- Email: [email protected]
Tb3+, Eu2+, Mn2+) phosphor nanofiber mat: Energy transfer, luminescence and tunable color properties
Author’s Accepted Manuscript A Single-composition CaSi 2O2N2:RE (RE=Ce3+/Tb3+, Eu2+, Mn2+) Phosphor Nanofiber Mat: Energy Transfer, Luminescence and Tunable Color Properties Bo Cui, Zhenhua Chen, Qinghong Zhang, Hongzhi Wang, Yaogang Li
PII: DOI: Reference:
www.elsevier.com/locate/yjssc
S0022-4596(17)30121-4 http://dx.doi.org/10.1016/j.jssc.2017.04.012 YJSSC19745
To appear in: Journal of Solid State Chemistry Received date: 4 January 2017 Revised date: 7 April 2017 Accepted date: 10 April 2017 Cite this article as: Bo Cui, Zhenhua Chen, Qinghong Zhang, Hongzhi Wang and Yaogang Li, A Single-composition CaSi 2O2N2:RE (RE=Ce3+/Tb3+, Eu2+ Mn2+) Phosphor Nanofiber Mat: Energy Transfer, Luminescence and Tunable Color Properties, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2017.04.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Single-composition CaSi2O2N2:RE (RE=Ce3+/Tb3+, Eu2+, Mn2+) Phosphor Nanofiber Mat: Energy Transfer, Luminescence and Tunable Color Properties Bo Cui1, Zhenhua Chen1, Qinghong Zhang1, Hongzhi Wang1* and Yaogang Li2 1
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials, Donghua University, Shanghai, 201620, People’s Republic of China, [email protected] 2 Engineering Research Center of Advanced Glasses Manufacturing Technology, MOE, Donghua University, Shanghai, 201620, People’s Republic of China ABSTRACT: CaSi2O2N2:RE (RE=Ce3+/Tb3+, Eu2+, Mn2+) phosphor nanofiber mat has been
prepared via the electrospinning process and further annealing treatment. The diameter of nanofiber precursors are generally in a range of 400~600 nm, which exhibits smooth and uniform surface morphology. After removing the polymer template by high temperature treatment, the fabricated mats have been transformed into a phosphor film and maintain uniform nanofiber network microstructures. Photoluminescence (PL) properties of CaSi2O2N2 phosphor nanofiber mat have been activated by Ce3+/Tb3+, Eu2+ and Mn2+. The emission color of CaSi2O2N2:Ce3+/Tb3+, Eu2+, Mn2+ phosphor mats can be adjusted by modifying the concentration of doped Tb3+/Mn2+/Eu2+ and the resultant energy transfer process. Owing to the energy transfer of Ce3+-Tb3+, the emitting color of the phosphor mat can be changed from blue to green. Meanwhile, we still found that the switching of emitting colors from blue to red and blue to yellow can be achieved via the energy transfer of Ce3+-Mn2+ and Ce3+-Eu2+, respectively. We demonstrated a white light emission phosphor nanofiber mat by using CaSi2O2N2 system, which exhibited promising applications for near-UV WLEDs. Keywords: A Single-composition, Phosphor Nanofiber Mat, Tunable Color Properties
Introduction White light-emitting diodes (W-LEDs) and field emission displays (FEDs) have attracted a lot of research attention for the past few years.1 Generally, the W-LEDs are obtained by combining several color phosphors.2 For different types of combined 1
yellow phosphor YAG:Ce and blue InGaN phosphor-converted (pc) W-LEDs, the applications have been limited by the low color rendering index (Ra < 80) and high color temperature (Tc > 7000 K) due to the absence of red phosphor composition.3 At the same time, the other challenge is the reabsorption among different phosphors which usually leads to poor luminous efficiency. According to these reasons, it is necessary to develop a single phosphor system which can produce white emission.4 In addition, the current available phosphor systems for FEDs could not meet the demands for some harsh operating conditions.5 Thus new phosphor system needs to be developed. The particle size, morphology and distribution of the phosphor particles are the three key factors for determining the PL properties. For example, Hu et al. reported that the oriented Gd2O3:Eu platelet film performed a much stronger emission property than the irregular Gd2O3:Eu powders.6 However, it is still a challenge to control the distribution of phosphor particles in the resin. Recently, electrospinning has been proved to be a simple and versatile method to fabricate highly oriented nanofiber film. Various types of materials, such as inorganic, organic and inorganic-organic composites have been fabricated into nanofiber films with uniform diameter, large aspect ratio and good specific surface area by electrospinning.7-10 After integrating the phosphor particles into nanofiber, the nanofiber film could be directly assembled into luminescence devices without mixing the phosphor particles into the resin. In the past few years, with the help of electrospinning technology, nanofiber mats such as YBO3:Eu3+,11 CaWO4:Tb3+ 12 have been successfully prepared and showed great potential for the applications of displays and W-LEDs. However, most of the final products are still broke into powder after high-temperature annealing treatment. Therefore, those current existing fabrication methods for electrospinning phosphor nanofiber film are still needing further improvement. In our previous research, CaSi2O2N2:Eu2+ electrospun nanofiber mat has been successfully obtained, but the photoluminescence quantum yield is still to be improved.13 In this paper, we report a method to synthesize CaSi2O2N2: RE (RE = Ce3+, Eu2+, Tb3+, Mn2+ and their combinations, in some cases) phosphor nanofiber mat by using the electrospinning and further high temperature annealing process. 2
According to the previous report, there are at least two disparate crystallographic phases in the CaSi2O2N2 host.14 We find that the Ce3+, Eu2+ and Mn2+ ions have broad band emissions at various sites in the CaSi2O2N2 host. Furthermore, the energy transfer properties from Ce3+ to Tb3+/Mn2+/ Eu2+ under UV and electron beam excitation with low-voltage are both discussed particularly in detail. The as-prepared RE modified CaSi2O2N2 nanofiber mat has extensive applications in W-LEDs due to the wide-range-tunable blue, green, reddish-orange and white emissions under UV excitation.
