Study On Synthesis, Modification And Potential Application Of The Oil-soluble Ⅱ-Ⅵ Group Quantum Dots

As a type of new nanomaterials, Quantum dots (QDs) have very uniqueoptical properties. And they also showed great potential in biomedical andoptoelectronic applications in recent research fields. In this thesis,according to the requirements of applications, our reseach was mainlyfocused on the efficient and “greener” synthesis route of CdSe andCdSeTe core/multi-shell quantum dots and doped quantum dots of Ⅱ-Ⅵgroup in the “cheap” paraffin liquid synthetic system, modification ofquantum dots with silica and applications of quantum dots on whitelight-emitting diodes (WLEDs). The main contributions were as follows:(1). Highly-fluorescent CdSe/CdS/ZnS core/multi-shell quantum dotswere successfully synthesized via a phosphine-free and continual precursorinjection method in paraffin liquid. The photoluminescence of as-preparedcubic zinc blended QDs were significantly improved compared to theCdSe core dots, and showed high quantum yield (PLQY) up to70%withexcellent optical stability. Besides, such continual precursor injectionmethod was also introduced to synthesis core/multi-shell tructure of thenear-infared CdSeTe QDs. Compared to the CdSeTe core QDs, theas-prepared CdSeTe/CdS/ZnS core/multi-shell QDs with tunable emissionwavelength between625nm and850nm and PLQY up to60%showedenhanced fluorescence as well as improved stabilities, which greatlyimproved the optical properties of the near-infrared QDs at long wavelength.(2). The monodispersed red-emitting CdS:Cu QDs were successfullysynthesized via a one-pot method in paraffin liquid. The as-prepared dopedQDs presented high-luminescence emission with PLQY of18%~30%inthe red region and strong excitation at near-UV and blue light, andsimultaneously showed no absorption to green-yellow light. TheCu-dopant emission could be simply tuned by the reaction temperature.After a ZnS layer was coated around CdS:Cu QDs, the resultantCdS:Cu/ZnS core/shell QDs displayed greatly improved PLQY up to40%~50%as well as better photochemical and thermal stabilities.Meanwhile, Mn-doped ZnSe QDs were also successfully synthesized inliquid paraffin by the “nuclei doping” process. The as-prepared Mn:ZnSeQDs showed highly-fluorescent Mn-dopant emission at580nm withPLQY up to26%. The fluorescence of Mn:ZnSe QDs could be tuned bymulti-injection of ZnSe precursors and controlling growth time of MnSecore. Besides, the synthesis route of Cu-doped ZnSe QDs in liquid paraffinwas also tentatively explored. The Cu:ZnSe QDs with tunable emissionbetween blue and green region were obtained via a phosphine-free method.(3). The fluorescent quenching during incorporating QDs into silicamatrix (QD@silica) was improved by optimizing key factor inprepparation together with an efficient post-treatment. Firstly, theQD@silica prepared with proper concentration of methylamine solution ascatalyst possessed uniform size distribution and could keep as much as70%of initial fluorescent intensity compared to other two commonly usedcatalysts, ammonia and dimathylamine. And other factors on preparationwere also studied to further optimize synthesis process. Moreover, a novelpost-treatment by combining thermal annealing together with UVphotoactivation process was carried out to further improve the fluorescentproperty of QD@silica. The nanoparticles obtained after the post-treatmentshowed steadily enhanced PLQY up to56%with improved stability. And the mechanism of fluorescent enhancment was also discussed.(4). In order to solve the disadvantage caused by the fluorescentmaterials concentrated near the chip in traditional LED encapsulation, wedirectly dispersed QDs into the epoxy resin to form a new type ofcomposites before encapsulation with excellent stability. The compositescould be used as both light conversion and encapsulation materials. Ared-light QDs-LED was obtained by encapsulating the red-emiting CdSecore/double shell QDs/eopxy composites onto a blue LED chip, whichproved the composites available for LED application. Then an RYGB“four-band” QDs-WLED was fabricated by combing the compositescontaining red-,yellow-and green-emitting QDs with a blue LED chip.The WLED showed improved color rendering of light in red andyellow-green region with color rendering index (CRI) Ra up to88, Tc of3865K and luminous efficiency of32lm/W under100mA forward biascurrent. Another fabricated WLED based on blue LED chip, YAG:Cephosphor and red-emitting CdS:Cu/ZnS QDs exhibited Ra up to86, and aluminous efficiency of37.43lm/W under120mA forward bias current.The color rensering of red and deep red regions was significantly improvedwithout re-absorption between phosphor of YAG:Ce and CdS:Cu/ZnS QDsdue to the large Stokes shift of the doped dots.