Article

Multiple Temperature-Sensing Behavior of Green and Red Upconversion Emissions from Stark Sublevels of Er3+ Baosheng Cao, Jinlei Wu, Xuehan Wang, Yangyang He, Zhiqing Feng and Bin Dong * Received: 7 September 2015; Accepted: 1 December 2015; Published: 10 December 2015 Academic Editor: Vittorio M. N. Passaro School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600, China; [email protected] (B.C.); [email protected] (J.W.); [email protected] (X.W.); [email protected] (Y.H.); [email protected] (Z.F.) * Correspondence: [email protected]; Tel.: +86-411-87656135; Fax: +86-411-87656331

Abstract: Upconversion luminescence properties from the emissions of Stark sublevels of Er3+ were investigated in Er3+ -Yb3+ -Mo6+ -codoped TiO2 phosphors in this study. According to the energy levels split from Er3+ , green and red emissions from the transitions of four coupled energy levels, 4 4 2H 2 4 4 4 4 2 2 11/2(I) / H11/2(II) , S3/2(I) / S3/2(II) , F9/2(I) / F9/2(II) , and H11/2(I) + H11/2(II) / S3/2(I) + S3/2(II) , were observed under 976 nm laser diode excitation. By utilizing the fluorescence intensity ratio (FIR) technique, temperature-dependent upconversion emissions from these four coupled energy levels were analyzed at length. The optical temperature-sensing behaviors of sensing sensitivity, measurement error, and operating temperature for the four coupled energy levels are discussed, all of which are closely related to the energy gap of the coupled energy levels, FIR value, and luminescence intensity. Experimental results suggest that Er3+ -Yb3+ -Mo6+ -codoped TiO2 phosphor with four pairs of energy levels coupled by Stark sublevels provides a new and effective route to realize multiple optical temperature-sensing through a wide range of temperatures in an independent system. Keywords: temperature sensing; upconversion emissions; Stark sublevel; rare earth; sensitivity

1. Introduction Optical temperature-sensing devices have been widely researched to promote their application in electrical power stations, oil refineries, coal mines, and fire detection, as they have been shown to overcome the interference of strong electromagnetic noise, hazardous sparks, or corrosive environments inaccessible to traditional temperature-measurement methods such as thermocouple detectors [1–5]. Sensors built based on the fluorescence intensity ratio (FIR) technique have attracted particular attention due to their ability to reduce dependence on measurement conditions and improve accuracy and resolution. FIR functions independent of fluorescence loss or fluctuations in excitation intensity can be applied to fluorescence systems in which two closely spaced energy levels with separations of the order of thermal energy are involved, following a Boltzmann-type population distribution [1,6,7]. Optical temperature sensors using the FIR technique are mainly focused on fluoride and oxides matrixes [8–14]. The fluoride matrixes possesses higher fluorescence efficiency and lower excitation power; however, the maximum operating temperature is usually low. On the contrary, the oxides matrices can operate at high temperature, although the fluorescence intensity is lower. Upconversion emissions of rare earth ion-doped materials are typically utilized to realize FIR measurement because of the large amount of coupled energy levels in many rare earth ions

Sensors 2015, 15, 30981–30990; doi:10.3390/s151229839

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Sensors 2015, 15, 30981–30990

