Color-Tunable Upconversion Luminescence and Multiple

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Color Tunable Upconversion Luminescence, Multiple Temperature Sensing and Optical Heating Properties of BaYO:Er /Yb Phosphors 3

4

9

3+

3+

Shuifu Liu, Hong Ming, Jun Cui, Songbin Liu, Weixiong You, Xinyu Ye, Youming Yang, Huaping Nie, and Ruixiang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04180 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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The Journal of Physical Chemistry

Color Tunable Upconversion Luminescence, Multiple Temperature Sensing and Optical Heating Properties of Ba3Y4O9:Er3+/Yb3+ Phosphors Shuifu Liu a, d，Hong Ming a, d，Jun Cui a, d，Songbin Liu a, d，Weixiong You c, d, Xinyu Yea, b, d,∗, Youming Yang a, b, d, Huaping Nie a, b, d, Ruixiang Wang a, b, d a

School of Metallurgy and Chemistry Engineering, Jiangxi University of Science and Technology,

Ganzhou 341000, P.R.China b

National Engineering Research Center for Ionic Rare Earth, Ganzhou 341000, P.R.China

c

School of Material Science and Engineering, Jiangxi University of Science and Technology, Ganzhou

341000, P.R.China d

Key Laboratory of Rare Earth Luminescence Materials and Devices of Jiangxi Province, Ganzhou

341000, P.R.China

ABSTRACT: Upconversion (UC) luminescence materials doped with rare earth (RE) ions are extensively investigated as optical temperature probes by using fluorescence intensity ratio (FIR) technique. However, most Er3+ doped materials are still suffering from low sensing sensitivity. In the present study, we attempt to develop high sensing sensitivity Er3+ doped materials based on the thermally coupled energy levels (TCLs) from Stark sublevels as well as the properties at subzero temperatures, which continues lack of research. Er3+/Yb3+ co-doped Ba3Y4O9 (BYO) phosphors were produced via a solid-state reaction. Excited by 980 nm, various output colors, including bright green, yellow and red, in BYO:Er3+/Yb3+ phosphors as well as the relative emission intensities could be regulated through altering Yb3+ concentrations. Subsequently, based on all twelve pairs of TCLs especially from Stark sublevels of 2H11/2, 4S3/2 and 4F9/2 of Er3+ ions, multiple temperature sensing performances are evaluated over a wide range of 73-573 K. The results show that the maximum sensitivity of the 2H11/2 and 4S3/2(1) levels is approximately one-fold higher than that of traditional TCLs of 2H11/2/4S3/2 at elevated temperature and the maximum sensitivity based 1

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on the 2H11/2(1) and 2H11/2(2) levels is more than twelve times higher than the traditional TCLs of 2

H11/2/4S3/2 at subzero temperature. Therefore, it is expected to realize high sensitivity temperature

detection from subzero to elevated temperatures by combining two pairs of different TCLs. In addition, the potential of Er3+/Yb3+ co-doped BYO phosphors to be used as optical heater is studied. The generated temperature can be accurately monitored by BYO:Er3+/Yb3+ phosphors and regulated by adjusting the excitation power, which indicate that BYO:Er3+/Yb3+ phosphors can be used as optical heating device.

INTRODUCTION Although contact temperature measurement has been widely used in various applications, some unavoidable disadvantages still exist, such as intense electromagnetic noise interference, dangerous sparks and inaccessibility to corrosive environments.1-4 Alternatively, non-contact temperature measurement can overcome those shortcomings of contact temperature measurement, in which non-contact temperature sensors are the preferred devices with high sensitivity, broad dynamic range and multiplexing capabilities.5 Among various ways to non-contact temperature sensing, optical temperature sensing based on fluorescence intensity ratio (FIR) technique recently became a research hotspot owing to that it can reduce the influence of measuring conditions and improve sensitivity and signal discriminability.6-8 The FIR technique is usually associated with the temperature dependence of the photoluminescence intensity from two thermally coupled energy levels (TCLs).9 Generally, TCLs population follows the Boltzmann distribution and the energy gap ∆ E between them should be larger than 200 cm-1 but lower than 2000 cm-1.10-11 UC luminescence materials doped with rare earth (RE) ions are extensively investigated due to large amount TCLs in some RE ions and easy acquisition of UC emission excited by commercially near-infrared (NIR) laser diode. Among RE ions, the temperature sensors based on the green UC emissions from TCLs of 2H11/2 and 4S3/2 of Er3+ ions have gained widely attention.12-17 Besides the intrinsic TCLs of RE ions, the energy level pairs formed from Stark sublevels can also be regarded as TCLs. Unlike the Stark energy levels of Tm3+, Nd3+, Yb3+ ions have been studied in optical thermometers,18-23 few reports focused on the Stark energy levels of Er3+ ions. Tripathi and coworkers investigated the temperature sensing performance based on the 2


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Stark sublevels of 4S3/2 of Er3+.24 It was concluded that not only the traditional TCLs, but also the Stark sublevels can be applied to detecting temperature. Cao et al. investigated the temperature sensing properties based on the Stark sublevels of Er3+ ions in Er3+/Yb3+/Mo6+ co-doped TiO2 phosphors.1 From their studies, using TCLs by Stark split is a promising route to achieve multiple optical temperature detection. However, only the temperature sensing behaviors of four pairs of TCLs were investigated, i.e.

