Temperature Sensing Of Rare Earth Doped Upconversion Materials Based On Fluorescence Intensity Ratio Technique

The rare earth doped upconversion material has many advantages,such as high spatial resolution,noninvasion,rapid response,high electromagnetic protection and all optical systems avoiding spark.Therefore,it has broad application in the field of optical temperature sensing.Although,the upconversion material as a temperature sensor has been extensively investigated,a quantitative analysis of temperaturedependent upconversion luminescence is still incomplete.In addition,highly sensitive and accurate temperature sensing become targets for researcher.The main research contents and results are as follows:The Gd2(Mo O4)3:Er3+/Yb3+film and powder are successfully prepared via a solgel method,respectively.The photo-thermal effect induced by laser is investigated for film and powder sample at different power.In view of film,weak absorption and rapid thermal diffusion to the substrate are responsible for this non-thermal phenomenon.For powder sample,strong absorption and slow thermal diffusion lead to heat accumulation.Then,the temperature-dependent mechanism of upconversion is investigated for film sample.The luminescent intensity as a function of temperature is illustrated by a comprehensive theoretical model including interaction of ion-ion(energy transfer and cross relaxation),ion-phonon(phonon-assisted energy transfer and non-radiative relaxation)and ion-photon(light absorption and radiation).Since the photo-thermal effect can be ignored,the Gd2(Mo O4)3:Er3+/Yb3+film is expected to realize highly accurate temperature sensing.The temperature sensing behaviors of the film are investigated in the 295-675 K range.Meanswhile,the photothermal behaviors of Gd2(MoO4)3:Er3+/Yb3+ powder are illustrated.Under 980 nm excitation,the temperature induced by laser is calculated,which varie s from 323 to 617 K as exciting power increased from 0.03 to 3.78 W.Due to the temperature sensing behavior is largely affected by the matrix material,we investigated temperature sensing behavior of rare earth doped Gd2(WO4)3 crystal.Since the 4S3/2 and 4F9/2 levels are split into the 4S3/2(2)/4S3/2(1)and 4F9/2(2)/ 4F9/2(1)levels,respectively,the green emission band of longer wavelength and the red emission band are strongly divided into two parts.As a result,the five pairs of thermally coupled levels are 2H11/2/4S3/2(2),2H11/2/4S3/2(1),2H11/2/4S3/2,4S3/2(2)/4S3/2(1)and 4F9/2(2)/4F9/2(1),respectively.Based on the five pairs of thermally coupled levels,the temperature sensing behaviors are investigated.Compared with 2H11/2 and 4S3/2 levels,a higher sensitivity is achieved based on the Stark sublevels of Er3+ ions.Traditionally,the fluorescence intensity ratio technique is associated with thermally coupled levels.This method has many limitations on the excatation and detection.Actually,the most pairs of levels for rare earth are non-thermally coupled levels.Furthermore,the fluorescence intensity ratio technique based on nonthermally coupled levels is also related to temperature.Therefore,based on nonthermally coupled levels,the temperature sensing behaviors of Gd2(WO4)3:Ho3+/ Tm3+/ Yb3+ phosphor are investigated,and the colors of luminescence are discussed in detail with the temperature rising.Up to now,rare earth doped Na Lu F4 has been known as one of the most excellent upconversion materials.Since red light has a high penetration depth in vivo,based on red light levels(Er3+:4F9/2 and Tm3+:3F2),the temperature sensing behavior of Na Lu F4:20%Yb3+/2%Er3+/ 1%Tm3+ is investigated.Meanwhile,based on blue light levels(Tm3+: 1D2 and 1G4)and green light levels(Er3+: 2H11/2 and 4S3/2),the temperature sensing behaviors are also investigated.Additionally,introducing Mn2+ for Na Lu F4:Yb3+/Er3+ phosphor rusults in enhancing the red to green intensity ratio,which acts as a bridge for energy transfer between 4S3/2 and 4F9/2 levels of Er3+.The energy transfer shows a temperaturedependent behavior.The temperature sensing behaviors of Na LuF4:Yb3+/Er3+ and Na Lu F4:Yb3+/Er3+/Mn2+ phosphors are ivestigated in temperature 295-595 K range.The sensitivity of Na Lu F4:Yb3+/Er3+/Mn2+ phosphors based on non-thermally coupled levels(4F3/2 and 4S3/2)is much higher than that of Na Lu F4:Yb3+/Er3+ phosphors based on both thermally(2H11/2 and 4S3/2)and non-thermally(4F3/2 and 4S3/2)coupled levels in the temperature 295-525 K range.