Influence of excitation power and doping concentration on the upconversion emission and optical temperature sensing behavior of Er3+: BaGd2(MoO4)4 phosphors

Ruoshan Lei; Degang Deng; Xin Liu; Feifei Huang; Huanping Wang; Shilong Zhao; Shiqing Xu

doi:10.1364/OME.8.003023

1. Introduction

In recent decades, the optical thermometers based on the rare earth (RE) ions doped upconversion (UC) luminescent phosphors have attracted growing interests, as they are able to make the temperature measurements of sub-micron scale or inaccessible objects, such as coal mines, power station, tissues and cells etc., in which the temperatures are undetectable by conventional contact thermometers [1–5]. Among the various optical thermometry strategies, fluorescence intensity ratio (FIR) technique has demonstrated particular advantages such as fast response, electromagnetic passivity, good accuracy and relatively high sensitivity [6, 7]. Generally, the technique measures temperature via analysis of the emission intensity ratio between two thermally coupled energy levels (TCLs) from RE ions, such as Er³⁺, Ho³⁺ and Tm³⁺, etc [7–9]. Especially, the ²H_11/2 to ⁴S_3/2 levels of Er³⁺ ion have been investigated most extensively due to their proper energy gap ( $Δ E$ = ~700cm⁻¹) and relatively high radiative transition efficiencies [9–11].

Most of the previous research works take the view that the FIR values from one pair of TCLs are independent on the excitation power of infrared laser due to their nearly equal photon number UC process [1, 10, 12]. Hence, the effect of excitation power on the performance of luminescence thermometry has been rarely analyzed. However, a few investigations have proposed that two levels of TCLs (such as ²H_11/2 and ⁴S_3/2 of Er³⁺) may exhibit different power dependences, since the excited states in RE ion can be populated and depopulated by a few different channels due to the abundant energy levels [12–15]. Hence, the pump power might also have influence on the optical temperature sensing property. Recently, Marciniak et al. have reported that the LiYbP₄O₁₂:0.1%Er³⁺ nanocrystals exhibit the maximal relative sensitivity value of 2.88%K⁻¹ at average power density smaller than 25mW/cm², but fall to only ~0.5%K⁻¹ at 50mW/cm². And they have suggested that this is resulted from the competition between thermalization population and nonradiative depopulation processes [16].

Although there has been few works concerning the impact of excitation power density on the sensing sensitivities of the optical thermometers, its influence on the measurement accuracy has been seldom considered yet. In fact, the possible dependence of FIRs on the pump power points out the weakness of the previous research works on the laser induced heating effect in UC luminescent phosphors, which show the potential bi-functional applications in optical thermometry and photothermal therapy [17, 18]. Normally, the researches obtained a calibration curve to describe the relationship between FIRs and temperatures at a low excitation power density. And then the FIR data measured under higher excitation power densities were substituted into the calibration curve to estimate the temperature rise of the sample. However, if the FIRs are susceptible to the pump power density, measurement errors could be caused owing to the alteration of the corresponding calibration curves with the excitation power density. Therefore, in-depth study is still urgent to explore the effect of excitation power density on the optical temperature sensing behaviors, especially the measurement error.

On the other hand, several materials have already been applied as the matrixes for RE ions in the application of luminescent thermometry. The double barium lanthanide molybdate BaGd₂(MoO₄)₄ has been suggested to be a good host owing to its high UC efficiency as well as the good physical and chemical stability [19–21]. For instance, Sun et al. have reported that Er³⁺, Yb³⁺, Eu³⁺ co-doped BaGd₂(MoO₄)₄ phosphors demonstrate high luminescence efficiency and have the potential applications in multi-color fluorescence imaging and anti-counterfeiting [21]. However, the investigation on the optical temperature sensing property of Er³⁺ ion in BaGd₂(MoO₄)₄ is still lacking, not to mention the effect of Er³⁺ doping concentration.

In this work, Er³⁺: BaGd₂(MoO₄)₄ phosphors with different concentrations were synthesized by a high temperature solid-state reaction method. We have studied the influences of the excitation power density and Er³⁺ concentration on the UC luminescence and optical temperature sensing behavior of BaGd₂(MoO₄)₄ phosphors based on the FIRs of ²H_11/2 to ⁴S_3/2 in detail. The measurement results obtained from two different calculation methods have been analyzed at high excitation power, which reveals that the internal heating induced by laser in the RE³⁺ doped materials may be overestimated in the past study. Ultimately, the dependence of the sensing sensitivity on Er³⁺ concentration has been investigated to explore the possible pathway to design optical temperature sensing materials.

2. Experimental methods

x mol% Er³⁺: BaGd₂(MoO₄)₄ powders with different concentrations (x = 1, 3, 5, 7 and 9) were successfully prepared by high temperature solid-state reaction method. Er₂O₃ (99.99%), BaCO₃ (A.R.), Gd₂O₃ (99.9%) and MoO₃ (A.R.) were used as the starting materials. The weighted materials were thoroughly mixed by ball milling in anhydrous alcohol for 4h. Then, the powders were sintered at 900 °C for 2h under air atmosphere.