and the easily accessible upconversion luminescence with near-infrared radiation from low-cost, commercially available diodes. Xu et al. [8], for example, reported the FIR of Ho3+ using two blue emissions from coupled energy levels of 5 G6 /5 F1 and 5 F2,3 /3 K8 and found that Ho3+ -Yb3+ -codoped CaWO4 possessed higher absolute sensitivity due to a larger energy gap between the thermally coupled 5 G6 /5 F1 and 5 F2,3 /3 K8 levels of Ho3+ ions. The paired energy levels of 3 F2 and 3 F3 in Tm3+ ions have also been used to investigate temperature-dependent red upconversion emissions and corresponding FIR properties [9]. The FIR properties of green upconversion emissions ascribed to paired energy levels of 2 H11/2 and 4 S3/2 in Er3+ -doped materials, in particular, have been quite widely studied [10–14]. In addition to the intrinsic thermally coupled energy levels of rare earth ions, the pair energy levels of Stark sublevels can also be thermally coupled and used to investigate FIR versus temperature characteristics [15–18]. Baxter et al. [17], for example, used the coupled energy levels of 2 F5/2(a) and 2 F5/2(b) by Stark split of 2 F5/2 levels in Yb3+ ions to study FIR properties of Yb3+ -doped silica fiber. Feng et al. [18] investigated the FIR properties of Er3+ -doped fluoride glass using coupled Stark sublevels of 4 S3/2(1) and 4 S3/2(2) in Er3+ ions. In this study, four thermally coupled energy levels of Er3+ ions based on the Stark sublevels were simultaneously observed in Er3+ -Yb3+ -Mo6+ -codoped TiO2 phosphors. FIR properties of the four coupled energy levels from green and red emissions in Er3+ -Yb3+ -Mo6+ -codoped TiO2 phosphors were studied as a function of temperature in the range of 307–673 K. The effects of the energy gap of thermally coupled energy levels, FIR value, and upconversion emission intensity on the sensitivity and accuracy of the optical temperature sensor are discussed in an effort to explore potential developments in optical temperature-sensor technology based on different FIR routes in an independent system. 2. Experimental Section The sol-gel method was used to prepare Er3+ -Yb3+ -Mo6+ -codoped TiO2 phosphors. The rare earth nitrates Er(NO3 )3 ¨ 5H2 O (99.99%) and Yb(NO3 )3 ¨ 5H2 O (99.99%) were purchased from Aladdin. Other chemicals including Iso-Propanol (i-PrOH), n-butyl titanate (Ti(OBu)4 ), acetylacetone (AcAc), and concentrated nitric acid (HNO3 ) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals are of analytical reagent and were used without any further purification. i-PrOH was first added as a solvent to modified titanium(IV) n-butoxide by facilitating a chelating reaction between Ti(OBu)4 and AcAc under agitation for 1 h at room temperature. Next, a mixture of deionized water, i-PrOH, and HNO3 was slowly added into the solution. The mixed solution was stirred for 6 h to form a clear and stable sol. The molar ratios of Ti(OBu)4 , AcAc, H2 O, and HNO3 were 3:3:6:1. Finally, Er, Mo, and Yb ions were introduced by adding Er(NO3 )3 ¨ 5H2 O, (NH4 )6 Mo7 O24 ¨ 5H2 O, and Yb(NO3 )3 ¨ 5H2 O in the molar ratio of 2:2:20:100 for Er:Mo:Yb:Ti. The codoped sols were dried at 373 K for 8 h to remove the solvent. The xerogels were then heated at a rate of 4 K/min and maintained at the sintering temperature of 1073 K for 1 h, then cooled to room temperature in the furnace. The sintered 2 mol % Er3+ –20 mol % Yb3+ –2 mol % Mo6+ -codoped TiO2 phosphors were finally milled into powders for structural analysis and spectral measurement. The phase structures of Er3+ -Yb3+ -Mo6+ -codoped TiO2 phosphor samples were analyzed by SHIMADZU XRD-6000 X-ray diffractormeter (XRD) with Cu-Kα radiation. A homemade temperature control system, which was composed of a small stove and an intelligent digital-display-type temperature control instrument, was used to adjust sample temperature from 307 to 673 K, at measurement and control accuracy of about ˘0.5 K. Temperature-dependent upconversion emissions from each sample were focused onto a Jobin Yvon iHr550 monochromator and detected with a CR131 photomultiplier tube by 976 nm laser diode (LD) excitation. The LD pump current varied from 0 to 2 A, and the spectral resolution of the experimental set-up was 0.1 nm.

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3. Results and Discussion Figure 1 shows XRD patterns of the Er3+ -Yb3+ -Mo6+ -codoped TiO2 phosphor samples. The XRD pattern observed was characteristic of the anatase phase of TiO2 (JCPDS No. 21-1272) and Sensors 2015, 15, page–page the face-centered cubic phase of Yb2 Ti2 O7 (JCPDS No. 17-0454) referenced below. There was no diffractionpeak peak of Mo compounds, anddiffraction the main peak toward Mo small angles, 6+ of Mo compounds, and the main peakdiffraction shifted toward smallshifted angles, indicating indicating stochastically Mo6+ stochastically located at the interstitial sites ofasthe matrixelement. lattice as a solution element. located at the interstitial sites of the matrix lattice a solution Sensors 2015, 15, page–page

peak of Mo compounds, and the main diffraction peak shifted toward small angles, indicating Mo6+ stochastically located at the interstitial sites of the matrix lattice as a solution element.