2

H11/2(1)/2H11/2(2),

4

S3/2(1)/4S3/2(2),

4

F9/2(1)/4F9/2(2), and

2

H11/2(1) +

2

H11/2(2)/4S3/2(1) + 4S3/2(2). Actually, besides these four pairs of TCLs, Er3+ ions possess other eight

pairs of TCLs, including 2H11/2(1) + 2H11/2(2)/4S3/2(1), 2H11/2(1) + 2H11/2(2)/4S3/2(2), 2H11/2(1)/4S3/2(1), 2

H11/2(1)/4S3/2(2), 2H11/2(2)/4S3/2(1), 2H11/2(2)/4S3/2(2), 2H11/2(1)/4S3/2(1) + 4S3/2(2), and 2H11/2(2)/4S3/2(1) +

4

S3/2(2). Up to now, there is no detailed report on the sensitivity of these eight pairs of TCLs. In consideration of potential applications, a high sensitivity temperature sensor over a wide

temperature range is desired for FIR technique. Although several methods have been reported by researchers to enhance the sensitivity of Er3+ doped systems,16-17 most of Er3+ doped materials are still suffering from low sensing sensitivity. It is worth noting that the sensitivity is related to ∆ E and the value of FIR to great extent and in general, larger energy gap and FIR contribute to higher sensitivity. Therefore, based on the TCLs from Stark sublevels may be a valid strategy to improve sensitivity owing to significantly differences in energy gap and FIR. Over the last decade, in relative to the interest on the temperature sensing properties at elevated temperatures, insufficient attention is paid to them at subzero temperatures, which may be because of poor sensitivity of the traditional TCLs of 2H11/2 and 4S3/2 levels of Er3+ ions at subzero temperatures. Since the sensing sensitivity of coupled Stark sublevels varies monotonically in a certain temperature range, it is expected that there may be a higher sensitivity at subzero temperatures. Unfortunately, there are only few reports about the temperature sensing properties of Er3+ doped system based on the Stark sublevels at subzero temperatures.25 Besides non-contact temperature measurement, photothermal therapy has garnered lots of attention in recent years.26-28 Conventional approaches to treat cancer including radiofrequency or microwave ablation relying on macroscopic heat sources,29-30 damage normal tissue and result in some serious systemic side effects.

31-32

Photothermal therapy can overcome these limitations of

conventional approaches to treat cancer. In photothermal therapy, the material with high 3



photothermal conversion efficiency is injected into the human body and gathered near the tumor tissue by a target recognition technology. Under the irradiation of an external light source (usually NIR laser), the material converts light energy into heat to kill cancer cells. In this process, the commercialized NIR laser is selected to be an efficient excitation source due to that NIR laser radiation is absorbed less by biological tissues and accompanied by large penetration depth.33 Some UC materials can absorb energy from NIR laser, convert it into heat, and at the same time, heat the material itself. Although the thermal effect leads to a decrease in UC luminescence efficiency, the heat generated can kill cancer in photothermal therapy.34-36 In the process, the temperature sensing properties can be employed to measure the temperature and then the temperature in the process of photothermal therapy in biological systems can be accurately controlled. Here, we report a series of Er3+/Yb3+ co-doped Ba3Y4O9 (BYO) phosphors with color tunable UC luminescence. To enhance the temperature detection sensitivity of temperature sensors, the strategy using FIR technique based on the TCLs from Stark sublevels of Er3+ ions were employed. It’s worth mentioning that most Stark sublevels have not been investigated. Especially, it is expected to improve the sensitivity at subzero temperatures, which continues lack of research. Based on all twelve pairs of TCLs especially from Stark sublevels of Er3+ ions, multiple optical temperature sensing properties of BYO:Er3+/Yb3+ phosphor are totally investigated by the FIR technique in a wide temperature range of 73-573 K. Moreover, we attempt to study the potential application of BYO:Er3+/Yb3+ phosphors in optical heater for photothermal therapy.

EXPERIMENTAL SECTION Materials and Synthesis. The phosphors with nominal composition of Ba3Y4-x-yO9: yEr3+/xYb3+ (x and y are the dopant concentration) were produced through a high-temperature solid-state reaction method. Y2O3 (99.99%), Er2O3 (99.99%), Yb2O3 (99.99%) and BaCO3 (99.9%) were employed as the raw materials. These raw materials in the desired ratio were mixed in an agate mortar for 30 min with moderate amount of ethanol and then dried at 60 °C for 2 h. The dried mixtures were ground again for 10 min moved into crucibles, and then calcined in an electric

4


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furnace at 1400°C for 6 h. After calcination, the samples were cooled naturally down to room temperature and ground into powder for subsequent analysis. Measurements and Characterization. Powder X-ray diffraction (XRD) experiments were conducted on a powder X-ray diffractometer (PANalytical X’Pert Pro, Holland) with Cu Kα radiation (λ = 0.1540598 nm, 40 kV, 40mA). All the patterns within the 10-90° 2θ range were collected in a scanning mode with a 0.01313° step size. The morphologies and chemical compositions of the samples were analyzed by a MIRA3 LMH (TESCAN) field-emission scanning electron microscope (FE-SEM) equipped with energy dispersive spectrum (EDS) as well as the selected-area electron diffraction. The UC emission spectra were recorded with a FluoroLog-3 spectrophotometer equipped with an adjustable laser diode (980 nm) as excitation source. The luminescence decay curves were measured using a FLS980 (Edinburgh) spectrometer with a tunable mid-band OPO pulse laser. The temperature dependence of the UC emission was performed on a temperature controlling stage. For low temperature spectra measurement, the phosphors were placed in liquid-helium cooling device, and measuring temperature was increased from 73 to 275 K. In addition to the temperature dependence of UC luminescence, all the measurements were performed at room temperature.

RESULTS AND DISCUSSION Crystal Structure and Morphology. In the present study, complex oxide BYO is selected as matrix to synthesized UC phosphors, which is owing to its excellent thermal stability and low phonon energy (~585 cm-1). The crystal structure of BYO was firstly proposed by Lopato et al. in 1972.37 BYO can be regarded as one of perovskite-derived type with a rhombohedral crystal phase. The host lattice exhibits the space group R3 (146) with lattice parameters a = b = 6.1100 Å, c = 25.1870 Å, α = β = 90°, γ = 120°, V = 817.31 Å3 and Z = 3. As illustrated in Fig. 1, there are four distinct kinds of Y atoms sites (namely Y1, Y2, Y3 and Y4), three kinds of Ba atoms sites (Ba1, Ba2, Ba3) in this crystal structure, all of which are locate at 3a sites. All of Y atoms are linked by six oxygen atoms forming YO6 octahedron. Among them, Y2 and Y4 occupy the normal octahedral sites, while Y1 and Y3occupy distorted prismatic sites. The Y2O9 units can be formed due to the octahedra and prisms share faces. Besides, the corresponding coordination numbers for 5



