A highly Mn²⁺-doped narrowband green phosphor toward wide color-gamut display applications

Abstract

Weak blue light absorption severely limits the application of Mn²⁺-activated narrowband green phosphors. Herein, a strategy to improve the absorption of Mn²⁺-activated phosphors is proposed by selecting a suitable host to achieve a high doping concentration of Mn²⁺. Specifically, a magnetoplumbite-type compound La_0.827Al_11.9O_19.09 with a unique layered structure is selected as the host to accommodate Mn²⁺ (La_0.827Al_11.9−xMn_xO_19.09,LAO:xMn²⁺) and a high doping concentration of x = 0.30 is obtained. Upon 450 nm excitation, the prepared phosphor LAO:0.30Mn²⁺ displays an intense narrowband green emission peaking at 517 nm with a full width at half-maximum (FWHM) of 28 nm, a color purity of 88% and an internal/external quantum yield of 90%/16%. Furthermore, it exhibits a low thermal quenching (6% at 423 K) and negligible color drifting (3.29‰ at 423 K). The LED device fabricated from the green phosphor LAO:0.30Mn²⁺, the commercial red phosphor K₂TiF₆:Mn⁴⁺ and a blue InGaN chip can emit bright white light with a corresponding correlated color temperature (CCT) of 5464 K and a color gamut value of 124% of the National Television Commission (NTSC) standard. These results indicate that the LAO:0.30Mn²⁺ phosphor has great potential for usage as a green component in wide-color gamut LCD backlight applications.

---

1. Introduction

Phosphor-converted white LEDs (pc-wLED_S) have been widely used in liquid-crystal display (LCD) backlight applications due to their low power consumption, high brightness, long lifetime, and eco-friendliness.^1–3 For display applications, the color gamut is a key factor affecting display performance.⁴ Generally, phosphors for display applications need to have a narrow emission band, an appropriate emission peak position, a high photoluminescence quantum yield (PL QY), and outstanding PL thermal stability. In particular, the narrower full width at half maximum (FWHM) of the emission band for phosphors, the larger color gamut and vivid images will be displayed. Currently, the optimal commercial pc-wLED backlight is packaged by combining an InGaN-based blue chip (∼460 nm) with a narrowband emissive green phosphor β-SiALON:Eu²⁺ and a red phosphor K₂SiF₆:Mn⁴⁺ (KSF:Mn⁴⁺).⁵ The red phosphor KSF:Mn⁴⁺ has an extremely narrow FWHM (∼7 nm) and can well meet commercial expectations, while the green phosphor β-SiALON:Eu²⁺ has a considerably broader FWHM of ∼54 nm, which has plenty of room for optimization. Thus, the development of high-performance green phosphors with narrower FWHM is crucial for achieving further wider color gamut display devices.

To date, several types of narrowband green emitting materials have the potential for LCD backlight, including quantum dots, Eu²⁺-activated phosphors, Mn²⁺-activated phosphors, etc. Unfortunately, each of them has its own flaws that prevent them from their wide commercial use. The green-emitting quantum dots, typically CdSe/ZnS and perovskite CsPbBr₃, have high PL QYs and ultra-narrow band emission (FWHM = ∼20 nm). However, their poor stability and toxicity of Cd/Pb content limit their practical applicability, especially in high-power density devices.⁶ The PL spectra of Eu²⁺-activated green phosphors generally have a broad FWHM, which is ascribed to the dipole-allowed 5d → 4f transition with strong electron–phonon coupling. For example, the nitride Ba[Li₂(Al₂Si₂)N₆]:Eu²⁺ has an FWHM of 57 nm,⁷ and NaBaB₉O₁₅:Eu²⁺ has an FWHM of 61 nm.⁸ Although several highly condensed narrowband green phosphors based on the unique structure of UCr₄C₄-type compounds have been reported, among which the RbNa[Li₃SiO₄]₂:Eu²⁺ green phosphor has the narrowest emission FWHM of 41 nm,⁹ their poor chemical stability limits their application. In comparison, transition metal Mn²⁺-activated phosphors have a distinct advantage in narrowband green emission.^10–14 Mn²⁺ ions commonly exhibit green emission in tetrahedral coordination with weak crystal field strength and red emission in octahedral coordination with strong crystal field strength, because the d–d transition of 3d electrons is extremely sensitive to the crystal field strength.¹⁵ Over the past few years, Mn²⁺-activated phosphors such as Zn₂SiO₄:Mn²⁺ and BaAl₁₂O₁₉:Mn²⁺ have been widely used as commercial green phosphors for PDP displays.^16,17 Besides, Mn²⁺-activated narrowband green phosphors have been continuously discovered, and the FWHM has achieved a leap from 43 nm to 18 nm, such as those of the oxynitride γ-AlON (FWHM = ∼43 nm),¹⁰ MgAl₂O₄:Mn²⁺ (FWHM = ∼35 nm),¹¹ BaZnAl₁₀O₁₇:Mn²⁺ (FWHM = ∼31 nm),¹² Sr₂MgAl₂₂O₃₆:Mn²⁺ (FWHM = ∼26 nm),¹³ and even 18 nm of ZnAl₂O₄:Mn²⁺.¹⁴ As a result, LEDs fabricated with Mn²⁺-activated green phosphors can theoretically achieve a large color gamut greater than the 110% NTSC standard. Nonetheless, the d–d parity-forbidden transition of Mn²⁺ severely limits its blue light absorption efficiency, resulting in significantly lower external PL QY than those of commercially available Eu²⁺- or Ce³⁺-activated phosphors. Generally, the introduction of a distorted crystal-field environment to loosen or break the selection rule limitation can effectively enhance the Mn²⁺ blue light absorption efficiency.^18,19 In contrast, a more effective strategy is to introduce as many luminescent centers into the host as possible to increase the absorption efficiency while avoiding the occurrence of concentration quenching.