6+ codoped 3+3+-Yb 6+ codoped 1. XRD pattern -Yb3+3+ -Mo TiO2. TiO . Figure Figure 1. XRD pattern of of ErEr -Mo 2

Figure 2 shows the upconversion emission spectra of Er3+-Yb3+-Mo6+-codoped TiO2 under 3+ -Yb 3+ -Mo pumpthe currents. Green and red upconversion emissions were observed in6+the wavelengths Figuredifferent 2 shows upconversion emission spectra of Er -codoped TiO2 under 6+ codoped TiO2. Figure 1. XRD pattern of Er3+-Yb3+-Moto 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → of 500–540 nm, 540–580 nm, and 620–710 nm, corresponding different pump currents. Green and red upconversion emissions were observed in the wavelengths 4I15/2 transitions of Er3+ ions, respectively. Each transition (2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2) 2H 4I 4I 3+ 3+ 6+ of 500–540was nm, 540–580 nm, and 620–710 nm, corresponding to Ñ , 3+4 Ssplit Ñ , and Figure shows upconversion emission spectra of, 4Er -codoped 2 under 11/2 15/2 3/2 2H11/2 4F9/2 levels divided2into two the emission peaks, which indicated S3/2,-Yb and-Mo of ErTiO into 15/2 4F 4 3+ 2 4 4 4 different pump currents. Green and red upconversion emissions were observed in the wavelengths 4S3/2(I)·(SI) and 4S11/2 4F,9/2(I)S Er ions, respectively. Each ( H ÑII), and I15/2 I15/2 , three transitions coupled Starkof sublevels of 2H11/2(I) ·(HI) and 2H11/2(II) ·(HII),transition 3/2(II)·(S ·(F I) Ñ 9/2 Ñ I15/2 3/2 4F9/2 → of 500–540 540–580 nm, and 620–710 nm, corresponding to 2environment H11/2 → 4I15/2, 4on S3/2Er →3+2 4ions. I15/2, and 4 I4F9/2(II))·nm, 4the 4F and (Fwas II), respectively, due to the emission effect of crystal field As LD and 4 F9/2 Ñ divided into two peaks, which indicated H , S , and 15/2 11/2 3/2 9/2 4I15/2 transitions of Er3+ ions, respectively. Each transition (2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2) 3+ split 2 Hnumber¨ (H 2 H emission 4S pump current increased from 0.8 to 2.0 A, the position and of upconversion peaks levels of Erwas into three coupled Stark sublevels of ) and ¨ (H ), ¨ (S 2 11/2 I 4F9/2 levels11/2(II) IIinto 3/2(I) I) 11/2(I) divided intowhereas two emission peaks, which indicated , 4S3/2, and of Er3+due splitto did not change, the intensity of green and red H emissions markedly increased the 4 4 4 2 2 4 4 4 F9/2(I) ¨ (F ) and F9/2(II) ¨ (FHII11/2(II) ), respectively, due Sto of·(Fcrystal field and S3/2(II) ¨ (Scoupled three Stark sublevels I) and ·(HII), S3/2(I)·(SI) and 3/2(II)the ·(SII),effect and F9/2(I) I) II ),inand increase excitation power.I of H11/2(I)·(H 4F9/2(II)·(F and on respectively, due to the effectcurrent of crystalincreased field environment on Er ions.A,Asthe the position LD environment Er3+II),ions. As the LD pump from 0.8 to3+2.0 and current increased from 0.8 to 2.0 A, the position and number of upconversion emission peaks number ofpump upconversion emission peaks did not change, whereas the intensity of green and red did not change, whereas the intensity of green and red emissions markedly increased due to the emissions increase markedly increased due to the increase in excitation power. in excitation power.

Figure 2. Upconversion emissions spectra of Er3+-Yb3+-Mo6+-codoped TiO2 with different pump currents. Inset shows corresponding upconversion emission intensity ratios versus the pump current.

3 3+-Yb3+-Mo6+-codoped TiO2 with different pump Figure 2. Upconversion emissions spectra of Er Upconversion emissions spectra of Er3+ -Yb3+ -Mo6+ -codoped TiO2 with different currents. Inset shows corresponding upconversion emission intensity ratios versus the pump current.

