| Recently,up conversion materials(absorbing low-energy long-wavelength excitation light and emitting high-energy short-wavelength emission light)have been applied in a variety of emerging and potential fields,including high-resolution biological imaging,super-resolution nanotechnology,photodynamic therapy,light triggered drug transport,remote light excitation,anti-counterfeiting,full-color display and solar energy collection.Lanthanide ions(Ln3+)have rich energy levels.Therefore,lanthanide ions doped up-conversion luminescent materials have unique optical properties,such as large anti-Stokes shift,sharp emission peak,low spontaneous fluorescence background signal,adjustable emission wavelength and lifetime,excellent anti photobleaching stability and continuous emission ability,which have attracted great attention of researchers.However,the luminescent intensity and conversion efficiency of Ln3+doped up-conversion materials are still too low.Therefore,researchers proposed various methods to enhance the intensity of up-conversion luminescence,and studied the internal enhancement mechanism.The methods included doping of unequal-ions(Mn2+,Li+,etc.),design of core-shell structure,local surface plasmon resonance(LSPR),modification of external excitation conditions,et al.Among them,Li+incorporation in the Ln3+doped fluoride and oxide luminescent materials can enhance the luminescent intensity simply.A deep understanding of the enhancement mechanism of Li+will be helpful to design and develop more effective methods to enhance the luminescence intensity,and to select a more suitable up-conversion host.This thesis focused on the incorporation methods of Li+and the enhancement mechanism Li+ doping in up-conversion materials.(1)Up-conversion materials(Y0.78-xYb0.20Er0.02Lix)2O3 with different lithium concentrations were prepared by combustion method.The excitation-emission spectra showed that the up-conversion luminescence intensity of Y2O3:Yb3+,Er3+could be enhanced by lithium doping.The neutron scattering results showed that Li+occupied the Y positions in yttrium oxide lattice,and oxygen defects were introduced due to charge balance,which was confirmed by XPS.FT-IR showed that the lithium doping did not lead to the change of the number of quenching groups adsorped in the surface,but the positron annihilation lifetime showed that Li+doping could reduce the total defect concentration,especially the concentration of large defect clusters(Y-O-O-Y-Y,Y-O-O-O-Y five defects,etc.),that is to say,reduce the concentration of quenching center,and then enhance the luminous intensity.At the same time,Li+doping could reak the symmetry of Er3+ coordination enviroments,and enhance the crystallinity of the sample.These results put forward new ideas on the enhancement mechanism,and provided new ideas for controlling upconversion luminescence and exploring new upconversion matrix materials through defect engineering.(2)The up-conversion luminescent sodium yttrium fluoride-based materials were studied.Firstly,the effects of lithium doping was studied,and it was found that the sample with 8 mol%Li+incorporation was still pure phase and exhibited largest improved emission intensity.This is because the appropriate doping of lithium ion can reduce the local coordination symmetry of Er3+,enable the forbidden 4f-4f transition,and enhance the luminescence intensity.Then,hexagonal phase sodium yttrium fluoride with good dispersity and uniform particle size was synthesized by thermal decomposition method,and core-shell inert coating layer was designed.Because the inert layer passivated the surface defects of the core,and reduced the number of quenching centers,the integral strength of red light and green light was increased by 26 times and 17 times,respectively.The inert layer also isolated the nucleus from the surrounding environments,which greatly enhances the luminous intensity. |
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