Rare-earth free narrow-band green-emitting KAlSi₂O₆:Mn²⁺ phosphor excited by blue light for LED-phosphor material

Abstract

A novel rare-earth free green-emitting KAlSi₂O₆:Mn²⁺ phosphor is synthesized by traditional solid-state reaction. The structure, photoluminescence and temperature-dependent properties are investigated. KAlSi₂O₆:Mn phosphor can emit green light peaking at 513 nm with narrow full-width at half-maximum of 30 nm upon 450 nm excitation. The quantum yields and Commission Internationale de I’Eclairage (CIE) color coordinates are 30.2% and (0.27, 0.64). The white LED is fabricated by KAlSi₂O₆:Mn and CaAlSiN₃:Eu (red phosphor) combined with blue LED chip. Its correlated color temperature, color coordinates, R_a and luminous efficiency are 4775 K, (0.35, 0.36), 85.1 and 110.2 lm W⁻¹, respectively. It is demonstrated that KAlSi₂O₆:Mn²⁺ phosphor as the potential material could be simulated to blue-LED for producing efficient white-light.

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1. Introduction

White light-emitting diodes (w-LED), as promising candidates to replace conventional incandescent and fluorescent lamps, have received increasing attention in recent years due to their admirable merits of high luminous efficiency, low energy cost and robustness.^1–3 The current most widely used method to obtain w-LED is InGaN blue chip combined with yellow phosphor (YAG:Ce).⁴ However, such design suffers some technical weaknesses in practical application. One of these problems is the hiatus in red components, which leads to high correlated color temperature (CCT) and low color rendering index (CRI) for white light. Besides this method, another kind of w-LEDs can be also fabricated by GaN-based blue chip combing with green and red phosphors which can solve above problems. Some nitrides/oxynitrides green phosphors like MSi₂O₂N₂:Eu²⁺, SiAlON:Eu²⁺ and Ba₂SiO₄:Eu²⁺ have been commercialized.^5–7 However, most |of those phosphors are doped by rare-earth and their synthesis conditions are harsh which will lead to high production cost. As an alternative, the non-rare-earth Mn²⁺ doped phosphors offer some important advantages in this respect and thus have gained increased attention recently.^8,9 As known to all, only Mn²⁺ in tetrahedron crystal field can emit green light. Mn²⁺ is featured by a 3d³ electron configuration with electrons located in an outer orbit, which causes its optical property heavily affected by the matrix; in other words, the green emitting wavelengths of spin-forbidden Mn²⁺: forbidden d–d transition (⁴T₁ → ⁶A₁) can be affected by the crystal field conditions which can emit bright light.¹⁰

In this work, a novel rare-earth free Mn-doped KAlSi₂O₆ (KAS) green phosphor excited by blue light is reported for the first time. A phosphor with narrow green emission band and nice temperature properties is obtained. As they are tetrahedrally coordinated Al and Si are in the same crystal coordinate and occupy crystal positions randomly. Relative appropriate radius and coordinate may provide a suitable factor for Mn²⁺ occupying the same position. The crystal phase formation, luminescence properties and thermal quenching properties are investigated.

2. Experimental section

K₂CO₃ (99.9%), H₂SiO₃ (AR), Al(OH)₃ (AR) and MnCO₃ (99.99%) as the raw materials were stoichiometric and 2 wt% NH₄F as the flux was also used in the synthesis process. These materials were ground in an agate mortar with a small amount of ethanol. The prepared samples were sintered at 800 °C for 2 h and 1420 °C for 6 h. When the material had cooled, they were ground and the samples were obtained. The phase formation and crystal structure were analyzed by the X-ray powder diffraction (XRD) (D2 PHASER X-ray Diffractometer, Germany) with graphite monochromator using Cu Kα radiation (λ = 1.54056 A), operating at 30 kV and 15 mA. The luminescent spectra were measured by Fluorolog R-3 Spectrophotometer.

3. Results and discussion

In order to determine the real structure of the synthesized samples, ICSD-161638 (KAlSi₂O₆) is used as the standard data to refine KAS and structural refinement of XRD is made using GSAS program. Fig. 1a illustrates the experimental and refined XRD patterns of KAS sample. By comparing the calculated data with the experimental spectra, we find that each peak is in good agreement. There is no impurity phase found in the samples, which reveals that it crystallizes in good single-phase. The calculated residual factor value is R_p = 7.43%, and R_wp = 8.55%. The appearance of KAlSi₂O₆ emerges homogeneous honeycomb morphology shown in the inset of Fig. 1a. XRD patterns of the as-prepared KAlSi₂O₆:x% Mn²⁺ (1 ≤ x ≤ 6) phosphors are given in Fig. 1b. It can be seen that all of the diffraction peaks of the samples can be basically indexed to the standard data, suggesting that KAlSi₂O₆:x% Mn²⁺ with different Mn²⁺ concentration can be formed in the single-phased structure. KAlSi₂O₆ belongs to I41/a (88) space group with tetragonal structure. Al and Si are in the same crystal coordinate and occupy crystal positions randomly. Different Al/Si–O tetrahedrons are connected by total of angle connection of O.¹¹ According to ionic radius,¹² Mn²⁺ may occupy an Al/Si crystal site, which is shown in Fig. 1c and d.

Fig. 1 (a) Rietveld refinement of the powder XRD profile of KAS with its JCPDS. Inset shows the SEM micrograph of KAlSi₂O₆:1% Mn²⁺ phosphor taken at a magnification of 1.50k×. (b) XRD patterns of series KAS:x% Mn²⁺ and standard data. (c) Crystal structure of KAlSi₂O₆. (d) Coordination relationship of Si|Al–O and Mn²⁺ occupying situation.

