A Novel Red-Emitting Na₂NbOF₅:Mn⁴⁺ Phosphor with Ultrahigh Color Purity for Warm White Lighting and Wide-Gamut Backlight Displays

Abstract

In this work, a novel red-emitting oxyfluoride phosphor Na₂NbOF₅:Mn⁴⁺ with an ultra-intense zero-phonon line (ZPL) was successfully synthesized by hydrothermal method. The phase composition and luminescent properties of Na₂NbOF₅:Mn⁴⁺ were studied in detail. The photoluminescence excitation spectrum contains two intense excitation bands centered at 369 and 470 nm, which match well with commercial UV and blue light-emitting diode (LED) chips. When excited by 470 nm blue light, Na₂NbOF₅:Mn⁴⁺ exhibits red light emission dominated by ZPL. Notably, the color purity of the Na₂NbOF₅:Mn⁴⁺ red phosphor can reach 99.9%. Meanwhile, the Na₂NbOF₅:Mn⁴⁺ phosphor has a shorter fluorescence decay time than commercial K₂SiF₆:Mn⁴⁺, which is conducive to fast switching of images in display applications. Profiting from the intense ZPL, white light-emitting diode (WLED) with high color rendering index of Ra = 86.2 and low correlated color temperature of T_c = 3133 K is realized using yellow YAG:Ce³⁺ and red Na₂NbOF₅:Mn⁴⁺ phosphor. The WLED fabricated using CsPbBr₃ quantum dots (QDs) and red Na₂NbOF₅:Mn⁴⁺ phosphor shows a wide color gamut of 127.56% NTSC (National Television Standard Committee). The results show that red-emitting Na₂NbOF₅:Mn⁴⁺ phosphor has potential application prospects in WLED lighting and display backlight.

1. Introduction

High luminescence efficiency, environmentally friendly features and long operating lifetimes are all advantageous performance aspects of white light-emitting diodes (WLEDs), which have obtained widespread attention. WLEDs have been widely used in solid-state illumination and liquid crystal display backlight [1,2,3,4,5,6]. At present, the wide color gamut WLED backlight is mainly composed of blue InGaN chip, K₂SiF₆:Mn⁴⁺ red phosphor and green β-SiALON:Eu²⁺ phosphor [7,8,9]. However, the long decay time (~8 ms) of K₂SiF₆:Mn⁴⁺ (KSF:Mn⁴⁺) red phosphor easily affects the image-retention performance of fast-response backlight displays [10,11,12]. Apparently, phosphor for LED backlight should not only possess a broad excitation band appropriate to LED chip emission and narrow band emission with high color purity, but also have appropriate decay time [13]. Therefore, the exploration of novel red-emitting phosphors with high color purity and short fluorescence lifetime for backlight displays is necessary.

For solid-state lighting, the commercial WLED is phosphor-converted light-emitting diode (LED) fabricated by a combination of InGaN chip and YAG:Ce³⁺ phosphor. However, due to the lack of red component, this type of WLED exhibits a cold white light emission with low color-rendering index (CRI, Ra < 80) and high correlated color temperature (CCT, T_c > 4500 K). It is thus clear that red phosphor is an important part of assembling high-CRI light sources. Presently, non-rare-earth Mn⁴⁺ red phosphors have been used in the packaging of WLEDs due to their high luminous efficacy and low cost [14,15,16]. Under UV or blue light excitation, Mn⁴⁺-doped oxide phosphors can emit a moderate-intensity deep red light in the range of 650–720 nm [17,18,19,20,21,22]. Fluoride phosphors (e.g., A₂BF₆:Mn⁴⁺; A = Na, K, Rb, Cs; B = Si, Ti, Ge) show strong red emission with high color purity around 630 nm [23,24,25,26]. The oxyfluoride compounds are regarded as succedaneous hosts for Mn⁴⁺ substitution because Mn⁴⁺ in some oxyfluoride hosts also presents parallel photoluminescent (PL) properties with Mn⁴⁺-activated fluoride phosphors. More interestingly, the oxyfluoride compounds may induce Mn⁴⁺ to exhibit excellent luminescence properties owing to distorted octahedral sites and F^- and O^2- mixed ligands [27]. Hence, the exploration for new Mn⁴⁺-doped red-emitting phosphors based on oxyfluorides is of great significance.

