Investigation into the temperature sensing behavior of Yb³⁺ sensitized Er³⁺ doped Y₂O₃, YAG and LaAlO₃ phosphors

Abstract

The optical temperature sensors based on Y₂O₃ (YAG/LaAlO₃):Yb³⁺/Er³⁺ have been successfully prepared by the sol–gel method. The sensing behavior of samples was studied as a function of temperature in the range 298–573 K for optical temperature measurement. The maximum sensor sensitivity derived from the ratio of fluorescence intensity (R) of the green upconversion (UC) emissions was approximately 0.0050 K⁻¹ (Y₂O₃). The influence of the different structures and bonding of the hosts on the temperature sensitivities of Er³⁺/Yb³⁺ co-doped Y₂O₃, YAG and LaAlO₃ was investigated in detail based on chemical bond theory. The calculated results could provide certain guiding significance for other relevant temperature sensing materials.

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1. Introduction

Almost all physical, chemical and biological processes are temperature dependent, making accurate temperature knowledge essential for their control and understanding.^1,2 There are many areas of industry where temperature measurements are essential, such as metallurgy, glass manufacturing, material modeling as well as food manipulation and testing.³ In most of these applications, contact-based temperature measurements (using thermocouples, thermistors or resistance thermometers) are not applicable. A fluorescent sensor that makes use of temperature-dependent fluorescence properties from a luminescent material can overcome the interference of strong electromagnetic noise, hazardous sparks, and a corrosive environment, which are inaccessible to traditional temperature-measurement methods.⁴ In this vast field, fluorescence thermometry appears as a tool for non-contact and non-invasive surface temperature measurements, and offers a number of advantages over conventional test methods (infrared pyrometry, thermocouples, thermistors). These include high accuracy, remote detection, high signal yield, cost-effectiveness, and quantitative global temperature/heating information.⁵ As two models of this technique, both the single-color fluorescence thermometry method and the two-color fluorescence thermometry technique are based on a thermophosphor whose fluorescence intensity is temperature dependent.⁶ Most luminescence-based thermometry mainly relies on the temperature-dependent fluorescence intensity of one transition, the accuracy of which is susceptible to errors introduced by optical occlusion, concentration inhomogeneities, excitation power fluctuations, or environment-induced non-radiative relaxation. Ratiometric detection, based on the intensity ratio of two independent emissions of the same phosphor, can circumvent these complications and give rise to more accurate self-referencing thermal sensing, and is thus gaining popularity.⁷

Yb³⁺/Er³⁺ co-doped UC fluorescence materials are of interest for optical temperature sensors. In this method, the temperature of a sample can be obtained by measuring the signal ratio of two fluorescence lines associated with the excited states of trivalent rare earth ions, for example the two energy levels ²H_11/2 and ⁴S_3/2 in the Er³⁺ ion could be used. It is important to choose an appropriate material that presents a highly efficient fluorescence signal and a good thermal stability for successful practical applications of the fluorescence intensity ratio method. In addition, when the phonon energy of the matrix in which the rare earth ions are embedded is low, the intensity of the UC emission will be strong. However, NaYF₄ has a low phonon energy, but its application is greatly restricted because of the poor chemical stability and low laser induced damage threshold of the hosts.^8–10 Therefore it has been recognized that the host matrix plays an important role in the fluorescence properties of the rare earth-doped material. Y₂O₃, Y₃Al₅O₁₂ (YAG) and LaAlO₃ have better stable chemical properties for practical applications than traditional fluoride materials,^11–15 which make Yb³⁺/Er³⁺ co-doped Y₂O₃, Y₃Al₅O₁₂ (YAG) and LaAlO₃ phosphors promising materials in optical temperature sensing. In addition, UC materials exhibit anti-Stokes emission upon low levels of irradiation in the near-infrared (NIR) spectral region with a sharp emission bandwidth, long lifetime, tunable emission, high photostability, and low cytotoxicity, which render them particularly useful for bio-imaging applications.¹⁶ In recent years, photothermal ablation (PTA) therapy has attracted much interest as a minimally invasive alternative to conventional approaches, such as surgery and chemotherapy, for therapeutic intervention at specific biological targets. In particular, NIR (λ = 700–1100 nm) laser induced PTA, which converts NIR optical energy into thermal energy, has gained increasing attention because the NIR laser is absorbed less by biological tissues, and the typical penetration depth of the NIR (such as 980 nm) light can be several centimeters in biological tissues. Therefore, Yb³⁺/Er³⁺ co-doped oxide phosphors could realize three functions: vivo imaging, temperature sensing, and PTA of cancer cells.^17,18

