Investigation on Upconversion Luminescence and Optical

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Investigation on upconversion luminescence and optical temperature sensing behaviour for Ba2Gd2Si4O13:Yb3+-Er3+/Ho3+/Tm3+ phosphors Jia Zhang, Xiumin Jiang, and Zhenghe Hua Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00882 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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Industrial & Engineering Chemistry Research

Investigation on upconversion luminescence and optical temperature sensing behaviour for Ba2Gd2Si4O13:Yb3+-Er3+/Ho3+/Tm3+ phosphors Jia Zhang1,2,*, Xiumin Jiang1, Zhenghe Hua1,2 1

Physics department and Jiangsu Key Laboratory of Modern Measurement Technology and

Intellige, Huaiyin Normal University, 111 West Chang Jiang Road, Huai'an 223300, China 2

Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin Normal University, 111 West Chang Jiang Road, Huai'an 223300, China

KEYWORDS: Luminescence; Temperature sensing; Sensitivity; Phosphor

ABSTRACT: To explore new phosphors for temperature sensing with high detection sensitivity, the Yb3+-Er3+/Ho3+/Tm3+ doped Ba2Gd2Si4O13 (BGS) were designed. Different strategies were introduced based on the upconversion (UC) luminescence. For BGS:0.2Yb3+,0.02Er3+, the fluorescence intensity ratio (FIR) of two green emissions of Er3+ shows a gradual enhancement with increasing temperature due to the thermally coupled levels. The piecewise expression of sensitivity was proposed in the temperature range of 293-553 K based on the Boltzmann distribution. For BGS:0.2Yb3+,0.01Ho3+, the FIR of the red to green emissions of Ho3+ changes with temperature, showing a linear relationship from 293 to 453 K. The absolute sensitivity was

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gained to be 0.0452 K-1. For BGS:0.2Yb3+,0.02Tm3+, the high absolute and relative sensitivities were both achieved by employing the thermally coupled (3F2,3F3) and 3H4 levels of Tm3+. The above study could have special reference to the development of new luminescent materials with high sensitivity.

1. INTRODUCTION Temperature is a fundamental parameter in many kinds of industrial processes and scientific research.1 Recently, noncontact temperature measurement methods have attracted much attention, since they are noninvasive, accurate and work in even strong electromagnetic fields.2,3 These strategies generally utilize rare earth (RE) ions as the activators, which are based on fluorescence intensity ratio (FIR) technique between two thermally coupled energy levels (TCELs). So far, several trivalent RE ions of this kind have been reported, such as Er3+ (2H11/2/4S3/2), Tm3+ (3F2,3/3H4), Dy3+ (4I15/2/4F9/2), Eu3+ (5D1/5D0), Nd3+ (4F5/2/4F3/2), and Pr3+ (3P1/3P0).4-10 And the Er3+ ion with upconcersion (UC) luminescence is mostly used. In these ions, the populations on the TCELs follow the Boltzmann distribution law, and the sensitivity for temperature sensing is in proportion to the energy difference (∆E) of the TCELs.11 So, a wide energy gap will favor the sensitivity. Unfortunately, the ∆E values for the trivalent RE ions change little due to their intra4f transitions. Moreover, the ∆E of the interested TCELs is bounded not exceeding a certain value in order to achieve thermal equilibrium.11 Owing to this restriction, it is necessary to design new thermometry strategy or improving the FIR technique to gain high detection sensitivity for temperature sensing at present. With regard to this, two ways were involved in this work. On one hand, a modified Boltzmann distribution formula was proposed to apply to the Yb3+-Er3+ and Yb3+-Tm3+ codoped phosphors according to the UC spectral characteristics. On


