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Synthesis and photoluminescence properties of a red-emitting phosphor, K2SiF6:Mn4+, for use in three-band white LED applications
Optical Materials 51 (2016) 50–55
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and photoluminescence properties of a red-emitting phosphor, K2SiF6:Mn4+, for use in three-band white LED applications Byul-Ee Yeo, Young-Sik Cho, Young-Duk Huh ⇑ Department of Chemistry, Dankook University, Gyeonggi-Do 448-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 September 2015 Received in revised form 23 October 2015 Accepted 8 November 2015
Keywords: Red-emitting phosphors K2SiF6:Mn4+ Photoluminescence
a b s t r a c t K2SiF6:Mn4+ phosphors were prepared by redox precipitation at room temperature from mixed aqueous solutions of SiO2, HF, KMnO4, and H2O2. The optimal conditions required to obtain the brightest red emission spectra of the K2SiF6:Mn4+ phosphors were examined. The K2SiF6:Mn4+ phosphors emitted deep red light with three strong peaks at 615, 630, and 650 nm under 465 nm excitation, the emission wavelength of a commercial blue LED. K2SiF6:Mn4+ and SrGa2S4:Eu2+ phosphors were used to produce the red and green emission bands under excitation from a commercial blue LED, respectively. The photoluminescence properties of the three-band white LEDs fabricated by coating K2SiF6:Mn4+ and SrGa2S4:Eu2+ phosphors onto the commercial blue LED chip were investigated. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Innovative white light-emitting diodes (white LEDs) based on blue LEDs show promise in a variety of eco-friendly and energysaving lighting applications [1–6]. By law, fluorescence lamps must be replaced with white LEDs in an effort to regulate the harmful mercury vapors contained in the fluorescence lamps. White LEDs with long lifetimes and low energy requirements are advantageous over fluorescence lamps. Most commercial phosphor-converted two-band white LEDs have been fabricated by coating Y3Al5O12: Ce phosphors onto a InGaN blue-emitting LED chips [7–9]. Y3Al5O12:Ce phosphors emitted very broad yellow emission bands around 530 nm under blue excitation at 465 nm, the wavelength generated by the InGaN blue-emitting LED chip. White emission light may be obtained through additive color mixing between the blue emission from InGaN LEDs and the yellow emission from Y3Al5O12:Ce phosphors; however, these two-band white LEDs cannot produce nature-equivalent colors recognized by the three red-, green-, and blue-color receptions of the cones in human eyes. Therefore, phosphor-converting three-band white LEDs comprising blends of green- and red-emitting phosphor coatings applied to a blue emitting InGaN chips must be developed to improve natural color production and color rendering. SrGa2S4:Eu2+ and b-SiAlON:Eu2+ present good candidate greenemitting phosphors for use in phosphor-converted three-band white LEDs [10–12]. Red-emitting phosphors, such as CaS:Eu2+, ⇑ Corresponding author. E-mail address: [email protected] (Y.-D. Huh). http://dx.doi.org/10.1016/j.optmat.2015.11.014 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.
Na5Y(MoO4)4:Eu3+, NaY(W, Mo)2O8:Eu3+, Sr2Si5N8:Eu2+, and CaAlSiN3:Eu2+, have been used in phosphor-converted three-band white LEDs [13–18]. Oxide phosphors based on trivalent europium ion activators, such as Na5Y(MoO4)4:Eu3+ and NaY(W, Mo)2O8:Eu3+, have sharp absorption and emission lines at 465 and 615 nm due to the 7F0 ? 5D2 and 5D0 ? 7F2 transitions, respectively, in Eu3+. The sharp absorption line of Eu3+ allows the Eu3+-based phosphors to be excited by a fraction of the blue LED emission. Therefore, oxides comprising trivalent europium ions cannot be used as redemitting phosphors in phosphor-converted three-band white LEDs due to their low brightness. Although CaS:Eu2+ has a broad excitation band at 465 nm, it suffers from low air and thermal stability. Silicon-based nitrides with divalent europium ion activators, such as Sr2Si5N8:Eu2+ and CaAlSiN3:Eu2+, are considered to be the best red-emitting phosphors for phosphor-converted three-band white LEDs; however, the harsh experimental conditions required for the preparation of these nitrides with divalent europium ions, which included high pressures and reactive H2 and NH3 gases at very high temperatures of 1750 °C, are not appropriate for large-scale production. Recently, K2SiF6:Mn4+ phosphors were used for the room-temperature preparation of warm white LEDs [19–24]. K2SiF6:Mn4+ phosphors have a strong broad absorption band at 465 nm due to the 4A2g ? 4T2g transition of Mn4+. They emit over a sharp and strong band in the deep red, at 632 nm, due to a vibrational shift and a 2Eg ? 4A2g transition in Mn4+ [19]. K2MnF6 was prepared from the mixed aqueous solutions of KF, HF, KMnO4. and H2O2 [25]. Mn4+ ion was formed by reduction of KMnO4 with H2O2. General Electric has developed TriGainTM to overcome the red conundrum for LED manufacturers. For the sharper and
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brighter red color, deep red-emitting K2SiF6:Mn4+ phosphor was used in LED display backlight applications [26]. K2SiF6:Mn4+ phosphor has been considered to be the best candidate for the largescale production of red-emitting phosphors in phosphorconverted three-band white LEDs. The optimal conditions needed to obtain the bright emission of the K2SiF6:Mn4+ phosphor have not previously been investigated. This study describes the optimal experiment conditions for preparing the K2SiF6:Mn4+ phosphor and the photoluminescence properties of phosphor-converted threeband white LEDs prepared by coating K2SiF6:Mn4+ and SrGa2S4: Eu2+ phosphors onto a blue-emitting InGaN chip.