Throughout the literature, we found that the sensitivities of different host materials based on the fluorescence intensity ratio (R) of Er³⁺ and Yb³⁺ were different. For example, the sensitivities in Al₂O₃ nanoparticles, La₂O₃ phosphor, LiNbO₃ particles and phosphate glass ceramics were 0.0051 K⁻¹, 0.0091 K⁻¹, 0.0016 K⁻¹ and 0.0075 K⁻¹, respectively.^12,19–21 Thus, it is necessary to understand which kind of material has higher sensitivity with high fluorescence efficiency and what the influencing factors are for the sensitivity of temperature sensing. In the present paper, we report the temperature sensing characteristics of Er³⁺/Yb³⁺ co-doped Y₂O₃, YAG and LaAlO₃ powders. Then the factors affecting the sensitivity of temperature sensing are given and discussed in detail according to the experimental and theoretically calculated results.

2. Experimental section

2.1. Synthesis

Synthesis of Yb³⁺/Er³⁺ co-doped Y₂O₃. The Y₂O₃:xEr³⁺, yYb³⁺ ((a) y = 0.05; x = 0.01, 0.02, 0.03; (b) x = 0.01; y = 0.01, 0.02, 0.03, 0.04, 0.05) (mole fraction) were synthesized by the sol–gel method. All the chemicals were commercially available and used without further purification. Rare earth nitrate (Y(NO₃)₃, Er(NO₃)₃ and Yb(NO₃)₃) stock solutions of 0.08 M, 0.05 M and 0.05 M, and citric acid (C₆H₈O₇·H₂O) were used as the starting materials. The solutions of Y(NO₃)₃, Er(NO₃)₃ and Yb(NO₃)₃ were mixed together with stirring. Then the citric acid was added to the solutions as a chelating agent for metal ions. The molar ratio of total metal ions to citric acid was 1 : 2. Highly transparent solutions were obtained after stirring for a few minutes. Then the resultant mixtures were dried at 90 °C for 24 h to obtain light-brown dried gels. After grinding in an agate mortar, the gels were fired in air at 1200 °C for 4 h to get the corresponding phosphors.

Synthesis of Er³⁺/Yb³⁺ co-doped YAG. The YAG:xEr³⁺, yYb³⁺ ((a) y = 0.05; x = 0.01, 0.02, 0.03; (b) x = 0.01; y = 0.01, 0.03, 0.05) (mole fraction) were prepared similarly to Y₂O₃:xEr³⁺, yYb³⁺ (x = 0.01; y = 0.01, 0.02, 0.03, 0.04, 0.05). However, Al(NO₃)₃·9H₂O was also necessary. A predetermined amount of Al(NO₃)₃·9H₂O (0.9378 g) was added to the solution in which Y(NO₃)₃, Er(NO₃)₃ and Yb(NO₃)₃ were mixed together, and then the other steps were followed as for the synthesis of Yb³⁺/Er³⁺ co-doped Y₂O₃.

Synthesis of Er³⁺/Yb³⁺ co-doped LaAlO₃. The LaAlO₃:xEr³⁺, yYb³⁺ ((a) y = 0.05; x = 0.01, 0.02, 0.03; (b) x = 0.01; y = 0.01, 0.03, 0.05) (mole fraction) were prepared similarly to YAG:xEr³⁺, yYb³⁺ (x = 0.01; y = 0.01, 0.03, 0.05) except that the gels were fired in air at 900 °C for 4 h.

