Structure, luminescence and temperature sensing in rare earth doped glass ceramics containing NaY(WO4)2 nanocrystals

Novel rare earth doped glass ceramics containing NaY(WO4)2 nanocrystals were fabricated for the first time. The appearance of sharp diffraction peaks and well-resolved lattice fringes certifies the precipitation of NaY(WO4)2 nanocrystals with high crystallinity. After the crystallization process, significant changes in the photoluminescence emission spectra and fluorescence lifetime of Sm3+ ions are observed, which are ascribable to the enrichment of Sm3+ ions in the highly disordered NaY(WO4)2 nanocrystals. Under 980 nm excitation, characteristic green and red upconversion emission signals were detected and the enhanced upconversion luminescence of Er3+ ions in the glass ceramics was attributable to the incorporation into the low energy phonon NaY(WO4)2 nanocrystals. Based on the dependence of upconversion intensity on the excitation power, the upconversion mechanism of Er3+–Yb3+ ions was proposed. The temperature-dependent fluorescence intensity ratio (FIR) of the thermally-coupled 2H11/2 and 4S3/2 energy levels was determined at a low power density of 0.4125 W cm−2. The maximum temperature sensitivity is 146 × 10−4 K−1 at 523 K, which is mainly attributed to the highly disordered structure of NaY(WO4)2 nanocrystals and exhibits promising potential for optical temperature sensors.


Introduction
In recent years, upconversion luminescence of active ions has attracted wide interest owing to its promising applications in bioimaging, solar cells, white LEDs and temperature sensing. [1][2][3][4] Especially, temperature sensors based on the FIR technique have garnered great attention due to their negligible electromagnetic interference, simple data processing and high sensitivity. [4][5][6][7] Currently, the research of high-performance temperature sensors mainly focuses on the optimization of active ions and host matrix. Er 3+ ion is the most extensively investigated because of its intense upconversion luminescence and appropriate bandgap (around 800 cm À1 ) between 2 H 11/2 and 4 S 3/2 energy levels, which exactly fall in the thermallycoupled energy range (200-2000 cm À1 ). 8 In order to obtain stronger upconversion luminescence, Yb 3+ is usually utilized as the preferred sensitizer for Er 3+ due to its high absorption crosssection at 980 nm and efficient Yb 3+ / Er 3+ energy transfer efficiency. 9 It is well known that the phonon energy and crystal structure of host matrix have an important role on the upconversion luminescence efficiency. A low phonon energy is very benecial to enhance the upconversion efficiency of activators as a result of the reduction in the multi-phonon relaxation rate. 10 Hexagonal NaYF 4 is regarded as the most efficient upconversion host matrix due to its low phonon energy and multisite characteristic. 11 The temperature sensing performance of Er 3+ ions in hexagonal NaYF 4 was studied and the maximum temperature sensitivity was 37 Â 10 À4 K À1 at 508 K. 12 By now, various host materials have been investigated for optical thermometry, however, the inuence of crystal structure on the temperature sensitivity is rarely investigated. 13 Recently, three Judd-Ofelt parameters 14,15 of Er 3+ ions were calculated and used to analyze the role of host matrix from oxyuoride glass to cubic NaYF 4 glass ceramic on the temperature sensitivity. It was found that the maximum temperature sensitivity of Er 3+ ions in the precursor glass (66 Â 10 À4 K À1 at 570 K) is twice more than the value in the cubic NaYF 4 glass ceramic (24 Â 10 À4 K À1 at 540 K). Based on the structural change during the temperature-induced crystallization process, it is concluded that the more disordered the local environment of active ions, the better FIR and temperature sensing sensitivity. Thus, oxide glass is regarded as an ideal host matrix doped with Er 3+ ions for optical thermometry due to its highly disordered local symmetry surroundings. 16 Unfortunately, a low upconversion efficiency of active ions can only be obtained in oxide glasses due to their high phonon energy.
