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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 29233−29237

Integrated Surface Modification to Enhance the Luminescence Properties of K2TiF6:Mn4+ Phosphor and Its Application in WhiteLight-Emitting Diodes Mu-Huai Fang,†,‡ Chia-Shen Hsu,†,§ Chaochin Su,*,§ Wenjing Liu,⊥ Yu-Hua Wang,⊥ and Ru-Shi Liu*,‡,|| ‡

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Institute of Organic and Polymeric Materials and ||Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan ⊥ Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China

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§

S Supporting Information *

ABSTRACT: Narrow-band Mn4+-doped fluoride phosphors have become a research hotspot worldwide. In this study, we propose integrated surface modification processes to enhance the performance and stability of the luminescence properties of K2TiF6:Mn4+ (KTF) phosphor. These integrated process are applied in the initial synthesis step, coating of the as-synthesized powder post-treatment process, and during the application of the phosphor in the white-light-emitting diode (WLED) device. Surface etching is conducted to remove impurities and small particles in KTF. Double-shell coating forms a stable protective layer outside the KTF. Atomic layer deposition is employed for the surface of the WLED device.

KEYWORDS: narrow-band-emitting, red fluoride phosphor, white-light-emitting diodes, surface coating, atomic layer deposition disadvantage of the Mn4+-doped fluoride phosphors is its chemical stability or water resistance. Unlike most of the oxide or nitride phosphors, fluoride phosphors can dissolve in water and thus is unstable in environments with high humidity and temperature. An increasing number of researchers aim to address this problem.20−22 In 2015, Nguyen et al. 23 successfully coated the surface of KSF with an alkyl phosphate layer to enhance its chemical stability. In 2017, Arunkumar et al.24 used hydrophobic oleic acid (OA) to provide protection to KSF phosphor. In the same year, Zhou et al.25 used alkyl trimethoxy-silane to enhance the stability of KTF. The most popular fluoride phosphor is KSF, which is already commercialized. However, to the best of our knowledge, KTF demonstrates an approximately 10% external quantum efficiency (EQE) compared with KSF, although KTF is more sensitive to the environment than KSF. Therefore, KTF is a promising and potential candidate in the practical application of LEDs. In our previous work, we use alkyl phosphate materials to coat KSF.23

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igh-level lighting and backlit devices are becoming increasingly popular worldwide. Phosphor materials, which determine color quality, are one of the most important components of light-emitting diodes (LEDs).1−5 Among the colored components, the red component is considerably more important than any other color because of the spillover of photons that are longer than 650 nm, to which the human eye is insensitive. Mn4+-doped fluoride phosphors have recently attracted a considerable attention worldwide.6−13 This material shows five extremely narrow-emission bandwidth peaks. The maximum emission peak is approximately 630 nm, indicating an excellent luminescent performance with zero waste-phonon ratio (WPR).14 In 2008, Adachi et al.15 have successfully synthesized a series of Mn4+-doped fluoride phosphors, including the most famous K2SiF6:Mn4+ (KSF) phosphor, by using the wet chemical etching method. In 2014, Zhu et al.16 used the cation-exchange method to synthesize the K2TiF6:Mn4+ (KTF) phosphor, which demonstrates an efficiency of higher than 100 lm/W in devices. Nowadays, most researchers use the coprecipitation method, a modified synthesis method, to synthesize fluoride phosphors; compared with the previous methods, the coprecipitation method produces phosphors with considerably uniform particles and high luminescent efficiency.17−19 However, the most serious © 2018 American Chemical Society

Received: July 19, 2018 Accepted: August 21, 2018 Published: August 21, 2018 29233

DOI: 10.1021/acsami.8b12170 ACS Appl. Mater. Interfaces 2018, 10, 29233−29237

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns, (b) SEM, (c) PLE spectra, (d) PL spectra, (e) relative intensity, and (f) quantum efficiency of KTF after treatment with HF solution of different concentrations.

(HF-free) sample, indicating the presence of impurities during the coprecipitation process. Nevertheless, the impurity peaks disappear after the etching process involving 12 and 24% HF, indicating that HF solution can effectively reduce the amount of impurities. To validate this finding, we used a scanning electron microscope (SEM) to determine the morphological changes in KTF (Figure 1b). The compound not treated with HF shows hexagonal plate morphology (KTF) and numerous small particles, which have possibly caused the impurity peaks in the XRD patterns. The small particles disappear after the etching process. Small particles usually reduce the emission intensity of phosphor materials compared with large particles; this phenomenon is attributed to the high defect concentration on the surface of small particles. Photoluminescence excitation (PLE) spectra and photoluminescence (PL) spectra are obtained (Figure 1c, d) to analyze the luminescent properties of the phosphors. The PLE spectrum is attributed to spin-allow 4A2g → 4T2g and 4A2g → 4 T1g electron transitions, and these spectra peaked at 460 and 350 nm, respectively. These results indicate that blue LED chips with 460 nm can effectively excite KTF phosphor. For its PL spectrum, the transition belongs to spin-forbidden 2Eg → 4 A2g, which shows the sharp-line spectrum with a maximum peak at approximately 630 nm. All its emission spectra are shorter than 700 nm and thus a zero WPR can be expected. As seen in the crystal structure, Ti4+ is coordinated by six F− ions to form an octahedral environment. As a result, the zerophonon line (ZPL) emission cannot be seen at approximately 630 nm due to the highly symmetrical environment of Ti4+.28 When dilute HF solution is applied, PL intensity is enhanced by more than 10% (Figure 1e). To quantitatively analyze the photoluminescence enhancement, we measured the internal quantum efficiency (IQE), absorption (abs.), and EQE (Figure 1f). IQE does not change, whereas the absorption values considerably increased after HF treatment, resulting in enhanced EQE. To understand the practical luminescent properties of the HF-treated KTF, we measured the cathodoluminescence (CL) spectra of the phosphor. An uneven luminescence property is observed on the surface of the original KTF phosphor (Figure 2a). Moreover, numerous small luminescent points are found in the CL spectra. By contrast, the 12% HF-treated sample