Figure 2. pump currents. Inset shows corresponding upconversion emission intensity ratios versus the pump current. 3

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The inset Figure 2 shows the upconversion emission intensity ratios of HI /HII , SI /SII , FI /FII , Sensors 2015, in 15, page–page and (HI + HII )/(SI + SII ) versus the pump current. All intensity ratios of HI /HII , SI /SII , FI /FII and The inset in Figure 2 shows the upconversion emission intensity ratios of HI/HII, SI/SII, FI/FII, and (HI + HII )/(S I + SII ) increased alongside the pump current, implying that the nonradiative processes (HI + HII)/(S I + S3+ versus the pump current. All intensity ratios of HI/HII, SI/SII, FI/FII and (HI + HII)/(SI + 3+ 3+ 6+ -codoped of Er in Er -Yb II)-Mo TiO2 phosphor can partially transform pump energy into heat SII) increased alongside the pump current, implying that the nonradiative processes of Er3+ in Er3+energy, 3+therefore elevating the phosphor temperature. The temperature variation induced by Yb -Mo6+-codoped TiO2 phosphor can partially transform pump energy into heat energy, therefore increasing the pump current caused changes in the intensity ratio [19]; this suggests that the elevating the phosphor temperature. The temperature variation induced by increasing the pump temperature-dependent intensity for ratio the four energy of HI /HII , SI /SII , FI /FII , current caused changes in the ratio intensity [19]; coupled this suggests thatlevels the temperature-dependent and (H + Sthe utilized for optical temperature I + HII )/(S II ) can intensity ratioI for fourbe coupled energy levels of HI/HII, SI/SII,sensing. FI/FII, and (HI + HII)/(SI + SII) can be Figure 3 shows a schematic energy level diagram of the Er3+ -Yb3+ -Mo6+ -codoped TiO2 utilized for optical temperature sensing. 3+-Yb3+-Mo6+-codoped Figure 3 shows a schematic energy level diagram of the Er TiO2the phosphors phosphors under 976 nm LD excitation. The upconversion mechanism of Er3+ after addition of 3+ 3+ 6+ 2 ´ under 976 nm LD excitation. The upconversion mechanism of Er after the addition of Mo Mo was reported in a previous study on the sensitization of the Yb -MoO4 dimer to Er6+3+was [20–22]. 3+-MoO42−2dimer reported in a previoussensitization study on the sensitization theYb Yb3+ to Er3+two [20–22]. Through Through a cooperative process in of the -MoO4 ´ dimer, excited Yb3+a ions cooperative sensitization process in the Yb3+-MoO 42− dimer, two excited Yb3+ ions nonradiatively nonradiatively transfer their energy to MoO4 2´ . This process is followed by a high excited state transfer their energy to MoO442−. This process 3+ is followed by a high excited state energy transfer energy transfer (HESET) to the F level of Er ions. After nonradiative relaxations from 4 F to (HESET) to the 4F7/2 level of Er3+ 7/2 ions. After nonradiative relaxations from 4F7/2 to the Stark sublevels7/2 the Stark sublevels of H , H , S and S , green upconversion emissions are produced by transitions I II I II emissions are produced by transitions of HI/HII/SI/SII → 4I15/2. of HI, HII, SI and SII, green upconversion of HIThe /HIInonradiative /SI /SII Ñ 4 Irelaxation nonradiative relaxation fromsubsequent SII to FI and FII levels subsequent 15/2 . The from SII to FI and FII levels and transitions of Fand I/FII → 4I15/2 4 transitions of red FI /F II Ñ I15/2 generate red emissions. generate emissions.

Figure 3. Schematic energy level diagram of Er -Yb3+-Mo 6+ TiO2 phosphors under 976 nm Figure 3. Schematic energy level diagram of Er3+ -Yb -Mo -codoped -codoped TiO2 phosphors under 976 nm LD excitation. Wavy arrows indicate nonradiative relaxation. LD excitation. Wavy arrows indicate nonradiative relaxation. 3+

3+

6+

In order to distinguish the effects of temperature from the pump current on the intensity ratio 3+-Mo 6+-codoped In order2),tothedistinguish theemission effects properties of temperature from the pump current on measured the intensity (Figure upconversion of Er3+-Yb TiO2 were 3+ 3+ 6+ 3+ different temperatures. Figureemission 4 shows the upconversion emissions spectra-codoped of Er -Yb3+TiO -Mo26+-were ratio under (Figure 2), the upconversion properties of Er -Yb -Mo codoped TiO2 different at measured temperatures between and 673 Changes in temperature no of measured under temperatures. Figure 4307 shows theK.upconversion emissionshad spectra 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 influence on the bands of green and red emissions from 3+ 3+ 6+ Er -Yb -Mo -codoped TiO2 at measured temperatures between 307 and 673 K. Changes in transitions of Er3+ between 500 to 580 nm and 620 to 700 nm, respectively; the intensity varied with temperature had no influence on the bands of green and red emissions from 2 H11/2 /4 S3/2 Ñ 4 I15/2 temperature, however. The inset in Figure 4 shows the intensity of green and red emissions and the and 4 F9/2 Ñ 4 I15/2 transitions of Er3+ between 500 to 580 nm and 620 to 700 nm, respectively; the intensity ratio of green to red emissions as a function of temperature. The intensity of red emissions intensity varied with temperature, however. The inset in Figure 4 shows intensity of green and decreased with increasing temperature, in accordance with the classical theorythe of thermal quenching. red emissions and the intensity ratio of green to red emissions as a function of temperature. The Temperature-dependent intensity of the red emissions can be expressed as follows [23]: intensity of red emissions decreased with increasing temperature, in accordance with the classical I 0 (1) I T   theory of thermal quenching. Temperature-dependent intensity of the red emissions can be expressed 1  A exp  E kT  as follows [23]: I p0q (1) I pTq “ 1 ` Aexp p´∆E1 {kTq