Rare earth doped double tungstate/molybdate phosphors have been extensively investigated due to their structural versatility, good thermal stability and low phonon energy. Recently, intense green upconversion luminescence was observed in the Er 3+ /Yb 3+ codoped NaY(WO 4 ) 2 phosphors and its temperature sensing behaviour was analysed by the FIR technique. 17 A maximum temperature sensing sensitivity about 112 Â 10 À4 K À1 was obtained at 515 K, which was about three times larger than that in the NaYF 4 nanocrystals 12 and exhibited promising potential for application on optical thermometry. Moreover, in order to assemble a practical optical ber temperature sensing probe, the phosphor is usually adhered to the end face of optical ber by organic resin (epoxy resin or silicone). Therefore, the operating temperature range, longterm reliability and lifetime of optical ber temperature sensor are reduced dramatically. Glass can be easily drawn into glass ber and conveniently spliced to standard silica ber. The combination of oxide glass and tungstate/molybdate phosphor is expected to improve the performance of temperature sensors. Therefore, the key problem is to develop an appropriate glass composition, in which tungstate/molybdate nanocrystals could be successfully separated from the precursor glass and intense upconversion emission could be achieved.
Here, transparent glass ceramics containing NaY(WO 4 ) 2 nanocrystals have been successfully prepared by the optimization of glass composition and thermal treatment condition. Sm 3+ ion usually acts as a spectroscopic probe to distinguish the local site symmetry around active ions in the host matrix. The spectral analysis of Sm 3+ ions in the precursor glass and glass ceramics containing NaY(WO 4 ) 2 nanocrystals has been carried out and used to estimate their potential performance for optical temperature sensing. Successively, the temperature dependent upconversion emission spectra in Er 3+ /Yb 3+ codoped glass and glass ceramics were systematically investigated. According to FIR technique, the inuence of host matrix on the temperature sensitivity was discussed in details.

Sample preparation
The glass samples were fabricated with molar composition of Firstly, high purity raw materials SiO 2 , H 3 BO 3 , Na 2 CO 3 , ZnO, WO 3 , Y 2 O 3 , Sm 2 O 3 , Er 2 O 3 and Yb 2 O 3 were weighed accurately and mixed thoroughly. Then, the well-mixed raw materials were put into a covered Al 2 O 3 crucible and were transferred to an electric furnace preheated to 1500 C for about 30 min. Finally, the glass melt was quenched into a cold stainless-steel plate and rapidly transferred to a pre-heated muffle furnace. Transparent precursor glasses (PG) were obtained. PG samples were cut into small pieces and heat-treated at 610 C, 630 C and 650 C for 2 h, respectively. The obtained glass ceramics were named as GC-610, GC-630 and GC-650, respectively. All the samples were polished for further characterization. The nal thickness of PG and glass ceramics is 1.5 mm.

Characterization
The crystallized phase before and aer thermal treatment was identied via X-ray diffraction (XRD) measurement (Bruker D2 PHASER Diffractometer), with Cu-K a radiation in 2q range from 10 to 80 with a step length 0.02 . The microstructure of GC-650 was examined on a transmission electron microscope (TEM, Philips-FEI-Tecnai G2 F30). Excitation and emission spectra of Sm 3+ ions at room temperature were recorded on a uorescence spectrophotometer equipped with a Hamamatsu R928 photomultiplier tube (Horiba J-Y Fluorolog-3). 450 W xenon lamp was used as the light source. The luminescence decay curves of 4 G 5/2 levels of Sm 3+ ions were recorded on the above apparatus excited at 370 nm. Upconversion luminescence Er 3+ /Yb 3+ co-doped samples were also performed on the same spectrophotometer and a 980 nm laser diode module (MDL-III-980, China) was used to excite the samples. The output spot size of 980 nm laser was 4 Â 6 mm 2 . In order to study the temperature-dependent upconversion luminescence of Er 3+ , the temperature controller (TAP-02) was used to heat the sample from 298 to 573 K and its temperature accuracy is 0.1 K.

