Yb3+ co-doped transparent phosphate glass-ceramics

Journal of Luminescence 195 (2018) 314–320

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Enhanced up-conversion luminescence and optical thermometry characteristics of Er3+/Yb3+ co-doped transparent phosphate glassceramics

T

⁎

Y. Chen, G.H. Chen , X.Y. Liu, T. Yang School of Material Science and Engineering, Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Glass ceramics Up-conversion Optical thermometry Fluorescence intensity ratio (FIR) Energy transfer

Novel Er3+/Yb3+ co-doped transparent glass-ceramics (GCs) containing orthorhombic NaZnPO4 nanocrystals (NCs) were successfully prepared for the first time by a conventional melt-quenching and subsequent heating. Under 980 nm laser prompting, the GC samples produced intense red and green up-conversion emissions. The emission intensities varied with Yb3+ concentration and heat treatment conditions. Optimum emission intensities were obtained for the sample with 2 mol% of Yb3+ heat treated at 580 °C for 4 h. Furthermore, the temperature dependent fluorescence intensity ratio (FIR) of thermally coupled emitting states (4S3/2, 2H11/2) in Er3+/Yb3+ co-doped GCs was evaluated under 980 nm. A high relative temperature sensitivity of 1.329% K−1 was obtained at 303 K and the maximal absolute temperature sensitivity at 612 K was evaluated to be 5.732 × 10−3 K−1. It is expected that the as-fabricated GCs containing NaZnPO4 NCs are an efficient up-conversion material with potential application in optical temperature sensor.

1. Introduction In recent years, trivalent rare-earth (RE) ions doped up-conversion (UC) GCs have emerged as a fascinating field of research due to their potential application in all-solid compact lasers, thermal imaging, fiber amplifiers, spectral conversion, photo-voltaic solar cells, drug delivery carriers cancer treatment, temperature sensors as well as optical heaters [1–15]. One of the especially interesting applications is as non-contact temperature sensors which are regarded as a promising tool for temperature detection because of their noninvasive operation mode, excellent accuracy, high spatial resolution and fast response [16,17]. UC is a non-linear optical phenomenon which involves the sequential absorption of two or more low energy (NIR) photons to emit a high energy (visible) photon. Nowadays, phosphate based materials have become an efficient luminescent productive hosts in the photoluminescence (PL) emission studies due to its peculiar structure, low phonon energy and excellent doping ability for RE ions [18]. Phosphates based GCs possess excellent physicochemical properties, thermal stability, optical stability, better color rendering index (CRI), and low phonon energy. This leads to produce the luminescent materials with high luminescence efficiency for practical applications. Among the phosphates based materials, the sodium zinc orthophosphate (NaZnPO4) having excellent coordination flexibility and strong Zn-O-Zn linkages within the lattice is

⁎

of particular interest, because the materials with such properties may improve the PL performance [19–21]. The host materials not only play the important role to improve the PL performance but also the interaction between the RE ions and the host lattice. Erbium ion (Er3+) is a well studied RE ion as dopants in the GCs technology because under NIR excitation it gives efficient visible UC emission. Er3+ ion has low absorption for 980 nm corresponding to the 4I15/2→4I11/2 transition while Yb3+ ions show a broad absorption cross section corresponding to the 2F7/2→2F5/2 transition. Additionally, the energy levels of Yb3+ (2F5/2) and Er3+ (4I11/2) are almost resonant which enables the possible efficient energy transfer (ET) from the Yb3+ to Er3+ ions which significantly enhances the UC luminescence efficiency [22–25]. Among many reported RE ions doped materials for temperature sensing based on the change of fluorescence intensity ratio (FIR) of two thermally-coupled energy levels (TCELs) with temperature, Er3+ has two sets of TCELs, a pair (2H11/2, 4S3/2) with agap of~780 cm−1 and the other pair (4G11/2, 2H9/2) with a gap of ~1530 cm−1 [26]. By making use of both sets of levels, Er3+ could be suitable for optical thermometry from very low to very high temperature with excellent sensitivity [3]. The high measurement accuracy and wide measurement temperature range can be achieved with the GCs samples, which indicates the Er3+/Yb3+ co-doped transparent GCs containing NaZnPO4 NCs has potential application in optical temperature sensor. To our