Experiment procedures Materials Main starting materials used in this study were Tb(NO3)3·6H2O, Ca(NO3)2·4H2O, Ce(NO3)3·6H2O, Eu(NO3)3·6H2O, MnCl2·4H2O, (the purity of all is 99.99%, from Diyang Chemical Ltd., Shanghai), tetraethyl orthosilicate Si(OC2H5)4 (TEOS, analytical reagent (A.R.) of 99wt%, from Sinopharm Group Co., Ltd.) and poly(vinylpyrrolidone) ( PVP; from Aldrich, Mw= 1 300 000). Electrospinning In a representative experiment, 1.7 mmol Ca(NO3)2, 0.085 mmol Ce(NO3)3, Tb(NO3)3, Eu(NO3)3 and MnCl2 (doping concentration of 5 mol.%) were dissolved in distilled water by magnetic stirring. Then the mixed solution consisting of 2 ml water, 8 ml ethanol and 0.36 mL of TEOS were added drop by drop to the as-prepared solution under magnetic stirring and kept for 1 h, forming into a transparent solution. Finally, a considerable volume of PVP (the weight percentage of PVP in the water/ethanol mixed solution is 6%) was added to adjust the solution’s viscoelastic behavior. A homogeneous liquid of hybrid suspension then was obtained after stirring for 6 h. The process of electrospinning was operated with the help of high voltage power supply (Model: JG50-1, Shanghai Shenfa Detection Instrument, China) with the applied voltage of approximately 10kV. A 10 mL hypodermic syringe was used to accommodate the precursor solution and the tip of syringe needle was connected with 3
the high voltage supplier, while the aluminum foil was chosen as the collector plate. The precursor solution’s flow rate was controlled accurately at 15 mL min-1 with assistance of the micro-syringe pump (Model 22, Harvard Apparatus, USA) and
the
constant distance from tip-to-collector was kept at 22 cm. The environmental temperature was kept in the range of 20~30 °C and the ambient humidity remained well below 20%. Synthesis of CaSi2O2N2: Ce3+, Tb3+, Eu2+, Mn2+ The as-prepared fibrous membranes were placed in vacuum oven at 50°C and dried for 24 h. In order to wipe off the organic ingredients, the fibrous membranes were annealed at 500 °C in air for 4 h. Then the temperature was set up to adapt the nitridation with flowing in pure NH3 (300 mL min-1) and maintained for 1~5 h. Finally, the sample was cooled down in the NH3 atmosphere to room temperature. The heating rate before 500 °C was 2 °C min-1, and then was changed to 1 °C min-1 after 500 °C. Characterization X-ray diffraction (XRD) measurements were carried out on the Rigaku-D/max 2550 PC diffractometer equipped with Cu-Kα radiation (λ= 0.15405 nm). The XRD data was collected with the 2θ range of 10~80°. The step size was set as 0.02 and the count time should be 1 s per step. The micro morphologies of the phosphor nanofiber mats were observed by using a FESEM (Field-Emission Scanning Electron Microscope) (Hitachi S4800). With the method of coating red/green/blue phosphors in silicon on top of the InGaN NUV LED chip (365 nm), the pc-LEDs were assembled and it could be used as the primary light. The efficiency and the photoluminescence (PL) spectra were measured under room temperature by using a fluorescent spectrophotometer (JASCO FP-6600) equipped with an integrating sphere.
Result and discussion Morphology of fiber precursor The preparation of the CaSi2O2N2: Ce3+/Tb3+, Eu2+, Mn2+ nanofiber mats contains 4
three primary steps, the preparation of nanofiber precursor, following removal of the organic compounds and the ultimate nitridation. After the process of electrospinning, the nanofiber shape has already formed with uniform diameter within a range of 400~600 nm, as shown in the Fig. 1. Although the nanofiber has part taken shape of the previous fiber form under ambient conditions on account of the condensation of TEOS precursor nanofiber and the loss continuously in the hydrolysis, it absorbed moisture as a catalyst yet from the air. Thus in this work, scanning electron microscope (SEM) observation was taken immediately after the formation of the present fiber precursor by electrospinning.
Fig. 1 SEM images of (a) as-formed nanofiber precursor and (b) with high magnification.
The pretreatment and nitridation of the precursor High annealing temperature was applied to remove the PVP templates. Then pure inorganic fibers were obtained after heat treatment. SEM measurement was taken to study the influence of temperature on the morphology of as-prepared inorganic fibrous film. The uniform structure of nanofiber mats is tending to be destroyed during violent high-temperature treatment. Consequently, in order to maintain the smooth and uniform morphology of the nanofiber mats, it is necessary to set the heating speed into a very slow rate and maintain the high-temperature reaction for a specific time at 500°C. Fig. 2 shows SEM images of the samples after removing the organic templates. The inorganic nanofiber mat retains the nanofiber morphology very well and the diameters of fiber decrease to 150~200 nm due to the loss of the organic components. 5
Fig. 2 SEM of the samples after removing the organic template (a) and with high magnification (b).
After the process of nitridation at 1300°C, the nanofiber mat can still remain flat and smooth (Fig. 3c). Furthermore, the nanofiber mat has enough strength to afford the packaging operation. In the SEM images (Fig. 3a, b), it can be found that CaSi2O2N2 nanofibers are composed of nanocrystallines with diameter of 150~200 nm. Some regions of the nanofibers shrunk when treated with niridation, manifesting that reduction nitridation process in the present system can obviously proceed through solid-state reactions assisted with liquid.
Fig. 3 SEM images of as-prepared CaSi2O2N2 nanofiber mat (a) with high magnification (b), and photograph of an as-prepared CaSi2O2N2 nanofiber mat slice(c).
Formation of CaSi2O2N2 nanofibers XRD patterns of the samples after the nitridation treatment are showed in Fig. 4. The XRD data corroborated that it is sufficient to get nanofiber mat with an almost pure phase of CaSi2O2N2:0.02Ce3+, 0.04Tb3+ after a short annealing time of 1 h at 1300°C. Without proper reaction temperature or enough annealing time, the XRD 6
data showed the peaks of unknown by-products. Meanwhile, the medium products could be removed after high temperature nitridation with an extreme low heating rate. This nanofiber fabrication approach with precursor mediator can be completed at 1300 °C with nitridation time of 1 h, while the common synthetic routes usually require reaction temperature of 1400~1600°C and reaction time of over 10 h.15,16
Fig. 4 X-ray diffraction patterns of the nanofiber mat synthesized at various nitridation conditions.
Fig.5 shows that rietveld method has been applied to refine XRD data by using Fullprof program. The refinement results show the values of Rp and Rwp are limited. It proves that relatively high phase purity can be obtained at the 1300 °C.
Fig.5 Observed and calculated synchrotron XRD profiles and their difference for the Rietveld refinement 7
Luminescence of CaSi2O2N2 nanofiber mat
Fig. 6 The PL and PLE spectra of nanofiber mats of CaSi2O2N2:0.02Ce3+, CaSi2O2N2:0.01Tb3+ (a) and CaSi2O2N2:0.02Ce3+, 0.04Tb3+ (b).
The PL and PLE spectra of CaSi2O2N2:0.02Ce3+ and CaSi2O2N2: 0.01Tb3+ are shown in Fig. 6(a). The excitation spectrum of CaSi2O2N2: 0.01Tb3+ composes of two parts. One contains a broad band in the range of 200~350 nm centered at 240 nm, generated by the spin-allowed transition from 4f to 5d of Tb3+. Another one contains a series of weak narrow bands ranging from 350 to 450 nm, which is mainly related to the characteristic f-f transitions of Tb3+.
