Mn⁴⁺-activated oxyfluoride K₃TaOF₆ red phosphor with intense zero phonon line for warm white light-emitting diodes

Abstract

The intense zero phonon line (ZPL) of the Mn⁴⁺:²E→⁴A₂ transition can further promote the color rendering and luminous efficiency for high-quality white-emitting diodes (w-LEDs). In this article, a Mn⁴⁺-activated K₃TaOF₆ oxyfluoride red phosphor was synthesized via a facile two-step method. Its phase and morphology were characterized by X-ray diffraction, SEM and TEM. The as-prepared K₃TaOF₆:Mn⁴⁺ exhibits an intense absorption of blue light and a strong emission band peaking at 628 nm with a color purity as high as 96.4%. Attributed to the distorted octahedral coordination environment of Mn⁴⁺ ions, an intense ZPL emission was detected at 620 nm. By theoretical calculation, Mn⁴⁺ ions in the K₃TaOF₆ host experience a strong crystal field. In addition, the temperature-dependent PL and thermoluminescence (TL) spectra suggest that thermal ionization dominates the thermal quenching phenomenon in this phosphor.

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1. Introduction

With the development of lighting and display technology in the past decades, there is an increasing requirement of the color quality of white lighting-emitting diode (w-LED) illuminant sources.^1–5 However, the commercially available w-LED devices, i.e., based on Y₃Al₅O₁₂:Ce³⁺ (YAG) yellow phosphors with InGaN blue chips are unsuitable for liquid crystal display (LCD) backlights due to their limited color gamut.^6–8 As an alternative, combining commercial CaAlSiN₃:Eu²⁺ red phosphors with β-sialon:Eu²⁺ green phosphors has been a popular model in the LCD backlight market.^9,10 Unfortunately, the excitation band of CaAlSiN₃:Eu²⁺ is so broad that serious re-absorption between β-sialon:Eu²⁺ and CaAlSiN₃:Eu²⁺ will be caused, which then brings the change of color and a reduction of luminous efficacy.¹¹ Moreover, the LCD backlight obtained via this approach can only reach ∼82% of the required standard proposed by National Television Standards Committee (NTSC). Conceivably, it is crucial to develop phosphors with appropriate peak position, narrow emission band, high quantum efficiency (QE) and excellent thermal stability to fulfil the application.

Thanks to the specific electronic structure of the Mn⁴⁺ (3d³ electron configuration) ion, obvious broadening of the color gamut of LED-lit LCD backlights has been observed by employing Mn⁴⁺-activated red phosphors.^12–18 Theoretically, both Stokes and anti-Stokes photon vibrational sidebands could be easily implemented in the transition of Mn⁴⁺:²E→⁴A₂.^16,17 Whereas, only when Mn⁴⁺ ions have distorted coordination geometries can the ZPL transition of Mn⁴⁺:²E→⁴A₂ occur.^19–22 Generally, the sharpest PL emission of the Mn⁴⁺ ion is located at ∼630 nm among a group of sharp emission peaks while the observed ZPL emission in a few cases peaks at ∼620 nm.^8,21,22 In fact, as part of the PL emission, the ZPL emission is of great benefit to improve the luminous efficiency because human eyes are more sensitive to shorter wavelengths.^8,12,14 In spite of the absence of distorted octahedral coordination sites, the oxyfluoride compounds still can provide distorted local environments by F/O mixed-ligands, which could bring rich spectroscopic properties, thus many researchers have focused on the oxyfluoride compounds.²² For instance, new fluorotungstate red-emitting phosphors A₂WO₂F₄:Mn⁴⁺ (A = Na, Cs) were successfully synthesized through a co-precipitation method and intense ZPL emissions were detected under blue light excitation.^22,23 Similarly, in the newly obtained fluoromolybdate red phosphor Cs₂MoO₂F₄:Mn⁴⁺, the ZPL emission is also observed despite the lower intensity compared with the phonon sidebands.²⁴

In this work, a fluorotantalate compound, i.e., K₃TaOF₆ which can offer a distorted octahedral polyhedron was taken as the matrix. The host material was synthesized by the solid-state reaction and samples of K₃TaOF₆:Mn⁴⁺ were prepared through the hydrothermal method. The high phase purity of the as-prepared samples was demonstrated by XRD and TEM analyses. As expected, an intense ZPL at 620 nm was observed in K₃TaOF₆:Mn⁴⁺ upon blue light excitation. In this article, the synthetic method, luminescence property and thermal quenching of this series of phosphors will be reported.