These alkyl phosphate materials greatly enhance the water resistance of KSF. Therefore, we used such method to provide protection to KTF. However, the water resistance of the asprepared sample and the coated sample did not significantly differ. This result may be attributed to the morphological difference between KTF and KSF. KSF is round-shaped, whereas KTF is a hexagonal plate, which folds up together with other KTFs, resulting in incomplete coating. When the method involving OA and alkyl trimethoxy-silane was applied on KTF, we still did not obtain the desired results. Few reports used SiO2 to coat the surface of phosphor materials, such as oxide and nitride phosphors.26,27 However, we cannot obtain good results when this method was applied to KTF. The possible reason is the lack of functional group on the surface of the fluoride phosphor; this functional group serves as a linkage between the surface of the phosphor and the coating materials. Moreover, a synthesized fluoride phosphor usually contains small particles and impurities, which reduce the luminescence intensity or result in uneven luminescence of the fluoride phosphors. The last critical issue in fluoride phosphors is the stability of the LED device. Although already packaged in epoxy resin, phosphor materials remain vulnerable to heat, moisture, and light, and thus their luminescence intensity may decrease and their color points may shift. This study aimed to develop an integrated system that can be applied in (1) the initial processes in the synthesis stage, (2) the coating of the as-synthesized powder post-treatment, and (3) the practical use of the phosphor in white-light-emitting diode (WLED) device. Different concentrations of hydrogen fluoride (HF) solution are used to etch the KTF phosphors; HF can not only reduce the number of small particles and impurities but also increase photoluminescence intensity. Moreover, reducing the amount of small particles and smoothing the surface of KTF through etching creates a good environment for the subsequent coating process. A double-layer coating consisting of OA and SiO2 is used to enhance the chemical stability of the as-prepared powders. Moreover, atomic layer deposition (ALD) is used to enhance the practical performance of the WLED device (Figure S1). X-ray diffraction (XRD) is used in structural examination before and after HF etching (Figure 1a). No obvious change in KTF is found. Some small peaks are observed in the original 29234

DOI: 10.1021/acsami.8b12170 ACS Appl. Mater. Interfaces 2018, 10, 29233−29237

Letter

ACS Applied Materials & Interfaces

Figure 3. (a) TEM of KTF, KTF@OA, and KTF@OA@SiO2. (b) Result of the water resistance test for KTF and KTF@OA@SiO2 after being placed in a ethanol/water solution at a ratio of 2:3 and excited by 365 nm UV light.

Figure 2. CL figures of (a) original KTF, (b) 12% HF-treated KTF, and (c) 24% HF-treated KTF.

showed improved properties (Figure 2b), and the 24% HFtreated sample showed the most uniform luminescence (Figure 2c). We used the double-layer coating process to address the problem on chemical stability (water resistance). Initially, we try to directly use OA to coat the surface of KTF, but the coated layer cannot sufficiently resist water attack (Figure S2). The possible reasons are as follows: (1) the hexagonal plate morphology, which causes the phosphors to easily fold together, resulting in incomplete coating; and (2) the weak intermolecular force of OA itself, which cannot resist prolonged soaking in water. Therefore, we use SiO2 as coating material, and we combined the OA and SiO2 coating processes to achieve an enhanced performance. A schematic of the coating process is shown in Figure S3. First, OA is used as the first layer of coating given that OA forms hydrogen bonds with the surface of KTF. The luminescent intensity remains the same (Figure S4). Subsequently, (3-aminopropyl)triethoxysilane (APTES) is added; APTES reacts with OA and forms F−OH hydrogen bonds on the surface, providing an environment for further chemical reaction. Finally, TEOS (tetraethoxysilane) is added to form the outer SiO2 layer. The detailed experimental steps are presented in the Supporting Information. For comparison, we directly use APTES and TEOS to form the SiO2 layer. However, the surface of KTF is not suitable for SiO2 coating. As a result, we obtain not only a SiO2-coated KTF but also numerous small SiO2 particles, which reduce the luminescent intensity of the phosphor (Figure S5). To ensure that the integrity and properties are maintained throughout the coating process, we analyzed the XRD patterns of KTF, KTF@OA, and KTF@OA@SiO2 (Figure S6). All peaks fit the standard pattern, indicating that the structure of the treated samples remained stable after the coating process. Furthermore, to assess the success of the SiO2 coating process, we obtain transmission electron microscope (TEM) images of the structures in question (Figure 3a). The results show that an OA layer is deposited on the surface of the KTF, consistent with previous results.24 After the SiO2 coating, the layer becomes thinner because of the SN2 chemical reaction between APTES and OA, resulting in the loss of OA. Moreover, infrared spectroscopy is performed to characterize the coating layer (Figure S7). The KTF shows a flat spectrum ranging from 1000 to 4000 cm−1. By contrast, the KTF@OA@SiO2 shows three peaks at approximately 1100, 1600, and 3000