Hexaaluminates are a class of hexagonal aluminate compounds that are frequently utilized as high-temperature catalysts, thermal barrier coatings, and phosphor hosts due to their unique layered structure and excellent physicochemical stability.^20–22 The structure of hexaaluminates consists of dense spinel blocks and mirror planes, stacked alternately along the c-axis. According to the difference in the charge and radius of the large cations positioned on the mirror planes, they can be classified as β-Al₂O₃ and magnetoplumbite (PbFe₁₂O₁₉) types.^23,24 The commercial blue-emitting phosphor BaMgAl₁₀O₁₇:Eu²⁺ is a typical representation of the β-Al₂O₃ structure,²⁵ whereas the magnetoplumbite structure is represented by the red-emitting phosphor CaAl₁₂O₁₉:Mn⁴⁺.²⁶ In this work, we selected a La_0.827Al_11.9O_19.09 (hereafter referred to as LAO) crystal as the host, which has a distorted magnetoplumbite structure that can also be viewed as a non-stoichiometric ratio compound related to La deficiency.²⁷ In comparison, the LaMgAl₁₁O₁₉ crystal has an analogous magnetoplumbite structure, but it can be regarded as a perfect crystal structure.²⁸ However, the LAO crystal can compensate for the additional charges induced by the partial lack of La on the mirror planes via structural rearrangements formed by a Frenkel defect mechanism.²⁹ Interestingly, this particular defective structure has the benefit of giving free electrons to enable the spontaneous reduction of high-valent dopant ions in the crystal.³⁰ Moreover, it has been established that Mn²⁺ preferentially occupies tetrahedral Al(2) sites in the spinel blocks of hexaaluminate structures, because the Al(2) sites have the longest Al–O bond length and the lowest bond-valence sum among all species of Al sites, making Al(2) much more suitable to accommodate the larger and less positive Mn²⁺ ions.^31,32 Besides, the Al(2) sites have a high degree of symmetry and are placed in densely packed spinel blocks with a highly rigid structure, resulting in a narrow FWHM and low PL thermal quenching for doped Mn²⁺ ions. Most importantly, the Al(2) sites between the adjacent spinel blocks have a large distance of 9.84 Å due to the separating effect of the mirror planes, which is favorable for suppressing the concentration quenching effect caused by the energy migration between the Mn²⁺ luminescent centers.^33,34

In this study, we prepared an Mn²⁺-activated green phosphor La_0.827Al_11.9−xMn_xO_19.09 (LAO:xMn²⁺) with a magnetoplumbite-type structure by a facile solid-state reaction method. We have meticulously investigated the impact of activator concentration on the luminescence properties. Under 450 nm excitation, the LAO:0.30Mn²⁺ sample exhibits a narrow green emission band peaking at 517 nm with an FWHM of 28 nm, a high PL QY and remarkable PL thermal stability. More significantly, using this phosphor as the green component, the fabricated LED achieves a high color gamut value of 124% NTSC, demonstrating the great potential of LAO:0.30Mn²⁺ as a green phosphor for LCD backlight applications.

2. Experimental

2.1. Materials and preparation

Mn²⁺-doped La_0.827Al_11.9O_19.09 phosphors were synthesized by a conventional high-temperature solid-state method. The feed ratio was calculated according to the formula La_0.827Al_11.9−xMn_xO_19.09 in which Al³⁺ was partially replaced by Mn²⁺. The starting raw materials were La₂O₃ (99.99%, Aladdin), Al₂O₃ (A. R., Aladdin), MnCO₃ (99.9%, Aladdin), and MnO₂ (99.9%, Aladdin), with 2 wt% H₃BO₃ (A. R., aladdin) serving as a flux. These raw materials were weighed and ground thoroughly in an agate mortar for 30 minutes. Afterward, the mixtures were transferred to an alumina crucible and sintered at 1550 °C for 4 h in a horizontal tube furnace under an H₂/N₂ (5%/95%) reducing atmosphere. After the furnace was slowly cooled to room temperature, the sintered product was taken out and re-ground for further characterization.

2.2. Characterization

The phase purity and crystal structure were characterized using an X-ray diffractometer (Miniflex 600, Rigaku) with Cu K_α1 radiation (λ = 1.54187 Å) at an interval of 0.02°. The XRD data were collected in a 2θ range from 5° to 80° with a scanning speed of 8 °C min⁻¹. The Rietveld method was employed for crystal structure refinement using the General Structure Analysis System (GSAS-II) program. EPR spectra were recorded on Bruker A300 at room temperature. The morphology and composition were characterized using a field-emission scanning electron microscope (Thermo Scientific, Apreo S) equipped with an energy dispersive spectroscopy (EDS) system. Diffuse reflection spectra were measured on a UV-Vis-NIR spectrophotometer (Agilent, Cary 5000) in the range of 200 to 800 nm and taking BaSO₄ as the reference. The excitation and PL spectra were measured using a spectrometer (FLS 980, Edinburgh Instrument) equipped with 450 W xenon (Xe) lamps. The PL decay curves were measured with the same instrument by using an OPO laser (NT242-1K, EKSPLA, 195–2600 nm, 3–6 ns pulse duration, and 1000 Hz pulse repetition rate) as the excitation source. The temperature-dependent emission spectra were measured via a thermal heating stage (77–873 K, THMS 600, Linkam Scientific Instruments) which is coupled to the FLS 980 spectrometer. For PL QY measurements, the samples were put inside a barium sulfate coated integrating sphere coupled to a PG2000-Pro spectrophotometer (194–1119 nm, 50 um slit) and a standard tungsten halogen lamp was used to calibrate the optical response of the instrument.

2.3. LED performance

A white LED was fabricated by integrating the narrowband green phosphor LAO:0.30Mn²⁺, red phosphor K₂TiF₆:Mn⁴⁺, and InGaN blue chip. The phosphors were thoroughly mixed with silicone (part A + B, SHIN-ETSU Chemical Co. Ltd), and then coated on the blue InGaN LED chip. The weight ratio of silicone to LAO:0.30Mn²⁺ and to K₂TiF₆:Mn⁴⁺ was 0.6 : 2 : 0.1. The packaged devices were further cured at 120 °C for 2 h in an oven, and then fixed in a 5054 PCB substrate to produce the desired LEDs. The photoelectric properties of the LEDs were measured using an integrating sphere spectroradiometer system (HASS-2000, 350–1100 nm, Everfine).