17
It has been reported that Tb3+ f-f emission
was observed from both the 5D3 and the 5D4 level18, while a spectral overlap with the Tb3+ f-f emission from the 5D3 and the Ce3+ emission is observed at the range from 370 to 470 nm in current work. The main emission peaks of 488, 542, 584 and 622 nm are distributed to 5D4→7FJ transitions and the main emission peaks of 382, 417, 439 and 463 nm are distributed to 5D3→7FJ under 240 nm excitation. The PLE and PL spectra of CaSi2O2N2: 0.02Ce3+, 0.04Tb3+ are shown in Fig. 6(b). It is noted that PLE spectra of the Tb3+ monitoring 544 nm and the Ce3+ monitoring 440 nm exhibit similar emission behavior, which proves the effect of energy transfer from Ce3+ to Tb3+. Meanwhile, following the decrease of Ce3+ emission intensity, the emission intensity of Tb3+ is substantially increased, further demonstrating the fact of energy transfer from Ce3+ to Tb3+. As demonstrated in Fig. 7, the energy transfer from Ce3+ to Tb3+ is initiated with 8
the excitation of Ce3+ ions from the ground state (2F7/2 and 2F5/2) to the 5d level under UV radiation at 330 nm. Then the non-radiative transition leads to the relaxation transition of excited Ce3+ ions to the underlying excited level 2D3/2. Afterwards, some Ce3+ ions jump back to the ground state through de-excitation process, accompanied by fluorescence-emissions at 430 nm. Meanwhile, part of excitation energy related to the transition process of 2D3/2 to ground state of the Ce3+ ions is transferred to the excited Tb3+ ions at level 5D3 and 5D4. Subsequently, the Tb3+ ions decay down to various levels at 7FJ (J=6, 5, 4, 3) through radiative transition, with emissions at 489 nm, 543 nm, 584 nm and 622 nm, respectively.
Fig.7 Schematic diagram of energy transfer process from Ce3+to Tb3+.
Fig. 8 The excitation (a) and emission (b) spectra for CaSi2O2N2:0.02Ce3+, zTb3+ nanofiber mats.
Fig. 8 shows the PL spectra of CaSi2O2N2:0.02Ce3+, zTb3+ (z = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) under UV excitation at 330 nm. The characteristic peaks of Tb3+ 9
and Ce3+ are exhibitted in the emission spectra with different Tb3+ concentrations. Due to the previously mentioned energy transition processes, the emission intensity of Ce3+ ions notably decreases while the amount of Tb3+ ions increases. Meanwhile, the increase of doped Tb3+ concentration from 0.01 to 0.03 M leads to the raise of emission intensity and the drop of Tb3+ ions due to the concentration quenching. Fig. 9 (a) and (b) show the excitation and emission spectra of Ce3+ and Ce3+, Mn2+ co-doped nanofiber mat phosphors. We don’t show the excitation and emission spectra of Mn2+ solely doped CaSi2O2N2 (without Ce). There is no obvious the excitation and emission spectra. A broad band from 350 to 600 nm is observed in the PL spectrum of CaSi2O2N2: 0.02Ce3+, which is due to the radiative transition from 5d level to ground state of the Ce3+ ions. Generated by 4f-5d transition of the Ce3+ ions, a broad absorption band ranging from 200 to 350 nm is presented in the excitation spectrum monitoring 440 nm. Fig. 9 (b) shows the PLE and PL spectra of CaSi2O2N2:0.02Ce3+, 0.2Mn2+. It’s noted that the PLE spectra of Mn2+ monitoring the red emission and Ce3+ monitoring the blue emission exhibit similar performance, proving the effect of energy transfer from Ce3+ to Mn2+ in CaSi2O2N2 systems.
Fig. 9 The excitation and emission spectra of nanofiber mats of (a) CaSi2O2N2:0.02Ce3+ and (b) CaSi2O2N2:0.02Ce3+, 0.2Mn2+.
10
Fig. 10 Schematic diagram of energy transfer process from Ce3+ to Mn2+.
As illustrated in Fig. 10, electrons of Ce3+ in ground state (2F5/2) are excited to the 5d excited state under blue excitation. Some excited electrons go back to the ground states (2F7/2 and 2F5/2) through radiative transfer resulting in yellow emission. At the same time, due to their similar energy levels, the 5d excited states of Ce3+ ions transfer to the 4T1 level of Mn2+ ions via non-radiative transfer, which results in the emission of red light generated by the 4T1 → 6A1 transitions. The emission spectra of Ce3+, Mn2+ co-doped CaSi2O2N2 further prove the effect of energy transfer from Ce3+ to Mn2+.
Fig. 11 The excitation (a) and emission (b) spectra of CaSi2O2N2: 0.02Ce3+, yMn2+ nanofiber mat.
Fig. 11 shows the normalized emission spectra of CaSi2O2N2 phosphors co-doped with Ce3+, Mn2+ having different Mn2+ concentrations (y = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 0.24). With the increasing of Mn2+ contents, the PL intensity of 11
Mn2+ at 630 nm is enhanced step by step, further demonstrating the impact of ETCe-Mn mechanism. Moreover, the red shift in Mn2+ emission is realized by increasing the Mn2+ concentration which is possibly related to the enhanced crystal field resulting from the augment in Mn2+ doping and shortened Mn-O distance.
Fig. 12 Excitation and emission spectra of (a) CaSi2O2N2 0.02Ce3+ and (b) CaSi2O2N2 0.05Eu2+ phosphor nanofiber mat, (c) the spectral overlap between excitation spectrum of CaSi2O2N2 0.05Eu2+ and emission spectrum of CaSi2O2N2 0.02Ce3+.
Fig. 12(a) shows the excitation (black line) and emission spectra (blue line) of Ce3+-doped CaSi2O2N2 phosphor. A broad excitation band can be seen in the range of 300~400 nm and the maximum intensity peak is located at 360 nm. Fig. 12(b) shows the excitation (black line) and emission spectra (orange line) of Eu2+-doped CaSi2O2N2 phosphor. The excitation spectrum (300~500 nm) exhibits similar broad bands, which is attributed to the host absorption. The transition is generated between the 5d level of crystal-field components and the ground state of Eu2+ ions. The normalized excitation spectrum of Eu2+-doped CaSi2O2N2 (black line) and the emission spectrum of Ce3+-doped CaSi2O2N2 (blue line) are illustrated in Fig. 12(c). An obvious spectral overlap is observed at the range from 400 to 500 nm, indicating the energy transfer is possible from Ce3+ to Eu2+ ions in CaSi2O2N2.
12
Fig. 13 The excitation (a) and emission (b) spectra of CaSi2O2N2:0.02Ce3+ yEu2+ and CaSi2O2N2:0.05Eu2+ phosphor nanofiber mat.
Fig. 13 shows the emission spectra of CaSi2O2N2:0.05Eu2+ phosphors and CaSi2O2N2:0.02Ce3+yEu2+ phosphors with different Eu2+ concentrations (y=0~0.02). It can be seen that the emission peaks of Ce3+ (around 450 nm) and Eu2+ (around 550 nm) merges with a certain extent, as Ce3+ and Eu2+ (y=0.002) co-doped into the nanofiber mats. The Eu2+ emission observed in the co-doped samples are caused by the direct excitation of the Eu2+ and the Ce to Eu energy transfer. With further incorporating of the Eu2+ ions, emission intensity of Ce3+ decreases significantly accompanied by the increasing of Eu2+ emission which is mainly attributed to the energy transfer from Ce3+ to Eu2+ ions.