Microstructure analysis
The XRD proles of PG and glass ceramics are presented in Fig. 1(a). In the PG, no obvious diffraction peaks are detected in the range of 10-80 , indicative of the amorphous feature of glass matrix. When the samples were thermal-treated at 610, 630 and 650 C for 2 h, intense diffraction peaks are observed and easily assigned to pure tetragonal NaY(WO 4 ) 2 (JCPDS No 48-0886). The full width at half-maximum (FWHM) of diffraction peaks becomes narrower with the increase of thermal treatment temperature, indicating the gradual growth of NaY(WO 4 ) 2 phase. The average crystalline size of NaY(WO 4 ) 2 nanocrystals could be calculated based on the Scherrer formula, 18 in which l is X-ray wavelength, m is FWHM of diffraction peaks, and q is diffraction angle. The average crystalline sizes were 15.0 nm, 17.8 nm and 20.9 nm for GC-610, GC-630 and GC-650, respectively. The Rietveld XRD renement of GC-650 was carried out by using TOPAS soware. The initial structural models was NaY 0.95 Yb 0.05 (WO 4 ) 2 . 19 Fig. 1(b) show the observed and calculated diffraction peaks as well as the difference of GC-650, which are nearly identical and suggests that GC-650 has a pure tetragonal phase. The tting reliability parameters of R exp , R wp , R p , and GOF are 3.94, 5.90, 4.82 and 1.5, respectively. The inset of Fig. 1(b) represents the crystal structure of NaY(WO 4 ) 2 drawn by Diamond soware. The crystal has a high structural disorder. Especially, Na and Y occupy the same sites in the structure randomly and link with eight oxygen atoms, forming NaO 8 and YO 8 edge-sharing polyhedral with S 4 site symmetry, while W 6+ ions are coordinated with four oxygen atoms. 20 Fig. 2(a) gives the dark eld TEM image of NaY(WO 4 ) 2 nanocrystals in the GC-650. One can observe plenty of dark and nearly spherical nanoparticles, corresponding to the precipitated NaY(WO 4 ) 2 nanocrystals, while the grey background is amorphous glass phase. The HRTEM image of an individual NaY(WO 4 ) 2 nanocrystal shows the well-resolved lattice fringes, which suggests that the precipitated NaY(WO 4 ) 2 nanocrystal possess high crystallinity. The measured interplanar distance is 0.309 nm, which can be well-indexed as the d-spacing value of (112) crystal plane of NaY(WO 4 ) 2 crystal (d (112) ¼ 0.308 nm). The crystalline size of NaY(WO 4 ) 2 is around 25 nm with a narrow size distribution, which is slightly larger than the value (20.9 nm) calculated from XRD data. The same phenomenon was reported in the PbF 2 glass ceramics, which was due to the broadening of diffraction peaks and suggested the presence of disorder of the precipitated nanocrystals. 21 Photoluminescence and decay lifetimes of Sm 3+ ions Fig. 3 shows the photoluminescence spectra of Sm 3+ ions in the glass samples before and aer thermal treatment. The excitation wavelength is 404 nm. Some strong emission bands at around 566, 602, 648 and 706 nm are observed, which correspond to the transitions from 4 G 5/2 energy level to 6 H 5/2 , 6 H 7/2 , 6 H 9/2 and 6 H 11/2 energy levels of Sm 3+ ions, respectively. 