3. Results and discussion

The crystal structure of LAO is shown in Fig. 1a. It is derived from magnetoplumbite (PbFe₁₂O₁₉) with the space group of P6₃/mmc. Similar to other hexaaluminates, LAO is an orderly layered structure stacked along the c-axis and its structural units include a densely packed spinel block (Al₁₁O₁₆) and a so-called mirror plane (LaO + AlO).^27,35 The spinel block (Al₁₁O₁₆) is composed of [Al(1)O₆] octahedra, [Al(2)O₄] tetrahedra, or [Al(3)O₆] octahedra, respectively. Among these polyhedra, the [Al(2)O₄] tetrahedra are located in the center of the spinel blocks and exhibit a corner-sharing with nearby [Al(1)O₆] and [Al(3)O₆] octahedra, while the [Al(1)O₆] and [Al(3)O₆] octahedra connect with each other by edge-sharing. In this way, a rigid 3D network structure is constructed from these polyhedra. Generally, the mirror plane will have large divalent and trivalent ions located inside it. Nevertheless, in contrast to most hexaaluminate structures, LAO is a non-stoichiometric compound with a portion of La deficiency on the mirror layer (LaO + AlO). To maintain charge balance, a series of point defects and vacancies spontaneously emerged and dispersed randomly across the mirror plane.^30,36–38 In the hexaaluminate structure, Mn²⁺ prefers to occupy the Al(2) site in the [Al(2)O₄] tetrahedron due to the lowest formation energy.²⁹ Additionally, the Mn²⁺ doping sites in the host must be separated by a relatively large distance to attenuate the energy migration between Mn²⁺ ions, thus suppressing the PL concentration quenching effect and achieving a higher Mn²⁺ doping concentration. As shown in Fig. 1b, the closest distance between Al(2) sites in the [Al(2)O₄] tetrahedron inside the spinel block is 3.43 Å. However, owing to the separating effect of mirror planes, the closest distance of Al(2) sites in [Al(2)O₄] between the spinel layers rises to 9.84 Å. Benefiting from this, we achieve a high Mn²⁺ doping concentration of x = 0.30 for the LAO host in this study.

Fig. 1 (a) Crystal structure of La_0.827Al_11.9O_19.09 (LAO) and coordination polyhedra of Al(1)–Al(6) atoms. (b) The distance between the isolated [Al(2)O₄] tetrahedron in the crystal structure.

The crystal structure and phase purity of the prepared LAO:xMn²⁺ samples were characterized by X-ray diffraction. The H₃BO₃ flux plays an important role in promoting the formation of a pure phase, which can only be obtained with the addition of 2 wt% H₃BO₃ (Fig. S1†). To investigate the impact of Mn²⁺ doping concentration on the luminescence properties, a series of LAO:xMn²⁺ (x = 0.00–0.40) samples were prepared and all of them were in the pure phase (Fig. S2†). A gradual shift of the diffraction peak corresponding to the (107) lattice plane towards a smaller angle is observed due to lattice expansion induced by the substitution of Mn²⁺ (r = 0.66 Å, CN = 4) for the smaller Al³⁺ (r = 0.53 Å, CN = 4).³⁹Fig. 2a shows the XRD pattern of the LAO:0.30Mn²⁺ sample and the diffraction peaks match well with the standard pattern of the LAO crystal (ICSD NO.38371). To further investigate the microstructure of the LAO:xMn²⁺ sample, a structural Rietveld refinement of the LAO:0.30Mn²⁺ sample was performed and compared with that of the undoped LAO sample. The crystallographic data derived from ICSD NO.38371 were used as the initial structural model for the refinement. The refinement results converge well, showing a low residual coefficient of R_wp = 6.34 and χ² = 1.34 (Fig. 2b). The corresponding cell parameters were calculated to be a = b = 5.5731(1) Å, c = 22.1039(3) Å, and V = 592.68(1) ų with a space group of P6₃/mmc. Compared with the undoped LAO sample (Fig. S3†), LAO:0.30Mn²⁺ displays considerable lattice expansion (Table S1†), which is consistent with the above observed XRD peak shift. Tables S2 and S3† show the detailed atomic coordinates of the undoped LAO and LAO:0.30Mn²⁺ samples, respectively, based on XRD Rietveld refinement. The valence state of the Mn element in the structure is determined to be divalent, because the EPR spectrum of the LAO:0.30Mn²⁺ sample exhibits sextet hyperfine peaks with a g factor of 2.03 (Fig. 2c), which are a result of a hyperfine coupling between the nuclear spin of Mn (I = +5/2) and the electrons in the 3d orbital of the Mn²⁺.⁴⁰ SEM images of the LAO:0.30Mn²⁺ sample show an irregular particle morphology stacked with dozens of minor sheets and the particle size distribution is in the range of several dozen micrometers (Fig. 2d and e). In addition, the smooth surface suggests that the LAO:0.30Mn²⁺ sample has a high degree of crystallinity. The atomic and mass ratios of La, Al, O, and Mn are determined by EDS and are closer to the theoretical elemental ratio (Fig. 2f). The corresponding elemental mapping images exhibit a uniform distribution of the related elements (Fig. 2g–k). These results demonstrate the formation of well-crystallized samples and the successful doping of Mn²⁺ ions.

Fig. 2 (a) XRD patterns of the LAO:0.30Mn²⁺ sample and pure LAO crystals (ICSD NO. 38371) for comparison. (b) Rietveld refinement results of the LAO:0.30Mn²⁺ sample. (c) EPR spectrum of Mn²⁺ ions in the LAO:0.30Mn²⁺ sample. (d and e) SEM images of the LAO:0.30Mn²⁺ sample. (f) The mass and atomic percent of each element are determined by EDS analysis. (g–k) The mapping images of La, Al, O, and Mn elements.

The PL spectra of LAO:xMn²⁺ (x = 0.05–0.40) samples were measured and are shown in Fig. 3a. All samples exhibit a narrowband green emission peaking at 517 nm under 450 nm blue-light excitation, which is attributed to the ⁴T₁(⁴G) → ⁶A₁(⁶S) transition of Mn²⁺. The integrated PL intensity of the LAO:xMn²⁺ samples increased gradually as the doping concentration rose from 0.05 to 0.30, and then progressively declined as a result of the concentration quenching effect (Fig. 3b). Therefore, we obtain a high activator doping concentration of x = 0.30 in the LAO host. Generally, the nonradiative energy migration among Mn²⁺ ions followed by quenching by the structural defects leads to PL quenching, and the energy migration efficiency is highly dependent on the distance between the dopant Mn²⁺ ions. Specifically, when the distance is small enough, the energy migration is highly efficient and the excited electrons will migrate a longer distance in the lattice and have a larger probability of being captured by the structural defects, thus inducing strong PL quenching. To better analyze the mechanism, the critical distance (R_c), which is defined as the point at which the probabilities of nonradiative energy transfer and radiative emission are equal, is introduced. The value of R_c can be calculated using the equation below.⁴¹