Recently, Mn⁴⁺-doped oxyfluoride red phosphors have been reported successively, such as ANaWO₂F₄:Mn⁴⁺ (A = Li, Na, K) [28], Na₂WO₂F₄:Mn⁴⁺ [29], Cs₂NbOF₅:Mn⁴⁺ and Rb₂NbOF₅:Mn⁴⁺ [30,31]. However, to the best of our knowledge, the study on the luminescence properties of Na₂NbOF₅:Mn⁴⁺ has not been reported. Herein, we synthesized a novel red-emitting oxyfluoride phosphor Na₂NbOF₅:Mn⁴⁺ for the first time and systematically investigated its crystal structure, composition and PL properties. Finally, white LED for indoor lighting and backlight displays was packaged by employing the as-prepared Na₂NbOF₅:Mn⁴⁺ phosphor as a red supplement.

2. Experimental Section

2.1. Sample Preparation

The starting materials Nb₂O₅ (99.99%), NaF (A.R.), HF solution (40 wt%, A.R.), ethanol (AR, 95%) and methyl alcohol (AR, 99.5%) were used without any purification. K₂MnF₆ was obtained through an optimized route reported by Verstraete [32].

The experimental process of synthesizing Na₂NbOF₅:xMn⁴⁺ (abbreviated as NNOF:xMn⁴⁺) red-emitting phosphors is shown in Figure 1. NaF (0.2688 g, 0.0064 mol), Nb₂O₅ (0.8058 g, 0.0032 mol) and 40% aqueous HF (2.88 mL) were added into a teflon pouch. Two pouches were placed in a 150 mL Teflon-lined stainless-steel autoclave filled with 50 mL deionized H₂O as backfill and heated at 150 °C for 24 h, which were then slowly cooled to room temperature at 10 °C/h. Different amounts of K₂MnF₆ were dissolved in the solution with ultrasonic vibration until a light gold solution was formed in the pouch. To research the effect of the concentration of Mn⁴⁺ on the obtained phosphors, a series of NNOF:xMn⁴⁺ samples with different concentrations of Mn⁴⁺ were prepared using the same method according to parameters listed in

Table 1. Synthesis parameters of NNOF:xMn⁴⁺ phosphors with different doping amounts of Mn⁴⁺.

Samples	The Molar Quantities of K2MnF6 (mol)	Actual Doping Amount of Mn4+ (x mol)
1	3.2 × 10−6	0.001
2	6.4 × 10−6	0.002
3	9.6 × 10−6	0.003
4	1.6 × 10−5	0.005
5	3.2 × 10−5	0.01
6	6.4 × 10−5	0.02
7	9.6 × 10−5	0.03

. After that, 5 mL of methanol was slowly injected into the pouch to obtain precipitation. The precipitate was further washed with ethanol, centrifuged three times to remove impurities and then dried in an oven at 60 °C for 3 h.

2.2. Characterization

The phase purity of the as-prepared samples was initially identified by taking X-ray diffraction (XRD) measurements from a X-ray powder diffractometer (Ultima IV-185, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å). The diffraction patterns were scanned at a scanning speed of 8°/min in the 2θ range from 10° to 80°. The infrared (IR) data was monitored by Fourier Transform Infrared Spectrometer (Bruker Tensor 27, Karlsruhe, Germany). The photoluminescence excitation (PLE) and emission (PL) spectra were obtained via a spectrophotometer (F-7000, HITACHI, Tokyo, Japan). Diffuse reflection spectrum was obtained using the spectrometer (Cary-5000, Varian, Palo Alto, CA, USA). The luminescence decay curve was recorded by a spectrometer (FS5, Edinburgh, UK). The morphology and elemental composition of the product were obtained by a scanning electron microscopy (SEM, JEOL JSM-6510, Tokyo, Japan) with an energy-dispersive spectrometer (EDS).