Fig. 2a shows the room-temperature photoluminescence (PL) excitation spectrum monitoring at 513 nm emission. The PL excitation shows five distinct peaks centered at 358, 382, 424, 450, and 465 nm that they are in good agreement with the well-known Mn²⁺ absorption transitions.¹³ Especially, it can match blue GaN chips well due to its relative strong 450 nm absorption. The PL emission spectra of KAlSi₂O₆:x% Mn²⁺ phosphor registered at 450 nm excitation wavelengths are shown in Fig. 2b. It can be easily seen that it is a green light emission peaking at 513 nm with full-width at half-maximum (FWHM) of 30 nm and the emission intensities have an obvious increasing trend with increasing Mn²⁺concentration, and maximizes is at x% = 3%, then the emission intensity decreases. Since in KAlSi₂O₆, the Mn²⁺ is tetrahedron coordinated, it produces a single broad-band green emission centered at 513 nm which is attributed to spin forbidden d–d transition (⁴T₁ → ⁶A₁) of Mn²⁺ and the small FWHM (30 nm) can get high color purity. The excitation transitions of Mn²⁺ from ground level ⁶A₁(⁶S) to ⁴E(⁴D), ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)], ⁴T₂(⁴G), and ⁴T₁(⁴G) excited levels are schematically shown in Fig. 2c. The electron could be relaxed from these excited states to the ⁴T₁(G) state by a non-radiative relaxation process and then transferred back to the ground state ⁶A₁(S) emitting the characteristic green (513 nm) light. Temperature-dependent relative emission intensity upon 450 nm excitation of KAS:3% Mn²⁺ is indicated in Fig. 2d. It can be clearly seen that with temperature increasing, the emission intensity decreases gradually due to the non-radiative transition probability by thermal activation dependent on temperature. The integrated emission intensity of KAS:3% Mn²⁺ is decreased to 67.6% (250 °C) compared with the initial value (25 °C), meanwhile, commercial Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺ (green phosphor) and YAG:Ce³⁺ (P46-Y3, yellow phosphor) are decreased to 65.3% and 27%. It indicates that the temperature-dependent properties of KAS:3% Mn²⁺ are better than YAG:Ce³⁺ (P46-Y3, yellow phosphor) and similar with Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺.

Fig. 2 Photoluminescence excitation of KAS:Mn²⁺ (a); photoluminescence emission spectra of KAS phosphors, inset: dependence of emission intensity of KAS:xMn²⁺ phosphors on Mn²⁺ content (b); the excitation and emission transitions of Mn²⁺ ions (c); temperature dependence of KAS:3% Mn²⁺ PL properties, inset: contradistinction of KAS:3% Mn²⁺, YAG:Ce³⁺ (P46-Y3, yellow phosphor) and commercial Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺ (green phosphor).

The color coordinates are highly useful in determining the exact emission color and color purity of the sample, as per the chromaticity diagram of the Commission Internationale de I’Eclairage (CIE). The CIE color coordinates of KAS:0.03Mn²⁺ phosphor were calculated to be x = 0.27 and y = 0.64 using an equidistant wavelength method and are illustrated in the CIE chromaticity diagram as shown in Fig. 3a. The color coordinates of KAS:Mn is much closer to standard green coordinates (0.21, 0.71). These values obtained are comparable to the coordinates (0.43, 0.52) and (0.31, 0.60) of commercially used YAG:Ce and β-SiAlON:Eu²⁺ (for green). The quantum yields are measured to be 41.4% at 460 nm, 30.2% at 450 nm, and 88.7% at 460 nm for β-SiAlON:Eu²⁺, KAS:0.03Mn²⁺, and YAG:Ce phosphors, respectively. The comparison of color coordinates to other yellow-green/green phosphors, this Mn doped phosphor exhibits high color purity, which enables to achieve a larger color gamut for devices. Furthermore, the relative strong absorption peaking at 450 nm can match with currently available blue-LED chips. To obtain warm white light, white light LED is fabricated by KAS:Mn phosphor blended with CaAlSiN₃:Eu red phosphor combined with blue-LED chip (∼455 nm). Its emission spectrum is shown in Fig. 3b. The chromaticity coordinates of this typical white LED is (0.35, 0.36) with CCT of 4775 K which is shown in Fig. 3a. The R_a and luminous efficiency is 85.1 and 110.2 lm W⁻¹ with drive voltage 3.2 V. It indicates that the narrow emission band and high color purity make the current sample a very attractive green phosphor for white LEDs.

Fig. 3 The CIE chromaticity coordinates and their respective quantum yields. The inset shows the phosphor simulated white light using blue (455 nm) LED (a); the PL spectrum of white light combined with blue chips, KAS:Mn and CaAlSiN₃:Eu.

4. Conclusions

In summary, we provide original insights for preparing a rare-earth free KAlSi₂O₆:Mn phosphor with comparable PL brightness levels useful for white-LEDs. This relatively inexpensive phosphor has an internal QY of 30.2% at 450 nm. It can emit green light peaking at 513 nm with FWHM = 30 nm and nice color coordinates (0.27, 0.64). The integrated emission intensity of KAS:3% Mn²⁺ is decreased to 67.6% (250 °C) of the initial value (25 °C). The chromaticity coordinates of fabricated white LED is (0.35, 0.36) with CCT of 4775 K. The R_a and luminous efficiency is 85.1 and 110.2 lm W⁻¹ with drive voltage 3.2 V. It is demonstrated that KAlSi₂O₆:Mn²⁺ phosphor could be simulated to blue-LED for producing efficient white-light.

Acknowledgements

This work is supported by Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120211130003), the National Natural Science Funds of China (Grant no. 51372105) and the Fundamental Research Funds for the Central Universities (no. lzujbky-2014-231).

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