where V is the volume per unit cell, X_c is the critical concentration of Mn²⁺, and N is the number of cation sites that can be occupied by activator ions. In this case, V = 592.681, X_c = 0.30, and N = 2, and the critical distance R_c was determined to be 12.36 Å. Notably, this value exceeds 5 Å, indicating a greater tendency for the concentration quenching mechanism toward multipolar electron interactions. In view of Dexter's theory, the multipolar interactions can be further subdivided into an adjacent ion energy transfer mechanism, dipole–dipole interactions, dipole–quadrupole interactions, and quadrupole–quadrupole interactions, corresponding to θ = 3, θ = 6, θ = 8 and θ = 10, respectively. The type of interaction can be estimated by linear fitting according to the following equation.^42,43where I represents the PL intensity, x is the activator concentration over the critical concentration, K and β are constants, and θ is used to describe the different modes of multipolar electron interactions. Fig. S4† shows a plot of the relationship between log(x) versus log(I/x), which can be well-fitted with a slope of −3.14 (−θ/3). Thus, the θ was calculated to be 9.42 and close to 10, which suggests that quadrupole–quadrupole interactions dominate the PL concentration quenching mechanism in LAO:xMn²⁺.

Fig. 3 (a) The PL spectra of the LAO:xMn²⁺ (x = 0.05–0.40) samples under excitation at 450 nm. (b) The evolution of PL peak intensity with Mn²⁺ doping concentration for the LAO:xMn²⁺ samples. (c) The PL and excitation spectra of the LAO:0.30Mn²⁺ sample and commercial green emitting β-SiALON:Eu²⁺ phosphor. The electroluminescence spectra of the InGaN chip are shown for comparison. (d) UV-vis diffuse reflectance spectra of the LAO:0.30Mn²⁺ sample. (e) The measured spectral data of the LAO:0.30Mn²⁺ sample under 450 nm excitation for calculating the internal and external PL QY. (f) The PL decay curve was measured by monitoring the emission at 517 nm of the LAO:0.30Mn²⁺ sample under excitation at 450 nm.

The emission peaks of the LAO:xMn²⁺ samples exhibit a subtle redshift of 4 nm and an imperceptible FWHM change with increasing Mn²⁺ concentration and the CIE chromaticity coordinates with a corresponding slight shift but remains stable in the green region (Fig. S5†). This redshift is probably due to the aggravated reabsorption with increasing dopant concentration, since there is an overlap between the emission and excitation spectra.^44,45 For a more detailed investigation of LAO:xMn²⁺ green phosphors, we singled out the LAO:0.30Mn²⁺ sample and compared it with the commercial green phosphor β-SiALON:Eu²⁺. As shown in Fig. 3c, under 450 nm excitation, the LAO:0.30Mn²⁺ sample exhibits intense green emission with an FWHM of 28 nm, ranging from 480 nm to 600 nm. Its excitation spectrum measured by monitoring the emission at 517 nm contains five distinct peaks. The peak at 280 nm is a charge transfer band from O²⁻ to Mn²⁺, while 361 nm, 384 nm, 426 nm, and 450 nm are attributed to the electronic transitions of Mn²⁺ from the ground state ⁶A₁(⁶S) to the ⁴E(⁴D), ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)], and ⁴T₂(⁴G) excited states, respectively. This is consistent with the results observed in the absorption spectra of the LAO:0.30Mn²⁺ sample (Fig. 3d). It is worth mentioning that the most intense absorption band is at 450 nm, which means that this phosphor can be excited by blue diode chips. In comparison, the commercial green phosphor β-SiALON:Eu²⁺ exhibits a broadband excitation in the range from 270 nm to 520 nm and a relatively narrow emission band peaking at 540 nm with an FWHM of 54 nm. It is obvious that the FWHM of the LAO:0.30Mn²⁺ sample is significantly narrower than that of β-SiALON:Eu²⁺. In addition, we also evaluated the color purity of the LAO:0.30Mn²⁺ phosphor since a narrower FWHM also implies a higher color purity. The CIE chromaticity coordinates of the LAO:0.30Mn²⁺ sample are (0.1798, 0.7442) determined by its PL spectra. The color purity is defined as the percentage of the linear distance between the chromaticity coordinates of the measured plots and the white light source as compared to the linear distance between the monochromatic light source and the white light source, which can be calculated using the following equation.⁴⁶

where (x, y) represent the CIE chromaticity coordinates of the LAO:0.30Mn²⁺ sample, (x_i, y_i) represent the CIE chromaticity coordinates of the white light source, and (x_d, y_d) represent the color coordinates of the corresponding monochromatic light source. The color purity of the LAO:0.30Mn²⁺ sample is determined to be 88%, which is promising to support its application in wide color gamut backlight displays. In addition, as shown in Fig. 3e, the internal PL QY of the LAO:0.30Mn²⁺ sample under 450 nm blue-light excitation at room temperature was found to be 90.0%, indicating high emission efficiency. The corresponding absorption efficiency is 17.9%. Fig. S6† shows the variation of internal PL QYs, absorption efficiency, and external PL QYs with Mn²⁺ doping concentration in LAO:xMn²⁺ (x = 0.05–0.40) samples and the detailed data are listed in Table S4.† The room-temperature PL decay curves of LAO:xMn²⁺ (x = 0.05–0.40) samples measured by monitoring emission at 517 nm exhibit single exponential behavior (Fig. S7†), and the fitted PL lifetime decreased from 6.05 to 4.58 ms with increasing x values. The shorter PL lifetime with increasing Mn²⁺ concentration in the range of x = 0.05–0.40 may be due to the exchange-coupled interaction between the paramagnetic Mn²⁺ ions, which enhances the radiative transition probability with increasing doping concentration.⁴⁷ The PL lifetime for the LAO:0.30Mn²⁺ sample is determined to be 5.37 ms (Fig. 3f).

