Upconversion Luminescence and Discussion of Sensitivity

Apr 17, 2018 - The diffraction peaks could be attributed to monoclinic Y2WO6 structure (JCPDS Card No.73-0118). No impurity phase is found when the RE...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Upconversion Luminescence and Discussion of Sensitivity Improvement for Optical Temperature Sensing Application Jia Zhang,* Baowei Ji, Guibin Chen, and Zhenghe Hua Physics Department and Jiangsu Key Laboratory of Modern Measurement Technology and Intelligent Systems, Huaiyin Normal University, 111 West Chang Jiang Road, Huai’an 223001, China S Supporting Information *

ABSTRACT: Upconversion (UC) based luminescent materials have promising applications in noncontact temperature sensors. How to improve the sensitivity is one main object at present. This work presented several strategies for optical temperature sensing based on UC spectra of the Y2WO6:Yb3+-Er3+/Ho3+/Tm3+ phosphors. The improvement for the relative (SR) and absolute (SA) sensitivities were discussed by using a fluorescence intensity ratio technique. It includes thermally coupled levels (TCLs) and non-TCLs. It was proposed that a piecewise expression could be employed to achieve high SA value for TCLs. However, improving the SR value is limited for TCLs. With regard to the non-TCLs, SR and SA are not restricted, but not easy to be improved synchronously. On the other hand, the morphology and UC spectra of the samples were also studied. The above investigation could be instructive to develop new luminescent materials with high sensitivity.

1. INTRODUCTION The upconversion (UC) luminescence in rare-earth (RE) doped materials has gained much attention due to their multiple applications, for instance, display technologies, optical temperature sensors, biomedical imaging, data storage, etc.1−6 Among them, luminescent temperature sensors have been widely studied. Conventional temperature sensors depend on the principle of liquid and metal expansion.7,8 They are limited in fast-moving objects, intracellular temperature, and harsh environment.8,9 Nowadays, noncontact temperature sensors based on optical thermometry methods have been widely developed. The fluorescence lifetime and fluorescence intensity ratio (FIR) techniques are regarded as two promising strategies.8,10 The lifetime-based measurement is independent of the sensor concentration,11 but it was not found suitable for higher temperature.12 The FIR technique includes two kinds of energy levels: thermally coupled levels (TCLs) and non-TCLs. The former one utilizes the change of transition intensities derived from two TCLs of the RE3+ with temperature. This method does not rely on the spectra losses and fluctuations of excitation intensity, which could show high accuracy and resolution.7,13,14 Generally, the energy gap between the TCLs (ΔETCL) should be higher than 200 cm−1 but lower than 2000 cm−1.8 Several RE ions meeting this requirement have been reported at present, such as Er3+ (2H11/2, 4S3/2), Nd3+ (4F7/2, 4 F3/2), Tm3+ (3F2,3, 4I15/2), Dy3+ (4F9/2, 4I15/2), and Eu3+ (5D0, 5 D1).8,15 Among these RE ions, the Er3+ is most used, and the energy difference between its TCLs 2H11/2/4S3/2 is about 700− 800 cm−1.8,16 Unfortunately, the sensor sensitivity of Er3+ is restricted owing to the limit of the energy difference of TCLs.17 © XXXX American Chemical Society

As an alternative strategy to improve the sensitivity, the nonTCLs are receiving much attention. The non-TCLs method generally utilizes two emitting energy levels derived from one or two luminescent ions, and a linear relationship between the FIR and temperature is usually obtained.18−21 For any strategy mentioned above, enhancing the sensitivity of the optical thermometer effectively is an important object for researchers. The tungstate compounds have been widely employed as the matrixes of UC phosphors due to the relatively low phonon energy. Thereinto, the Y2WO6-based phosphors could serve in lighting, displays, and other fields. For instance, Li22 and Qian23 et al. studied the Y2WO6:Eu3+ and Y2WO6:Sm3+ phosphors for light-emitting diodes; the controllable morphology via selfassembly of RE activated Y2WO6 phosphors was reported;24−27 the UC spectra of Tm3+-Yb3+ codoped Y2WO6 were evaluated by Rai et al.28 Nevertheless, the research on temperature sensors on the basis of the UC luminescence of Yb3+-Er3+/ Ho3+/Tm3+ doped Y2WO6 phosphors is still lacking. In this paper, two points are focused on for these phosphors. On one hand, the detailed UC spectra of Yb3+-Er3+/Ho3+/Tm3+ activated Y2WO6 was introduced. On the other hand, the sensitivities of these phosphors were discussed by employing the decay-lifetime and the FIR technique with TCLs and nonTCLs. More importantly, we proposed effective approaches to increase the sensitivities for the FIR technique, which are instructive to develop other new luminescent materials with high sensitivities for temperature sensing. Received: January 13, 2018

A

DOI: 10.1021/acs.inorgchem.8b00102 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

2. EXPERIMENTAL SECTION Powder samples of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3), Y1.98‑yWO6:yYb3+,0.02Ho3+ (0.1 ≤ y ≤ 0.4), and Y1.996‑zWO6:zYb3+,0.004Tm3+ (0.1 ≤ z ≤ 0.4) were synthesized by the solid-state reaction method. Y2O3 (99.99%), Yb2O3 (99.99%), Er2O3 (99.9%), Ho2O3 (99.99%), and Tm2O3 (99.99%) were used as starting reactants. All the above reagents were provided by Sinopharm Chemical Reagent Co., Ltd. Appropriate amounts of raw materials were thoroughly mixed in an agate mortar. The mixture was calcined at 1623 K for 2 h in air. The phase purity was examined by an ARL X’TRA powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 35 mA. The morphologies of the samples were inspected by field emission scanning electron microscopy (FESEM, FEI, Quanta FEG, operated at 5.0 kV), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) (JEOL 2100F, operated at 200.0 kV). The luminescence spectra were measured on an EI-FS5 fluorescence spectrophotometer. The decay curves excited at 980 and 518 nm were measured with a 980 nm pulsed laser and a 5 W microsecond xenon flashlamp as excitation sources, respectively. The temperature-dependent measurement was carried out using an EI-FS5 fluorescence spectrophotometer with the samples mounted on a heating device. The temperature of the heating device can vary from room temperature to 573 K in steps of 0.1 K.

