Letter Cite This: ACS Appl. Mater. Interfaces 2018, 10, 17508−17511
www.acsami.org
Ultrafast Self-Crystallization of High-External-Quantum-Efficient Fluoride Phosphors for Warm White Light-Emitting Diodes Wenli Zhou,†,‡ Mu-Huai Fang,† Shixun Lian,‡ and Ru-Shi Liu*,†,∥ †
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China ∥ Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on January 14, 2019 at 08:17:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: In this study, we used HF (as good solvent) to dissolve K2GeF6 and K2MnF6 and added ethanol (as poor solvent) to cause ultrafast self-crystallization of K2GeF6:Mn4+ crystals, which had an unprecedentedly high external quantum efficiency that reached 73%. By using the red phosphor, we achieved a high-quality warm white light-emitting diode with color-rendering index of Ra = 94, R9 = 95, luminous efficacy of 150 lm W−1, and correlated color temperature at 3652 K. Furthermore, the good−poor solvent strategy can be used to fast synthesize other fluorides. KEYWORDS: self-crystallization, fluoride, red phosphor, quantum efficiency, Mn4+
C
urrently, great efforts have been made to exploit Mn4+activated red fluoride phosphors.1−6 Synthesis of Mn4+doped fluorides is unpractical by solid-state reaction and can be alternatively performed in solutions. At present, many strategies, including chemical etching,7 coprecipitation,8,9 hydrothermal processes,10 and cation exchange,1,11 have been developed to fabricate them. Generally, the PL efficiency of fluorides synthesized by chemical etching is low, possibly because the activator Mn4+ does not evenly distribute into the six-coordinated cation lattice of hosts. Through coprecipitation, fluorides can exhibit relatively highly efficient red-line emissions; however, fluorides would undergo some complex synthesis processes, for instance, ice bath and dropwise addition. Hydrothermal methods can produce good morphology, but quantum efficiency (QE) would still be low. Wang’s group utilized a H3PO4−KHF2 couple as the fluorine source to synthesize K2SiF6:Mn4+ (KSFM) microcrystals during hydrothermal process.11 No HF was used in their method and the QE was too low at only 28%. Such low QE was possibly due to the lack of HF to stabilize the [MnF6]2− group in the reaction. For cation exchange reaction, the benefits were obvious: short reaction time (minutes), high internal QE (IQE = 93%), suitable particle size, and uncomplicated process.13 Whereas, when increasing the Mn4+ concentration (5.5% to 6.5%) in K2TiF6:Mn4+ (KTFM) to increase the absorption efficiency (54% to 60%), the IQE suffered a serious decrease (93% to 78%), thereby decreasing the external QE (EQE = 50% to 46%). The decrease in IQE may be related to concentration © 2018 American Chemical Society
quenching effects and the increase in surface states (defects) of KTFM crystals when increasing HF volume to dissolve more K2MnF6. High-EQE phosphor is distinctly a key to producing high luminous efficacy (LE) warm white LEDs. To the best of our knowledge, the IQE of Mn4+-doped red phosphors can be optimized to over 90%, but the EQE over 60% is difficult to be achieved before. Related data collected from published literatures are compiled in Table S1. Additionally, fabrication of the fluoride involves in usage of HF that is harmful to environment and health, therefore simple and fast production process could contribute to decreasing volatilization of HF. In this work, we report an ultrafast self-crystallization of redemitting fluorides in a two solutions mixture at room temperature (RT). The idea source and possible mechanism of good-poor solvent (GPS, Figure S2) strategy are stated in the Supporting Information for the readership. We first dissolved the fluoride resources [K 2 GeF 6 (KGF) and K2MnF6] in 48% HF solution (good solvent) and then added ethanol (poor solvent); bright yellow precipitates instantly appeared (Figure 1). Under the 460 nm light irradiation, the yellow solution emitted harsh red lights (Movie S1); this result indicated that Mn4+ was incorporated into the octahedron sites of KGF crystals. Received: March 1, 2018 Accepted: May 11, 2018 Published: May 11, 2018 17508
