Luminescence And Temperature Sensing Properties Of Er³⁺/Yb³⁺Co-Doped Upconversion Phosphors

Due to their high optical stability and long emission lifetime,Up-conversion luminescent materials doped with rare earth ions have found extensive applications in various fields such as anti-counterfeiting identification,display lighting,biological tracking,and optical temperature sensing.For the study of Up-conversion luminescent materials,there are still many deficiencies,such as the lack of further thinking on improving luminous efficiency,which seriously limits the further application of luminescent materials.Therefore,how to improve its luminous efficiency has always been a hot spot for researchers.The conventional contact temperature sensor is no longer adequate to meet the requirements,due to its limitations in both usage environment and temperature measurement range.With advantages such as non-contact,high sensitivity,large-scale imaging,wide dynamic range,and fast response speed,optical temperature sensors solve previously unsolvable technical problems.For rare earth ions,temperature is closely related to their luminescent properties.Changes in ambient temperature can change the crystal field of rare earth ions,which ultimately affects its luminous intensity.Therefore,we chose the fluorescence intensity ratio（FIR）technique based on rare earth ions to realize the characterization of the temperature measurement.Molybdate materials can effectively absorb and transfer energy to rare earth ions due to molybdate radicals.And it has a wide and strong charge transfer band in the near ultraviolet region,thereby improving the luminous efficiency.Fluoride materials are also an ideal host material due to their advantages such as low phonon energy,long emission lifetime,and good thermal stability.Therefore,in this paper,we choose the popular molybdate material NaGd（MoO₄）₂ and the new fluoride material K₃ScF₆ as host materials and investigate their Up-conversion emission properties.The NaGd（MoO₄）₂:Er³⁺/Tm³⁺/Yb³⁺phosphor was produced by the hydrothermal method,and the K₃ScF₆:Er³⁺/Yb³⁺/Bi³⁺phosphor was produced by the solid-state reaction method.The phase and structure of the phosphors were analyzed by SEM and XRD,and the luminescence performance of the phosphors was tested by fluorescence spectrometer.The main research contents are as follows:Phosphors NaGd（MoO₄）₂:Er³⁺/Yb³⁺and NaGd（MoO₄）₂:Tm³⁺/Yb³⁺co-doped with different concentrations of rare earth ions were prepared by the hydrothermal method.XRD analysis shows that the NaGd（MoO₄）₂ phosphor obtained by the hydrothermal method is a pure tetragonal phase.SEM test results show that most of the prepared samples have smooth surfaces,complete crystallinity,ellipsoidal spherical particles with a particle size of about 1-4μm,and agglomeration.Under 980 nm excitation,as the concentration of rare earth ions increases,the luminous intensity of the phosphor climbs up and then decline.We got the best doping concentrations of NaGd（MoO₄）₂:0.03Er³⁺/0.20Yb³⁺and NaGd（MoO₄）₂:0.005Tm³⁺/0.20Yb³⁺respectively.The temperature-sensing properties of Er³⁺were investigated using thermally coupled levels（TCLs）.The results show that the maximum absolute sensitivity（S_A）of NaGd（MoO4）2:0.03Er³⁺/0.20Yb³⁺phosphor is 0.0167 K^-1（573 K）;the maximum relative sensitivity（S_R）is 0.0135 K^-1（298 K）.The temperature sensing properties of Tm³⁺were investigated using non-thermally coupled levels（NTCLs）.The results show that the maximum absolute sensitivity（S_A）of NaGd（MoO₄）₂:0.005Tm³⁺/0.20Yb³⁺phosphor is 0.0152 K^-1（573 K）,and the maximum relative sensitivity（S_R）is 0.0167 K^-1（373 K）.NaGd（MoO₄）₂:0.03Er³⁺/0.005Tm³⁺/x Yb³⁺phosphors co-doped with different Yb³⁺concentrations were prepared by hydrothermal method.Under 980 nm excitation,as the concentration of Yb³⁺increases,the luminous intensity of the phosphor climbs up and then decline.We got the best doping concentrations of NaGd（MoO₄）₂:0.03Er³⁺/0.005Tm³⁺/0.25Yb³⁺.We compared and calculated the temperature sensitivity between various thermally coupled energy levels and non-thermally coupled energy levels,the S_AMAXis 0.042 K^-1（573 K）,and the S_RMAX is 0.0198 K^-1（298 K）.Both are based on the non-thermally coupled energy levels of²H_11/2(Er³⁺green emission at 531 nm)and ¹G₄(Tm³⁺blue emission at 477 nm).In summary,regardless of TCLs energy levels or NTCLs energy levels,the temperature sensitivity of the combination of different ion energy levels is always much greater than the temperature sensitivity between individual ion energy levels.And the temperature sensitivity of the NTCLs energy level is much greater than that of the TCLs energy level,indicating that the Er/Tm/Yb three-doped system has great potential in the field of temperature sensing.A series of K₃ScF₆:Er³⁺/Yb³⁺and K₃ScF₆:Er³⁺/Yb³⁺/Bi³⁺（Na⁺）phosphors were prepared by solid-state reaction method.XRD tests show that the prepared samples belong to the cubic cryolite structure,and the peak positions are shifted by the Bi³⁺ion doping.SEM test showed that the prepared phosphor samples showed irregular particle shape,and the size range was about 15-40μm.Under 980 nm excitation,as the concentration of rare earth ions increases,the luminous intensity of the phosphor climbs up and then decline.We got the best doping concentrations of K₃ScF₆:0.05Er³⁺/0.15Yb³⁺.In order to further enhance the luminous efficiency of K₃ScF₆:Er/Yb phosphor,we carried out Er/Yb/Bi（Na）triple doping.The results show that the introduction of Bi³⁺and Na⁺does not cause changes in the crystal structure and has little effect on the size and morphology.Compared with pure samples,the introduction of Bi³⁺resulted in 2.5-fold and 3.6-fold enhancements in green and red luminescence intensities,respectively;the introduction of Na⁺resulted in 4.5-fold and 8.9-fold enhancements in green and red luminescence intensities,respectively.And we found that the introduction of Bi³⁺and Na⁺made the phosphor realize the transition from green light to red light emission,and the measured CCT value phosphor powder realized the transition from cold light to warm light.Finally,the temperature sensing properties of Er³⁺were investigated by using TCLs,and the results showed that the K₃ScF₆:0.05Er³⁺/0.15Yb³⁺/0.10Bi³⁺phosphor had a maximum absolute sensitivity（S_A）of 0.0088 K^-1（573 K）and a maximum relative sensitivity（S_R）of 0.0174 K^-1（298 K）;K₃ScF₆:0.05Er³⁺/0.15Yb³⁺/0.10Na⁺phosphor maximum absolute sensitivity（S_A）is0.0092 K^-1（573 K）,maximum relative sensitivity（S_R）is 0.0183 K^-1（298 K）.This indicates that K₃ScF₆ has potential applications in non-contact optical temperature sensors.