Novel SrMg2La2W2O12:Mn4+ far-red phosphors with high quantum efficiency and thermal stability towards applications in indoor plant cultivation LEDs

Abstract

Novel Mn⁴⁺-activated far-red emitting SrMg₂La₂W₂O₁₂ (SMLW) phosphors were prepared by a conventional high-temperature solid-state reaction method. The SMLW:Mn⁴⁺ phosphors showed a broad excitation band peaking at around 344 nm and 469 nm in the range of 300-550 nm. Under 344 nm near-ultraviolet light or 469 nm blue light, the phosphors exhibited a far-red emission band in the 650-780 nm range centered at about 708 nm. The optimal Mn⁴⁺ doping concentration in the SMLW host was 0.2 mol% and the CIE chromaticity coordinates of SMLW:0.2% Mn⁴⁺ phosphors were calculated to be (0.7322, 0.2678). In addition, the influences of crystal field strength and nephelauxetic effect on the emission energy of Mn⁴⁺ ions were also investigated. Moreover, the internal quantum efficiency of SMLW:0.2% Mn⁴⁺ phosphors reached as high as 88% and they also possessed good thermal stability. Specifically, the emission intensity at 423 K still maintained about 57.5% of the initial value at 303 K. Finally, a far-red light-emitting diode (LED) lamp was fabricated by using a 365 nm near-ultraviolet emitting LED chip combined with the as-obtained SMLW:0.2% Mn⁴⁺ far-red phosphors.

Fig. 1. (a) The Rietveld refinement for the XRD patterns of SMLW:0.2% Mn⁴⁺ phosphors. (b) The crystal structure of SMLW:0.2% Mn⁴⁺ phosphors.

Fig. 2. (a) The XRD patterns of SMLW:xMn⁴⁺ (x = 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) phosphors and the standard PDF card SMLW (JCPDS # 35-0259). (b) The local XRD patterns in the 2θ range from 30.5 to 34.5 degree.

Fig. 3. (a–c) FE-SEM images and (d) elemental mapping of as-prepared SMLW:0.2% Mn⁴⁺ phosphors.

Fig. 4. (a) PLE (λ_em = 708 nm) and PL (λ_ex = 344 nm and 469 nm) spectra of SMLW:0.2% Mn⁴⁺ phosphors. (b) The emission spectrum of SMLW:0.2% Mn⁴⁺ and absorption spectrum of P_FR.

Fig. 5. (a) Tanabe–Sugano energy level diagram of Mn⁴⁺ ions in the SMLW host. (b) The simple energy level diagram of Mn⁴⁺ ions. (c) The relationship between the ²E_g energy level of Mn⁴⁺ ions and the calculated nephelauxetic ratio β₁ in different hosts.

Fig. 6. (a) PL spectra of SMLW:xMn⁴⁺ (x = 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) phosphors excited at 344 nm. (b) The emission intensity of SMLW:xMn⁴⁺ as a function of Mn⁴⁺ doping concentration. (c) The dependence of log(I/x) on log(x). (d) The lifetime decay curves of SMLW:xMn⁴⁺ (x = 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%) phosphors under the 344 nm excitation while monitored at 708 nm.

Fig. 7. (a) CIE chromaticity coordinates diagram of SMLW:0.2% Mn⁴⁺ phosphors (λ_ex = 344 nm). Inset (i) and (ii) represent the photographs of the SMLW:0.2% Mn⁴⁺ under daylight and 365 nm near-UV light, respectively. (b) The excitation line of BaSO₄ reference and the emission spectrum of SMLW:0.2% Mn⁴⁺ phosphors excited at 344 nm.

Fig. 8. (a) The temperature-dependent PL spectra of SMLW:0.2% Mn⁴⁺ phosphors. (b) The normalized PL intensities of SMLW:0.2% Mn⁴⁺ phosphors at different temperature from 303 to 503 K. (c) The configuration diagram of Mn⁴⁺ in the octahedron. (d) The plot of ln(I₀/I − 1) versus 1/kT.

Fig. 9. (a) EL spectrum of fabricated far-red emitting LED lamp by using a 365 nm near-UV chip combined with SMLW:0.2% Mn⁴⁺ phosphors driven by 60 mA current. Inset (ii) and (i) were the photographs of the far-red LED lamp with and without current, respectively. (b) CIE chromaticity coordinates diagram based on the EL spectrum of fabricated far-red emitting LED lamp excited at 365 nm.