Study On Fluorescence Modulation Of Up-and Down-conversion And Thermosensitivity In Rare Earth Doped Lutetium-based Materials

In recent years,rare earth ions have attracted the widespread attention of researchers because of their unique luminescent properties.Up-conversion luminescent materials have wide application prospects in bioimaging and photodynamic therapy due to their low toxicity,good biological applicability,low spontaneous fluorescence,high photostability,long excitation lifetime and narrow emission bandwidth.If the emission wavelength is also modulated in the biological window（600-1100 nm）,the signal-to-noise ratio and detection depth can be further improved.In this paper,the down-conversion phenomenon of Yb³⁺/Ho³⁺co-doped Lu₂O₃ was studied.The green light monochromism was improved by using Lu₂O₃:Yb³⁺/Ho³⁺.The red up-conversion luminescence of Lu₂O₃:Yb³⁺/Eu³⁺was studied.LuVO₄:Yb³⁺/Er³⁺synthesized by high temperature solid-state and hydrothermal method.The up-conversion luminescence and temperature sensitivity of Yb³⁺/Er³⁺co-doped LuVO₄ were systematically analyzed.The specific research contents are as follows:The first part:Lu₂O₃:1 mol%Ho³⁺/x mol%Yb³⁺was synthesized by high temperature solid-state method and the down-conversion（quantum tailoring）phenomenon was studied.The energy transfer between Ho³⁺and Yb³⁺is proved by the spectra and dacay curves in the visible and near infrared regions.Effective near infrared quantum tailoring can be observed when the ⁵G₆/⁵F₁ level of Ho³⁺is directly excited at 446 nm wavelength.By analyzing the relationship between the initial energy transfer rate and the concentration of Yb³⁺,it is confirmed that the energy transfer mode between Ho³⁺and Yb³⁺is a two-step energy transfer.When the concentration of Ho³⁺is fixed at 1 mol%and Yb³⁺is 30 mol%,the energy transfer efficiency can reach 62%.Near infrared photons are emitted not only by activator Yb³⁺,but also by sensitizer Ho³⁺.The ⁵I₅→⁵I₈ and ⁵F₄/⁵S₂→⁵I₆ transitions of Ho³⁺and Yb³⁺:2F_5/2→2F_7/2 transitions are all within the optimum spectral response range of crystal silicon solar cells.The results show that Lu₂O₃:Ho³⁺/Yb³⁺is an excellent quantum tailoring material and has a good application prospect in improving the conversion efficiency of solar cells.The second part:The integral area ratio of green emission and red emission intensity increased by 4.3 time adopt three-doped Eu³⁺in Lu₂O₃:Yb³⁺/Ho³⁺powders and the green up-conversion monochrome was successfully enhanced.The steady-state fluorescence spectra and attenuation curves show that there exists an energy transfer process from Ho³⁺⁵F₄/⁵S₂ level to Eu³⁺⁵D₀ level and from Ho³⁺⁵I₆ level to Eu³⁺⁷F₆ level.In addition,the population source of red emission level Ho³⁺⁵F₅ in Yb³⁺/Ho³⁺co-doped system under 980nm excitation was also studied.The third part:Lu₂O₃:Yb³⁺/Eu³⁺/Ho³⁺was synthesized by using Ho³⁺as intermediate ion.Experiments show that the energy transfer of Ho³⁺ions through directly excited Yb³⁺is first filled into the green emission level ⁵F₄/⁵S₂.Subsequently,Ho³⁺at the ⁵F₄/⁵S₂ level transits to the ⁵D₀ level of Eu³⁺at the ground state through energy transfer.With the help of Ho³⁺ion acting as a bridge,this energy transfer process is more effective than the synergistic energy transfer process from Yb³⁺to Eu³⁺ion.Compared with Lu₂O₃:5 mol%Yb³⁺/1 mol%Eu³⁺,the red up-conversion luminescence intensity of Eu³⁺in Lu₂O₃:5 mol%Yb³⁺/1 mol%Eu³⁺/0.5 mol%Ho³⁺materials increased by 8 times.The fourth part:LuVO₄ with different Yb³⁺and Er³⁺doping concentration was synthesized by traditional high temperature solid-state method.The up-conversion spectra,near-infrared spectra and lifetime curves show that there is an effective energy transfer process from Yb³⁺to Er³⁺.The optimum doping concentration of Yb³⁺and Er³⁺is 20 mol%and 2 mol%respectively.In the visible region,the optical temperature measurement of LuVO₄:Yb³⁺/Er³⁺is studied by using the fluorescence intensity ratio of two thermal coupling levels ²H_11/2 and ⁴S_3/2 of Er³⁺ions.The absolute sensitivity reaches the maximum at 423 K,which is 0.82%K^-1.In the near infrared region,the fluorescence intensity ratios of ⁴I_13/2→⁴I_15/2 transition peaks 1（at 1595 nm）and 3（at 1660 nm）and 2（at 1637 nm）and3（at 1637 nm）of Er³⁺are well fitted with temperature.The maximum absolute sensitivity is 1.85%K^-1 and 0.62%K^-1,respectively.These results indicate that LuVO₄:Yb³⁺/Er³⁺is a potential temperature measurement material based on FIR in the visible and near infrared regions.The fifth part:LuVO₄:20 mol%Yb³⁺/2 mol%Er³⁺@SiO₂ was synthesized by hydrothermal method.The optical temperature measurement properties of LuVO₄:20 mol%Yb³⁺/2 mol%Er³⁺@SiO₂ nanomaterials were studied in detail.In order to avoid the superheating effect of 980 nm radiation on biological tissues,the up-conversion emission spectra（UC）and optical temperature measurements of the prepared samples were studied using 915 nm as excitation wavelength.In the visible region,the optical temperature measurement performance was studied by using the fluorescence intensity ratio（FIR）of Er³⁺two thermal coupling levels ²H_11/2 and ⁴S_3/2.In this case,the relative sensitivity S_R is1077/T².In the near infrared（NIR）region,abnormal phenomena of ⁴I_13/2→⁴I_15/2 were observed,which gradually increased with the increase of temperature.More importantly,the FIR of splitting peak 2（1496 nm）and peak 1（1527 nm）varies regularly with the increase of temperature,and can also be used for temperature measurement.The combination of visible and near infrared optical temperature measurement can provide a self-reference temperature measurement method,which makes the temperature measurement more accurate.