Figure 2. (a) SEM image of Y2WO6, inset presents its particle size distribution; (b) enlarged SEM image of Y2WO6; (c) TEM image of Y2WO6, inset shows the enlarged TEM image for one particle; (d) HRTEM image of Y2WO6, inset shows its Fourier transformation pattern.

3. RESULTS AND DISCUSSION 3.1. XRD and Morphology Analysis. Figure 1 shows the XRD patterns of the typical Y1.88WO6:0.1Yb3+,0.02Er3+,

to the surface. The smooth surface is helpful to reduce the nonradiation and scattering, which is beneficial to the enhancement of the phosphors’ luminescence efficiency.29 The smooth surface of the particles can be also verified from the TEM image, as shown in Figure 2c. The morphology characteristics are in agreement with those observed from Figure 2a, and a very smooth surface is found in the inset of Figure 2c. The HRTEM image at the edge of a particle is given in Figure 2d. The lattice fringes are clearly visible with a spacing of 0.298 nm, in agreement with the lattice spacing of (013) for Y2WO6, which corresponds to the strongest diffraction peak in the XRD patterns. The Fourier transformation pattern (see inset of Figure 2d) can be well indexed according to the Y2WO6 structure. To investigate the influence of the RE dopants on the morphology, Figure S1a-c presents the SEM images of the typical Y1.88WO6:0.1Yb3+,0.02Er3+, Y1.68WO6:0.3Yb3+,0.02Ho3+, and Y1.896WO6:0.1Yb3+,0.004Tm3+ phosphors, respectively. It can be found that no obvious difference in shape and size appears for these samples, which may be due to their similar crystal structure. 3.2. Spectral Characteristics. 3.2.1. Luminescence of Yb3+-Er3+ Codoped Y2WO6. Figure 3a shows the emission spectra of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3) under 980 excitation. The green emissions at 524 and 545 nm have been found. They can be ascribed to the Er3+2H11/2-4I15/2 and 4 S3/2-4I15/2 transitions, respectively.30 The red ones are assigned to the Er3+4F9/2-4I15/2 transition.30 It can be also found the Er3+ emission intensity becomes very weak when Yb3+ is absent, but it can be increased greatly by codoping Yb3+. This is owing to the efficient energy transfer (ET) from Yb3+ to Er3+, on the basis of the large absorption cross section of Yb3+ at 980 nm and large energy overlap between Yb3+ and Er3+.31 Moreover, the strongest green emission around 545 nm is obtained for x = 0.1, while the red one around 675 nm is for x = 0.3, indicating that the population mechanisms for green and red emissions are different and FIR (R R/G ) will change with Yb 3+ concentration. To clearly observe this point, Figure 3b shows their normalized (for 545 nm) emission spectra. The inset presents the integrated intensities of the 2H11/2-4I15/2 (from 510

Figure 1. XRD patterns of Y 1 . 8 8 WO 6 :0.1Yb 3 + ,0.02Er 3 + , Y1.68WO6:0.3Yb3+,0.02Ho3+ and Y1.896WO6:0.1Yb3+,0.004Tm3+.

Y1.68WO6:0.3Yb3+,0.02Ho3+, and Y1.896WO6:0.1Yb3+,0.004Tm3+ samples. The diffraction peaks could be attributed to monoclinic Y2WO6 structure (JCPDS Card No.73-0118). No impurity phase is found when the RE ions are doped into the host lattice. Figure 2a,b demonstrates the SEM images of the Y2WO6 host. It can be found that most of the particles exhibit a polyhedron shape, and there is a degree of aggregation between the particles. To evaluate the dimensional homogeneity of particle size, the inset of Figure 2a presents the particle size distribution, which was taken from statistical analysis for approximately 120 particles. The sizes of most particles are between 1.8 and 2.8 μm, and the average particle size was obtained to be about 2.35 μm. Figure 2b shows the SEM image of one particle under higher magnification to learn the details on the particles’ surface. It has been found that the particle of polyhedron has a smooth surface, and no small particles adhere B

DOI: 10.1021/acs.inorgchem.8b00102 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) Emission spectra of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3) excited at 980 nm; (b) normalized emission spectra of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3) under 980 excitation, inset shows the emission intensities of both the 2H11/2-4I15/2 and 4F9/2-4I15/2 transitions as a function of Yb3+ concentration; (c) emission spectra of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3) excited at 518 nm, inset depicts the intensity ratio (I657/I545) as a function of Yb3+ concentration; (d) normalized (for 545 nm) emission spectra of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3) under 518 nm excitation; (e) energy level diagram for Yb3+-Er3+; (f) decay curves of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3).