Corresponding author. E-mail address: [email protected] (G.H. Chen).

https://doi.org/10.1016/j.jlumin.2017.11.049 Received 15 June 2017; Received in revised form 25 October 2017; Accepted 23 November 2017 Available online 26 November 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

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knowledge, there have been no reports on up-conversion luminescence and optical thermometry characteristics of Er3+/Yb3+ co-doped transparent NaZnPO4 glass- ceramics. In the present work, a novel stable and environment friendly Er3+/ Yb3+ ions co-doped transparent phosphate glass ceramics have been prepared via conventional melt quenching technique and characterized by differential scanning calorimetry (DSC), XRD, TEM, transmittance spectrum, photoluminescence spectra, decay time, and energy level diagram. The effects of Yb3+ content and heat treatment temperature in Er3+/Yb3+ co-doped NaZnPO4 transparent GCs on enhancing the UC luminescence properties were investigated. Furthermore, the optical temperature sensing study in the Er3+/Yb3+ co-doped NaZnPO4 transparent GCs has been carried out based on FIR technique. 2. Experimental 2.1. Sample preparation

Fig. 1. DSC curve of Yb3+/Er3+ (2.0/0.1 mol%) co-doped glass at a heating rate of 10 K/ min.

The samples were prepared by a melt-quenching method with the specially designed composition (in mol%): 20Na2O–42ZnO–38(P2O5+ B2O3)–xYb2O3–0.05Er2O3 (x = 0, 0.5, 1, 1.5,). The analytical reagents comprising Na2CO3, ZnO, NH4H2PO4, H3BO3, (≧99.5%) and high purity Yb2O3 and Er2O3 (≧99.99%) (Guo-Yao Co. Ltd, Shanghai, China) were used as starting materials. The chemicals were mixed thoroughly calcined at 600 °C for 60 min to remove vapor and melted in a covered alumina crucible at 1250 °C in air for 100 min in an electric furnace. The melt was poured into a 350 °C pre-heated copper mold to form the precursor glass (denoted as PG). The as-quenched glass was annealed in a muffle furnace at 420 °C for 10 h and then cooled down naturally to room temperature to release thermal stress. The glass samples were cut into 10 × 10 × 1 mm3 and polished for spectral measurement. Afterwards, the PGs were then heat-treated at 560 °C, 580 °C and 600 °C for 4 h respectively to form GCs via glass crystallization, which were labeled as GC560, GC580 and GC600. Simultaneously, the PG was heattreated at 580 °C for 2 h, 4 h and 6 h with a heating rate of 5 K/min to form GCs. All samples were polished optically for further characterization.

To investigate the structure and the purity of PG and GCs, these samples were characterized by XRD measurements and the results are shown in Fig. 2(a)-(b). There are a broad diffuse humps without any sharp peaks in the XRD pattern of PG sample, indicating its amorphous structure. However, after heat treatment, some intense diffraction peaks are found to superimpose on the broad diffuse humps. All these diffraction peaks can be perfectly indexed to orthorhombic NaZnPO4 phase (JCPDS No. 49-1185) and no any other diffraction peaks were found, indicating that pure GCs containing NaZnPO4 have been successfully fabricated. It is observed that raising temperature or holding time indeed enhance the formation of NaZnPO4 phase. The average crystallite size D of GC580 is estimated to be 21 nm according to the Scherrer equation [27].