2.2. Characterization

The crystal structures were obtained from powder X-ray diffraction (XRD) measurements performed on a Rigaku-Dmax 2500 diffractometer at a scanning rate of 15° min⁻¹ in the 2θ range from 10° to 80°, with graphite monochromatized Cu Kα radiation (λ = 0.15405 nm). The morphology and size of the obtained samples were examined by a field emission-scanning electron microscope (FE-SEM, XL30, Philips). All the measurements were performed at room temperature. The UC luminescence spectra were measured by an Andor Shamrock SR-750 fluorescence spectrometer. A CCD detector combined with a monochromator was used for signal collection from 400 to 750 nm. A diode laser tuned to 980 nm was used as the pump source. The diode was coupled to a fiber (core diameter 200 mm, numerical aperture 0.22). The luminescence spectra were recorded by exciting the samples with the 980 nm LD using an Andor SR-500i spectrometer (Andor Technology Co, Belfast, UK). The Yb³⁺/Er³⁺ co-doped Y₂O₃, YAG and LaAlO₃ samples were filled in an iron sample cell and the temperature of the sample was increased from 298 K to 573 K by heating with resistive wire elements. A copper-constant thermocouple buried in the sample was used to monitor the sample's temperature with a measurement error of ±1.5 K.

3. Results and discussion

3.1. Structural investigations

Fig. 1 shows the X-ray diffraction patterns of the Yb³⁺/Er³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors. The results indicated that all diffraction peaks of Y₂O₃ phosphor could be readily indexed to the cubic phase of Y₂O₃ (JCPDS no. 41-1105), and the sample was of good purity. The XRD patterns of YAG phosphor were basically consistent with those of the cubic phase of YAG (JCPDS no. 09-0310), and no other impurities were observed. In contrast to the standard card (JCPDS no. 31-0022), the X-ray diffraction patterns of LaAlO₃ phosphor indicated that LaAlO₃ phosphor exhibited a pure hexagonal phase. Insets show the FE-SEM images of the Yb³⁺/Er³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors, respectively. All phosphors were microcrystalline with flaky morphology.

Fig. 1 X-ray diffraction patterns for Yb³⁺/Er³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors after firing. The standard data for Y₂O₃ (JCPDS card no. 41-1105), YAG (09-0310) and LaAlO₃ (31-0022) are shown for reference. Insets show the FE-SEM images of the Yb³⁺/Er³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors.

3.2. UC luminescence

We chose Y₂O₃ to investigate relative properties among Yb³⁺/Er³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors. Fig. 2 presents the UC emission spectra of the Yb³⁺/Er³⁺ co-doped Y₂O₃ samples with different dopant concentrations under the excitation of a 980 nm diode laser. The results indicated that the UC emission intensity and spectral properties depended on the dopant concentrations. The spectra exhibited two distinct emission bands corresponding to green emission and red emission of Er³⁺ ions. Emissions were centered at 522, 563 and 660 nm, corresponding to green emissions from ²H_11/2 and ⁴S_3/2 and red emission from ⁴F_9/2 excited states to the ⁴I_15/2 ground state of Er³⁺ ions, respectively. When keeping the Er³⁺ ion concentration fixed at 0.01, and decreasing the concentration of Yb³⁺ (from 0.05 to 0.01), the intensity of green UC emission increased apparently with the red emission decreasing. This phenomenon can be explained by the following: in Yb³⁺/Er³⁺ co-doped materials, the energy transfer (ET), as a dominant mechanism, is due to the large absorption cross section of Yb³⁺ and Er³⁺ ions. When the concentration of Yb³⁺ decreased, the co-doping of Yb³⁺ greatly reduced and ET between Yb³⁺ and Er³⁺ greatly reduced, which decreased the population of the ⁴F_9/2 level and relatively increased the population of ²H_11/2 and ⁴S_3/2 levels. As a result, the ⁴F_9/2–⁴I_15/2 transitions of Er³⁺ were considerably decelerated, resulting in the lessening of red emission in the Er³⁺/Yb³⁺ co-doped Y₂O₃ phosphor.²² The insets show the UC emission images of the five samples of Y₂O₃:xEr³⁺, yYb³⁺ (x = 0.01; y = 0.01, 0.02, 0.03, 0.04, 0.05) under 980 nm excitation. In particular, the green emission was strong enough to be seen by the naked eye in the Y₂O₃:0.01Er³⁺, 0.01Yb³⁺ sample. In addition, the UC emission spectra of the Yb³⁺/Er³⁺ co-doped YAG and LaAlO₃ samples with different dopant concentrations under the excitation of the 980 nm diode laser were provided in Fig. S1 and S2 (ESI),† respectively. The spectra exhibited two distinct emission bands, corresponding to green emissions from ²H_11/2 and ⁴S_3/2 and red emission from ⁴F_9/2 excited states to the ⁴I_15/2 ground state of Er³⁺ ions and the UC emission intensities of LaAlO₃ (0.01Er³⁺, 0.03Yb³⁺) and YAG (0.01Er³⁺, 0.05Yb³⁺) were relatively stronger than those of the other samples.

