| Since the 21st century,upconversion luminescent(UCL)materials have been widely used in social production practices due to their unique optical properties and stable chemical properties,such as temperature sensors,photodynamic therapy,biological cell imaging,optical information storage.UCL refers to the process of continuously absorbing two or more low-energy photons to radiate higher-energy photons,which can be understood as the absorption of near-infrared light into visible light or ultraviolet light visible to the naked eye.In the scientific life,in order to improve the luminescence efficiency of UCL materials,appropriate synthesis methods can be adopted,appropriate matrix materials can be selected,appropriate doping ions can be added,which can be alkali metal ions or other lanthanide ions,change the traditional single excitation mode,adding surfactants and so on.Therefore,in this paper,a series of lanthanide ion-doped Na3Gd(VO4)2 and Ba4Bi3F17 UC fluorescence materials were prepared by sol-gel method and hydrothermal method,respectively,and their UCL properties and mechanisms were explored.In this paper,novel Na3Gd(VO4)2:Ln3+UCL phosphors(UCLPs)were prepared by a sol-gel means.Excited at 980 nm,Na3Gd(VO4)2:15%Yb3+/2%Er3+,Na3Gd(VO4)2:15%Yb3+/2%Ho3+,and Na3Gd(VO4)2:15%Yb3+/2%Tm3+exhibited a green,red and blue fluorescent band,respectively.The phenomenon was that different activators lead to different characteristic the emission bands.On the other hand,Na3Gd(VO4)2:15%Yb3+/2%Er3+,Na3Gd(VO4)2:15%Er3+/2%Ho3+,and Na3Gd(VO4)2:15%Er3+/2%Tm3+yielded the same green fluorescent band by irradiation at 1550 nm,which might be attributed to the main emission supplied by Er3+ ions.Interestingly,a higher Sa(5.54×10-3 K-1)for Na3Gd(VO4)2:15%Yb3+/2%Er3+excited at 1550 nm in the range from 298 to 463 K could be obtained,which had an advantage over that the Sa(5.46×10-3 K-1)of Na3Gd(VO4)2:15%Yb3+/2%Er3+at 980 nm excitation.UCL intensity of Na3Gd(VO4)2:15%Yb3+/2%Er3+under dual-wavelength(980 nm and 1550 nm)excitation was 1.74 times the of sum under the excitation at 980 nm and 1550 nm,indicating there was a dual-wavelength synergistic effect in this system.The possible energy transfer mechanism of Na3Gd(VO4)2:15%Yb3+/2%Er3+under single and multi-wavelength excitation was also discussed in detail using energy level diagram.To further improve the UCL intensity,Na3Gd(VO4)2:15%Yb3+/2%Er3+phosphors dopped with other rare-earth ions(La3+/Lu3+/Y3+)were synthesized and their UCL characteristics were investigated under excitation at mono-or multi-wavelengths,respectively.The results showed that the UCL intensities could be promoted under single-wavelength due to the symmetry change of lattice and multi-wavelengths excitation because of the lattice change and the energy loss caused by multi-photon relaxation decreases.Ba4Bi3F17 is one of the excellent host materials for high-efficient UCL,but it is still challenging to obtain high-purity red emission via lanthanide doping.In this work,nearly pure red emission was achieved in the Er3+/Tm3+and Er3+/Ho3+-doped Ba4Bi3F17 matrix by using a 1550 nm laser diode(LD).In order to improve the UCL intensity,optical-inert ions Li+or Na+ were doped into the system.Meanwhile,red-togreen luminescent intensity ratio was also enhanced.On basis of the analysis of the dependence of UCL intensities as a function of the excitation power and energy level of Ln3+,all possible red emission mechanisms were discussed. |
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