22 The whole emission intensity of Sm 3+ ions aer thermal treatment is greatly enhanced with the increase of thermal treatment temperature and obvious Stark splitting appear at 602 nm, which is attributed to the segregation of Sm 3+ ions into the low phonon energy NaY(WO 4 ) 2 nanocrystals by occupying the sites of Y 3+ ions. 23 It is notable that the strongest photoluminescence  emission band in the PG originates from 4 G 5/2 / 6 H 7/2 transition at 602 nm, while the strongest photoluminescence emission band in the GC-650 becomes 4 G 5/2 / 6 H 9/2 transition at 648 nm, which is completely on the contrary to the luminescent behaviour of Sm 3+ ions in YF 3 glass ceramics. 24 Sm 3+ ions are very sensitive to the subtle change of external environment and acts as a spectroscopic probe to investigate the local site symmetry around active ions in the host matrix. According to the selection rules of electronic transitions, 25 4 G 5/2 / 6 H 5/2 transition belongs to magnetic dipole transition due to DJ ¼ 0, which is nearly independent of the local environmental change around Sm 3+ ions, while 4 G 5/2 / 6 H 9/2 transition is electric transition because of DJ ¼ 2 and which is highly sensitive to external environmental change. In general, the intensity ratio b between 4 G 5/2 / 6 H 9/2 and 4 G 5/2 / 6 H 5/2 transitions is used to evaluate the local site symmetry around Sm 3+ ions. When Sm 3+ ions are situated in the surroundings close to inversion symmetry, 4 G 5/2 / 6 H 5/2 will be stronger, while in the surroundings far away from inversion symmetry, the 4 G 5/2 / 6 H 9/2 will be predominant. The calculated b values in the PG, GC-610, GC-630 and GC-650 are 1.25, 1.76, 1.98 and 2.18, respectively. The gradual increase of b value demonstrates that Sm 3+ ions are far away from inversion symmetry and enter into a more disordered surrounding aer thermal treatment process, which favours the strengthening of photoluminescence intensity for the hypersensitive transitions and is very benecial to enhance the temperature sensing sensitivity. 26,27 Due to the relative photoluminescence intensity of Sm 3+ ions change greatly aer thermal treatment, the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates were calculated in order to evaluate the colorimetric performance calculated from the emission spectra. The calculated chromaticity coordinates were A (0.542, 0.448), B (0.573, 0.421), C (0.576, 0.418) and D (0.587, 0.409) for the PG, GC-610, GC-630 and GC-650, respectively, which were labelled in the Fig. 3(b). It is found that the colour changes from yellow orange to reddish orange aer thermal treatment, which indicates that glass ceramic doped Sm 3+ ions could be served as alternative red phosphors in the white LED application.
The luminescence decay curves of Sm 3+ ions in different samples are investigated and presented in the Fig. 4. The excitation wavelength is 370 m and the emission wavelength is 602 nm. All the decay curves are well described with a twoexponential equation, 28 in which I is the emission intensity, A 1 and A 2 are the tting parameters, s 1 and s 2 are the slow and rapid lifetimes for exponential components, respectively. The average uorescence lifetime can be calculated as: The tting average uorescence lifetime of Sm 3+ ions are 2.00, 2.13, 2.32 and 2.64 ms for PG, GC-610, GC-630 and GC-650, respectively. The gradual increase of uorescence lifetime of Sm 3+ ions in the glass ceramics can be ascribed to that much more Sm 3+ ions partition into the low phonon energy NaY(WO 4 ) 2 nanocrystals and the non-radiative transition rate of Sm 3+ ions reduces.