X-ray powder diffraction spectroscopy (XRD) was performed to investigate the phase structure of the samples using a Bruker D2 PHASER Diffractometer with Cu K_α radiation (λ = 1.5406Å). The morphology observation of the samples was carried out on a HITACHI TM 3000 field emission scanning electron microscope (FESEM). The UC luminescence spectra were measured by a FLUOROLOG3/Jobin Yvon spectrofluorometer under a 980nm continuous wave laser diode. A TAP-02 temperature controlling system was applied to adjust the temperature of the sample, which was also monitored by a thermocouple with a temperature accuracy of 0.1K, according to the manufacturer.

3. Results and discussion

The phase of the samples with different concentrations was carefully checked by XRD analysis. As shown in Fig. 1(a), the detected diffraction peaks of all products are consistent well with those appeared in standard pattern of the monoclinic phase BaGd₂(MoO₄)₄ (JCPDS file No. 36-0192). No secondary phases can be observed, indicating that Er³⁺ ions are incorporated into the host lattice. Moreover, a small range of 2θ between 28° and 32° is demonstrated to illustrate the effect of Er³⁺ doping on the structure of host lattice. As shown in Fig. 1(b), the peak positions shift towards larger angles with the increment of Er³⁺ concentration, revealing the shrinkage of the host lattice. This is resulted from the substitution of Gd³⁺ (R_Gd³⁺ = 0.1053nm) ions by the smaller Er³⁺ (R_Er³⁺ = 0.100nm) ions. Furthermore, the sharp diffraction peaks demonstrate the samples are well crystallized.

Fig. 1 (a) XRD patterns of the x mol%Er³⁺ doped BaGd₂(MoO₄)₄ phosphors with JCPDS File No. 36-0192 as a reference. (b) The enlarged XRD patterns for the samples.

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As representatives, Fig. 2(a) and 2(b) show FESEM images of 1% Er³⁺: BaGd₂(MoO₄)₄ and 9% Er³⁺: BaGd₂(MoO₄)₄ phosphors, respectively. It can be found that the samples exhibit the non-uniform morphologies with the aggregated large particles in sub-micrometer range. Besides, Er³⁺ concentration has little impact on the particle sizes.

Fig. 2 SEM images of 1%Er³⁺: BaGd₂(MoO₄)₄ (a) and 9%Er³⁺: BaGd₂(MoO₄)₄ (b) phosphors, respectively.

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Figure 3 shows the UC emission spectra of the samples with different doping concentrations, which were obtained upon 980nm excitation with laser power density of 16.7mW/mm² at room temperature. Three typical emission bands peaking at 534, 556 and 658nm can be detected, which are due to ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺ ions, respectively. The emission intensity of ²H_11/2→⁴I_15/2 transition is lower than that of ⁴S_3/2→⁴I_15/2 transition, which is resulted from the fact that the nonradiative relaxation from ²H_11/2 to ⁴S_3/2 level is efficient due to the relatively small energy gap ( $Δ E$ = ~700cm⁻¹). It can be noticed that the overlap between the two green emission bands can be neglected, which is in favor of temperature sensing based on the two emissions. Besides, it is clear that the green emissions dominate while the intensity of red emission is rather tiny. As shown in the inset of Fig. 3, the intensities of all the emissions enhance with increasing Er³⁺ concentrations from 1 to 7mol%. Compared with 1% Er³⁺: BaGd₂(MoO₄)₄ powders, the emission intensities from ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions are improved by about 3.2, 4.9 and 11.5 times for 7% Er³⁺: BaGd₂(MoO₄)₄ powders, respectively. With further increasing Er³⁺ concentration, the reduction of UC emission intensities is attributed to the concentration quenching effect. Hence, the optimum Er³⁺ concentration for UC luminescence is 7mol%.

Fig. 3 UC emission spectra of Er³⁺: BaGd₂(MoO₄)₄ phosphors on increasing Er³⁺ concentration under 980nm excitation. Inset shows the green and red emission intensities as a function of Er³⁺ ion concentration.

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Figure 4 depicts the energy diagram and the corresponding UC processes of Er³⁺ ions in BaGd₂(MoO₄)₄. Under 980nm excitation, ⁴F_7/2 level can be populated by the ground state absorption (GSA) and then the following excited-state absorption (ESA) or energy transfer UC (ETU) processes. Subsequently, the Er³⁺ ion in ⁴F_7/2 level relaxes rapidly to the lower ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels. Finally, the green (534nm and 556nm) and red (658nm) emissions are produced with the radiative transitions from ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels to ⁴I_15/2 level, respectively. Furthermore, the extremely weak red emission verifies that the nonradiative transition process of ⁴S_3/2→⁴F_9/2 is mainly responsible for the population of ⁴F_9/2 level, which is less likely to occur due to the large energy gap between ⁴S_3/2 and ⁴F_9/2 levels ( $Δ E$ = ~3200cm⁻¹) [19].

