Yb3+ glass ceramic

Journal Pre-proof Highly sensitive optical thermometer based on FIR technique of transparent 3+ 3+ NaY2F7:Tm /Yb glass ceramic SunYueZi Chen, WeiHui Song, JiangKun Cao, FangFang Hu, Hai Guo PII:

S0925-8388(20)30374-1

DOI:

https://doi.org/10.1016/j.jallcom.2020.154011

Reference:

JALCOM 154011

To appear in:

Journal of Alloys and Compounds

Received Date: 27 December 2019 Revised Date:

21 January 2020

Accepted Date: 22 January 2020

Please cite this article as: S. Chen, W. Song, J. Cao, F. Hu, H. Guo, Highly sensitive optical 3+ 3+ thermometer based on FIR technique of transparent NaY2F7:Tm /Yb glass ceramic, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

SunYueZi Chen: Investigation, Writing - Original Draft WeiHui Song: Investigation JiangKun Cao: Writing - Original Draft FangFang Hu: Writing - Review & Editing Hai Guo: Conceptualization, Resources, Writing - Review & Editing, Supervision

Highly Sensitive Optical Thermometer Based on FIR Technique of Transparent NaY2F7:Tm3+/Yb3+ Glass Ceramic SunYueZi Chena, WeiHui Songa, JiangKun Caoa, FangFang Hua, Hai Guoa∗ a

Department of Physics, Zhejiang Normal University, Jinhua, Zhejiang, 321004, China

Abstract Non-contact temperature sensors based on up-conversion photoemission with quick response and in situ monitoring exhibits scientific and technological potential. However, to achieve highly efficient up-conversion and good temperature sensing performance from doped disordered glass matrix has led to limited success. Here, we report a nano-structured NaY2F7:Yb3+/Tm3+ glass ceramic for intense up-conversion in the first bio-window and sensitive optical response on temperature. Evidenced by detailed structural and optical characterization, the incorporation of Yb3+/Tm3+ into NaY2F7

nano-crystals

results

in

the

greatly

enhanced

and

long-lived

photoluminescence with noteworthy Stark splitting. More interestingly, the emission intensity ratio of thermally coupled 3F2,3 → 3H6 and 1G4 → 3F4 transition of Tm3+ is highly temperature dependent and repeatable due the protection of silicate glass network. Thus, NaY2F7:Yb3+/Tm3+ glass ceramic shows excellent temperature sensing performances, including high sensitivity, and excellent repeatability. In particular, the maximal relative and absolute sensitivity reach 1.63 %K−1 (415 K) and 10.01 %K-1 (567 K), respectively. These results reveal that Yb3+/Tm3+ co-doped NaY2F7 GC could be promising in the application of up-converting materials for high performance optical temperature sensor.

Keywords: Optical thermometry; glass ceramic; up-conversion; fluorescence intensity ratio (FIR).

∗

Corresponding author E-mail: [email protected] (H. Guo)

1. Introduction Recently, considerable investigations have been focused on rare earth (RE) ions doped up-conversion materials, owing to their extensive applications in diverse fields, such as laser cooling, optical temperature sensor [1-3] and biomedical image. Er3+, Ho3+, Tm3+ are usually used as activators in up-conversion materials for their abundant excited states [4-9]. Unfortunately, these dopants exhibit insufficient absorption cross sections, which results in low up-conversion efficiency and limited pumping sources [10]. To solve this problem, Yb3+ ion as an efficient sensitizer is brought into up-conversion materials on account of its efficient absorption near 980 nm [10-12]. More recently, rare earth ions activated non-contact thermometry, which is normally constructed on photoemission from thermally coupled energy states have been attracting much attention on account of its unmatched advantages, especially compared to traditional contact-mode temperature sensor [4, 13-20]. They are suitable for operating in harsh environments and detecting fast moving objects [11]. Particularly, optical thermometry based on fluorescence intensity ratio (FIR) technique is recognized as one of the most valuable optical thermometry strategies, due to the unique merits of contactless feature, fast response, as well as excellent resolution [21]. In order to obtain temperature-dependent up-conversion luminescence for temperature monitoring, the number of particles on adjacent energy levels should be able to be re-arranged under the influence of temperature. Thus, the value of energy gap should be proper to, on one hand, remain thermal activated distribution and, on the other hand, avoid strong overlap between two emissions Thus, thermally coupled energy levels (TCELs) of dopant with suitable energy gap is a vital parameter that affects the performance of optical temperature sensor, especially for the relative sensitivity. Normally, in FIR technique, the energy gap ( ∆ E ) between TCELs of rare earth ions usually ranges from 200 to 2000 cm-1. As known to us, Er3+ ions with 2