Fig. 14 (a) CIE chromaticity coordinates of CaSi2O2N2:Ce3+/Tb3+, Eu2+, Mn2+ samples. The lighting optical images with a drive current of 100 mA: three LEDs with nanofiber mats of CaSi2O2N2: 0.02Ce3+, 0.03Tb3+ (b), CaSi2O2N2: 0.02Ce3+, 0.01Eu2+ (c) and CaSi2O2N2: 0.02Ce3+, 0.12Mn2+ (d).
We summarized the CIE chromaticity coordinates of the phosphor nanofiber 13
mats. As shown in Fig. 14(a), the changes in color can be obtained by altering the types of doped ion. The linear variation of chromaticity coordinates have already been realized by UV LED chip with Ca2Si2O2N2 nanofiber mat by different doping ions. Three lighted LEDs integrated with as-prepared different ions doped Ca2Si2O2N2 nanofiber mat are shown in Fig. 14(b)-(d), which proves the previous conclusion and the success of the fabrication of these phosphor nanofiber mats. The chromaticity coordinates of three LED lamps exhibiting correlated color temperatures (CCTs) of 6666, 3740 and 1500K under a drive Current of 100 mA could be obtained, marked in CIE1931 with values of (0.2948, 0.4031), (0.4388, 0.495), and (0.4357, 0.2290) (Table 1). Table 1 Electroluminescence properties of LED lamps using CaSi2O2N2 nanofiber mats with different doped ions
Correlated color Sample
CIE value (x, y) temperature (K)
CaSi2O2N2: 0.02Ce3+, 0.03Tb3+
6666
(0.2948, 0.4031)
CaSi2O2N2: 0.02Ce3+, 0.01Eu2+
3740
(0.4388, 0.4954)
CaSi2O2N2: 0.02Ce3+, 0.12Mn2+
1500
(0.4357, 0.2290)
Conclusion In conclusion, phosphor nanofiber mats consisting of CaSi2O2N2: 0.02Ce3+/Tb3+, Eu2+, Mn2+ have been successfully fabricated by electrospinning procedure and subsequent high-temperature nitridation treatment. The as-prepared nanofiber precursors exhibit smooth, uniform morphology and tunable diameter (400~600 nm). CaSi2O2N2 nanofiber could be well crystallized and retain its uniform nanofiber type morphology after high-temperature treatment. Energy transfer and photoluminescence in CaSi2O2N2:Ce3+/ Tb3+, Eu2+, Mn2+ have been discussed as a function of Tb3+, Mn2+, Eu2+ concentrations. In CaSi2O2N2:Ce3+/Tb3+, Mn2+ phosphor system, the energy transfer of Ce3+ to Tb3+ or Mn2+ can be excited through near UV-LED. Accordingly, the emitting colors of the phosphor nanofiber mat have been changed very well from 14
the blue to green or red under UV excitation. When Ce3+ and Eu2+ were co-doped in CaSi2O2N2, the emission spectra exhibited a combination of emission bands of Ce3+ and Eu2+. With the raise of Eu2+ concentration, the emission intensity of Eu2+ was increased, whereas that of Ce3+ simultaneously decreased monotonically. The emitting colors of the as-prepared phosphor nanofiber mat were well controlled from the blue to yellow under UV excitation.
Acknowledgements We gratefully acknowledge the financial support by Natural Science Foundation of China (No. 51572046), the Shanghai Natural Science Foundation (15ZR1401200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Program of Shanghai Academic Research Leader (16XD1400100), Science and Technology Commission of Shanghai Municipality (16JC1400700), the Program of Introducing Talents of Discipline to Universities (No.111-2-04) and the Fundamental Research Funds for the Central Universities(2232014A3-06).
References 1.
C. Liu, S. Zhang, Z. Liu, H. Liang, S. Sun, Y. Tao, J. Mater. Chem. C, 1 (2013) 1305-1308.
2.
S. Su, W. Liu, R. Duan, L. Cao, G. Su, C. Zhao, J. Alloys compd. 575 (2013) 309-313.
3.
X. Li, J. D. Budai, F. Liu, J. Y. Howe, X. J. Wang, Z. Gu, R. S. Meltzer, Z. Pan, Light: Sci. Appl. 2 (2013) e50.
4.
M. S. Kim, L. K. Bharat, J. S. Yu, J. Lumin. 142 (2013) 92-95.
5.
M. Shang, D. Geng, D. Yang, X. Kang, Y. Zhang, Inorg. Chem. 52 (2013) 3102-3112.
6.
L. F. Hu, R. Z. Ma, T. C. Ozawa, T. Sasaki, Angew. Chem. Int. Ed. 48 (2009) 3846-3849.
7.
Y. Q. Liu, X. P. Zhang, Y. N. Xia, H. Yang, Adv. Mater. 22 (2010) 2454-2457.
8.
H. Wu, Y. Sun, D. D. Lin, R. Zhang, C. Zhang, W. Pan, Adv. Mater. 21 (2009) 227-231.
9.
S. Agarwal, A. J. Greiner, H. Wendorff, Adv. Funct. Mater. 19 (2009) 2863-2879.
10. A. Greiner, J. H. Wendorff, Angew. Chem. Int. Ed. 46 (2007) 5670-5703. 11. H. W. Song, H. Q. Yu, G. H. Pan, X. Bai, B. Dong, X. T. Zhang, S. K. Hark, Chem. Mater. 20 (2008) 4762-2767. 15
12. Z. Y. Hou, C. X. Li, J. Yang, H. Z. Lian, P. P. Yang, R. T. Chai, Z. Y. Cheng, J. Lin, J. Mater. Chem. 19 (2009) 2737-2746. 13. Y. X. Gu, Q. H. Zhang, H. Z. Wang, Y. G. Li, J. Mater. Chem. 21 (2011) 17790-17797 14. H. H. Chia, M. C. Bing, H. L. Chung, J. Am. Ceram. Soc. 94 (2011) 2878-2883. 15. Y. X. Gu, Q. H. Zhang, Y. G. Li, H. Z. Wang, R. J. Xie, Mater. Lett. 63 (2009) 1448-1450. 16. H. A. Höppe, F. Stadler, O. Oeckler, W. Schnick, Angew. Chem. Int. Ed. 43 (2004) 5540-5542. 17. L.J. Nugent, R.D. Baybarz, J.L. Burnett, J.L. Ryan, J. Phys. Chem. 77 (1973) 1528-1539. 18. L. X. Yang, X. Xu, L. Y. Hao, X. F. Yang, J. Y. Tang, R. J. Xie. Opt. Mater. 33 (2011) 1695-1699.
