Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-018-9045-9
Properties optimization of Er3+ doped lead–sodium–yttrium–fluoride phosphor through combining LCS–HSR method Hongwei Liu1 · Liping Lu1 · Xiyan Zhang1 Received: 7 November 2017 / Accepted: 3 April 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract Up-conversion luminescence materials sensitive to 1550 nm are of broad application prospects. One of the most important properties is that the phosphor should be ultrafine and dispersed well with excellent luminescence intensity in order to meet the matching application requirements of those image and detection devices. The low-temperature combustion synthesis method, a most popular method for the synthesis of ultrafine oxides, was adopted to synthesize the E r3+ doped lead–sodium– yttrium–fluoride phosphor using glycine as the fuel. Orthogonal experiments were carried out to determine the optimum cation molar-proportion. Dispersing agent ( NH4)2SO4 was adopted to improve the dispersion state of the ultrafine phosphor and the high-temperature solid-state reaction process was performed to optimize its morphology and luminescence properties. The sample presents E r3+ characteristic emission peaks and its luminescence mechanism excited at 1550 nm was discussed.
1 Introduction Upconversion luminescence (UCL) materials possess the excellent property of converting the infrared light to visible light; therefore, they have broad application prospects in the field of upconversion lasers [1], infrared detection [2], infrared imaging [3], bio-detection, etc [4]. The 1550 nm laser has so many excellent advantages, such as high eyesafety, strong smoke penetrability and excellent stability, etc [5, 6], hence it has prominent application prospect in laser communications. In future, the laser working wavelength of many detection devices might be 1550 nm, therefore the research of UCL materials sensitive to 1550 nm is significant apparently. Considered from the phosphor-device matching requirements, the UCL phosphor should be ultrafine as well as bright because the particle size will affect the image resolution of the device greatly. Very few reports are available to help known about the synthesis of the UCL materials sensitive to 1550 nm. Our group has been devoting ourselves * Liping Lu
[email protected] * Xiyan Zhang
[email protected] 1
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, Jilin, People’s Republic of China
to this kind of phosphor, and barium/lead-based complex fluorides have been synthesized. In this paper, lead-based complex fluoride was synthesized and the synthesis conditions were studied in detail. Compared with barium-based fluoride, lead-based fluoride has lower phonon energy. Lower phonon energy contributes to the higher luminescence efficiency during the upconversion process; therefore lead-based complex fluoride would have better luminescence properties. As for its preparation method, there are two main methods. The first one is high-temperature solid-state reaction (HSR) method which is known for its excellent luminescence properties, and the other one is lowtemperature combustion synthesis (LCS) method which is known for its prominent particle size. The HSR method has the outstanding advantages of purer phase and higher luminescence intensity, but a known shortcoming of the bigger particle size which could be improved by the grinding process with a significant decrease of its luminescence intensity, unfortunately. Researchers have paid much attention on the decrease of the particle size, and the LCS method is the promising candidate for the synthesis of ultrafine oxides [7, 8] and compound oxides [9, 10] phosphor, however, nonoxide phosphors synthesized directly by the LCS method were seldom reported except for several literature of our group [11–14]. In this paper, a combining LCS–HSR preparation process was adopted to synthesize a kind of lead-based quaternary-fluoride phosphor. The technological parameters
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were studied systematically with the aim of obtaining both ultrafine particle size and excellent luminescence intensity, in order to improve the overall matching application performance of the phosphor when used in the imaging or detection devices.
2 Experimental 2.1 Preparation This kind of lead-based quaternary-fluoride phosphor can be represented as PbxNayYzF2x+y+3z+3m:Er m wherein the x, y, z and m can be changed freely in a relatively wide range. Firstly, the phosphor samples were synthesized by the LCS method. Because there are too many free variables in the molar-ratio, orthogonal experiments were performed to determine the optimal cation molar-proportion of the Pb–Na–Y–Er quaternary-fluoride, with Er, Y, Pb mol% and fuel amount as the “factors” of L 25(54) orthogonal experiments. The first step is to dissolve the rare earth starting materials according to the stoichiometric ratio shown in Table 1. Er2O3 (99.999%) and Y2O3 (99.999%) were dissolved in 8 mol L−1 nitric acid and then were continually heated to get the concentrated rare earth nitrate solution. Na (wherein Na mol% = 100% − Er–Y–Pb mol%) was introduced in the above solution by dissolving N a2SiF6 which can decompose into SiF4 gas to help the powder product looser when in the combustion process. Then, the fuel glycine, PbF2 and NH4HF2were added directly into the above concentrated solution and ground homogenously for getting the combustion precursor. The second step is low-temperature combustion synthesis process. The muffle furnace should be preheated to 650 °C in advance in order to ignite the above combustion precursor effectively. Once the precursor was ignited, a spontaneous combustion reaction would keep on until the end. The combustion duration would be approximately 3–5 min, and then a pink–white-like loose powder was obtained. Secondly, the HSR method was adopted to compare the properties with that of LCS method. The sample’s formula is as same as the optimal formula of LCS method, and the calcining conditions adopted 750 °C for 1.5 h, according to our previous research [15]. Finally, LCS–HSR process was carried out to improve the overall application performance of the phosphor. The optimal Pb–Na–Y–Er cation molar-proportion was determined through the orthogonal experiments and orthogonal analysis results. In order to improve the dispersion and homogeneity of the phosphor samples, the dispersing agent (NH4)2SO4would be added into the dissolving process on the basis of the optimal formula. And, because the LCS process
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is too fast to get complete crystal lattice, HSR heat treatment was performed to repair the lattice imperfection in order to enhance the luminescence intensity.