3. Results and Discussion

3.1. X-ray Diffraction and Structure Analysis

Figure 2A shows the XRD patterns of Na₂NbOF₅:Mn⁴⁺ (NNOF:Mn⁴⁺) red phosphors doped with different doping amounts of Mn⁴⁺ and the enlarged XRD patterns in 2θ region of 27.5–28.5°. All the diffraction peaks of the samples matched with the Na₂NbOF₅ standard card (ICSD-48165, space group Pcnb (60), a = 5.089(1) Å, b = 5.512(1) Å, c = 18.207(4) Å, cell volume V = 510.72(18) ų) and no impurity phase was found. The main diffraction peak moved to a higher angle with the increase in Mn content. According to Bragg’s diffraction law, the diffraction peak will move to a higher angle when small ions replace large ions into the lattice. The result indicated that the smaller Mn⁴⁺ (r = 0.53 Å, CN = 6) replaced the larger Nb⁵⁺ (r = 0.64 Å, CN = 6) into the lattice. However, when tetravalent Mn replaces pentavalent Nb into the lattice, charge mismatch occurs. A positive charge is required in the structure to maintain electrical neutrality. Positively charged oxygen vacancies are most likely to appear in the structure. This possible charge compensation can be represented by the following equation according to the Kröger–Vink notation [30]:

$${\mathrm{K}}_2{\mathrm{MnF}}_6 \stackrel{{\mathrm{Na}}_2{\mathrm{NbOF}}_5}{\to } 2{\mathrm{K}}_{\mathrm{Na}}^{\times }+{\mathrm{Mn}}_{\mathrm{Nb}}^{\prime }+{\mathrm{V}}_{\mathrm{O}}^{••}+5{\mathrm{F}}_{\mathrm{F}}^{\times }+{ \mathrm{F}}_{\mathrm{i}}^{\prime }$$

(1)

where ${\mathrm{Mn}}_{\mathrm{Nb}}^{\prime }$ is the negative charge defect produced by the substitution of Nb⁵⁺ with Mn⁴⁺, ${\mathrm{V}}_{\mathrm{O}}^{••}$ is the oxygen vacancy and ${ \mathrm{F}}_{\mathrm{i}}^{\prime }$ is the fluorine interstitial ion. The charge-balance is achieved by fluorine interstitial ion and oxygen vacancy.

Figure 2B depicts the simulated structure of the NNOF unit cell, where six twisted [NbOF₅]²⁻ octahedra are regularly distributed in the cell. Figure 2C clearly depicts the coordination environment surrounding Nb. It is noticeable that Nb⁵⁺ coordinates six O²⁻/F⁻ to form a distorted [NbOF₅]²⁻ octahedron, and the bond lengths of Nb-O₁/F₁, Nb-O₂/F₂, Nb-F₃, Nb-F₄, Nb-F₅, Nb-F₆ bonds are 1.765, 1.931, 1.974, 2.095, 1.953, 1.925 Å, respectively. At the same time, each bond angle of the [NbOF₅]²⁻ octahedron is significantly different from the ideal bond angle (90°) of the regular octahedron.

Figure 3 shows the IR spectrum of NNOF:Mn⁴⁺ at room temperature. The wide band at 3433 cm⁻¹ is due to the vibration of the O–H bonds, and the small peak at 1626 cm⁻¹ is attributable to the bending vibration of the O–H bonds in the water adhering to the surface of the NNOF:Mn⁴⁺ minute particles. The IR spectrum shows two strong sharp peaks at 925 and 528 cm⁻¹, which are consistent with the Nb–O and Nb–F bonds in the structure, respectively [33].