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the other hand, the non-thermal coupled levels of Ho3+ were introduced in the Yb3+-Ho3+ codoped phosphor. UC is a process by converting long-wavelength radiation to short-wavelength radiation via long-lived intermediate states.12 Trivalent RE ions are the suitable candidates for UC luminescence. And Er3+/Tm3+/Ho3+ ions are mostly used owing to the abundant energy levels and narrow emission lines.13 To enhance their luminescence, Yb with a larger absorption cross section at near infrared (NIR) is usually employed as a photosensitizer.14 Besides the luminescent ions, the selection of the matrix is another key factor to obtain desirable UC luminescence.15 High UC luminous efficiency is generally gained in phosphor materials with low phonon frequency, such as sulphide and fluoride. However, sulphide based phosphor could suffer some disadvantages, for instance, poor chemical, low durability and effect of saturation at higher pump power.16,17 The fluorides are sensitive to oxygen surface contamination, which may affect the optical properties and limit their application.18 In contrast, oxides could show high chemical stability, environmental friendliness and more suitable to be used in the higher temperature environments.19,20 In 2010, Wieczorek et al. reported a new silicate Ba2Gd2Si4O13 (BGS) which contains finite zigzag-shaped Si4O13 chains and Gd2O12 dimers.21 Later, the down-shifting luminescence of Eu2+/Eu3+/Ce3+/Tb3+/Dy3+/Sm3+ activated phosphors upon vacuum ultraviolet or ultraviolet excitation were studied.22-26 To the best of our knowledge, the UC luminescence properties of BGS-based phosphors were rarely reported. In this paper, to improve the detection sensitivity for temperature sensing, a series of Yb3+-Er3+/Ho3+/Tm3+ doped BGS phosphors were designed, and their UC luminescence and temperature-dependence of FIR were investigated. 2. EXPERIMENTAL


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The Ba2Gd1.8-xSi4O13:0.2Yb3+,xEr3+ (BGS:0.2Yb3+,xEr3+, 0.01 ≤ x ≤ 0.06), Ba2Gd1.83+ 3+ ySi4O13:0.2Yb ,yHo zSi4O13:0.2Yb

3+

(BGS:0.2Yb3+,yHo3+,

0.01

≤

y

≤

0.06)

and

Ba2Gd1.8-

,0.08Tm3+ (BGS:0.2Yb3+,zEr3+, 0 ≤ z ≤ 0.04) samples were prepared by the

solid-state reaction method. The raw materials were BaCO3 (99%), SiO2 (99%), Gd2O3 (99.99%), Yb2O3 (99.99%), Ho2O3 (99.99%), Er2O3 (99.9%) and Tm2O3 (99.99%). Stoichiometric amounts of the raw materials were ground in an agate mortar and then calcined at 1020 °C for 4 h in air. The phase purity was determined 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 photoluminescence spectra and decay curves were recorded on an EI-FS5 fluorescence spectrophotometer. The spectral resolution and wavelength accuracy used are 0.1 and ± 0.5 nm, respectively. The pulse width of the 980 nm pulse laser diode (LD) is 35 µs, and these decay curves were all measured under room temperature. The temperature dependent measurement was carried out using the EI-FS5 fluorescence spectrophotometer with a heating device whose temperature can vary from room temperature to 575 K with the step of 0.1 K. 3. RESULTS AND DISCUSSION 3.1 XRD analysis To identify the phase composition of the as-prepared samples, Figure 1 presents the XRD patterns

of

the

typical

BGS:0.2Yb3+,0.02Er3+,

BGS:0.2Yb3+,0.01Ho3+

and

BGS:0.2Yb3+,0.02Tm3+ phosphors. It can be found the diffraction peaks could be indexed to the standard BGS as reported in Ref. 21. No obvious impurity from any secondary phase is observed, indicating that the introduction of RE ions doesn’t change the crystal structure of the


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host obviously. Besides, the other samples in this work were also determined to be single-phase by XRD analysis.