DOI: 10.1021/acsami.8b03525 ACS Appl. Mater. Interfaces 2018, 10, 17508−17511
Letter
ACS Applied Materials & Interfaces
P3m ̅ 1 (Figure S3); this result indicated that the P3m ̅ 1 is more stable relative to P63mc phase in terms of energy. R2 is a mixture of both. From SEM images, R1 appeared octahedral at around 20 μm in size (Figure 2c and Figure S4) and had a clear and smooth surface. The particle size of R5 was approximately 0.6 μm (Figure 2b and Figure S4). R2 consisted of big and small particles (Figure S4), which should be P3̅m1 and P63mc KGFM according to the XRD result (Figure 2a). KGFM particles were not stable enough under the electron beam, but we still clearly observed the crystal lattices of R1 and R5 (Figure S5). Additionally, all of the elements of KGFM were uniformly distributed across the crystals (Figure S6). These results indicated that the fluorides were well-crystallized by the GPS method. The IQE, AE, and EQE of P3̅m1 KGFM with different doping Mn4+ concentrations (x, which were determined using inductively coupled plasma technique) are shown in Table S3. As x increased, the AE consistently increased, whereas the IQE peaked (96%) when x = 6.75%. Additionally, the AE increase was also supported by diffuse reflectance spectra (Figure S7). Therefore, the cross variation of both causes the best EQE value (73%) at x = 11.74%, which was higher than that (54%) synthesized by coprecipitation.9 Such high EQE value of KGFM has never been recorded in published literature; moreover, this value is higher than that of commercial KSFM (61%) and KTFM (65%) phosphors. In addition to ethanol, methanol, acetone and propanol can also drive the ultrafast self-crystallization of the KGFM in fluoride/HF solution. The phases of KGFM can be controlled by varying their volumes (Figures S8−S10). From the above results, the GPS strategy has at least three advantages for the synthesizing Mn4+-doped fluorides: (1) simple operation; (2) extremely short reaction time (in seconds); and (3) activator Mn4+ would not be of nonequilibrium segregation or
Figure 1. (a) Schematic good−poor solvent strategy for the fabrication of the K2GeF6:Mn4+ phosphor. (b) Pictures of three key states during the fabrication process. (1) K2GeF6 and K2MnF6 powders are dissolved in HF and form a yellow transparent solution; (2) adding ethanol causes ultrafast self-crystallization of K2GeF6:Mn4+, which emits intensive red light under 460 nm light radiation.
Morphology and optical properties of the KGFM crystals were affected by the volume ratio (R) of ethanol and HF. Therefore, we fixed the volume of HF solvent at 5 mL and varied the volume of ethanol at 5, 10, and 25 mL, and the asprepared products were named R1, R2, and R5, respectively. The X-ray diffraction (XRD) results showed that the phases of these products changed from P3̅m1 to P63mc with the increasing volume of ethanol (Figure 2a). R1 is a pure phase of P3̅m1, which was determined by refining its synchrotron XRD (Figure 2b). R5 was in pure P63mc phase when using Bruker D2 measurement, whereas the same R5 sample measured by synchrotron XRD partially was transferred to
Figure 2. (a) XRD patterns of KGFM products, (b) refined synchrotron XRD of R1, SEM images of (c) R1 and (d) R5 samples; the scale bars are (c) 10 μm and (d) 1 μm, respectively. 17509
DOI: 10.1021/acsami.8b03525 ACS Appl. Mater. Interfaces 2018, 10, 17508−17511
Letter
ACS Applied Materials & Interfaces
Figure 3. (a) Excitation and emission spectra of R1 sample. (b) PL intensity of P3̅m1 KGFM as a function of concentration (x) of Mn4+ ions. (c) Integrated PL intensity as a function of temperature for the R1−11 and commercial KSFM samples. (d) CIE coordination. (e) Photographs of the lighted LEDs, and (f) EL spectra of three LED packages.