Fig. 2 The UC luminescence spectra of Y₂O₃:xEr³⁺, yYb³⁺ (x = 0.01; y = 0.01, 0.02, 0.03, 0.04, 0.05). The insets show the UC emission images of the five samples under 980 nm excitation.

Fig. 3 reveals the dependence of the intensity of UC peaks on the excitation power of the 980 nm diode laser. The intensity of UC luminescence increased remarkably with the increase of excitation power. The visible luminescence emission intensity of the UC phosphor and the infrared excitation power obey the relation I_μp ∝ (P_pump)ⁿ, where n is the number of pump photons used to populate the UC emission state and I_μp and P_pump represent the UC emission intensity and power, respectively.^23–25 It can be observed that the slopes of the three fitted lines corresponding to 522, 563 and 660 nm emissions are 2.48, 2.34 and 1.84, respectively. The results indicated that two IR excitation photons were required to produce one UC photon in the system of Y₂O₃:0.01Yb³⁺, 0.01Er³⁺.^23,26 At the same time, we obtained the values of YAG (0.01Er³⁺, 0.05Yb³⁺) and LaAlO₃ (0.01Er³⁺, 0.03Yb³⁺). The slopes of the three fitted lines of YAG corresponding to 523, 559 and 669 nm emissions are 1.90, 2.17 and 1.76, respectively (Fig. S3, ESI†). For LaAlO₃, the slopes of the three fitted lines corresponding to 520, 540 and 651 nm emissions are 1.86, 1.75, and 1.65 (Fig. S4, ESI†), respectively. The results indicated that two IR excitation photons were required to produce one UC photon in the systems of Y₂O₃, YAG and LaAlO₃.^27,28

Fig. 3 The log–log plots of UC emission intensity of Y₂O₃:0.01Yb³⁺, 0.01Er³⁺ as a function of the 980 nm laser pumping power.

It is well known that there are two main emission bands corresponding to green emission and red emission, respectively, for Er³⁺ ions. The mechanism of UC luminescence in Er³⁺-doped materials has been extensively studied.^27–30 A possible UC mechanism was proposed and is shown in Fig. 4. The Yb³⁺ ions at ground state can be excited to ²F_5/2 (²F_7/2 → ²F_5/2) through a ground absorption (GSA) process. Then the metastable energy level ⁴I_11/2 of Er³⁺ ions is populated by a first Yb³⁺ → Er³⁺ energy transfer (ET) step: ²F_5/2 (Yb³⁺) + ⁴I_15/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺); Er³⁺ in the ⁴I_11/2 state is next excited to the ⁴F_7/2 state by another ET: ²F_5/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_7/2 (Er³⁺) or the excited state absorption (ESA): ⁴I_11/2 (Er³⁺) + one photon → ⁴F_7/2 (Er³⁺). Then multiphonon relaxation (MPR) happens and populates the lower levels ²H_11/2 (⁴S_3/2). After the first-level excitation, the same laser pumps the excited Er³⁺ from the ⁴I_11/2 and ⁴I_13/2 levels to the ⁴F_7/2 and ⁴F_9/2 levels, respectively. Subsequent MPR processes populate the ²H_11/2 (⁴S_3/2). The radiative relaxation from the thermally coupled levels ²H_11/2 (⁴S_3/2) and the ⁴F_9/2 level to the ground state ⁴I_15/2 of Er³⁺ results in the green emissions and red emissions, respectively.^22,31

Fig. 4 The energy level diagrams of Yb³⁺/Er³⁺ co-doped Y₂O₃ under excitation of 980 nm, and possible UC processes.

