Luminescence properties of Na₂SiF₆:Mn⁴⁺ red phosphors for high colour-rendering white LED applications synthesized via a simple exothermic reduction reaction

Abstract

Na₂SiF₆:Mn⁴⁺ red phosphor is reported in this paper. Na₂SiF₆:Mn⁴⁺ was synthesized via a simple exothermic reduction reaction approach using Na₂SiF₆ and NaMnO₄ as raw materials. H₂O₂ was dropped into 0.02 mol L⁻¹ of NaMnO₄ in HF solution to reduce Mn⁷⁺ to Mn⁴⁺ at room temperature. The phosphors can produce sharp red emissions in a broad absorption range (from 500 nm to 700 nm). The white LEDs were fabricated with blue GaN chips merged with a mixture of Na₂SiF₆:Mn⁴⁺ and commercial YAG:Ce yellow phosphors for white light.

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Introduction

Solid-state white light-emitting diodes (LEDs) are new generation lighting sources to replace traditional fluorescent lamps and incandescent light because of their various advantages such as low power consumption, long service life, fast response, high efficiency, small size, safety, and so on.^1–13 At present, the most mature method for the preparation of white LEDs is a blue chip combined with a Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) yellow phosphor. However, due to the lack of a red light component in the emission spectrum of YAG:Ce³⁺, it is hard to obtain white LEDs with a high color-rendering index (CRI, R_a > 80) and low color temperature (CC < 4000 K) for application in indoor lighting.^14–16 During this year, great efforts have been made to explore novel rare-earth doped red phosphors to solve these problems. Of these, Eu²⁺-doped alkaline silicates, typically including Sr₂SiO₄:Eu²⁺ and Sr₃SiO₅:Eu²⁺, show a very broad excitation band and an effective absorption covering the blue spectra in the range of 450–470 nm.¹⁷ But these phosphors still have several drawbacks, such as hygroscopicity and bad thermostability.¹⁸ Now, Eu²⁺-doped (oxy)nitrite compounds, for example LiSr₄B₃O_(9−3x/2)N_x:Eu²⁺, Sr₂Si₅N₈:Eu²⁺, CaAlSiN₃:Eu²⁺, are the most successful red phosphors for white LEDs.^19–21 These phosphors were researched and have wide applications, but they are difficult to prepare under the rigorous synthesis conditions required for these compounds. To avoid the above drawbacks, an ideal red phosphor should possess relatively narrow absorption and emission bands, low-production costs and easy synthesis.

Recently, narrow-band red-emitting fluoride phosphors doped with Mn⁴⁺ have drawn increasing attention due to their good thermal stability and potential applications in white LED devices. Mn⁴⁺ is a transition metal ion with an outer 3d³ electron configuration. A spectral feature of Mn⁴⁺ is that it exhibits both broadband excitation and a sharp emission band due to its distinct electronic structure. Its optical properties are strongly influenced by local crystal field environments. Zhu et al. synthesized the K₂TiF₆:Mn⁴⁺ red phosphor, which exhibited the most intense excitation band at ∼468 nm with a bandwidth of ∼50 nm.²² Particularly, they achieved a photoluminescence quantum yield as high as 98% for K₂TiF₆:Mn⁴⁺, which is perfect for blue-chip excitation. Many researchers prepared all sorts of red fluoride phosphors, such as K₂SiF₆:Mn⁴⁺, Na₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, BaSiF₆:Mn⁴⁺, ZnSiF₆:Mn⁴⁺ and so on, which are synthesized via wet chemical etching of silicon wafers in aqueous HF solution with the addition of oxidizing agents NaMnO₄ or KMnO₄ at room temperature.^23–28 The red phosphors have broadband excitation and a highly intense sharp emission band, which offset the lack of a red light component for white LEDs.