Fig. 4 Simplified energy level diagram of Er³⁺ ions in BaGd₂(MoO₄)₄ along with the main UC processes under 980nm excitation.

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To shed further light onto the involved UC luminescence mechanism, the dependence of UC emission intensity on the pump power density is studied. As well known, the relation between the UC emission intensity I_Vis and excitation power P_NIR for an unsaturation luminescence process can be expressed as [22]:

I_{v i s} \propto P_{N I R}^{n}

where n denotes the number of the absorbed photons to populate a particular emitting level. Figure 5 shows the logarithmic dependences of ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 emission intensities on excitation powers in 7% Er³⁺:BaGd₂(MoO₄)₄ powders at 293K and 573K, respectively. The slopes for the fitting lines confirm that both the green and red UC emissions belong to two-photon UC processes, as discussed above. Noteworthily, there are two interesting phenomenon concerning the slope values. Firstly, the slopes for the ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions are not identical, indicating that their dependencies on pump power densities are different. Secondly, it can be noticed that the slope values for 534nm and 556nm emissions change in response to temperature. For instance, the slope values of the fitted lines for 534nm emission are 1.84 and 1.63 at 293 and 573K, respectively. This is resulted from the competition between the thermal population and the nonradiative relaxation of ²H_11/2 and ⁴S_3/2 levels [12, 23, 24]. Hence, the FIR of ²H_11/2 to ⁴S_3/2 (I₅₃₄/I₅₅₆) is believed to be also susceptible to pump power density, which may lead to the change of temperature sensing performance with power density.

Fig. 5 Ln-ln plots of the UC emission intensities versus excitation powers of 980nm laser diode for 7%Er³⁺: BaGd₂(MoO₄)₄ powders at 293K (a) and 573K(b).

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Considering the most intense UC luminescence intensity, 7% Er³⁺: BaGd₂(MoO₄)₄ phosphor is a better luminous one among the samples for practical application. Herein, it is chosen to perform an in-depth study of the temperature sensing behavior. Figure 6(a) shows the temperature-dependent UC luminescence spectra under the 980nm excitation, which are normalized by the peak at 556nm. The excitation power density is set to be 16.7mW/mm² to avoid the possible laser induced thermal effect. With respect to ⁴S_3/2→⁴I_15/2 transition (556nm), the emission intensity of ²H_11/2→⁴I_15/2 transition (534nm) gradually enhances as a function of temperature due to the population redistribution of ²H_11/2 level from ⁴S_3/2 level induced by thermal excitation. According to the theory proposed by Wade et al., the integrated emission intensity ratio of ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions could be described as follows [25]:

F I R = B \exp (- \frac{Δ E}{k T^{}}) + C

where

Δ E

is the fitting energy gap between ²H_11/2 and ⁴S_3/2 levels, k and T are the Boltzmann constant (0.695K⁻¹) and absolute temperature, respectively. B is a pre-exponential constant, which depends on the optical material and intrinsic spectroscopic parameters. C is often used as an offset for the overlap of emission spectra and also stray light. As shown in Fig. 6(b), the FIR values continuously increase with the rise of temperature. The best fitting curve by using Eq. (2) is given as the red solid line, which is expressed as FIR = 15.5exp (−920/T) + 0.06. Furthermore, Fig. 6(c) shows the monolog plot of the FIR value as a function of inverse absolute temperature. It can be found that the logarithm of FIR varies almost linearly in the range of 293-573K, which suggests that the relative population of ²H_11/2 and ⁴S_3/2 levels is mainly governed by the Boltzmann type distribution [26].

Fig. 6 (a) The temperature-dependent UC spectra of 7%Er³⁺: BaGd₂(MoO₄)₄ in the temperature range of 293 to 573K, where the intensity was normalized at 556nm. (b) The plots of FIRs versus the absolute temperatures and the fitting curves by Eq. (2). (c) Monolog natural logarithm plots of FIRs as a function of the inverse temperature at two different excitation densities.