2.2 Measurements The XRD patterns of the samples were tested through a Rigaku UltimaIVX-ray diffractometer, using Cu Ka1as the radiation source, with an accelerating voltage of 40 kV and a working current of 20 mA. The morphology photos of the samples were observed by a JEOL JSM-6701F scanning electronic microscope. The infrared up-conversion spectra were measured by a SHIMADZU RF-5301PC fluorescence spectrometer coupled with an excitation source of 1550 nm 100 mW laser.
3 Results and discussion 3.1 Optimization of the cation molar‑proportion Table 1 shows the L 25 (54) orthogonal experiments layouts. The upconversion emission spectra of all of the above orthogonal samples are shown in Fig. 1, measured at the same conditions, with a 1550 nm laser as the excitation source (the power density is 180 mW/cm2), with a slit width of 1.5 nm and a distance between the sample and the laser source of 25 mm. The luminescence intensity results of all of the above orthogonal samples were also recorded in the same Table 1 (the sixth column), and then the orthogonal analysis could be performed. The orthogonal analysis results are also listed in the same Table 1 (at the bottom of Table 1: Kn and kn−). k1−, k2−, k3−,k4−and k5− are the average values of the corresponding intensities of every level of different factors (A, B, C and D). The largest indicates the optimal level of every factor. According to the L25 (54) orthogonal analysis result, the optimal levels are A3B5C1D4. Figure 2 is the line chart of different levels of every factor, corresponding to the Table 1. It shows that both the 4, factor A and D have found their inflection point A 3 and D respectively; the factor B and C only presented the change tendency but without an inflection point, which means that there should be further experiments to determine the optimal level. Therefore, on the basis of those rules analyzed from L25 4 (5 ), a further optimization experiments for Y 3+ and P b2+ have been performed. Their emission spectra of different Y3+ or Pb2+ concentrations are shown in Fig. 3, from which we can confirm that the optimum concentration of Y 3+ and 2+ Pb are 15 and 18%, respectively. Accordingly, the optimum levels of the above four factors have been confirmed, and the optimum conditions are
Journal of Materials Science: Materials in Electronics Table 1 Experimental layouts and results of orthogonal experiments L25 (54)
Trial
A Er mol%
B Y mol%
C Pb mol%
D Glycineamount (to the stoi.)
Total intensity (red + green) /a.u
No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9 No. 10 No. 11 No. 12 No. 13 No. 14 No. 15 No. 16 No. 17 No. 18 No. 19 No. 20 No. 21 No. 22 No. 23 No. 24 No. 25 K1 K2 K3 K4 K5 k 1− k2− k3− k4− k5−
5 5 5 5 5 8 8 8 8 8 11 11 11 11 11 14 14 14 14 14 17 17 17 17 17 1494.489 2825.948 2927.276 2699.585 2679.208 298.898 565.19 585.455 539.917 535.842
3 6 9 12 15 3 6 9 12 15 3 6 9 12 15 3 6 9 12 15 3 6 9 12 15 1844.583 1890.647 2684.626 2843.787 3362.863 368.917 378.129 536.925 568.757 672.573
18 24 30 36 42 24 30 36 42 18 30 36 42 18 24 36 42 18 24 30 42 18 24 30 36 3795.482 3166.088 2838.333 1800.217 1026.386 759.096 633.218 567.667 360.043 205.277
0.7 1 1.3 1.6 1.9 1.3 1.6 1.9 0.7 1 1.9 0.7 1 1.3 1.6 1 1.3 1.6 1.9 0.7 1.6 1.9 0.7 1 1.3 1519.93 2022.02 3014.69 3157.28 2912.59 303.985 404.404 602.938 631.456 582.518
67.509 132.784 467.545 495.175 331.476 658.121 701.07 305.646 162.452 998.659 760.078 76.848 206.34 999.232 884.778 257.374 224.617 974.754 760.064 482.776 101.501 755.328 730.341 426.864 665.174
Bold values indicate the name of every factor The numbers in column A, B, C and D indicate different values of the factors 3 B5 C1 D4 Optimum conditions from L25(54) are: A
(Er:Y:Pb:Na mol% = 11:15:18:56 mol%, glycine amount = 1.6 stoi.)
