Synthesis And Properties Of Bismuth-based Fluoride Upconversion Luminescent Materials

Rare earth doped upconversion luminescent materials have advantages such as strong anti-interference ability,high optical stability,narrowband emission,and tunable emission.They are widely used in medical imaging,solar cells,biological probes,laser anti-counterfeiting,fluorescence thermometry,and other fields.Rare earth doped up conversion luminescent temperature sensitive materials have attracted much attention due to their fast response,high sensitivity,and strong anti-interference ability.However,currently,there are two main problems in the research of rare earth doped up conversion luminescence temperature sensitive materials:first,the luminescence efficiency is low,and thermal quenching occurs easily at high temperatures,so the temperature measurement sensitivity cannot meet practical requirements;The second is that the detection temperature range is not wide enough.In this paper,bismuth-based fluorides were selected as matrix materials,and ion substitution strategies were used to enhance up conversion luminescence to study the mechanism of luminescence enhancement;Design dual emission center materials through multi-ion doping to explore ways to broaden the temperature detection range and high sensitivity.（1）A series of NaBiF₄:Er³⁺/Yb³⁺/Lu³⁺(Al³⁺)luminescent materials were prepared by the coprecipitation method.The results indicate that both Lu³⁺and Al³⁺can enter the lattice and cause a decrease in cell volume.After Lu³⁺and Al³⁺enter the lattice,they will increase the particle size and increase the crystallinity of the material,which is also a reason for the enhanced luminescence.The overall luminescence intensity of Na Bi F₄:Er³⁺/Yb³⁺/Lu³⁺has increased by three times,and the overall luminescence intensity of Na Bi F₄:Er³⁺/Yb³⁺/Al³⁺has increased by 54 times.The enhancement of luminescence intensity is mainly attributed to the lattice distortion caused by the entry of Lu³⁺and Al³⁺into the lattice.At the same time,it was also found that the fluorescence lifetime of Er³⁺(⁴S_3/2)was extended after replacing with Lu³⁺,while the lifetime of Er³⁺was shortened after replacing with Al³⁺.This article also evaluates the temperature detection performance of the material based on the thermal coupling energy level of Er³⁺(²H_11/2/⁴S_3/2)using fluorescence intensity ratio technology.The maximum relative sensitivity of Na Bi F₄:Er³⁺/Yb³⁺/Lu³⁺is 1.93%K^-1（141 K）,and the maximum relative sensitivity of NaBiF₄:Er³⁺/Yb³⁺/Al³⁺is 2.48%K^-1（121 K）.（2）A series of NaBiF₄:Tm³⁺/Yb³⁺/Lu³⁺(Al³⁺)luminescent materials were prepared by coprecipitation method.The selected alternative ion is Lu³⁺(Al³⁺).The results indicate that the particle size increases after Lu³⁺(Al³⁺)substitution.The overall luminescence intensity of Na Bi F₄:Tm³⁺/Yb³⁺/Lu³⁺has increased by three times,and the overall luminescence intensity of Na Bi F₄:Tm³⁺/Yb³⁺/Al³⁺has increased by one time compared to the unreplaced state,mainly due to lattice distortion caused by ion substitution.After Al³⁺substitution,the fluorescence lifetime of Tm³⁺（¹G₄）is shortened,but the fluorescence lifetime of Tm³⁺after Lu³⁺substitution is actually extended.This article also evaluated the temperature detection performance of the material based on the ³F₃/¹G₄ non-thermal coupling energy level of Tm³⁺.The maximum relative sensitivity of Na Bi F₄:Tm³⁺/Yb³⁺/Lu³⁺is 1.25%K^-1（296 K）,and the maximum relative sensitivity of Na Bi F₄:Tm³⁺/Yb³⁺/Al³⁺is 1.1%K^-1（316 K）.（3）Based on the design idea of multi-ion doping,Nd³⁺was selected as the second emission center to prepare a series of Na Bi F₄:Er³⁺/Yb³⁺/Nd³⁺luminescent materials.The results show that Nd³⁺can enter the lattice of the matrix lattice to replace Bi³⁺,resulting in a decrease in the cell volume,and the size of the doped Nd³⁺material also increases.As the concentration of Nd³⁺increases,the characteristic emission of Er³⁺decreases to varying degrees,and there is no significant change in the characteristic emission of Nd³⁺.This is mainly due to the fact that Yb³⁺will transfer energy to Nd³⁺after Nd³⁺doping under 980 nm excitation to achieve Nd³⁺characteristic emission,while the energy transferred to Er³⁺decreases,resulting in a weakening of Er³⁺characteristic emission.Moreover,it was found that the lifetime of Er³⁺was prolonged after Nd³⁺doping.In this paper,based on the fluorescence intensity ratio technology,the related properties of materials in the field of temperature detection were tested.It was found that the maximum relative sensitivity based on the Er³⁺thermal coupling energy level(²H_11/2/⁴S_3/2)was 0.91%K^-1（301 K）,and the maximum relative sensitivity based on the Nd³⁺thermal coupling energy level(⁴F_5/2/⁴F_3/2)was 2.08%K^-1（301 K）.