2. Experimental SiO2 (Aldrich), HF (40%, Aldrich), KMnO4 (Aldrich), and H2O2 (35%, Aldrich) were used as received. The experimental conditions used to prepare the K2SiF6:Mn4+ phosphors are summarized in Table 1. The photoluminescence properties of the K2SiF6:Mn4+ products were examined as functions of HF, KMnO4, and H2O2 concentrations. In a typical process used to synthesize sample 10, 1.00 g SiO2 was added to a 70 mL HF aqueous solution to prepare the H2SiF6 solution. Then, 3.951 g KMnO4 were added to the H2SiF6 solution. Finally, 12.8 mL H2O2 were very slowly added to the mixed solution. The final mixed solution was stirred at approximately 700 rpm for 1 h at room temperature. The products were collected by centrifugation at 4000 rpm for 10 min, washed with water several times, and finally dried for 24 h at 60 °C in an oven. For the comparison, the product was also washed with ethanol several times, and dried for 24 h under the vacuum at room temperature. The SrGa2S4:Eu2+ phosphor was synthesized using a procedure reported previously [27]. A blue InGaN chip (NSPBS500S, Nichia, kmax = 465 nm) was used to fabricate the three-band white LED. The three-band white LED was prepared by coating a phosphor film onto the outer sphere of a blue InGaN LED. The quantities of SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors in the coating were controlled by adjusting the thickness of the coating layer applied using the applicator. The phosphors were mixed with PAS ink (800 series, Jujo) and were applied to a poly(ethylene terephthalate) (PET) film. The crystal structures of the K2SiF6:Mn4+ phosphors were examined using powder X-ray diffraction (XRD, PANalytical, X’pert-pro MPD) with Cu Ka radiation. The excitation and emission spectra of the K2SiF6:Mn4+ phosphors and photoluminescence spectra of the three-band white LEDs were obtained using a spectrum analyzer (DARSA, PSI). The 39K and 55Mn signals of the K2SiF6:Mn4+ phosphors were measured by ICP-MS (DRC-e, Perkin Elmer, MA, USA).
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3. Results and discussion K2SiF6:Mn4+ phosphors were prepared through a precipitation reaction of an H2SiF6 solution with aqueous KMnO4 and the simultaneous reduction of KMnO4 by H2O2 at room temperature. The ionic radii of Mn2+, Mn3+, and Mn4+ were 0.083, 0.065, and 0.054 nm, respectively [28]. Because the ionic radius of Mn4+ differed only slightly from that of Si4+ (0.040 nm), the Mn4+ ions could occupy the Si4+ sites in the host lattices of K2SiF6. Only a small fraction of the activator Mn4+ ions were substituted into the Si4+ sites in K2SiF6; therefore, the bulk crystal structure of the K2SiF6:Mn4+ phosphor was very similar to that of the host K2SiF6. Fig. 1 shows a typical X-ray diffraction (XRD) pattern obtained from the K2SiF6:Mn4+ product. All XRD peaks matched those in cubic K2SiF6 (JCPDS 07-0217, a = 0.813 nm). The XRD patterns confirmed that the K2SiF6:Mn4+ phosphor was successfully prepared without impurities. Because K2SiF6 assumes a cubic crystal structure, the Mn4+ activator ions substituted into the Si4+ sites of K2SiF6 were located at the centers of the octahedral SiF26 lattice. The octahedral geometry of the Mn4+–F interactions split the d orbitals into three low T2g states and two high Eg states. The electronic ground state of Mn4+, with a 3d3 electronic configuration, was 4A2g. The 4T2g and 4 T1g states were the two low energy electronic ground states with the same spin multiplicity. The calculated energy differences 4A2g 4 T2g and 4A2g 4T1g for Mn4+ in K2SiF6 were 23,229 and 31,251 cm 1, respectively [29]. These energy differences corresponded to 430 and 320 nm, indicating that the K2SiF6:Mn4+ phosphor had a strong absorption band that allowed transitions in the near-UV and blue region of the visible spectrum, as shown in Fig. 2. The emission spectrum consisted of a series of three sharp emission lines between 600 and 650 nm, which corresponded to three vibronic energy shifts within the 2Eg ? 4A2g electronic transition of Mn4+ in the K2SiF6:Mn4+ structure [19]. When we examine the emission spectrum with higher resolution by decreasing the slit width of spectrophotometer, three emission peaks are decoupled to five emission peaks. However, we used the emission spectra with lower resolution to find the optimal conditions for the brightest for the K2SiF6:Mn4+ phosphors. Therefore, K2SiF6:Mn4+ could be used as a bright red-emitting phosphor in a phosphor-converted three-band white LED based on a blue-emitting InGaN chip. The chromaticity coordinates of the K2SiF6:Mn4+ phosphor were x = 0.696 and y = 0.304, close to the National Television Standard Committee (NTSC) red coordinates of x = 0.67 and y = 0.33 [30]. Most red-emitted phosphors with trivalent europium activator ions provide strong emission
Table 1 Summary of the experimental conditions used to prepare the K2SiF6:Mn4+ phosphors. Sample number
SiO2 (mmol)
HF (mL)
KMnO4 (mmol)
H2O2 (mmol)
PL data Fig. 3(a) Fig. 3(b) Figs. 3(c) and 5(b) Fig. 3(d) Fig. 3(e) Fig. 5(a) Figs. 5(c) and 6(a) Fig. 5(d) Fig. 6(b) Fig. 6(c) Fig. 6(d)
1 2 3
16.6 16.6 16.6
30 50 70
20 20 20
100 100 100
4 5 6 7
16.6 16.6 16.6 16.6
90 110 70 70
20 20 15 25
100 100 100 100
8 9 10 11
16.6 16.6 16.6 16.6
70 70 70 70
30 25 25 25
100 125 150 175
Fig. 1. An XRD pattern typical of the K2SiF6:Mn4+ product. The XRD pattern and Miller indices of the cubic K2SiF6 crystal (JCPDS 07-0217), obtained from the database, are included.
