Study On The Upconversion Thermal Enhancement Mechanism And Temperature Sensing Of Monoclinic Double Perovskite CsK₂BiCl₆ Material

In recent years,the upconversion luminescence of rare earth ion doped materials has received great attention in the application of temperature sensing.Especially lead-free halide perovskite has become a hot research material due to its unique properties:tunable band structure,higher defect tolerance,strong light absorption ability,excellent photoelectric conversion efficiency,low toxicity,and high stability.However,there are still several problems in the study of the mechanism of luminescence thermal enhancement in upconversion materials for temperature sensing applications:（1）the research materials are single,and most of them are cubic phase halide perovskite and oxide perovskite materials;（2）The upconversion luminescence band is unified,and the phenomenon is generally similar,and the thermal quenching phenomenon of upconversion luminescence seriously affects its practical application in the lighting field;（3）There are many existing mechanisms for thermal enhanced upconversion luminescence,and there are some disagreements that do not have universality;（4）The non-contact optical temperature measurement mode is relatively single,with low sensitivity and poor reusability;This paper is based on the high-temperature solid-state method after grinding,and CsK₂BiCl₆ is selected as the matrix material to successfully prepare rare earth ion Yb³⁺/Ho³⁺,Yb³⁺/Er³⁺doped upconversion fluorescent materials.By studying its monoclinic crystal system,up conversion luminescence intensity and lifetime application,the following achievements have been achieved:（1）The material is monoclinic crystal system,which is different from cubic double perovskite.After doping rare earth ions,up conversion luminescence intensity is stronger;（2）The material doped with Yb/Ho exhibited abnormal short wavelength thermal enhancement;Moreover,the upconversion luminescence of the matrix material doped with rare earth ions did not exhibit thermal quenching,but instead exhibited abnormal thermal enhancement;（3）The mechanism of thermal enhancement is the reduction of surface defects and lattice distortion as the temperature increases;（4）The non-contact optical temperature measurement of the prepared material has achieved a high sensitivity.The specific content includes the following three parts:1、A monoclinic CsK₂BiCl₆:x Yb³⁺,2%Ln³⁺（Ln=Ho,Er,）double perovskite material was synthesized using a simple self grinding method and high-temperature sintering.Further temperature dependent photon number testing was conducted on CsK₂BiCl₆:15%Yb³⁺,2%Ln³⁺（Ln=Ho,Er,）to determine the photon number change process and energy transfer path of upconversion luminescence heat enhancement.It was observed that under excitation at 980nm,as the temperature increased from 298to 573 K,the energy levels of Ho³⁺/Er³⁺exhibited high thermal enhancement and anti thermal quenching of upconversion luminescence.Moreover,in the Yb³⁺/Ho³⁺doped system,the material exhibits abnormal short wavelength blue high magnification luminescence thermal enhancement.Through the doping of other rare earth ions,it was found that high magnification thermal enhancement was achieved in all bands.2、For the first time,CsK₂BiCl₆,a monoclinic type positive thermal expansion double perovskite material with high thermal enhancement and reverse thermal quenching,was synthesized under sustained excitation at 980nm The material doped with 15%Yb³⁺/2%Ho³⁺exhibits a light green to white luminescence characteristic under 980nm excitation;The upconversion fluorescence intensity of this material at the short wavelength Ho（467 nm,494 nm）of blue light significantly increased by about 721 times and 246 times,respectively.This is also the first time to achieve ultra-high magnification thermal enhancement in the blue light band in Yb/Ho rare earth ion doped materials.In situ XRD,in situ TEM,and in situ EPR tests were conducted on it,and it was found that the desorption of OH on the surface of the material caused the recovery of Yb quenching point after temperature rise.The improvement of Yb Yb co upconversion and energy transfer efficiency resulted in a high magnification thermal enhancement of the material’s upconversion luminescence.From the lattice distortion,it is proved that the lattice distortion occurs during the heating process of the material,the field environment of rare earth ionic crystal changes,and the phonon energy of lattice vibration decreases,which makes the upconversion luminescence of the material highly thermal enhanced.The Yb Yb cooperative upconversion and low energy phonon assisted enhancement of Yb/Ho short wavelength luminescence at high temperatures.3、Microscale CsK₂BiCl₆:15%Yb³⁺,2%Ln(Ln=Ho³⁺/Er³⁺)was synthesized by high-temperature solid-state method.Because its UCL intensity is extremely sensitive to temperature,and the radiation transitions corresponding to different energy levels vary with temperature,it is obtained that CsK₂BiCl₆:15%Yb³⁺,2%Ho³⁺shows an absolute sensitivity of 2.7%K^-1（573 K）and a relative sensitivity of 2.69%K^-1（298K）in the non thermally coupled level FIR temperature measurement of a single luminescent center,which can achieve high sensitivity temperature detection in both high and low temperature regions simultaneously,This result is superior to the sensitivity of most materials reported.Similarly,using the high sensitivity of its fluorescence lifetime decay curve to temperature,a relative sensitivity of 0.797%for CsK₂BiCl₆:15%Yb³⁺,2%Ho³⁺at 393K,and 3.02%K^-1for CsK₂BiCl₆:15%Yb³⁺,2%Er³⁺at 298K was obtained,achieving a multimode,higher sensitivity,and wider temperature measurement range of reusable advanced optical temperature measurement modes.This research will promote the application of monoclinic lead free halide double perovskite materials in the field of temperature sensing.