where τ1 and τ2 are the fast and slow components of the luminescent lifetimes, A1 and A2 are the fitting parameters, respectively. The corresponding τi and Ai (i = 1, 2) were demonstrated in Table S1. The average decay lifetime can be calculated by36

to 540 nm) and 4F9/2-4I15/2 (from 630 to 700 nm) emissions as a function of Yb3+ concentration. On the whole, both the transition intensities increase with increasing Yb3+ content, which reveals the introduction of Yb3+ is helpful to the population on the 4F9/2 level of Er3+. In previous reports,8,32 the increase of the population the Er3+4F9/2 level could be realized by the cross-relaxation (CR) processes described as 4F7/2 + 4 I11/2 → 24F9/2 and 4S3/2 + 4I13/2 → 4F9/2 + 4I11/2 in the phosphors where the Yb3+ concentration is fixed and the Er3+ concentration is increased, because an increase of Er3+ content increases the Er3+-Er3+ interaction owing to the shorter ion−ion distance, resulting in larger CR rate.32 However, the RR/G in the present phosphors is still enlarged by increasing the Yb3+ concentration when the Er3+ content is fixed. Thus, it can be predicted some other ET mechanisms may exist. To interpret this, the down-shifting luminescence of the Yb3+-Er3+ doped Y2WO6 phosphors was studied. Figure 3c presents the emission spectra of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3) under 518 nm excitation (the Er3+4I15/2-4H11/2 emission). Inset presents the FIR (I657/I545) as a function of Yb3+ concentration. A gradual enhancement with increasing Yb3+ concentration has been found. The normalized (for 545 nm) emission spectra in Figure 3d further reflect this point, agreeing with the spectra by exciting at 980 nm in Figure 3b. It has been witnessed in the previous references that the CR process Er3+(4S3/2) + Yb3+(2F7/2) → Er3+(4I13/2) + Yb3+(2F5/2) becomes more efficient at higher Yb3+ concentration (see Figure 3e).33−35 This can be also verified in the Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3) phosphors by measuring the decay curves (see Figure 3f). All the decay curves have been fitted via a doubleexponential equation: I = A1 exp( −t /τ1) + A 2 exp(−t /τ2)

= (A1τ12 + A 2 τ22)/(A1τ1 + A 2 τ2)

(2)

So, 24.9, 21.4, 17.2, 13.1, and 12.4 μs were obtained for x = 0, 0.05, 0.1, 0.2 and 0.3, respectively. The decreasing lifetime for the 4S3/2 level with increasing Yb3+ concentration could be owing to the above CR process, which further decreases the 4 S3/2-4I15/2 radiative transition intensity largely. Immediately, an energy back transfer (marked as CRB in Figure 3e) from Yb3+ to Er3+ could occur by Er3+(4I13/2) + Yb3+(2F5/2) → Er3+(4F9/2) + Yb3+(2F7/2). This CRB process is the main reason for the increased population on the Er3+4F9/2 level, which causes the increased FIR of the 4S3/2-4I15/2 to 4F9/2-4I15/2 emissions with the Yb3+ content increased as mentioned above. On the other hand, the 2H11/2 level of Er3+ is almost not involved in the above CR process, so when the 4S3/2 → 4I15/2 emission is normalized, the 2H11/2 → 4I15/2 transition intensity also increases gradually with increasing Yb3+ concentration, as shown in Figure 3b and its inset. In addition, it can be noticed in Figure 3c that the intensity of the green 4S3/2-4I15/2 transition exhibits a gradual decrease with the Yb3+ content increased, which is different from that in the UC luminescence spectra in Figure 3a. This is due to the different luminescence mechanisms. For the down-shifting luminescence, the Er3+ is directly excited by 518 nm and then transfers the energy to Yb3+ via CR process (see Figure 3e). So the green emission intensity of Er3+ decreases as the Yb3+ content increases. But, for the UC luminescence, the ET of Yb3+ → Er3+ plays a main role in the populations on Er3+ levels. Thus, with increasing

(1) C

DOI: 10.1021/acs.inorgchem.8b00102 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Emission spectra of Y1.88WO6:0.1Yb3+,0.02Er3+ excited at 980 nm under different temperatures; (b) dependence of relative intensities of 524, 545, and 675 nm emissions on the absolute temperature; dependence of (c) I524/I545 and (d) I524/I675 on the absolute temperature, the insets in panels (c) and (d) show temperature-induced switching of R(I524/I545) and R(I524/I675) (alternating between 303 and 563 K), respectively.

= 0.0022T − 0.479 by fitting these experimental data, indicating the FIR for 524 and 675 nm emissions changes with temperature linearly. To evaluate the repeatability of measurement, the insets in Figure 4c,d depict the temperature-induced switching for R(I524/I545) and R(I524/I675) (alternating between 303 and 563 K), respectively. Both the FIRs are repeatable and reversible for several cycling processes. According to the mechanism of the thermally coupled character of the Er3+2H11/2 and 4S3/2, it can be concluded that the decay lifetime for the 4S3/2 level will decrease with increasing temperature due to the ET from the 4S3/2 to 2H11/2 level. Figure 5a demonstrates the decay curves of Y 1.88WO6 :0.1Yb3+,0.02Er3+ by exciting at 980 nm and monitoring at 545 nm at various temperatures. These decay curves can be reproduced by eq 1. The corresponding τi and Ai (i = 1, 2) are shown in Table S2. Via eq 2, the average lifetimes were gained to be 214.8, 210.8, 204.5, 198.8, 191.6, 186.3, 180.9, 175.3, and 169.5 μs when the temperature is 303, 333, 363, 393, 423, 453, 483, 513, and 543 K, respectively. The decrease of decay lifetime as rising temperature could be interpreted by an Arrhenius-type model shown as follows,11,38 which attributes the temperature dependence of the excited state exclusively to a thermally activated nonradiative decay.

