Er3+: β-NaGdF4 nanocrystals: Phase-separation-controlled crystallization, optical spectroscopy and upconverted temperature sensing behavior

Journal of Alloys and Compounds 638 (2015) 21–28

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Bulk glass ceramics containing Yb3+/Er3+: b-NaGdF4 nanocrystals: Phase-separation-controlled crystallization, optical spectroscopy and upconverted temperature sensing behavior Daqin Chen a,⇑, Zhongyi Wan a, Yan Zhou a, Ping Huang b,⇑, Jiasong Zhong a, Mingye Ding a, Weidong Xiang c, Xiaojuan Liang c, Zhenguo Ji a,⇑ a

College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, PR China Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China c College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, PR China b

a r t i c l e

i n f o

Article history: Received 23 January 2015 Received in revised form 21 February 2015 Accepted 23 February 2015 Available online 14 March 2015 Keywords: Glass ceramics Upconversion NaGdF4 Optical temperature sensor

a b s t r a c t Lanthanide doped hexagonal b-NaGdF4 nanocrystals embedded transparent bulk glass ceramics were successfully fabricated via a phase-separation-controlled crystallization route. Elemental mapping in the scanning transmission electron microscope and optical spectroscopy analysis demonstrated the partition of the active centers into the b-NaGdF4 crystalline lattice. As a result, upconversion luminescence of the glass ceramic co-doped with Yb3+ and Er3+ is about 60 times as high as that of the precursor glass, attributing to the modification of Yb3+/Er3+ surrounding from phase-separated amorphous nanoparticle to b-NaGdF4 crystalline lattice with low phonon energy and high crystallinity after crystallization. Furthermore, the temperature-dependent green upconversion emissions assigned to 2H11/2 ? 4I15/2 (520 nm) and 4S3/2 ? 4I15/2 (540 nm) transitions were investigated, and the corresponding fluorescence intensity ratio of these two thermally coupled emitting-states greatly enhanced with increase of temperature. Using such fabricated glass ceramic as an optical thermometric medium, the maximum sensitivity reached as high as 0.0037 K1 at 580 K. It is expected that the investigated Er3+/Yb3+ codoped glass ceramic might be a very promising candidate for accurate optical temperature sensors. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Currently, there has been an increasing trend on searching efficient lanthanide (Ln3+) doped upconversion (UC) luminescence material. Especially, series of Ln3+-doped UC nanocrystals (NCs) have been developed because of their potential applications in biological fluorescence marker and diagnosis [1–9]. However, there are several restrictions for the applications of micro-/nano-size phosphors (powders) in other numerous optoelectronics fields, such as three-dimensional display, solid-state laser, optical amplifier, sensor, solar cell as well as photosynthesis [10–15]. For example, the scattering of the phosphors is serious. To fabricated bulk materials, UC NCs are generally dispersed in organic polymer. Unfortunately, the easy agglomeration of NCs and the instability of organic bulk materials are still a big challenge. Therefore,

⇑ Corresponding authors. Tel./fax: +86 571 87713542 (D.Q. Chen, Z.G. Ji). Tel./fax: +86 571 87713535 (P. Huang). E-mail addresses: [email protected] (D. Chen), [email protected] (P. Huang), [email protected] (Z. Ji). http://dx.doi.org/10.1016/j.jallcom.2015.02.170 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

developing transparent inorganic bulk materials with excellent UC performance and high ability to transmit light is highly desirable so far. Inorganic monocrystal, transparent ceramics and glass are the most common bulk hosts for Ln3+ doping to realize UC luminescence [16–18]. However, the grown of monocrystal is extremely complex and time-consuming. In addition, due to the existence of phase transformation, some materials may not be readily grown as bulk single-crystal [19]. As for transparent ceramics, technical challenges remain, including complicated and time-consuming procedures to avoid porosity, limited compositions, segregation of doping agents, etc. [20]. On the other hand, transparent glass is easily fabricated, but its luminescent property is usually poor because of the high phonon energies ascribed to the stretching vibration of the network-forming oxide glass matrix (such as silicate, phosphate and borate glasses), which increases the nonradiative deexcitation probability of the optically active centers. As an alternative, transparent glass ceramic (GC) composites have caused wide attention over the recent years [21–27]. GCs can be obtained with the simple preparation process as the common glass. The key

22

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 21–28

factor for the efficient luminescence is the partition of the active centers into the precipitated crystalline phase with nanometer sizes by controlling crystallization conditions. In terms of UC luminescence, precipitating fluoride NCs is a good choice because of their relatively low phonon energy, which can efficiently reduce the probability of the non-radiative relaxation. Notably, NaREF4 (RE = Gd, Y, Lu) sodium lanthanide tetrafluorides have been reported to be the ideal UC host for Ln3+ activators such as Yb3+/Er3+(Tm3+, Ho3+) [28–36]. NaLnF4 exists in two phase structures, i.e., cubic (a) and hexagonal (b), and the UC efficiency of Ln3+-doped b-NaREF4 is much higher than that of a-NaREF4 [29– 31]. As a consequence, hexagonal b-NaREF4 co-doped with Yb3+/ Er3+(Tm3+, Ho3+) has been widely studied. However, there are seldom reports concerning the synthesis and optical properties of the b-NaREF4 NCs embedded bulk GCs so far. Recently, Andreas Herrmann et al. have investigated the crystallization behavior of b-NaGdF4 NCs in glass and the related optical properties using Sm3+ as the structural probe [37,38]. Xu et al. have studied UC luminescence of Er3+-doped GCs containing b-NaGdF4 NCs under 1530 nm near-infrared laser excitation [39]. Notably, for the most of oxyfluoride GCs reported previously, the entering of activators into fluoride lattice is generally based on the Ln3+ diffusion [40– 45]. Unfortunately, such process highly relies on the heating temperature. Generally, for the low crystallization temperature, the diffusion rate of Ln3+ among glass matrix is slow, which impedes their incorporation into crystals. Whereas, for the high crystallization temperature, the growth of the crystals is very fast, which degrades the transparency of GC and further affects its optical properties. Therefore, precisely controlling crystallization to realize optimal microstructure and excellent optical performance of GC still a formidable task. Herein, we report the successful elaboration of hexagonal b-NaGdF4 NCs embedded transparent bulk GCs via the phase-separation-controlled crystallization route. X-ray diffraction, transmission electron microscopy, high-angle annual dark-field scanning transmission electron microscopy, elemental mapping as well as steady state and time-resolved emission spectra were used to investigate microstructure evolution of GCs during crystallization. Furthermore, Eu3+ was used as a structural probe to confirm the incorporation of Ln3+ activators into the precipitated b-NaGdF4 Lattice. As a result, greatly enhanced UC luminescence was realized in the Yb/Er: b-NaGdF4 NCs embedded GCs. Finally, the influence of temperature on the green UC emission bands of the thermally coupled 2H11/2 and 4S3/2 emitting-states in GC was studied to explore its possible application in the optical temperature sensors.

Cubed S-Twin transmission electron microscope operated at 200 kV. The emission, excitation spectra and decay curves of the Eu3+ doped transparent glass and glass ceramics were recorded on an Edinburgh Instruments FLS920 spectrofluoremeter equipped with both continuous (450 W) and pulsed xenon lamps. The UC emission spectra of samples were detected using the Hamamatsu R943-02 photomultiplier tube and the Spex 1000 M monochromator with an adjustable laser diode (980 nm) as the excitation source. Visible UC decay curves were measured with a customized UV to mid-infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable mid-band OPO pulse laser as excitation source (410–2400 nm, 10 Hz, pulse width 65 ns, Vibrant 355II, OPOTEK). The temperature dependent green UC emission spectra were recorded on an Edinburgh Instruments FS5 spectrofluoremeter equipped with a 980 nm laser diode and a homemade temperature controlling stage.

3. Results and discussions XRD patterns of the obtained PG and GCs are presented in Fig. 1. For PG, only diffuse humps are observed, verifying its amorphous characteristics. After crystallization treatment at 400–750 °C for 2 h, sharp diffraction peaks appears for all the GC samples. When the heating temperature lower than 700 °C, both cubic and hexagonal structure NaGdF4 crystals coexist in glass, with further increasing temperature to 750 °C, the cubic crystals completely disappear and pure hexagonal NaGdF4 crystals embedded GC is realized. Notably, it was seldom reported that fluoride crystallization occurred at such low temperature (as low as 400 °C) in the aluminosilicate glass matrix previously. TEM and HRTEM images for the PG and GCs are shown in Fig. 2. Spherical nanoparticles with sizes of 20–30 nm in PG are observed (Fig. 2a), which indicates that phase-separation occurs in PG after high-temperature melt-quenching by combined with XRD result of PG shown in Fig. 1. The corresponding SAED pattern (inset of Fig. 2b) shows a halo pattern, characteristic of an amorphous phase structure of PG, which is verified by the HRTEM micrograph of an individual non-crystalline particle (Fig. 2b). After crystallization treatment at 400 °C for 2 h, the morphology and size of the particles in GC400 (not shown here) are similar to those in PG. However, HRTEM micrograph of an individual particle in GC400 (Fig. 2c) shows the presence of many tiny crystals among one particle, indicating the phase-separated amorphous droplets in PG are converted into crystalline NaGdF4 particles after 400 °C heating. Apparently, it can be well explained that the crystallization occurs at a relatively low temperature (as low as 400 °C) at the present system since the NaGdF4 nanoparticles are in situ crystallized from these phase-separated amorphous droplets without the requirement of long-range ionic diffusion among glass matrix.

JCPDS No 27-0688: cubic NaGdF4

2. Experimental section

GC750

Intensity (a.u.)

The materials were prepared with the following composition (in mol%): 70.1SiO2–4.3Al2O3–2.3Na2O–3.0Gd2O3–1.8AlF3–18.5NaF. The Ln3+ activators were introduced by addition of LnF3 (EuF3, YbF3 and ErF3) compounds. The chemicals were mixed thoroughly and melted in a covered alumina crucible at 1590 °C for 45 min in the ambient atmosphere. The melt was poured into a 300 °C pre-heated copper mold and cooled down naturally to room temperature to form the precursor glass (PG), which was then heat-treated to 400, 450, 500, 600, 650, 700 and 750 °C with a heating rate of 10 K/min, and hold for 2 h to form glass ceramic (denoted as GC400, GC450, GC500, GC600, GC650, GC700 and GC750 respectively) through fluoride crystallization. To identify the crystallization phase, X-ray diffraction (XRD) analysis was carried out with a powder diffractometer (DMAX2500 RIGAKU) using Cu Ka radiation (k = 0.154 nm). The microstructure of GC sample was studied using a transmission electron microscopy (TEM, JEM-2010) equipped with an energy dispersive X-ray spectroscopy (EDS) and the selected area electron diffraction (SAED). TEM specimen was prepared by directly drying a drop of a dilute ethanol dispersion solution of glass pieces on the surface of a carbon coated copper grid. Scanning transmission electron microscopy (STEM) operated in the high-angle annular dark-field (HAADF) mode, which is sensitive to the atomic number (Z) of the sample (scaling proportionally to Z2) [32], is performed on an FEI aberration-corrected Titan

GC700 GC650 GC600 GC500 GC450 GC400 PG

JCPDS No 27-0699: hexagonal NaGdF4 10

20

30

40

50

60

70

80

2θ (degree) Fig. 1. XRD patterns of PG and GCs obtained by heating at various temperatures (400–750 °C) for 2 h. Bars represent standard cubic and hexagonal NaGdF4 crystal data.

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 21–28

(a)

(b)

(c)

(d)

(e)

(f)

23

Fig. 2. TEM images of samples with (a) PG and (d) GC750, insets of (a) and (d) are the corresponding SEAD patterns; (b), (c) and (e) are HRTEM micrographs of an individual particle in the PG, GC450 and GC750 samples respectively. (f) Photograph of GC750 sample.

Increasing crystallization temperature induces growth up of both cubic and hexagonal NaGdF4 NCs, and finally pure hexagonal bNaGdF4 NCs with sizes of 50–80 nm are formed homogeneously among glass matrix when the temperature further increases to 750 °C, as revealed in Fig. 2d. High-resolution TEM (HRTEM) image of GC750 (Fig. 2e) exhibits clear-cut b-NaGdF4 lattice structure with high crystallinity. The photograph of GC750 sample, presented in Fig. 2f, demonstrates its high transparency. In a further experiment, scanning transmission electron microscopy (STEM) observations operated in the high-angle annular

dark-field (HAADF) mode have been carried out to investigate the microstructures of PG and GC750 samples, as shown in Fig. 3a and b. The Na, Gd, Yb and F STEM elemental mappings of the precursor glass, presented in Fig. 3c–g, shows the localization of Na, Gd and Yb in the phase-separated amorphous droplets. After glass crystallization, similar results are observed, as demonstrated in Fig. 3h–l. F element is equally distributed among the particles and the glass matrix due to its high content. Note that for STEM–HAADF images, contrary to previous TEM images, the particles appear with a brighter contrast than the aluminosilicate

24

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 21–28

(b)

(a)

(c)

(d)

(h)

(i)

Na

Na

(e)

Gd

(f)

Yb

(g)

(j)

Gd

(k)

Yb

(l)

F

F

Fig. 3. (a and c) STEM–HAADF images of PG with associated (d) Na, (e) Gd, (f) Yb and (g) F elemental mapping; (b and h) STEM–HAAD images of GC750 with associated (i) Na, (j) Gd, (k) Yb and (l) F elemental mapping, showing the localization of Na, Gd, and Yb in the phase-separated amorphous dropets and the precipitated nanocrystals.

5

7

D0→ F2

λ ex =393 nm

PG GC750

Intensity (a.u.)

glass matrix, owing to the much larger atomic numbers of Gd/Yb (Z = 64/70) segregated in the amorphous particles or NaGdF4 NCs than those of Si/Al (Z = 14/13) distributed in the glass matrix. For the present aluminasilicate oxyfluoride glasses, [SiO4] tetrahedra was partially substituted by [AlO4] one in the glass network, and charge compensation by modifying ions (Na+ and Gd3+ herein), located in the interstices of the glass framework, is required due to valence difference between Al3+ and Si4+ [46]. F ions in the glasses are coordinated exclusively to the network modifier [47,48]. Therefore, since Si4+/Al3+ ions are tightly constrained with O2 ions in glass network, the modifier Na+/Gd3+ and F ions are highly possible to be separated from glass matrix during melt-quenching, forming Na+/Gd3+/F-enriched amorphous particles. According to the phase diagram of NaF–GdF3, cubic NaGdF4 is a meta-stable phase while hexagonal NaGdF4 is the thermodynamically stable one [49–51]. After the formation of Na+/Gd3+/F-enriched amorphous droplets by phase-separation from glass matrix, both meta-stable cubic NaGdF4 and stable hexagonal NaGdF4 NCs nucleated directly in one amorphous droplet at the crystallization temperature of as low as 400 °C. With increase of crystallization temperature from 400 °C to 700 °C, these grains grew up, and finally converted into stable hexagonal NaGdF4 modification when heating temperature increased to 750 °C. To study the surroundings of the Ln3+-doped ions in the PG and GC750 samples, optical spectroscopy of Eu3+, acting as a structural probe, is recorded under 393 nm excitation corresponding to Eu3+: 7 F0 ? 5L6 absorption transition, as shown in Fig. 4. Typical emission

5

7

5

D 3,2,1→ FJ

7

D 0→ F1 5

7

D 0→ F 4

5

7

D 0→ F 3

400

450

500

550

600

650

700

Wavelength (nm) Fig. 4. Emission spectra of the Eu3+-doped PG and GC750 samples under 393 nm excitation.

lines of Eu3+ intra-4f transitions from the excited states to the lower ones, i.e., 5D3 ? 7FJ, 5D2 ? 7FJ, 5D1 ? 7FJ and 5D0 ? 7FJ (J = 0–4) are detected. Compared to that in PG, the spectrum changes remarkably in GC750, i.e., the emission bands become structured (Stark-splitting) and narrowed, the 5D3,2,1 ? 7FJ emission intensities greatly enhance, and the emission ratio of electric dipolar 5D0 ? 7F2 transition to magnetic dipolar 5D0 ? 7F2 one

25

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 21–28

λ ex=393nm λ em=554nm

λ ex =393nm λ em =611nm

τ =2.79 ms τ =3.34 ms

PG GC750

τ =3.96 ms τ =6.12 ms

Intensity (a.u.)

Intensity (a.u.)

PG GC750

(b)

(a) 0

5

10

15

20

25

0

4

8

12

16

Time (ms)

Time (ms) λ ex =393nm λ em =507nm

λ ex=393nm λ em =463nm PG τ =0.97 ms GC750 τ =1.16 ms

Intensity (a.u.)

Intensity (a.u.)

PG τ =1.52 ms GC750 τ =2.05 ms

(c) 0

(d) 2

4

6

8

10

12

14

0

3

6

9

12

Time (ms)

Time (ms)

Fig. 5. Decay curves of the Eu3+ of (a) 5D0, (b) 5D1, (c) 5D2 and (d) 5D3 emitting-states for PG and GC700 samples under 393 nm excitation.

4

4

S3/2→ I15/2

λex=980 nm

PG GC750

(a)

PG GC750

4

4

H11/2→ I15/2

Intensity (a.u.)

F9/2→ I15/2

τ=0.85ms

τ=0.42ms

2

Intensity (a.u.)

4

λex=980 nmλem=540 nm

(b) 500

550

600

650

0

700

1

Wavelength (nm) PG

(d)

540nm n=1.985 650nm n=1.885

3

4

GC750 540nm n=1.910 650nm n=1.898

Log[Intensity (a.u.)]

Log[Intensity (a.u.)]

(c)

2

Time (ms)

2.5

2.6

2.7

Log[pomp power (mW)]

2.8

1.8

2.0

2.2

2.4

2.6

Log[pump power (mW)]

Fig. 6. (a) UC emission spectra and (b) decay curves (kem = 540 nm) of the Yb3+/Er3+-doped PG and GC750 under 980 nm near-infrared laser excitation. (c and d) Log–Log plots of UC emission intensity versus pump power for PG and GC750.

26

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 21–28

4

20

2

15

F 7/2 H S

ET3

4

4

2

10

3/2

F

9/2

3

-1

5/2

4 11/2

2 ET

Energy (×10 cm )

F

4

F 5/2

I

9/2

ET1

4

I

11/2

4

I

650nm

520nm

540nm

980nm

13/2

5

4

0

2 F 7/2

Yb

3+

I

15/2

Er3+

Fig. 7. Energy level diagrams of Er3+ and Yb3+, showing the possible UC mechanisms in the present GC sample.

4

4

2

4 H11/2→ I15/2 S3/2→ I15/2

(a) 563K 533K 503K 473K 443K 413K 383K 353K 323K 303K

480

500

1.2

λ ex=980 nm

(b)

Experimental data Fitting curve

1.0 0.8

FIR

Normlized Intensity (a.u.)

decreases from 1.80 to 1.38. All these results indicate the alternation of Eu3+ environment from amorphous particles to b-NaGdF4 NCs with high crystal-field symmetry and low phonon energy (400 cm1) [29] after crystallization treatment. To prove this point, the decay behaviors of Eu3+ 5D0, 5D1, 5D2 and 5D3 emitting states in the PG and GC750 samples under 393 nm excitation were investigated, as revealed in Fig. 5. Obviously, the decay rates of GC750 are much slower than those of PG due to the low-phonon-energy environment of Eu3+ in b-NaGdF4 NCs. Fig. 6a shows UC emission spectra of the Yb3+/Er3+ co-doped PG and GC750 samples under 980 nm near-infrared laser excitation.

Both PG and GC750 samples exhibit green (540 nm) and red (650 nm) UC emissions attributed to 2H11/2/4S3/2 ? 4I15/2 and 4 F9/2 ? 4I15/2 transitions of Er3+ ions, respectively. Evidently, the luminescence of GC750 is much stronger than that of PG, i.e., about 60 times as high as that of PG one. This is attributed to the modification of Yb/Er surrounding from phase-separated amorphous nanoparticle to b-NaGdF4 crystalline lattice with low phonon energy after crystallization, which greatly decease Er3+ nonradiative relaxation probability. For UC materials, a long lifetime usually means a highly-efficient UC luminescence, i.e., a low non-radiative deactivation probability of Ln3+ activators [52]. As exhibited in Fig. 6b, taking Eu3+: 2H11/2/4S3/2 emitting states as an example, the decay lifetime of GC750 is obviously longer than that of PG, which agrees with the result of UC emission spectra. As shown in Fig. 6c and d, the dependence of green or red UC emission intensity on the pump power is nearly quadratic for both PG and GC750 samples, indicating that two-photon UC process is responsible for the population of the Er3+ 2H11/2/4S3/2 and 4F9/2 states. Fig. 7 shows the energy level diagrams of Yb3+ and Er3+ as well as possible UC mechanisms. Under the 980 nm laser excitation, Yb3+ ions are promoted to the 2F5/2 state though ground-state absorption. Afterwards, electrons in the Er3+: 4I15/2 ground state are excited to higher 4F7/2 one though successive energy transfers (ET1 and ET2) from Yb3+ to Er3+, from which the green-emitting 2 H11/2/4S3/2 and red-emitting 4F9/2 states are populated by nonradiative multi-phonon relaxations. In addition, the population of 4 F9/2 level can also be achieved via the following energy transfer process (ET3): 4I13/2(Er3+) + 2F5/2(Yb3+) ? 4F9/2(Er3+) + 2F7/2(Yb3+), where the Er3+: 4I13/2 state can be populated through non-radiative transition from the 4I11/2 one. Finally, to explore the possible application of the present investigated GC in optical thermometry, UC emission spectra of the Yb3+/Er3+ co-doped b-NaGdF4 embedded GC under 980 nm excitation ranging from 500 to 580 nm are recorded at different

0.6 FIR=7.71exp(-1135/T)

0.4 0.2 520

540

560

300

580

350

400

Wavelength (nm) 0.3

36

-4

Ln (FIR)

-0.3

-1

Slope=-1135±27.197 Intercept=2.043 ± 0.067

Sensitivity (10 K )

(c)

0.0

-0.6 -0.9 -1.2 Experimental data Fitting line

-1.5

450

500

550

600

T (K)

(d)

33 30 2

S=d(FIR)/dT=(FIR) ×exp(-E/T )

27 24 21

-1.8 18

21

24

27 -4

-1

1/T (10 K )

30

33

300

350

400

450

500

550

600

650

T (K)

Fig. 8. (a) Normalized UC emission spectra of the Yb3+/Er3+ co-doped GC750 sample in the wavelength range of 500–580 nm at different temperatures (303–563 K). Inset shows the UC luminescent photograph of GC750. (b) Dependence of FIR on absolute temperature, (c) monolog plot of FIR as a function of inverse absolute temperature, and (d) dependence of sensor sensitivity on absolute temperature.

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 21–28 Table 1 Optical thermometry parameters in some typical Yb3+/Er3+ co-doped UC materials. UC hosts

DE (cm1)

C

Tmax (K)

Maximal sensitivity (K1)

Refs.

Silicate glass Fluorotellurite glass NaYF4 NCs a-NaYF4 GC b-NaYF4 GC750

895 770 752 775 789

7.92 11.2 8.06 4.89 7.71

550 550 535 560 580

0.0031 0.0055 0.0040 0.0024 0.0037

[55] [56] [57] [13] This work

temperatures from 303 to 563 K, as depicted in Fig. 8a. It can clearly observed that these spectra exhibit two distinct emission bands around 520 nm and 540 nm assigned to the 2H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2 transitions of Er3+ ion, respectively. The fluorescent intensity ratio (FIR) of these two UC emissions shows a remarkable dependence on the temperature (Fig. 8b), owing to the thermal coupling between 2H11/2 and 4S3/2 states of Er3+. Based on Boltzmann distribution theory, FIR of two thermally coupled states can be expressed as the following equation [53,54]:

    I520 g H AH rH wH  DE  DE ¼ C exp FIR ¼ ¼ exp kB T kB T I540 g S AS rS wS

ð1Þ

where I520 and I540 are the integrated UC intensities corresponding to the 2H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2 transitions, respectively, g, A, r and w are the degeneracy, the spontaneous radiative transition rate, the emission cross-section and the angular frequency of fluorescent transitions from the 2H11/2 or 4S3/2 excited state to the 4 I15/2 ground state of Er3+, respectively, C is the constant, DE is the energy gap between 2H11/2 and 4S3/2 states, kB is the Boltzmann constant, and T is the absolute temperature. According to the expression of the FIR, the value of Ln(I520/I540) versus the inverse absolute temperature (1/T) is plotted in Fig. 8c. The slope and intercept are fitted to be 1135 ± 27.197 and 2.043 ± 0.067, respectively. As a consequence, the energy gap DE and the pre-exponential constant are evaluated to be about 789 cm1 and 7.71, respectively. These two parameters are vital factors for the sensor sensitivity (S) of temperature detection, as defined by the following equation [53,54]:

S¼

      dðFIRÞ DE DE  DE ¼ C exp : ¼ FIR dT kB T kB T 2 kB T 2

ð2Þ

The calculated curve of sensor sensitivity as a function of absolute temperature is plotted in Fig. 8d. It can be seen that the sensitivity keeps increasing in our experimental temperature range, and the maximal value of about 0.0037 K1 is realized at the temperature of 580 K. Impressively, the repeatability of temperature sensor is quite good after several cycle experiments. As a comparison, the optical thermometry parameters in some related Er3+/Yb3+ co-doped UC materials [13,55–57] are tabulated in Table 1. Compared to silicate glass, the present glass ceramic exhibits greatly improved UC performance owing to the incorporation of lanthanide activators (Yb3+/Er3+) into low-phonon-energy b-NaYF4 crystalline lattice. On the other hand, due to the protecting role of the aluminosilicate glass matrix, the b-NaYF4 NCs embedded GC shows better stability than pure NaYF4 micro-/nanocrystals. Evidently, it is expected that such transparent bulk Yb3+/Er3+: b-NaGdF4 embedded GC fabricated via phase-separation-controlled crystallization is a very promising UC material for the application in optical temperature sensors. 4. Conclusions In summary, transparent bulk glass ceramics containing Ln3+: b-NaGdF4 nanocrystals were successfully prepared. Different to

27

the cases previously reported, phase-separation occurred in the precursor glass after high-temperature melt-quenching. The segregation of Na, Gd, F as well as Ln3+ dopants in the phase-separation amorphous particle was evidenced by elemental mapping and optical spectroscopy analysis. After crystallization, these amorphous particles in situ transformed into both cubic and hexagoanl NaGdF4 nanocrystals at a low temperature (400 °C) and finally converted into pure hexagonal NaGdF4 phase at 750 °C. Owing to the low-phonon-energy and high crystallinity of b-NaGdF4 nanocrystals where Yb3+ and Er3+ ions resided, the upconversion luminescent intensity of glass ceramic is about 60 times stronger than that of precursor glass. The green UC emission bands observed around 520 nm and 540 nm assigned to the 2 H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2 transitions of Er3+ ion, respectively, have been explored for possible application in optical temperature sensors by the fluorescence intensity ratio technique. The results suggest the present transparent bulk glass ceramic is a novel candidate as the optical thermometric medium with a sensor sensitivity of about 0.0037 K1. Acknowledgement This work was supported by the National Natural Science Foundation of China (21271170, 61372025 and 51372172). References [1] F. Wang, X.G. Liu, Chem. Soc. Rev. 38 (2009) 976. [2] F. Wang, Y. Han, C. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, X.G. Liu, Nature 463 (2010) 1061. [3] M. Haase, H. Schäfer, Angew. Chem. Int. Ed. 50 (2011) 5808. [4] J. Zhou, Z. Liu, F.Y. Li, J. Zhou, Z. Liu, F.Y. Li, Chem. Soc. Rev. 41 (2012) 1323. [5] Y.S. Liu, D.T. Tu, H.M. Zhu, X.Y. Chen, Chem. Soc. Rev. 42 (2013) 6924. [6] S. Gai, C. Li, P. Yang, J. Lin, Chem. Rev. 114 (2014) 2343. [7] G. Chen, H. Qiu, P.N. Prasad, X. Chen, Chem. Rev. 114 (2014) 5161. [8] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Appl. Phys. Lett. 83 (2003) 284. [9] M.A.R.C. Alencar, G.S. Maciel, C.B. de Araújo, A. Patra, Appl. Phys. Lett. 84 (2004) 4753. [10] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185. [11] E. Heumann, S. Bär, K. Rademaker, G. Huber, S. Butterworth, A. Diening, W. Seelert, Appl. Phys. Lett. 88 (2006) 061108. [12] F. Auzel, Chem. Rev. 104 (2004) 139. [13] S. Jiang, P. Zeng, L.Q. Liao, S.F. Tian, H. Guo, Y.H. Chen, C.K. Duan, M. Yin, J. Alloys Comp. 617 (2014) 538. [14] D.Q. Chen, Y.S. Wang, M.C. Hong, Nano Energy 1 (2012) 73. [15] C. Ming, F. Song, L. An, X. Ren, Appl. Phys. Lett. 102 (2013) 141903. [16] J.J. Ju, T.Y. Kwon, S.I. Yun, M. Cha, H.J. Seo, Appl. Phys. Lett. 69 (1996) 1358. [17] L. Fu, H. Xia, Y. Dong, S. Li, X. Gu, J. Zhang, D. Wang, H. Jiang, B. Chen, J. Alloys Comp. 617 (2014) 584. [18] J.S. Kumar, K. Pavani, M.P.F. Graca, M.J. Soares, J. Alloys Comp. 617 (2014) 108. [19] R.H. Page, K.I. Schaffers, P.A. Waide, J.B. Tassano, J. Opt. Soc. Am. B 15 (1998) 996. [20] G.L. Messing, A.J. Stevenson, Science 322 (2008) 383. [21] D.Q. Chen, W.D. Xiang, X.J. Liang, J.S. Zhong, H. Yu, M.Y. Ding, H.W. Lu, Z.G. Ji, J. Eur. Ceram. Soc. 35 (2015) 859. [22] D.Q. Chen, Z.Y. Wan, Y. Zhou, Y. Chen, H. Yu, H.W. Lu, Z.G. Ji, P. Huang, J. Alloys Comp. 625 (2015) 149. [23] Y.L. Wei, H.M. Yang, X.M. Li, L.J. Wang, H. Guo, J. Am. Ceram. Soc. 97 (2014) 2012. [24] Y.L. Wei, X.Y. Liu, X.N. Chi, R.F. Wei, H. Guo, J. Alloys Comp. 578 (2013) 385. [25] S. Chenu, E. Véron, C. Genevois, G. matzen, T. Cardinal, A. Etienne, D. Massiot, M. Allix, Adv. Opt. Mater. 2 (2014) 364. [26] R. Zhang, H. Lin, Y.L. Yu, D.Q. Chen, J. Xu, Y.S. Wang, Laser Photonics Rev. 8 (2014) 158. [27] S. Zhou, N. Jiang, B. Wu, J. Hao, J. Qiu, Adv. Funct. Mater. 19 (2009) 2081. [28] X.Y. Huang, J. Alloys Comp. 628 (2015) 240. [29] A. Aebischer, M. Hostettler, J. Hauser, K. Krämer, T. Weber, H.U. Güdel, H.B. Bürgi, Angew. Chem. Int. Ed. 45 (2006) 2802. [30] K. Krämer, D. Biner, G. Frei, H.U. Güdel, M.P. Hehlen, S.R. Lüthi, Chem. Mater. 16 (2004) 1244. [31] P. Ptacek, H. Schäfer, K. Kömpe, M. Haase, Adv. Funct. Mater. 17 (2007) 3843. [32] D.Q. Chen, P. Huang, Dalton Trans. 43 (2014) 11299. [33] Q. Liu, Y. Sun, T. Yang, W. Feng, C. Li, F. Li, J. Am. Chem. Soc. 133 (2011) 17122. [34] P. Ghosh, A. Patra, J. Phys. Chem. C 112 (2008) 3223. [35] P. Ghosh, A. Patra, J. Phys. Chem. C 112 (2008) 19283. [36] F. Liu, E. Ma, D.Q. Chen, Y.L. Yu, Y.S. Wang, J. Phys. Chem. B 110 (2006) 20843. [37] A. Herrmann, M. Tylkowski, C. Bocker, C. Rüssel, Chem. Mater. 25 (2013) 2878. [38] A. Herrmann, M. Tylkowski, C. Bocker, C. Rüssel, J. Mater. Sci. 48 (2013) 6262.

28

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 21–28

[39] S.L. Zhao, X.L. Wang, X. Sun, G.H. Jia, L.H. Huang, D.G. Deng, F.X. Xin, S.Q. Xu, CrystEngComm 15 (2013) 7346. [40] K. Hirao, K. Tanaka, M. Makita, N. Soga, J. Appl. Phys. 78 (1995) 3445. [41] S. Ye, B. Zhu, J. Chen, J. Luo, J.R. Qiu, Appl. Phys. Lett. 92 (2008) 141112. [42] D.Q. Chen, Y.S. Wang, Y.L. Yu, F. Liu, P. Huang, Appl. Phys. Lett. 91 (2007) 051920. [43] D.Q. Chen, Y.S. Wang, K.L. Zheng, T.L. Guo, Y.L. Yu, P. Huang, Appl. Phys. Lett. 91 (2007) 251903. [44] D.Q. Chen, Y.L. Yu, P. Huang, F.Y. Weng, H. Lin, Y.S. Wang, Appl. Phys. Lett. 94 (2009) 041909. [45] A.C. Yanes, A. Santana-Alonso, J. Méndez-Ramons, J. del-Castillo, V.D. Rodríguez, Adv. Funct. Mater. 21 (2011) 3136. [46] P.E. McMillan, Glass-Ceramics, Academic Press, London, 1979. [47] T.J. Kiczenski, L. Du, J.F. Stebbins, J. Non-Cryst. Solids 337 (2004) 142.

[48] [49] [50] [51] [52] [53] [54] [55] [56] [57]

J. Cheng, W. Deng, M. Wang, Physica B 407 (2012) 2778. E. Thoma, H. Insley, G.M. Hebert, Inorg. Chem. 5 (1966) 1222. D. Tu, Y. Liu, H. Zhu, R. Li, L. Liu, X. Chen, Angew. Chem. Int. Ed. 52 (2013) 1128. D.Q. Chen, P. Huang, Y.L. Yu, F. Huang, A.P. Yang, Y.S. Wang, Chem. Commun. 47 (2011) 5801. D.Q. Chen, Y.L. Yu, F. Huang, P. Huang, A.P. Yang, Z.X. Wang, Y.S. Wang, Chem. Commun. 47 (2011) 11083. K.Z. Zheng, Z.Y. Liu, C.J. Lv, W.P. Qin, J. Mater. Chem. C 1 (2013) 5502. B. Dong, B.S. Cao, Y.Y. He, Z. Liu, Z.P. Li, Z.Q. Feng, Adv. Mater. 24 (2012) 1987. Q. Zhou, C.R. Li, Z.F. Liu, S.F. Li, C.L. Song, Opt. Mater. 30 (2007) 513. G.S. Maciel, L.de S. Menezes, A.S.L. Gomes, C.B. de Araújo, Y. Messaddeq, A. Florez, M.A. Aegerter, IEEE Photon. Technol. Lett. 7 (1995) 1474. S.H. Zhou, K.M. Deng, X.T. Wei, G.C. Jiang, C.K. Duan, Y.H. Chen, M. Yin, Opt. Commun. 291 (2013) 138.