2. Sample preparation

The raw materials used for the synthesis were Ta₂O₅ (A.R.), KF (A.R.), K₂TaF₇ (A.R.), KMnO₄ (99.99%), KHF₂ (A.R.), HF (49%) and H₂O₂ (30%).

The Mn⁴⁺-activated fluorotantalate K₃TaOF₆:Mn⁴⁺ phosphor was prepared via the solid-state reaction and hydrothermal method in sequence (Fig. 1(a)).

Fig. 1 (a) Synthesis schematic diagram of K₃TaOF₆ and K₃TaOF₆:Mn⁴⁺ (b) X-ray diffraction patterns of K₃TaOF₆:Mn⁴⁺ samples.

Step I: pure K₃TaOF₆ host powder was synthesized by the following stoichiometric equation:²⁵

3K₂TaF₇ + Ta₂O₅ + 9KF → 5K₃TaOF₆ (1)

The mixtures were heated at 900 °C for 0.5 h under an Ar atmosphere. As presented in Fig. 1(b), the main XRD diffraction peaks of the as-obtained host are consistent with the standard XRD pattern of K₃TaOF₆ (PDF No. 29-1052). Then powders of the obtained K₃TaOF₆ and prefabricated K₂MnF₆ in different molar ratios were mixed and ground for 0.5 h in an agate mortar. K₂MnF₆ was obtained from previous work.¹⁹

Step II: loaded the mixtures into Teflon-lined autoclaves and added a drizzle of HF. The autoclaves were heated to 180 °C, kept for 1 h, and then cooled down to room temperature. Finally, the product was collected by centrifugation, subsequently washed with ethanol for three times and dried at 120 °C for 8 h.

The obtained diffraction peaks of K₃TaOF₆:Mn⁴⁺ also agree well with the standard data of K₃TaOF₆ and no impurity phase was observed with the increasing doping amount of K₂MnF₆, illustrating the high phase purity of the as-obtained K₃TaOF₆:Mn⁴⁺ samples.

3. Results and discussion

3.1 Phase, morphology and composition

Fig. 2(a) shows the XRD Rietveld refinement results of K₃TaOF₆:0.04Mn⁴⁺. The experimental data are consistent with calculated patterns deriving from the K₃NbOF₆ matrix (ICSD code: 26634). The good weight-and profile-R-parameters (R_p = 5.94% and R_wp = 8.56%) prove the high match of K₃TaOF₆ to K₃NbOF₆. Then, the cell volumes for K₃TaOF₆:xMn⁴⁺ as determined by Rietveld refinement of powder XRD data is presented in Fig. S1.† According to the results, the volumes gradually decrease with the change of x from 0 to 0.12, due to the Ta⁵⁺ and Mn⁴⁺ ions have the different radius. In the structure of K₃TaOF₆, each Ta atom may be coordinated by six F atoms and one O atom and form a highly disordered octahedron of [TaOF₆]³⁻. Once the Mn⁴⁺ ions invade the crystal lattice of K₃TaOF₆, the similar ionic radii between Mn⁴⁺ and Ta⁵⁺ ions make Mn⁴⁺ ions easy to occupy the sites of Ta⁵⁺ ions. However, the substitution of Mn⁴⁺ ions for Ta⁵⁺ ions would inevitably bring some point defects. The doped phase can be represented through the formula K₃Ta_{(1 − x)}Mn_xO_{(1 − y)}(Vox)_yF₆, with oxygen vacancies (Vox) in the lattice (alternately F vacancies). Meanwhile, Ta⁵⁺ and Mn⁴⁺ cations would coexist which would require electrical compensation with O or F vacancies. The highly-distorted octahedral coordination environment of Mn⁴⁺ ions may induce the local symmetry to lose the inversion centre, which is expected to heighten the emission intensity of ZPL, while the crystal defects would has serious effect for the thermal stability.²⁶