The PL thermal quenching behavior of phosphors is a key factor in evaluating its potential device performance. Hence, the temperature-dependent PL spectra of the LAO:0.30Mn²⁺ sample were measured from 298 to 473 K with a temperature interval of 25 K under 450 nm excitation (Fig. 4a). With increasing temperature, the integrated PL intensity decreases from the initial value of 100% at 298 K to 94% at 423 K and 89% at 473 K, indicating excellent PL thermal stability. The decreased intensity is attributed to the accelerated lattice vibration and relaxation in the luminescent center at high temperatures, resulting in enhanced nonradiative transition.⁴⁸ The activation of thermal quenching (ΔE) was estimated using the Arrhenius equation.

where I₀ and I_T are the PL intensity at initial room temperature and a given temperature (T), respectively, ΔE represents the activation energy, A is the constant for a given host, and k is the Boltzmann constant. In general, a larger activation energy ΔE implies better thermal stability. Based on ln(I₀/I_T − 1) versus 1/kT, the activation energy ΔE was calculated to be 0.16 eV (Fig. S8†). This value is comparable to those of previously reported hexaaluminate materials.¹³Fig. 4b shows the normalized PL spectra of the LAO:0.30Mn²⁺ sample with increasing temperature, and the emission-peak position remains unchanged at 517 nm, while the FWHM increases gradually from 28 nm (298 K) to 34 nm (473 K) (the inset in Fig. 4b). The broadening of the FWHM with temperature can be explained by the configuration coordinate model and electronic Boltzmann distribution.⁴⁹where hv is the average phonon energy, S is the Huang–Rhys factor, which represents the electron–phonon coupling strength, and k is the Boltzmann constant. It is evident that temperature has a positive correlation with FWHM. As the temperature gradually increases, more excited state electrons diffuse to higher vibrational energy levels and then make a radiative transition from these different energy levels to the ground state, resulting in a broadened emission band. The variation of FWHM with temperature is well-fitted using eqn (5) as shown in Fig. 4c, and the values of S and hv are calculated to be 2.32 and 18.60 meV, respectively. Fig. 4d gives the temperature-dependent PL decay curves of the LAO:0.30Mn²⁺ sample and all curves can be well-fitted with a single exponential decay function. As the temperature increases from 298 K to 473 K, the PL lifetime decreases slightly from 5.37 to 5.22 ms due to the increase in nonradiative transition probability at high temperatures. These findings support that the LAO:0.30Mn²⁺ sample has a low PL thermal quenching. Moreover, the chromaticity stability of the LAO:0.30Mn²⁺ sample with increasing temperature is shown in Fig. 4e. The CIE chromaticity coordinates of the LAO:0.30Mn²⁺ sample shift from (0.1798, 0.7442) to (0.1823, 0.7290) and then to (0.1837, 0.7216) when the temperature changed from 298 K to 423 K and then to 473 K. Specifically, the chromaticity coordinate shift value (ΔD) can be calculated using the following equation.^50,51

u′ = 4x/12(3 − 2x + y) (7)

v′ = 9y/(3 − 2x + 12y) (8)

w′ = 1 − u′ − v′ (9)

where (x, y) represent the chromaticity coordinates in CIE 1931, (u′, v′) represent the chromaticity coordinates in u′v′ uniform color space, and 0 and t are the chromaticity shifts at 298 K and given temperature conditions, respectively. As a result, the shifts of the chromaticity coordinates were determined to be 3.29‰ at 423 K and 4.86‰ at 473 K in comparison with the initial value of 298 K. This shift value is lower than those of most phosphors, demonstrating excellent chromaticity stability of the LAO:0.30Mn²⁺ phosphor.^50–54Fig. 4f depicts the variation in the PL intensity of the LAO:0.30Mn²⁺ sample after three heating–cooling cycles at the temperature range from 298 K to 473 K. As we can see, the emission intensity can recover its initial value well after each cycle, proving good stability of the LAO:0.30Mn²⁺ sample. The first reason for the high thermal stability of the LAO:0.30Mn²⁺ phosphor is its high structural rigidity, resulting in a relatively weak electron–phonon coupling strength as indicated by its narrow emission bandwidth (FWHM = 28 nm). This means that the lattice thermal variation can be effectively restricted, while simultaneously minimizing the horizontal offset ΔR between two parabolas representing the lowest excited state ⁴T₁ and the ground state ⁶A₁ energy levels. Consequently, this leads to a larger thermal activation energy (ΔE) required for nonradiative transition and reduces emission loss during the heating process.⁴⁸ In addition, our prepared LAO:0.30Mn²⁺ phosphor exhibits excellent crystallinity, as evidenced by SEM analysis, along with an ultra-high internal PL QY of 90%. This implies a reduced presence of structural defects within the crystal lattice. With increasing temperature, the energy migration among Mn²⁺ ions will become more efficient, thus facilitating easier trapping of excited electrons by structural defects. Therefore, the relatively low defect density in our samples also contributes to its remarkable thermal stability.

Fig. 4 (a) Temperature-dependent PL spectra of the LAO:0.30Mn²⁺ sample at 298–473 K. The inset shows the variation of normalized integrated emission intensities with temperature. (b) The normalized PL spectra of the LAO:0.30Mn²⁺ sample at 298–473 K. The inset shows the variation of FWHM with temperature. (c) Temperature-dependence of the FWHM (experimental and fitted results) in LAO:0.30Mn²⁺. (d) Temperature-dependent PL decay curves of the LAO:0.30Mn²⁺ sample at 298–473 K. All the curves exhibit a single exponential decay behavior, and the inset shows the fitted PL lifetime at various temperatures. (e) The CIE coordinates of the LAO:0.30Mn²⁺ sample at 298–473 K. The inset shows the CIE shift as a function of temperature. (f) The PL intensity corresponds to three heating and cooling cycles of the LAO:0.30Mn²⁺ sample.