4

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where T is the absolute temperature, and I(T) and I(0) are the fluorescence intensities at temperatures Sensors 2015, 15, page–page of T and 0 K, respectively; ∆E1 is the activation energy, k is the Boltzmann constant, and A is a constant. The temperature-dependent intensity emissions fits well to Equation (1), where T is the absolute temperature, and I(T) and I(0) areof thered fluorescence intensities at temperatures 1 whereof∆E = 0.074 eV. T and 0 K, respectively; ΔE′ is the activation energy, k is the Boltzmann constant, and A is a constant. (FI+FII) Conversely, the intensity intensity of greenofemissions increased increasing temperature, which The temperature-dependent red emissions fits well with to Equation (1), where ΔE′(FI+FII) = 0.074 eV. does 3+ Conversely, the intensity of green emissions increased withdue increasing which does not satisfy the classical theory of thermal quenching, likely to the temperature, increased Yb absorption not satisfyat theelevated classical theory of thermal[22,24]. quenching,Alikely due totheoretical the increaseddescription Yb3+ absorption cross-section temperatures general of crossthe green section at elevated [22,24]. A general theoretical description of the green upconversion upconversion emissiontemperatures can be given by [22]: emission can be given by [22]:

„ ˆ ˙ hν 2 ´2  h     Igreen “B (2) (2) I green  B11´exp exp  ´ kT   kT    wherewhere B is Ba isconstant, and hνhνis isthe participatingin in multiphonon-assisted a constant, and thephonon phonon energy energy participating thethe multiphonon-assisted excitation. The dependence of green upconversion emissions on temperature fits to Equation excitation. The dependence of green upconversion emissions on temperature fits wellwell to Equation (2). (2). The IThe temperature,causing causing color to turn from to green green /Iredvalue valueincreased increased with with temperature, thethe color to turn from red red to green with with green I/I red elevated temperature. elevated temperature.

3+3+-Yb3+ Figure 4. Upconversion emissions spectra ofof ErEr -Mo6+6+ -codoped TiO at different temperatures. Figure 4. Upconversion emissions spectra -Yb3+-Mo -codoped TiO 2 at2different temperatures. shows the integrated intensity greenand and red red emissions intensity ratio of green to redto red Inset Inset shows the integrated intensity ofofgreen emissionsand andthe the intensity ratio of green emissions a function of temperature.The Thesolid solid lines lines for intensity of redof red emissions as a as function of temperature. forthe thetemperature-dependent temperature-dependent intensity and green emissions are fitting curves by Equations (1) and (2). and green emissions are fitting curves by Equations (1) and (2).

According to previous research [1], the relative population of two “thermally coupled” energy

According previous the relative of two “thermally coupled” levels with to separation of research the order [1], of thermal energypopulation follows a Boltzmann-type population energy levels with separation of the order of thermal energy follows a Boltzmann-type population distribution, causing variation in the transitions of two closely spaced levels at elevated temperature distribution, causing in the transitions of two closelyare spaced levels at two elevated if pumped throughvariation a continuous light source. After populations thermalized closelytemperature spaced levels,through the FIR of upconversionlight emissions (R)After related to the transitions of both levels can closely be written if pumped a continuous source. populations are thermalized at two spaced asthe follows: levels, FIR of upconversion emissions (R) related to the transitions of both levels can be written as follows: I upper NN upper E˙ ˆ Iupper ´∆E upper  C exp R  (3)  (3) R“ I “ N “ Cexp kT   kT lower lower Ilower Nlower , Ilower, , N Nupper upper, ,and Nlower are the intensity and number ions foroftheions upper wherewhere IupperIupper , Ilower and Nlower arefluorescence the fluorescence intensity and of number for the lower thermalizing energy levels, respectively; ΔE is the energy between coupled levels, upperand and lower thermalizing energy levels, respectively; ∆E is thegap energy gaptwo between two coupled and CC is is a constant relative to the degeneracy, emission cross-section, and angular frequency of levels, and a constant relative to the degeneracy, emission cross-section, and angular frequency corresponding transitions. Equation (3) suggests that FIR is related to the energy gap ΔE and of corresponding transitions. Equation (3) suggests that FIR is related to the energy gap ∆E and temperature T. Figure 5 shows FIR plots of (HI + HII)/(SI + SII), HI/HII, SI/SII, and FI/FII as a function of temperature T. Figure 5 shows FIR plots of (HI + HII )/(SI + SII ), HI /HII , SI /SII , and FI /FII as a function of inverse absolute temperature from 307 to 673 5K. The inset shows corresponding upconversion