CIE chromaticity coordinates of CaSi2O2N2:Ce3+/Tb3+, Eu2+, Mn2+ samples. The lighting optical images with a drive current of 100 mA: three LEDs with nanofiber mats of CaSi2O2N2: 0.02Ce3+, 0.03Tb3+ (b), CaSi2O2N2: 0.02Ce3+, 0.01Eu2+ (c) and CaSi2O2N2: 0.02Ce3+, 0.12Mn2+ (d).
16
PII: DOI: Reference:
www.elsevier.com/locate/yjssc
S0022-4596(17)30121-4 http://dx.doi.org/10.1016/j.jssc.2017.04.012 YJSSC19745
To appear in: Journal of Solid State Chemistry Received date: 4 January 2017 Revised date: 7 April 2017 Accepted date: 10 April 2017 Cite this article as: Bo Cui, Zhenhua Chen, Qinghong Zhang, Hongzhi Wang and Yaogang Li, A Single-composition CaSi 2O2N2:RE (RE=Ce3+/Tb3+, Eu2+ Mn2+) Phosphor Nanofiber Mat: Energy Transfer, Luminescence and Tunable Color Properties, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2017.04.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Single-composition CaSi2O2N2:RE (RE=Ce3+/Tb3+, Eu2+, Mn2+) Phosphor Nanofiber Mat: Energy Transfer, Luminescence and Tunable Color Properties Bo Cui1, Zhenhua Chen1, Qinghong Zhang1, Hongzhi Wang1* and Yaogang Li2 1
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials, Donghua University, Shanghai, 201620, People’s Republic of China, [email protected] 2 Engineering Research Center of Advanced Glasses Manufacturing Technology, MOE, Donghua University, Shanghai, 201620, People’s Republic of China ABSTRACT: CaSi2O2N2:RE (RE=Ce3+/Tb3+, Eu2+, Mn2+) phosphor nanofiber mat has been
prepared via the electrospinning process and further annealing treatment. The diameter of nanofiber precursors are generally in a range of 400~600 nm, which exhibits smooth and uniform surface morphology. After removing the polymer template by high temperature treatment, the fabricated mats have been transformed into a phosphor film and maintain uniform nanofiber network microstructures. Photoluminescence (PL) properties of CaSi2O2N2 phosphor nanofiber mat have been activated by Ce3+/Tb3+, Eu2+ and Mn2+. The emission color of CaSi2O2N2:Ce3+/Tb3+, Eu2+, Mn2+ phosphor mats can be adjusted by modifying the concentration of doped Tb3+/Mn2+/Eu2+ and the resultant energy transfer process. Owing to the energy transfer of Ce3+-Tb3+, the emitting color of the phosphor mat can be changed from blue to green. Meanwhile, we still found that the switching of emitting colors from blue to red and blue to yellow can be achieved via the energy transfer of Ce3+-Mn2+ and Ce3+-Eu2+, respectively. We demonstrated a white light emission phosphor nanofiber mat by using CaSi2O2N2 system, which exhibited promising applications for near-UV WLEDs. Keywords: A Single-composition, Phosphor Nanofiber Mat, Tunable Color Properties
Introduction White light-emitting diodes (W-LEDs) and field emission displays (FEDs) have attracted a lot of research attention for the past few years.1 Generally, the W-LEDs are obtained by combining several color phosphors.2 For different types of combined 1
yellow phosphor YAG:Ce and blue InGaN phosphor-converted (pc) W-LEDs, the applications have been limited by the low color rendering index (Ra < 80) and high color temperature (Tc > 7000 K) due to the absence of red phosphor composition.3 At the same time, the other challenge is the reabsorption among different phosphors which usually leads to poor luminous efficiency. According to these reasons, it is necessary to develop a single phosphor system which can produce white emission.4 In addition, the current available phosphor systems for FEDs could not meet the demands for some harsh operating conditions.5 Thus new phosphor system needs to be developed. The particle size, morphology and distribution of the phosphor particles are the three key factors for determining the PL properties. For example, Hu et al. reported that the oriented Gd2O3:Eu platelet film performed a much stronger emission property than the irregular Gd2O3:Eu powders.6 However, it is still a challenge to control the distribution of phosphor particles in the resin. Recently, electrospinning has been proved to be a simple and versatile method to fabricate highly oriented nanofiber film. Various types of materials, such as inorganic, organic and inorganic-organic composites have been fabricated into nanofiber films with uniform diameter, large aspect ratio and good specific surface area by electrospinning.7-10 After integrating the phosphor particles into nanofiber, the nanofiber film could be directly assembled into luminescence devices without mixing the phosphor particles into the resin. In the past few years, with the help of electrospinning technology, nanofiber mats such as YBO3:Eu3+,11 CaWO4:Tb3+ 12 have been successfully prepared and showed great potential for the applications of displays and W-LEDs. However, most of the final products are still broke into powder after high-temperature annealing treatment. Therefore, those current existing fabrication methods for electrospinning phosphor nanofiber film are still needing further improvement. In our previous research, CaSi2O2N2:Eu2+ electrospun nanofiber mat has been successfully obtained, but the photoluminescence quantum yield is still to be improved.13 In this paper, we report a method to synthesize CaSi2O2N2: RE (RE = Ce3+, Eu2+, Tb3+, Mn2+ and their combinations, in some cases) phosphor nanofiber mat by using the electrospinning and further high temperature annealing process. 2
According to the previous report, there are at least two disparate crystallographic phases in the CaSi2O2N2 host.14 We find that the Ce3+, Eu2+ and Mn2+ ions have broad band emissions at various sites in the CaSi2O2N2 host. Furthermore, the energy transfer properties from Ce3+ to Tb3+/Mn2+/ Eu2+ under UV and electron beam excitation with low-voltage are both discussed particularly in detail. The as-prepared RE modified CaSi2O2N2 nanofiber mat has extensive applications in W-LEDs due to the wide-range-tunable blue, green, reddish-orange and white emissions under UV excitation.