3.2. Morphology and Composition Identification

Figure 4A exhibits the SEM image of NNOF:Mn⁴⁺ phosphor. The obtained powder is composed of the irregular particle with clear edges and corners, indicating good crystallization of the sample. As shown in the EDS spectrum (Figure 4B), NNOF:Mn⁴⁺ red phosphor is composed of Na, Nb, O, F and Mn elements. The small amount of Mn in the test results indicates that Mn⁴⁺ has been successfully doped into the NNOF matrix. The atom percentages of Na, Nb, O and F are 21.72%, 12.67%, 10.86% and 54.11%, respectively, which are close to the stoichiometric ratio of 2:1:1:5 in the matrix. These data further confirmed the successful preparation of NNOF:Mn⁴⁺ phosphor. In Figure 4C–H, the EDS element mapping chart further proved the existence and uniform distribution of Na, Nb, O, F and Mn elements, and further confirmed the composition of NNOF:Mn⁴⁺.

3.3. Photoluminescence Properties

The emission spectra of NNOF:Mn⁴⁺ phosphors with different Mn⁴⁺ concentrations are shown in Figure 5A. When the concentration is 0.003, the luminous intensity is at the highest value. Due to the effect of concentration quenching, the luminous intensity of phosphor decreases with the increasing of Mn⁴⁺ concentration [34].

The red phosphor excited by blue chip used in the WLED requires a wide absorption in the blue region and an effective emission near the ideal red light within 650 nm. Figure 5B shows the PLE and PL spectra of NNOF:Mn⁴⁺ at room temperature. Obviously, two intense excitation bands centered at 369 (27,100 cm⁻¹) and 470 nm (21,277 cm⁻¹) can be observed in the excitation spectrum, which are caused by the spin allowed ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ transitions of the Mn⁴⁺ ions, respectively [35,36]. Under 470 nm excitation, NNOF:Mn⁴⁺ exhibits a narrow peak emission distributed between 575 and 675 nm. The results of excitation and emission spectra attested that the prepared NNOF:Mn⁴⁺ samples can be excited by blue light effectively and produce effective red emission. Meanwhile, the NNOF:0.003Mn⁴⁺ exhibits photoluminescence quantum yields (PLQYs) of 68.3% under 470 nm blue light excitation. The PLQY was obtained according to the method found in the reported work [37]. Notably, the intensity of zero-phonon line (ZPL) emission is higher than that of phonon sideband, which is different from most of previously reported emission spectra of Mn⁴⁺. The sharp ZPL emission peaking at 620 nm and Stokes/anti-Stokes phonon sidebands are derived from the coupling of antisymmetric v₃, v₄, and v₆ to the ZPL. The emission spectrum of Mn⁴⁺ doped phosphor is usually dominated by the anti-Stokes/Stokes phonon sideband, and the vibronic transition v₆ is always at the highest peak in the PL spectrum. Meanwhile, the ZPL of Mn⁴⁺:²E → ⁴A₂ is generally very weak. Interestingly, the intensity of ZPL in the emission spectrum of NNOF:Mn⁴⁺ is higher than that of the v₆ sideband. Herein, the low symmetry of the Mn⁴⁺ center is considered to be the main reason for the intense ZPL in NNOF:Mn⁴⁺ [29,38,39]. The ultra-high ZPL emission is conducive to improving the color purity of red phosphors [40].

Figure 6 shows the diffuse reflectivity spectra of pristine NNOF and NNOF:0.003Mn⁴⁺ phosphor, from which it can be observed that the NNOF:0.003Mn⁴⁺ phosphor has an absorption band at 470 nm corresponding to the ⁴A₂ → ⁴T₂ electron transition of Mn⁴⁺. Due to the strong intrinsic absorption of pristine NNOF, the absorption band near 369 nm, which responds to the ⁴A₂ → ⁴T₁ electron transition of Mn⁴⁺, was completely covered. Compared with Mn⁴⁺ and Mn²⁺, Mn³⁺ is rarely encountered in the literature about luminescence. The Mn³⁺ ion has the corresponding characteristic absorption band due to the 3d⁴ configuration. In Figure 6, the ⁵E′ → ⁵T₂ and ⁵E′ → ⁵E″ absorption bands of Mn³⁺ can be clearly observed in the 500–1100 nm range. The presence of Mn³⁺ impurity ions will lead to the reduction of quantum efficiency of phosphor [32].