Fig. 2 (a) Rietveld refinements of the observed XRD patterns for the K₃TaOF₆:0.04Mn⁴⁺. (b) and (c) TEM and HRTEM diagram of the K₃TaOF₆:0.04Mn⁴⁺. (d)–(i) SEM image of K₃TaOF₆:0.04Mn⁴⁺ and EDS mapping images of K, Ta, O, F and Mn elements.

TEM analysis in Fig. 2(b) demonstrates that the whole particle of the K₃TaOF₆:0.04Mn⁴⁺ sample exhibits the crystalline nature. Moreover, with the help of HRTEM, lattice fringes could be observed clearly in Fig. 2(c) and two interplanar spacings of 5.15 Å and 4.45 Å were obtained, which are corresponding to the (1 1 1) and (2 0 0) plane of K₃TaOF₆, respectively. From Fig. 2(d), it can be seen that the represented particles of K₃TaOF₆:0.04Mn⁴⁺ have irregular morphology with the size of particle being ∼3 μm and exhibit a degree of particle agglomeration. Moreover, the energy dispersive X-ray spectroscopy (EDS) elemental mapping technique was used to confirm the composition uniformity of K₃TaOF₆:Mn⁴⁺, as presented in Fig. 2(e)–(i). The elemental mapping images exhibit that K, Ta, O, F and Mn are homogeneously distributed within the phosphor particle. In addition, the atom percentages of K, Ta, O and F element are 26.2%, 10.3%, 11.5% and 52.0%, respectively, which is close to 3 : 1 : 1 : 5 of K₃TaOF₆.

3.2 Room-temperature photoluminescence

Fig. S2(a)† shows the diffuse reflection spectrum (DRS) of K₃TaOF₆:xMn⁴⁺ phosphors. The absorption in the blue and UV region arises from the spin-allowed transition of the Mn⁴⁺ ion in d³ configuration. Based on the DR spectrum of non-doped sample, the optical band gap (E_gap) of the K₃TaOF₆ host calculated from the Kubelka–Munk's equations²⁷ is approximately 4.04 eV, as denoted in Fig. S2(b).† When the Mn⁴⁺ ions are embedded into the host lattice, some well-localized energy states belonging to the ground configuration and excited configuration of impurity are introduced between the valence and the conduction bands of the host. Such a wide band gap could provide enough space to accommodate the energy levels of the activator, which can restrain the overlap between the excited state and conduction band.