In this report, a simple reduction reaction was presented to synthesize red phosphor Na₂SiF₆:Mn⁴⁺ with sharp emission bands. The phosphors could be used for commercial applications, and exhibited intense emission, a short reaction time, and simple post-processing, as well as controllability of reactant size and other advantages. The crystal structure of the host, concentrations of HF and NaMnO₄, and photoluminescence properties are discussed. The prepared fluoride red phosphors are potential candidates, and present a substantial advance for white LED applications.

Experimental

Synthesis

Red phosphor Na₂SiF₆:Mn⁴⁺ was synthesized using a directly mixed method as shown in Fig. 1. The raw materials consisted of a 10 ml HF (wt. 40%) solution, a Na₂SiF₆ (A. R.), H₂O₂ (wt. 35%) solution, and NaMnO₄ (A. R.) powders. The NaMnO₄ and HF (wt. 40%) solutions were mixed thoroughly by magnetically stirring in a Teflon-cup. The HF solution thoroughly dissolves NaMnO₄ powders to obtain solution A at room temperature, and then solution B was formed by adding Na₂SiF₆ to the resulting solution A under continuous stirring for 20 min, meanwhile, this releases a lot of gas and gives off heat in solution B. Thereafter, 2 ml H₂O₂ was added to the deep purple solution, which rapidly turned milky orange. After the reaction, the resulting products were filtered, washed with ethanol, and dried at 70 °C to room temperature. The sample shows that it emits brilliant red light under blue light (365 nm) excitation (Fig. 1a), and shows a pale pink tint under natural light illumination (Fig. 1b).

Fig. 1 Synthesis of the Na₂SiF₆:Mn⁴⁺ phosphor, and samples of Na₂SiF₆:Mn⁴⁺: (a) UV light (λ_ex = 365 nm), (b) room light, and (c) EDX spectrum and SEM image.

Characterization

The crystalline phases of the samples were analyzed using an X-ray diffractometer (XRD, TD-3500, Dandong, China) with Cu Kα radiation in the range of 2θ = 10°–70° with a step angle of 0.06°, a tube voltage of 30 kV and a tube current of 20 mA. The photoluminescent (PL) and PL excitation (PLE) spectra were measured by using a fluorescence spectrophotometer (F-7000, Hitachi, Japan) with a Xe lamp as the excitation light source. The excitation and emission spectra were scanned in the range of 330–500 nm and 550–700 nm, respectively. The surface morphology was observed using a scanning electron microscope (SEM, Quanta 250, FEI, USA).

The Mn⁴⁺ doping percentage in Na₂SiF₆ was estimated from the EDX results and the SEM image of the sample before and after the reaction indicates that the particle size is primarily in the range of 5–10 μm (Fig. 1c). All energy peaks correspond to the elements in Na₂SiF₆:Mn⁴⁺, except for the peaks at 0.25 and 0.5 keV, which correspond to organic matter from the preparation process. ICP-OES analyses show that the Mn⁴⁺ concentrations of the samples are 0.5–4.0 at%, which are increased by increasing the initial NaMnO₄ concentration.

Results and discussion

Composition and structure

Fig. 2 shows the XRD patterns of the red phosphors of the Na₂SiF₆:Mn⁴⁺ samples. It can be seen that all these compounds correspond to Na₂SiF₆ (JCPDS No. 33-1280) with a hexagonal structure (space group – P321, the lattice parameter values are a = 0.8859 nm and c = 0.5038 nm). There are no other special obvious diffraction peaks. Therefore, the doped Mn⁴⁺ ion does not change the main phase of the crystal structure.

Fig. 2 XRD patterns of the Na₂SiF₆:Mn⁴⁺ doped with various Mn⁴⁺ contents: (a) 0.005, (b) 0.01, (c) 0.015, (d) 0.02, (e) 0.03 and (f) 0.04 M.