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Fig. 2. (a) Excitation (kem = 632 nm) and (b) emission spectra (kex = 465 nm) of the K2SiF6:Mn4+ phosphor.
lines at 615 nm due to the 5D0 ? 7F2 transition of Eu3+. The chromaticity coordinates of phosphors with trivalent europium ions, such as the Na5Y(MoO4)4:Eu3+ phosphor, are x = 0.671 and y = 0.328 [14]. The K2SiF6:Mn4+ phosphor showed better chromaticity coordinates than those of the well-known phosphors with trivalent europium ions. This result indicated that the K2SiF6:Mn4+ phosphor is the best red-emitting phosphor among phosphorconverted three-band white LEDs. The brightest emission profile was sought by investigating the optimal experimental conditions needed for the preparation of a K2SiF6:Mn4+ phosphor. Fig. 3 shows the emission spectra of the K2SiF6:Mn4+ phosphors synthesized using different volumes of the HF solution. The emission intensities of the K2SiF6:Mn4+ phosphors dramatically increased as the volume of HF solution increased from 30 to 50 mL, and a slight increase was obtained as the HF solution was further increased up to 70 mL. At HF solution volumes up to 110 mL, however, the emission intensities decreased slightly. The brightest red emission was obtained from the K2SiF6:Mn4+ phosphor prepared using 70 mL of the HF solution. Fig. 4 shows SEM images of K2SiF6:Mn4+ phosphors prepared with different amounts of HF used. The mean size of K2SiF6:Mn4+ phosphor is slightly increased from 10 lm to 14 lm as increasing the amount of HF from 30 to 110 mL. The some aggregated particles K2SiF6:Mn4+ phosphor is also observed in K2SiF6:Mn4+ phosphor prepared with larger amount of HF used. As the particle size is
Fig. 3. Emission spectra of the K2SiF6:Mn4+ phosphors prepared with different volumes of the HF solution. The inset shows the relative intensity at 632 nm as a function of the HF solution volume: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, and (e) sample 5.
increased slightly with the HF amounts, the size effect on the brightness of K2SiF6:Mn4+ phosphor is negligible. Fig. 5 shows the emission spectra of the K2SiF6:Mn4+ phosphors prepared with different amounts of KMnO4. The brightest emission was obtained from the K2SiF6:Mn4+ phosphor prepared using 25 mmol KMnO4. Only Mn4+ ion was incorporated in K2SiF6 host. Mn4+ ion was formed by reduction from KMnO4 with H2O2. H2O2 can reduce only small portion of Mn7+ ions in KMnO4 to Mn4+ ions. It is important to examine the concentration of Mn4+ ions of K2SiF6: Mn4+ phosphors. Lv et al. measured the concentrations of Mn4+ ions for K2TiF6:Mn4+ phosphors through ICP-AES [31]. We measured the relative atomic amounts of Mn and K by using ICP-MS. The atomic ratio of Mn to K is 0.0125 for sample 3. It indicates that approximately 1% of Mn7+ ions in KMnO4 are reduced to Mn4+ ions in our reaction system. Unfortunately, we did not measure the relative atomic amounts of Mn and Si by using ICP-MS, due to the severe interference for the measurement of Si [32,33]. Based on the formula of K2Si(1 x)F6:xMn4+ phosphors, we calculated that mol% of Mn4+ ions is 2.50 (x = 0.025) from ICP-MS data of sample 3. Calculated mol% of Mn4+ ions of K2Si(1 x)F6:xMn4+ phosphors are 2.08, 2.50 3.34, 3.77 for samples 6, 3, 7, 8, respectively. We observed concentration quenching effect of Mn4+ ions as activator, as shown in Fig. 5. The maximum concentration of Mn4+ ion for the brightest emission of K2SiF6:Mn4+ phosphors is 3.34 mol%. The optimal H2O2 concentration was sought by measuring the emission spectra of the K2SiF6:Mn4+ phosphors prepared with different amounts of H2O2, as shown in Fig. 6. The concentration of Mn4+ ion depends on both amounts of KMnO4 and H2O2. Therefore, it is important to find the optimal reactant conditions for the brightest emission of K2SiF6:Mn4+ phosphors. We also prepared the K2SiF6:Mn4+ phosphor washed with ethanol and dried under vacuum at room temperature. This product shows brighter emission by factor of 1.31 than that washed with water and then dried at 60 °C in an oven. Fig. 7 shows the emission spectra of the K2SiF6:Mn4+ phosphors prepared with different washing and drying conditions. When the K2SiF6:Mn4+ products washed with water and then dried at 60 °C in an oven, the hydrolysis processes resulted in lower brightness. Fig. 8 shows the emission spectra of the green-emitting SrGa2S4:Eu2+ and red-emitting K2SiF6:Mn4+ phosphors under excitation at 465 nm, the exact emission wavelength of the blue-emitting InGaN chip. SrGa2S4:Eu2+ has a broad absorption band between 350 nm and 500 nm due to the 4f7 ? 4f65d1 transition of the Eu2+ ions [27]. The emission spectrum of SrGa2S4:Eu2+ showed a single band around 535 nm due to a transition from the lowest excited state, 4f65d1, to the 4f7 ground state of the Eu2+ ions in the SrGa2S4 host lattice, as shown in Fig. 8(a) [27]. The emission spectrum of K2SiF6:Mn4+ included three sharp deep red emission peaks between 600 and 650 nm, as shown in Fig. 8(b). These results indicated that the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors could be used as green- and red-emitting phosphors excited by the blue InGaN chip in a phosphor-converted three-band white LED, respectively. The feasibility of using the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors was investigated by fabricating phosphor-converted threeband LEDs by coating different amounts of SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors onto a blue LED chip. Fig. 9 shows the photoluminescence spectra of the four three-band LED fabricated in this study. The amounts of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors were adjusted by varying the thickness of the phosphor layer applied to the PET film. Distinct emission peaks were obtained at 465, 535 nm, and a series of peaks around 635 nm, originating from the blue LED chip, green-emitting SrGa2S4:Eu2+, and redemitting K2SiF6:Mn4+ phosphors, respectively. The relative amounts SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors were fixed to 1:1 to obtain white light, as defined by the Commission Interna-
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Fig. 4. SEM images of K2SiF6:Mn4+ phosphors prepared with different volumes of the HF solution: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, and (e) sample 5.