Experiment procedures Materials Main starting materials used in this study were Tb(NO3)3·6H2O, Ca(NO3)2·4H2O, Ce(NO3)3·6H2O, Eu(NO3)3·6H2O, MnCl2·4H2O, (the purity of all is 99.99%, from Diyang Chemical Ltd., Shanghai), tetraethyl orthosilicate Si(OC2H5)4 (TEOS, analytical reagent (A.R.) of 99wt%, from Sinopharm Group Co., Ltd.) and poly(vinylpyrrolidone) ( PVP; from Aldrich, Mw= 1 300 000). Electrospinning In a representative experiment, 1.7 mmol Ca(NO3)2, 0.085 mmol Ce(NO3)3, Tb(NO3)3, Eu(NO3)3 and MnCl2 (doping concentration of 5 mol.%) were dissolved in distilled water by magnetic stirring. Then the mixed solution consisting of 2 ml water, 8 ml ethanol and 0.36 mL of TEOS were added drop by drop to the as-prepared solution under magnetic stirring and kept for 1 h, forming into a transparent solution. Finally, a considerable volume of PVP (the weight percentage of PVP in the water/ethanol mixed solution is 6%) was added to adjust the solution’s viscoelastic behavior. A homogeneous liquid of hybrid suspension then was obtained after stirring for 6 h. The process of electrospinning was operated with the help of high voltage power supply (Model: JG50-1, Shanghai Shenfa Detection Instrument, China) with the applied voltage of approximately 10kV. A 10 mL hypodermic syringe was used to accommodate the precursor solution and the tip of syringe needle was connected with 3
the high voltage supplier, while the aluminum foil was chosen as the collector plate. The precursor solution’s flow rate was controlled accurately at 15 mL min-1 with assistance of the micro-syringe pump (Model 22, Harvard Apparatus, USA) and
the
constant distance from tip-to-collector was kept at 22 cm. The environmental temperature was kept in the range of 20~30 °C and the ambient humidity remained well below 20%. Synthesis of CaSi2O2N2: Ce3+, Tb3+, Eu2+, Mn2+ The as-prepared fibrous membranes were placed in vacuum oven at 50°C and dried for 24 h. In order to wipe off the organic ingredients, the fibrous membranes were annealed at 500 °C in air for 4 h. Then the temperature was set up to adapt the nitridation with flowing in pure NH3 (300 mL min-1) and maintained for 1~5 h. Finally, the sample was cooled down in the NH3 atmosphere to room temperature. The heating rate before 500 °C was 2 °C min-1, and then was changed to 1 °C min-1 after 500 °C. Characterization X-ray diffraction (XRD) measurements were carried out on the Rigaku-D/max 2550 PC diffractometer equipped with Cu-Kα radiation (λ= 0.15405 nm). The XRD data was collected with the 2θ range of 10~80°. The step size was set as 0.02 and the count time should be 1 s per step. The micro morphologies of the phosphor nanofiber mats were observed by using a FESEM (Field-Emission Scanning Electron Microscope) (Hitachi S4800). With the method of coating red/green/blue phosphors in silicon on top of the InGaN NUV LED chip (365 nm), the pc-LEDs were assembled and it could be used as the primary light. The efficiency and the photoluminescence (PL) spectra were measured under room temperature by using a fluorescent spectrophotometer (JASCO FP-6600) equipped with an integrating sphere.
Result and discussion Morphology of fiber precursor The preparation of the CaSi2O2N2: Ce3+/Tb3+, Eu2+, Mn2+ nanofiber mats contains 4
three primary steps, the preparation of nanofiber precursor, following removal of the organic compounds and the ultimate nitridation. After the process of electrospinning, the nanofiber shape has already formed with uniform diameter within a range of 400~600 nm, as shown in the Fig. 1. Although the nanofiber has part taken shape of the previous fiber form under ambient conditions on account of the condensation of TEOS precursor nanofiber and the loss continuously in the hydrolysis, it absorbed moisture as a catalyst yet from the air. Thus in this work, scanning electron microscope (SEM) observation was taken immediately after the formation of the present fiber precursor by electrospinning.
Fig. 1 SEM images of (a) as-formed nanofiber precursor and (b) with high magnification.
The pretreatment and nitridation of the precursor High annealing temperature was applied to remove the PVP templates. Then pure inorganic fibers were obtained after heat treatment. SEM measurement was taken to study the influence of temperature on the morphology of as-prepared inorganic fibrous film. The uniform structure of nanofiber mats is tending to be destroyed during violent high-temperature treatment. Consequently, in order to maintain the smooth and uniform morphology of the nanofiber mats, it is necessary to set the heating speed into a very slow rate and maintain the high-temperature reaction for a specific time at 500°C. Fig. 2 shows SEM images of the samples after removing the organic templates. The inorganic nanofiber mat retains the nanofiber morphology very well and the diameters of fiber decrease to 150~200 nm due to the loss of the organic components. 5
Fig. 2 SEM of the samples after removing the organic template (a) and with high magnification (b).
After the process of nitridation at 1300°C, the nanofiber mat can still remain flat and smooth (Fig. 3c). Furthermore, the nanofiber mat has enough strength to afford the packaging operation. In the SEM images (Fig. 3a, b), it can be found that CaSi2O2N2 nanofibers are composed of nanocrystallines with diameter of 150~200 nm. Some regions of the nanofibers shrunk when treated with niridation, manifesting that reduction nitridation process in the present system can obviously proceed through solid-state reactions assisted with liquid.
Fig. 3 SEM images of as-prepared CaSi2O2N2 nanofiber mat (a) with high magnification (b), and photograph of an as-prepared CaSi2O2N2 nanofiber mat slice(c).
Formation of CaSi2O2N2 nanofibers XRD patterns of the samples after the nitridation treatment are showed in Fig. 4. The XRD data corroborated that it is sufficient to get nanofiber mat with an almost pure phase of CaSi2O2N2:0.02Ce3+, 0.04Tb3+ after a short annealing time of 1 h at 1300°C. Without proper reaction temperature or enough annealing time, the XRD 6
data showed the peaks of unknown by-products. Meanwhile, the medium products could be removed after high temperature nitridation with an extreme low heating rate. This nanofiber fabrication approach with precursor mediator can be completed at 1300 °C with nitridation time of 1 h, while the common synthetic routes usually require reaction temperature of 1400~1600°C and reaction time of over 10 h.15,16
Fig. 4 X-ray diffraction patterns of the nanofiber mat synthesized at various nitridation conditions.
Fig.5 shows that rietveld method has been applied to refine XRD data by using Fullprof program. The refinement results show the values of Rp and Rwp are limited. It proves that relatively high phase purity can be obtained at the 1300 °C.
Fig.5 Observed and calculated synchrotron XRD profiles and their difference for the Rietveld refinement 7
Luminescence of CaSi2O2N2 nanofiber mat
Fig. 6 The PL and PLE spectra of nanofiber mats of CaSi2O2N2:0.02Ce3+, CaSi2O2N2:0.01Tb3+ (a) and CaSi2O2N2:0.02Ce3+, 0.04Tb3+ (b).
The PL and PLE spectra of CaSi2O2N2:0.02Ce3+ and CaSi2O2N2: 0.01Tb3+ are shown in Fig. 6(a). The excitation spectrum of CaSi2O2N2: 0.01Tb3+ composes of two parts. One contains a broad band in the range of 200~350 nm centered at 240 nm, generated by the spin-allowed transition from 4f to 5d of Tb3+. Another one contains a series of weak narrow bands ranging from 350 to 450 nm, which is mainly related to the characteristic f-f transitions of Tb3+.