increased the temperature to 473 K (Figure 3c). Above 498 K, the relative intensity decreased sharply. The activation energy (Ea) of R1−11 was estimated to be 1.08 eV, which was four times than that of nitride phosphors (∼0.25 eV)16 and higher than those of other fluorides (∼0.70 eV for KTFM).1 However, the thermal stability of R1−11 was weaker than that of commercial KSFM phosphor in the high-temperature area (Figure 3c). To estimate the potentials of the KGFM phosphor as a red component for warm WLEDs, we employed blue chip, commercial YAG, and the best KGFM powder together to encapsulate LED packages. At 3.0 V and 20 mA, these LEDs showed high-brightness warm white light (Figure 3e). The chromaticity coordinates of the three typical LEDs were close to the blackbody radiation locus (BBR), as marked in Figure 3d. As the amount of KGFM increased, the electroluminescence (EL) spectra of the WLEDs show an increased red component (Figure 3f), and the warmer white-light can be observed. The important photoelectric performance parameters of the LEDs are listed in Table S4. LED2 and LED3 exhibited Ra > 90 and CCT < 4,000 K, and the R9 (rendition of the red color) were 95 and 84, respectively. A cooler LED1 with CCT = 4,221 K generated high Ra (= 94) and R9 (= 94). The LE of LED1 and LED2 reached 158 and 150 lm/W at 20 mA, respectively, which were higher than that using nitrides or KTFM red phosphors. The good photoelectric performance parameters of the LEDs suggested that the highly efficient K2GeF6:Mn4+ red phosphor can significantly improve the EL performance of WLEDs. In summary, we developed a simple good−poor solvent strategy for fast fabrication of Mn4+-activated fluorides. The
concentration gradient in KGFM crystals, therefore causing high EQE. These advantages are crucial for its scale production. The strategy can be further applied to fabricate KSFM (Figure S11) and KTFM (Figure S12). Similarly, the morphology can be controlled by varying the amount of ethanol. Especially, the method can only produce one pure phase for KSFM (Fm3̅m) and KTFM (P3̅m1), even if we decreased the R to 0.5 or increased it to 10. Their PL spectra showed characteristic transitions of Mn4+; however, no zero-phonon-line (ZPL) was observed. Both P3m ̅ 1 and P63mc KGFM gave characteristic excitation and emission spectra,12,13 as shown in Figure 3a and Figure S13. The 4A2 → 4T2 band of Mn4+ was not beyond 520 nm, indicating no significant reabsorption between KGFM and Y3Al5O12:Ce3+ (YAG) would occur for producing warm whiteLEDs. At RT, the PL spectrum of P63mc gains an intensive ZPL at 620 nm, which we also observed in our previous reports.14,15 As the x increased, the PL intensity of P3̅m1 KGFM exhibited a maximum at x = 11.74% (Figure 3b); the trend was similar to that of EQE. Such small concentration quenching effect could be related to (1) even distribution of Mn4+ at KGFM crystal lattice, (2) smooth surface of KGFM microcrystals, (3) high crystallinity due to the ultrafast self-crystallization. Furthermore, we measured the decay curves of the eight samples at room temperature. The single exponential fitting results show that the lifetimes (τ) of Mn4+ in KGFM were in the range of 6.65−6.56 ms as x ≤ 11.74%. τ does evidently decrease (to 6.45 ms) when x = 12.76% (Figure S14) because of the enhanced nonradiative transition possibility among Mn4+ ions. For the highest EQE sample (R1−11), its integrated PL intensity kept 99% of its initial value (at 298 K) when we 17510
DOI: 10.1021/acsami.8b03525 ACS Appl. Mater. Interfaces 2018, 10, 17508−17511
Letter
ACS Applied Materials & Interfaces
Alkali-Germanate System: Structural Determination and Electronic Calculations. Inorg. Chem. 2016, 55, 10310−10319. (7) Xu, Y. K.; Adachi, S. Properties of Na2SiF6:Mn4+ and Na2GeF6:Mn4+ Red Phosphors Synthesized by Wet Chemical Etching. J. Appl. Phys. 2009, 105, 013525. (8) Hoang Duy, N.; Lin, C. C.; Fang, M. H.; Liu, R. S. Synthesis of Na2SiF6:Mn4+ Red Phosphors for White LED Applications by Coprecipitation. J. Mater. Chem. C 2014, 2, 10268−10272. (9) Wei, L. L.; Lin, C. C.; Fang, M. H.; Brik, M. G.; Hu, S. F.; Jiao, H.; Liu, R. S. A Low-temperature Co-Precipitation Approach to Synthesize Fluoride Phosphors K2MF6:Mn4+ (M = Ge, Si) for white LED applications. J. Mater. Chem. C 2015, 3, 1655−1660. (10) Huang, L.; Zhu, Y.; Zhang, X.; Zou, R.; Pan, F.; Wang, J.; Wu, M. HF-Free Hydrothermal Route for Synthesis of Highly Efficient Narrow-Band Red Emitting Phosphor K2Si1−xF6:xMn4+ for Warm White Light-Emitting Diodes. Chem. Mater. 2016, 28, 1495−1502. (11) Han, T.; Lang, T.; Wang, J.; Tu, M.; Peng, L. Large Micro-sized K2TiF6:Mn4+ Red Phosphors Synthesised by A Simple Reduction Reaction for High Colour-rendering White light-emitting Diodes. RSC Adv. 2015, 5, 100054−100059. (12) Wei, L.-L.; Lin, C. C.; Wang, Y.-Y.; Fang, M.-H.; Jiao, H.; Liu, R.-S. Photoluminescent Evolution Induced by Structural Transformation Through Thermal Treating in the Red Narrow-Band Phosphor K2GeF6:Mn4+. ACS Appl. Mater. Interfaces 2015, 7, 10656− 10659. (13) Adachi, S.; Takahashi, T. Photoluminescent Properties of K2GeF6:Mn4+ Red Phosphor Synthesized from Aqueous HF/KMnO4 Solution. J. Appl. Phys. 2009, 106, 013516. (14) Jin, Y.; Fang, M. H.; Grinberg, M.; Mahlik, S.; Lesniewski, T.; Brik, M. G.; Luo, G. Y.; Lin, J. G.; Liu, R. S. Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for Warm White LightEmitting Diodes. ACS Appl. Mater. Interfaces 2016, 8, 11194−11203. (15) Wu, W. L.; Fang, M. H.; Zhou, W.; Lesniewski, T.; Mahlik, S.; Grinberg, M.; Brik, M. G.; Sheu, H.-S.; Cheng, B. M.; Wang, J.; Liu, R. S. High Color Rendering Index of Rb2GeF6:Mn4+ for Light-Emitting Diodes. Chem. Mater. 2017, 29, 935−939. (16) Wang, S. S.; Chen, W. T.; Li, Y.; Wang, J.; Sheu, H. S.; Liu, R. S. Neighboring-Cation Substitution Tuning of Photoluminescence by Remote-Controlled Activator in Phosphor Lattice. J. Am. Chem. Soc. 2013, 135, 12504−12507.