3.4. Decay Curves, Chromaticity Coordinates (CIE) and Color Purity

The decay time of phosphor is non-negligible in the application of display backlight. Long decay time phosphors may cause a certain degree of lag in image conversion. Figure 7A shows the photoluminescence decay curve of NNOF:0.003Mn⁴⁺ phosphor. The data of the luminescence decay curve conforms to the mono-exponential decay mode, as shown in the following formula:

$$I(t) = I_0+\mathrm{A} \exp (-\frac{t}{\tau })$$

(2)

where I₀ and I(t) are the initial luminous intensity and the luminous intensity at time t, respectively, and τ represents the fluorescent lifetime. τ is then calculated to be 3.32 ms. The millisecond scale of the lifetime indicates that Mn⁴⁺ ions present forbid transitions in the intra-d-shell [8]. The NNOF:0.003Mn⁴⁺ phosphor with short fluorescence decay time (<5 ms) will be a hopeful red component for fast-response backlight displays. Herein, the decay time of NNOF:0.003Mn⁴⁺ is shorter than that of K₂SiF₆:Mn⁴⁺. To specify the reason for the shorter life of NNOF:0.003Mn⁴⁺, the distortion of the coordination polyhedron (DI) is calculated. The distortion degree of polyhedron is characterized by bond angle variance (σ²) and mean quadratic elongation (λ), which can be determined by following formula [29]:

$${\sigma }^2=\frac{1}{6}{{\displaystyle \sum }}_{\mathrm{i}=1}^6{\left(l_{\mathrm{i}}/l_0\right)}^2$$

(3)

$$\lambda =\frac{1}{11}{{\displaystyle \sum }}_{\mathrm{i}=1}^{12}{\left({\theta }_{\mathrm{i}}/{\theta }_0\right)}^2$$

(4)

where l₀ is the distance from the center to the vertex of the regular octahedron with the same volume as the octahedron structure, l_i is the bond length of the studied octahedron, θ₀ is the ideal bond angle (90°) of the regular octahedron and θ_i is the bond angle of the twisted octahedron. The corresponding values are shown in

Table 2. Polyhedral distortion index and corresponding zero-phonon line (ZPL) intensities in Na₂NbOF₅ and K₂SiF₆ matrices.

Compounds	σ2	λ	ZPL Intensity
Na2NbOF5	43.6362	1.0184	Very strong
K2SiF6	0.0000	1.0000	Very weak

. The results indicate that the coordination environment of Mn⁴⁺ in NNOF:xMn⁴⁺ is extremely distorted compared to K₂SiF₆:Mn⁴⁺. In fact, it is understandable that NNOF has a higher degree of distortion since the anion coordinated with the cation is mixed-anion with unequal radius. It has been reported that Mn⁴⁺ exhibits good luminescence properties in a highly symmetric structure [28]. Nevertheless, mixed anion coordination offers more possibilities for luminescent behavior. We believe that the rapid decay may be due to the low symmetry of Mn⁴⁺.

The CIE chromaticity coordinate of NNOF:0.003Mn⁴⁺ sample is shown in Figure 7B. It is observed that the prepared NNOF:0.003Mn⁴⁺ red phosphor emits a strong red light under the excitation of 365 nm ultraviolet lamp. The CIE chromaticity coordinates are (0.6819, 0.3179). To further understand the chromatic behaviors of the phosphor, its color purity was found by using the following formula [41]:

$$\mathrm{Color} \mathrm{purity}=\frac{\sqrt{{(x - x_{\mathrm{i}})}^2- (y -{ y}_{\mathrm{i}}{)}^2}}{\sqrt{{{(x}_{\mathrm{d}}{ - x}_{\mathrm{i}})}^2{ - (y}_{\mathrm{d}} - y_{\mathrm{i}}{)}^2}} \times 100\%$$

(5)