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inverse absolute temperature from 307 to 673 K. The inset shows corresponding upconversion emission intensity intensity and and the the intensity intensity ratio ratio relative relative to to temperature. temperature. The experimental data fits fits well well to to emission Equation (3). (3). Energy Energy gaps gaps ∆E ΔE of of the the four four coupled coupled energy energylevels levelsof of(H (HII ++ HIIII)/(S HH I/H II, SI/S II,/S and Equation )/(SI I++SIIS),II ), /H , S I II I II , F I /F II are calculated in Table 1. The decreased intensity of two red emissions with elevated and FI /FII are calculated in Table 1. The decreased intensity of two red emissions elevated temperature, shown shown in in the the inset inset of of Figure Figure 5d, 5d, can can also also be be fitted fitted to to Equation Equation (1). (1). The activation activation energy energy temperature, of FFII and FFIIII levels is calculated calculated as as ∆E ΔE′1 FI FI = ΔE′1 FII = 0.080 eV, which is the of = 0.069 eV and ∆E = 0.080 eV, consistent with the FII 1 average activation energy of (F I + F II ) level (ΔE′ (FI+FII) = 0.074 eV) shown in Figure 4. average activation energy of (FI II ) level (∆E (FI+FII) = 0.074 eV) shown in Figure 4.

Figure 5. 5. Cont. Cont. Figure

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Figure 5. FIR II)/(S I + IS+ II);SII (b) HI/H (c)IIS; I(c) /SII;Sand FI/F(d) II as function of inverse FIR plots plots of of(a) (a)(H (HI I++HH ); (b) HIII;/H ; and FIa/F of II )/(S I /SII(d) II as a function temperature in the range of range 307–673 Insets K. show corresponding upconversion emission emission intensity inverse temperature in the of K. 307–673 Insets show corresponding upconversion and intensity relative to relative temperature. FIR plots areFIR fitted by are Equation (3) and the temperatureintensity and ratio intensity ratio to temperature. plots fitted by Equation (3) and the dependent intensities of red emissions in (d) are fitted by Equation (1). temperature-dependent intensities of red emissions in (d) are fitted by Equation (1). Table 1. 1. Energy gap of coupled coupled energy energy levels levels ∆E, ΔE,pre-exponential pre-exponentialfactor factorC, C,maximum maximumsensitivity sensitivitySSmax max, temperature of maximum sensitivity TTmax max and and upconversion upconversion emission emission intensity intensity for for the four coupled energy levels of (HII ++ H I + SIIS),IIH /HI /H II, SII I/S II Iand I/FII.FI /FII . HIIII)/(S )/(S ), IH ,S /SII Fand I+

Coupled Energy Levels I + HII)/(SI + SII) I/HII Coupled Energy Levels (HI + H(H HI /HH II )/(SI + SII ) II

II SIS /SI/S II

FFI/F /FII

ΔE (eV) ∆E (eV) C C ´4 ¨ K´1 ) Smax (10 −4·K−1) Smax (10 Tmax (K) Tmax (K) Upconversion intensity Upconversion intensity

0.0110 0.0110 0.98 0.98 41.4 41.4 64 64 Low Low

0.0093 0.0093 1.61 1.61 81.0 81.0 54 54 Highest Highest

0.0558 0.0558 9.2 9.2 76.7 76.7 324 Higher 324 Higher

0.0107 0.0107 1.6 1.6 69.7 69.7 62 Higher 62 Higher

I

II

For optical temperature-sensing applications, it is crucial to know the rate at which the FIR For optical temperature-sensing applications, it is crucial to know the rate at which the FIR changes with temperature, known as the absolute sensitivity Sa , which is expressed as follows [1]: changes with temperature, known as the absolute sensitivity Sa, which is expressed as follows [1]:

11 dR dR ∆E E SSaa “  R dT “ kT 22 R dT kT

(4) (4)

Equation (4) makes clear that the appropriate selection of two thermally coupled energy levels Equation (4) makes clear that the appropriate selection of two thermally coupled energy levels with a suitable energy difference ∆E is very important. Larger ∆E benefits absolute sensitivity with a suitable energy difference ΔE is very important. Larger ΔE benefits absolute sensitivity and and accurate measurement of emission intensity, due to the decrease of fluorescence peak overlap accurate measurement of emission intensity, due to the decrease of fluorescence peak overlap originating from the two individual thermally coupled energy levels. Knowing this, the absolute originating from the two individual thermally coupled energy levels. Knowing this, the absolute sensitivity Sa when using coupled energy levels of (HI + HII )/(SI + SII ) (with the largest possible sensitivity Sa when using coupled energy levels of (HI + HII)/(SI + SII) (with the largest possible ΔE = ∆E = 0.0558 eV) is higher than those using the other three coupled levels, as shown in Table 1. The 0.0558 eV) is higher than those using the other three coupled levels, as shown in Table 1. The energy energy gap ∆E must be not too large, though, or thermalization no longer occurs. gap ΔE must be not too large, though, or thermalization no longer occurs. Considering practical applications, it is extremely useful to be aware of variations in sensitivity Considering practical applications, it is extremely useful to be aware of variations in sensitivity with temperature. Relative sensitivity Sr is expressed [25]: with temperature. Relative sensitivity Sr is expressed [25]: ∆E dR dR “ R E S r  dT  R kT 22 dT kT Sr “

(5) (5)

Compared to absolute sensitivity Sa , relative sensitivity Sr is dependent on not only energy gap Compared to absolute sensitivity Sa, relative sensitivity Sr is dependent on not only energy gap ∆E, but also the intensity ratio FIR. Equation (3) indicates that larger FIR causes larger C. Thus, larger ΔE, but also the intensity ratio FIR. Equation (3) indicates that larger FIR causes larger C. Thus, larger ∆E and FIR (or C) contribute to higher Sr . Table 1 also shows pre-exponential factor C values for the ΔE and FIR (or C) contribute to higher Sr. Table 1 also shows pre-exponential factor C values for the four pair energy levels (HI + HII )/(SI + SII ), HI /HII , SI /SII , and FI /FII . The coupled energy levels of four pair energy levels (HI + HII)/(SI + SII), HI/HII, SI/SII, and FI/FII. The coupled energy levels of (HI + HII)/(SI + SII) processed larger relative sensitivity Sr than those of HI/HII, FI/FII, or SI/SII. Sr as a function 30987 7

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(HI + HII )/(SI + SII ) processed larger relative sensitivity Sr than those of HI /HII , FI /FII , or SI /SII . Sr of temperature for the four coupled energy levels calculated by Equation (5) is shown in Figure 6, in as a function of temperature for the four coupled energy levels calculated by Equation (5) is shown in accordance with the above results in the measured temperature range 307–673 K. Figure 6, in accordance with the above results in the measured temperature range 307–673 K.

Figure as aafunction functionofoftemperature temperaturefor forthe thefour four coupled energy levels Figure6.6. Relative Relative sensitivities sensitivities S r as coupled energy levels of of (HI (H ), HII,I /H SI /SFIII/F and FI /FII . symbols Closed symbols the experimental data theare lines + IH+IIH )/(S I + SIII+ ), SHIII/H SI/SIIII, and II. Closed are the are experimental data and theand lines the II )/(S are the theoretical calculated by Equation theoretical valuesvalues calculated by Equation (5). (5).

Maximumsensitivity sensitivitySmax Smaxand andtemperature temperatureTTmax max, at which the sensor has maximum sensitivity Maximum , at which the sensor has maximum sensitivity max, are of utmost importance because these two parameters indicate the highest sensitivity SSmax , are of utmost importance because these two parameters indicate the highest sensitivity propertiesand andoptimum optimumoperating operatingtemperature temperaturerange rangeofofoptical opticalthermal thermalsensors. sensors. According Accordingtoto properties Equation(5), (5),SSmax max and and TTmax max can follows: dSrr{dT dT“ 00 asasfollows: Equation can be be calculated calculated by by dS

S max“ Smax

0.54C 0.54C E k ∆E{k

Tmax “ Tmax

1 E

1 ∆E 22 kk

(6) (6) (7) (7)