Figure

1

XRD

patterns

of

BGS:0.2Yb3+,0.02Er3+,

BGS:0.2Yb3+,0.01Ho3+

and

BGS:0.2Yb3+,0.02Tm3+ 3.2 UC luminescence spectra

Figure 2 Emission spectra of BGS:0.2Yb3+,xEr3+ (0.01 ≤ x ≤ 0.06) under 980 nm excitation, inset shows the 4S3/2-4I15/2 emission intensity of Er3+ as a function of Er3+ concentration and the digital photograph under 980 nm LD irradiation


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Figure 2 shows the emission spectra of BGS:0.2Yb3+,xEr3+ (0.01 ≤ x ≤ 0.06) under 980 nm excitation. Three main emission peaks have been found in the region of 500-700 nm. The green emissions around 520 and 546 nm could be attributed to the 2H11/2-4I15/2 and 4S3/2-4I15/2 transitions of Er3+, respectively.27 The red emission from 630 to 700 nm can be assigned to the 4F9/2-4I15/2 transition of Er3+.28 The inset shows the 4S3/2-4I15/2 emission intensity of Er3+ as a function of Er3+ concentration, which reveals the sample for x = 0.02 exhibits the strongest emission intensity. To determine the emission color of Yb3+-Er3+ codoped BGS, the Commission International del'Eclairage (CIE) chromaticity coordinates of the typical BGS:0.2Yb3+,0.02Er3+ sample were calculated using the emission spectra from 500 to 700 nm, which were obtained to be (0.360, 0.627). Thus, a yellow-greenish emission has been obtained, as can be understood from the digital photograph under 980 nm LD irradiation in Figure 2.

Figure 3 Emission spectra of BGS:0.2Yb3+,yHo3+ (0.01 ≤ x ≤ 0.06) under 980 nm excitation, inset shows the digital photograph under 980 nm LD irradiation Figure 3 presents the emission spectra of BGS:0.2Yb3+,yHo3+ (0.01 ≤ x ≤ 0.06) under 980 nm excitation. The strongest emission peaks are located in the red range from 630 to 680 nm, attributed to the 5F5-5I8 transition of Ho3+.29 In the green region of 520-570 nm, weak emission


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peaks belonging to the (5F4,5S2)-5I8 transitions of Ho3+ are also observed. With increasing Ho3+ concentration, both the green and red emissions demonstrate continuous decrease. The CIE chromaticity coordinates of the typical BGS:0.2Yb3+,0.01Ho3+ sample were calculated to be (0.701, 0.298), indicating a red emission as can be found from the digital photograph under 980 nm LD irradiation in Figure 3. Figure 4 (a) illustrates the emission spectra of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) under 980 nm excitation. The very intense emission peaks located in the NIR region could be ascribed to the 3H4-3H6 transition of Tm3+.30 Besides, the weak emission peaks of Tm3+ are also observed in the near ultraviolet (NUV) and visible region. The blue emissions at 451 and 475 nm can be assigned to the 1D2-3F4 and 1G4-3H6 transitions of Tm3+, respectively. The red (at 651 nm) and NIR (at 792 nm) emissions can be attributed to the 1G4-3F4 and (3F2,3F3)-3H6 transitions of Tm3+, respectively.30,31 A very weak emission peak at 366 nm is also found, which could be assigned to the 1D2-3H6 transition of Tm3+.32 Moreover, the 8S7/2-6P7/2 transition of Gd3+ appears at 312 nm, which reveals an energy transfer (ET) from Tm3+ to Gd3+. In the BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) phosphors, the strongest intensity of the 1G4-3H6 emission is obtained at z = 0.01, but that of the 3H4-3H6 emission at z = 0.02. This observation indicates that the change tendency in intensity for different transitions of Tm3+ shows some difference. To understand this point, Figure 4(b) presents the normalized (for 475 nm) emission spectra of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04). It can be found that the 1G4-3F4 and 1D2-3H4 emission peaks of Tm3+ overlap well for different Tm3+ concentration, but the (3F2,3F3)-3H6 and 3H4-3H6 transitions exhibit gradual enhancement with increasing Tm3+ concentration. To well interpret this phenomenon, the downshifting luminescence spectra of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) were measured. Figure S1 shows the excitation spectrum of the typical BGS:0.2Yb3+,0.04Tm3+ sample by monitoring at


