Highly efficient far-red emitting Mn4+-activated Li3La3W2O12 phosphors for plant growth LED lighting

Abstract

High-efficiency far-red emitting phosphors with an emission peak of around 730 nm are urgently needed for indoor plant growth light-emitting diode (LED) lighting. In this article, we report on the synthesis, crystal structure, and luminescence properties of a new highly efficient Mn4+ ion-activated Li3La3W2O12 (LLWO) far-red emitting phosphor. A group of LLWO:Mn4+ phosphors doped with different Mn4+ ion concentrations have been prepared via the conventional high-temperature solid-state reaction route. X-ray diffraction Rietveld refinement reveals that the LLWO:Mn4+ phosphor has the monoclinic structure with space group P21/n, along with lattice parameters of a = 5.5396 Å, b = 5.6026 Å, c = 7.8802 Å, α = γ = 90°, β = 90.0988°, and V = 244.57 Å3. Interestingly, LLWO:Mn4+ phosphors can generate bright narrowband far-red emission in the 650–800 nm wavelength range peaking at 719 nm due to the spin- and parity-forbidden 2Eg→4A2g transition of Mn4+ ions, matching well with the absorption spectrum of phytochrome PFR. The excitation spectrum monitored at 719 nm has a broad excitation band with the range from 250 to 550 nm centered at 339 nm and 472 nm, indicating that LLWO:Mn4+ phosphors could be efficiently pumped by both ultraviolet and blue LED chips. The highest far-red emission is achieved for the sample doped with 1.2 mol% Mn4+ ion, and this optimal LLWO:1.2%Mn4+ sample shows a high internal quantum efficiency of 88.4% and a decay lifetime of 2.289 ms. Commission Internationale de l’Eclairage (CIE) color coordinates of LLWO:1.2%Mn4+ are determined to be (0.7326, 0.2674). Besides, the crystal field analysis and temperature-dependent emission spectra of the as-prepared samples have been investigated in detail. The emission intensity of LLWO:1.2%Mn4+ sample at 423 K remains about 40% of that at 303 K. Finally, a LED device is fabricated by integrating the LLWO:1.2%Mn4+ phosphors and a 460 nm blue LED chip, which shows bright blue and far-red dual emission bands under 20–300 mA driving currents. This work demonstrates that these newly developed far-red emitting LLWO:Mn4+ phosphors have excellent application prospects in plant growth LED lighting.