A3B5C1D4 (Er:Y:Pb:Na mol% = 11:15:18:56 mol%, glycine amount = 1.6 stoi.) which didn’t occur in the L25(54) layouts; therefore it is necessary to perform a comparing verification experiment between the orthogonal analysis optimum sample (marked as sample No. 26) and the best sample of layouts (marked as sample No. 14). Figure 4 is the comparative XRD patterns of sample Nos. 26 and 14, with the related standard cards JCPDS
No. 16-0334 [hexagonal Na(Y1.5Na0.5)F6] and JCPDS No. 37-0949 (tetragonal P b0.52Y0.48F2.48) at the bottom. It can be known from Fig. 4 that the orthogonal optimum sample No. 26 possesses the purer phase, in which the impurities peaks (marked as “*”) decrease apparently compared with the sample No. 14. These impurities are confirmed as NaF(200), PF2(113) (213), PF3(101) and Y2O3(111).
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Fig. 1 Upconversionemission spectra of samples of L25 (54) at the excitation of 1550 nm
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Fig. 2 Line chart of different levels of every factor 10
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Figure 5 is the comparative upconversion emission spectra of sample Nos. 26 and 14. The orthogonal optimum sample No. 26 presents higher luminescence intensity compared with that of sample No. 14. All of the above spectra present Er 3+characteristic emissions peaking at ~ 540 and ~ 660 nm, corresponding to the 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively; and the related upconversion luminescence mechanism would be discussed in the last sections.
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Fig. 5 Comparative emission spectra of sample Nos. 26 and 14
It can be confirmed from both the comparative XRD patterns and the comparative emission spectra that the optimum
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Journal of Materials Science: Materials in Electronics 3500 3000 2500 2000 1500 1000 500 0 3500 3000 2500 2000 1500 1000 500 0
3.2 Properties comparison between LCS and HSR method
LCS sample
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16-0334>Na(Y1.5Na0.5)F6-Sodium Yttrium Fluoride
37-0949>Pb0.52Y0.48F2.48-Lead Yttrium Fluoride
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Fig. 6 Comparative XRD patterns of LCS sample and HSR sample
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conditions A3B5C1D4determined from the L 25 (54) orthogonal analysis results are reasonable.
In order to improve the overall application performance of the phosphor by using the LCS method, the properties of the sample synthesized by the LCS method and HSR method was compared and discussed. The sample’s formula of HSR method adopted the orthogonal optimum A 3B5C1D4 of LCS method, and the calcining conditions adopted the optimal parameters determined by our previous research [15], calcined at 750 °C for 1.5 h. Figure 6 is the comparative XRD patterns of LCS sample and HSR sample. It can be seen that the X-ray diffraction intensity of HSR sample is much stronger than that of LCS sample, especially at the main diffraction peak of Pb0.52Y0.48F2.48 (2θ = 26.757°). Correspondingly, the upconversion luminescence property of HSR sample should be better because of its purer crystal lattice, just as shown in Fig. 7. The HSR sample possesses more excellent luminescence intensity than that of LCS sample. As for the above phase purity and luminescence intensity, the HSR method is indeed better than the LCS method. However, in order to match the application requirements of detection devices, the particle morphology and the homogeneity are much more important for its image resolution. Figure 8 is the comparative SEM photos of LCS sample and HSR sample. It can be seen from Fig. 8 that the particle morphology and the homogeneity of LCS sample is superior to that of HSR sample. It can be explained from the LCS synthesis mechanism. The LCS method is on the basis of redox reaction, the metal nitrate and the organic fuel were adopted respectively as oxidant and reducer in this method [16]. Just because the LCS process begins from solution state, the E r3+ ions distribution can realize excellent space homogeneity at the molecular level, which would improve the quenching concentration, and therefore enhance the luminescence intensity. The redox reaction precursor was ignited at a preheated temperature and then a spontaneous combustion reaction will start, and there would release a lot of gas during the combustion process, including CO2, H2O, NOx, and
Fig. 8 Comparative SEM photos of LCS sample and HSR sample
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also SiF4 decomposed from the starting material Na2SiF6, which would benefit for the ultrafine particle size and its dispersing performance. Therefore, the morphology of LCS sample is more suitable for the matching application with image devices. Furthermore, it can also be seen that the phosphor would agglomerate easily because the small particle size and hence the large surface area. Therefore, the optimization experiments should be carried out to improve the dispersing state in the following experiments.