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Ba1, Ba2 and Ba3 are six, six and three, respectively. Because of Er3+ and Yb3+ radii are close to that of Y3+ (Table 1), it is rational to assume that Er3+ and Yb3+ doped ions could occupy the Y3+ sites in BYO host. In order to confirm this conjecture, the radius percentage differences between Er3+, Yb3+ and the possibly substituted ions in BYO are calculated and listed in Table 1 using the following expression:38

Dr =100 ×

Rm (CN ) − Rd (CN ) (1) Rm (CN )

where Dr represents the radius percentage difference; CN is the coordination number; Rm(CN) and Rd(CN) are the radius of host cation and doping ion, respectively. The values of Dr between Er3+, Yb3+ and Ba2+ are 34.1% and 35.7%, while Dr between Er3+, Yb3+ and Y3+ are 1.11% and 3.56%. As a result, the doping ions of Er3+ and Yb3+ will occupy the Y3+ sites in the host lattice.

Fig. 1 Crystal structure of BYO and coordinate environments of Y (white ball) and Ba (green ball) with O (red ball) atoms. Table 1 The radius percentage differences between the doping ions (Er3+, Yb3+) and the cation of host (Ba2+, Y3+). Dr (%)

Ions

Wyckoff sites

Radius (CN)

Ba2+

3a

1.35 Å (6)

0

0

3+

3a

0.90 Å (6)

0

0

3+

3a

0.89 Å (6)

34.1

1.11

Yb3+

3a

0.868 Å (6)

35.7

3.56

Y

Er

2+

Ba

Y3+

The XRD results of BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) are presented in Fig. 2. All diffraction peaks of these samples are well consistent with the rhombohedral phase of Ba3Y4O9 (JCPDS No.

6


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01-089-5891). No impurities or second phases are observed, which indicate that the doping ions of Er3+ and Yb3+ were introduced into the BYO host successfully.

Fig. 2 XRD patterns of BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) samples.

In order to further determine the phase composition of these samples, the Rietveld structural refinements of the powder diffraction data for the un-doped BYO and BYO:0.1Er3+/xYb3+ (x = 0.1, 0.4, 0.7) compounds were performed. The initial structure model is constructed with crystallographic data of BYO (JCPDS No. 01-089-5891) and the detailed refinement parameters are summarized in Table 2. Fig. 3 shows the observed, calculated, and different patterns of BYO and BYO:0.1Er3+/xYb3+ (x = 0.1, 0.4, 0.7). The final weighted R factors (Rwp) of all the samples are less than 6%, which confirm the phase purity of these samples. Moreover, the refined lattice parameters obtained in the doping samples are slightly smaller than those in the pure phase, which is owing to that the Er3+ and Yb3+ ions with the smaller ionic radii (as shown in Table 1) successfully enter the host lattice through occupying Y3+ sites.

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Fig. 3 Observed, calculated, and different XRD patterns of (a) un-doped BYO, (b) BYO:0.1Er3+/0.1Yb3+, (c) BYO:0.1Er3+/0.4Yb3+ and (d) BYO:0.1Er3+/0.7Yb3+. Table 2 Main Parameters of refinement processing and the corresponding results of the un-doped BYO and BYO:Er3+/Yb3+ (x = 0.1, 0.4, 0.7). Compound

BYO

BYO:0.1Er3+/0.1Yb3+

BYO:0.1Er3+/0.4Yb3+

BYO:0.1Er3+/0.7Yb3+

space group

R3 (146)

R3 (146)

R3 (146)

R3 (146)

a = b (Å)

6.1106(2)

6.1088(4)

6.1043(4)

6.1012(3)

c (Å)

25.1847(3)

25.1795(7)

25.1678(7)

25.1602(7)

α, β, γ (deg)

90°, 90°, 120°

90°, 90°, 120°

90°, 90°, 120°

90°, 90°, 120°

V (Å3)

814.40(2)

813.75(4)

812.17(3)

811.71(6)

Z

3

3

3

3

2θ range

10-90°

10-90°

10-90°

10-90°

Rp (%)

4.18

4.54

4.22

4.43

Rwp (%)

5.12

5.84

5.42

5.68

χ2

1.96

2.05

1.99

2.21

The morphologies and chemical compositions of as-synthesized phosphors were obtained by SEM and EDS. As shown in Fig. 4a, BYO:0.1Er3+/0.4Yb3+ powder exhibits a rugby-like shape with particle size in the range 1-6 µm. However, some particles agglomerates formed due to the 8


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high sintering temperature. As shown in Fig. 4b, the actual chemical compositions of as-synthesized sample consist of Ba, Y, Er, Yb and O elements and the atomic ratio of Y/(Er and Yb) is about 7:1, which agrees with the nominal composition ratio. Fig. 5 displays the elemental distribution mappings for O, Ba, Y, Er and Yb. As one can see from the figure, the distribution of Y, Er, Yb are uniformly dispersed and homogeneous. It is well known that homogenous distribution of activator can minimize the concentration quenching and improve the UC emission.39-40

Fig. 4 (a) SEM image and particle size distribution (inset) and (b) EDS spectrum of BYO: 0.1Er3+/0.4Yb3+ sample.

9



Fig. 5 SEM image (a) and corresponding element mappings of (b) O, (c) Ba, (d) Y, (e) Er, (f) Yb of BYO:0.1Er3+/0.4Yb3+ sample.

UC Luminescence of BYO:Er3+/Yb3+. Fig. 6a presents the UC spectra of BYO:Er3+/Yb3+ samples under 980 excitation. A weak blue emission, two relatively weak green emissions and a primary red emission bands can be discovered, which corresponding to the 2H9/2 → 4I15/2 (411 nm), 2H11/2 → 4I15/2 (537 nm), 4S3/2 → 4I15/2 (560 nm) and 4F9/2 → 4I15/2 (663 nm) transitions, respectively. It is noteworthy that the appearance of blue emission indicates that BYO:Er3+/Yb3+ is an ideal UC luminescent material because the blue emission generally disappears in Er3+/Yb3+ co-doped system owing to that the three or four-photon emission is low efficiency and strong scattering of host lattices.41-42 Fig. 6b illustrates the energy level model and possible UC luminescence mechanism.