H11/2/4S3/2 TCELs (gap of 720 cm-1) have been extensively chosen as optical probe of

temperature sensor [12]. However, the small energy gap of Er3+ ions hinders the further improvement of sensitivity of optical temperature sensors. Interestingly, 3

F2,3/3H4 TCELs (gap of 2000 cm-1) of Tm3+ present a larger energy gap, which is of

preferable sensitivity as well as accuracy and appropriate for FIR technique [5, 14, 22-27]. Host materials are another critical parameter for the application of up-conversion materials in FIR-based temperature sensors. Host materials should present low phonon energy for RE doping and good stability for long term use, especially under measured temperature range. In general, oxide materials bear good physical and chemical stability, while fluoride materials can provide low phonon energy environment for dopants and lead to efficient up-conversion luminescence. Thus, novel materials combining the advantages of both oxide and fluoride materials should be favorable host for RE doping, to achieve high performance optical temperature sensor. Herein, by constructing fluoride nano-crystals in disordered oxide silicate glasses, we experimentally realized a RE doped nano-structured hybrid optical material with good transmittance, efficient up-conversion and excellent optical sensing performance, via simple melting-quenching method and subsequent thermal treatment [28]. The structural and optical properties of such composite materials were investigated via X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), high resolution transmission electron microscope (HRTEM), transmittance spectra, up-conversion spectra and decay curves. More importantly, the temperature sensing performance was investigated via FIR technique in detail.

2. Experimental procedure 2.1 Sample synthesis The bulk samples were fabricated via melting-quenching method. Silicate glass with

nominal

composition

50SiO2-10Na2O-15Al2O3-3CaO-15NaF-7YF3-0.2YbF3-0.01TmF3 (in mol%) were

chosen, based on our previous investigation. Analytical purity SiO2, Na2CO3, Al2O3, CaCO3, NaF (from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and

99.99% TmF3, YF3, YbF3 (from AnSheng Inorganic Materials Co., Ltd., Ganzhou, Jiangxi, China) were selected as raw materials. NaF and Na2O were chosen as flux. Typically, 25 g of the well ground and mixed reagents were melted for 1 h in 1500 °C in air. The molten liquid was quenched on a 300 °C copper plate and pressed by the other to accelerate the quenching process and form solid precursor glass (PG) sample. After isothermal treatment at 680 °C for 2 h, nano-structured glass ceramic (GC680) was prepared. At last, PG and GC680 samples were optically polished with thickness of 1.5 mm for optical and structural characterization.

2.2 Samples Characterization The phases of PG and GC680 were examined by using a Rigaku MiniFlex / 600 XRD device (Tokyo, Japan), where the radiation of CuKa (λ = 0.154056 nm) works at 45 kV and 15 mA. Absorption features were investigated via a U-3900 ultraviolet-visible spectrophotometer. The microstructure analysis of GC680 was performed in a JOEL JEM-2010 transmission electron microscopy. Photoemission spectra and decay curves of PG and GC680 under the excitation of 980nm were recorded by using an Edinburgh FS5 spectrofluorometer. Temperature dependent up-conversion spectra (from 307 to 567 K) of GC680 under the excitation of 980 nm were obtained in a FS5 spectrofluorometer. 3. Results and discussion 3.1 Structural properties The phases of samples were investigated by XRD and the results are presented in Fig. 1(a). Interestingly, it can be observed that several weak diffraction peaks over diffuse hump are displayed on XRD patterns of quenched PG sample. While intense diffraction peaks over diffuse hump were observed in thermally treated GC680 sample, which suggests the in-situ precipitation of nano-crystals inside amorphous matrix. Further investigation indicates that these peaks match the standard data for

orthorhombic NaYb2F7. It’s worth noting that there are two diffraction peaks located around 2θ = 46.507° in PDF card and the GC sample exhibits the same peaks as well [29]. Thus, we speculate that the formed nano-particles in GC680 nm should be NaY2F7 nano-crystals. It is worth to mention the protection of fluoride nano-crystals by silicate glass network can significantly enhance the long-term stability of such hybrid materials, especially in harsh environment. The size for NaY2F7 nano-crystals in bulk sample is ~ 31 nm, which is obtained by the equation:

D = kλ β cosθ

(1)

where k = 0.89, λ (= 0.154056 nm) stands for the wavelength of CuKa radiation, θ is Bragg angle and β stands for the corrected half-width of diffraction peak. Additionally, the crystal volume is estimated to be 16% of host for GC680. Because the size of the precipitated NaY2F7 nanocrystals is much smaller than the wavelengths of visible light, the light scattering of NaY2F7 nanocrystals in visible wavelength region can be expected to be negligible [22]. As shown in the transmittance spectra of GC680 sample (Fig. 1(b)), high transparency (88% @ 600 nm) are observed. Obviously, two typical absorption bands of Tm3+ centered at 675 and 798 nm were detected, which are attributed to typical transitions of Tm3+ from 3

H6 to 3F2,3 and 3H4, respectively [16]. As presented in Fig. 1(c), the TEM images of the nanostructured GC680 samples

clearly present the uniform distribution of NaY2F7 nano-crystals in silicate glass matrices. The average size of nano-crystal is about 32 nm, matching the results from Fig. 1(a). The SAED patterns in the inset of Fig. 1(c) directly prove the hybrid feature of such GC680 sample, which is the combination of nano-crystals and amorphous glass. Furthermore, Fig. 1(d) presents the HRTEM of GC680 sample. Clear lattice fringes indicate the good crystallinity of this glass ceramic. The inter-planar distance (d) obtained is 0.314 nm, which is consistent with (009) crystal plane of NaY2F7 crystals (d009=0.314 nm). These results further support the formation of NaY2F7 in nano-structured GC680 sample.

3.2 Room temperature emission properties As shown in Fig. 2(a), PG and GC680 present intense blue and near infrared emission of Tm3+ ions, upon 980 nm excitation at room temperature. The emission band with peak at 800 nm could be assigned to the 3H4 → 3H6 transition of Tm3+. While the visible emission bands located at 480, 649, and 700 nm could be attributed to characteristic transition of Tm3+ ions from 1G4 → 3H6 (480 nm), 3F4 (649 nm) and 3

F2,3 → 3H6 (700 nm), respectively [30]. After thermal treatment on quenched PG

sample, significant variation emerged in the emission spectra of GC680. On one hand, the emission intensity is highly enhanced after thermally triggered crystallization of NaY2F7. In detail, the blue (480 nm), red (649, 700 nm) and NIR (800 nm) emission of GC680 are enhanced by a factor of 7, 8, 14 and 10 times, respectively, which can be clearly seen in Fig. 2(b). On the other hand, obvious Stark splitting emerged in the emission spectra of GC680, suggesting the successful incorporation of Tm3+ into NaY2F7 nano-crystals with high symmetry environment. Thus, we can conclude that Yb3+/Tm3+ ions are enriched into NaY2F7 nano-crystals with low phonon energy and high symmetry, which greatly increase the energy transfer (ET) possibility from Yb3+ to Tm3+ and decease Tm3+ non-radiative relaxation probability, so as to produce improved luminescence in GC680 [7, 31]. To understand the up-conversion mechanism of Yb3+/Tm3+ in these glass samples, the dependence of up-conversion luminescence intensities on pump power was measured for PG and GC680, respectively. As shown in Fig. 3, the 480 (1G4 → 3H6), 649 (1G4 → 3F4) nm emissions are dominated by three-photon process. While 700 (3F2,3 → 3H6) nm is two photon process in PG and GC680 samples. The up-conversion process of Tm in PG and GC680 is illustrated in Fig. 4. Initially, Yb3+ ions at 2F7/2 state are excited to 2F5/2 state via ground-state absorption, under the excitation of 980 nm light. Subsequently, these Yb3+ ions relax to ground state by a non-radiative relaxation process, transferring the energy to the adjacent Tm3+ ions, promoting them to 3H5 state from 3H6 state, which can be presented as ET1 process: 3H6 (Tm3+) + 2F5/2 (Yb3+) → 3H5 (Tm3+) + 2F7/2 (Yb3+). Then, Tm3+ ions at 3

H5 excited state relax into 3F4 level non-radiatively. Via ET2 process, Tm3+ ions at

3

F4 state are excited to 3F2 state by ET2 process: 3F4 (Tm3+) + 2F5/2 (Yb3+) → 3F2

(Tm3+) + 2F7/2 (Yb3+). After that, Tm3+ ions at 3F2 state non-radiatively return to 3H4 state. And Tm3+ ions at 3H4 state are further pumped into 1G4 state by ET3 process: 3

H4 (Tm3+) + 2F5/2 (Yb3+) → 1G4 (Tm3+) + 2F7/2 (Yb3+). Finally, the 1G4 → 3H6, 1G4 →