Fig. 5. Emission spectra of the K2SiF6:Mn4+ phosphors prepared with different amounts of KMnO4. The inset shows the relative intensity at 632 nm as a function of the KMnO4 content: (a) sample 6, (b) sample 3, (c) sample 7, and (d) sample 8.
Fig. 6. Emission spectra of the K2SiF6:Mn4+ phosphors prepared with different volumes of the H2O2 solutions. The inset shows the relative intensity at 632 nm as a function of the H2O2 solution volume: (a) sample 7, (b) sample 9, (c) sample 10, and (d) sample 11.
tional de l’Eclairage (CIE), with chromaticity coordinates of x = 0.35 and y = 0.33, as shown in Fig. 10(b). Setting the relative molar ratios of SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors to 1:0.67 yielded bluish white light with CIE coordinates of x = 0.33 and y = 0.30, as shown in Fig. 10(a). Greenish white light with CIE coordinates of x = 0.35 and y = 0.43 was obtained at a SrGa2S4:Eu2+:K2SiF6:Mn4+ phosphor molar ratio of 0.83:1.33, as shown in Fig. 10(c). The use of large quantities of SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors, in
a molar ratio of 1.33:1.33, yielded reddish white light with CIE coordinates of x = 0.48 and y = 0.39, as shown in Fig. 10(d). These results revealed that we could fine-tune the color and CIE coordinates of the LEDs by varying the amounts of both SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors. Fig. 10 shows the CIE chromaticity coordinates of the photoluminescence spectra of the four threeband LEDs fabricated. The CIE chromaticity coordinates of the National Television Standard Committee (NTSC), that is, blue
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Fig. 7. Emission spectra of the K2SiF6:Mn4+ phosphors (a) washed with water and then dried at 60 °C in an oven and (b) washed with ethanol and dried under vacuum at room temperature.
Fig. 10. CIE chromaticity coordinates of the photoluminescence spectra of four three-band LED fabricated by coating different molar ratios of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors onto the blue LED chip. The molar ratios of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors were (a) 1:0.67, (b) 1:1, (c) 0.83:1.33, and (d) 1.33:1.33. The NTSC chromaticity coordinates in the red, green, and blue are included.
(0.283, 0.682), and (0.696, 0.304), respectively. The triangular area of the three-band LED was 96.5% the area of NTSC. This result indicated that the color rendering of the three-band LED fabricated by combing the blue LED chip, green-emitting SrGa2S4:Eu2+, and redemitting K2SiF6:Mn4+ phosphors was excellent and may be used as a display white light source. 4. Conclusions Fig. 8. Emission spectra of the (a) green-emitting SrGa2S4:Eu2+ and (b) red-emitting K2SiF6:Mn4+ phosphors under excitation at 465 nm.
Fig. 9. Photoluminescence spectra of four three-band LED fabricated by coating different relative quantities of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors onto the blue LED chip. The molar ratios of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors were (a) 1:0.67, (b) 1:1, (c) 0.83:1.33, and (d) 1.33:1.33.
(x = 0.48 and y = 0.08), green (x = 0.21 and y = 0.71), and red (x = 0.67 and y = 0.33), are shown in Fig. 10 [30]. The color purity of a display panel may be depicted, for reference purposes, using the NTSC triangle formed by connecting the chromaticity coordinate positions. The CIE chromaticity coordinates of the blue LED, SrGa2S4:Eu2+, and K2SiF6:Mn4+ phosphors are (0.119, 0.092),
K2SiF6:Mn4+ phosphors were prepared via a precipitation reaction from mixed solution containing SiO2, HF, KMnO4, and H2O2 at room temperature. The K2SiF6:Mn4+ phosphor had a strong broad absorption band around 465 nm, the exact wavelength of the blue-emitting InGaN chip. The K2SiF6:Mn4+ phosphor emitted deep red light characterized by three sharp emission lines between 600 and 650 nm. K2SiF6:Mn4+ could be used as a bright redemitting phosphor in a phosphor-converted three-band white LED using the blue-emitting InGaN chip. The optimal condition for obtaining the brightest emission from the K2SiF6:Mn4+ phosphor was determined experimentally. Mixtures of the greenemitting SrGa2S4:Eu2+ and red-emitting K2SiF6:Mn4+ phosphors were coated onto the blue-emitting InGaN chip to fabricate a phosphor-converted three-band white LED. Fine color tuning in the three-band LED was accomplished by varying the amounts of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors in the coating solution. The phosphor-converted three-band white LED prepared with SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors showed excellent color rendering and may be applied as white light sources in displays. Acknowledgment This study was supported by the research fund of Dankook University in 2015. References [1] S. Nakamura, The roles of structural imperfections in InGaN-based blue lightemitting diodes and laser diodes, Science 281 (1998) 956–961. [2] S. Nakamura, III–V nitride based light-emitting devices, Solid State Commun. 102 (1997) 237–243.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and photoluminescence properties of a red-emitting phosphor, K2SiF6:Mn4+, for use in three-band white LED applications Byul-Ee Yeo, Young-Sik Cho, Young-Duk Huh ⇑ Department of Chemistry, Dankook University, Gyeonggi-Do 448-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 September 2015 Received in revised form 23 October 2015 Accepted 8 November 2015