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To further investigate the impact of pump power, the relationship between the FIRs and temperatures at a higher excitation power density of 116.7mW/mm² is also presented in Fig. 6(b) and 6(c). The best fit by using Eq. (2) is FIR = 23.9exp(−1132.6/x) + 0.254, which has been presented as the blue solid line. The correlation coefficient R value is 0.997, indicating the perfect fitting. Obviously, the FIR data obtained at different pump power densities could not be fitted with the same calibration curve (Fig. 6(b) and 6(c)). Besides, at a given temperature, the FIR obtained at 116.7mW/mm² is larger than that at 16.7mW/mm². There are two reasons can be taken into consideration. Firstly, as illustrated in Fig. 5, the ²H_11/2 and ⁴S_3/2 levels have slightly different dependencies on the excitation power. This is related to the complex UC processes. Though the two-photon UC process is mainly responsible for the populations of ²H_11/2 and ⁴S_3/2 levels, three-photon or even four-photon processes may also co-exist [13, 14]. Hence, the ²H_11/2 and ⁴S_3/2 levels are not populated by exactly the same UC processes, giving rising to their different power dependencies. As a result, the FIRs of ²H_11/2 to ⁴S_3/2 are susceptible to pump power density, leading to the change of calibration curves with powers. Secondly, a high excitation power density would result in a heating effect due to the electron-phonon coupling and nonradiative relaxation processes. Hence, the optical heating generated by laser may also contribute to the variation of fitting curves at different power densities, which provides an extra temperature rise of the sample with the increasing power density [6, 10, 27].

The above results point out that unavoidable measurement error can be caused, when the calibration curve and FIR data are not obtained upon the equal excitation power density. However, this error has often been neglected in the previous studies about the optical heating effect and temperature-feedback UC luminescent materials for photothermal therapy. For the purpose of verification, we have studied the impacts of excitation power and calibration curve on the measurement results based on the TCLs of ²H_11/2 and ⁴S_3/2 in 7% Er³⁺: BaGd₂(MoO₄)₄. Accordingly, Eq. (2) has been converted as follows:

T = \frac{Δ E / k}{\ln B - \ln (F I R - C)}

where the values of B, C and △E/k are obtained respectively based on the fitting results. For instance, the △E/k values are 920K and 1132.6K for 16.7mW/mm² and 116.7mW/mm², respectively. The FIR data measured at the high excitation density of 116.7mW/mm² are substituted into the calibration curve obtained at the low density of 16.7mW/mm² (FIR = 15.5exp (−920/T) + 0.06, thus T = 920/[ln15.5-ln(FIR-0.06)]). As shown in Fig. 7(a), there are obvious temperature differences between the calculated values and the experimental ones (reading by a thermocouple). Moreover, the temperature discrepancy gets larger with the increasing temperature, because the population redistribution processes of TCLs become more complicated at higher temperatures [12, 23, 24]. Consequently, when the experimental temperature is at 573K, the temperature of the sample is calculated to be 618K at 116.7mW/mm². Is it true that the laser induced heating effect can give rise to an extra temperature rise of up to 45K?

Fig. 7 Experimental temperature measured from the thermocouple versus calculated temperature using Eq. (3). For better observation, the dashed line as guide is drawn, which corresponds to y = x. (a) The calibration curve obtained at 16.7mW/mm² was used to calculate the temperature of the sample excited at 116.7mW/mm²; (b) The temperatures were calculated with the calibration curves and FIR values obtained upon the same excitation conditions.

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For clarification, the FIR data are also substituted into the corresponding calibration curves obtained at the same pump power densities. As shown in Fig. 7(b), the temperature differences between the calculated values and experimental ones are quite small. When the experimental temperature is at 573K, the calculated temperature is ~577K at 116.7mW/mm². It indicates that the laser induced temperature rise is ~4K, which is much smaller than the value of 45K estimated above. The result reveals that it is inappropriate to use the calibration curve fitted at a low pump power density to calculate the temperature of the object excited at a high power density, which could lead to the large temperature measurement error due to the inconsistent calibration curves at different power densities. Besides, the temperature deviations of the calculated values from the experimental ones are ± 1.2 and 4K for the excitation power densities of 16.7mW/mm² and 116.7mW/mm², respectively. The larger deviation at 116.7mW/mm² is mainly attributed to the stronger laser induced heating effect.

Moreover, we have studied the temperature sensing properties of the samples containing different Er³⁺ ions concentration. Figure 8 shows that the fitting results change with the doping concentration, when the excitation power density was fixed to be 16.7mW/mm². The best fits are FIR = 12.3exp(−906/T) + 0.16, FIR = 16exp(−920/T) + 0.21, FIR = 20.4exp(−1146/T) + 0.23, FIR = 15.5exp(−920/T) + 0.06, FIR = 16.4exp(−1113/T) + 0.22 for 1% Er³⁺, 3% Er³⁺, 5% Er³⁺, 7% and 9% Er³⁺ ions doped samples, respectively (Fig. 8(a)). Accordingly, the △E values can be calculated to be 630cm⁻¹ (1%Er³⁺: BaGd₂(MoO₄)₄), 639cm⁻¹ (3%Er³⁺: BaGd₂(MoO₄)₄), 796cm⁻¹ (5%Er³⁺: BaGd₂(MoO₄)₄), 639cm⁻¹ (7%Er³⁺: BaGd₂(MoO₄)₄) and 773cm⁻¹ (9%Er³⁺: BaGd₂(MoO₄)₄), respectively. Theoretically, △E is a constant in the same host. However, Eq. (2) is a theoretical model and mainly considers the Boltzmann distribution between TCLs and radiative transitions from TCLs to ground state, without including many other factors such as nonradiative relaxation and energy transfer [3, 12]. In the real photoluminescence process, the nonradiative relaxation and energy transfer processes are also active, and affect the FIR values from TCLs. Hence, the small variation of the fitted △E values with Er³⁺ concentration may be related with the complex dynamic process for the population between ²H_11/2 and ⁴S_3/2 levels, which can be influenced by the dopant concentration and the crystal field symmetry around Er³⁺ ions, etc. The similar phenomenon has also been observed in some other kinds of materials [28, 29]. Even so, the fitted △E values for the x% Er³⁺: BaGd₂(MoO₄)₄ powders are in agreement with the experimental value obtained from the diffuse reflection spectrum (~724cm⁻¹, not shown), indicating that the Boltzmann distribution plays the key role in the population of ²H_11/2 and ⁴S_3/2 levels for the developed samples.