The crystal structure and cation polyhedra arrangements of Na₂SiF₆ are shown in Fig. 3a. The crystal structure of the Na₂SiF₆ unit cell demonstrates a hexagonal structure. The center atom Si⁴⁺ is consistent of the type of octahedron, which is coordinated by six fluorine atoms with a Si–F distance as marked in Fig. 3b. Each Si atom is surrounded by six fluorine atoms resulting in only one kind of SiF₆²⁻ octahedral structure. Therefore, when doped with Mn⁴⁺ atoms, the Si⁴⁺ atoms of the octahedron were replaced by Mn⁴⁺ atoms to obtain the MnF₆²⁻ octahedral structure in Fig. 3c. The crystal field strength of the octahedral structure is changed with the ingress of Mn⁴⁺, because the radius of Mn⁴⁺ (0.053 nm) is larger than Si⁴⁺ (0.04 nm), which leads to the lattice expansion in the MnF₆²⁻ octahedra, and neighboring lattice contraction. These effects cause unsymmetrical vibrations of the crystal field of the octahedral structure, and the electronic transition of ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g, and finally results in red light emission.

Fig. 3 Crystal structure and cation polyhedron arrangements of the Na₂SiF₆:Mn⁴⁺ phosphor host.

Photoluminescence properties

The PLE and PL spectra of the Na₂SiF₆:Mn⁴⁺ (nMn = 0.02 mol l⁻¹) red phosphors are shown in Fig. 4. The left curve corresponds to the PL excitation spectrum recorded in the range of 330 to 500 nm with monitoring at 631 nm emission. The PL excitation spectrum exhibits two broad bands with a weak peak at 360 nm and a strong peak at 460 nm, which are ascribed to the spin-allowed transitions of ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g respectively. Under 460 nm excitation, the right curve shows the sharp red emission of Na₂SiF₆:Mn⁴⁺ from 500 to 700 nm originating from the spin-forbidden ²E_g/⁴A_2g transitions.^22,29 There are seven peaks at ∼596, 607, 611, 620, 629, 632, and 645 nm, which are related to the transitions of the ν3(t_1u), ν4(t_1u), ν6(t_2u), zero phonon line (ZPL), ν6(t_2u), ν4(t_1u), and ν3(t_1u) vibronic modes, respectively.^23–28

Fig. 4 Excitation and emission spectra, where the excitation spectrum is monitored at λ_em = 631 nm and the emission spectra excited by 460 nm.

Fig. 5 shows the PLE and PL spectra of the Na₂SiF₆:Mn⁴⁺ phosphors synthesized with different concentrations of NaMnO₄. The inset shows the emission intensity, measured at λ_em = 631 nm with excitation at λ_ex = 460 nm, versus NaMnO₄ concentration. The concentration of NaMnO₄ has a pronounced effect on the luminescence intensities of Na₂SiF₆:Mn⁴⁺. We observed that the luminescence intensities of Na₂SiF₆:Mn⁴⁺ increased gradually when the NaMnO₄ concentration increased from 0.005 to 0.02 mol l⁻¹. The highest luminescence intensity was found with the optimized NaMnO₄ concentration of 0.02 mol l⁻¹.

Fig. 5 Excitation and emission spectra with a NaMnO₄ concentration of (a) 0.005, (b) 0.01, (c) 0.015, (d) 0.02, (e) 0.03, and (f) 0.04 mol l⁻¹; the inset curve shows the maximum PL intensities.

The effects of HF concentration on the luminescence of Na₂SiF₆:Mn⁴⁺ are shown in Fig. 6. With the increase in HF concentration, the excitation and emission intensities of Na₂SiF₆:Mn⁴⁺ increased gradually from 10 to 40 wt%. Hence, we can determine the optimized luminescence intensity by the solution with HF concentration at 40 wt%. According to the luminous theory, the results indicated that high HF concentrations might make Mn⁴⁺ easy to introduce into the host.

Fig. 6 Excitation and emission spectra of the Na₂SiF₆:Mn⁴⁺ phosphor with HF concentrations of (a) 10, (b) 20, (c) 30, and (d) 40 wt%.