10


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Fig. 6 The UC luminescence spectra (a), and the simplified energy diagram and possible populating pathways of BYO:Er3+/Yb3+ samples (b). Numbers in red circles represent the numbers of photons required in population of corresponding excited states.

Concentration-Dependent UC Luminescence. The UC luminescence spectra of BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) samples are displayed in Fig. 7a. The blue, green and red emissions increase simultaneously with the increase of Yb3+ content and reach the highest photoluminescence intensity at x = 0.6, x= 0.4 and x = 0.6, respectively. When Yb3+ concentration exceeds threshold, Yb3+ sublattice-mediated energy dissipation processes become more pronounced in comparison with the enhancement produced by increasing Yb3+ ions, thus quenching the UC emission. It is noteworthy that the optimum Yb3+ concentrations of green and red emissions are different, which is due to there exist another route to populate the 4F9/2 level (2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+)) in comparison to the population of green emitting levels. This conclusion can be confirmed by the downconversion luminescence spectra 11



when excited by 491 nm. As we can see from the Fig. 8, the red emission is rather weak than green emission, which is opposite to UC emission. As presented in Fig. 7a, compared with green emission, the red emission is more strongly. The population of 4F9/2 level only can be achieved via non-radiative relaxation from 4F7/2 state when pumping at 491 nm. However, under 980 nm excitation, in addition to the non-radiative relaxation from 4F7/2 state, the energy transfer (ET) process: 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+) is another way to realize the population of 4F9/2 state. These results demonstrate that the second route is primarily responsible for the red emission. In addition, it is observed that the green emission decline faster compared to red emission in the inset of Fig. 7a, which originates from the ET from Er3+ to Yb3+ happened in the 4S3/2 level.43 The intensity ratios of Green/Blue (G/B), Red/Blue (R/B), Red/Green (R/G) are displayed in Fig. 7b. It is observed that the ratios of G/B, R/B keep declining and R/G keeps increasing with increasing Yb3+ concentration, respectively.

Fig. 7 (a) The UC emission spectra of BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) samples under 980 nm excitation, the inset presents the blue, green and red emissions versus Yb3+ concentration; (b) the ratios of G/B, R/B and R/G versus Yb3+ concentration.

12


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Fig. 8 Luminescence spectra of BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) samples excited by 491 nm light.

Fig. 9 displays the decay profiles of UC emission at 560 nm and 663 nm of BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) phosphors under 980 nm excitation. Both the experimental intensity data of 560 nm and 663 nm emissions can be fitted to the expression:44-45

 t   t  I (t ) = I 0 + A1 exp  −  + A2 exp  −  (2)  τ1   τ2  where I(t) and I0 represent the intensities at t and 0, A1and A2 are fitting constants, t represents time, τ1 and τ2 represent the rapid and slow decay times, respectively. Moreover, the average lifetime τ are determined by the following equation:44

τ=

A1τ 12 + A2τ 22 (3) A1τ 1 + A2τ 2

The lifetimes of 560 nm and 663 nm emissions are calculated to be 211.13, 198.32, 184.56, 179.63, 164.74, 158.32, 144.43 µs and 279.33, 270.32, 256.28, 238.56, 212.56, 184.16, 168.27 µs for different Yb3+ contents at x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 in BYO:0.1Er3+/xYb3+, respectively. The Er3+ doped concentration of each sample is fixed to 0.1, thus the shortening of lifetimes is due to the interaction between Er3+ and Yb3+. The interaction can be attributed to the back-energy-transfer (BET) process. Increasing Yb3+ concentration results in enhancement of BET process, which results from resonance between appropriate energy levels of Er3+ with the corresponding Yb3+ energy levels.

13



Fig. 9 Luminescence decay curves for BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) of 537 nm(a) and 663 nm(b) emissions.

Based on the above analysis as well as energy level diagram in Fig. 6b, the variation tendencies of UC emission and emission intensity ratios with Yb3+ doping concentration in BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) can be understood. With increasing Yb3+ content, more Yb3+ ions can absorb energy from excitation source and donate their energy to the luminescence center, thus the UC luminescence intensity is enhanced. At higher Yb3+ concentrations, the BET process is accelerated because the distance between Er3+ and Yb3+ ions becomes shorter. In addition, the interaction between Yb3+ ions is enhanced, which result in the energy mainly migrate to defects or other quenching centers and ET from Yb3+ to Er3+ is less efficient.46-47 The BET process (4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+)) suppresses the population of green-emitting levels 4S3/2, which leads to decreasing green emission. At the same time, the electrons population of 4I13/2 level increase and then absorb the energy from Yb3+ ions through the energy transfer process: 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+), resulting in the enhancement of red emission. Moreover, this energy transfer process is demonstrated to be primarily population pathway for red emission in Fig. 8. Thus, it is easy to understand why the R/G ratio keeps 14


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increasing with the increase of Yb3+ content. At lower concentrations of Yb3+ ion, ET process: 2

F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+) is more favorable due to the lifetime of the

4

I11/2 state being much shorter than that of 4I13/2 level, which promote the red emission.48 At higher

concentrations of Yb3+ ion, the green emission would be decreased faster than red emission, which is attributed to the increasing rate of BET process. Besides the BET process as discussed above, the cross relaxation (CR) process is regarded as another reason for the ratio of R/G enhancement. It is well known that CR process is associated with the distance between activators. For BYO:Er3+/Yb3+ sample, Yb3+ enter the host lattice through substituting Y3+, resulting in the lattice constants decrease with increasing Yb3+ concentration and then the distance between two nearby Er3+ ions becomes shorter. In general, three possible CR processes are responsible for the increase of R/G ratio. The first one, red emission enhancement is originated from the process: 4S3/2 (Er3+) + 4