Upconversion luminescence and mechanism of Er 3+ ions Fig. 5(a) shows the upconversion emission spectra of Er 3+ /Yb 3+ ions in different samples at room temperature. The excitation wavelength is 980 nm and its excitation power is 99 mW. Thus, the power density is 0.4125 W cm À2 , which is enough low and the heating effect caused by 980 nm excitation may be ignored. 29 Strong green upconversion emission as well as weak red upconversion emission are recorded, which are originated from the 2 H 11/2 / 4 I 15/2 (533 nm), 4 S 3/2 / 4 I 15/2 (555 nm) and 4 F 9/2 / 4 I 15/2 (658 nm) transitions of Er 3+ ions, respectively. With the increase of thermal treatment temperature, the upconversion intensity is signicantly raised and its intensity in GC-650 is 60 times higher than that of PG. The red-to-green emission ratios are 0.263, 0.242, 0.138 and 0.117 for the PG and GC-610, GC-630 and GC-650, respectively. The decrease of red-to-green emission ratio is mainly attributed to the gradual incorporation of Er 3+ and Yb 3+ into the low-phonon-energy NaY(WO 4 ) 2 nanocrystals, which effectively reduce the populations of 4 F 9/2 energy level through the non-radiative 4 S 3/2 / 4 F 9/2 transition. Furthermore, intense Stark splitting of Er 3+ ions due to high crystal eld effect also verify the above viewpoint. In order to analyze upconversion mechanism of Er 3+ ions, the double logarithmic curves of upconversion intensity as a function of excitation power is determined and shown in the inset of Fig. 5(a). For an unsaturated process, the correlation of upconversion intensity I and excitation power P satisfy the following formula, 30 I f P n , in which n is the number of the pump photon absorbed in the upconversion process for one visible photon emitted. Thus, the n values are equal to the slopes in the inset of Fig. 5(a) and are 1.95, 1.89 and 1.83 for 533 nm, 555 nm, and 658 nm, respectively. These values demonstrate that a double-photon absorption process is responsible for the populations on the 2 H 11/2 , 4 S 3/2 and 4 F 9/2 energy levels of Er 3+ ions. Based on the above data, the upconversion mechanism of Er 3+ , Yb 3+ ions was proposed and plotted in the Fig. 5(b). Due to its wide absorption cross-section at 980 nm of Yb 3+ ions, 980 nm light is mainly absorbed by Yb 3+ and the latter efficiently transfers the energy to the adjacent Er 3+ ions. 21 Subsequently, Er 3+ ions are excited to 4 F 7/2 energy level through absorbing double 980 nm photons from Yb 3+ ions. Then, the 2 H 11/2 , 4 S 3/2 and 4 F 9/2 energy levels are successively populated by the rapid non-radiative relaxation from 4 F 7/2 energy level, from which green and red upconversion emissions appear. Moreover, the population of 4 F 9/2 energy level may be also from the energy transfer process of 4 I 13/2 energy level.

Temperature dependence upconversion luminescence of Er 3+ ions and temperature sensing performance
In order to investigate the potential of Er 3+ /Yb 3+ codoped glass ceramics containing NaY(WO 4 ) 2 nanocrystals for optical thermometry based on FIR technique, the dependence of green upconversion luminescence in GC-650 on the temperature was measured at the temperature range from 298 to 573 K and the change of the integrated upconversion intensity of 2 H 11/2 (I H ), 4 S 3/2 (I S ) as well as 2 H 11/2 + 4 S 3/2 (I H + I S ) of Er 3+ ions versus temperature is shown in the Fig. 6(a). It is notable that the integrated upconversion intensity I H of 2 H 11/2 / 4 I 15/2 transition rst increase and then decrease slowly with the enhancement of environmental temperature. The maximum value of I H occurs at 423 K. On the contrary, the integrated upconversion intensity I S of 4 S 3/2 / 4 I 15/2 transition decreases dramatically, which results in the FIR of two energy levels (I H /I S ) rise from 0.81 to 4.49. In general, when the temperature increases from 298 to 573 K, the upconversion emission become weak due to thermal quenching effect. In our experiment, the whole green upconversion intensity (I H + I S ) reduces by 1/2 and intense green emission can be observed at 573 K. This is very benecial to improve the accuracy of optical data. In order to better reveal the relative change of FIR, the green upconversion emissions were normalized to the intensity at 555 nm and shown in the Fig. 6(b). Clearly, the upconversion emission wavelength changes little, however, the FIR change signicantly. Owing to the small bandgap between 2 H 11/2 and 4 S 3/2 , they are thermally coupled and 2 H 11/2 energy level may be easily populated via thermal excitation from 4 S 3/2 energy level and achieve thermal equilibrium, which follows the Boltzmann distribution law. The FIR of upconversion emission is proportional to the ratio of the populations corresponding energy levels and is expressed as: in which I H and I S are the integrated upconversion intensities for the 2 H 11/2 / 4 I 15/2 and 4 S 3/2 / 4 I 15/2 transitions, respectively. The integrated ranges for both transitions are 515-540 nm and 540-570 nm, respectively. N( 2 H 11/2 ) and N( 4 S 3/2 ) are the population numbers of 2 H 11/2 and 4 S 3/2 energy levels, respectively. DE is the bandgap between the 2 H 11/2 and 4 S 3/2 . k is the Boltzmann constant and T is the absolute temperature. The correlation between ln(FIR) and the inverse absolute temperature 1/T is described in Fig. 6(c). The linear tting of the experimental values ln(FIR) as a function of the 1/T gives a slope about 1073.76. From this slope, the value of DE is calculated to be approximately 746.2 cm À1 , which is very close to the energy gap between the 2 H 11/2 and 4 S 3/2 levels calculated from the upconversion emission spectra.