D=

kλ β cos θ

(1)

where k = 0.89, λ (0.154056 nm) represents the wavelength of CuKa radiation, θ is the Bragg angle of the XRD peak and β represents the corrected half width of diffraction peak. Transmittance, a vital parameter of transparent GCs, was recorded between 350 and 1100 nm at room temperature. As shown in Fig. 3, the transmittance obviously deceases with the increment in temperature or holding time. The GC580 remains relative high transparent with a transmittance of approximately 73–76% in the wavelength range of 500–600 nm, indicating that the size of the NaZnPO4 NCs is much smaller than the wavelength of visible light and that the NaZnPO4 NCs are distributed uniformly in the glass matrix. A battery of obvious absorption peaks located at 377 nm, 487 nm, 522 nm, and 658 nm, corresponding to the transitions of Er3+ from the ground state 4I15/2 to 4 G11/2, 4F7/2, 2H11/2 and 4F9/2 respectively, were observed, and the broad absorption band at around 975 nm can be mainly ascribed to the transition from 2F7/2 to the 2F5/2 multiplet state of Yb3+ ions with a wide absorption cross section around 980 nm, apart from weak contributions from the 4I15/2→4I11/2 transition of Er3+ [28]. Hence, the 980 nm can be chosen as the excitation source. It is found that the transmittance of all the samples tends to decrease at short wavelengths, which is predominately due to the degree of crystallization increasing. This phenomenon could be explained by Henry theory, in which the intensity of scattered light in GCs follows a λ−8R7 relationship, where λ is the wavelength of light and R is the average radius of NCs in GCs [29]. Therefore, it can be inferred that the transmittance of GCs with larger crystal size decreases more promptly with wave length. Typically, the average crystallite size of NaZnPO4NCs is estimated to be about 21 nm for GC580. Fig. 3(c) and (d) depicts TEM and HRTEM images of GC580 to provide further details of the microstructure morphology of the

2.2. Characterization The thermal properties of the PG were analyzed by differential scanning calorimetry (STA-449-F3-Jupiter, Netzsch). The crystallization phases of GCs were identified via X-ray powder diffractometer (D8-Advance, BRUKE) with CuKα radiation at room temperature. The microstructure of GCs was studied using a transmission electron microscopy(TEM, JEM-2010). The optical transmittance spectra were recorded on METASH UV–vis–NIR spectrophotometer (UV-6000) in the wavelength range from 350 to 1100 nm. The decay curve measurements were recorded on an Edinburgh Instruments FS5 spectrofluorometer. The photoluminescence spectra were recorded on a FuoroSENS9000A Fluorescence spectrometer with a 980 laser excitation with a very low power of 156 mW to avoid the possible laser induced heating. The temperature dependent emission spectra were recorded on an Edinburgh Instruments FS5 spectrofluorometer equipped with a homemade temperature controlling stage. 3. Results and discussion 3.1. Structure and morphology The representative DSC curve of PG sample is shown in Fig. 1. It is observed that the glass transition temperature (Tg), the initial crystallization temperature (Tx) corresponding to the onset of crystallization and the temperature of crystallization peak (Tp) are 460 °C, 560 °C and 629 °C, respectively. In order to obtain transparent GCs, it is suitable for selecting heat treatment temperature between 560 °C and 629 °C. 315

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Fig. 2. XRD patterns of Yb3+/Er3+ (2.0/ 0.1 mol%) co-doped precursor glass (PG)and glass ceramic (GC) (a)under different temperature and (b) under different holding time.

3.2. UC under 980 nm excitation

specimens. TEM bright-field image shows that the formed NaZnPO4 NCs are homogeneously dispersed in glass matrix seen in Fig. 3(c) and the diameters of the NCs are in the range of 18–24 nm, which are close to that estimated by the Scherrer formula. The HRTEM micrograph displays well-demarcated lattice patterns of GC580, and the value of the related interplanar spacing d is about 0.325 nm, corresponding to the (122) lattice plane of the orthorhombic phase (d122 = 0.329 nm) of the NaZnPO4 lattice, as shown in Fig. 3(d).

The room temperature emission spectra of Er3+ single doped and Yb3+/Er3+ co-doped PGs under 980 nm laser excitation with a low laser power of 156 mW are shown in Fig. 4(a). According to the emission spectrum of Fig. 4(a), the Er3+ single doped PG shows luminescence is very weak and hardly can see any emission bands in the range of 500–700 nm. And all the Yb3+/Er3+ co-doped PGs show several

Fig. 3. Transmittance spectra of PG and GC (a) under different temperature, (b) different holding time, (c) TEM image of GC580 and (d) HRTEM image of GC580.