Keywords: Red-emitting phosphors K2SiF6:Mn4+ Photoluminescence
a b s t r a c t K2SiF6:Mn4+ phosphors were prepared by redox precipitation at room temperature from mixed aqueous solutions of SiO2, HF, KMnO4, and H2O2. The optimal conditions required to obtain the brightest red emission spectra of the K2SiF6:Mn4+ phosphors were examined. The K2SiF6:Mn4+ phosphors emitted deep red light with three strong peaks at 615, 630, and 650 nm under 465 nm excitation, the emission wavelength of a commercial blue LED. K2SiF6:Mn4+ and SrGa2S4:Eu2+ phosphors were used to produce the red and green emission bands under excitation from a commercial blue LED, respectively. The photoluminescence properties of the three-band white LEDs fabricated by coating K2SiF6:Mn4+ and SrGa2S4:Eu2+ phosphors onto the commercial blue LED chip were investigated. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Innovative white light-emitting diodes (white LEDs) based on blue LEDs show promise in a variety of eco-friendly and energysaving lighting applications [1–6]. By law, fluorescence lamps must be replaced with white LEDs in an effort to regulate the harmful mercury vapors contained in the fluorescence lamps. White LEDs with long lifetimes and low energy requirements are advantageous over fluorescence lamps. Most commercial phosphor-converted two-band white LEDs have been fabricated by coating Y3Al5O12: Ce phosphors onto a InGaN blue-emitting LED chips [7–9]. Y3Al5O12:Ce phosphors emitted very broad yellow emission bands around 530 nm under blue excitation at 465 nm, the wavelength generated by the InGaN blue-emitting LED chip. White emission light may be obtained through additive color mixing between the blue emission from InGaN LEDs and the yellow emission from Y3Al5O12:Ce phosphors; however, these two-band white LEDs cannot produce nature-equivalent colors recognized by the three red-, green-, and blue-color receptions of the cones in human eyes. Therefore, phosphor-converting three-band white LEDs comprising blends of green- and red-emitting phosphor coatings applied to a blue emitting InGaN chips must be developed to improve natural color production and color rendering. SrGa2S4:Eu2+ and b-SiAlON:Eu2+ present good candidate greenemitting phosphors for use in phosphor-converted three-band white LEDs [10–12]. Red-emitting phosphors, such as CaS:Eu2+, ⇑ Corresponding author. E-mail address: [email protected] (Y.-D. Huh). http://dx.doi.org/10.1016/j.optmat.2015.11.014 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.
Na5Y(MoO4)4:Eu3+, NaY(W, Mo)2O8:Eu3+, Sr2Si5N8:Eu2+, and CaAlSiN3:Eu2+, have been used in phosphor-converted three-band white LEDs [13–18]. Oxide phosphors based on trivalent europium ion activators, such as Na5Y(MoO4)4:Eu3+ and NaY(W, Mo)2O8:Eu3+, have sharp absorption and emission lines at 465 and 615 nm due to the 7F0 ? 5D2 and 5D0 ? 7F2 transitions, respectively, in Eu3+. The sharp absorption line of Eu3+ allows the Eu3+-based phosphors to be excited by a fraction of the blue LED emission. Therefore, oxides comprising trivalent europium ions cannot be used as redemitting phosphors in phosphor-converted three-band white LEDs due to their low brightness. Although CaS:Eu2+ has a broad excitation band at 465 nm, it suffers from low air and thermal stability. Silicon-based nitrides with divalent europium ion activators, such as Sr2Si5N8:Eu2+ and CaAlSiN3:Eu2+, are considered to be the best red-emitting phosphors for phosphor-converted three-band white LEDs; however, the harsh experimental conditions required for the preparation of these nitrides with divalent europium ions, which included high pressures and reactive H2 and NH3 gases at very high temperatures of 1750 °C, are not appropriate for large-scale production. Recently, K2SiF6:Mn4+ phosphors were used for the room-temperature preparation of warm white LEDs [19–24]. K2SiF6:Mn4+ phosphors have a strong broad absorption band at 465 nm due to the 4A2g ? 4T2g transition of Mn4+. They emit over a sharp and strong band in the deep red, at 632 nm, due to a vibrational shift and a 2Eg ? 4A2g transition in Mn4+ [19]. K2MnF6 was prepared from the mixed aqueous solutions of KF, HF, KMnO4. and H2O2 [25]. Mn4+ ion was formed by reduction of KMnO4 with H2O2. General Electric has developed TriGainTM to overcome the red conundrum for LED manufacturers. For the sharper and
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brighter red color, deep red-emitting K2SiF6:Mn4+ phosphor was used in LED display backlight applications [26]. K2SiF6:Mn4+ phosphor has been considered to be the best candidate for the largescale production of red-emitting phosphors in phosphorconverted three-band white LEDs. The optimal conditions needed to obtain the bright emission of the K2SiF6:Mn4+ phosphor have not previously been investigated. This study describes the optimal experiment conditions for preparing the K2SiF6:Mn4+ phosphor and the photoluminescence properties of phosphor-converted threeband white LEDs prepared by coating K2SiF6:Mn4+ and SrGa2S4: Eu2+ phosphors onto a blue-emitting InGaN chip.
2. Experimental SiO2 (Aldrich), HF (40%, Aldrich), KMnO4 (Aldrich), and H2O2 (35%, Aldrich) were used as received. The experimental conditions used to prepare the K2SiF6:Mn4+ phosphors are summarized in Table 1. The photoluminescence properties of the K2SiF6:Mn4+ products were examined as functions of HF, KMnO4, and H2O2 concentrations. In a typical process used to synthesize sample 10, 1.00 g SiO2 was added to a 70 mL HF aqueous solution to prepare the H2SiF6 solution. Then, 3.951 g KMnO4 were added to the H2SiF6 solution. Finally, 12.8 mL H2O2 were very slowly added to the mixed solution. The final mixed solution was stirred at approximately 700 rpm for 1 h at room temperature. The products were collected by centrifugation at 4000 rpm for 10 min, washed with water several times, and finally dried for 24 h at 60 °C in an oven. For the comparison, the product was also washed with ethanol several times, and dried for 24 h under the vacuum at room temperature. The SrGa2S4:Eu2+ phosphor was synthesized using a procedure reported previously [27]. A blue InGaN chip (NSPBS500S, Nichia, kmax = 465 nm) was used to fabricate the three-band white LED. The three-band white LED was prepared by coating a phosphor film onto the outer sphere of a blue InGaN LED. The quantities of SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors in the coating were controlled by adjusting the thickness of the coating layer applied using the applicator. The phosphors were mixed with PAS ink (800 series, Jujo) and were applied to a poly(ethylene terephthalate) (PET) film. The crystal structures of the K2SiF6:Mn4+ phosphors were examined using powder X-ray diffraction (XRD, PANalytical, X’pert-pro MPD) with Cu Ka radiation. The excitation and emission spectra of the K2SiF6:Mn4+ phosphors and photoluminescence spectra of the three-band white LEDs were obtained using a spectrum analyzer (DARSA, PSI). The 39K and 55Mn signals of the K2SiF6:Mn4+ phosphors were measured by ICP-MS (DRC-e, Perkin Elmer, MA, USA).
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3. Results and discussion K2SiF6:Mn4+ phosphors were prepared through a precipitation reaction of an H2SiF6 solution with aqueous KMnO4 and the simultaneous reduction of KMnO4 by H2O2 at room temperature. The ionic radii of Mn2+, Mn3+, and Mn4+ were 0.083, 0.065, and 0.054 nm, respectively [28]. Because the ionic radius of Mn4+ differed only slightly from that of Si4+ (0.040 nm), the Mn4+ ions could occupy the Si4+ sites in the host lattices of K2SiF6. Only a small fraction of the activator Mn4+ ions were substituted into the Si4+ sites in K2SiF6; therefore, the bulk crystal structure of the K2SiF6:Mn4+ phosphor was very similar to that of the host K2SiF6. Fig. 1 shows a typical X-ray diffraction (XRD) pattern obtained from the K2SiF6:Mn4+ product. All XRD peaks matched those in cubic K2SiF6 (JCPDS 07-0217, a = 0.813 nm). The XRD patterns confirmed that the K2SiF6:Mn4+ phosphor was successfully prepared without impurities. Because K2SiF6 assumes a cubic crystal structure, the Mn4+ activator ions substituted into the Si4+ sites of K2SiF6 were located at the centers of the octahedral SiF26 lattice. The octahedral geometry of the Mn4+–F interactions split the d orbitals into three low T2g states and two high Eg states. The electronic ground state of Mn4+, with a 3d3 electronic configuration, was 4A2g. The 4T2g and 4 T1g states were the two low energy electronic ground states with the same spin multiplicity. The calculated energy differences 4A2g 4 T2g and 4A2g 4T1g for Mn4+ in K2SiF6 were 23,229 and 31,251 cm 1, respectively [29]. These energy differences corresponded to 430 and 320 nm, indicating that the K2SiF6:Mn4+ phosphor had a strong absorption band that allowed transitions in the near-UV and blue region of the visible spectrum, as shown in Fig. 2. The emission spectrum consisted of a series of three sharp emission lines between 600 and 650 nm, which corresponded to three vibronic energy shifts within the 2Eg ? 4A2g electronic transition of Mn4+ in the K2SiF6:Mn4+ structure [19]. When we examine the emission spectrum with higher resolution by decreasing the slit width of spectrophotometer, three emission peaks are decoupled to five emission peaks. However, we used the emission spectra with lower resolution to find the optimal conditions for the brightest for the K2SiF6:Mn4+ phosphors. Therefore, K2SiF6:Mn4+ could be used as a bright red-emitting phosphor in a phosphor-converted three-band white LED based on a blue-emitting InGaN chip. The chromaticity coordinates of the K2SiF6:Mn4+ phosphor were x = 0.696 and y = 0.304, close to the National Television Standard Committee (NTSC) red coordinates of x = 0.67 and y = 0.33 [30]. Most red-emitted phosphors with trivalent europium activator ions provide strong emission
Table 1 Summary of the experimental conditions used to prepare the K2SiF6:Mn4+ phosphors. Sample number
SiO2 (mmol)
HF (mL)
KMnO4 (mmol)
H2O2 (mmol)
PL data Fig. 3(a) Fig. 3(b) Figs. 3(c) and 5(b) Fig. 3(d) Fig. 3(e) Fig. 5(a) Figs. 5(c) and 6(a) Fig. 5(d) Fig. 6(b) Fig. 6(c) Fig. 6(d)
1 2 3
16.6 16.6 16.6
30 50 70
20 20 20
100 100 100
4 5 6 7
16.6 16.6 16.6 16.6
90 110 70 70
20 20 15 25
100 100 100 100
8 9 10 11
16.6 16.6 16.6 16.6
70 70 70 70
30 25 25 25
100 125 150 175
Fig. 1. An XRD pattern typical of the K2SiF6:Mn4+ product. The XRD pattern and Miller indices of the cubic K2SiF6 crystal (JCPDS 07-0217), obtained from the database, are included.