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792 nm. Two main excitation peaks appear, and the strongest excitation peak is located at 355 nm, which can be assigned to the 3H6-1D2 transition of Tm3+. The emission spectra of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) from 635 to 835 nm by exciting at 355 nm are presented in Figure S2(a). Three main emission peaks around 664, 753 and 792 nm are observed, which could be attributed to the 1D2-3H4, 1D2-3F3 and 3H4-3H6 transitions of Tm3+, respectively. It can be also found that the emission peaks are gradually enhanced with increasing Tm3+ concentration, but the 3H4-3H6 emission intensity shows a faster increase rate compared with those of the other two emissions. This point can be further understood from the normalized (for 664 nm) emission spectra in Figure S2(b). It reveals the 3H4-3H6 transition intensity shows a continuous increase as the Tm3+ concentration increases. As shown in the energy level diagram for Yb3+-Tm3+ ions in Figure 4(c), the electron on 1D2 level could be transferred to 3H4 level by emitting a 664 nm photon, and then it jumps to the ground state 3H6 level after emitting a 792 nm photon or other levels by non-radiative relaxation. Besides, the population on 3H4 level can be also derived from the upper levels of Tm3+ by non-radiative relaxation. Anyway, the intensity ratio of the 1D2-3H4 to 3H4-3H6 emissions should stay the same even if the Tm3+ concentration increases when no other ET processes exist. However, the spectra in Figure S2(b) indicate a different result. Based on the above analysis, one can predict that the cross-relaxation (CR) between the Tm3+ levels may play an important role in the increase of the populations on (3F2,3F3) and 3H4 levels of Tm3+. With regard to the efficient CR process for the upper level of Tm3+, the 1D2 and 1G4 levels can be considered. Firstly, to determine if the

1

D2 level is involved, the decay curves of

BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) by exciting at 355 nm (3H6 → 1D2 transition) and monitoring 664 nm (1D2→3H4 transition) were shown in Figure S3. All the decay curves could be well-fitted by a single-exponential function as I = A exp(−t / τ ) , where τ and A are the


8

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luminescent lifetime and fitting parameter, respectively. The corresponding τ values were obtained to be 20.4, 20.2, 20.6 and 19.6 µs for z = 0.01, 0.02, 0.03 and 0.04, respectively. The decay lifetime of 1D2 level is oscillated with the Tm3+ doping concentration, indicating the probable CR process involves the 1D2 level little. Secondly, to interpret whether the 1G4 level is included in the CR process, the decay curves of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) by exciting at 980 nm and monitoring 475 nm are depicted in Figure S4). All the curves could be well fitted by the above single-exponential equation, and the lifetimes have been obtained to be 316.4, 300.4, 283.8 and 258.8 µs for z = 0.01, 0.02, 0.03 and 0.04, respectively. It is obvious that the lifetime of the 1G4 level exhibits a continuous decrease with increasing Tm3+ concentration. Thus, probable CR process 1G4 + 3 H 6 → 3 F4 + 3 F2,3 for Tm3+ is proposed here, marked as CR in Figure 4(c). The phonon-assisted ET rate (P(∆E)) can be expressed as P (∆E ) = P (0)e−α∆E / hω , where P(∆E) represents the probability of ET, P(0) represents the probability when ∆E is zero, α is host constant, ∆E is the energy gap in the ET process, ћω is the highest phonon energy in the material.33,34 The energy mismatch ∆E largely affects the phonon-assisted ET rate in a luminescent material. The highest phonon energy ћω in a phosphate compound was reported to be about 1100 cm-1,35 and the energy mismatch in the above CR process can be calculated to be approximately 1000 cm-1 from the UC spectra. As a result, this CR could occur with assistance of one phonon. On the other hand, the CR becomes more efficient for higher Tm3+ concentration, which helps to pump the (3F2,3F3) and 3H4 levels efficiently. By using the emission spectra from 400 to 730 nm of the typical BGS:0.2Yb3+,0.02Tm3+ sample, the CIE chromaticity coordinates were obtained to be (0.153, 0.135). Hence, the blue emission was achieved, as can be seen from the digital photograph under 980 nm LD irradiation in Figure 4(b).