3.3. Temperature sensing behavior

The temperature-dependent UC photoluminescent properties of Y₂O₃:Er³⁺, Yb³⁺ phosphor were investigated in terms of fluorescence intensities from two closely spaced energy levels (thermally coupled levels) in order to establish its potential as an optical temperature sensor. It has been shown that the relative population of such thermally coupled levels follows a Boltzmann type population distribution.^31,32 Fig. 5 represents the UC emission spectra of Y₂O₃:0.01Er³⁺, 0.01Yb³⁺ phosphor from 298 K to 573 K; the pumping power of the 980 nm diode laser was set at 1.3 W. It was interesting to find that the fluorescence intensity ratio of the two emissions from ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions increased with increasing temperature. This is because the state of ²H_11/2 can be populated effectively from ⁴S_3/2 by a thermalization process, which will lead to the relative variation of intensity between the two thermally coupled levels to ensure a quasi-thermal equilibrium according to the Boltzmann distribution. Fig. 8(a) and 9(a) show the green UC emission spectra of LaAlO₃ (0.01Er³⁺, 0.03Yb³⁺) and YAG (0.01Er³⁺, 0.05Yb³⁺) at 298 K, 423 K and 548 K, respectively. It also can be observed that the fluorescence intensity ratio of the two emissions from ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions increased with increasing temperature. Since the emission intensities are proportional to the population of each energy level, the ratio of fluorescence intensity from two thermally coupled energy levels can be written as follows.^33–35where

Fig. 5 Temperature evolution of the Er³⁺ UC emission spectra excited by a 980 nm laser for the Y₂O₃:0.01Er³⁺, 0.01Yb³⁺ phosphor from 298 K to 573 K.

N, g, σ, ω are the number of ions, the degeneracy, the emission cross-section and the angular frequency of fluorescence transitions from the upper (²H_11/2) and lower (⁴S_3/2) thermalizing energy levels to level ⁴I_15/2, respectively. I_2j and I_1j are the integrated intensities of the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, respectively. ΔE is the energy difference between the two thermally linked levels, k_B is the Boltzmann constant and T is the absolute temperature.³⁴ Fig. 6(a) shows the fluorescence intensity (integrated area below the fluorescence curves) ratio of green UC emissions I₅₂₂ and I₅₆₃ of the Y₂O₃ phosphor as a function of inverse absolute temperature in the range of 298–573 K on a monolog scale. The experimental data were fitted to a straight line with a slope of about 1145.5, which represents the value of ΔE/k_B. The R of green UC emissions for the ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions relative to temperature in the range of 298–573 K is presented in Fig. 6(b). We can observe that the value of R increases with the sample's temperature. The experimental data are fitted to eqn (1), which matches well, and the value of coefficient B is found to be 10.53. Fig. 8(b) and 9(b) show the fluorescence intensity ratio of green UC emission of LaAlO₃ (0.01Er³⁺, 0.03Yb³⁺) and YAG (0.01Er³⁺, 0.05Yb³⁺) as a function of inverse absolute temperature in the range of 298–573 K on a monolog scale. The values of ΔE/k_B are 575.27 and 800.38, respectively. Fig. 8(c) and 9(c) show the dependence of R for LaAlO₃ (0.01Er³⁺, 0.03Yb³⁺) and YAG (0.01Er³⁺, 0.05Yb³⁺) on the temperature in the range of 298–573 K. The values of B for the best fit curves of the experimental data are found to be 3.44 and 2.48, for LaAlO₃ (0.01Er³⁺, 0.03Yb³⁺) and YAG (0.01Er³⁺, 0.05Yb³⁺), respectively.