Fig. 3(a) displays the 3D PL emission spectra of K₃TaOF₆:0.04Mn⁴⁺ with different excitation wavelengths at room temperature. Under various photon irradiation, the shape and position of characteristic peaks almost unchanged, indicating the good color stability of K₃TaOF₆:0.04Mn⁴⁺. Then, iso-intensity contours at the longer excitation wavelengths were recorded to typical 2D photoluminescence excitation (PLE) and PL spectra (Fig. 3(b) and (c)) for further analysis. The emission spectrum excited by 470 nm blue light shows spin-forbidden ²E_g→⁴A_2g transition peaks with phonon-coupled vibronic Stokes/anti-Stokes modes of MnOF₆ octahedron. Obviously, an intense ZPL was recorded at about 620 nm, credited to the extremely distorted coordination environment of Mn⁴⁺ ions. The PLE spectrum detecting at 628 nm shows two wide excitation bands at 470 nm and 370 nm, assigned to the spin-allowed ⁴A_2g→⁴T_2g and ⁴A_2g→⁴T_1g transitions of Mn⁴⁺ ions, respectively.^19,20 Notably, a substantial overlap between the broad blue absorption band and the emission region of InGaN blue chip were found, indicating that K₃TaOF₆:0.04Mn⁴⁺ could be a promising candidate for LCD. Moreover, the obtained CIE chromaticity coordinates of (0.691, 0.309) calculated from the emission spectrum are quite close to the red color standard values of (0.67, 0.33) and the correlated color temperature calculated to be 4451 K, as shown in Fig. 3(d).²⁸ Besides, the color purity of this red phosphor was evaluated to be 96.4%, attributing to its narrow emission band.²⁹ Consequently, K₃TaOF₆:0.04Mn⁴⁺ is a promising red phosphor which has potential application in backlit display field. To achieve the optimization of the luminescence performance of K₃TaOF₆:Mn⁴⁺, a group of samples with various molecular concentration of Mn⁴⁺ ions from 0.02 to 0.12 were synthesized. As displayed in Fig. S3(a),† under the 470 nm blue-light excitation, the integrated intensity exhibits an increase first and the optimal doping concentration of Mn⁴⁺ ions is 4 mol%, then concentration quenching occurs. It is accepted that non-radiative energy transfer between the adjoining luminescence centers contributed to this phenomenon. Fig. S3(b)† shows the luminescent decay curves of K₃TaOF₆:xMn⁴⁺ measured under excitation at 470 nm. Evidently, the luminescence of Mn⁴⁺ ions decays exponentially in K₃TaOF₆ host and the calculated lifetime of this group of samples declines from 4.38 to 3.31 ms with the increase of the concentration. It is believed that the gradually enhanced nonradiative transition between Mn⁴⁺ ions is responsible for the shortening of the lifetime. Specifically, a lifetime less than 5 ms could efficiently avoid the image-retention phenomenon in the application of backlight display.

Fig. 3 (a) The contour plots for the PL emission spectra of K₃TaOF₆:0.04Mn⁴⁺ with various excitation wavelength. (b) and (c) The sliced spectra for excitation and emission bands. (d) CIE coordinates of the light emitted by the sample.

The influence of crystal field strength on the energy levels of Mn⁴⁺ in K₃TaOF₆ could be analyzed via the Tanabe–Sugano diagram. According to Tanabe–Sugano diagram,¹⁷ the values of the crystal field strength D_q and the Racah parameters B and C of K₃TaOF₆:Mn⁴⁺ phosphor were calculated to be ∼2127 cm⁻¹, 527 cm⁻¹ and 3932 cm⁻¹, respectively. Thus, the D_q/B is equivalent to 4.03, implying a high crystal field strength in K₃TaOF₆ host lattice (Fig. S4(a)†).

The nephelauxetic effect caused by the chemical bonds between Mn⁴⁺ and F⁻/O²⁻ligand often determines the emission energy of Mn⁴⁺:²E→⁴A_2g. Therefore, a parameter β₁ was adopted to quantitatively describe the nephelauxetic effect in the spectroscopy of the Mn⁴⁺:²²

where the free ion values of B₀ and C₀ of the Mn⁴⁺ ion are 1160 cm⁻¹ and 4303 cm⁻¹, respectively. The value of β₁ for Mn⁴⁺ in K₃TaOF₆ is then calculated to be 1.02. Notably, the data point followed the empirical line well, as shown in Fig. S4(b).†

3.3 Temperature-dependent luminescence

To get a better study on temperature-dependent luminescence behavior of K₃TaOF₆:Mn⁴⁺, the PL spectra of K₃TaOF₆:0.04Mn⁴⁺ phosphor excited by 470 nm blue light at various temperature (290–435 K) were measured. The temperature-dependent emission intensity spectra and the corresponding intensity trend were plotted in Fig. 4(a) and (b). Clearly, the integrated PL intensity of K₃TaOF₆:0.04Mn⁴⁺ decreases with the increasing of temperature and the red phosphor shows a remarkable thermal quenching because of the severer non-radiative transition at higher temperature. The thermal activation energy (ΔE_a) can be determined by Arrhenius equation:²²

Fig. 4 (a) and (b) Temperature-dependent emission intensity and the corresponding integrated emission intensity of K₃TaOF₆:0.04Mn⁴⁺. (c) Configurational coordination diagram for Mn⁴⁺ in K₃TaOF₆. (d) CIE shift of K₃TaOF₆:0.04Mn⁴⁺ with various temperature.