Fig. 8 (a) The plots of FIRs versus absolute temperatures and the fitting curves by Eq. (2) for the Er³⁺ ions doped BaGd₂(MoO₄)₄ phosphors with different concentrations. (b) Monolog plots of FIRs as a function of inverse absolute temperature.

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As is known, the absolute (S_a) sensitivity is a key parameter to identify the optical thermometry performance. The S_a is defined as the change rate of FIR with respect to temperature [1–3]:

S_{a} = \frac{d F I R}{d T}

Figure 9(a) depicts the evolution of S_a values of the samples as a function of temperature. All the S_a curves demonstrate maxima with increasing temperature. The relationship between the dopant concentration and the maximal S_a values is presented in Fig. 9(b). As shown, the maximal S_a values increase first, maximize at 5mol%Er³⁺, and then reduce. Consequently, for 5%Er³⁺ doped sample, the maximal S_a value is 110.5 × 10⁻⁴K⁻¹ at 564K, which is among the highest values of the inorganic optical thermometric materials based on TCLs (Table 1).

Fig. 9 (a) Dependence of S_a values on absolute temperature for Er³⁺: BaGd₂(MoO₄)₄ phosphors with different concentration. (b) The variation of the maximal S_a values with respect to Er³⁺ doping concentration. (c) Temperature-induced switching of FIRs measured for 5%Er³⁺:BaGd₂(MoO₄)₄ powders at 16.7mW/mm² (alternating between 293 and 573K)

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Table 1. Optical sensing sensitivities of different RE³⁺ ions doped materials based on TCLs.

View Table

The dependence of the maximal S_a values on the doping concentration can be explained as follows. As shown in Fig. 1, the substitution of Gd³⁺ ions by Er³⁺ ions can lead to the slight distortion of the host lattice, which benefits the radiation from ²H_11/2→⁴I_15/2 hypersensitive transition [30, 31]. Ref [32]. also demonstrates that the reduction of the crystal field symmetry around Er³⁺ ions benefits the enhancement of temperature sensing sensitivity. Hence, a higher sensing sensitivity is expected for increasing Er³⁺ concentration. However, with the further increasing concentration, the luminescence re-absorption originating from the overlap between the absorption and green UC emission bands enhances, which affects the radiative transfer processes and leads to the reduction of sensitivity values [33]. Overall, the results suggest that it may be a promising way to modulate the temperature sensing property of UC luminescent materials by changing the doping concentration.

Additionally, the temperature uncertainty △T, which is a significant parameter for judging the accuracy of optical thermometer, should be taken into account. It can be expressed by the following formula [1, 2, 34]:

Δ T = Δ (F I R) | \frac{d T}{d F I R} | = \frac{Δ (F I R)}{S_{a}}

where △FIR is the standard deviation of FIR, S_a is the absolute sensing sensitivity. The minimal temperature uncertainties are calculated to be 1.35K for 1% Er³⁺: BaGd₂(MoO₄)₄, 1.13K for 3% Er³⁺: BaGd₂(MoO₄)₄, 0.82K for 5%Er³⁺: BaGd₂(MoO₄)₄, 1.01K for 7%Er³⁺: BaGd₂(MoO₄)₄ and 1.07K for 9%Er³⁺: BaGd₂(MoO₄)₄, respectively. It can be noticed that 5%Er³⁺: BaGd₂(MoO₄)₄ powders exhibit the smallest temperature uncertainty. This is because a larger sensing sensitivity is in favor of smaller measurement uncertainty. Finally, Fig. 9(c) presents the dependence of FIRs measured for 5% Er³⁺: BaGd₂(MoO₄)₄ powders on the absolute temperature in five cycling processes. A minimal change of FIR values is observed, indicating a high stability for continuous use. So, the developed phosphor is suitable for optical thermometry.