Fig. 6 (a) The monolog plot of the fluorescence intensity ratio R versus 1/T for the Y₂O₃ phosphor; (b) R (I₅₂₂/I₅₆₃) of Er³⁺ green UC emissions for the ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transitions relative to the temperature range of 298–573 K.

It is important to know the rate at which the fluorescence intensity ratio changes for a given change in temperature for sensing applications. The temperature sensitivity S can be defined as follows:^36–38

Eqn (2) suggests that using pairs of energy levels with larger energy differences increases the sensitivity of the fluorescence intensity ratio. However special attention must be given to ensuring that the levels are not too far apart to prevent thermalization from being observed. The sensitivity of Y₂O₃:Yb³⁺/Er³⁺ as a function of temperature is shown in Fig. 7. It can be observed that the sensitivity of Y₂O₃:0.01Yb³⁺, 0.01Er³⁺ reached the maximum of about 0.0050 K⁻¹ at the temperature of 563 K. It can be seen from Fig. 8(d) and 9(d) that the sensitivity of LaAlO₃ (0.01Er³⁺, 0.03Yb³⁺) and YAG (0.01Er³⁺, 0.05Yb³⁺) reached maxima of about 0.0032 K⁻¹ at the temperature of 281 K and 0.0017 K⁻¹ at the temperature of 404 K, respectively. In addition, relative parameters for Er³⁺ and Yb³⁺/Er³⁺ co-doped materials are shown in Table 1. It can be seen that the sensitivity alters from 0.0016 K⁻¹ to 0.0091 K⁻¹, as determined by B and ΔE/k_B.

Fig. 7 The sensor sensitivity S = dR/dT as a function of temperature for the Y₂O₃:Yb³⁺/Er³⁺ phosphor. S₂ represents the value of experimental sensitivity.

Fig. 8 Temperature sensing based on LaAlO₃:0.01Er³⁺, 0.03Yb³⁺. The excitation power was 1.3 W. (a) UC spectra of the LaAlO₃:Yb³⁺/Er³⁺ for green emissions at 298 K, 423 K and 548 K; (b) the monolog plot of R as a function of inverse absolute temperature; (c) R between the green emissions at temperatures ranging from 298 K to 573 K; (d) the sensor sensitivity S = dR/dT as a function of temperature. S₂ represents the value of experimental sensitivity.

Fig. 9 Temperature sensing based on YAG:0.01Er³⁺, 0.05Yb³⁺. The excitation power was 1.3 W. (a) UC spectra of the YAG:Yb³⁺/Er³⁺ for green emissions at 298 K, 423 K and 548 K; (b) the monolog plot of R as a function of inverse absolute temperature; (c) R between the green emissions at temperatures ranging from 298 K to 573 K; (d) the sensor sensitivity S = dR/dT as a function of temperature. S₂ represents the value of experimental sensitivity.

Table 1 Summarized performance of Er³⁺ and Er³⁺/Yb³⁺ co-doped materials showing temperature sensing parameters. Values of the maximum sensitivity, the temperature range as well as the excitation wavelength are also included

Host	Temperature range [K]	Excitation wavelength (nm)	Maximum sensitivity [K^−1]	Ref.
Fluorotellurite glass	300–550	800	0.0054	42
Oxy-fluoride ceramics glass	303–823	980	0.0047	43
La2O2S phosphor	290–573	971	0.0080	30
LiNbO3 particles	285–453	980	0.0075	21
CaLa2ZnO5 phosphor	298–513	980	0.0059	44
Ga2S3:La2O3 glass	293–498	1060	0.0052	45
Al2O3 nanoparticles	295–973	978	0.0051	12
Er–Mo:Yb2Ti2O7 nanophosphor	290–610	976	0.0074	38
Er–Mo:Yb3Al5O12 nanocrystals	295–973	980	0.0048	46
GaWO4 phosphor	303–873	980	0.0073	47
Yttrium silicate powders	300–600	975 (pulsed)	0.0070	48
Yttrium silicate powders	300–600	975	0.0056	49
Silicate glass	296–723	978	0.0033	13
Silicate glass	296–673	978	0.0023	50
Phosphate glass ceramics	298–450	975	0.0016	20
La2O3 phosphor	303–600	980	0.0091	19
Y2O3	298–573	980	0.0050	This work
YAG	298–573	980	0.0017	This work
LaAlO3	298–573	980	0.0032	This work