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Fig. 4. (a) UC emission spectra of the x mol%Yb3+/0.1 mol% Er3+ co-doped PG (x = 0, 1, 2,3). (b) UC emission spectra of 2% Yb3+/0.1% Er3+co-doped GCs of heat-treated for 4 h at 560 °C, 580 °C and 600 °C. (c) the 2 mol% Yb3+/0.1 mol% Er3+co-doped GCs of heat-treated at 580 °C for 2 h, 4 h and 6 h. (d) Energy level diagram of Yb3+ and Er3+ ions and the possible relevant energy transfer.

bands centered at 522 nm, 546 nm and 658 nm assigned to 2H11/ 4 4 4 4 4 3+ , respec2→ I15/2, S3/2→ I15/2 and F9/2→ I15/2 transitions of Er 3+ tively. It indicates that Yb promotes the emission of Er3+. The emission intensity of luminescence materials is known to be dependent on the doping concentration. It is noticed that the relative intensity of 4 F9/2→4I15/2 transition of Er3+ varies as a function of Yb3+ concentration (0, 1.0, 2.0, 3.0 mol%). When the Yb3+ ion concentration is 2 mol%, the maximum luminous intensity is reached, as shown in Fig. 4(a). The decrease in emission intensity for 3 mol% Yb3+ sample is due to the concentration quenching effect [30]. The UC emission spectra for the PG and GCs obtained at various heat treatment conditions upon laser excitation of 980 nm are shown in Fig. 4(b) and (c). The UC emission spectrum of 2.0 mol% Yb3+/0.1 mol % Er3+ co-doped NaZnPO4 GCs is composed of intense red and green emissions in the visible region. From Figs. 4(b) and (c), it is clearly observation the highest emission intensity is achieved for the sample heat treated at 580 °C for 4 h (GC580). According to the above results of emission spectra, the possible UC processes are systematically sketched in Fig. 4(d) with the energy level diagrams of Er3+ and Yb3+, and the possible relevant transitions are also displayed. Among the energy level diagrams of Fig. 4(d), ET, ground state absorption (GSA), excited state absorption (ESA) and cross-relaxation (CR). Due to the fact that the absorption cross-section at around 980 nm is markedly larger for Yb3+ than Er3+ ions, the energy transfer processes (ET) from Yb3+ to Er3+ ions in NaZnPO4 GCs is supposed to be much efficient than ESA processes. The primary process is the ET from 2F5/2 energy level of Yb3+ to 4 I11/2, 4F9/2, 4F7/2, 2H9/2 and 4G9/2 energy level of Er3+ ions. The 2H11/2

and 4S3/2 states are populated through non-radiative relaxation from the 4F7/2, 2H9/2 and 4G9/2 state of Er3+ ions, and then produced the two green emission bands at 522 nm and 546 nm by radiative transition to the ground state, whereas another part is dissipated by non-radiative relaxation that feeds lower energy excited states and gives rise to 658 nm red luminescence related to the 4F9/2→4I15/2 transition. From Fig. 4(a), it can be seen that ytterbium concentration affects markedly relative integrated intensities of luminescence bands corresponding to the 2H11/2, 4S3/2→4I15/2 (green) and 4F9/2→4I15/2 (red) transitions of Er3+. Especially, the Yb3+ content has stronger influence on the intensity of the red emission than the green emission. When the Yb3+ concentration is relatively high, cross-relaxation processes contribute to the feeding of the 4F9/2 state and a red emission due to 4F9/ 4 2→ I15/2 transition occurs. On this occasion, several possible cross-relaxation routes, describing as CR1-CR4 shown in Fig. 4(d) are expressed as follows: CR1: 4I11/2 (Er3+) + 4I11/2 (Er3+)→4I15/2 (Er3+) + 4F7/2 (Er3+) CR2: 4S3/2 (Er3+) + 4I15/2 (Er3+)→4I9/2 (Er3+) + 4I13/2 (Er3+) CR3: 4F7/2 (Er3+) + 4I13/2 (Er3+)→4I11/2 (Er3+) + 4F9/2 (Er3+) CR4: 2H9/2 (Er3+) + 4I11/2 (Er3+)→4F7/2 (Er3+) + 4F9/2 (Er3+) The decay curves for PG and GCs with different heat treatment conditions under 980 nm excitation and monitoring the emission at 658 nm are shown in Fig. 5(a) and (b). The luminescence lifetime of all samples can be calculated by the following equation [8]:

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Fig. 5. Decay curves of 2 mol% Yb3+/0.1 mol% Er3+ co-doped PG and GCs. (a) heat-treated for 4 h at 560 °C, 580 °C and 600 °C. (b) heat-treated at 580 °C for 2 h, 4 h and 6 h.

τ=

∫ tI (t ) dt ∫ I (t ) dt

R=

(2)

I522 −ΔE ⎞ = A exp ⎛ I546 ⎝ KB T ⎠ ⎜

⎟

(3)

where R is the symbol of FIR under different temperatures, I522 and I546 are the integrated intensities (the integrated areas below the emission curves) for the emissions from the upper (2H11/2) and lower (4S3/2) thermally coupled levels, respectively, KB is the Boltzmann constant, ΔE is the effective energy gap between the 2H11/2 and 4S3/2 levels, T is the absolute temperature and A is a temperature-independent constant [33]. Fig. 6(b) shows the temperature dependence of FIR in the test temperature range of 303–753 K for GC580, and the experimental data can be well fitted with the following equation:

where I(t) is the luminescence intensity as a function of time t, and τ represents the decay time of the excited state. The measured decay time (τ) obtained by fitting the Eq. (2), and the lifetimes of monitoring the emission at 658 nm are about 0.508, 0.628, 0.922 and 0.530 ms for PG, GC560, GC580 and GC600, respectively. In the same way, the lifetimes of monitoring the emission at 658 nm are about 0.508, 0.813, 0.922 and 0.658 ms for PG, GC580/2 h, GC580/4 h and GC600/6 h, respectively. The longer lifetime of GC580 sample than that of PG and other GCs may be attributed to the reduced non-radiative relaxation rate of Er3+ in NaZnPO4 NCs with lower phonon energy. In a word, dramatic enhancement of UC emission and the increased lifetime are all support the incorporation of Er3+ ions into NaZnPO4 NCs after crystallization.

R = 12.8 × exp( −1218.4/ T )

(4)

The effective energy gap ΔE between 2H11/2 and 4S3/2 levels of Er3+ ions obtained from Eq. (4) is 855 cm−1. The absolute temperature sensitivity Sa and the relative temperature sensitivity Sr are two important parameters, which are defined as the absolute and relative change of the FIR value with respect to temperature variation [34–36]:

3.3. UC luminescence for optical thermometry In based on FIR of optical temperature measurement technology, it is important to choice two appropriate thermal coupling levels (TCELs) from rare earth ions and investigate how their UC luminescence intensity ratio varies with temperature. Since the energy gap between the 2 H11/2 and 4S3/2 energy levels is relatively small, the 2H11/2 energy level can be populated from the 4S3/2 energy level by thermal excitation to achieve thermal equilibrium and the relative distributions follows the Boltzmann distribution, that leading to variation in the FIR value of UC emission from 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions of Er3+ ions with the increase of temperature. From the UC spectrum, one knows that both the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions were split into two peaks. This may be the effect of crystal field and the stark split effect accounts for the multiple peaks for a single transition [31,32]. According to the above content, to evaluate the possibility of thermometry application, the green emission of UC emission spectra of 2 mol% Yb3+/0.1 mol% Er3+co-doped GC580 measured in the temperature range from 303 to 753 K under 980 nm laser excited are shown in Fig. 6(a). It is observed that the green UC emission intensity of the 2H11/ 4 4 S3/2→4I15/2 2→ I15/2 transition are increases remarkably but the transition is slow increase with the temperature increases. Moreover, it is important that the center wavelengths of the green UC emission bands remain basically same with testing temperature increases. The FIR of the emissions from 2H11/2→4I15/2 and 4S3/2→4I15/2, could be mathematically written as:

Sa =

dR ΔE = R⋅ dT KB T 2

(5)

Sr =

1 dR ΔE = R dT KB T 2

(6)

The temperature dependence of the relative sensitivity Sr and the absolute sensitivity Sa of the GC580 sample inferred from Eqs. (4)–(6) is displayed in Fig. 6(c). The Sr value deduced from Eq. (6) is 1218.4/T2, and the Sr value derived from Eq. (6) is 1.329% K−1 at 303 K. Furthermore, the absolute temperature sensitivity Sa at 612 K is evaluated to be 5.732 × 10−3 K−1, this shows Yb3+/Er3+co-doped NaZnPO4 GCs for potential application as an optical temperature sensor. Moreover, the data reproducibility of the potential temperature sensor is quite good after several repeating experiments, which is attributed to the protective role of the phosphate glass matrix. A comparison of sensor sensitivity of the different Yb3+/Er3+ co-doped materials has been presented in Table 1. The observed result revels that the developed Yb3+/Er3+co-doped NaZnPO4 GCs possesses larger relative sensitivity Sr and is more suitable for temperature sensing applications within the wide temperature range of 303–753 K [37–40]. On the whole, it is expected that these transparent GCs containing NaZnPO4 NCs could be used in optical temperature sensor.

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Table 1 ΔE and Sr of several typical FIR-based optical temperature sensors of Yb3+/ Er3+co-doped hosts. Sensing materials

Temperature range (K)

ΔE (cm−1)

Sr (%K−1)

Ref.

GCs (NaYb2F7:Yb3+, Er3+) Ba(Zr, Ca)TiO3 ceramics GCs (Sr2YbF7:Yb3+, Er3+) BaMoO4:Yb3+, Er3+ phosphors SrWO4: Yb3+, Er3+ phosphors Tungsten-tellurite: Yb3+, Er3+ glass Lead-free fluorogermanate: Yb3+, Er3+ glasses LaF3: Yb3+, Er3+ phosphors GCs (NaZnPO4:Yb3+, Er3+)

300–773 200–443 300–500 300–575

852 789 786 813

1213.6/T2 1135.5/T2 1129.8/T2 1170.0/T2

[37] [38] [39] [40]

299–518

601.99

/

[41]

300–690

678.94

/

[42]

303–623

/

/

[43]

300–515

549

/

[44]