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Figure 4 (a) Emission spectra of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) under 980 nm excitation; (b) normalized (for 475 nm) emission spectra of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤


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0.04) under 980 nm excitation, inset shows the digital photograph under 980 nm LD irradiation; (c) Energy level diagram for Yb3+-Tm3+ ions 3.3 Temperature sensing behavior The emission spectra of the typical BGS:0.2Yb3+,0.02Er3+ phosphor under 980 excitation at various temperatures are shown in Figure 5(a). The intensities of the 4S3/2-4I15/2 and 4F9/2-4I15/2 emissions have been observed to decrease gradually with increasing temperature. However, the 2

H11/2-4I15/2 transition exhibits an opposite change tendency in intensity. To clearly interpret this

point, Figure 5(b) represents their normalized (for 546 nm) emission spectra. It can be found that the positions of the two UC emission peaks in the green region barely change with temperature, whereas their FIR varies when the temperature rises. The changing FIR is mainly due to the different populations on the 2H11/2 and 4S3/2 TCELs at different temperature. According to Ref. 14,30,36, the FIR for the Er3+ green UC emissions at 520 and 546 nm can be related to the following equation: R = N exp(

−∆E ) KT (1)

where R is the FIR of the Er3+ 2H11/2-4I15/2 and 4S3/2-4I15/2 emissions, N is the proportionality constant, ∆E is the energy gap between two thermally coulpled levels, K = 0.695 K-1cm-1 is the Boltzmann constant, and T is absolute temperature. Figure 5(c) shows the plot of logarithmic of FIR versus inverse absolute temperature. The experimental data can be well fitted by

R ( I 520 / I 546 ) = 9.02 ∗ exp(−1069.4 / T ) . So, the ∆E can be calculated to be 743.2 cm-1. To evaluate the repeatability of measurement, Figure S5 presents the dependence of R(I520/I546) for BGS:0.2Yb3+,0.02Er3+ on the absolute temperature by repeating measurement for five times.


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Good repeatability has been found in this phosphor. To further understand this point, the relative standard deviation (RSD) was employed, which is expressed as

1 RSD = R

∑

n i =1

( Ri − R ) 2 n −1

× 100% (2)

where Ri is the FIR for the i-th measurement, n is the number of iterations, R is the mean value of FIR for all the measurements. The RSD value as a function of temperature is given in the inset of Figure 5(c). The RSD value decreases with increasing temperature, so this phosphor is more suitable to apply in the temperature range from 333 to 553 K with regard to the repeatability.

Figure 5 (a) Emission spectra of BGS:0.2Yb3+,0.02Er3+ under 980 excitation at various temperatures; (b) normalized (for 546 nm) emission spectra of BGS:0.2Yb3+,0.02Er3+ under 980 excitation at various temperatures; (c) dependence of FIR of I662/I540 on the absolute temperature, inset shows its RSD as a function of the absolute temperature; (d) absolute sensitivity as a function of the absolute temperature from 293 to 553 K


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According to Ref. 37, the absolute (SA) sensitivity can be calculated by SA =

d ( R) ∆E ) (3) = R( d (T ) KT 2

It can be seen the SA value is proportional to R and ∆E. To improve the absolute sensitivity, it is effective to increase these two factors. As the ∆E is generally fixed for Er3+ in a certain host due to the intra-4f transitions, the only way is to enhance R. A simple method by changing the FIR between the TCELs can be considered here. If one takes R1 = I 520 / I 546 and R2 = I 546 / I 520 , the expression of SA will both refer to equation (3). Correspondingly, two different variations in sensitivity with temperatures for BGS:0.2Yb3+,0.02Er3+ (marked as SA1 and SA2, respectively) can be obtained (see Figure 5(d)). The SA1 value increases and the SA2 value decreases gradually with increasing temperature, which converge at about 487 K. To make the sensitivity in the temperature range of 293-553 K as high as possible, the piecewise expression is proposed, i.e., SA = SA1 when T ≥ 487 K; SA = SA2 when T ≤ 487 K. The resultant curve for SA is shown in Figure