Yb3+ concentration, the Er3+ emission intensity is increased first. But when the Yb3+ concentration reaches a certain value, the CRB from Er3+ to Yb3+ will play an important role, which decreases the Er3+ emission intensity. This is why an optimal Yb3+ concentration exists in Figure 3a. Figure 4a presents the UC emission spectra of Y1.88WO6:0.1Yb3+,0.02Er3+ by exciting at 980 nm under different temperatures. It can be noticed the red 4F9/2-4I15/2 transition intensity of Er3+ shows a decrease as the temperature rises and the green transition intensities exhibit some difference. Figure 4b depicts the relative intensities of the 2 H11/2-4I15/2 emission for 524 nm (integrated from 510 to 540 nm), the 4S3/2-4I15/2 transition of 545 nm (integrated from 541 to 570 nm) and the 4F9/2-4I15/2 transition of 675 nm (integrated from 640 to 685 nm) as a function of the absolute temperature. It can be noticed that the Er3+4F9/2-4I15/2 and 4S3/2-4I15/2 emissions show a continuous decrease, but the 2H11/2-4I15/2 one has been largely enhanced with increasing temperature. The relative population of the 2H11/2 and 4S3/2 TCLs follows the Boltzmann distribution; that is, the FIR of the 2H11/2-4I15/2 and 4S3/2-4I15/2 emissions can be expressed as37 R=

I524 ⎛ −ΔE ⎞ ⎟ = N exp⎜ ⎝ kT ⎠ I545

(3)

τ(T ) = τ0/(1 + Ce−ΔE / kT )

where ΔE is the energy gap between the H11/2 and S3/2 levels, k = 0.695 K−1 cm−1 is the Boltzmann constant, T is the absolute temperature, and N is the proportionality constant. Figure 4c demonstrates the dependence of I524/I545 on temperature. By eq 3, these experimental data could be reproduced, and the ΔE can be calculated to be 724 cm−1. This value is close to the splitting of 700−800 cm−1 between 2H11/2 and 4S3/2 levels.8,16 Figure 4d demonstrates the dependence of I524/I675 on the absolute temperature. Linear relationship can be gained with R 2

4

(4)

where τ(T) and τ0 are the decay lifetimes at temperature T and 0 K, respectively, C is a rate constant, ΔE is the energy difference between the emitting level and upper excited state level, and T is absolute temperature. Figure 5b demonstrates the change of Er3+ lifetime with the absolute temperature. By fitting the data, a function τ = (0.0044 + 0.0131 exp(−1164.2/ T))−1 has been obtained. So ΔE equals 809 cm−1. It is close to that by employing the FIR strategy. Hence, evaluating the D

DOI: 10.1021/acs.inorgchem.8b00102 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) Decay curves of Y1.88WO6:0.1Yb3+,0.02Er3+ under different temperatures; (b) change of Er3+ lifetime with the absolute temperature.

variety of Er3+ decay lifetime with temperature could be another strategy for temperature sensing. 3.2.2. Luminescence of Yb3+-Ho3+ Codoped Y2WO6. Figure 6a presents the emission spectra of Y1.98‑yWO6:yYb3+,0.02Ho3+ (0.1 ≤ y ≤ 0.4) by exciting at 980 nm. The strongest transition is found at 662 nm. It belongs to the 5F5-5I8 transition of Ho3+.39 The other two around 540 and 758 nm could be attributed to the (5F4,5S2)-5I8 and (5F4,5S2)-5I7 transitions of Ho3+.39 Moreover, the optimal Yb3+ concentration was determined to be y = 0.3. The emission spectra of Y1.68WO6:0.3Yb3+,0.02Ho3+ under 980 excitation at various temperatures are shown in Figure 6b. The green and red emission intensities exhibit different change regulation with temperature. The inset depicts the relative intensities of the (5F4,5S2)-5I8 transition of 540 nm (integrated from 525 to 565 nm) and 5F5-5I8 transition of 662 nm (integrated from 625 to 685 nm) as a function of the absolute temperature. The intensity for 540 nm emission is decreased continuously when the temperature increases. However, the red transition around 662 nm is strengthened at first and then decays beyond 413 K. To interpret this result, Figure 6c demonstrates the energy level diagram of Yb3+-Ho3+. In our previous reference, the main UC ET processes have been discussed.40 According to the Mott−Seitz model, the nonradiative de-excitation probability (Knr) could be approximately expressed as18

⎛ ΔE ⎞ ⎟ K nr ∝ exp⎜ − ⎝ kT ⎠

Figure 6. (a) Emission spectra of Y1.98‑yWO6:yYb3+,0.02Ho3+ (0.1 ≤ y ≤ 0.4) under 980 nm excitation; (b) emission spectra of Y1.68WO6:0.3Yb3+,0.02Ho3+ by exciting at 980 nm under different temperatures, inset shows the relative intensities of the 540 and 662 nm emissions as a function of the absolute temperature; (c) energy level diagram of Yb3+-Ho3+; (d) dependence of I662/I540 on the absolute temperature.