self-crystallization reaction can be achieved in seconds. The particle size and phase-control of KGFM depend on the volume ratio of HF (good solvent) and ethanol (poor solvent). The fabricated KGFM crystals (∼20 μm) showed high IQE (93%), high AE (78%), and an unprecedentedly high EQE of up to 73%, which was higher than that of the commercial KSFM and KTFM. Additionally, the successful synthesis of KSFM and KTFM demonstrates the universality of this approach. Finally, by using the best KGFM phosphor, we encapsulated a warm white LED with CCT = 3652 K, Ra = 94, R9 = 95, and LE = 150 lm/W. Therefore, we believe such a simple and ultrafast synthesis strategy can be adopted for mass production of fluoride materials.
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03525. Experimental details, idea source, possible mechanism, XRD, SEM, TEM, elements mapping and PL spectra of K2GeF6:Mn4+ crystals, and tables (PDF) Movie S1, showing the ultrafast fabrication of K2GeF6:Mn4+ (AVI)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (R.S.L.). ORCID
Wenli Zhou: 0000-0002-6975-2206 Shixun Lian: 0000-0001-6524-2703 Ru-Shi Liu: 0000-0002-1291-9052 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan (contract MOST 104-2113-M-002-012MY3). W. Zhou appreciates the support from the National Natural Science Foundation of China (21501058) and the Research Foundation of Education Bureau of Hunan Province (Grant 16B152).
■
REFERENCES
(1) Zhu, H.; Lin, C. C.; Luo, W.; Shu, S.; Liu, Z.; Liu, Y.; Kong, J.; Ma, E.; Cao, Y.; Liu, R. S.; Chen, X. Highly Efficient Non-Rare-Earth Red Emitting Phosphor for Warm White Light-Emitting Diodes. Nat. Commun. 2014, 5, 5312. (2) Kim, M.; Park, W. B.; Bang, B.; Kim, C. H.; Sohn, K.-S. Radiative and Non-Radiative Decay Rate of K2SiF6:Mn4+ Phosphors. J. Mater. Chem. C 2015, 3, 5484−5489. (3) Kim, M.; Park, W. B.; Bang, B.; Kim, C. H.; Sohn, K.-S. A Novel Mn4+-activated Red Phosphor for Use in Light Emitting Diodes, K3SiF7:Mn4+. J. Am. Ceram. Soc. 2017, 100, 1044−1050. (4) Li, J.; Yan, J.; Wen, D.; Khan, W. U.; Shi, J.; Wu, M.; Su, Q.; Tanner, P. A. Advanced Red Phosphors for White Light-Emitting Diodes. J. Mater. Chem. C 2016, 4, 8611−8623. (5) Zhou, Q.; Dolgov, L.; Srivastava, A. M.; Zhou, L.; Wang, Z.; Shi, J.; Dramicanin, M. D.; Brik, M. G.; Wu, M. Mn2+ and Mn4+ Red Phosphors: Synthesis, Luminescence and Applications in WLEDs. A review. J. Mater. Chem. C 2018, 6, 2652−2671. (6) Singh, S. P.; Kim, M.; Park, W. B.; Lee, J.-W.; Sohn, K.-S. Discovery of A Red-Emitting Li3RbGe8O18:Mn4+ Phosphor in the 17511
DOI: 10.1021/acsami.8b03525 ACS Appl. Mater. Interfaces 2018, 10, 17508−17511