The colorimetric coordinates (x, y) for the Na₂SiF₆:Mn⁴⁺ phosphor series are calculated using an equidistant wavelength method and are summarized in Table 1. They are depicted in the CIE chromaticity diagram shown in Fig. 7. The colorimetric coordinate of the sample with (nMn⁴⁺) = 0.005 mol l⁻¹ locates in the orange-red region with the value being (0.6364, 0.3566). The colorimetric coordinate shifts to the crimson region and the value fluctuates in a small area for different Mn doping amounts (0.01–0.04 mol l⁻¹). However, the position of the colorimetric coordinates depends on the form of the emission spectrum above.

Table 1 Wavelength and colorimetric coordinates (x, y) with different Mn⁴⁺ concentrations in the Na₂SiF₆:Mn⁴⁺ phosphor series

n (Mn^4+)	Peaks	CIE coordinates (x, y)
		x	y
0.005	629	0.6364	0.3566
0.010	630	0.6708	0.3274
0.015	630	0.6769	0.3222
0.020	631	0.6770	0.3219
0.030	631	0.6762	0.3224
0.040	631	0.6709	0.3268

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Fig. 7 Coordinate location points of Na₂SiF₆:Mn⁴⁺ (0.005 to 0.04 mol l⁻¹) in the CIE chromaticity diagram.

Fabrication of white LEDs using InGaN-based blue LEDs with Na₂SiF₆:Mn⁴⁺ red phosphors

The electroluminescence spectra of the white LEDs under different drive currents are shown in Fig. 8a. We have fabricated white LEDs using blue InGaN chips, commercial YAG:Ce³⁺ yellow phosphors and Na₂SiF₆:Mn⁴⁺ red phosphors as shown in Fig. 8b, and one white LED was illuminated under 120 mA drive current as shown in Fig. 8c, with the luminous efficacy of 101.5 lm W⁻¹ and CRI of 86.5. Table 2 shows different relevant photoelectric parameters, such as the CCT, CRI and chromaticity coordinate for the fabricated white LEDs. The results indicate that the Na₂SiF₆:Mn⁴⁺ red phosphors are potential candidates current mature white LED applications.

Fig. 8 Luminescence spectra based on the Na₂SiF₆:Mn⁴⁺ phosphor under various drive currents (a); white LEDs (b) and bright white-light (c).

Table 2 Photoelectric parameters of typical white LEDs fabricated with different phosphors

White LEDs
Phosphor composition	Y3Al5O12:Ce^3+	Y3Al5O12:Ce^3+ and Na2SiF6:Mn^4+
CCT (K)	5290	4986
R a	75.8	86.5
R 9	8.8	79.1
Chromaticity coordinate	x = 0.342; y = 0.326	x = 0.351; y = 0.321
Current (mA)	120	120
Efficacy (lm W^−1)	106.9	101.5

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Conclusions

In summary, Na₂SiF₆:Mn⁴⁺ red fluoride phosphors were synthesized by using a simple exothermic reduction reaction method. Na₂SiF₆:Mn⁴⁺ had a micrometer-sized hexagonal structure, and the particle size can be controlled from the raw materials used. The prepared phosphors had strong blue-light absorption and intense narrow-band red emission. The optimal concentration of NaMnO₄ in the reaction system for the synthesis of Na₂SiF₆:Mn⁴⁺ is 0.02 mol l⁻¹, which is much lower than those in the previously reported studies, and the spectra of the Na₂SiF₆:Mn⁴⁺ phosphor were affected with disparate HF concentrations. A significant improvement in the CRI of the white LEDs fabricated using the Na₂SiF₆:Mn⁴⁺ red phosphor was observed. The obtained Na₂SiF₆:Mn⁴⁺ phosphors, with efficient red luminescence under blue light excitation, a short reaction time, high yield, and simple post-processing, are a potential candidate for improving the color reproducibility of white LEDs.

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51402032), the China Postdoctoral Science Foundation funded project (Grant No. 2014M562297), the Chongqing Youth Science and Technology talent cultivation project (Grant No. cstc2014kjrc-qnrc40006) and the Chongqing Education Commission funded project (Grant No. KJ1401102).

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