where I₀ is the emission intensity at temperature 290 K, C and k is constant. So the calculated ΔE_a can be determined to be 0.218 eV. With the increasing of temperature, most electrons at ⁴T_2g state will absorb the heat and climb to the crossover point of the ⁴T_2g and ⁴A_2g states once the accumulated energy exceeds ΔE_a. Then, these electrons will return to the ground state ⁴A_2g by a non-radiative way and give rise to thermal quenching, as demonstrated in configurational coordinate diagram (Fig. 4(c))

Besides, color stability is another critical parameter for phosphors, which can be described with CIE shift (ΔE). The calculated equation is as follows:^30,31

where u = 4x/(3 − 2x + 12y), v = 9y/(3 − 2x + 12y) and w = 1 − u − v. u and v are the chromaticity coordinates in uv uniform color space. As shown in Fig. 4(d), the low ΔE of K₃TaOF₆:Mn⁴⁺ red phosphor further proves its high color stability and the potential for display devices.

Killer centers would be formed when the Mn⁴⁺ ions occupy the sites of Ta⁵⁺ ions, which may accelerate the thermal quenching process. In K₃TaOF₆:Mn⁴⁺, there are two kinds of killer centers, such as O or F vacancies. To further study the nature of the traps, EPR was adopted to probe the point defects and impurity ions. As shown in Fig. 5(a), the EPR spectra of the K₃TaOF₆:0.04Mn⁴⁺ before/after irradiation were recorded. When the specimen was illuminated with blue-light (450 nm), an almost symmetric EPR signal appeared at g = 1.998. The negative g shift is attributed to electrons trapped in K₃TaOF₆ host.^32–34 In addition to EPR analysis, to study the thermal ionization process in K₃TaOF₆:0.04Mn⁴⁺, the thermoluminescence (TL) spectrum monitored by temperature (90–450 K) was collected with the sample exposure to UV light for 120 s, as shown in Fig. 5(b). Obviously, there is one broad band locating at 100–250 K, corresponding to the trapping energy level. Generally, shallow traps are easier to capture and release the electrons or holes than the deep ones. As a consequence, when the temperature reaches to a certain value, the captured electrons or holes would escape from the shallow traps, which leads to thermal quenching. In Fig. 5(b), there are two Gaussian peaks locating at around 156 K and 200 K respectively for K₃TaOF₆:0.04Mn⁴⁺. The phenomenon that the integrated PL intensity is sensitive to the heating temperature and decreases dramatically with the increasing temperature strongly indicates that thermal ionization is the real villain for thermal quenching of K₃TaOF₆:0.04Mn⁴⁺. The trap depth E was estimated to range from 0.31 to 0.40 eV according to the equation of E = T_m/500, where T_m is the peak temperature. To get a better understand of the process, the charging for TL measurement and thermal quenching induced by thermal ionization was illustrated in Fig. 5(c).

Fig. 5 (a) The EPR spectra of K₃TaOF₆:Mn⁴⁺ before/after irrational. (b) TL spectrum and theoretically fitted spectra of K₃TaOF₆:Mn⁴⁺ sample. (c) Schematic illustration of thermal quenching.

4. Conclusions

In summary, Mn⁴⁺-activated fluorotantalate red phosphor K₃TaOF₆ was successfully obtained. This phosphor shows intense red emission peaking at 628 nm along with a high intensity ZPL emission under blue light excitation. The optimal doping concentration of Mn⁴⁺ ions is 4 mol%. By conducting the TL and EPR analysis, thermal ionization may play a dominant role in the thermal quenching.

Conflicts of interest

The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 51902226, 61905099, 52002288), the National Natural Science Foundation of Guangdong Province (No. 2019A1515012072), the Innovation Projects of Department of Education of Guangdong Province (No. 2018KQNCX266), Innovative Leading Talents of Jiangmen [Jiangmen (2019) 7], the Science and Technology Projects of Jiangmen (No. 2020JC01012, 2020JC01018), the research and development fund of Wuyi university joint Hong Kong-Macao (No. 2019WGALH04, No. 2019WGALH09).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra05174a

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