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Abstract
A series of Sn2+-Dy3+ co-doped fluorophosphate glasses (FPGs) were prepared by the melt quenching method. The luminescent properties and energy transfer mechanisms of the FPGs glasses were investigated through photoluminescence and decay lifetime analysis. By controlling the concentration of Dy3+, the FPGs present a white light with a CIE chromaticity coordinate of (0.311, 0.330), which is very close to the standard equal energy white light illumination. The corresponding quantum efficiency, CRI and the brightness are 56.3%, 75 and 6706 cd/m2, respectively. Furthermore, the physical and chemical stability and thermal properties were also analyzed using differential scanning calorimetry and a thermal conductivity detector. The corresponding values of the ∆T (Tx-Tg) and thermal conductivity are 155 °C and 3.02~3.31 W/m·K, respectively. These results demonstrate that the FPGs can be a promising candidate for tunable white light phosphors.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays, white light emitting diodes (W-LEDs) play an increasingly significant role in general lighting, optical communication, optical amplifiers and waveguides due to its excellent properties such as small size, high brightness, long lifetime and friendly-environment [1–3]. One of the most popular and economical ways to obtain the white light was the combination of blue LED chips with yellow emitting phosphor powders, of which organic resins are usually used as package materials [4, 5]. However, the current commercial packaging materials degenerate easily and then turn yellow due to the poor physical and chemical stability and low thermal conductivity, adversely affecting the device performance such as color coordinate and operation lifetime [6–8]. To solve these problems, glass phosphors for W-LEDs have been extensively studied given to their advanced performances and excellent stability. Comparing with the other phosphors, including glass-ceramic and transparent ceramic phosphors, glass phosphors have lower production cost and simpler manufacture procedure [6–8]. Moreover, the package materials can be minimized or even avoided by using LED chips directly exciting glass phosphors. Therefore, glass phosphors are more suitable for white light emitting applications.
In recent years, considerable interest has been attracted for photoluminescent properties of glass phosphors, especially all kinds of host glasses (oxides, fluorides, chlorides, bromides, iodides and sulfide) doped with rare earth ions (Eu3+, Er3+, Tm3+, Ho3+ and Dy3+) [9–13]. Among numerous host glasses, fluorophosphates glass is considered to be a suitable host for optically active ions because of its high transparency, low melting point, low nonlinear refractive index and good rare-earth ion solubility [14]. On the other hand, comparing with the other rare earth ions, the Dy3+ ion is especially attractive in the glass phosphors application because of their interesting properties like high susceptibility and high thermal neutron absorption cross-section [15]. However, the Dy3+ ion usually exhibits two emission peaks in the yellow and blue regions, thus the emission color of which cannot be tuned and the pure white light is hardly to generate independently [16]. The Sn2+ ion is an efficient emission material and may act as an effective sensitizer and activator [17]. Therefore, the Sn2+-Dy3+ co-doping strategy is probably a way to obtain tunable white light emission [18, 19].
In this work, the Sn2+-Dy3+ co-doped fluorophosphate glasses (FPGs) were prepared for white light emitting phosphors by the melt quenching method. The photoluminescence properties, quantum efficiency, the thermal stability, luminescent lifetime, CIE chromaticity coordinates and CRI of the FPGs were investigated. Integrated with a commercial UV LED chip, the FPGs show high quantum efficiency (QE), tunable emission spectrum, high thermal conductivity and good CIE chromaticity coordinates. In addition, in order to further examine and develop the potentialities of the FPGs as a tunable white light source, the energy transfer from Sn2+ to Dy3+ ions was investigated systematically as well. The results show that the Sn2+-Dy3+ co-doped fluorophosphate glass phosphors may be a promising candidate for commercial white light emitting applications.
