Materials Letters 243 (2019) 73–76
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Effect of microstructure on up-conversion luminescent of Tb3+/Yb3+ co-doped phosphate glass and glass-ceramic Mengjie Zhao a, Hongbo Zhang a,⇑, Xiangyu Zou a, Wentao Jia a, Chunhui Su a,b,⇑ a b
School of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China Changchun Normal University, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 20 September 2018 Received in revised form 3 January 2019 Accepted 3 February 2019 Available online 12 February 2019 Keywords: Microstructure Phosphate Up-conversion luminescent Luminescence quantum yield
a b s t r a c t Tb3+/Yb3+ co-doped precursor glass was synthesized by melt quenching method, and transparent glassceramic was prepared by heat treatment the precursor glass. Comparing the X-ray diffraction patterns of precursor glass and glass-ceramic showed that the Na3.6Y1.8(PO4)3 crystals were formed after heat treatment. By the scanning electron microscopy images of glass-ceramic, the grainy crystal grains were observed dispersed in the glass matrix. The structure of precursor glass and glass-ceramic was studied by FTIR spectra and Raman spectra, indicating that phosphate group was formed in glass-ceramic. The upconversion luminescent intensity of glass-ceramic was obviously enhanced than precursor glass. Moreover, the luminescence quantum yield of the glass-ceramic sample showed higher than precursor glass, which can be attributed to the formation of phosphate groups in glass-ceramic. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Rare earth (RE) ions doped glass-ceramics with excellent thermal stability, chemical stability and mechanical strength, and allow high concentration of rare earth ions doping, so it becomes a kind of excellent up-conversion luminescence matrix material [1–4]. Besides, it also inherits the fine properties of glasses and crystals, such as high transparency and suitable crystal environment for RE ions [5]. RE ions such as Ho3+, Tm3+, and Er3+ doped into up-conversion materials as luminescent centers [6,7]. Yb3+ ions act as sensitizer, that is, the donor, which transfers energy to the luminescent center, namely the energy receptor [8,9]. Up-conversion property of Er3+/Yb3+ and Tm3+/Yb3+ co-doped glass-ceramics investigated comprehensively [10,11]. In recent years, many studies focused on Tb3+/Yb3+ co-doped glass-ceramics [12,13]. However, to the best of our knowledge, there are no reports about the up-conversion luminescence properties and preparation of Tb3+/Yb3+ co-doped glass-ceramics containing Na3.6Y1.8(PO4)3 crystals. In this paper, the microstructures of glass and glass-ceramics samples were discussed. It was shown that phosphate groups were formed in glass-ceramics sample. The effect of the phosphate group on the quantum efficiency is also discussed.
The precursor glass (PG) sample was prepared with the following composition: 20.1Na2CO3-35.8SiO2-38.7H3BO3-2.0P2O52.0Y2O3-0.2Tb4O7-1.2Yb2O3 (in mol%). For each batch, about 20 g raw materials were fully mixed and melted in a corundum crucible at 1400 for 2 h, and then annealing at 450 for 2 h. The obtain PG sample was cut 1 cm 1 cm 2 cm block and then heat treated at 630 for 3 h to synthesis the glass-ceramic (GC) combined with our previous work [14]. Polishing the prepared PG and GC samples for subsequent testing. X-ray diffraction (XRD) patterns were performed on a Rigaku2500PCX(Japan) X-ray diffraction apparatus with Cu Ka radiation. The SEM images and X-ray energy dispersive spectroscopy (EDS) of samples were obtained by scanning electron microscopy (SEM, JEOL, JSM-7610F) operated at 10 kV. The Fourier transform infrared spectra were measured by a spectrometer (FTIR-8400S). Raman spectra were recorded by a T6400 micro-Raman spectrometer (RERU-YQ-006) with the excitation wavelength of 514.5 nm. The transmittance spectra were obtained by spectrophotometer (SHIMADZU, UVmini-1240). The up-conversion luminescence spectra were measured by a spectrometer (Hitachi F-4500) equipped with a 980 nm pump laser. The luminescence quantum yield was measured by an Absolute PL Quantum Yield Measurement System (C9920-02G, Hamamatsu Photonics K.K., Japan).
⇑ Corresponding authors. E-mail address:
[email protected] (H. Zhang). https://doi.org/10.1016/j.matlet.2019.02.021 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
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3. Results and discussion Fig. 1(a) shows the XRD patterns of PG and GC. The XRD pattern of PG does not show any sharp diffraction peaks having just one halo, indicating the amorphous structure of PG sample. After heat treatment, GC showed several sharp diffraction peaks attribute to Na3.6Y1.8(PO4)3 crystalline phase (JCPDS No. 47-0972). The average crystal size of Na3.6Y1.8(PO4)3 in GC can be calculated by Scherrer equation [15]:
D¼
kk bCOSh
ð1Þ
where k is 0.89, k is 0.15403 nm represents the wavelength of Xray, b is the corrected half width of diffraction peak and h is the angle of diffraction. The mean grain size is about 97 nm. SEM image of GC presented in Fig. 1(b). The SEM image demonstrates that the formed grainy crystal grains were dispersed in glass phase. The EDS spectra taken from the individual crystals, as shown in Fig. 1(c), reveal that the crystalline phase to be Na-, P-, Si, Y-, Tb- and Ybrich for the GC sample [16]. Fig. 2(a) shows the FTIR spectra of PG and GC in the frequency region between 400 and 1500 cm1. For PG, bands are located at 457, 707, 1020, 1250, and 1400 cm1. The GC shows three new bands at 550, 617, and 670 cm1. The bands at 707, and 1400 cm1 are due to O-B-O bending vibrations and B-O asymmetric stretching modes, respectively [17]. The bands at 457, 1020, and 1250 cm1 are generally assigned to the Si-O-Si bending vibrations, stretching vibrations and the asymmetric stretching of Si-O bonds of SiO4 tetrahedron, respectively [18]. The band at 550, 617, and 670 cm1 can be attributed to P-O-P bending vibration, phosphate network bending vibration and symmetric stretching of P-O-P [19], which indicated the existence of phosphate group.