I13/2 (Er3+) → 4F9/2 (Er3+) + 4I11/2 (Er3+),49 denoted as CR1. The second one, the enhanced red

emission is ascribed to CR2 process: 4F7/2 (Er3+) + 4I11/2 (Er3+) → 4F9/2 (Er3+) +4F9/2 (Er3+).50 The last one, red emission enhancement is attributed to CR3 process: 4S3/2 (Er3+) + 4I15/2 (Er3+) → 4I9/2 (Er3+) + 4I13/2 (Er3+), and subsequent 4I13/2 (Er3+) + 2F5/2 (Yb3+) → 4F9/2 (Er3+) + 2F7/2 (Yb3+).51 However, compared with CR process, the BET process may play a dominant role, which is due to the Er3+ doped concentration is too low. Similarly, the G/B ratio keeps declining, which is due to the population of 2H9/2 state mainly via the ET from the 4F9/2 state. Interestingly, the R/B ratio decreases with the increasing Yb3+ content. The result indicates that increasing Yb3+ doping concentration tends to enhance the multi-photon UC emission. According to the above analysis, there probably exists a competition between different intermediate metastable energy levels.52 At higher Yb3+ concentrations, the electrons are easily excited to the upper levels (2H9/2) from the lower levels (4F9/2) within the populating process. Hence the increase of Yb3+ content or content ratio of Yb3+ to Er3+ ions can usefully facilitate the three-photon blue emission.53-54 Furthermore, the calculated chromaticity coordinates are illustrated in Fig. 10b and summarized in Table 3. The chromaticity coordinates move from yellow-green region (x = 0.3783 and y = 0.6076) to orange region (x = 0.5343 and y = 0.4218) with the increase of Yb3+ content, which means that the output color of BYO: Er3+/Yb3+ samples can be easily tuned through controlling Yb3+ doped content. 15



Page 16 of 32

Fig. 10 The luminescence photographs of BYO:0.1Er3+/xYb3+ (x = 0.1-0.7) samples under 980 nm excitation (a); the CIE chromaticity coordinates of BYO:0.1Er3+/xYb3+ (x = 0.1, 0.3, 0.5 and 0.7) samples (b). Table 3 Chromaticity coordinates (x, y) of the BYO:0.01Er3+/xYb3+ (x = 0.1-0.7) samples under 980 nm laser excitation. Sample

x

y

3+

3+

0.3783

0.6076

3+

3+

0.4073

0.5796

3+

3+

BYO:0.1Er /0.3Yb

0.4169

0.5622

BYO:0.1Er3+/0.4Yb3+

0.4412

0.5326

BYO:0.1Er /0.1Yb BYO:0.1Er /0.2Yb

3+

3+

0.4513

0.5155

3+

3+

BYO:0.1Er /0.6Yb

0.4813

0.4759

BYO:0.1Er3+/0.7Yb3+

0.5343

0.4218

BYO:0.1Er /0.5Yb

Pump-Power Dependence of UC Luminescence. To get deeper understand the UC luminescence mechanism in BYO:Er3+/Yb3+ samples, the power-dependent UC emission intensities (I) were investigated. For the UC process, I depend on the pump-power (P) is follows the relationship of I ∝ P , where n represents the required photon numbers to populate the n

excited state. The values of n can be obtained from linear fit of Ln-Ln plots of I versus P. As is illustrated in Fig. 11a, the slopes for blue (2H9/2 → 4I15/2), green (2H11/2, 4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) emissions are fitted to be 2.35, 1.91 and 1.96, respectively. The results illuminate that the UC emission of blue, green and red are three, two and two-photon absorption process, respectively. 16


Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As presents in Fig. 11, all of slopes are slightly lower than the actual required photon numbers. These results is derived from the competition among the different intermediate state and the energy loss in the UC process.55-56 In addition, the slopes decline gradually with increasing Yb3+ content, which is probably owing to the saturation of UC processes become easier at higher content ratios of Yb3+ to Er3+.57 At higher Yb3+ concentrations, the electrons populating in intermediate state of Er3+ ions are easily jump to upper excited state via absorbing energy of Yb3+ ions, which subsequently result in energy saturation.

Fig. 11 The pump-power dependence of UC luminescence intensity of (a) BYO:Er3+/0.1Yb3+, (b) BYO:Er3+/0.4Yb3+ and (c) BYO:Er3+/0.7Yb3+ samples under 980 nm excitation.

Multiple Optical Temperature Sensing Behaviors. Using FIR technique based on the TCLs, it is needful to study the FIR of TCLs varies with the temperature. For Er3+ ions, the

∆ E between 2H11/2 and 4S3/2 is about 800 cm-1.9 The small energy gap leads to the 2H11/2 level can easily thermally populated from 4S3/2 level. Excited by 980 nm with low power (~ 0.036 W), the green UC luminescence spectra of BYO:Er3+/Yb3+ sample were obtained in a temperatures range of 298-573 K. As displayed in Fig. 12a, the UC emission is corresponding to the 2H11/2 → 4I15/2 transition keeps increasing as the temperature rises, which is due to the population of TCLs (2H11/2 and 4S3/2) abide by Boltzmann distribution law.58 The FIR from the TCLs can be written as follows:59-61

FIR =

I H g H ωH δ H  −∆E   −∆E  = exp   = B exp   (4) IS g S ωS δ S  kT   kT 

where IH and IS denote the integrated intensities of the radiative relaxations from 2H11/2 and 4S3/2 to 4

I15/2 level, respectively. g, ω and δ are the degeneracy factors, the frequency of emission and the 17



radiative transition rate, respectively. ∆ E represents the energy gap between the 2H11/2 and 4S3/2 levels, k is the Boltzmann constant, T is the absolute temperature. The pre-exponential parameter is defined as B =

g H ωH δ H

g S ωS δ S

.

Fig. 12 (a) Normalized UC emission spectra of BYO:Er3+/Yb3+ under the excitation of 980 nm laser diode with power of 0.036 W at various temperatures of 298-573 K, (b) monolog plot of the FIR from TCLs versus inverse absolute temperature, (c) temperature dependence of FIR, and (d) the SA and SR of BYO:Er3+/Yb3+.