4. Conclusion

In summary, a series of Er³⁺ doped BaGd₂(MoO₄)₄ phosphors were synthesized by a high temperature solid-state reaction method. The samples emit intense green emissions and tiny red emission originating from the intra-4f radiative transitions of Er³⁺ ions under 980nm excitation. The maximal UC luminescence is achieved at Er³⁺ concentration of 7mol%. Moreover, the FIR of the two green emissions (²H_11/2,⁴S_3/2→⁴I_15/2) and its temperature dependence are found to be susceptible to excitation powers, leading to the inconsistent calibration curves at different power densities. Consequently, the substitution of FIR data measured at a higher power density into the calibration curve obtained at a lower power density can lead to a large thermometric error. Meanwhile, Er³⁺ doping concentration is found to affect the temperature sensing sensitivity significantly. The maximum S_a of 110.5 × 10⁻⁴K⁻¹ is achieved for 5mol%Er³⁺ doped phosphor at 564K, which is higher than that of most RE ions doped materials based on TCLs. Overall, the present work opens a guidance to avoid the unnecessary thermometric error and a method to tune the optical sensing property of UC luminescent materials.

Funding

Natural Science Foundation of Zhejiang Province (No. LY18E020008 and No. LD18F050001); National Natural Science Foundation of China (No. 61605192 and No. 51602301).

References

1. M. Quintanilla and L. M. Liz-Marzán, “Guiding rules for selecting a nanothermometer,” Nano Today 19, 126–145 (2018). [CrossRef]

2. S. Balabhadra, M. L. Debasu, C. D. S. Brites, R. A. S. Ferreira, and L. D. Carlos, “Upconverting nanoparticles working as primary thermometers in different media,” J. Phys. Chem. C 121(25), 13962–13968 (2017). [CrossRef]

3. X. F. Wang, Q. Liu, Y. Y. Bu, C. S. Liu, T. Liu, and X. H. Yan, “Optical temperature sensing of rare-earth ion doped phosphors,” RSC Advances 5(105), 86219–86236 (2015). [CrossRef]

4. Y. Q. Zhang, S. Xu, X. P. Li, J. S. Zhang, J. S. Sun, H. P. Xia, R. N. Hua, and B. J. Chen, “Temperature sensing, excitation power dependent fluorescence branching ratios, and photothermal conversion in NaYF₄:Er³⁺/Yb³⁺ @NaYF₄:Tm³⁺/Yb³⁺core-shell particles,” Opt. Mater. Express 8(2), 368–384 (2018). [CrossRef]

5. Z. Huang, Z. Q. Nie, M. B. Xie, Y. X. Wang, and D. Y. Li, “Excellent optical thermometry based on upconversion emission in SrMoO₄:Er³⁺ phosphor,” Opt. Mater. Express 7(7), 2404–2410 (2017). [CrossRef]

6. A. K. Soni and V. K. Rai, “Thermal and pump power effect in SrMoO₄:Er³⁺-Yb³⁺ phosphor for thermometry and optical heating,” Chem. Phys. Lett. 667, 226–232 (2017). [CrossRef]

7. L. Marciniak, A. Bednarkiewicz, and W. Strek, “Tuning of the up-conversion emission and sensitivity of luminescent thermometer in LiLaP₄O₁₂:Tm,Yb nanocrystals via Eu³⁺ dopants,” J. Lumin. 184, 179–184 (2017). [CrossRef]

8. S. S. Zhou, S. Jiang, X. T. Wei, Y. H. Chen, C. K. Duan, and M. Yin, “Optical thermometry based on upconversion luminescence in Yb³⁺/Ho³⁺ co-doped NaLuF₄,” J. Alloys Compd. 588, 654–657 (2014). [CrossRef]

9. L. Mukhopadhyay, V. K. Rai, R. Bokolia, and K. Sreenivas, “980 nm excited Er³⁺/Yb³⁺/Li⁺/Ba²⁺: NaZnPO₄ upconverting phosphors in optical thermometry,” J. Lumin. 187, 368–377 (2017). [CrossRef]

10. P. Du, L. Luo, X. Huang, and J. S. Yu, “Ultrafast synthesis of bifunctional Er³⁺/Yb³⁺-codoped NaBiF₄ upconverting nanoparticles for nanothermometer and optical heater,” J. Colloid Interface Sci. 514, 172–181 (2018). [CrossRef] [PubMed]

11. J. Cao, F. Hu, L. Chen, H. Guo, C. Duan, and M. Yin, “Wide-range thermometry based on green up-conversion luminescence of K₃LuF₆:Yb³⁺/Er³⁺ bulk oxyfluoride glass ceramics,” J. Am. Ceram. Soc. 100(5), 2108–2115 (2017). [CrossRef]