To evaluate the practical application of the LAO:0.30Mn²⁺ phosphor in devices, white LED1 was fabricated by coating LAO:0.30Mn²⁺ green and KTF:Mn⁴⁺ red phosphors on 450 nm InGaN chips. Besides, white LED2 using a phosphor blend of green β-SiAlON:Eu²⁺ and red KTF:Mn⁴⁺ was also packaged for comparison. Fig. 5a and b show the electroluminescence (EL) spectra and chromaticity coordinates of these two LEDs, respectively. Driven by a voltage of 3.0 V and a current of 60 mA, LED1 shows white light emissions with chromaticity coordinates of (0.3329, 0.3160), a correlated color temperature (CCT) of 5464 K, and a luminous efficacy of 18 lm W⁻¹, while LED2 shows white light emissions with chromaticity coordinates of (0.3393, 0.3307), a CCT of 5646 K, and a luminous efficacy of 89 lm W⁻¹. The lower luminous efficacy of LED1 is attributed to the weak absorption of the LAO:0.30Mn²⁺ phosphor, which requires a large amount of phosphor to be coated on the LED chip, thus inducing serious light scattering and reabsorption losses. Noteworthily, the color gamut of LED1 reached 124% of the NTSC 1931 standard and 93% of the Recommendation 2020 (Rec. 2020) standard, whereas the corresponding color gamut of LED2 was only 94% and 70%, respectively (Fig. 5c and d). The significantly larger color gamut of LED1 comes mainly from the narrower green emission of the LAO:0.30Mn²⁺ phosphor. The EL spectra and the chromaticity coordinates of these two LEDs under various drive currents from 20 mA to 100 mA are shown in Fig. S9,† and the small variation in color coordinates proves that LED1 has similar excellent stability to LED2 under different input powers. Table S5† summarizes the main spectral parameters and device performances of various well-known narrowband green phosphors. It can be seen that the LAO:0.30Mn²⁺ phosphor has an ideal emission peak position, a narrow emission band, high PL thermal stability, and a wide color gamut. These results demonstrate that LAO:0.30Mn²⁺ has great potential for usage as a green phosphor to achieve high-quality wide-color gamut LCD backlight displays.

Fig. 5 Electroluminescence spectra of the LEDs fabricated using a blue InGaN chip, a red KTF:Mn⁴⁺ phosphor, along with (a) the green emitting LAO:0.30Mn²⁺ sample (LED1), and (b) the commercial green emitting β-SiALON:Eu²⁺ phosphor (LED2), respectively. The chromaticity coordinates of the fabricated (c) LED1 and (d) LED2 in CIE 1931, and the corresponding color space (yellow line) compared with the NTSC 1931 standard (purple line) and Rec. 2020 standard (black line).

4. Conclusions

In summary, we have successfully prepared a green phosphor LAO:xMn²⁺ (peak wavelength at ∼517 nm) with a narrow FWHM of 28 nm by the solid-state reaction method. Thanks to the separating effect of the mirror layer that makes the isolated Mn²⁺ stay far away from each other, we have achieved a high doping concentration of x = 0.30 in the LAO host. Under 450 nm excitation, the LAO:0.30Mn²⁺ phosphor shows a color purity of 88% and an internal/external PL QY of 90%/16%. Moreover, the phosphor shows excellent PL thermal stability with an emission intensity loss of 6% at 423 K, and 11% at 473 K compared to its initial value at 298 K. The corresponding temperature-dependent PL lifetime proves that LAO:0.30Mn²⁺ possesses a low nonradiative transition probability, probably due to its highly rigid structure and good crystal quality. Moreover, it has an excellent resistance to color drifting with chromaticity shifts of 3.39 ‰ at 423 K and 4.86 ‰ at 473 K. Finally, using the narrowband emitting LAO:0.30Mn²⁺ phosphor as the green component and the K₂TiF₆:Mn⁴⁺ phosphor as the red component, combined with a 450 nm InGaN blue chip, the fabricated white LED device can emit bright white light and achieve a wide color gamut of 124% NTSC 1931 standard with a CCT of 5464 K. These exciting results indicate that the prepared LAO:0.30Mn²⁺ green phosphor has promising applications in wide color gamut displays.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Natural Science Foundation of China (No. 11974351) and Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (No. 2021ZZ114).

