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

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 Tc = 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.


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, K 2 SiF 6 :Mn 4+ red phosphor and green β-SiALON:Eu 2+ phosphor [7][8][9]. However, the long decay time (~8 ms) of K 2 SiF 6 :Mn 4+ (KSF:Mn 4+ ) 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:Ce 3+ 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 Mn 4+ 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, Mn 4+ -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., A 2 BF 6 :Mn 4+ ; 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 Mn 4+ substitution because Mn 4+ in some oxyfluoride hosts also presents parallel photoluminescent (PL) properties with Mn 4+ -activated fluoride phosphors. More interestingly, the oxyfluoride compounds may induce Mn 4+ to exhibit excellent luminescence properties owing to distorted octahedral sites and Fand O 2mixed ligands [27]. Hence, the exploration for new Mn 4+ -doped red-emitting phosphors based on oxyfluorides is of great significance.
The experimental process of synthesizing Na 2 NbOF 5 :xMn 4+ (abbreviated as NNOF:xMn 4+ ) red-emitting phosphors is shown in Figure 1. NaF (0.2688 g, 0.0064 mol), Nb 2 O 5 (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 H 2 O 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 K 2 MnF 6 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 Mn 4+ on the obtained phosphors, a series of NNOF:xMn 4+ samples with different concentrations of Mn 4+ were prepared using the same method according to parameters listed in Table 1. 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. solved in the solution with ultrasonic vibration until a light gold solution was formed in the pouch. To research the effect of the concentration of Mn 4+ on the obtained phosphors, a series of NNOF:xMn 4+ samples with different concentrations of Mn 4+ were prepared using the same method according to parameters listed in Table 1. 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.

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). Figure 2A shows the XRD patterns of Na 2 NbOF 5 :Mn 4+ (NNOF:Mn 4+ ) red phosphors doped with different doping amounts of Mn 4+ and the enlarged XRD patterns in 2θ region of 27.5-28.5 • . All the diffraction peaks of the samples matched with the Na 2 NbOF 5 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) Å 3 ) 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 Mn 4+ (r = 0.53 Å, CN = 6) replaced the larger Nb 5+ (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]:

X-ray Diffraction and Structure Analysis
where Mn Nb is the negative charge defect produced by the substitution of Nb 5+ with Mn 4+ , V •• O is the oxygen vacancy and F i 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 [NbOF 5 ] 2− octahedra are regularly distributed in the cell. Figure 2C Figure 3 shows the IR spectrum of NNOF:Mn 4+ 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:Mn 4+ 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].  Figure 4A exhibits the SEM image of NNOF:Mn 4+ phosphor. The obtained powder is composed of the irregular particle with clear edges and corners, indicating good crystal-  Figure 3 shows the IR spectrum of NNOF:Mn 4+ 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:Mn 4+ 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].   Figure 3 shows the IR spectrum of NNOF:Mn 4+ 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:Mn 4+ 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].  Figure 4A exhibits the SEM image of NNOF:Mn 4+ phosphor. The obtained powder is composed of the irregular particle with clear edges and corners, indicating good crystal-  Figure 4A exhibits the SEM image of NNOF:Mn 4+ 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:Mn 4+ red phosphor is composed of Na, Nb, O, F and Mn elements. The small amount of Mn in the test results indicates that Mn 4+ 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:Mn 4+ 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:Mn 4+ . lization of the sample. As shown in the EDS spectrum ( Figure 4B), NNOF:Mn 4+ red phosphor is composed of Na, Nb, O, F and Mn elements. The small amount of Mn in the test results indicates that Mn 4+ 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:Mn 4+ 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:Mn 4+ .

Photoluminescence Properties
The emission spectra of NNOF:Mn 4+ phosphors with different Mn 4+ 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 Mn 4+ 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:Mn 4+ 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 4 A2 → 4 T1 and 4 A2 → 4 T2 transitions of the Mn 4+ ions, respectively [35,36]. Under 470 nm excitation, NNOF:Mn 4+ exhibits a narrow peak emission distributed between 575 and 675 nm. The results of excitation and emission spectra attested that the prepared NNOF:Mn 4+ samples can be excited by blue light effectively and produce effective red emission. Meanwhile, the NNOF:0.003Mn 4+ 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 Mn 4+ . The sharp ZPL emission peaking at 620 nm and Stokes/anti-