3

F4, 3F2 → 3H6, 3F3 → 3H6 and 3H4 → 3H6 electronic transitions of Tm3+ ions produce

emissions around 480, 649, 678, 700 and 800 nm, respectively [32, 33]. In PG sample, most of Yb3+ and Tm3+ ions are dispersed in glass matrix due to the small amount of formed nano-crystals and quick quenching process, which is evidenced by the weak diffraction peak on the XRD patterns of PG sample. Thus, ET from Yb3+ to Tm3+ is suppressed by the long Tm3+-Yb3+ distance. However, with the growth of NaY2F7 nano-crystals and the incorporation of Yb3+/Tm3+ into these nano-crystals during thermal treatment, the Tm3+-Yb3+ distance will be highly shortened and the ET between them will be strengthened, which consequently lead to improved up-conversion emission and Stark splitting [10]. As far as we know, the decay behavior of up-conversion luminescence will, to some extent, reflect the enrichment of rare earth into formed fluoride nano-crystals in nano-structured glass. Thus, the decay curves of 480, 649, and 800 nm emissions were recorded for PG and GC680, as plotted in Fig. 5(a-c). The lifetime ( τ ) of samples is evaluated by the equation:

τ = ∫ tI (t) dt

∫ I (t )dt

(2)

where I(t) is up-conversion luminescence intensity upon time t. Clearly, τ

for

GC680 is longer than that of PG samples. In detail, the τ of 480, 649, and 800 nm are 672, 660 and 895 µs for GC680, respectively. However, the τ of 480, 649, and 800 nm are 488, 450 and 477 µs for PG, respectively. As we all know, the low phonon energy environment decreases the non-radiative transition possibility of rare earth ions and thus favors long lived up-conversion luminescence [34]. Therefore, the prolonged lifetime in GC680 sample further reflect the enrichment of Yb3+/Tm3+ into NaY2F7 nano-crystals.

3.3 Temperature-dependent up-conversion emission The temperature dependent up-conversion spectra of GC680 with temperature ranging from 307 to 567 K were recorded. As known to us, the 3F2 and 3F3 levels of Tm3+ presents very small energy gap of 450 cm-1, which results in severe emissions overlap [16]. Hence, these two energy levels could be considered as one level. Here, the ~600 and 700 emission bands from TCELs of Tm3+ at 3F2,3 and 1G4 are chosen as temperature signal. As presented in Fig. 6(a), three distinct emission bands located at ~649, ~678 and ~700 nm were observed under the excitation of 980 nm, which can be assigned to 1G4 → 3F4, 3F2 → 3H6 and 3F3 → 3H6 transitions of Tm3+ ion, respectively. The integral emission intensity at 678 and 700 nm peaks are labeled as I700+678 (from 668 to 730 nm). With increasing temperature gradually from 307 to 563 K, the integral emission intensity at 649 nm peak (labeled as I649, from 600 to 668 nm) gradually decreases, while I700+678 monotonously enhance, which proves the 3F2,3 and 1

G4 excited states are TCELs. Therefore, the I700+678 and I649 should obey the

Boltzmann law and present an opposite temperature dependent behavior, as exhibited in Fig. 6(b). The FIR is defined as following equation, ∆E

∆E

− I N ω A g ω A − FIR = H = H H H = H H H e kBT + C = B × e kBT + C IL g LωL AL g LωL AL

(3)

where I, N, g, ω and A is the photoemission intensity, the amount of activators, the degeneracy, the angular frequency and the spontaneous radiative transition rate of 3

F2,3 → 3H6 (H) and 1G4 → 3F4 (L), respectively; B = g H ωH AH g LωL AL ; ∆E is the

energy gap between 3H4 and 3F3,2 states; K B and T stands for the Boltzmann constant and absolute temperature, respectively. The FIR from 307 to 567 K for GC680 was calculated and is shown in Fig. 6(c). Based on equation (3), the FIR from temperature dependent emission spectra can be written as follow,

FIR = 5431× exp(−

3637 ) + 0.249 T

(4)

The effective energy gap ∆E was obtained from Eq. (3) is 2525 cm-1. B was calculated

to be 5431, and C is 0.249. The degree of fitting reaches 99.998%. For thermometry applications, sensitivity is a vital parameter, which can be classified

as

absolute

sensitivity

(SA)

and

relative

sensitivity

(SR),

SA =

d (FIR) ∆E = [ ( FIR ) − C ] dT k BT 2

(5)

SR =

1 d ( FIR ) ∆E C (1 − ) × = 2 FIR dT k BT FIR

(6)