The absolute sensitivity (S a ) and relative sensitivity (S r ) are usually used to evaluate the temperature sensing performance and dened as the rate at which the absolute and relative FIR change with the temperature, which could be calculated using the following formula: The corresponding sensitivity curves are demonstrated in Fig. 6(d). Table 1 lists the FIR parameters, relative sensitivity S r , the maximum sensitivity S a and the temperatures for the maximum sensitivity in different host matrix. Because S r depends only on the energy gap DE, the S r decreases gradually with the increase of temperature. Furthermore, due to its moderate energy gap, S r achieved here is comparable to most of values of Er 3+ ions in other host matrix. The absolute temperature sensitivity S a increases rapidly with the increase of temperature and achieves the maximum value 146 Â 10 À4 K À1 at 523 K. Clearly, the temperature sensitivity S a in our work is much larger than the reported data in uorides, glass and glass ceramics and slightly larger than the value in tungstates and molybdates. According to Judd-Ofelt theory, S. F. León-Luis proposed that the host matrices with highly distorted local symmetry environments for Er 3+ ions could obtain large uorescence intensity ratios and high temperature sensitivities. 16 Generally, rare earth ions are situated in a symmetric environment in the uoride, while rare earth ions are located in the asymmetric surroundings in the double tungstates/molybdates due to its highly disordered structure. 41 The above spectral analysis of Sm 3+ ions in the NaY(WO 4 ) 2 glass ceramic also conrmed this point. Thus, our experiment provides a new evidence for the viewpoint that the disordered local surroundings around active ions are responsible for the enhancement of the optical sensitivity.

Conclusions
An optimized glass composition and an appropriate thermal treatment condition leads to the preparation of a novel glass ceramics, composed of NaY(WO 4 ) 2 nanocrystals dispersed  homogeously in the amorphous glass matrix. The microstructure of these nanocrystals were analysed in details based on the XRD and TEM data. The inuence of thermal treatment temperature on the luminescence of Sm 3+ and Er 3+ /Yb 3+ ions is studied thoroughly. During the devitrication process, Sm 3+ and Er 3+ /Yb 3+ ions are gradually segregated into the precipitated low phonon energy NaY(WO 4 ) 2 nanocrystals with a high disorder, which results in the enhancement of photoluminescence emission intensity, well-resolved Stark splitting and long luminescence decay lifetimes. According to the dependence of upconversion intensity on the excitation power, the upconversion mechanism of Er 3+ -Yb 3+ ions was put forward. The temperature sensing performance was investigated based on the FIR technique and a maximum sensitivity 146 Â 10 À4 K À1 was achieved at 523 K. The established correlation of structure, luminescence and temperature sensing provides a new approach to optimize the best host materials for temperature sensing applications.

Conflicts of interest
There are no conicts to declare.