Highly efficient far-red emitting Mn4+-activated Li3La3W2O12phosphors for plant growth LED lighting

High-efficiency far-red emitting phosphors with emission peak 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→4A2gtransition 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. CIE color coordinates of Li3La3W2O12:1.2%Mn4+are determined to be (0.7326, 0.2674). Besides, the crystal field analysis and temperature-dependent 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.