17
It has been reported that Tb3+ f-f emission
was observed from both the 5D3 and the 5D4 level18, while a spectral overlap with the Tb3+ f-f emission from the 5D3 and the Ce3+ emission is observed at the range from 370 to 470 nm in current work. The main emission peaks of 488, 542, 584 and 622 nm are distributed to 5D4→7FJ transitions and the main emission peaks of 382, 417, 439 and 463 nm are distributed to 5D3→7FJ under 240 nm excitation. The PLE and PL spectra of CaSi2O2N2: 0.02Ce3+, 0.04Tb3+ are shown in Fig. 6(b). It is noted that PLE spectra of the Tb3+ monitoring 544 nm and the Ce3+ monitoring 440 nm exhibit similar emission behavior, which proves the effect of energy transfer from Ce3+ to Tb3+. Meanwhile, following the decrease of Ce3+ emission intensity, the emission intensity of Tb3+ is substantially increased, further demonstrating the fact of energy transfer from Ce3+ to Tb3+. As demonstrated in Fig. 7, the energy transfer from Ce3+ to Tb3+ is initiated with 8
the excitation of Ce3+ ions from the ground state (2F7/2 and 2F5/2) to the 5d level under UV radiation at 330 nm. Then the non-radiative transition leads to the relaxation transition of excited Ce3+ ions to the underlying excited level 2D3/2. Afterwards, some Ce3+ ions jump back to the ground state through de-excitation process, accompanied by fluorescence-emissions at 430 nm. Meanwhile, part of excitation energy related to the transition process of 2D3/2 to ground state of the Ce3+ ions is transferred to the excited Tb3+ ions at level 5D3 and 5D4. Subsequently, the Tb3+ ions decay down to various levels at 7FJ (J=6, 5, 4, 3) through radiative transition, with emissions at 489 nm, 543 nm, 584 nm and 622 nm, respectively.
Fig.7 Schematic diagram of energy transfer process from Ce3+to Tb3+.
Fig. 8 The excitation (a) and emission (b) spectra for CaSi2O2N2:0.02Ce3+, zTb3+ nanofiber mats.
Fig. 8 shows the PL spectra of CaSi2O2N2:0.02Ce3+, zTb3+ (z = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) under UV excitation at 330 nm. The characteristic peaks of Tb3+ 9
and Ce3+ are exhibitted in the emission spectra with different Tb3+ concentrations. Due to the previously mentioned energy transition processes, the emission intensity of Ce3+ ions notably decreases while the amount of Tb3+ ions increases. Meanwhile, the increase of doped Tb3+ concentration from 0.01 to 0.03 M leads to the raise of emission intensity and the drop of Tb3+ ions due to the concentration quenching. Fig. 9 (a) and (b) show the excitation and emission spectra of Ce3+ and Ce3+, Mn2+ co-doped nanofiber mat phosphors. We don’t show the excitation and emission spectra of Mn2+ solely doped CaSi2O2N2 (without Ce). There is no obvious the excitation and emission spectra. A broad band from 350 to 600 nm is observed in the PL spectrum of CaSi2O2N2: 0.02Ce3+, which is due to the radiative transition from 5d level to ground state of the Ce3+ ions. Generated by 4f-5d transition of the Ce3+ ions, a broad absorption band ranging from 200 to 350 nm is presented in the excitation spectrum monitoring 440 nm. Fig. 9 (b) shows the PLE and PL spectra of CaSi2O2N2:0.02Ce3+, 0.2Mn2+. It’s noted that the PLE spectra of Mn2+ monitoring the red emission and Ce3+ monitoring the blue emission exhibit similar performance, proving the effect of energy transfer from Ce3+ to Mn2+ in CaSi2O2N2 systems.
Fig. 9 The excitation and emission spectra of nanofiber mats of (a) CaSi2O2N2:0.02Ce3+ and (b) CaSi2O2N2:0.02Ce3+, 0.2Mn2+.
10
Fig. 10 Schematic diagram of energy transfer process from Ce3+ to Mn2+.
As illustrated in Fig. 10, electrons of Ce3+ in ground state (2F5/2) are excited to the 5d excited state under blue excitation. Some excited electrons go back to the ground states (2F7/2 and 2F5/2) through radiative transfer resulting in yellow emission. At the same time, due to their similar energy levels, the 5d excited states of Ce3+ ions transfer to the 4T1 level of Mn2+ ions via non-radiative transfer, which results in the emission of red light generated by the 4T1 → 6A1 transitions. The emission spectra of Ce3+, Mn2+ co-doped CaSi2O2N2 further prove the effect of energy transfer from Ce3+ to Mn2+.
Fig. 11 The excitation (a) and emission (b) spectra of CaSi2O2N2: 0.02Ce3+, yMn2+ nanofiber mat.
Fig. 11 shows the normalized emission spectra of CaSi2O2N2 phosphors co-doped with Ce3+, Mn2+ having different Mn2+ concentrations (y = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 0.24). With the increasing of Mn2+ contents, the PL intensity of 11
Mn2+ at 630 nm is enhanced step by step, further demonstrating the impact of ETCe-Mn mechanism. Moreover, the red shift in Mn2+ emission is realized by increasing the Mn2+ concentration which is possibly related to the enhanced crystal field resulting from the augment in Mn2+ doping and shortened Mn-O distance.
Fig. 12 Excitation and emission spectra of (a) CaSi2O2N2 0.02Ce3+ and (b) CaSi2O2N2 0.05Eu2+ phosphor nanofiber mat, (c) the spectral overlap between excitation spectrum of CaSi2O2N2 0.05Eu2+ and emission spectrum of CaSi2O2N2 0.02Ce3+.
Fig. 12(a) shows the excitation (black line) and emission spectra (blue line) of Ce3+-doped CaSi2O2N2 phosphor. A broad excitation band can be seen in the range of 300~400 nm and the maximum intensity peak is located at 360 nm. Fig. 12(b) shows the excitation (black line) and emission spectra (orange line) of Eu2+-doped CaSi2O2N2 phosphor. The excitation spectrum (300~500 nm) exhibits similar broad bands, which is attributed to the host absorption. The transition is generated between the 5d level of crystal-field components and the ground state of Eu2+ ions. The normalized excitation spectrum of Eu2+-doped CaSi2O2N2 (black line) and the emission spectrum of Ce3+-doped CaSi2O2N2 (blue line) are illustrated in Fig. 12(c). An obvious spectral overlap is observed at the range from 400 to 500 nm, indicating the energy transfer is possible from Ce3+ to Eu2+ ions in CaSi2O2N2.
12
Fig. 13 The excitation (a) and emission (b) spectra of CaSi2O2N2:0.02Ce3+ yEu2+ and CaSi2O2N2:0.05Eu2+ phosphor nanofiber mat.
Fig. 13 shows the emission spectra of CaSi2O2N2:0.05Eu2+ phosphors and CaSi2O2N2:0.02Ce3+yEu2+ phosphors with different Eu2+ concentrations (y=0~0.02). It can be seen that the emission peaks of Ce3+ (around 450 nm) and Eu2+ (around 550 nm) merges with a certain extent, as Ce3+ and Eu2+ (y=0.002) co-doped into the nanofiber mats. The Eu2+ emission observed in the co-doped samples are caused by the direct excitation of the Eu2+ and the Ce to Eu energy transfer. With further incorporating of the Eu2+ ions, emission intensity of Ce3+ decreases significantly accompanied by the increasing of Eu2+ emission which is mainly attributed to the energy transfer from Ce3+ to Eu2+ ions.