References

1. M.-H. Fang, J. L. Leaño and R.-S. Liu, Control of Narrow-Band Emission in Phosphor Materials for Application in Light-Emitting Diodes, ACS Energy Lett., 2018, 3, 2573–2586.
2. M. Zhao, Q. Zhang and Z. Xia, Narrow-band emitters in LED backlights for liquid-crystal displays, Mater. Today, 2020, 40, 246–265.
3. Y. Zhang, L. Luo, G. Chen, Y. Liu, R. Liu and X. Chen, Green and red phosphor for LED backlight in wide color gamut LCD, J. Rare Earths, 2020, 38, 1–12.
4. I. Kim and K. Chung, Wide Color Gamut Backlight from three-band white LED, J. Opt. Soc. Korea, 2007, 11, 67–70.
5. S. Li, L. Wang, D. Tang, Y. Cho, X. Liu, X. Zhou, L. Lu, L. Zhang, T. Takeda, N. Hirosaki and R.-J. Xie, Achieving High Quantum Efficiency Narrow-Band β-Sialon:Eu²⁺ Phosphors for High-Brightness LCD Backlights by Reducing the Eu³⁺ Luminescence Killer, Chem. Mater., 2017, 30, 494–505.
6. X. Zhang, H.-C. Wang, A.-C. Tang, S.-Y. Lin, H.-C. Tong, C.-Y. Chen, Y.-C. Lee, T.-L. Tsai and R.-S. Liu, Robust and Stable Narrow-Band Green Emitter: An Option for Advanced Wide-Color-Gamut Backlight Display, Chem. Mater., 2016, 28, 8493–8497.
7. P. Strobel, S. Schmiechen, M. Siegert, A. Tücks, P. J. Schmidt and W. Schnick, Narrow-Band Green Emitting Nitridolithoalumosilicate Ba[Li₂(Al₂Si₂)N₆]:Eu²⁺ with Framework Topology whj for LED/LCD-Backlighting Applications, Chem. Mater., 2015, 27, 6109–6115.
8. Y. Zhuo, S. Hariyani, J. Zhong and J. Brgoch, Creating a Green-Emitting Phosphor through Selective Rare-Earth Site Preference in NaBaB₉O₁₅:Eu²⁺, Chem. Mater., 2021, 33, 3304–3311.
9. H. Liao, M. Zhao, Y. Zhou, M. S. Molokeev, Q. Liu, Q. Zhang and Z. Xia, Polyhedron Transformation toward Stable Narrow-Band Green Phosphors for Wide-Color-Gamut Liquid Crystal Display, Adv. Funct. Mater., 2019, 29, 1901988.
10. Q. Dong, F. Yang, J. Cui, Y. Tian, S. Liu, F. Du, J. Peng and X. Ye, Enhanced narrow green emission and thermal stability in γ-AlON:Mn²⁺, Mg²⁺ phosphor via charge compensation, Ceram. Int., 2019, 45, 11868–11875.
11. E. H. Song, Y. Y. Zhou, Y. Wei, X. X. Han, Z. R. Tao, R. L. Qiu, Z. G. Xia and Q. Y. Zhang, A thermally stable narrow-band green-emitting phosphor MgAl₂O₄:Mn²⁺ for wide color gamut backlight display application, J. Mater. Chem. C, 2019, 7, 8192–8198.
12. H. Li, Y. Liang, S. Liu, W. Zhang, Y. Bi, Y. Gong and W. Lei, Highly Efficient Green-Emitting Phosphor BaZnAl₁₀O₁₇:Mn²⁺ with Ultra-Narrow Band and Extremely Low Thermal Quenching for Wide Color Gamut LCD Backlights, Adv. Opt. Mater., 2021, 9, 2100799.
13. Y. Zhu, Y. Liang, S. Liu, H. Li and J. Chen, Narrow-Band Green-Emitting Sr₂MgAl₂₂O₃₆:Mn²⁺ Phosphors with Superior Thermal Stability and Wide Color Gamut for Backlighting Display Applications, Adv. Opt. Mater., 2019, 7, 1801419.
14. D. Q. Trung, N. Tu, N. V. Quang, M. T. Tran, N. V. Du and P. T. Huy, Non-rare-earth dual green and red-emitting Mn-doped ZnAl₂O₄ phosphors for potential application in plan-growth LEDs, J. Alloys Compd., 2020, 845, 156326.
15. Q. Zhou, L. Dolgov, A. M. Srivastava, L. Zhou, Z. Wang, J. Shi, M. D. Dramićanin, M. G. Brik and M. Wu, Mn²⁺ and Mn⁴⁺ red phosphors: synthesis, luminescence and applications in WLEDs. A review, J. Mater. Chem. C, 2018, 6, 2652–2671.
16. C. Bertail, S. Maron, V. Buissette, T. Le Mercier, T. Gacoin and J.-P. Boilot, Structural and Photoluminescent Properties of Zn₂SiO₄:Mn²⁺ Nanoparticles Prepared by a Protected Annealing Process, Chem. Mater., 2011, 23, 2961–2967.
17. J. Zhou, Y. Wang, B. Liu and Y. Lu, Effect of H₃BO₃ on structure and photoluminescence of BaAl₁₂O₁₉:Mn²⁺ phosphor under VUV excitation, J. Alloys Compd., 2009, 484, 439–443.
18. L. Wu, B. Wang, Y. Zhang, L. Li, H. R. Wang, H. Yi, Y. F. Kong and J. J. Xu, Structure and photoluminescence properties of a rare-earth free red-emitting Mn(2+)-activated KMgBO₃, Dalton Trans., 2014, 43, 13845–13851.
19. L. J. Xu, C. Z. Sun, H. Xiao, Y. Wu and Z. N. Chen, Green-Light-Emitting Diodes based on Tetrabromide Manganese(II) Complex through Solution Process, Adv. Mater., 2017, 29, 1605739.
20. C. Meisheng, W. Liangshi, Z. Na, L. Zhiqi, L. Dianqing and C. Aifan, La-Hexaaluminate Catalyst Preparation and Its Performance for Methane Catalytic Combustion, J. Rare Earths, 2006, 24, 690–694.
21. X. Chen, Y. Zhao, W. Huang, H. Ma, B. Zou, Y. Wang and X. Cao, Thermal aging behavior of plasma sprayed LaMgAl₁₁O₁₉ thermal barrier coating, J. Eur. Ceram. Soc., 2011, 31, 2285–2294.
22. C. Huang, D. Huang, S. Xu, J. Zhang, S. Liang, J. Hu, K. Xu, D. Chen and H. Zhu, Highly Efficient and Thermally Stable Ba_0.75Al₁₁O_17.25:Eu²⁺, Mn²⁺ Phosphor with Dual Narrowband Blue and Green Emissions: A Promising Candidate for Wide Gamut Displays, Adv. Photonics Res., 2022, 3, 2200069.
23. S. Liu, Z. Wang, H. Cai, Z. Song and Q. Liu, Highly efficient near-infrared phosphor LaMgGa₁₁O₁₉:Cr³⁺, Inorg. Chem. Front., 2020, 7, 1467–1473.
24. C. Dou, F. Zhao, S. Liu, Z. Song and Q. Liu, Achieving efficient violet-light-excited blue phosphors by nitridation for violet-chip-based full-spectrum lighting, Inorg. Chem. Front., 2023, 10, 2430–2437.
25. Z. Xiying, Z. Weidong, C. Xiangzhong, H. Huaqiang and H. Xiaowei, Preparation of BaMgAl₁₀O₁₇:Eu²⁺ and Its Phase Behavior in Microemulsion System, J. Rare Earths, 2006, 24, 736–739.
26. T. Murata, T. Tanoue, M. Iwasaki, K. Morinaga and T. Hase, Fluorescence properties of Mn⁴⁺ in CaAl₁₂O₁₉ compounds as red-emitting phosphor for white LED, J. Lumin., 2005, 114, 207–212.