Photoluminescence Properties
The emission spectra of NNOF:Mn 4+ phosphors with different Mn 4+ 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 Mn 4+ concentration [34]. Stokes phonon sidebands are derived from the coupling of antisymmetric v3, v4, and v6 to the ZPL. The emission spectrum of Mn 4+ 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 Mn 4+ : 2 E → 4 A2 is generally very weak. Interestingly, the intensity of ZPL in the emission spectrum of NNOF:Mn 4+ is higher than that of the v6 sideband. Herein, the low symmetry of the Mn 4+ center is considered to be the main reason for the intense ZPL in NNOF:Mn 4+ [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:0.003Mn 4+ phosphor, from which it can be observed that the NNOF:0.003Mn 4+ phosphor has an absorption band at 470 nm corresponding to the 4 A2 → 4 T2 electron transition of Mn 4+ . Due to the strong intrinsic absorption of pristine NNOF, the absorption 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:Mn 4+ 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 4 A 2 → 4 T 1 and 4 A 2 → 4 T 2 transitions of the Mn 4+ ions, respectively [35,36]. Under 470 nm excitation, NNOF:Mn 4+ exhibits a narrow peak emission distributed between 575 and 675 nm. The results of excitation and emission spectra attested that the prepared NNOF:Mn 4+ samples can be excited by blue light effectively and produce effective red emission. Meanwhile, the NNOF:0.003Mn 4+ 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 Mn 4+ . The sharp ZPL emission peaking at 620 nm and Stokes/anti-Stokes phonon sidebands are derived from the coupling of antisymmetric v 3 , v 4 , and v 6 to the ZPL. The emission spectrum of Mn 4+ doped phosphor is usually dominated by the anti-Stokes/Stokes phonon sideband, and the vibronic transition v 6 is always at the highest peak in the PL spectrum. Meanwhile, the ZPL of Mn 4+ : 2 E → 4 A 2 is generally very weak. Interestingly, the intensity of ZPL in the emission spectrum of NNOF:Mn 4+ is higher than that of the v 6 sideband. Herein, the low symmetry of the Mn 4+ center is considered to be the main reason for the intense ZPL in NNOF:Mn 4+ [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:0.003Mn 4+ phosphor, from which it can be observed that the NNOF:0.003Mn 4+ phosphor has an absorption band at 470 nm corresponding to the 4 A 2 → 4 T 2 electron transition of Mn 4+ . Due to the strong intrinsic absorption of pristine NNOF, the absorption band near 369 nm, which responds to the 4 A 2 → 4 T 1 electron transition of Mn 4+ , was completely covered. Compared with Mn 4+ and Mn 2+ , Mn 3+ is rarely encountered in the literature about luminescence. The Mn 3+ ion has the corresponding characteristic absorption band due to the 3d 4 configuration. In Figure 6, the 5 E → 5 T 2 and 5 E → 5 E" absorption bands of Mn 3+ can be clearly observed in the 500-1100 nm range. The presence of Mn 3+ impurity ions will lead to the reduction of quantum efficiency of phosphor [32].

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:0.003Mn 4+ phosphor. The data of Figure 6. Diffuse reflection spectra of pristine NNOF and NNOF:0.003Mn 4+ , 4 A 2 and 4 T 2 are the ground state and excited state of Mn 4+ , respectively, 5 E and 5 E" are two Jahn-Teller split 5 E ground states and 5 T 2 is the excited state of Mn 3+ .