Equation pre-exponential factor factor CCand andsmaller smallerenergy energydifference differenceΔE ∆Eof Equation(6) (6)indicates indicates that that aa larger larger pre-exponential ofcoupled coupledenergy energy levels help to increase S . Equation (7) shows that T is relative to the max max levels help to increase Smax. Equation (7) shows that Tmax is relative to the energy energy difference ∆E, inthe which thewith sensor withΔE a larger ∆E hasTmax a higher Tmax . for Smax for difference ΔE, in which sensor a larger has a higher . Smax and Tmax theand fourTmax coupled the four levels coupled are1.shown in Table Thefound highest was found (HI + energy HII )/ max energy areenergy shown levels in Table The highest Tmax1.was for T (H I + H II)/(S I + SII)for coupled (Slevels + S ) coupled energy levels used for thermal sensing, due to a larger ∆E. The relatively larger I IIused for thermal sensing, due to a larger ΔE. The relatively larger C and smallest ΔE in FI/FII Ccoupled and smallest in FIused /FII coupled energy levels used for thermal sensing resulted energy∆E levels for thermal sensing resulted in the highest sensitivity Smaxin . the highest sensitivity S . max Temperature measurement error can be calculated using the relation [8,26]: Temperature measurement error can be calculated using the relation [8,26]:

kT 2 R  kT 2 ∆R ∆T “ ∆R RE“ S r T  R

(8) (8)

R∆E Sr Larger Sr and smaller ΔR imply better accuracy. As shown in Figure 6, larger Sr at a higher Larger Sr for andcoupled smaller energy ∆R imply better accuracy. As 6, larger Sr at higher temperature levels of (H I + HII)/(S I +shown SII) ledintoFigure a better accuracy inathe high temperature for coupled energy levels of (H + H )/(S + S ) led to a better accuracy in the I canIIbe expected I II in the low temperature range high temperature range. Likewise, better accuracy using temperature be expected in the low temperature range using HI/HII, SI/SII range. and FI/FLikewise, II coupled better energyaccuracy levels forcan thermal sensing. HI /HIIThe , SI /S and F /F coupled energy levels for thermal sensing. II I of IItwo coupled energy levels ΔE should be large enough to avoid overlap of the separation

two fluorescence emissions and to produce efficient luminescence for feasible and accurate intensity measurement. The efficient luminescence of Er3+-doped materials also contributes to the ready detection of luminescence and ΔR accuracy, where only low excitation power is needed. Table 1 30988 8

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The separation of two coupled energy levels ∆E should be large enough to avoid overlap of the two fluorescence emissions and to produce efficient luminescence for feasible and accurate intensity measurement. The efficient luminescence of Er3+ -doped materials also contributes to the ready detection of luminescence and ∆R accuracy, where only low excitation power is needed. Table 1 shows where (HI + HII )/(SI + SII ) coupled energy levels had the highest accuracy of all samples, due to a larger ∆E and the strongest luminescence intensity; conversely, SI /SII coupled energy levels had the lowest accuracy, evidenced by a smaller ∆E and the lowest luminescence intensity, which are altogether consistent with the results shown in Figure 5. 4. Conclusions The green and red upconversion emissions by transitions of Er3+ Stark sublevels were observed in Er3+ -Yb3+ -Mo6+ -codoped TiO2 phosphors in this study. There are four coupled energy levels of Er3+ ions due to the effect of the crystal field environment on Er3+ , each of which was utilized to study temperature-dependent upconversion emission properties. Based on the FIR technique, the optical temperature-sensing behaviors of sensing sensitivity, measurement error, and operating temperature for the four coupled energy levels were discussed in detail, with all closely related to the energy gap of the coupled energy levels, FIR value, and luminescence intensity. High sensitivity and negligible error are obtainable through the use of different coupled energy levels for optical sensing, throughout a wide range of temperature in an independent system. The utilization of coupled energy levels by Stark split is a new and effective method in the realization of multiple optical temperature measurement. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No 11004021, 11204024, 11274057, and 11474046), Natural Science Foundation of Liaoning Province (Grant No. 2013020089), Program for Liaoning Excellent Talents in University, and Fundamental Research Funds for the Central Universities (Grant No. DC201502080202 and DC201502080406). Author Contributions: This work was carried out in collaboration between all authors. Baosheng Cao, Jinlei Wu, Xuehan Wang and Zhiqing Feng carried out the experiments and analyzed the data. Baosheng Cao, Yangyang He and Bin Dong interpreted the results and wrote the paper. Baosheng Cao, Jinlei Wu, Xuehan Wang and Zhiqing Feng were involved in the experimental design, data acquisition and revision of the manuscript. All of the authors have read and approved the final version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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