Synthesis and Temperature Sensing of CaF2∶Er3＋，Yb3＋Nanoparticles with Upconversion Fluorescence

LIU Guo-feng，FU Zuo-ling，2?

(1.Coherent Light and Atomic and Molecular Spectroscopy Laboratory，Key Laboratory of Physics and Technology for Advanced Batteries，College of Physics，Jilin University，Changchun 130012，China，2.Department of Physics，Georgia Southern University，Statesboro，USA)?Corresponding Author，E-mail:zfu＠GeorgiaSouthern.edu

Synthesis and Temperature Sensing of CaF2∶Er3＋，Yb3＋Nanoparticles with Upconversion Fluorescence

LIU Guo-feng1，FU Zuo-ling1，2?

(1.Coherent Light and Atomic and Molecular Spectroscopy Laboratory，Key Laboratory of Physics and Technology for Advanced Batteries，College of Physics，Jilin University，Changchun 130012，China，2.Department of Physics，Georgia Southern University，Statesboro，USA)?Corresponding Author，E-mail:zfu＠GeorgiaSouthern.edu

CaF2∶Yb3＋，Er3＋upconversion nanoparticles(UCNPs)were synthesized by hydrothermal method.The green and red light from2H11/2/4S3/2→4I15/2and4F9/2→4I15/2of Er3＋were investigated in detail by using the power depended luminescence intensity under 980 nm excitation.Afterwards，the as-obtained sample was proved that it had good temperature sensing property in the range of 293－573 K based on thermal coupled levels(TCLs)of Er3＋(2H11/2/4S3/2)，simultaneously the maximum sensitivity of sample was obtained at 483 K(0.002 85 K－1)by utilizing fluorescence intensity ratio(FIR)technique.All the experiment data indicate that the sample has great luminescence property and excellent potential in temperature sensing.

rare earth ions；temperature sensing；upconversion luminescence

1 Introduction

The rare earth ions-doped upconversion nanomaterials have been paid more attention for their potential application in cell imaging，temperature sensing，solar cell and so on[1-3].The reason is that the 4fnenergy manifolds can be protected by 5d to avoid the effect from surrounding environment.In addition，the rare earth ions also possess other superior characteristic，for example，narrow emission band，long fluorescence lifetime，etc.[4-6].Furthermore，the UCNPs can be excited by near infrared(NIR)，which is scattered little in vivo and has less harm for biology tissues，emitting the visible and NIR light. Simultaneously the biology window locates in 750－1 100 nm，thus theNIR has deeppenetration depth，which is promising for biology imaging and temperature sensing[7].

Inparticular，thenon-contacttemperature measuring based on the FIR technique of UCNPs has attracted more attention for its great accuracy[8]. The FIR technique is the upconversion luminescence (UCL)intensity ratio，which is from the transitions of TCLs.In general，the energy gap of TCLs ranges from 200－2 000 cm－1to avoid the overlap and ensure quasi thermal equilibrium based on the Boltzmann distribution theory[8].In fact，the energy gap of Er3＋is about 800 cm－1，lying in the range[5，9]. Compared to halide，sulfide and oxide，the fluoride becomes the most efficient matrix in rare earth doped materials based on excellent physical chemistry stable，low phonon energy and non-radiative transition[7，10-13].To sum up，Er3＋-doped fluoride UCNPs will play more and more important role in the temperature sensor and scientific research.Herein，in this work，CaF2∶Yb3＋，Er3＋UCNPs are prepared by hydrothermal method，the UCL and the temperature sensing property of sample are investigated in detail based on FIR technique.The results indicate that the sample has great luminescence property and excellent potential for temperature sensing.

2 Experiments

2.1 Raw Materials

Y2O3and Yb2O3were purchased from Sinopharm Chemical Reagent Co.，Ltd.(99.99%)，then they were dissolved in HNO3solution to obtain Y(NO3)3(0.05 mol/L)and Yb(NO3)3(0.05 mol/L)，Ca(NO3)2·4H2O and NaF were purchased from Beijing Chemical Reagent Co.，Ltd. (99.5%).All the chemicals were commercially available and used without further purification.

2.2 Synthesis of Sample

CaF2∶xEr3＋，yYb3＋((a)y＝3%；x＝1%，2%，3%；(b)x＝1%；y＝3%，5%，7%，10%， 15%)was prepared by hydrothermal method.Ca-(NO3)2·4H2O and deionized water(18 mL)were added to beaker A，then the calculated quantity of Er(NO3)3and Yb(NO3)3solution were added and stirred to transparent.Next，NaF(4 mmol)and deionized water(20 mL)were added to beaker B and stirred to transparent.Finally，the solution of beaker B was dropped in beaker A under stirring with the production of precipitate.The obtained solution was poured in a 60 mL Teflon-lined autoclave and heated to 180℃for 12 h.After returning to room temperature，the production was obtained by centrifugation and washed with ethanol and deionized water for 3 times.Finally，the production was dried at 60℃for 12 h.