12. X. F. Wang, Q. Liu, P. Q. Cai, J. Wang, L. Qin, T. Y. Vu, and H. J. Seo, “Excitation powder dependent optical temperature behavior of Er³⁺ doped transparent Sr_0.69La_0.31F_2.31 glass ceramics,” Opt. Express 24(16), 17792–17804 (2016). [CrossRef] [PubMed]

13. L. P. Li, L. J. Zheng, W. Xu, Z. Liang, Y. Zhou, Z. G. Zhang, and W. W. Cao, “Optical thermometry based on the red upconversion fluorescence of Er³⁺ in CaWO₄:Yb³⁺/Er³⁺ polycrystalline powder,” Opt. Lett. 41(7), 1458–1461 (2016). [CrossRef] [PubMed]

14. Y. Tian, R. Hua, J. Yu, J. Sun, and B. Chen, “The effect of excitation power density on frequency upconversion in Yb³⁺/Er³⁺ codoped Gd₆WO₁₂ nanoparticles,” Mater. Chem. Phys. 133(2–3), 617–620 (2012). [CrossRef]

15. F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev. 104(1), 139–174 (2004). [CrossRef] [PubMed]

16. L. Marciniak, K. Waszniewska, A. Bednarkiewicz, D. Hreniak, and W. Strek, “Sensitivity of a nanocrystalline luminescent thermometer in high and low excitation density regimes,” J. Phys. Chem. C 120(16), 8877–8882 (2016). [CrossRef]

17. Y. L. Wei, C. H. Su, H. B. Zhang, J. Shao, and Z. L. Fu, “Thermal sensor and optical heater of upconversion phosphor: Yb³⁺/Er³⁺ codoped KY(MoO₄)₂,” Physica B 525, 149–153 (2017). [CrossRef]

18. R. Dey, A. Pandey, and V. K. Rai, “Er³⁺-Yb³⁺ and Eu³⁺-Er³⁺-Yb³⁺ codoped Y₂O₃ phosphors as optical heater,” Sens. Actuators B Chem. 190, 512–515 (2014). [CrossRef]

19. H. Y. Du, Y. J. Lan, Z. G. Xia, and J. Y. Sun, “Synthesis and upconversion luminescence properties of Yb³⁺/Er³⁺ codoped BaGd₂(MoO₄)₄ powder,” Mater. Res. Bull. 44(8), 1660–1662 (2009). [CrossRef]

20. J. Y. Sun, Y. J. Lan, Z. G. Xia, and H. Y. Du, “Sol-gel synthesis and green upconversion luminescence in BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphors,” Opt. Mater. 33(3), 576–581 (2011). [CrossRef]

21. J. Y. Sun, W. Zhang, W. H. Zhang, and H. Y. Du, “Synthesis and two-color emission properties of BaGd₂(MoO₄)₄:Eu³⁺,Er³⁺,Yb³⁺ phosphors,” Mater. Res. Bull. 47(3), 786–789 (2012). [CrossRef]

22. Y. N. Bao, X. S. Xu, J. L. Wu, K. C. Liu, Z. Y. Zhang, B. S. Cao, and B. Dong, “Thermal-induced local phase transfer on Ln³⁺-doped NaYF₄ nanoparticles in electrospun ZnO nanofibers: enhanced upconversion luminescence for temperature sensing,” Ceram. Int. 42(10), 12525–12530 (2016). [CrossRef]

23. L. Xing, Y. Xu, R. Wang, and W. Xu, “Influence of temperature on upconversion multicolor luminescence in Ho³⁺/Yb³⁺/Tm³⁺-doped LiNbO₃ single crystal,” Opt. Lett. 38(14), 2535–2537 (2013). [CrossRef] [PubMed]

24. J. F. Suyver, A. Aebischer, S. García-Revilla, P. Gerner, and H. U. Güdel, “Anomalous power dependence of sensitized upconversion luminescence,” Phys. Rev. B 71(12), 125123 (2005). [CrossRef]

25. S. A. Wade, S. F. Collins, and G. W. Baxter, “The fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4756 (2003). [CrossRef]

26. Z. H. Feng, L. Lin, Z. Z. Wang, and Z. Q. Zheng, “Low temperature sensing behavior of upconversion luminescence in Er³⁺/Yb³⁺ codoped PLZT transparent ceramic,” Opt. Commun. 399, 40–44 (2017). [CrossRef]

27. X. Zhu, W. Feng, J. Chang, Y. W. Tan, J. Li, M. Chen, Y. Sun, and F. Li, “Temperature-feedback upconversion nanocomposite for accurate photothermal therapy at facile temperature,” Nat. Commun. 7, 10437 (2016). [CrossRef] [PubMed]

28. X. Wang, Y. Wang, Y. Bu, X. Yan, J. Wang, P. Cai, T. Vu, and H. J. Seo, “Influence of doping and excitation powers on optical thermometry in Yb³⁺-Er³⁺ doped CaWO₄,” Sci. Rep. 7(1), 43383 (2017). [CrossRef] [PubMed]