temperature, which can increase the population of the electrons on the 5F5 level. So, the (5F4,5S2)-5I8 emission intensity has been enhanced first with increasing temperature. Meanwhile, the relaxation 2 will decrease the (5F4,5S2)-5I8 radiative transition probability, which can be understood from the change of the decay lifetime for the (5F4,5S2) levels. The corresponding decay curves of Y1.68WO6:0.3Yb3+,0.02Ho3+ excited at 980 nm and monitored at 540 nm under different temperatures are shown in Figure S2. The corresponding τi and Ai (i = 1, 2) are shown in Table S3. By eq 2, the average lifetimes were 48.2, 47.5, and 42.3 μs, respectively. The decreasing lifetime implies the intensified nonradiative relaxation from the (5F4,5S2) to other levels. Nevertheless, the 5 F5-5I8 transition starts to decrease when the temperature continues to rise owing to the increasing nonradiative relaxation from the 5F5 to lower levels or other sinking states under the condition that the nonradiative relaxations 1 and 2 cannot effectively supply the energy that lost on the 5F5 level. The above luminescence characteristics reveal the FIR of the Ho3+ 662 and 540 nm emissions will change with temperature, which could be considered to apply to the temperature sensing. Figure

(5)

where ΔE is the energy difference between two levels, k is the Boltzmann constant, and T is the absolute temperature. Hence, the nonradiative relaxation processes of 5I6 → 5I7 (1) and (5F4,5S2) → 5F5 (2) will be intensified with increasing E

DOI: 10.1021/acs.inorgchem.8b00102 Inorg. Chem. XXXX, XXX, XXX−XXX

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Tm3+3H4-3H6 transition.40 The bluish green peaks around 485 nm are ascribed to the Tm3+1G4-3H6 emission. The weak red peaks in the range of 640−730 nm are derived from the 1G4-3F4 and (3F2,3F3)-3H6 emissions of Tm3+.41,42 It can be also found the Tm3+ emission intensity shows a gradual decrease with Yb3+ concentration increased. Figure 8a presents the emission spectra of Y1.896WO6:0.1Yb3+,0.004Tm3+ under 980 excitation for different temperatures. The dependence of the relative intensities of 473, 485, 691, and 795 nm emissions on the absolute temperature is shown in Figure 8b. It is interesting to find all the emission intensities increase first and then decrease, or rather, the intensities of the 485 and 795 nm emissions are enhanced continuously before 413 K and those of 473 and 691 nm until T = 453 K. A possible reason can be combined with the phonon-assisted ET from Yb3+ to Tm3+. Figure 8c shows the energy level diagram of Yb3+-Tm3+. In our previous references, the specific upconversion luminescence has been discussed.40,43 The phonon density of states in a material can be described as 1 = exp(ℏω / kT ) − 1 , where ℏω is the phonon energy, k is

6d presents the dependence of I662/I540 on the absolute temperature. With increasing temperature, the I662/I540 value is enhanced gradually, and these experimental data could be fitted linearly by y = 0.011T − 1.48. 3.2.3. Luminescence of Yb3+-Tm3+ Codoped Y2WO6. Figure 7 depicts the emission spectra of Y1.996‑zWO6:zYb3+,0.004Tm3+

Boltzmann constant, and T is absolute temperature.44,45 The phonon density of states will increase when the temperature rises, which will be beneficial to the ET from Yb3+ to Tm3+ when the Yb3+ absorbs 980 nm photons from the excitation source. However, with the temperature further increased, the nonradiative de-excitation probability (see eq 5) of the excited

Figure 7. Emission spectra of Y1.996‑zWO6:zYb3+,0.004Tm3+ (0.1 ≤ z ≤ 0.4) under 980 nm excitation.

(0.1 ≤ z ≤ 0.4) by exciting at 980 nm. In the near-infrared (NIR) range, the transition peaks are assigned to the

Figure 8. (a) Emission spectra of Y1.896WO6:0.1Yb3+,0.004Tm3+ under 980 excitation at various temperatures; (b) dependence of relative intensities of the 473, 485, 691, and 795 nm emissions on the absolute temperature; (c) energy level diagram of Yb3+-Tm3+; plot of logarithmic of (d) I691/I795 and (e) I473/I485 versus inverse absolute temperature. F

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Figure 9. (a) Relative sensitivities for (I524/I545)-Er3+, (I691/I795)-Tm3+, and (I473/I485)-Tm3+ as a function of the absolute temperature; (b) absolute sensitivities of Y1.88WO6:0.1Yb3+,0.02Er3+ for 524 and 545 nm emissions as a function of the absolute temperature; absolute sensitivities of Y1.896WO6:0.1Yb3+,0.004Tm3+ for (c) 691 and 795 nm emissions and (d) 473 and 485 nm emissions as a function of the absolute temperature; (e) relative sensitivities for (I524/I675)-Er3+ and (I662/I540)-Ho3+ as a function of the absolute temperature; (f) relative lifetime-sensitivity of Y1.88WO6:0.1Yb3+,0.02Er3+ as a function of the absolute temperature.

when the temperature reaches 453 K, the nonradiative relaxation from (3F2,3F3) to lower levels or other sinking states could play an important role gradually, which starts to decrease the lifetime of (3F2,3F3) levels and the (3F2,3F3)-3H6 transition intensity. Due to the thermal coupling effect, Figure 8c,d shows the plots of logarithmic of I691/I795 and I473/I485 versus inverse absolute temperature according to Boltzmann distribution in eq 3. The ΔE was obtained to be 1517.7 and 344.1 cm−1, respectively. 3.3. Discussion of Sensitivities. For the temperature sensing application, the relative (SR) and absolute (SA) sensitivities are very important parameters. As analyzed above, the FIR technique involves two kinds, i.e., TCLs and non-TCLs. Here, the discussion about the sensitivities is given separately. First, the SR and SA for TCLs can be calculated by the following formulas:48,49