303–753

855

1218.4/T2

This work

concentration and the thermal treatment temperature were increased, and the ultimate emission intensity was achieved for the sample with 1 mol% Yb3+thermally-treated at 580 °C for 4 h. The UC mechanisms and possible UC processes were proposed, in which ET played a dominant role. The FIR of the two green UC emission bands centered at 522 and 546 nm, originating from the transitions from two thermal coupling energy levels 2H11/2 and 4S3/2 to 4I15/2, was recorded for optical thermometry in the temperature range of 303–753 K. The relative sensitivity for Yb3+/Er3+co-doped NaZnPO4 GCs reached 1.329% K−1 at 303 K, corresponding to an effective ΔE of 855 cm−1, and the absolute temperature sensitivity at 612 K was 5.732 × 10−3 K−1. The enhanced emissions, good temperature sensitivity and the advantage of low phonon energy of transparent NaZnPO4 GCs, all recommend that the Yb3+/Er3+ co-doped NaZnPO4 GCs provides a promising candidate with high sensitivity in optical temperature sensing. Acknowledgments This work was financially supported by National Natural Science Foundation of China (No. 51362005) and Natural Science Foundation of Guangxi Province (2017GXNSFDA198023). References [1] J. Wang, R.R. Deng, A. Mark, D. Mac, B.L. Chen, J.K. Yuan, F. Wang, D.Z. Chi, S.A.H. Tzi, P. Zhang, Y. Han, X.G. Liu, Nat. Mater. 13 (2014) 157–162. [2] F.F. Hu, X.T. Wei, Y.G. Qin, S. Jiang, X.Y. Li, S.S. Zhou, Y.H. Chen, C.K. Duan, M. Yin, J. Alloy. Compd. 674 (2016) 162–167. [3] Y.F. Tang, X.M. Yang, Y.C. Xu, J. Mater. Sci. Mater. Electron. 26 (2015) 2311–2315. [4] V.D. Rodríguez, V.K. Tikhomirov, J. Méndez-Ramos, J. del-Castillo, C. GörllerWalrand, J. Nano. Sci. Nano. Technol. 9 (2009) 2072. [5] Z.W. Wei, L.N. Sun, J.L. Liu, J.Z. Zhang, H.R. Yang, Y. Yang, L.Y. Shi, Biomater 35 (2014) 387–392. [6] S. Zheng, W. Chen, D. Tan, J. Zhou, Q. Guo, W. Jiang, C. Xu, X. Liu, J. Qiu, Nanoscale 6 (2014) 5675. [7] B.R. Reddy, I. Kamma, P. Kommidi, Appl. Opt. 52 (2013) 33–39. [8] H. Suo, C.F. Guo, Z. Yang, L. Li, J. Mater. Chem. C 3 (2015) 7379–7385. [9] B. Dong, B. Cao, Y. He, Z. Liu, Z. Li, Z. Feng, Adv. Mater. 24 (2012) 1987. [10] H. Suo, C.F. Guo, L. Li, Ceram. Int. 41 (2015) 7017–7020. [11] A. Sedlmeier, D.E. Achatz, L.H. Fischer, H.H. Gorris, O.S. Wolfbeis, Nanoscale 4 (22) (2012) 7090. [12] L.H. Fischer, G.S. Harms, O.S. Wolfbeis, Angew. Chem. Int. Ed. 50 (2011) 4546–4551. [13] J.K. Cao, X.M. Li, Z.X. Wang, Y.L. Wei, L.P. Chen, H. Guo, Sens. Actuators B. Chem. 224 (2016) 507–513. [14] S. Jiang, P. Zeng, L.Q. Liao, S.F. Tian, H. Guo, Y.H. Chen, C.K. Duan, M. Yin, J. Alloy. Compd. 617 (2014) 538–541. [15] X.Y. Li, Y.L. Yu, Z.Q. Zheng, Ceram. Int. 42 (2016) 490–494. [16] S.S. Zhou, S. Jiang, X.T. Wei, Y.H. Chen, C.K. Duan, M. Yin, J. Alloy. Compd. 588 (2014) 654–657. [17] X. Tian, X. Wei, Y. Chen, C. Duan, M. Yin, Opt. Express 22 (2014) 30333–30345.

Fig. 6. (a) UC emission spectra of GC580 under the excitation of 980 nm at various temperatures from 303 to 753 K. (b) Experimental data and the fitting line of the FIR of the two thermally coupled levels (2H11/2 versus 4S3/2) in the range from 303 to 753 K. (c) The curve of relative sensitivity Sr and absolute sensitivity Sa of GC580 in the temperature range of 303–753 K.

4. Conclusions New transparent GCs containing NaZnPO4 NCs were successfully synthesized for the first time. The formation of NaZnPO4 NCs was confirmed by XRD, TEM, HRTEM, and photoluminescence measurements. The mean size of the NaZnPO4 NCs was estimated to be 21 nm from XRD and TEM results. Under 980 nm laser excitation the GCs displayed intense luminescence, and the emissions centered at 658 nm (4F9/2→4I15/2), 546 nm (4S3/2→4I15/2) and 522 nm (2H11/2→4I15/2) in GCs were highly enhanced, compared with PG. The intensities of red and green UC emissions showed an increasing trend as the Yb3+ 319

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