Elsevier

Materials Letters

Volume 141, 15 February 2015, Pages 27-30
Materials Letters

Enhanced luminous efficiency of deep red emitting K2SiF6:Mn4+ phosphor dependent on KF ratio for warm-white LED

https://doi.org/10.1016/j.matlet.2014.11.025Get rights and content

Highlights

  • K2SiF6:Mn4+ was synthesized from SiO2 powders through redox reaction in HF/KMnO4 solution.

  • An intense absorption band in blue and a bright emission in red indicated the phosphor K2SiF6:Mn4+.

  • This phosphor and the synthesis method are expected to be promising candidates.

Abstract

We demonstrate a simple method of K2SiF6:Mn4+ red phosphor and discuss its promising application in warm-white lighting emitting diodes. The K2SiF6:Mn4+ was synthesized from SiO2 powders through redox reaction in HF/KMnO4 solution. To further increase emission efficiency, KF was introduced in the solution. An intense absorption band in blue and a bright emission in red indicated the phosphor K2SiF6:Mn4+. This phosphor and the synthesis method are expected to be promising candidates for application in white LEDs.

Introduction

Solid-State Lighting (SSL) has received attention in recent years because of its low energy cost, long lifetime, and absence of mercury, and because it is an environment friendly device [1], [2], [3]. These features make SSL a next generation lighting source. Generally, white light-emitting diodes (WLEDs) are fabricated from a blue-emitting InGaN chip with yellow-emitting Y3Al5O12: Ce3+ phosphor [4], [5], [6]. The commercialized WLEDs have serious problems of poor color rendering index (CRI) value and low correlated color temperature of 3000–4500 K [7]. Although this WLED system possesses high luminous efficiency and a simple structure, its CRI is still too low due to the lack of red emitting components [8].

Ziegler et al. reported a blue LED using silica-coated InP/ZnS QDs with Sr0.94Al2O4:Eu0.06 green phosphor and YAG: Ce3+ yellow phosphor [9], which showed a luminous efficacy of 15 lm/W and a CRI of 86 at 3900 K. Lin et al. developed a high efficiency WLED with a luminous efficiency of 68 lm/W and a high CRI of 93 at 3007 K using YAG:Ce3+ and CaSiAlN3:Eu2+, respectively [10]. In addition, Kim et al. reported a luminous efficiency of 54.71 lm/W and a CRI of 80.56 at 7864 K using InP/GaP/ZnS QDs with YAG:Ce3+ phosphor [11]. In discharge-based fluorescent lighting, two activator ions Eu2+ and Mn4+ are used for narrow-line red emission that can simultaneously optimize both the luminous efficacy of radiation and CRI of warm-white lamps [12], [13]. Red phosphor using Eu2+ has several advantages such as high emission output, chemical stability, wide excitation band and small thermal quenching. However, this method also has several drawbacks, including very high synthesis temperature, very high pressure, N2/H2 and H2 atmospheres, and gas pressure sintering (GPS) equipment [14], [15]. On the other hand, Mn4+-doped red phosphors are the most promising candidates for improving the color rendering of WLEDs, owing to their ease of handing, the abundance of raw materials required for the method, and environment friendly nature. Mn4+-doped phosphors have a spin-allowed parity-forbidden 4A24T2 transition causing sufficient absorption of blue and ultraviolet LED radiation [16].

In this article, we present a simple method to prepare the K2SiF6:Mn4+ red phosphor, and demonstrate its application in warm-white LED. SiO2 powders were used as Si source for the synthesis of K2SiF6:Mn4+ undergoing redox reaction in HF/KMnO4 solution. Then, H2O2 was used to effectively reduce Mn7+ to Mn4+, which not only diminishes the reaction time but also eases the recovery of the product [17], [18].

Section snippets

Experimental

For synthesis of K2SiF6:Mn4+ phosphor, 1 g of SiO2 powder was dissolved in 95 mL HF (48%, Aldrich, USA) in an ambient environment for 2 h to form H2SiF6 solution. 6 g of KMnO4 and varied amounts of KF were dissolved in the solution; the color of the solution quickly turned from colorless to deep purple. 20 mL hydrogen peroxide (30%) was then added slowly dropwise. The purple solution rapidly turned colorless and ultimately became transparent pink water. This was followed by stirring for 1, 3, 6, and

Results and discussion

The samples produced with different stirring times in the synthetic solutions were identified using XRD as shown in Fig. 1. The pure K2SiF6 phase was obtained with stirring for 3 h at room temperature. All the diffraction peaks were indexed to the cubic K2SiF6 (JCPDS card no. 07-0217) with a unit cell a=8.13 Å and space group Oh5-Fm3m. To investigate the effect of the synthetic parameters such as stirring time and concentration of KF on the properties of the red phosphor K2SiF6:Mn4+, two types of

Conclusions

In conclusion, we directly synthesized red-emitting K2SiF6:Mn4+ phosphor from KF and SiO2 in a HF/KMnO4 mixed solution. The reactions involved etching of SiO2 in HF and reduction of KMnO4 by HF. The formation mechanism of red phosphor K2SiF6:Mn4+ was investigated in detail. The emission intensity of the prepared phosphor indicated typical Mn4+ excitation and emission peak. These results suggest that KSF-2 phosphor can be a promising candidate for warm-white LED.

Acknowledgment

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (NRF-2013R1A1A2059280).

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    These authors contributed equally to this work.

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