2.3 Characterization

All the measurements were carried out at the room temperature except the temperature sensing experiment.Rigaku-Dmax2500Xdiffractometer equipped with Cu Kα radiation(λ＝0.154 06 nm) wasusedforperformingtheX-raydiffraction (XRD)pattern of sample，the 2θ range was from 10°to 80°，and the rate was set 15(°)/min.The morphology was performed by a field emission-scanning electron microscope(FE-SEM，XL30，Philips).A 980 nm semiconductor laser(fiber core diameter of 200 mm，numerical aperture of 0.22)was used for exciting the sample to obtain the UCL，which the signal was collected by A CCD detector combined with a monochromator.The sample was filled in an iron sample cell and the temperature was adjusted from 293 K to 573 K.A copper-constant thermocouple buried in the sample was used to detect the sample temperature.

3 Results and Discussion

3.1 Composition and Morphology

XRD pattern of the sample CaF2∶1%Er3＋，5%Yb3＋is presented in Fig.1.It can be observed that all the diffraction peaks are consistent with JCPDS card(No.65-0535)without any impurities existence，indicating the as-prepared sample is pure cubic phase and Yb3＋/Er3＋have entered into matrix. The inset represents the FE-SEM image of sampleCaF2∶1%Er3＋，5%Yb3＋，which is uniformly distributed and the average diameter is approximately 40 nm.

Fig.1 XRD patterns and FE-SEM image of CaF2∶1%Er3＋，5%Yb3＋UCNPs

3.2 UCL and Energy DiagramThe high quantum efficiency of rare earth ionsdoped CaF2UCNPs is achieved based on the low phonon energy and non-radiative transition ratio[5，9，14]. The emission spectra of different doping concentrations(Yb3＋/Er3＋)of sample under the excitation of 980 nm are shown in Fig.2.It can be found that the UCL intensity is affected greatly by doping concentration.The fluorescence intensity at fixed Yb3＋mole fraction(3%)decreases with the rise of the mole fraction of Er3＋(from 1%to 3%)because of the concentration quenching，which is shown in Fig. 2(a).Correspondingly，the fluorescence intensity rises firstly and then decays with the rise of Yb3＋

mole fraction(from 3%to 15%)at fixed Er3＋mole fraction(1%)in Fig.2(b)based on the same reason.Thus the highest intensity is obtained at the mole fraction of 1%Er3＋5%Yb3＋.It can be observed that the spectrum is composed of two emission bands，which are green emission(2H11/2/4S3/2→4I15/2)and red emission(4F9/2→4I15/2).The centers are located at 522/540 nm(green)and 656 nm (red)，respectively.In particular，the two green emissions can be used to investigate the temperature sensing according to FIR technique.

Fig.2 Emission spectra of CaF2∶x Er3＋，y Yb3＋.(a)y＝3%，x＝1%，2%，3%，marked by A，B，C.(b) x＝1%，y＝3%，5%，7%，10%，15%，marked by D，E，F，G，H.

Both the green and red transitions are from Er3＋，a possible mechanism can be used to explain CaF2∶Yb3＋/Er3＋upconversion process under 980 nm excitation in detail[15-16]，which is shown in Fig. 3.A 980 nm photon can be absorbed by Yb3＋，the population at ground state(2F7/2)can be populated to2F5/2state(ground state absorption，GSA). Then，4I15/2state of Er3＋is populated to4I11/2state by the first energy transfer(ET，2F5/2＋4I15/2→2F7/2＋4I11/2)from Yb3＋to Er3＋or GSA(4I15/2＋hν→4I11/2).Subsequently，the second ET(2F5/2＋4I11/2→2F7/2＋4F7/2)between Yb3＋and Er3＋or excite state absorption(ESA，4I11/2＋hν→4F7/2)populates the population from4I11/2to4F7/2state.Then，the population is populated to4S3/2/2H11/2state via multiphonon relaxation(MPR)process.Finally，the green emissions(522/540 nm)take place owing to the radiative transition from4S3/2/2H11/2to4I11/5state.In fact，part population of4I11/2level populate to4I13/2level by non-radiative transition，then the population are populated to4F9/2state by ET(4I13/2＋2F5/2→4F9/2＋2F7/2)or ESA(4I13/2＋hν→4F9/2).The red emission(656 nm)can be observed due to the radiative transition from4F9/2to4I11/5state.