state levels also increases, which can weaken the emission of Tm3+. Thus, the change of the emission intensities in Figure 8b could be the result of the competition between the phononassisted ET from Yb3+ to Tm3+ and the nonradiative deexcitation of the excited state levels of Tm3+. On the other hand, it is known that the 3H4 and (3F2,3F3) levels, as well as 1 G4(a) and 1G4(b) levels, are thermally coupled.46,47 When the temperature increases, the populations of electrons on the (3F2,3F3) and 1G4(a) levels will be increased owing to the thermal coupling with the 3H4 and 1G4(b) level, respectively. This is also the reason that the 795 and 473 nm emissions are enhanced until the temperature reaches a higher value compared with the 691 and 485 nm emissions. To further interpret the thermal coupling in Tm3+, the decay curves of Y1.896WO6:0.1Yb3+,0.004Tm3+ under various temperatures were measured. Figure S3a shows the decay curves excited at 980 nm and monitored at 806 nm (Tm3+3H4 level). By fitting with eq 1, τi and Ai (i = 1, 2) were obtained and given in Table S4. The average lifetimes were calculated to be 289.6, 288.0, 285.7, 280.3, 276.8, 274.7, 252.3, 230.7, and 208 μs when the temperature is 303, 333, 363, 393, 423, 453, 483, 513, and 543 K, respectively. Figure S3b depicts the average decay lifetime as a function of temperature. The lifetime for the Tm3+3H4 level demonstrates a slow decrease within 453 K but a quick decay beyond 453 K. Figure S4 presents the decay curves by exciting at 980 nm and monitoring 690 nm ((3F2,3F3) levels), which have been fitted by eq 1. The τi and Ai (i = 1, 2) were demonstrated in Table S5. The average decay lifetimes were calculated to be 77.3, 108.9, and 102.3 μs for T = 303, 423, and 513 K, respectively. The change of the lifetime could be interpreted as follows. As presented above, the lifetime of the 3 H4 level is much larger than that of (3F2,3F3) levels at room temperature. When the temperature increases continuously, the ET from the 3H4 to (3F2,3F3) levels will be intensified. So, the decay lifetime of the (3F2,3F3) levels will be prolonged at first, which comes closer to the lifetime of the 3H4 level. However,

SR =

1 d(R ) ΔE = R d(T ) kT 2

(6)

SA =

⎛ ΔE ⎞ d(R ) = R⎜ 2 ⎟ ⎝ kT ⎠ d(T )

(7)

Figure 9a presents the relative sensitivities for (I524/I545)-Er3+, (I691/I795)-Tm3+, and (I473/I485)-Tm3+ as a function of the absolute temperature with regard to TCLs on the basis of Figures 4c, 8d, and 8e, respectively. All the SR values decrease with increasing temperature, and SR for (I691/I795)-Tm3+ shows the highest value under the same temperature. According to eq 6, the relative sensitivity for TCLs is only proportional to the energy gap ΔE at a certain temperature. Thus, increasing ΔE value is the only way to improve the relative sensitivity. However, the ΔE between two TCLs must be less than 2000 cm−1 to allow the upper level of activators to have a minimum population in the temperature range of interest.8,50 As a result, G

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the definitions, the relative and absolute sensitivities at T2 temperature could be described as

the relative sensitivity for FIR technique by employing TCLs is restricted theoretically. On the other hand, the ΔE value changes little for a trivalent optically active ion such as the widely used Er3+ and Tm3+, although it is doped into different hosts. That is to say, further increasing the relative sensitivity of these active ions by using TCLs is very hard. Nevertheless, it is possible to enhance the absolute sensitivity for an active ion with TCLs because the SA is proportional to not only ΔE but also R according to eq 7. Especially, enlarging the R value is an effective way when the ΔE is restricted. In the previous references, the striving direction to increase the R value is mainly focused on enlarging the proportionality constant N in eq 3 where the R refers to the FIR of the upper level to lower level for TCLs. However, this is not urgently necessary because an equivalent but simple strategy to improve the R value by adjusting the ratio between the TCLs can be adopted in the actual measurement. This process makes no difference to the relative sensitivity but greatly affects the absolute sensitivity. Figure 9b shows the absolute sensitivities for 524 and 545 nm emissions of Y1.88WO6:0.1Yb3+,0.02Er3+ as a function of the absolute temperature. The FIRs of both I524/I545 and I545/I524 are employed, and the resultant absolute sensitivities are marked as SA1 and SA2 in Figure 9b, respectively. It can be found that the SA1 and SA2 curves converge at about 437 K. On the basis of this observation, we propose that the absolute sensitivity SA could be also described by a piecewise expression; that is, SA = SA1 when T ≥ 437 K and SA = SA2 when T ≤ 437 K. The corresponding SA curve as a function of temperature is also demonstrated in Figure 9b. By this way, the absolute sensitivity of an active ion could be largely enhanced. Figures 9c,d represent the absolute sensitivities of Y1.896WO6:0.1Yb3+,0.004Tm3+ for 691 and 795 nm emissions (SA1 and SA2 curves were obtained by using FIR = I691/I795 and I795/I691, respectively) and 473 and 485 nm emissions (SA1 and SA2 curves were obtained by using FIR = I473/I485 and I485/I473, respectively; SA is the corresponding piecewise expression) as a function of temperature. Similarly, the absolute sensitivities of Y1.896WO6:0.1Yb3+,0.004Tm3+ can be optimized effectively by this process. Second, for the non-TCLs, the linear relationship is usually obtained as mentioned in the Introduction. Scheme 1 shows a diagram for calculating sensitivities of this kind. According to

SR (T2) =

R − R1 R /R 1 d(R ) 1 = 2 = − 1 2 R d(T ) R 2*ΔT ΔT ΔT

(8)

