Novel Mn4+-activated LiLaMgWO6 far-red emitting phosphors: high photoluminescence efficiency, good thermal stability, and potential applications in plant cultivation LEDs

Abstract

Double perovskite-based LiLaMgWO₆:Mn⁴⁺ (LLMW:Mn⁴⁺) red phosphors were synthesized by traditional solid-state route under high temperature, and they showed bright far-red emission under excitation of 344 nm. The crystal structure, luminescence performance, internal quantum efficiency, fluorescence decay lifetimes, and thermal stability were investigated in detail. All samples exhibited far-red emissions around 713 nm due to the ²E_g → ⁴A_2g transition of Mn⁴⁺ under excitation of near-ultraviolet and blue light, and the optimal doping concentration of Mn⁴⁺ was about 0.7 mol%. The CIE chromaticity coordinates of the LLMW:0.7% Mn⁴⁺ sample were (0.7253, 0.2746), and they were located at the border of the chromaticity diagram, indicating that the phosphors had high color purity. Furthermore, the internal quantum efficiency of LLMW:0.7% Mn⁴⁺ phosphors reached up to 69.1%, which was relatively higher than those of the reported Mn⁴⁺-doped red phosphors. Moreover, the sample displayed good thermal stability; the emission intensity of LLMW:0.7% Mn⁴⁺ phosphors at 423 K was 49% of the initial value at 303 K, while the activation energy was 0.39 eV. Importantly, there was a broad spectral overlap between the emission band of LLMW:Mn⁴⁺ phosphors and the absorption band of phytochrome P _FR under near-ultraviolet light. All of these properties and phenomena illustrate that the LLMW:Mn⁴⁺ phosphors are potential far-red phosphors for applications in plant cultivation LEDs.

Fig. 1. XRD patterns of LLMW:Mn⁴⁺ phosphors with different Mn⁴⁺ concentrations. The standard XRD pattern of NaLaMgWO₆ (PDF #88-1761) is shown as a reference.

Fig. 2. (a) Rietveld refinements of LLMW:0.7% Mn⁴⁺ phosphors. (b) Crystal structure of LLMW:0.7% Mn⁴⁺ phosphors.

Fig. 3. Representative SEM images of LLMW:0.7% Mn⁴⁺ phosphors.

Fig. 4. (a) PLE and PL spectra of LLMW:0.7% Mn⁴⁺ phosphors. (b) Comparison of the emission spectrum of LLMW:0.7% Mn⁴⁺ phosphors (λ_ex = 344 nm) and the absorption spectrum of phytochrome P_FR. (c) PL spectra of LLMW:xMn⁴⁺ phosphors under 344 nm excitation. (d) Normalized PL intensity of LLMW:xMn⁴⁺ phosphors as a function of Mn⁴⁺ concentration. (e) Plot of log(I/x) vs. log(x) for the 713 nm emission of Mn⁴⁺ ions in LLMW:xMn⁴⁺ phosphors excited at 344 nm. (f) Decay curves of LLMW:xMn⁴⁺ phosphors excited at 344 nm and monitored at 713 nm.

Fig. 5. (a) Tanabe–Sugano diagram of the Mn⁴⁺ ion. (b) The relationship between the energy of the ²E_g state and the parameter β.

Fig. 6. (a) The CIE chromaticity diagram of LLMW:0.7% Mn⁴⁺ phosphors at 344 nm excitation. The inset shows the photograph of LLMW:0.7% Mn⁴⁺ phosphors under a 365 nm UV lamp. (b) The excitation line of BaSO₄ and the emission spectrum of LLMW:0.7% Mn⁴⁺ phosphors collected using an integrating sphere. The inset shows a magnification of the emission spectrum in the 650–780 nm wavelength range.

Fig. 7. (a) Temperature-dependent PL spectra of LLMW:0.7% Mn⁴⁺ phosphors excited at 344 nm. The inset shows the normalized PL intensity of LLMW:0.7% Mn⁴⁺ phosphors as a function of temperature. (b) Plot of ln(I₀/I − 1) vs. 1/kT. (c) The configurational coordinate diagram of Mn⁴⁺.

Fig. 8. The EL spectrum of the fabricated red-emitting LED device with a 365 nm NUV chip and LLMW:0.7% Mn⁴⁺ red phosphors under 60 mA current. The inset shows the fabricated LED device and the corresponding luminescent image.