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:0.003Mn 4+ phosphor. The data of the luminescence decay curve conforms to the mono-exponential decay mode, as shown in the following formula: where I 0 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 Mn 4+ ions present forbid transitions in the intra-d-shell [8]. The NNOF:0.003Mn 4+ 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:0.003Mn 4+ is shorter than that of K 2 SiF 6 :Mn 4+ . To specify the reason for the shorter life of NNOF:0.003Mn 4+ , the distortion of the coordination polyhedron (DI) is calculated. The distortion degree of polyhedron is characterized by bond angle variance (σ 2 ) and mean quadratic elongation (λ), which can be determined by following formula [29]: where l 0 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, θ 0 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. The results indicate that the coordination environment of Mn 4+ in NNOF:xMn 4+ is extremely distorted compared to K 2 SiF 6 :Mn 4+ . 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 Mn 4+ 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 Mn 4+ .
Materials 2021, 14, x FOR PEER REVIEW 8 of 11 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 Mn 4+ . The CIE chromaticity coordinate of NNOF:0.003Mn 4+ sample is shown in Figure 7B. It is observed that the prepared NNOF:0.003Mn 4+ 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]: Hereon, (x, y) represents the CIE coordinates of NNOF:0.003Mn 4+ red phosphor, (xi, yi) represents the chromaticity coordinate of the equal-energy white light source with the value of (0.3333, 0.3333), and (xd, yd) stands for the CIE coordinates of the corresponding dominant wavelength of the illuminant. The calculated color purity of NNOF:0.003Mn 4+ phosphor is about 99.9%, which is higher than the reported color purity of Mn 4+ -doped red phosphors, such as Cs2NbOF5:Mn 4+ (99%) [30], K2LiAlF6:Mn 4+ (89%) and K2NaAlF6:Mn 4+ (97%) [42,43]. Obviously, the stronger ZPL emission can bring about a higher color purity. NNOF:Mn 4+ red phosphor is very suitable for the application in LED backlight due to its ultra-high color purity.  Figure 8A shows the EL spectra and photographs of the packaged WLEDs. Curve (i) is the spectrum of WLED produced by YAG:Ce 3+ phosphor coupled with InGaN blue chip (3 V, 20 mA), and curve (ii) is the EL spectrum of WLED with the addition of NNOF:Mn 4+ red phosphor as a contrast. Compared with the scheme of blue chip + YAG:Ce 3+ (Ra = 72,  The CIE chromaticity coordinate of NNOF:0.003Mn 4+ sample is shown in Figure 7B. It is observed that the prepared NNOF:0.003Mn 4+ 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]:

Electroluminescence (EL) Performance of the Packaged WLEDs
Hereon, (x, y) represents the CIE coordinates of NNOF:0.003Mn 4+ 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:0.003Mn 4+ phosphor is about 99.9%, which is higher than the reported color purity of Mn 4+ -doped red phosphors, such as Cs 2 NbOF 5 :Mn 4+ (99%) [30], K 2 LiAlF 6 :Mn 4+ (89%) and K 2 NaAlF 6 :Mn 4+ (97%) [42,43]. Obviously, the stronger ZPL emission can bring about a higher color purity. NNOF:Mn 4+ red phosphor is very suitable for the application in LED backlight due to its ultra-high color purity. Figure 8A shows the EL spectra and photographs of the packaged WLEDs. Curve (i) is the spectrum of WLED produced by YAG:Ce 3+ phosphor coupled with InGaN blue chip (3 V, 20 mA), and curve (ii) is the EL spectrum of WLED with the addition of NNOF:Mn 4+ red phosphor as a contrast. Compared with the scheme of blue chip + YAG:Ce 3+ (Ra = 72, CCT = 6297 K), the LED device with added NNOF:Mn 4+ red component emits highbrightness warm white light (Ra = 86.2, CCT = 3133 K), indicating that the addition of NNOF:Mn 4+ can improve the color rendering index (CRI) and correlated color temperature (CCT). Even more to the point, the luminous efficiency of WLED using NNOF:Mn 4+ 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:0.003Mn 4+ 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:Mn 4+ red phosphor in the field of LED backlight displays, the EL spectrum of the WLED constructed with green-emitting CsPbBr 3 quantum dots (abbreviated as CPB QDs) and NNOF:Mn 4+ 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:Mn 4+ red phosphor for its application in the field of backlight displays. application potential of the synthesized NNOF:Mn 4+ red phosphor in the field of LED backlight displays, the EL spectrum of the WLED constructed with green-emitting CsP-bBr3 quantum dots (abbreviated as CPB QDs) and NNOF:Mn 4+ 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:Mn 4+ red phosphor for its application in the field of backlight displays.

Conclusions
A novel Mn 4+ doped oxyfluoride phosphor was successfully synthesized by hydrothermal method. The prepared NNOF:Mn 4+ 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:Mn 4+ can reach as high as 99.9%. Moreover, the WLED fabricated by using NNOF:Mn 4+ red phosphor and commercial YAG:Ce 3+ 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:Mn 4+ red

Conclusions
A novel Mn 4+ doped oxyfluoride phosphor was successfully synthesized by hydrothermal method. The prepared NNOF:Mn 4+ 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:Mn 4+ can reach as high as 99.9%. Moreover, the WLED fabricated by using NNOF:Mn 4+ red phosphor and commercial YAG:Ce 3+ 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:Mn 4+ red phosphor and green-emitting CPB QDs. These results show that NNOF:Mn 4+ red phosphor has potential application prospects in lighting or display backlights.