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Many host materials co-doped with Yb³⁺/Er³⁺ have been studied. It can be seen that ΔE differs little among different host materials. The shield of the outermost electrons 5s²5p⁶ keeps the levels of trivalent rare earth ions from the influence of the crystal field of the hosts. Generally, the energy gap ΔE is decided by the even-parity of the crystal field of the hosts and is related to the symmetries of the crystal system. Then the S value is mainly determined by B, which can be rewritten as follows:³⁰

It can be observed that B is determined only by Ω_{λ = 2, 4, 6}.³⁰ The crystal parameter Ω_λ represents several properties of materials. The Ω₆ parameter is related to the rigidity of the medium in which the ions are situated. The Ω₂ is related to symmetry. It is sensitive to changes in constituents of materials, because these lead to changes in crystal structures, bond lengths, covalency and nephelauxetic effects. It is more sensitive to the environments than others among J–O parameters. Thus B is mainly determined by Ω₂ and the Ω₂ parameter is strongly affected by covalent chemical bonding.³⁹ This means that sensitivity is associated with covalent chemical bonding. The covalency between O²⁻ and Er³⁺ is different in the three samples.⁴⁰ Since the Er³⁺ ions occupy the sites of Y and La ions, we calculated bond lengths and bond covalency of Y–O and La–O according to chemical bond theory.⁴¹ The calculated mean values of bond lengths and bond covalency are listed in Table 2. In addition, the related parameters for sensitivity of the different host materials (Y₂O₃, YAG and LaAlO₃) are shown in Table 2. It can be observed that the values of B and S increase with increasing average bond covalency. This means that the sensitivity is proportional to the bond covalency. These experimental and theoretical results are important in increasing our understanding of the temperature-dependent luminescence of doped rare earth ions and in developing their future applications.

Table 2 Mean value of chemical parameters and calculated B and S values in Yb³⁺/Er³⁺ co-doped Y₂O₃, YAG and LaAlO₃

Crystal	Bond	d̄^μ (Å)^a	f̄c^b	B	S (K^−1)
a Average bond length.b Average bond covalency.
Y2O3	Y–O	2.285	0.155	10.527	0.00498
LaAlO3	La–O	2.665	0.122	3.4341	0.00323
YAG	Y–O	2.367	0.066	2.4547	0.00166

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4. Conclusions

In conclusion, Yb³⁺ sensitized Er³⁺ doped Y₂O₃, YAG and LaAlO₃ phosphors have been successfully prepared by the sol–gel method. We studied the temperature sensing behavior of samples and obtained the ratio of fluorescence intensity from two thermally coupled energy levels, which increased with increasing temperature. In addition, we quantified the sensitivity of the samples. The maxima of sensitivity of Y₂O₃, YAG and LaAlO₃ were 0.0050 K⁻¹, 0.0017 K⁻¹, and 0.0032 K⁻¹, respectively. We found that the sensitivity was strongly affected by bond covalency. Correspondingly, we calculated bond lengths and bond co-valencies of Y–O and La–O according to chemical bond theory. We found that the values of B and S increased with increasing average bond covalency, and that the calculated values were basically in good agreement with our experimental results. The experimental and theoretical results are important in increasing our understanding of the temperature-dependent luminescence of doped rare earth ions and in developing their future applications.

Acknowledgements

This work was supported by the Science and Technology Development Planning Project of Jilin Province (20130522173JH), partially sponsored by the China Postdoctoral Science Foundation, supported by the National Foundation for Fostering Talents of Basic Science (no. J1103202) and by the Outstanding Young Teacher Cultivation Plan in Jilin University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05986k

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