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Fig. 2. (a) Excitation (kem = 632 nm) and (b) emission spectra (kex = 465 nm) of the K2SiF6:Mn4+ phosphor.
lines at 615 nm due to the 5D0 ? 7F2 transition of Eu3+. The chromaticity coordinates of phosphors with trivalent europium ions, such as the Na5Y(MoO4)4:Eu3+ phosphor, are x = 0.671 and y = 0.328 [14]. The K2SiF6:Mn4+ phosphor showed better chromaticity coordinates than those of the well-known phosphors with trivalent europium ions. This result indicated that the K2SiF6:Mn4+ phosphor is the best red-emitting phosphor among phosphorconverted three-band white LEDs. The brightest emission profile was sought by investigating the optimal experimental conditions needed for the preparation of a K2SiF6:Mn4+ phosphor. Fig. 3 shows the emission spectra of the K2SiF6:Mn4+ phosphors synthesized using different volumes of the HF solution. The emission intensities of the K2SiF6:Mn4+ phosphors dramatically increased as the volume of HF solution increased from 30 to 50 mL, and a slight increase was obtained as the HF solution was further increased up to 70 mL. At HF solution volumes up to 110 mL, however, the emission intensities decreased slightly. The brightest red emission was obtained from the K2SiF6:Mn4+ phosphor prepared using 70 mL of the HF solution. Fig. 4 shows SEM images of K2SiF6:Mn4+ phosphors prepared with different amounts of HF used. The mean size of K2SiF6:Mn4+ phosphor is slightly increased from 10 lm to 14 lm as increasing the amount of HF from 30 to 110 mL. The some aggregated particles K2SiF6:Mn4+ phosphor is also observed in K2SiF6:Mn4+ phosphor prepared with larger amount of HF used. As the particle size is
Fig. 3. Emission spectra of the K2SiF6:Mn4+ phosphors prepared with different volumes of the HF solution. The inset shows the relative intensity at 632 nm as a function of the HF solution volume: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, and (e) sample 5.
increased slightly with the HF amounts, the size effect on the brightness of K2SiF6:Mn4+ phosphor is negligible. Fig. 5 shows the emission spectra of the K2SiF6:Mn4+ phosphors prepared with different amounts of KMnO4. The brightest emission was obtained from the K2SiF6:Mn4+ phosphor prepared using 25 mmol KMnO4. Only Mn4+ ion was incorporated in K2SiF6 host. Mn4+ ion was formed by reduction from KMnO4 with H2O2. H2O2 can reduce only small portion of Mn7+ ions in KMnO4 to Mn4+ ions. It is important to examine the concentration of Mn4+ ions of K2SiF6: Mn4+ phosphors. Lv et al. measured the concentrations of Mn4+ ions for K2TiF6:Mn4+ phosphors through ICP-AES [31]. We measured the relative atomic amounts of Mn and K by using ICP-MS. The atomic ratio of Mn to K is 0.0125 for sample 3. It indicates that approximately 1% of Mn7+ ions in KMnO4 are reduced to Mn4+ ions in our reaction system. Unfortunately, we did not measure the relative atomic amounts of Mn and Si by using ICP-MS, due to the severe interference for the measurement of Si [32,33]. Based on the formula of K2Si(1 x)F6:xMn4+ phosphors, we calculated that mol% of Mn4+ ions is 2.50 (x = 0.025) from ICP-MS data of sample 3. Calculated mol% of Mn4+ ions of K2Si(1 x)F6:xMn4+ phosphors are 2.08, 2.50 3.34, 3.77 for samples 6, 3, 7, 8, respectively. We observed concentration quenching effect of Mn4+ ions as activator, as shown in Fig. 5. The maximum concentration of Mn4+ ion for the brightest emission of K2SiF6:Mn4+ phosphors is 3.34 mol%. The optimal H2O2 concentration was sought by measuring the emission spectra of the K2SiF6:Mn4+ phosphors prepared with different amounts of H2O2, as shown in Fig. 6. The concentration of Mn4+ ion depends on both amounts of KMnO4 and H2O2. Therefore, it is important to find the optimal reactant conditions for the brightest emission of K2SiF6:Mn4+ phosphors. We also prepared the K2SiF6:Mn4+ phosphor washed with ethanol and dried under vacuum at room temperature. This product shows brighter emission by factor of 1.31 than that washed with water and then dried at 60 °C in an oven. Fig. 7 shows the emission spectra of the K2SiF6:Mn4+ phosphors prepared with different washing and drying conditions. When the K2SiF6:Mn4+ products washed with water and then dried at 60 °C in an oven, the hydrolysis processes resulted in lower brightness. Fig. 8 shows the emission spectra of the green-emitting SrGa2S4:Eu2+ and red-emitting K2SiF6:Mn4+ phosphors under excitation at 465 nm, the exact emission wavelength of the blue-emitting InGaN chip. SrGa2S4:Eu2+ has a broad absorption band between 350 nm and 500 nm due to the 4f7 ? 4f65d1 transition of the Eu2+ ions [27]. The emission spectrum of SrGa2S4:Eu2+ showed a single band around 535 nm due to a transition from the lowest excited state, 4f65d1, to the 4f7 ground state of the Eu2+ ions in the SrGa2S4 host lattice, as shown in Fig. 8(a) [27]. The emission spectrum of K2SiF6:Mn4+ included three sharp deep red emission peaks between 600 and 650 nm, as shown in Fig. 8(b). These results indicated that the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors could be used as green- and red-emitting phosphors excited by the blue InGaN chip in a phosphor-converted three-band white LED, respectively. The feasibility of using the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors was investigated by fabricating phosphor-converted threeband LEDs by coating different amounts of SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors onto a blue LED chip. Fig. 9 shows the photoluminescence spectra of the four three-band LED fabricated in this study. The amounts of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors were adjusted by varying the thickness of the phosphor layer applied to the PET film. Distinct emission peaks were obtained at 465, 535 nm, and a series of peaks around 635 nm, originating from the blue LED chip, green-emitting SrGa2S4:Eu2+, and redemitting K2SiF6:Mn4+ phosphors, respectively. The relative amounts SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors were fixed to 1:1 to obtain white light, as defined by the Commission Interna-
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Fig. 4. SEM images of K2SiF6:Mn4+ phosphors prepared with different volumes of the HF solution: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, and (e) sample 5.