5(d). Relative to some other Yb3+-Er3+ codoped oxide-based

phosphors such as

CaWO4:Yb3+,Er3+, BaTiO3:Yb3+,Er3+, Gd2O3:Yb3+,Er3+ and Ba5Gd8Zn4O21:Yb3+,Er3+,37-39 the present phosphor shows a much improved sensitivity. Figure 6(a) illustrates the emission spectra of the typical BGS:0.2Yb3+,0.01Ho3+ phosphor under 980 excitation at various temperatures. With increasing temperature, both the green (5F4,5S2)-5I8 and red 5F5-5I8 transitions of Ho3+ have been decreased gradually. The relative intensities of the two emissions as a function of temperature in Figure 6(b) further display the different decrease rates. It can be found that the (5F4,5S2)-5I8 emission intensity of Ho3+ exhibits a larger decay rate. The reason will be discussed later. In this case, it can be predicted that the FIR for the 650 and 538 nm emissions will change with temperature. Figure 6(c) presents the dependence of the FIR on the absolute temperature. As the temperature changes from 293 to 453


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K, the FIR value increases gradually, and a linear relation of I 650 / I 538 = 0.0452T + 14.37 can be achieved by fitting these experimental data. However, the FIR shows an oscillation beyond 453 K. According to the definition of S A = d ( R ) / d (T ) , the absolute sensitivity of this phosphor is determined to be 0.0452 K-1. Hence, high sensitivity can be achieved in Yb3+-Ho3+ codoped BGS. To interpret the mechanism for the different decay rate of the (5F4,5S2)-5I8 and 5F5-5I8 emission intensities with temperature, the energy level diagram for Yb3+-Ho3+ ions is depicted in Figure 6(d). The detailed UC ET processes have been discussed in the previous references.40,41 It was reported that the (5F4,5S2) levels are populated by two successive ETs from Yb3+, but the 5F5 level could be pumped by non-radiative relaxation from the (5F4,5S2) levels (marked as ① in Figure 6(d)) or from the 5I7 level after the energy has been relaxed from 5I6 level (marked as ② in Figure 6(d)). Both ① and ② non-radiative relaxations are beneficial to the population on 5F5 level. The non-radiative relaxation rate of the excited state can be written as42,43 WNR = WNR (0)(1+ < n >) ∆E / hω (4) < n >=

1 (5) exp(hω / kT ) − 1

where WNR(0) is the non-raditive relaxation rate at 0 K, hω is the phonon energy, is the phonon density of states which strongly depends on phonon energy and temperature, k is Boltzmann’s constant and T is absolute temperature. When the temperature increases, the phonon density of states increases correspondingly, which will further result in the increased nonradiative relaxation rate for ① and ②.


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Figure 6 (a) Emission spectra of BGS:0.2Yb3+,0.01Ho3+ under 980 excitation at various temperatures; (b) relative intensities of the 538 and 650 nm emissions as a function of the absolute temperature; (c) dependence of FIR for 650 and 538 nm emissions on the absolute temperature; (d) energy level diagram for Yb3+-Ho3+ ions Figure 7(a) presents the emission spectra from 615 to 850 nm of the typical BGS:0.2Yb3+,0.02Tm3+ phosphor under 980 excitation at various temperatures. It can be found that the intense emission peaks in the NIR region have been decreased when the temperature rises. To clearly observe the change of the weak 1G4-3F4 (651 nm), 1D2-3H4 (664 nm) and (3F2,3F3)-3H6 (792 nm) transition intensities, the enlarged spectra from 615 to 730 nm are shown in Figure 7(b). Both the 651 and 664 nm emissions exhibit continuous decrease with increasing temperature, but the 695 nm one has been gradually enhanced. Figure 7(c) depicts the relative