Hereon, (x, y) represents the CIE coordinates of NNOF:0.003Mn⁴⁺ red phosphor, (x_i, y_i) represents the chromaticity coordinate of the equal-energy white light source with the value of (0.3333, 0.3333), and (x_d, y_d) stands for the CIE coordinates of the corresponding dominant wavelength of the illuminant. The calculated color purity of NNOF:0.003Mn⁴⁺ phosphor is about 99.9%, which is higher than the reported color purity of Mn⁴⁺-doped red phosphors, such as Cs₂NbOF₅:Mn⁴⁺ (99%) [30], K₂LiAlF₆:Mn⁴⁺ (89%) and K₂NaAlF₆:Mn⁴⁺ (97%) [42,43]. Obviously, the stronger ZPL emission can bring about a higher color purity. NNOF:Mn⁴⁺ red phosphor is very suitable for the application in LED backlight due to its ultra-high color purity.

3.5. Electroluminescence (EL) Performance of the Packaged WLEDs

Figure 8A shows the EL spectra and photographs of the packaged WLEDs. Curve (i) is the spectrum of WLED produced by YAG:Ce³⁺ phosphor coupled with InGaN blue chip (3 V, 20 mA), and curve (ii) is the EL spectrum of WLED with the addition of NNOF:Mn⁴⁺ red phosphor as a contrast. Compared with the scheme of blue chip + YAG:Ce³⁺ (Ra = 72, CCT = 6297 K), the LED device with added NNOF:Mn⁴⁺ red component emits high-brightness warm white light (Ra = 86.2, CCT = 3133 K), indicating that the addition of NNOF:Mn⁴⁺ can improve the color rendering index (CRI) and correlated color temperature (CCT). Even more to the point, the luminous efficiency of WLED using NNOF:Mn⁴⁺ as red component can reach as high as 106.05 lm/W.

As shown in Figure 8B, the white triangle region is the color gamut composed of standard red (0.67, 0.33), blue (0.21, 0.71) and green (0.14, 0.08) coordinates, which is defined by the National Television Standards Committee (NTSC). When the chromaticity coordinates (0.6819, 0.3179) of the prepared NNOF:0.003Mn⁴⁺ phosphor are matched with the standard blue and green coordinates, we can obtain a larger gamut with a calculated value of 102.63% NTSC as depicted in the red dotted triangle in Figure 8B. To prove the application potential of the synthesized NNOF:Mn⁴⁺ red phosphor in the field of LED backlight displays, the EL spectrum of the WLED constructed with green-emitting CsPbBr₃ quantum dots (abbreviated as CPB QDs) and NNOF:Mn⁴⁺ red phosphor is shown in curve (iii) in Figure 8A. The color gamut of the produced WLED device is shown in the black frame in Figure 8B, which is calculated as 127.56% of the NTSC color gamut and overlaps with NTSC by 99.46%. These results show the prospect of the as-prepared NNOF:Mn⁴⁺ red phosphor for its application in the field of backlight displays.

4. Conclusions

A novel Mn⁴⁺ doped oxyfluoride phosphor was successfully synthesized by hydrothermal method. The prepared NNOF:Mn⁴⁺ red phosphor can be matched well with commercial UV and InGaN blue chips because of its wide excitation band in the near ultraviolet and blue regions. When excited by blue light, the sample exhibited ultra-intense ZPL emission at 620 nm. Remarkably, the color purity of NNOF:Mn⁴⁺ can reach as high as 99.9%. Moreover, the WLED fabricated by using NNOF:Mn⁴⁺ red phosphor and commercial YAG:Ce³⁺ produced warm white light emission with low CCT value of 3133 K, high Ra value of 86.2 and luminous efficiency of 106.05 lm/W. Finally, a white LED with a wide color gamut of 127.56% NTSC was packaged on a InGaN blue chip using NNOF:Mn⁴⁺ red phosphor and green-emitting CPB QDs. These results show that NNOF:Mn⁴⁺ red phosphor has potential application prospects in lighting or display backlights.