cm−1, which correspond to the vibrations of Si−O, CO, and C−O−O−H.29,30 This finding indicates that some of the OA molecules possibly remain between the KTF and SiO2, forming a double-layer-coated structure; note that the OA deposited outside the SiO2 layer is dissolve by the solvent. To understand the influence of the coating process, we assessed the chemical stability and water resistance of the coatings by placing the KTF and KTF@OA@SiO2 into an ethanol/water solution with a ratio of 2:3 (Figure 3b). When the powder is placed into the solution, the KTF is instantly dissolved accompanied by the decay of luminescent intensity, which is excited by 365 nm UV light. By contrast, the KTF@OA@SiO2 maintained its strong luminescent intensity in the solution. The difference between these two samples increases with prolonged mixing time. After 2 h, KTF has lost most of its luminescence whereas KTF@OA@SiO2 still provides a bright red emission, indicating the efficiency of the protective layer formed using the double-layer coating process. To assess the practical application of phosphors, we should not only package them in LED package but also measure their performance when exposed to long-term blue light excitation and high humidity (Figure 4). One of the most serious concerns in LED is the long-term stability of the fluoride phosphors. When stability is lost, LED devices display reduced brightness, and their color points shift. Although phosphor materials have been mixed with epoxy resin, they remain vulnerable to moisture, heat, and light. Therefore, we use ALD to deposit a considerably thin and transparent materials on the surface of the LED chip itself. Figure 4a, b shows the changes in color points on the CIE diagram over a certain period of time. The coated LED shows smaller color shift compared with the uncoated LED, indicating that the Al2O3 layer can effectively diminish color point shifts. In terms of lifespan of the device, the coated LED demonstrates a better luminescent performance compared with the uncoated LED after a certain period (Figure 4c). In summary, integrated surface modification processes are proposed to enhance the luminescence properties of KTF phosphors; these processes are applicable in the synthesis and post-treatment stages up the use of phosphors in LED devices. As-prepared samples are treated with dilute HF solution to remove small particles and impurity, resulting in enhanced photoluminescence intensity and improve coating quality. 29235

DOI: 10.1021/acsami.8b12170 ACS Appl. Mater. Interfaces 2018, 10, 29233−29237

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Figure 4. (a) CIE x position, (b) CIE y position, and the change of relative intensity of LED package of lifetime measurement. (c) Lifespan of ALD-coated and non-ALD-coated devices.

Moreover, a double-layer coating process is proposed. Because of the lack of ligands at the surface of KTF, OA acts as the precursor first, and then APTES and TEOS are used to generate a thin SiO2 layer on the surface. Some of the OA located between the SiO2 layer and the KTF particle will be dissolved, resulting in a double-layer-coated KTF structure. The luminescent intensity of KTF@OA@SiO2 particle remains high after 2 h, whereas that of the uncoated particle quickly diminishes. Moreover, a LED packaging experiment is conducted to analyze the practical application of the phosphor. The ALD coating process is used to further protect the surface of the LED device itself from moisture. After exposure to extreme environmental conditions for 144 h, the sample coated via ALD shows a smaller color point shift compared with the uncoated sample. Moreover, the former shows a higher luminescence intensity compared with the latter, indicating the success of the three modification processes. This study provides comprehensive information on the moisture-sensitive properties of fluoride phosphors, and it may serve as reference to researchers who aim to improve the stability of phosphors. The processes we proposed, which are applicable in the initial stages of synthesis of phosphor up to their application in LED, help improve the stability of KTF. These methods are also expected to be useful when working on other luminescent materials with low chemical stability.

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Typical synthesis process, characterization, design of the experiment, water resistance test, SEM image, XRD, IR spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.S.). *E-mail: [email protected] (R.S.L.). ORCID

Mu-Huai Fang: 0000-0003-1475-0200 Yu-Hua Wang: 0000-0002-5982-8799 Ru-Shi Liu: 0000-0002-1291-9052 Author Contributions †

M.H.F. and C.S.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan (Contract MOST 107-2113-M-002008-MY3).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b12170. 29236

DOI: 10.1021/acsami.8b12170 ACS Appl. Mater. Interfaces 2018, 10, 29233−29237

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DOI: 10.1021/acsami.8b12170 ACS Appl. Mater. Interfaces 2018, 10, 29233−29237