Fig. 5. Emission spectra of the K2SiF6:Mn4+ phosphors prepared with different amounts of KMnO4. The inset shows the relative intensity at 632 nm as a function of the KMnO4 content: (a) sample 6, (b) sample 3, (c) sample 7, and (d) sample 8.
Fig. 6. Emission spectra of the K2SiF6:Mn4+ phosphors prepared with different volumes of the H2O2 solutions. The inset shows the relative intensity at 632 nm as a function of the H2O2 solution volume: (a) sample 7, (b) sample 9, (c) sample 10, and (d) sample 11.
tional de l’Eclairage (CIE), with chromaticity coordinates of x = 0.35 and y = 0.33, as shown in Fig. 10(b). Setting the relative molar ratios of SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors to 1:0.67 yielded bluish white light with CIE coordinates of x = 0.33 and y = 0.30, as shown in Fig. 10(a). Greenish white light with CIE coordinates of x = 0.35 and y = 0.43 was obtained at a SrGa2S4:Eu2+:K2SiF6:Mn4+ phosphor molar ratio of 0.83:1.33, as shown in Fig. 10(c). The use of large quantities of SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors, in
a molar ratio of 1.33:1.33, yielded reddish white light with CIE coordinates of x = 0.48 and y = 0.39, as shown in Fig. 10(d). These results revealed that we could fine-tune the color and CIE coordinates of the LEDs by varying the amounts of both SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors. Fig. 10 shows the CIE chromaticity coordinates of the photoluminescence spectra of the four threeband LEDs fabricated. The CIE chromaticity coordinates of the National Television Standard Committee (NTSC), that is, blue
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Fig. 7. Emission spectra of the K2SiF6:Mn4+ phosphors (a) washed with water and then dried at 60 °C in an oven and (b) washed with ethanol and dried under vacuum at room temperature.
Fig. 10. CIE chromaticity coordinates of the photoluminescence spectra of four three-band LED fabricated by coating different molar ratios of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors onto the blue LED chip. The molar ratios of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors were (a) 1:0.67, (b) 1:1, (c) 0.83:1.33, and (d) 1.33:1.33. The NTSC chromaticity coordinates in the red, green, and blue are included.
(0.283, 0.682), and (0.696, 0.304), respectively. The triangular area of the three-band LED was 96.5% the area of NTSC. This result indicated that the color rendering of the three-band LED fabricated by combing the blue LED chip, green-emitting SrGa2S4:Eu2+, and redemitting K2SiF6:Mn4+ phosphors was excellent and may be used as a display white light source. 4. Conclusions Fig. 8. Emission spectra of the (a) green-emitting SrGa2S4:Eu2+ and (b) red-emitting K2SiF6:Mn4+ phosphors under excitation at 465 nm.
Fig. 9. Photoluminescence spectra of four three-band LED fabricated by coating different relative quantities of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors onto the blue LED chip. The molar ratios of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors were (a) 1:0.67, (b) 1:1, (c) 0.83:1.33, and (d) 1.33:1.33.
(x = 0.48 and y = 0.08), green (x = 0.21 and y = 0.71), and red (x = 0.67 and y = 0.33), are shown in Fig. 10 [30]. The color purity of a display panel may be depicted, for reference purposes, using the NTSC triangle formed by connecting the chromaticity coordinate positions. The CIE chromaticity coordinates of the blue LED, SrGa2S4:Eu2+, and K2SiF6:Mn4+ phosphors are (0.119, 0.092),
K2SiF6:Mn4+ phosphors were prepared via a precipitation reaction from mixed solution containing SiO2, HF, KMnO4, and H2O2 at room temperature. The K2SiF6:Mn4+ phosphor had a strong broad absorption band around 465 nm, the exact wavelength of the blue-emitting InGaN chip. The K2SiF6:Mn4+ phosphor emitted deep red light characterized by three sharp emission lines between 600 and 650 nm. K2SiF6:Mn4+ could be used as a bright redemitting phosphor in a phosphor-converted three-band white LED using the blue-emitting InGaN chip. The optimal condition for obtaining the brightest emission from the K2SiF6:Mn4+ phosphor was determined experimentally. Mixtures of the greenemitting SrGa2S4:Eu2+ and red-emitting K2SiF6:Mn4+ phosphors were coated onto the blue-emitting InGaN chip to fabricate a phosphor-converted three-band white LED. Fine color tuning in the three-band LED was accomplished by varying the amounts of the SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors in the coating solution. The phosphor-converted three-band white LED prepared with SrGa2S4:Eu2+ and K2SiF6:Mn4+ phosphors showed excellent color rendering and may be applied as white light sources in displays. Acknowledgment This study was supported by the research fund of Dankook University in 2015. References [1] S. Nakamura, The roles of structural imperfections in InGaN-based blue lightemitting diodes and laser diodes, Science 281 (1998) 956–961. [2] S. Nakamura, III–V nitride based light-emitting devices, Solid State Commun. 102 (1997) 237–243.
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