2. Experimental
A series of Sn2+-Dy3+ co-doped fluorophosphate glasses with a basic composition of 58(NaPO3-NaF)-20Al(PO3)3-22BaF2 (mol)-2.5 wt% SnO-x Dy2O3 were prepared by the melt quenching method, where x = 0, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 wt% referred as FPG0, FPG10, FPG15, FPG20, FPG25, FPG30, FPG35 and FPG40, respectively. All glass precursors mixed in stoichiometric proportions were put into alumina crucibles, then melted by an electrical furnace at 800-900 °C in air atmosphere. After 0.5 h of melting, the bubble free and transparent melts were poured into a preheated mould, and then annealed at a muffle furnace near the glass transition temperature Tg (360 °C) for 5 h, and then cooled to room temperature at a rate of 1 °C/min to remove the internal thermal strain. Finally, these glass samples were cut and polished into the size of 10 mm × 10 mm × 2 mm for measurements.
The characteristic temperature parameters were detected by differential scanning calorimetry (DSC) (STA 8000, PerkinElmer, USA) at a heating rate of 10 °C/min under a N2 atmosphere. The UV-visible (VIS) transmission spectrum was measured by using a UV/VIS/near infrared (NIR) spectrophotometer (Lambda 950, PerkinElmer, USA) in the range of 200-800 nm. The thermal conductivities were measured by the heat-flow method using a detector (DRL-III, XiangYi, China). The emission spectrum, excitation spectrum, fluorescence decay curves, and QE were recorded by a fluorescence spectrometer (FLS920P, Edinburgh, UK). A 280 nm UV-LED chip was used for luminescence characterization. A spectrometer (PR650, Photo Research, USA) was used to get the CIE coordinates and CRI. All measurements were performed at room temperature.
3. Results and discussion
Figure 1(a) shows the DSC curve of the FPG0 sample, which can be found that the glass transition temperature Tg, the onset crystallization temperature Tx and the value of Tx-Tg (ΔT) are 360 °C, 515 °C and 155 °C, respectively. Generally, the higher ΔT means a larger working range during operations for phosphor manufacturing and an effective guarantee of the amorphous state. Meanwhile, Fig. 1(b) shows the relationship between temperature and thermal conductivity of the FPG0. It is observed that the thermal conductivity of the FPG ramps up rapidly with the rise of temperature, and ranges from 3.02 W/m·K to 3.31 W/m·K, which is much higher than that of the traditional commercial phosphors and the packaging epoxy resin of LED [20].
Fig. 1 Physical properties of FPG: (a) DSC profile, (b) the relationship between thermal conductivity and temperature.
Figure 2(a) and Fig. 2(b) shows the optical transmittance and absorption spectra of different FPGs from FPG0 to FPG40 in UV and VIS spectral regions. From the transmittance spectra of Fig. 2(a), it can be observed that all the samples have a high transmittance more than 88% in the VIS region and a strong absorption in the UV region, which means a highly efficient output in the future white light emitting process. Meanwhile, it is worth noting that the vast major of FPGs transmittance curves have obvious fluctuation in the spectral region from 325 nm to 450 nm except for that of SnO single-doped sample FPG0, which coincides with the absorption spectra as shown in Fig. 2(b). Compared with the FPG0 sample, the other samples exhibit five distinct absorption peaks appeared at 450, 387, 364, 350 and 324 nm, and these absorption peaks increases gradually with the increasing Dy3+ doping concentration, which can be generally attributed to the electronic transition of Dy3+ ion from the 6H15/2 ground state to the 4I15/2, 4K17/2, 4P3/2, 6P7/2 + 4M15/2 and 4L19/2 excited states, respectively [21].
The excitation and emission spectra of the FPGs are shown in Fig. 3(a) and Fig. 3(b), respectively. The excitation spectrum of FPG0 shown with black line in Fig. 3(a) covers a broad UV wavelength region from 250 to 325 nm with a maximum peak at about 295 nm, which is corresponding to the 1S0→3P1 electronic transition of Sn2+ ion and just partly overlaps with that of Dy3+ ions in this spectral region. Thus, 295 nm was selected as excitation wavelength for simultaneously and efficiently exciting the Sn2+ and Dy3+ ions as far as possible. The results are shown in the Fig. 3(b), from which it can be found that FPG0 presents a broad emission band spectrum from 340 to 700 nm marked in black under the excitation of 295nm UV light. Owing to the stokes shifting and numerous energy level splitting of Sn2+ ions, photon energies for emission almost cover whole visible region from 700 nm (1.77 eV) to 325 nm (3.81 eV).