Fig. 12b shows the value of Ln(FIR) as a function of absolute temperature ranging from 298 to 573 K. The data point can be fitted with a straight line, which confirms the suitability of the phosphor to temperature sensing applications. The slope and intercept of fitted line are about -1186.8 and 2.26, respectively. The dependence of FIR on the absolute temperature is shown in Fig. 12c. The pre-exponential constant B value is found to be 9.59 according to the fitted curve. Subsequently, the value of ∆E is calculated to be 822 cm-1. For the practical application of thermometers, the absolute and relative sensitivity SA and SR are two important parameters to assess the temperature sensing performance, which can be defined as:62-64

SA =

d ( FIR) ∆E = ( FIR) ⋅ 2 (5) dT kT 18


Page 18 of 32

Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SR =

1 d ( FIR) ∆E = 2 (6) ( FIR) dT kT

According to the formulas (5) and (6), larger energy gap ∆E can result in larger SA and SR. Fig. 12d describes the SA and SR versus temperature from 298 to 573 K. SA increases with increasing temperature, which indicates that the present materials could be serve as temperature sensors. The maximum SA is about 45.8×10-4 K-1 at 573 K. The temperature sensing performance of BYO:Er3+/Yb3+ phosphors are evaluated in comparison with those of other Er3+/Yb3+ co-doped systems. As listed in Table 4, the values of SA and SR in BYO:Er3+/Yb3+ phosphor are higher than those in the most other Er3+/Yb3+ co-doped systems. Table 4

∆E , SA-max and SR of some typical Er3+/Yb3+ co-doped FIR-based UC temperature sensing materials.

Sensing materials K3LuF6: Er3+/Yb3+ glass ceramics Ba3Y4O9:Er3+/Yb3+ phosphors 3+

∆E

range (K)

(cm-1)

300-773

870

1256/T2

298-573

822

273-333

SR (%K-1)

SA-max (10-4

Tmax (K)

Ref.

37.6

625

[62]

1186.8/T2

45.8

573

812

1171/T2

42

328

[60]

303-563

789

1135/T2

37

580

[12]

300-500

786

1129.8/T2

62.1

560

[63]

298-693

774

1117.4/T2

23

560

[58]

300-603

766

1102/T2

65

603

[64]

300-510

747

1079/T2

45

480

[9]

300-543

694

1002/T2

255

543

[61]

300-600

567

817/T2

56

400

[14]

300-900

519

746.4/T2

39

300

[15]

K-1)

This work

3+

NaYF4:Er /Yb nanoparticles

NaGdF4:Er3+/Yb3+ glass ceramics Sr2YbF7:Er3+ glass ceramics NaYF4:Er3+/Yb3+ glass ceramics NaZnPO4:Er3+/Yb3+/Li+ phosphors NaLnTiO4:Er3+/Yb3+ phosphors SrMoO4:Er3+/Yb3+ phosphors Y2SiO5:Er3+/Yb3+ phosphors Gd2O3:Er3+/Yb3+ phosphors

Temperature

As shown in Fig. 13(a), it is obvious to observe that the 2H11/2, 4S3/2 and 4F9/2 levels split into 2

H11/2(1)/2H11/2(2), 4S3/2(1)/4S3/2(2) and 4F9/2(1)/4F9/2(2) levels, respectively. The green UC emissions

peaked at 522 nm, 537 nm, 549 nm and 560 nm are attributed to the radiative transitions from 19



Stark sublevels of the 2H11/2 and 4S3/2 (2H11/2(1)/2H11/2(2) and 4S3/2(1)/4S3/2(2)) to the 4I15/2 state, respectively, while the Stark sublevels of the 4F9/2 level (4F9/2(1)/4F9/2(2)) results in red emissions centered at 664 nm and 677 nm. These pairs of Stark sublevels (2H11/2(1)/2H11/2(2), 4S3/2(1)/4S3/2(2), 4

F9/2(1)/4F9/2(2)) can be regarded as TCLs due to the narrow energy gap. Since 2H11/2 and 4S3/2 levels

are thermally coupled, thus any pair of 2H11/2/4S3/2(1), 2H11/2/4S3/2(2), 2H11/2(1)/4S3/2, 2H11/2(2)/4S3/2, 2

H11/2(1)/4S3/2(1), 2H11/2(2)/4S3/2(1), 2H11/2(1)/4S3/2(2), 2H11/2(2)/4S3/2(2) is TCLs. Thus, the temperature

sensing performances could be evaluated based on these TCLs at different temperatures (298-573 K). The FIR versus temperature is obtained through fitting as follows:

FIR522/537 = I522 / I537 = 3.19exp(−253.8 / T ) (7-1) FIR549/560 = I549 / I560 = 1.02exp(−129.6 / T ) (7-2) FIR664/677 = I664 / I677 = 3.28exp(−146.3/ T ) (7-3) FIR(522+537)/549 = I (522+537) / I 549 = 17.49exp(−1078.3 / T ) (7-4)

FIR(522+537)/560 = I (522+537) / I 560 = 18.78exp(−1231.1/ T ) (7-5) FIR522/(549+560) = I 522 / I (549+560) = 7.21exp(−1254.1/ T ) (7-6) FIR537/(549+560) = I 537 / I (549+560) = 2.27 exp(−1002 / T ) (7-7)

FIR522/549 = I522 / I549 = 13.74exp(−1167.9 / T ) (7-8) FIR537/549 = I537 / I549 = 4.35exp(−917.8 / T ) (7-9) FIR522/560 = I522 / I560 = 14.78exp(−1321.6 / T ) (7-10) FIR537/560 = I537 / I560 = 5.17exp(−1067.7 / T ) (7-11) Subsequently, the absolute sensitivity SA can be expressed as follows:

S522/537 = 253.8FIR522/537 / T 2 (8-1) S549/560 = 129.6 FIR549/560 / T 2 (8-2) S664/677 = 146.3FIR664/677 / T 2 (8-3)

S(522+537) /549 = 1078.3FIR( 522+537) /549 / T 2 (8-4) 20


Page 20 of 32

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S(522+537) /560 = 1231.1FIR(522+537 ) /560 / T 2 (8-5)

S522/( 549+560) = 1254.1FIR522/ ( 549+560) / T 2 (8-6) S537/ (549+560) = 1002FIR537/( 549+560) / T 2 (8-7) S522/549 = 1167.9 FIR522/549 / T 2 (8-8) S537/549 = 917.8FIR537/549 / T 2 (8-9)