The Raman spectra of PG and GC samples in the range 400– 2000 cm1 are shown in Fig. 2(b). A band is observed around 1124 cm1 which is assigned for the asymmetric stretching of SiO bonds of SiO4 tetrahedron. The peak in the range of 1300– 1600 cm1 are assigned to stretching vibrations of B-O bands in the [BO3] triangle. The Raman spectrum of GC appears new bands around 607 cm1, 733 cm1, 1036 cm1, 1158 cm1, and 1207 cm1. The bands are attributed to bending vibration and symmetric stretching of O-P-O and formation of the terminal phosphate groups [18,20]. The results of FTIR and Raman spectra confirmed the presence of phosphate in the GC sample. Fig. 3(a) shows the transmission spectra of PG and GC ranging from 200 to 1000 nm. According to the spectra, the transmittance of GC was less than the PG due to the crystal precipitated in glass matrix. However, the GC sample still maintain high transparency, is about 80% in green light range. The transmittance spectrum of PG contains two obvious absorption peaks located at 912 and 980 nm. The peak at 912 nm is assigned to the transitions from the 7F6 ground state of Tb3+ to a virtual intermediate state (V) [21], and the peak at 980 nm is attributed to the 2F7/2?2F5/2 transition of Yb3+ ions. Moreover, the PG contains another absorption peak located at 371 nm compared to GC, which is assigned to the populated to the excited level 5D3 from the 7F6 ground state of Tb3+. Fig. 3(b) presents the up-conversion luminescence (UCL) spectra of Tb3+/Yb3+ co-doped PG and GC samples, under the excitation of 980 nm. In the UCL spectra, a serious of emission peaks centered at 413 nm, 438 nm, 488 nm, 544 nm, 585 nm, and 623 nm are attributed to 5D3?7F5, 5D3?7F4, 5D4 ? 7F6, 5D4 ? 7F5, 5D4 ? 7F4 and 5D4 ? 7F3 transitions of Tb3+ ions. Compared with PG, the emission peaks of GC at 413 nm and 438 nm are weaker, but the emission peaks at 488 nm, 544 nm, 585 nm, and 623 nm are significantly enhanced. This phenomenon is due to 5D3 energy level cross relaxed to 5D4 energy level.
Fig. 1. (a) XRD patterns of PG and GC; (b)SEM image of GC; (c) EDS spectra taken from the crystal of GC.
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Fig. 2. (a) FTIR spectra and (b) Raman spectra of PG and GC.
Fig. 3. (a) Transmittance spectra, (b) UCL spectra of PG and GC; (c) Dependence of emission intensity on pumping power and (d) UCL mechanism of GC.
The luminescence quantum yield (LQY) of the PG and GC were measured under excitation at 980 nm. The LQY of the PG and GC samples are 14.7% and 33.8%. The LQY of GC sample is enhanced than PG sample. This may be due to the generated phosphate in the GC sample has relative high phonon energy can provide the phonon energy for energy transfer from Yb3+ to Tb3+ ions, which improve the energy transfer efficiency. The double logarithmic curve of emission intensity (Iem) and excitation power (Iex) by 980 nm laser excitation showed in Fig. 3 (c). The Iem is related to Iex by Formula (2):
Iem / Inex
ð2Þ
Here n is the number of photons absorbed. And the experiment results of n for 5D4 up-conversion emissions of Tb3+ are all closed to 2. The Tb3+ ions continuity absorbed two photons, excited from ground state to 5D4 energy level. The possible up-conversion emission channels are diagrammed in Fig. 3(d), can be described as follows: 22F5/2 (Yb3+) + 7F6 (Tb3+) ? 22F7/2 (Yb3+) + 5D4 (Tb3+). Then, Tb3+ ions at 5D4 level radiatively relax to 7F6(488 nm), 7 F5(544 nm), 7F4(585 nm) and 7F3(623 nm), respectively.
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4. Conclusions In summary, Tb3+/Yb3+ co-doped glass and glass–ceramic containing Na3.6Y1.8(PO4)3 crystals were successfully fabricated. Comparing the Raman and Fourier transform infrared spectra of PG and GC, it is shown that the presence of phosphate in the GC sample. Moreover, the up-conversion luminescent properties and quantum yield under 980 nm light excitation were investigated, and the quantum yield of GC sample is 33.8%, which is higher than PG sample. The change of microstructure leads to the enhancement of up-conversion luminescence intensity, indicated that the formation of phosphate has a positive effect on the luminescent quantum yield of samples. Conflict of interest We declare that we have no conflict of interest. Acknowledgments This work was supported by the ‘‘111” Project of China (D17017) and Key Research Project of Jilin Provincial Science and Technology Department (20150204051GX, 20160204027GX). References [1] M.H. Imanieh, I.R. Martín, A. Nadarajah, J.G. Lawrence, V. Lavín, J. GonzálezPlatas, J. Lumin. 172 (2016) 201–207.
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