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emission intensities of the (3F2,3F3) and 3H4 TCELs to ground state 3H6 level as a function of the absolute temperature. It can be seen that the intensity of the 3H4-3H6 emission is decreased by about 60% and that of the (3F2,3F3)-3H6 emission is enhanced by more than 11 times at 553 K relative to those at 293 K, respectively. Figure 7(d) shows the dependence of FIR of I695/I792 on the absolute temperature, which has been fitted by R ( I 695 / I 792 ) = 2.72 ∗ exp(−2339.6 / T ) . So, the

∆E was obtained to be 1626.0 cm-1, which is close to the experimentally observed splitting of about 1760 cm-1 in the present phosphor. Figure 7(e) shows the absolute sensitivity as a function of the temperature from 293 to 553 K by using R(I695/I792) and R(I792/I695) (marked as SA1 and SA2, respectively). High SA2 varies from 19.0 to 0.188 K-1 when the temperature changes from 293 to 553 K, which is much larger than SA1.

Figure 7 (a) Emission spectra of BGS:0.2Yb3+,0.02Tm3+ under 980 excitation at various temperatures, (b) shows the enlarged spectra from 615 to 730 nm; (c) relative intensities of 695 and 792 nm emissions as a function of the absolute temperature; (d) dependence of FIR of


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I695/I792 on the absolute temperature; (e) absolute sensitivity as a function of the absolute temperature from 293 to 553 K For the actual application, it is also very important to evaluate the relative sensitivity (SR) of the materials, which is calculated by the following formula:44,45 SR =

1 d ( R) R d (T )

(6)

Figure 8 presents the SR as a function of the temperature for the BGS:0.2Yb3+,0.02Er3+, BGS:0.2Yb3+,0.01Ho3+ and BGS:0.2Yb3+,0.02Tm3+ phosphors. It can be seen that the all SR values decrease with increasing temperature. The BGS:0.2Yb3+,0.02Tm3+ phosphor shows the highest relative sensitivity and the BGS:0.2Yb3+,0.01Ho3+ shows the lowest one at any temperature. At 293 K, the maximum SR values are obtained to be 1.25%, 0.16% and 2.73% K-1 for BGS:0.2Yb3+,0.02Er3+, BGS:0.2Yb3+,0.01Ho3+ and BGS:0.2Yb3+,0.02Tm3+, respectively. Table 1 shows a list of SR values, TCELs and host for different RE ions doped temperature sensing materials. It can be found the BGS:0.2Yb3+,0.02Er3+ and BGS:0.2Yb3+,0.02Tm3+ exhibit higher SR compared with the other Er3+ and Tm3+ activated phosphors, respectively. But the BGS:0.2Yb3+,0.01Ho3+ demonstrates a lower SR relative to the other Ho3+ activated phosphors. In addition, it can be also found the maximum SR of BGS:0.2Yb3+,0.02Tm3+ in this work has been improved compared with some other RE doped phosphor materials with down-shifting luminescence, such as Gd2Ti2O7:Eu3+ and Y4Al2O9:Dy3+.


17


Page 18 of 27

Figure 8 Relative sensitivities of BGS:0.2Yb3+,0.02Er3+, BGS:0.2Yb3+,0.01Ho3+ and BGS:0.2Yb3+,0.02Tm3+ as a function of the absolute temperature Table 1 SR values, TCELs and host for different RE ions doped temperature sensing materials RE ions

Host

Er3+

Y2 O3

Er3+

Al2O3

Ho3+

CaMoO4

5

F3/3K8

Y2 O3

5

3

F3/ K8

1067.76/T

2

[46]

Ho

3+

TCELs

SR

Ref.