S522/560 = 1321.6FIR522/560 / T 2 (8-10) S537/560 = 1067.7 FIR537/560 / T 2 (8-11) The variation trends of sensitivities with temperatures are presented in Fig. 13b-d and the relevant calculation parameters are summarized in Table 5. The energy gaps of all TCLs at elevated temperatures range from 90 cm-1 to 915 cm-1. As shown in Fig.13b-d, the sensitivities based on TCLs by Stark sublevels of 2H11/2/4S3/2(1), 2H11/2(1)/4S3/2(1), 2H11/2(2)/4S3/2(1), 2H11/2(2)/4S3/2(2) and 2H11/2(2)/4S3/2 first increase and then gradually decline with elevation of temperature; the sensitivities based on TCLs by Stark sublevels 2H11/2/4S3/2(2), 2H11/2(1)/4S3/2(2) and 2H11/2(1)/4S3/2 keep increasing monotonically with rising of temperature; the sensitivities based on the TCLs of 2

H11/2(1)/2H11/2(2), 4F9/2(1)/4F9/2(2) and 4S3/2(1)/4S3/2(2) keep declining monotonically with increasing

temperature. Among them, the sensitivities based on the TCLs of Stark sublevels 2H11/2/4S3/2(1), 2

H11/2/4S3/2(2), 2H11/2(1)/4S3/2(1), and 2H11/2(1)/4S3/2(2) are much higher than that of traditional TCLs of

2

H11/2/4S3/2. Particularly, the maximum SA of the TCLs by Stark sublevels (2H11/2/4S3/2(1)) is

calculated to be 88.3×10-4 K-1 at 523 K, which is approximately one-fold higher than the value based on the TCLs of 2H11/2 and 4S3/2. As indicated in Equation (5), absolute sensitivity SA relates to energy gap and the value of FIR. Therefore, higher sensitivity of the TCLs of 2H11/2/4S3/2(1), 2

H11/2/4S3/2(2), 2H11/2(1)/4S3/2(1) and 2H11/2(1)/4S3/2(2) probably result from larger FIR value and larger

energy gap.

21



Page 22 of 32

Fig. 13 (a) Luminescence spectra of BYO:Er3+/Yb3+, the sensitivity based on TCLs of (b) 2H11/2 and 4S3/2, 2H11/2

and 4S3/2(1), 2H11/2 and 4S3/2(2), 2H11/2(1) and 4S3/2(1), 2H11/2(1) and 4S3/2(2), (c) 2H11/2 and 4S3/2, 2H11/2(1) and 4S3/2, 2H11/2(2) and 4S3/2, 2H11/2(2) and 4S3/2(1), 2H11/2(2) and 4S3/2(2), (d) 2H11/2(1) and 2H11/2(2), 4S3/2(1) and 4S3/2(2), 4F9/2(1) and 4F9/2(2) versus absolute temperature. Table 5

∆E and SA-max of various TCLs of Er3+ ion. TCLs

∆E (cm-1)

SA-max (10-4 K-1)

Tmax (K)

H11/2/4S3/2

822

45.8

573

2

H11/2/4S3/2(1)

747

88.3

523

2

H11/2/4S3/2(2)

853

82.1

573

2

H11/2(1)/4S3/2(1)

809

63.2

523

2

H11/2(1)/4S3/2(2)

915

59.2

573

H11/2(1)/2H11/2(2)

176

38.7

298

H11/2(1)/4S3/2

869

30.7

573

2

H11/2(2)/4S3/2(1)

636

26.1

473

2

H11/2(2)/4S3/2(2)

739

23.6

523

4

F9/2(1)/4F9/2(2)

101

16.5

298

2

694

12.4

473

2

2

2

H11/2(2)/4S3/2

22


Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

S3/2(1)/4S3/2(2)

90

9.9

298

The temperature sensing properties based on these twelve pairs of TCLs were also investigated at temperatures from 73 to 273 K. The variation trends of the sensitivities with temperature are displayed in Fig. 14, and the relevant calculation results are summarized in table 6. The energy gaps of all TCLs at subzero temperature are range from 62 cm-1 to 761 cm-1. It is clearly observed that the sensitivities based on the TCLs of 2H11/2(1)/2H11/2(2), 4F9/2(1)/4F9/2(2), 4

S3/2(1)/4S3/2(2) keep declining monotonically as the temperature rises. Although the sensitivity

based on the TCLs of 2H11/2(1)/2H11/2(2), 4F9/2(1)/4F9/2(2), 4S3/2(1)/4S3/2(2) are limited at elevated temperatures, it is very high at subzero temperatures. On the contrary, the sensitivities of other TCLs are much lower at low temperatures than that at elevated temperatures (Fig. 14b). In particular, the sensitivity of traditional TCLs of 2H11/2 and 4S3/2 continues to decrease as the temperature decreases and is only 6.6×10-4 K-1 at 73K. As shown in Table 6, the corresponding maximum sensitivity values of 2H11/2(1)/2H11/2(2), 4F9/2 (1)/4F9/2(2) and 4S3/2(1)/4S3/2(2) are 83.9×10-4 K-1, 66.4×10-4 K-1 and 41.6×10-4 K-1 at 73 K, respectively. Among them, the sensitivity of 2

H11/2(1)/2H11/2(2) is more than twelve times higher than that of the traditional TCLs of 2H11/2 and

4

S3/2 (6.6×10-4 K-1 at 73 K). Therefore, combining the two pairs of 2H11/2(1)/2H11/2(2) and

2

H11/2/4S3/2(1), high sensitivities can be achieved by using BYO:Er3+/Yb3+ to sense temperatures.

23



Page 24 of 32

Fig. 14 The sensitivity based on the TCLs of (a) 2H11/2(1) and 2H11/2(2), 4F9/2(1) and 4F9/2(2), 4S3/2(1) and 4S3/2(2), (b) 2

H11/2 and 4S3/2(1), 2H11/2 and 4S3/2(2), 2H11/2(1) and 4S3/2(1), 2H11/2(1) and 4S3/2(2), 2H11/2 and 4S3/2, 2H11/2(2) and 4S3/2(1),

2

H11/2(2) and 4S3/2(2), 2H11/2(1) and 4S3/2, 2H11/2(2) and 4S3/2 at various temperatures from 73 to 273K.