Fig. 3 Luminescence properties of FPGs: (a) the excitation spectra monitored at 575 nm, (b) emission spectra excited at 295 nm for different concentration of Dy3+ ion.
The excitation spectra of Sn2+-Dy3+ co-doped FPGs glasses monitored at 575 nm mainly exhibit five spectral peaks at 324, 350, 364, 387 and 450 nm in Fig. 3(a), which are attributed to the electronic transitions of Dy3+ ions from ground state 6H15/2 to excited states 4L19/2 (324 nm), 6P7/2 (350 nm), 6P5/2 (364 nm), 4I13/2 (387 nm) and 4I15/2 (450 nm), respectively [21]. Besides, excited by a 295 nm UV light, emission spectra shown in Fig. 3(b) include a broad band blue emission with peak at 405 nm from Sn2+, as well as three sharp emission peaks at 480, 574 and 662 nm, which are ascribed to the 4F9/2→6H15/2, 4F9/2→6H13/2 and 4F9/2→6H11/2 transitions of Dy3+ ions, respectively [16].
Furthermore, by comparing Fig. 3(a) and Fig. 3(b), there is a strong spectral overlap between the emission of Sn2+ and the excitation of Dy3+ in the range of 325-450 nm. Consequently, it is expected that the energy transfer process can efficiently occur from Sn2+ to Dy3+ ions. What is more, with the Dy3+ ions concentrations decreasing, the blue emission intensity from Sn2+ ions enhanced gradually. The result shows that there is an energy transfer process from Sn2+ to the neighboring Dy3+ ions in the FPGs. Importantly, with the increasing of Dy3+ ions doping concentration, the relative intensities of excitation and emission peaks all firstly increase and then decrease, and reach the maximum when Dy3+ ions contents is 2.5 wt%, namely FPG25. The reasons for this phenomenon, on the one hand is due to the increasing of the efficiency of energy transfer from Sn2+ to Dy3+ ions in the beginning. However, in some cases the excitation energy may be quenched with the further increasing of Dy3+ ions concentration [22]. On the other hand, FGD25 has the highest excitation peak at 295 nm based on Fig. 3(a), so when exciting at 295 nm, FGD25 should have the highest emission intensity.
To further evaluate the energy transfer from Sn2+ to Dy3+ in FPGs, the decay curves of emissions were measured with excitation at 295 nm and monitoring at 420, 483 and 575 nm, as shown in Fig. 4(a), Fig. 4(b) and Fig. 4(c), respectively. The decay processes of these samples can be characterized by the average lifetime τ, which is defined as [17]:
where I(t) stands for the intensity at time t. The calculated lifetimes τ are shown in Table 1. With increasing Dy3+ content, the lifetime monitoring at 420 nm is gradually decrease, which can be attributed to the energy transfer process from Sn2+ to Dy3+ ions. The simplified energy levels diagram for the energy transfer process from Sn2+ to Dy3+ are shown in Fig. 4(d). The above analysis of luminescence spectra and decay lifetimes demonstrates the presence of the energy transfer process from 5P3/2 of Sn2+ to 4F9/2 of Dy3+ ions. As shown in Fig. 4(b), Fig. 4(c) and Table 1, it can be observed that lifetimes with respect to FPGs monitoring at 483 and 575 nm also decrease with increasing Dy3+ content.Fig. 4 The decay curves of FPGs monitoring at 420 (a), 483 (b), 575 nm (c) and (d) simplified energy levels diagram for the energy transfer process of Sn2+ to Dy3+ ions.
Table 1. The values of decay lifetime, CIE coordinates,η and QE of FPGs.