27. N. Iyi, Z. Inoue, S. Takekawa and S. Kimura, The crystal-structure of Lanthanum hexaaluminate, J. Solid State Chem., 1984, 54, 70–77.
28. J. Yu, X. Luo, Z. Wang, S. Yu, J. Wu, M. Li, X. Li, T. Tang, J. Wen and L. Zhao, A novel blue emitting europium doped lanthanum magnesium hexaaluminate long persistent phosphor and analysis of traps distribution, J. Lumin., 2022, 243, 118671.
29. S. R. Jansen, J. M. Migchels, H. T. Hintzen and R. Metselaar, Eu-Doped Barium Aluminum Oxynitride with the β-Alumina-Type Structure as New Blue-Emitting Phosphor, J. Electrochem. Soc., 1999, 146, 800.
30. J. Hu, E. Song, Y. Zhou, S. Zhang, S. Ye, Z. Xia and Q. Zhang, Non-stoichiometric defect-controlled reduction toward mixed-valence Mn-doped hexaaluminates and their optical applications, J. Mater. Chem. C, 2019, 7, 5716–5723.
31. T. R. Wagner and M. O'Keeffe, Bond lengths and valences in aluminates with the magnetoplumbite and β-alumina structures, J. Solid State Chem., 1988, 73, 211–216.
32. M. Bellotto, G. Artioli, C. Cristiani, P. Forzatti and G. Groppi, On the Crystal Structure and Cation Valence of Mn in Mn-Substituted Ba-β-Al₂O₃, J. Catal., 1998, 179, 597–605.
33. F. Zhao, H. Cai, Z. Song and Q. Liu, Structural Confinement for Cr³⁺ Activators toward Efficient Near-Infrared Phosphors with Suppressed Concentration Quenching, Chem. Mater., 2021, 33, 3621–3630.
34. F. Zhao, H. Cai, Z. Song and Q. Liu, Structural Confinement toward Controlling Energy Transfer Path for Enhancing Near-Infrared Luminescence, Chem. Mater., 2021, 33, 8360–8366.
35. A. L. N. Stevels, Red Mn²⁺-luminescence in hexagonal aluminates, J. Lumin., 1979, 20, 99–109.
36. J.-G. Park and A. N. Cormack, Crystal/Defect Structures and Phase Stability in Ba Hexaaluminates, J. Solid State Chem., 1996, 121, 278–290.
37. Z. Wu and A. N. Cormack, Defects in BaMgAl₁₀O₁₇:Eu²⁺ blue phosphor, J. Electroceram., 2003, 10, 179–191.
38. K. C. Mishra, Theoretical Investigation of Structure and Stability of Reidinger Defects in Barium Magnesium Aluminate, J. Electrochem. Soc., 2005, 152, H84.
39. R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751–767.
40. V. Singh, R. P. S. Chakradhar, J. L. Rao, S. J. Dhoble and S. H. Kim, Electron Paramagnetic Resonance and Photoluminescence Studies of LaMgAl₁₁O₁₉:Mn²⁺ Green Phosphors, J. Electron. Mater., 2014, 43, 4041–4047.
41. S. Liang, P. Dang, G. Li, M. S. Molokeev, Y. Wei, Y. Wei, H. Lian, M. Shang, A. A. Al Kheraif and J. Lin, Controllable two-dimensional luminescence tuning in Eu²⁺, Mn²⁺ doped (Ca,Sr)₉Sc(PO₄)₇ based on crystal field regulation and energy transfer, J. Mater. Chem. C, 2018, 6, 6714–6725.
42. Z. Zhou, J. Zheng, R. Shi, N. Zhang, J. Chen, R. Zhang, H. Suo, E. M. Goldys and C. Guo, Ab Initio Site Occupancy and Far-Red Emission of Mn(4+) in Cubic-Phase La(MgTi)(1/2)O(3) for Plant Cultivation, ACS Appl. Mater. Interfaces, 2017, 9, 6177–6185.
43. K. Li, J. Fan, M. Shang, H. Lian and J. Lin, Sr₂Y₈(SiO₄)₆O₂:Bi³⁺/Eu³⁺: a single-component white-emitting phosphor via energy transfer for UV w-LEDs, J. Mater. Chem. C, 2015, 3, 9989–9998.
44. D. Zhang, X. Zhang, Z. An, R. Dong, Z. Shi, Y. Song and H. Zou, Photoluminescence and Color-Tunable Properties of Na(4)Ca(4)Mg(21)(PO(4))(18):Eu(2+),Tb(3+)/Mn(2+) Phosphors for Applications in White LEDs, Inorg. Chem., 2020, 59, 14193–14206.
45. H. Chen, T. Seto and Y. Wang, An efficient blue phosphor with high thermal stability for lighting and optical pressure sensor applications, Inorg. Chem. Front., 2022, 9, 1644–1654.
46. X. Huang, H. Guo and B. Li, Eu³⁺-activated Na₂Gd(PO₄)(MoO₄): A novel high-brightness red-emitting phosphor with high color purity and quantum efficiency for white light-emitting diodes, J. Alloys Compd., 2017, 720, 29–38.
47. A. P. Vink, M. A. de Bruin, S. Roke, P. S. Peijzel and A. Meijerink, Luminescence of exchange coupled pairs of transition metal ions, J. Electrochem. Soc., 2001, 148, E313–E320.
48. P. Dang, W. Wang, H. Lian, G. Li and J. Lin, How to Obtain Anti-Thermal-Quenching Inorganic Luminescent Materials for Light-Emitting Diode Applications, Adv. Opt. Mater., 2022, 10, 2102287.
49. J. Yan, M. G. Brik, C. Liu, D. Hou, W. Zhou, B. Zhang, Y. Huang, Y. Tao and H. Liang, VUV–vis photoluminescence, low-voltage cathodoluminescence and electron-vibrational interaction of Mn²⁺ in Ba₂MgSi₂O₇, Opt. Mater., 2015, 43, 59–65.
50. C. Li, H. Zheng, H. Wei, S. Qiu, L. Xu, X. Wang and H. Jiao, A color tunable and white light emitting Ca(2)Si(5)N(8):Ce(3+),Eu(2+) phosphor via efficient energy transfer for near-UV white LEDs, Dalton Trans., 2018, 47, 6860–6867.
51. W. Wang, H. Yang, M. Fu, X. Zhang, M. Guan, Y. Wei, C. C. Lin and G. Li, Superior thermally-stable narrow-band green emitter from Mn²⁺-doped zero thermal expansion (ZTE) material, Chem. Eng. J., 2021, 415, 128979.
52. X. Zhang, L. Huang, F. Pan, M. Wu, J. Wang, Y. Chen and Q. Su, Highly thermally stable single-component white-emitting silicate glass for organic-resin-free white-light-emitting diodes, ACS Appl. Mater. Interfaces, 2014, 6, 2709–2717.
53. C. Li, X.-M. Wang, Z.-P. Yang and H. Jiao, Ca₈Mg₇Si₉N₂₂:Ce³⁺—A Yellow-Emitting Nitride Phosphor for White Light Emitting Diodes, ACS Appl. Electron. Mater., 2020, 2, 936–943.
54. Z. Leng, H. Bai, Q. Qing, H. He, J. Hou, B. Li, Z. Tang, F. Song and H. Wu, A Zero-Thermal-Quenching Blue Phosphor for Sustainable and Human-Centric WLED Lighting, ACS Sustainable Chem. Eng., 2022, 10, 10966–10977.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi02182c

---