Table 6

2

∆E and SA-max of various TCLs of Er3+ ion at temperatures from 73 to 273 K. TCLs

∆E (cm-1)

SA-max (10-4 K-1)

Tmax (K)

H11/2(1)/2H11/2(2)

132

83.9

73

4

F9/2(1)/4F9/2(2)

86

66.4

73

4

S3/2(1)/4S3/2(2)

62

41.6

73

2

H11/2/4S3/2(1)

581

29.1

273

2

H11/2/4S3/2(2)

692

22.5

273

2

H11/2(1)/4S3/2(1)

628

17.7

273

2

H11/2(1)/4S3/2(2)

761

13.4

273

H11/2/4S3/2

669

12.8

273

2

H11/2(2)/4S3/2(1)

487

11.6

273

2

H11/2(2)/4S3/2(2)

575

9.2

273

2

716

7.7

273

2

H11/2(1)/4S3/2

24


Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

H11/2(2)/4S3/2

553

5.2

273

Laser Induced Optical Thermal Effect. Excited by NIR laser, in addition to absorb energy to generate the UC emission through decay radiatively channels, the UC luminescence materials can convert some energy into heat through non-radiative processes. Thermal effect of the Stark sublevels of 2H11/2 and 4S3/2(1) induced by the laser are investigated due to high temperature sensitivity in pair of TCLs. The FIR of 2H11/2, 4S3/2(1) → 4I15/2 transition versus power is displayed in Fig. 15a. As we can see, the value of FIR increases along with increasing excitation power. The linear fitting of experimental data gives a linear equation as follows:

FIR = 2.01P + 0.413 (9) where P is excitation power. Combined equations (7-4) with (9), the temperature can be defined as:

T = − 1078.3 / Ln (0.12 P + 0.024) (10) According to the above study of temperature sensing behavior, each FIR value represents a specific temperature. The material temperature can be obtained via the following equation (11), which can be derived from equation (4).

   ∆E  1 T =   (11)  LnB − Ln( FIR)   k  The material temperatures calculated by equation (10) under different excitation powers are plotted in Fig. 15(b). The temperature varies from 293.2 to 329.6 K as the excitation power increases from 0.013 to 0.123 W. The results calculated by equation (10) and equation (11) are well matched, which indicate that the material temperature can be precisely controlled by adjusting the excitation power. Non-radiative relaxation from the upper energy level to a lower energy level is the primary reason for the heat can generate in material when excited by laser. As the excitation power increases, the population of intermediary level increases, which leads to the rate of the non-radiative relaxation enhanced to produce heat. UC luminescence material could serve as an ideal heater in photothermal therapy. According to the literature,65 cancer cells can be effectively killed when the temperature of the tumor is raised to 42-45°C. When the excitation power is in the range 0.075-0.084 W, the temperature of as-prepared phosphors can be controlled in the range 42-45°C. At the same time, this phosphor 25



can monitor the temperature of photothermal therapy agents in biological systems. Therefore, BYO:Er3+/Yb3+ could be used in photothermal therapy.

Fig. 15 (a) FIR and (b) temperature versus excitation power in BYO:Er3+/Yb3+.

CONCLUSIONS A series of Er3+/Yb3+ co-doped BYO phosphors were prepared by a high-temperature solid-state reaction method. Excited by 980 nm, multicolor visible emissions including green, yellow and red can be obtained by varying the content of Yb3+. The possible UC mechanisms are proposed to be two and three-photon processes for various color formation, respectively. UC emissions arising from Stark energy levels to ground state transitions (2H11/2(1) → 4I15/2 (522 nm), 2

H11/2(2) → 4I15/2 (537 nm), 4S3/2(1) → 4I15/2 (549 nm) and 4S3/2(2) → 4I15/2 (560 nm), 4F9/2(1) → 4I15/2

(664 nm) and 4F9/2(2) → 4I15/2 (677 nm)) were observed. Using the FIR technique, multiple temperature sensing performances based on the twelve pairs of TCLs are investigated in a wide temperature range 73-573 K. The maximum sensitivities of the TCLs by Stark sublevels (2H11/2/4S3/2(1)) is calculated to be 88.3×10-4 K-1 at 523K, which is approximately one-fold higher 26


Page 26 of 32

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than the value based on the TCLs of 2H11/2/4S3/2 (45.8×10-4 K-1). The sensitivities based on the TCLs of

2

H11/2(1)/2H11/2(2),

4

F9/2(1)/4F9/2(2),

4

S3/2(1)/4S3/2(2) keep increase monotonically with

decreasing temperature. Among them, the maximum sensitivity based on the TCLs (2H11/2(1)/2H11/2(2)) by Stark split of 2H11/2 level is 83.9×10-4 K-1 at 73 K, which is more than twelve times higher than that of the traditional TCLs of 2H11/2/4S3/2 (6.6×10-4 K-1). The sensitivities based on some Stark sublevels are significantly higher than those based on the traditional TCLs, both at subzero and elevated temperatures. Furthermore, the NIR laser induced thermal effect of BYO:Er3+/Yb3+ is also investigated. The temperatures from 293.2 to 329.6 K can be accurately regulated through adjusting the excitation power from 0.013 to 0.123 W. These results indicates that BYO:Er3+/Yb3+ could be used as an optical thermometer in non-contact temperature measurements and optical heater in photothermal therapy.

AUTHOR INFORMATION Corresponding Author ∗

E-mail: [email protected].

ORCID Xinyu Ye: 0000-0001-6536-4348

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This project supported by the Natural Science Foundation of China (51304086, 11464017), Foundation of Science and Technology Pillar Program in Industrial Field of Jiangxi Province (20123BBE50075), Natural Science Funds for Distinguished Young Scholar of Jiangxi Province (20171BCB23064), Science & Technology Major Project of Jiangxi Province (20165ABC28010) and the Program of Qingjiang Excellent Young Talents of Jiangxi University of Science and Technology.

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