| In this thesis, there are two sections. One is about of the Up-conversion mechanism of Er3+ doped and Yb3+/Er3+ co-doped ferroelectric Ba Ti O3 nanocrystals synthesized by Sol-Gel precursor monomer method. The other is about of modulation of size and upconverison for Na YF4:Yb3+/Tm3+ nanoparticles synthesized by coprecipitation method by hydroxyl carboxylic chelants sodium tartrate(ST), sodium gluconate(GAS) and aminocarboxylic chelant ethylenediamine tetraacetic acid disodium(EDTA). The results are as follows:(1) Dependences of crystal structures and surface morphology on calcination temperature and Ln3+ doped ratio were studied. After calcinating at 900℃ for 2h, the Ba Ti O3: Ln3+ crystals were pure cubic phase with high crystallinity. The doped Ln3+ did not affect the crystal structure of Ba Ti O3 nanocrystals at low doping concentration. The weak XRD pattern of Ba2 Ti O4 appeared at 5% Ln3+ doping concentration, while the Ln3+ doping concentration reached 20%, the products were mixture of Ba2 Ti O4 and Ba Ti O3, and the crystallinity decreases. At low Ln3+ concentration, the size distribution was relative even and average size was about 100 nm. Rodlike microparticles were observed while Ln3+ doping concentration increases.(2) The upconversion mechanism of Er3+ doped and Yb3+/Er3+ co-doped Ba Ti O3 were discussed. Under excitation of 980 nm, it was observed three narrow upconversion emissions around 530 nm, 552 nm and 664 nm, which are origin of 4H11/2â†'4I15/2, 4H3/2â†'4I15/2, and 4F9/2â†'4I15/2 transitions respectively. The strongest green emission was obtained at 0.5% Er3+ concentration for Er3+ single-doped Ba Ti O3. In Yb3+/Er3+ co-doped Ba Ti O3:Ln3+ nanocrystals, the presence of Yb3+ can induce energy back transfer between Yb3+ and Er3+, results in massive Er3+ populate at 4I13/2. Thus the red emission enhanced dramatically while green emission decreases.(3) Size modulation and crystalline phase transformation were compared in Na YF4:Yb3+/Tm3+ nanocrystals synthesized by ST, GAS, EDTA-assisted respectively. The size of Na YF4:Yb3+/Tm3+ nanocrystals decrease from 152 nm to 35 nm as ST/Ln3+ ratio increases from 0 to 4. ST exhibits effective size modulation which is comparable with EDTA, while GAS’s chelation with Ln3+ is much weaker, indicating that not all hydroxyl carboxylic chelants are useful for size modulation. ST-assisted synthesis processes do not prevent the crystalline phase transformation of products. After annealing at 500℃ for 5h under the protection of N2, the crystalline phase of Na YF4:Yb3+/Tm3+ nanocrystals transformed completely from cubic to hexagonal phase, which has higher upconversion efficiency. For products synthesized by GAS or EDTA assisted, cubic phase still exists after same annealing process.(4) The effect of chelants on the upconversion properties of Na YF4:Yb3+/Tm3+ nanocrystal was studied. Under excitation of 980 nm, Na YF4:Yb3+/Tm3+ nanocrystals exhibit strong blue upconversion emission, the optimal Tm3+ doping concentration was 0.5%. The upconversion of Na YF4:Yb3+/Tm3+ nanocrystals increase as the nanoparticle size decrease(the ST/Ln3+ ratio increase). The strongest upconversion was observed at ST/Ln3+=3, while average size of Na YF4:Yb3+/Tm3+ nanocrystal was 38 nm, which is half times and four times stronger than samples assisted by EDTA and GAS at same ratio respectively, two times stronger than samples without chelant. More important, hydroxyl carboxylic chelant ST also takes advantage of non-toxicity, biological degradability, and environmental protection over aminocarboxylic chelant EDTA. Considering that ST has comparable size modulation with EDTA, it can be concluded that ST is a promising chelant for Ln3+, especially in case of using large chelant/Ln3+ ratio to pursue upconversion nanoparticles(UCNPs) with strong upconverison and smaller nanoparticle size... |
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