Fig. 9 SEM photos of samples with different amount of dispersing agent. a Untreated b 3 wt% c 6 wt% d 9 wt% e 12 wt% f 15 wt%
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Journal of Materials Science: Materials in Electronics
3.3 Improvement of the dispersing state of the LCS sample (NH4)2SO4 was adopted as the dispersing agent to improve the dispersing state of the ultrafine LCS sample in order to match the application requirements of the image devices. A series of weight percent (NH4)2SO4 was added into the rare earth nitrate solution during the dissolving process. Figure 9 are SEM photos of samples with different amount of dispersing agent ( NH4)2SO4. It can be seen that
Journal of Materials Science: Materials in Electronics
3.4 Optimization of the luminescence intensity of the LCS sample by HSR process After obtaining the optimum cation molar-proportion and improving efficiently the particle’s dispersion state, the dominant technical problem we should resolve is to enhance
Intensity/a.u.
Fig. 10 XRD patterns of samples with different amount of (NH4)2SO4
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the agglomeration phenomenon weakens gradually with the increase of dispersing agent. This is because the adsorption of S O42− on the surface of the particles brings about an increase of the Zeta potential, correspondingly an increase of the repulsive force; therefore the particles are easy to be dispersed well [17, 18]. It can be known from Fig. 9 that when it is up to 9 wt%, the particle size has reduced to circa 300–400 nm, which is suitable to be matched with those detecting and imaging devices. Continue to increase the amount of dispersing agent, and the particle size would become smaller. But too much dispersing agent will affect the phase purity because the strong affinity between S O42− ions and cations makes S O42− ions difficult to be removed [19], correspondingly resulting in the decrease of its luminescence intensity. It can be seen that when it is up to 12 and 15 wt%, the particles become smaller and more dispersed, but there appears the impurity phase of BaSO3 and BaSO4 (as shown in Fig. 10) and the luminescence intensity also decreases gradually (as shown in Fig. 11). Considered from both the particle size and the luminescence intensity, the optimum weight percent of dispersing agent should be 9 wt%.
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0wt% 3wt% 6wt% 9wt% 12wt% 15wt%
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Fig. 11 Emission spectra of samples with different amount of (NH4)2SO4
its luminescence intensity, without growing up greatly its particle size. The optimum sample synthesized at the above determined optimal conditions was then calcined by HSR process. What should be noted in the HSR process is that the calcining temperature shouldn’t be too high and the calcining duration shouldn’t be too long, because with the completing of the crystal lattice in the calcining process, its particle size will also grow. Figure 12 shows the upconversion emission spectra of a series of samples calcined at different temperatures (450, 475, 500, 525 and 550 °C) through HSR process. With the increase of calcining temperature, the sample presents higher luminescence intensity which proves that HSR calcining process indeed repairs its lattice imperfection. However PbSO4
PbSO3
15 wt% 12 wt% 9 wt% 6 wt% 3 wt% 0% 16-0334>Na(Y1.5Na0.5)F6-Sodium Yttrium Fluoride
37-0949>Pb0.52Y0.48F2.48-Lead Yttrium Fluoride
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Fig. 14 Comparison XRD patterns of samples before-after HSR heating
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Fig. 13 Emission spectra of samples calcining for different t by HSR process
the calcining temperature shouldn’t be too high because of the glass transition phenomenon, which will result in the luminescence quenching, just as shown in the 500 °C spectrum of Fig. 12. It can be determined that the optimum heating temperature should be 525 °C. Figure 13 shows the upconversion emission spectra of a series of samples calcined at 525 °C for different duration (5, 10, 15 and 20 min) through HSR process, in comparison with that of the untreated sample. These spectra show that the samples after HSR treating all present higher luminescence intensity than that of untreated samples. The phosphors would be slightly ground before the heat treatment, which will destruct the structure of the particles. During the different calcining duration, the surface defects of the particles would be repaired; therefore the luminescence
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property of the samples would be improved and the particle size wouldn’t be grown up excessively. It can be seen from Fig. 13 that with the extension of calcining time, the intensity grows firstly because the repairing effect has been increasing, but decreased at 20 min because of the appearance of glass transition. Therefore, the optimum duration is 15 min. In order to confirm the optimization result, the beforeafter HSR heating samples were measured at the same conditions. Figure 14 is the comparison XRD patterns. The sample after HSR heating presents more intensive diffraction peaks, especially at the dominant peaks (2θ = 26.757° and 2θ = 43.494°), and the diffraction peaks (2θ = 38.841°, 56.099°) of the impurity—NaF were weaker after HSR heating. Therefore, the sample after HSR heating possesses more intensive luminescence intensity as shown in Fig. 15.