29. S. F. León-Luis, U. R. Rodríguez-Mendoza, I. R. Martín, E. Lalla, and V. Lavín, “Effects of Er³⁺ concentration on thermal sensitivity in optical temperature fluorotellurite glass sensors,” Sens. Actuators B Chem. 176, 1167–1175 (2013). [CrossRef]

30. S. F. Leon-Luis, U. R. Rodríguez-Mendoza, P. Haro-Gonzalez, I. R. Martín, and V. Lavín, “Role of the host matrix on the thermal sensitivity of Er³⁺ luminescence in optical temperature sensors,” Sens. Actuators B Chem. 174, 176–186 (2012). [CrossRef]

31. W. Xu, Y. Cui, Y. W. Hu, L. J. Zheng, Z. G. Zhang, and W. W. Cao, “Optical temperature sensing in Er³⁺-Yb³⁺ codoped CaWO₄ and the laser induced heating effect on the luminescence intensity saturation,” J. Alloys Compd. 726, 547–555 (2017). [CrossRef]

32. X. Ming, Q. Y. Meng, S. C. Lü, and W. J. Sun, “The hydrothermal synthesis and morphology-dependent optical temperature sensing properties of Er³⁺ doped NaGd(WO₄)₂ phosphor,” J. Lumin. 192, 196–202 (2017). [CrossRef]

33. W. A. Pisarski, J. Pisarska, R. Lisiecki, and W. Ryba-Romanowski, “Sensitive optical temperature sensor based on up-conversion luminescence spectra of Er³⁺ ions in PbO–Ga₂O₃–XO₂ (X = Ge, Si) glasses,” Opt. Mater. 59, 87–90 (2016). [CrossRef]

34. M. D. Dramićanin, “Sensing temperature via downshifting emissions of lanthanide-doped metal oxides and salts. A review,” Methods Appl. Fluoresc. 4(4), 042001 (2016). [CrossRef] [PubMed]

35. B. Dong, D. P. Liu, X. J. Wang, T. Yang, S. M. Miao, and C. R. Li, “Optical thermometry through infrared excited green upconversion emissions in Er³⁺ -Yb³⁺ codoped Al₂O₃,” Appl. Phys. Lett. 90(18), 181117 (2007). [CrossRef]

36. S. F. León-Luis, U. R. Rodriguez-Mendoza, E. Lalla, and V. Lavin, “Temperature sensor based on the Er³⁺ green upconverted emission in a fluorotellurite glass,” Sens. Actuators B Chem. 158(1), 208–213 (2011). [CrossRef]

37. K. Zheng, W. Song, G. He, Z. Yuan, and W. Qin, “Five-photon UV upconversion emissions of Er³⁺ for temperature sensing,” Opt. Express 23(6), 7653–7658 (2015). [CrossRef] [PubMed]

38. P. Singh, P. K. Shahi, A. Rai, A. Bahadur, and S. B. Rai, “Effect of Li⁺ ion sensitization and optical temperature sensing in Gd₂O₃: Ho³⁺/Yb³⁺,” Opt. Mater. 58, 432–438 (2016). [CrossRef]

39. G. R. Chen, R. S. Lei, H. P. Wang, F. F. Huang, S. L. Zhao, and S. Q. Xu, “Temperature-dependent emission color and temperature sensing behavior in Tm³⁺/Yb³⁺:Y₂O₃ nanoparticles,” Opt. Mater. 77, 233–239 (2018). [CrossRef]

Sample	Transitions	S_a (10⁻⁴K⁻¹)	T (K)	Reference
Er:BaGd₂(MoO₄)₄	²H_11/2,⁴S_3/2→⁴I_15/2	110.5	293-573	This work
Er/Yb: Al₂O₃	²H_11/2,⁴S_3/2→⁴I_15/2	51	295-973	[35]
Er: fluorotellurite glass	²H_11/2,⁴S_3/2→⁴I_15/2	54	327-540	[36]
Er/Yb: NaLuF₄	²H_11/2,⁴S_3/2→⁴I_15/2	73	303-503	[37]
Er/Yb/Sr: CaWO₄	²H_11/2,⁴S_3/2→⁴I_15/2	107	298-573	[28]
Ho/Yb: Gd₂O₃	⁵F₄, ⁵S₂→⁵I₈	92	300–673	[38]
Tm/Yb: Y₂O₃	¹G_4(a),¹G_4(b)→³H₆	28	303-573	[39]

Influence of excitation power and doping concentration on the upconversion emission and optical temperature sensing behavior of Er³⁺: BaGd₂(MoO₄)₄ phosphors

Abstract

1. Introduction

2. Experimental methods

3. Results and discussion

4. Conclusion

Funding

References

Cited By

Figures (9)

Tables (1)

Equations (5)

Optical Materials Express