SA(T2) =

R − R1 d(R ) = 2 d(T ) ΔT

(9)

where ΔT = T2 − T1. Set ΔT = 1; thus, high SR(T2) and SR(T2) values can be gained only when increase the R2/R1 and R2 − R1 per unit temperature, respectively. Theoretically speaking, the SR(T2) and SA(T2) values are not limited, but it is a challenge to obtain very large R2/R1 and (R2 − R1) per unit temperature synchronously. On the basis of the above discussion, the absolute sensitivities of (I524/I675)-Er3+ and (I662/I540)-Ho3+ in Figures 4d and 6d at any temperature were achieved to be 0.0022 and 0.011 K−1, respectively. Their relative sensitivities as a function of temperature are depicted in Figure 9e. Unfortunately, the high SR and SA were not obtained synchronously in the same sample. Besides the sensitivities for FIR technique, the relative lifetime-sensitivity based on eq 4 was also studied, which can be described as SR = ΔE kT 2

⎛ τ (T ) ⎞ 1 d(τ ) Ce−ΔE / kT ΔE = = ⎜1 − ⎟ 2 −ΔE / kT τ d(T ) τ0 ⎠ kT ⎝ 1 + Ce (10)

As mentioned above, the decay lifetime of an active ion decreases with rising temperature in the present case, i.e., τ(T) < τ0, and thus the relative lifetime-sensitivity obtained by eq 10 will lower than the FIR relative sensitivity by eq 6 for the same luminescent ion with regard to TCLs. The relative lifetimesensitivity of Y1.88WO6:0.1Yb3+,0.02Er3+ as a function of the absolute temperature is given in Figure 9f. When the temperature rises, SR is enhanced at first and reaches the maximum 0.1%/K at the temperature of 488 K. So, the relative lifetime-sensitivity is lower than the SR for (I524/I545)-Er3+ in Figure 9a, which is in agreement with the above analysis. Due to the above discussion, some suggestions can given for developing new luminescent materials with high sensitivities: (1) For TCLs, selecting luminescent ions with ΔETCL as large as possible on the premise that the thermally coupled effect can work effectively; for a selected luminescent ion such as Er3+ and Tm3+, one must chose a suitable host material to achieve strong luminescence intensity in order to reduce error in measurement. (2) For non-TCLs, selecting suitable luminescent ions and host materials to obtain R2/R1 and (R2 − R1) values per unit temperature as high as possible. (3) The relative lifetime-sensitivity is generally lower than the FIR relative sensitivity for TCLs in the same active ion.

Scheme 1. Diagram for Calculation of Sensitivities for NonTCLs

4. CONCLUSIONS In conclusion, the Y2WO6:Yb3+-Er3+/ Ho3+/Tm3+ samples were synthesized by a solid-state reaction method. In the Yb3+-Er3+ codoped phosphors, it was found the FIR of red to green emissions increases with increasing Yb3+ content because of the H