2

H11/2/4S3/2

886.08/T2

[9]

2

H11/2/4S3/2

964.1/T2

[9]

648.8/T2

[9]

Tm3+

NaLuF4

3

F2,3/3H4

2386.93/T2

[31]

Tm3+

KLuF4

3

F2,3/3H4

1249.85/T2

[10]

Eu3+

Gd2Ti2O7

5

D1/5D0

2471.9/T2

[47]

Dy3+

Y4Al2O9

4

I15/2/4F9/2

1937.6/T2

[48]

Er3+

BGS

2

H11/2/4S3/2

1069.4/T2

This work

3+

BGS

Tm3+

BGS

Ho

5

5

5

F5/( F4, S2) 0.16% (293 K) This work 3

F2,3/3H4

2339.6/T2

This work

4. CONCLUSIONS


18

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In conclusion, the BGS:Yb3+-Er3+/Ho3+/Tm3+ samples were prepared by conventional solidstate reaction method, and their luminescence properties for optical temperature sensing were studied. By XRD analysis, the prepared samples were indentified to be single-phase when the RE ions are introduced. Three emission peaks belonging to Er3+ were observed in the BGS:0.2Yb3+,xEr3+ (0.01 ≤ x ≤ 0.06) phosphors. By investigating the temperature dependence of FIR for the typical BGS:0.2Yb3+,0.02Er3+ phosphor, it has been found that the FIR for the 2H11/24

I15/2 and 4S3/2-4I15/2 transitions of Er3+ increases with increasing temperature. High absolute

sensitivity was obtained from 293 to 553 K. For the Yb3+-Ho3+ codoped BGS, the red and green emissions of Ho3+ were observed in the range from 500 to 700 nm, and the FIR of the two emissions changes with temperature. The corresponding reason has been interpreted. The absolute sensitivity was gained to be 0.0452 K-1. The (3F2,3F3)-3H6 and 3H4-3H6 transitions of Tm3+ exhibit an increasing FIR when the temperature rises due to the TCELs of (3F2,3F3) and 3

H4. The high absolute sensitivity was gained at 293 K. For the BGS:0.2Yb3+,0.02Er3+,

BGS:0.2Yb3+,0.01Ho3+ and BGS:0.2Yb3+,0.02Tm3+ phosphors, the relative sensitivities were calculated and the maximum values at 293 K were obtained to be 1.25%, 0.16% and 2.73% K-1 at 293 K, respectively.

ASSOCIATED CONTENT

Supporting Information. Figure S1 Excitation spectrum of BGS:0.2Yb3+,0.04Tm3+ monitored at 792 nm Figure S2 (a) Emission spectra of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) by exciting at 355 nm; (b) normalized (for 664 nm) emission spectra of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04)


19


Page 20 of 27

Figure S3 Decay curves of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) Figure S4 Decay curves of BGS:0.2Yb3+,zTm3+ (0.01 ≤ z ≤ 0.04) Figure S5 Dependence of R(I520/I546) for BGS:0.2Yb3+,0.02Er3+ on the absolute temperature by repeating measurement for five times AUTHOR INFORMATION

Corresponding Author *E-mail address: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 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).

ABBREVIATIONS UC, upconversion; RE, rare-earth; FIR, fluorescence intensity ratio; TCELs, thermally coupled levels; NIR, near-infrared.

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(48) Boruc, Z.; Kaczkan, M.; Fetlinski, B.; Turczynski, S.; Malinowski, M. Blue emissions in Dy3+ doped Y4Al2O9 crystals for temperature sensing. 2012, 37, 5214-5216. SYNOPSIS

High sensitivities have been obtained in the Ba2Gd2Si4O13:Yb3+-Er3+/Ho3+/Tm3+ phosphors by employing the fluorescence intensity ratio technique, which is based on the upconversion luminescence.


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Investigation on Upconversion Luminescence and Optical

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