Journal of Materials Science: Materials in Electronics
Fig. 16 Comparison SEM photos of samples before-after HSR heating. a Sample before HSR heating, b sample after HSR heating (525 °C 15 min)
Figure 16 presents the comparison SEM photos of samples before-after HSR heating. The particle size of the sample after HSR heating grows slightly, from ~ 300 to ~ 800 nm. Although the particles grow up, it is still in the appropriate range of sizes (~ 1 µm) which meets the requirements of those matching applications, and especially it possesses more complete crystal lattice and more intensive luminescence intensity. Then we combined the changes of particle size and crystal lattice with the improvement of luminescence intensity in Fig. 15, we could come to the conclusion that these changes have positive influence on the luminescence properties, which confirms that HSR heating process indeed optimize the whole luminescence properties of LCS sample to match excellently the application requirements of image and detection devices.
3.5 Up‑conversion luminescence mechanism discussion As for the upconversion luminescence mechanism sensitive to 1550 nm, there are two dominant physical mechanisms, the excited state absorption mechanism and the energy transfer mechanism [20, 21], just as stated in our previous literature [14]. Figure 17 is a simplified upconversion luminescence mechanism diagram [14]. Luminescence excited at 1550 nm was contributed from the triple-photon accumulated process. As for the excited state absorption mechanism, step 1, 2 and 3 occurred sequentially. Firstly, the electrons at the Er3+ ground state 4I15/2 will transit to its excited 4I13/2 state (step ①) when excited at 1550 nm. Secondly, the electrons at the excited 4I13/2 state will continue to transit to further excited state 4I9/2 (step ②). When they arrive at the state 4 I9/2, a part of them would stay there waiting for the next excited state absorption process; while the other part of
The left part represents those who transfer energy to other
The right part represents those who receive energy from other
Ĺ’
4 4
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~520nm
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I11/2
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I13/2
~655nm
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I9/2
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F7/2
2 H 4 11/2 S3/2 4 F9/2
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Ċ
4
3+
Er
I15/2
Fig. 17 Er3+ energy level diagram of the 1550 nm upconversion luminescence
them would relax to lower excited state 4I11/2 through the non-radiative process. Thirdly, the electrons at the excited 4 I9/2, 4I11/2 state will continue to transit to further excited state 2H11/2, 4F9/2 (step ③, ③′), separately, when absorb the third photon. After the above triple-photon absorption process, the following radiative transitions 2H11/2→4I15/2, 4 S3/2→4I15/2and 4F9/2→4I15/2 would produce ~ 520, ~ 540 and ~ 660 nm emission, separately, in which the electrons at the 4S3/2 state contributed from the non-radiative transitions 2 H11/2→4S3/2. It can be seen from the emission spectrum that there appears only ~ 540 nm and ~ 660 nm, and 660 nm is more intensive, which indicates that the above non-radiative
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t ransitions2H11/2→4S3/2 and 4I9/2→4I11/2 play a dominant role in the excitation process. As for another important energy transfer mechanism, energy transfer would happen between some appropriate energy levels. That means some Er3+ ions at the excited states can transfer their energy to the other excited ions to help them transit to further excited states, while themselves return to the ground state, just as shown in the process I, II and III of Fig. 17.
4 Conclusions A kind of Er3+ doped lead–sodium–yttrium–fluoride phosphor was synthesized by the LCS method, and orthogonal experiments were performed to determine the optimum cation molar-proportion. Dispersing agent and HSR heating process were adopted to optimize its particle morphology and luminescence properties. The determined optimum experiment conditions are as follows: Er:Y:Pb:Na mol%= 11:15:18:56 mol%, glycine amount = 1.6 stoi., (NH4)2SO4 dispersing agent amount = 9 wt%, and the HSR heating condition is 525 °C for 15 min. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 61307118, 51602027), Jilin Province Education Department Project (Grant No. JJKH20170607KJ).
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