Fig.3 Energy level diagram of Er3＋and possible UC processes under 980 nm excitation

3.3 Temperature Sensing Property

In order to study the property of temperature sensing of CaF2∶1%Er3＋，5%Yb3＋，we obtain the upconversion emission spectra of sample under different temperature(293，373，463 K)，which are shown in Fig.4(a).It can be observed that the fluorescence intensity decreases with the rise of temperature owing to the thermal quenching，which the nonradiation transition is strong in the high temperature. In addition，the FIR of 522 nm(2H11/2→4I15/2)and 540 nm(4S3/2→4I15/2)increases with the rising temperature in Fig.4(a).As we all know，the TCLs (2H11/2/4S3/2)gap of Er3＋is approximately 800 cm－1，and the population distribution of them follows Boltzmann theory[5，8].The population at lower level (4S3/2)of TCLs can be populated to the upper level (2H11/2)by thermalization process at high temperature，which becomes active with increasing temperature，resulting in the emission intensity of 522 nm increasing and 540 nm reducing，relatively.In addition，the FIR is calculated and presented in Fig.4 (b).Due to the luminescence intensity is relative to the population of every energy level，thus the FIR can be expressed as follows[17-19]:

Fig.4 (a)Emission spectra of sample at 293，373，463 K.(b)KFIRas a function of temperature.(c)lnKFIRandare line- arly dependent.(d)Sensor sensitivityas a function of temperature for sample.

Where，all the terms have been mentioned in the Eq.(1).Thus，the relationship between lnKFIRandcan be described by a line.In fact，the experiment data have been fitted by a straight line in the Fig.4(c)，which the slope is 1170.8and the intercept is 1.8223 lnB.Furthermore，the temperature of sample can be calculated by FIR，which canbe obtained by spectra measurement.

As an excellent temperature sensor，the sensitivity is an important parameter to judge the temperature sensing property.According to previous report，the sensitivity S can be defined as follows[21-23]:

The sensitivities of sample under different temperature are presented in Fig.4(d).The solid line represents the theoretical calculation values and dotted line denotes the actual experimental values.The maximum sensitivity is obtained at 483 K for 0.002 85 K－1.

4 Conclusion

CaF2∶Yb3＋，Er3＋UCNPs are successfully synthesized by hydrothermal method.XRD pattern indicates that the samples are pure cubic phase with an average grain size of 40 nm.The intense green emission can be observed under the irradiation of 980 nm，which comes from the transition of Er3＋.A possible upconversion process is proposed to explain the emission spectrum.In addition，the transitions of TCLs(2H11/2/4S3/2)of Er3＋are discussed in detail for their application in temperature sensing by utilizing the FIR technique.The experiment data indicates that the FIR is proportion to the temperature，which is consistent with the Boltzmann distribution theory.The maximum sensitivity is 0.002 85 K－1at 483 K.All the experimental results indicate that CaF2∶Yb3＋，Er3＋UCNPs have great potential to regard as a temperature sensor.

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劉國鋒(1990－)，男，山東濰坊人，博士研究生，2014年于山東師范大學獲得學士學位，主要從事稀土發光材料方面的研究。

E-mail:gfliu14＠163.com

付作嶺(1979－)，女，內蒙古赤峰人，博士，教授，2007年于中國科學院長春應用化學研究所獲得博士學位，主要從事稀土發光材料方面的研究。

E-mail:zlfu＠jlu.edu.cn

上轉換納米粒子CaF2∶Er3＋，Yb3＋的合成及其溫敏特性

劉國鋒1，付作嶺1，2?
(1.吉林大學物理學院相干光與原子分子光譜教育部重點實驗室，新型電池物理與技術教育部重點實驗室，吉林長春 130012；2.美國南佐治亞大學物理系，佐治亞州斯泰茨伯勒)

采用水熱合成法制備了CaF2∶Yb3＋，Er3＋上轉換納米粒子。在980 nm激發下，研究了來源于Er3＋的2H11/2/4S3/2→4I15/2躍遷的綠光發射和來源于4F9/2→4I15/2躍遷的紅光發射。由于Er3＋具有一對熱耦合能級(2H11/2/4S3/2)，所合成的樣品在293～573 K溫度范圍內有良好的溫敏特性。利用熒光強度比(FIR)技術，測得樣品在483 K時具有最大靈敏度0.002 85 K－1。

稀土離子；溫度傳感；上轉換發光

2016-11-09；

2016-12-20

吉林省科技發展計劃(20160101294JC)；中國博士后科學基金；國家基礎科學人才培養基金(J1103202)資助項目Supported by Science and Technology Development Planning Project of Jilin Province(20160101294JC)；China Postdoctoral Science Foundation；National Found for Fostering Talents of Basic Science(J1103202)

O482.31

A

10.3788/fgxb20173802.0133

1000-7032(2017)02-0133-06