to88.2when the mass ratio of Ca8Mg(SiO4)4Cl2:Eu2+phosphor and ZnCdTe/CdSe QDs was1:1.5.(6) The CdTe/Y nanocomposites were successful synthesized through the ion exchange method, using zeolite Y as host material. The influences of the experimental conditions on photocatalytic activity of the CdTe/Y were investigated systematically. The results showed that the framework structure of zeolite Y was not destroyed and that CdTe QDs were successfully introduced into zeolite Y. Approximately87.7%degradation rate was achieved within30min of irradiation of20mg·L-1methyl blue solution in the presence of CdTe/Y under UV light. |
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