Fig. 14 (a) CIE chromaticity coordinates of CaSi2O2N2:Ce3+/Tb3+, Eu2+, Mn2+ samples. The lighting optical images with a drive current of 100 mA: three LEDs with nanofiber mats of CaSi2O2N2: 0.02Ce3+, 0.03Tb3+ (b), CaSi2O2N2: 0.02Ce3+, 0.01Eu2+ (c) and CaSi2O2N2: 0.02Ce3+, 0.12Mn2+ (d).
We summarized the CIE chromaticity coordinates of the phosphor nanofiber 13
mats. As shown in Fig. 14(a), the changes in color can be obtained by altering the types of doped ion. The linear variation of chromaticity coordinates have already been realized by UV LED chip with Ca2Si2O2N2 nanofiber mat by different doping ions. Three lighted LEDs integrated with as-prepared different ions doped Ca2Si2O2N2 nanofiber mat are shown in Fig. 14(b)-(d), which proves the previous conclusion and the success of the fabrication of these phosphor nanofiber mats. The chromaticity coordinates of three LED lamps exhibiting correlated color temperatures (CCTs) of 6666, 3740 and 1500K under a drive Current of 100 mA could be obtained, marked in CIE1931 with values of (0.2948, 0.4031), (0.4388, 0.495), and (0.4357, 0.2290) (Table 1). Table 1 Electroluminescence properties of LED lamps using CaSi2O2N2 nanofiber mats with different doped ions
Correlated color Sample
CIE value (x, y) temperature (K)
CaSi2O2N2: 0.02Ce3+, 0.03Tb3+
6666
(0.2948, 0.4031)
CaSi2O2N2: 0.02Ce3+, 0.01Eu2+
3740
(0.4388, 0.4954)
CaSi2O2N2: 0.02Ce3+, 0.12Mn2+
1500
(0.4357, 0.2290)
Conclusion In conclusion, phosphor nanofiber mats consisting of CaSi2O2N2: 0.02Ce3+/Tb3+, Eu2+, Mn2+ have been successfully fabricated by electrospinning procedure and subsequent high-temperature nitridation treatment. The as-prepared nanofiber precursors exhibit smooth, uniform morphology and tunable diameter (400~600 nm). CaSi2O2N2 nanofiber could be well crystallized and retain its uniform nanofiber type morphology after high-temperature treatment. Energy transfer and photoluminescence in CaSi2O2N2:Ce3+/ Tb3+, Eu2+, Mn2+ have been discussed as a function of Tb3+, Mn2+, Eu2+ concentrations. In CaSi2O2N2:Ce3+/Tb3+, Mn2+ phosphor system, the energy transfer of Ce3+ to Tb3+ or Mn2+ can be excited through near UV-LED. Accordingly, the emitting colors of the phosphor nanofiber mat have been changed very well from 14
the blue to green or red under UV excitation. When Ce3+ and Eu2+ were co-doped in CaSi2O2N2, the emission spectra exhibited a combination of emission bands of Ce3+ and Eu2+. With the raise of Eu2+ concentration, the emission intensity of Eu2+ was increased, whereas that of Ce3+ simultaneously decreased monotonically. The emitting colors of the as-prepared phosphor nanofiber mat were well controlled from the blue to yellow under UV excitation.
Acknowledgements We gratefully acknowledge the financial support by Natural Science Foundation of China (No. 51572046), the Shanghai Natural Science Foundation (15ZR1401200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Program of Shanghai Academic Research Leader (16XD1400100), Science and Technology Commission of Shanghai Municipality (16JC1400700), the Program of Introducing Talents of Discipline to Universities (No.111-2-04) and the Fundamental Research Funds for the Central Universities(2232014A3-06).
References 1.
C. Liu, S. Zhang, Z. Liu, H. Liang, S. Sun, Y. Tao, J. Mater. Chem. C, 1 (2013) 1305-1308.
2.
S. Su, W. Liu, R. Duan, L. Cao, G. Su, C. Zhao, J. Alloys compd. 575 (2013) 309-313.
3.
X. Li, J. D. Budai, F. Liu, J. Y. Howe, X. J. Wang, Z. Gu, R. S. Meltzer, Z. Pan, Light: Sci. Appl. 2 (2013) e50.
4.
M. S. Kim, L. K. Bharat, J. S. Yu, J. Lumin. 142 (2013) 92-95.
5.
M. Shang, D. Geng, D. Yang, X. Kang, Y. Zhang, Inorg. Chem. 52 (2013) 3102-3112.
6.
L. F. Hu, R. Z. Ma, T. C. Ozawa, T. Sasaki, Angew. Chem. Int. Ed. 48 (2009) 3846-3849.
7.
Y. Q. Liu, X. P. Zhang, Y. N. Xia, H. Yang, Adv. Mater. 22 (2010) 2454-2457.
8.
H. Wu, Y. Sun, D. D. Lin, R. Zhang, C. Zhang, W. Pan, Adv. Mater. 21 (2009) 227-231.
9.
S. Agarwal, A. J. Greiner, H. Wendorff, Adv. Funct. Mater. 19 (2009) 2863-2879.
10. A. Greiner, J. H. Wendorff, Angew. Chem. Int. Ed. 46 (2007) 5670-5703. 11. H. W. Song, H. Q. Yu, G. H. Pan, X. Bai, B. Dong, X. T. Zhang, S. K. Hark, Chem. Mater. 20 (2008) 4762-2767. 15
12. Z. Y. Hou, C. X. Li, J. Yang, H. Z. Lian, P. P. Yang, R. T. Chai, Z. Y. Cheng, J. Lin, J. Mater. Chem. 19 (2009) 2737-2746. 13. Y. X. Gu, Q. H. Zhang, H. Z. Wang, Y. G. Li, J. Mater. Chem. 21 (2011) 17790-17797 14. H. H. Chia, M. C. Bing, H. L. Chung, J. Am. Ceram. Soc. 94 (2011) 2878-2883. 15. Y. X. Gu, Q. H. Zhang, Y. G. Li, H. Z. Wang, R. J. Xie, Mater. Lett. 63 (2009) 1448-1450. 16. H. A. Höppe, F. Stadler, O. Oeckler, W. Schnick, Angew. Chem. Int. Ed. 43 (2004) 5540-5542. 17. L.J. Nugent, R.D. Baybarz, J.L. Burnett, J.L. Ryan, J. Phys. Chem. 77 (1973) 1528-1539. 18. L. X. Yang, X. Xu, L. Y. Hao, X. F. Yang, J. Y. Tang, R. J. Xie. Opt. Mater. 33 (2011) 1695-1699.
CIE chromaticity coordinates of CaSi2O2N2:Ce3+/Tb3+, Eu2+, Mn2+ samples. The lighting optical images with a drive current of 100 mA: three LEDs with nanofiber mats of CaSi2O2N2: 0.02Ce3+, 0.03Tb3+ (b), CaSi2O2N2: 0.02Ce3+, 0.01Eu2+ (c) and CaSi2O2N2: 0.02Ce3+, 0.12Mn2+ (d).
16