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(2) Marciniak, L.; Prorok, K.; Bednarkiewicz, A. Size dependent sensitivity of Yb3+,Er3+ upconverting luminescent nano-thermometers. J. Mater. Chem. C 2017, 5, 7890−7897. (3) Stepuk, A.; Casola, G.; Schumacher, C. M.; Kramer, K. W.; Stark, W. Purification of NaYF4-based upconversion phosphors. Chem. Mater. 2014, 26, 2015−2020. (4) Wang, R.; Li, X.; Zhou, L.; Zhang, F. Epitaxial seeded growth of rare earth nanocrystals with efficient 800 nm near infrared to 1525 nm short wavelength infrared downconversion photoluminescence. Angew. Chem. 2014, 126, 12282−12286. (5) Kore, B. P.; Kumar, A.; Pandey, A.; Kroon, R. E.; Terblans, J. J.; Dhoble, S. J.; Swart, H. C. Spectroscopic investigation of upconversion properties in green emitting BaMgF4:Yb3+,Tb3+ Phosphor. Inorg. Chem. 2017, 56, 4996−5005. (6) Galleani, G.; Santagneli, S. H.; Ledemi, Y.; Messaddeq, Y.; Janka, O.; Pöttgen, R.; Eckert, H. Ultraviolet upconversion luminescence in a highly transparent triply-doped Gd3+-Tm3+-Yb3+ fluoride-phosphate glasses. J. Phys. Chem. C 2018, 122, 2275−2284. (7) Cai, J.; Zhao, L.; Hu, F.; Wei, X.; Chen, Y.; Yin, M.; Duan, C.-K. Temperature sensing using thermal population of low-lying energy levels with (Sm0.01Gd0.99)VO4. Inorg. Chem. 2017, 56, 4039−4046. (8) Li, L.; Guo, C.; Jiang, S.; Agrawal, D. K.; Li, T. Green upconversion luminescence of Yb3+-Er3+ co-doped CaLa2ZnO5 for optically temperature sensing. RSC Adv. 2014, 4, 6391−6396. (9) Brites, C. D. S.; Lima, P. P.; Silva, N. J. O.; Millán, A.; Amaral, V.; Palacio, F.; Carlos, L. D. Lanthanide-based luminescent molecular thermometers. New J. Chem. 2011, 35, 1177−1183. (10) Collins, S. F.; Baxter, G. W.; Wade, S. A.; Sun, T.; Grattan, K. T. V.; Zhang, Z. Y.; Palmer, A. W. Comparison of fluorescence-based temperature sensor schemes: Theoretical analysis and experimental validation. J. Appl. Phys. 1998, 84, 4649. (11) Peng, H.; Stich, M. I. J.; Yu, J.; Sun, L.-N.; Fischer, L. H.; Wolfbeis, O. S. Luminescent europium(III) nanoparticles for sensing and imaging of temperature in the physiological range. Adv. Mater. 2010, 22, 716−719. (12) Rai, V. K.; de Araujo, C. B. Limit of accuracy for fluorescence lifetime temperature sensing. Spectrochim. Acta, Part A 2008, 71, 116− 118. (13) Vetrone, F.; Naccache, R.; Zamarron, A.; Juarranz de la Fuente, A.; Sanz-Rodriguez, F.; Martinez Maestro, L. M.; Rodriguez, E. M.; Jaque, D.; Sole, J. G.; Capobianco, J. A. Temperature sensing using fluorescent nanothermometers. ACS Nano 2010, 4, 3254−3258. (14) del Rosal, B.; Ximendes, E.; Rocha, U.; Jaque, D. In vivo luminescence nanothermometry: from materials to applications. Adv. Opt. Mater. 2017, 5, 1600508. (15) Du, P.; Luo, L.; Park, H.-K.; Yu, J. S. Citric-assisted sol-gel based Er3+/Yb3+-codoped Na0.5Gd0.5MoO4: A novel highly-efficient infraredto-visible upconversion material for optical temperature sensors and optical heaters. Chem. Eng. J. 2016, 306, 840−848. (16) Tian, Y.; Tian, Y.; Huang, P.; Wang, L.; Shi, Q.; Cui, C. Effect of Yb3+ concentration on upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ co-doped YNbO4 nanoparticles prepared via molten salt route. Chem. Eng. J. 2016, 297, 26−34. (17) Lu, H.; Hao, H.; Shi, G.; Gao, Y.; Wang, R.; Song, Y.; Wang, Y.; Zhang, X. Optical temperature sensing in β-NaLuF4:Yb3+/Er3+/Tm3+ based on thermal, quasi-thermal and non-thermal coupling levels. RSC Adv. 2016, 6, 55307−55311. (18) Zheng, S.; Chen, W.; Tan, D.; Zhou, J.; Guo, Q.; Jiang, W.; Xu, C.; Liu, X.; Qiu. Lanthanide-doped NaGdF4 core−shell nanoparticles for non-contact self-referencing temperature sensors. Nanoscale 2014, 6, 5675−5679. (19) Zhang, X.; Xu, J.; Guo, Z.; Gong, M. Luminescence and energy transfer of dual-emitting solid solution phosphors (Ca,Sr)10Li(PO4)7:Ce3+,Mn2+ for ratiometric temperature sensing. Ind. Eng. Chem. Res. 2017, 56, 890−898. (20) Ding, M.; Zhang, H.; Chen, D.; Xi, J.; Ji, Z.; Hu, Q. Colortunable luminescence, energy transfer and temperature sensing behavior of hexagonal NaYF4:Ce3+/Tb3+/Eu3+microcrystals. J. Alloys Compd. 2016, 672, 117−124.

cross-relaxation process. In the emission spectra of Y1.88WO6:0.1Yb3+,0.02Er3+ upon 980 nm excitation under various temperatures, high absolute sensitivity was achieved by using the piecewise expression for FIR. In this sample, another strategy for thermal sensing involving decay lifetime of the Er3+4S3/2 level was also proposed, but the sensitivity was low. In the Yb3+-Ho3+ codoped phosphor, the FIR of red to green emissions of Ho3+ was employed to characterize its relationship with temperature. The mechanism was interpreted by the decay lifetime and energy level diagram of Yb3+-Ho3+. In the Yb3+-Tm3+ doped phosphor, the TCLs of Tm3+ were investigated by the Boltzmann distribution formula, and high sensitivity values were obtained. On the basis of the above investigation, several suggestions have been given for developing new luminescent materials with high sensitivities based on the FIR technique.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00102. Figure S1: SEM images of Y1.88WO6:0.1Yb3+,0.02Er3+, Y1.68WO6:0.3Yb3+,0.02Ho3+, and Y1.896WO6:0.1Yb3+,0.004Tm3+. Figure S2: Decay curves of Y1.68WO6:0.3Yb3+,0.02Ho3+ under different temperatures. Figure S3: (a) Decay curves of Y1.896WO6:0.1Yb3+,0.004Tm3+ with 980 nm excitation and 795 under various temperatures; (b) average decay lifetime as a function of temperature. Figure S4: Decay curves of Y1.896WO6:0.1Yb3+,0.004Tm3+ under different temperatures. Table S1 τi and Ai (i = 1, 2) of Y1.98‑xWO6:xYb3+,0.02Er3+ (0 ≤ x ≤ 0.3) excited at 518 nm and monitored at 545 nm. Table S2: τi and Ai (i = 1, 2) of Y1.88WO6:0.1Yb3+,0.02Er3+ excited at 980 nm and monitored at 545 nm at various temperature. Table S3: τi and Ai (i = 1, 2) of Y1.68WO6:0.3Yb3+,0.02Ho3+ excited at 980 nm and monitored at 540 nm at different temperatures. Table S4: τi and Ai (i = 1, 2) of Y1.896WO6:0.1Yb3+,0.004Tm3+ excited at 980 nm and monitored at 806 nm at different temperature. Table S5: τi and Ai (i = 1, 2) of Y1.896WO6:0.1Yb3+,0.004Tm3+ excited at 980 nm and monitored at 690 nm at different temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jia Zhang: 0000-0002-9305-5873 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51602117) and Natural Science Foundation of Jiangsu Province of China (No. BK20140456).

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REFERENCES

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