In addition, according to the decay curves of FPGs shown in Fig. 4, the energy transfer efficiency η of Sn2+→Dy3+ ions can also be obtained. The value can be estimated by the following formula [23]:
where τ1 and τ0 are decay lifetime of Sn2+ monitoring at 420 nm with and without doping Dy3+ cation, respectively. The results are listed in Table 1, from which it can be found that the η gradually increases with increasing Dy3+ doping content. This phenomenon indicates that the increase of Dy3+ ions concentration is beneficial to energy transfer from Sn2+ to Dy3+ions.Quantum efficiency is another important performance parameter, and the details of QE values of the FPGs with different doping concentration are also listed in Table 1. Therein, the QE is calculated by the following formula [17]:
where ε is the photons emitted by the sample, and α is the photons absorbed by the sample. The integral part of Lemission is the luminescence emission spectrum of Sn2+-Dy3+ co-doped FPGs samples. The integral part of Ehost is the spectrum of the light used for excitation with only the host material in the sphere, and the integral part of Esample is the spectrum of the light used to excite the Sn2+-Dy3+ co-doped FPGs samples in the sphere. As shown in Table 1, the maximum QE of the FPGs is 81.3% for the single-doped sample FPG0. Among the Sn2+-Dy3+ co-doped FPGs, the maximum QE is 73.3% for FPG5, and the minimum value is only 39.4% for FPG40. The QE value decreases gradually with the increase of Dy3+ doping content, which is just the opposite of that of energy transfer efficiency. One of the reason, may be the concentration quenching of Dy3+ ions. On the other hand, the decay lifetime of Dy3+ ions is much longer than that of Sn2+ ions, which leads to a decrease of the quantum efficiency obtained by the calculation based on the lifetime of the 4F9/2 energy levels of Dy3+ ions [16].Figure 5(a) depicts the CIE chromaticity diagram of the Sn2+-Dy3+ co-doped FPGs under the 280 nm UV-LED chip excitation, and the corresponding CIE chromaticity coordinates are listed in Table 1. From the Fig. 5(a) and Table 1, it can be seen that the color coordinates of the FPGs gradually change from blue to white region with the increasing of Dy3+ ions, which is interesting and significant for the LED applications because of easy color tunable properties. Figure 5(b) shows the actual light-emitting photos of Sn2+-Dy3+ co-doped FPGs excited by a 280nm UV-LED chip, from which almost pure white light can be clearly observed. The corresponding spectra are omitted here due to the high similarity with that of Fig. 3(b). Besides, Fig. 5(c) also shows the change curve of luminous intensity with the increasing of driving voltage of 280 nm LED chip for the FPG25 sample. It can be seen that the maximum brightness is up to 6706 cd/m2 at 7 V and 10 mA, and the corresponding CIE chromaticity coordinate and the quantum efficiency are (0.311, 0.330) and 56.3%, respectively. These performance characteristics is superior to that of the other similar light emitting phosphors. The results indicate that the Sn2+-Dy3+ co-doped fluorophosphate glasses can produce excellent white light and is very suitable for indoor illumination when combined with commercial UV-LED chips directly.
Fig. 5 The light-emitting properties of the FPGs under 280 nm UV-LED chip excitation: (a) CIE chromaticity coordinate, (b) Actual light-emitting photos and (c) brightness.
4. Conclusions
In summary, a series of Sn2+-Dy3+ co-doped fluorophosphate glasses with excellent physical and optical properties were prepared by melt quenching method. The thermal conductivity of FPGs ranges from 3.02 to 3.31 W/m·K, which is higher than that of the common commercial phosphors and the packaging materials of LED. Under 280 nm UV light excitation, the emission colors of FPGs can be changed from blue to white by properly adjusting the concentration of Dy3+ ions. The tunable emission performance is attributed to the energy transfer from 5P3/2 of Sn2+ to 4F9/2 of Dy3+ ions. When the doping concentration of Dy3+ is 2.5 wt%, a white-light emission with CIE coordinates of (0.311, 0.330) is realized under the 280 nm UV-LED chip excitation. The corresponding quantum efficiency and the brightness are 56.3% and 6706 cd/m2, respectively. These investigations indicate that the Sn2+-Dy3+ co-doped fluorophosphate glass is a promising candidate as a tunable phosphor for commercial white light emitting applications.
Funding
Incubation Foundation of the National Natural Science Foundation of Nanjing University of Posts and Telecommunications (NY215143); the State Key Laboratory of Transient Optics and Photonics of the Chinese Academy of Sciences (SKLST201606); the Research Center of Optical Communications Engineering & Technology (Grant No. ZXF20170103).
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