| Rare earth (RE) ions doped upconversion (UC) phosphors have received more and more attention due to their novel optical properties and wide applications in solid-state laser, displays, biological imaging, solar cells, and optical storages, etc. Among the various lanthanise ions studied for UC luminescence, Ho3+ has attracted considerable attention owing to its abundant energy levels and dominant green and red UC emssion. Hexagonal NaREF4 (RE=Y, Gd, Lu) are regarded as ideal UC host materials because of their unique crystal structure, low phonon energy as well as high photochemical stability. In comparison with the NaYF4 or NaLuF4 counterpart, Ln3+ doped NaGdF4 nanocrystals can not only yield comparable UC emissions, but also serve as efficient energy migration-mediated hosts. Hence, Ln3+ doped NaGdF4 nanocrystals have been widely studied as bio-imaging and magnetic resonance imaging agents. Previous studies have obtained that the morphology, materials size and crystal structure can influence the UC luminescence properties. Therefore, control the synthesis of RE3+ doped materials and tune the emission of UC phosphors have very significant to enlarge its application scope. At the same time, high luminescent efficiency is the basis of the application of various RE3+ doped materials. However, the application of upconversion fluorescence is still constrained because of its insuffucient intensity. Therefore, to find methods to improve the upconversion efficiency not only have important research of theoretical, but also have important contemporary relevance. In this work, NaGdF4:Ho3+ is the research object, significant UC luminescence enhancement and tunable emssion were achieved by co-doping other ions and changing the dopant concentration. The main contents are as follows:1. Hexagonal phased NaGdF4:Yb3+/Ho3+ nanorods are successfully synthesized through solvothermal method. When NaGdF4:Yb3+/Ho3+ nanocrystals co-doped with different concentration of Yb3+, not only the naorods size is changed from 25 nm to 125 nm, but also the UC emission tuned from green to yellow. Under 980 nm excitation, when the concentration of Yb3+ increases from 5 mol% to 15 mol%, the UC emission intensity of Ho3+ ion increases. The green UC emission intensity decreases if further increases the concentration of Yb3+ ions from 20 mol% to 50 mol%. It is also noted that green UC emission intensity is much stronger than red emission when the concentration of Yb3+ ions increases from 5 mol% to 20 mol%. The intensity of green emission decreases and red emission increases gradually if the concentration of Yb3+ ions continuously increases from 30 mol% to 50 mol%. As a consequence, the intensity ratio of red to green is changed from 0.78 to 1.76 with the concentration increases of Yb3+ions from 5 mol% to 50 mol%.2. The NaGdF4:Yb3+/Ho3+/Li+ nanocrystals are synthesized through solvothermal method when doped Li+. The XRD show that the size of nanorods structure has small changes via change the concentration of Gd3+ ions. Under 980 nm excitation, the strongest UC emssion was observed in the sample with a Li+ concentration of 5 mol% and the green and red emission were about 3 and 2 times stronger than the Li+-free sample. |
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