According to the definition of sensitivity, SA and SR of NaY2F7:Yb3+/Tm3+ glass ceramic were obtained and are shown in Fig. 6(d). Yb3+/Tm3+ co-doped NaY2F7 glass ceramic show relatively high SA and SR in a wide temperature range from 307 to 567 K, presenting the maximal SR (labeled as TSR-max) value of 1.63 %K−1 at 415 K and the maximal SA value of 10.01 %K-1 at 567 K. To further prove the great potential of such GC as optical temperature sensor, we evaluated the repeatability of the temperature-dependent FIR, as presented in Fig. 6(e). The yoyo experiment on the temperature dependent FIR reveals good stability of this optical temperature sensor. These results illustrate the NaY2F7:Yb3+/Tm3+ glass ceramic also have great potential in optical thermometry applications.

4. Conclusion In all, highly transparent bulk NaY2F7:Yb3+/Tm3+ glass ceramic was successfully manufactured via melting-quenching method. Detailed characterizations on structural and optical properties suggest the formation of NaY2F7 nano-crystals and the prioritized enrichment of Yb/Tm into formed nano-crystals. Therefore, highly enhanced and long lived up-conversion emission with obvious Stark splitting is obtained after controllable thermal treatment, via convenient commercial 980 laser excitation. More fascinatingly, benefiting from the large energy gap of Tm3+ between TCELs of 3F2,3 and 1G4, excellent temperature sensing performances were achieved, including high sensitivity, good repeatability and favorable temperature sensing range. In particular, the maximum relative sensitivity SR reaches 1.63 %K−1 at 415 K and absolute sensitivity SA is 10.01 %K-1 at 567 K. Hence, novel NaY2F7:Yb3+/Tm3+ glass

ceramic could be potential candidate for highly sensitive optical thermometry, especially considering the efficient up-conversion, good stability and excellent temperature sensing performance.

Acknowledgments This work was supported by NSFC (No. 11974315 and 11804303), the Natural Science Foundation of Zhejiang Province (Grant No. LZ20E020002).

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Table caption Table 1. Temperature sensing performance of typical optical materials via FIR technique. Typical materials

Range (K)

SR-max

TSR-max

(%K-1)

(K)

Ref.

NaY2F7:Yb3+/Tm3+ GC

307-567

1.63

415

This work

Sr2GdF7:Yb3+/Tm3+ GC

293-563

1.97

353

[22]

Ba3Y4O9:Yb3+/Ho3+/Tm3+

350-510

1.49

417

[33]

Sr2YF7:Yb3+/Tm3+ GC

303-663

1.16

428

[16]

NaLuF4:Ho3+ GC

390-780

0.83

500

[35]

Sr2YbF7:Er3+ GC

300-500

0.62

560

[10]

KYb2F7:Er3+ GC

300-480

0.45

590

[11]

Figure captions Fig. 1. (a) XRD patterns of samples and the diffraction peaks for orthorhombic NaYb2F7 (JCPDS card No. 43-1126); (b) Transmittance spectra of quickly quenched PG and thermal treated sample; TEM (c) Micrograph and HRTEM (d) of GC680. The inset is SAED patterns. Fig. 2. (a) Emission spectra of bulk samples (λex=980 nm); (b) The intensity ratio of GC680 compared to PG at different emission wavelength. Fig. 3. Dependence of luminescence intensities on pump power for (a) PG and (b) GC680 sample, respectively. Fig. 4 Energy level diagrams of NaY2F7:Yb3+/Tm3+ nano-crystals and feasible ET processes. Fig. 5. Decay curves for (a) 480 nm, (b) 649 nm and (c) 800 nm in PG and GC (λex=980 nm), respectively. Fig. 6. (a) Emission spectra of GC680 upon 980 nm excitation (533 mW/mm2) within temperature range from 307 to 567 K; (b) Temperature dependent FIR between 3

F2,3 → 3H6 and 1G2 → 3F4 transitions of Tm3+ ions in GC680; (c) Monolog plot

of FIR on inverse absolute temperature (K); (d) SA and SR of GC680; (e) Cyclic diagram of plots of FIR (I700+678/I649) versus temperature ranging from 307-567 K.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Highlights

1. Highly transparent bulk NaY2F7:Yb3+/Tm3+ glass ceramic was successfully manufactured. 2. The formation of NaY2F7 nano-crystals and the enrichment of Yb/Tm into nano-crystals were confirmed. 3. SR-MAX reaches 1.63 %K−1 (415 K) and SA-MAX is 10.01 %K-1 (567 K). 4. NaY2F7:Yb3+/Tm3+ glass ceramic could be potential candidate for highly sensitive optical thermometry.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: