Novel SrLaAlO4:Mn4+ deep-red emitting phosphors with excellent responsiveness to phytochrome PFR for plant cultivation LEDs: synthesis, photoluminescence properties, and thermal stability

Abstract

Herein, novel rare-earth-free Mn⁴⁺-doped SrLaAlO₄ deep-red emitting phosphors were successfully synthesized via a traditional solid-state reaction method. The crystal structure and phase purity of the as-prepared samples were confirmed by XRD Rietveld refinement. Photoluminescence properties of SrLaAlO₄:Mn⁴⁺ phosphors were examined in detail using photoluminescence spectra, decay lifetimes, temperature-dependent emission spectra and internal quantum efficiency measurements. The excitation spectrum obtained by monitoring at 730 nm contained two excitation bands centered at 364 and 520 nm within the range of 200-550 nm due to the Mn⁴⁺-O^2- charge-transfer band and the ⁴A_2g → ⁴T_1g, ⁴T_2g transitions of the Mn⁴⁺ ions. Under the 364 nm excitation, the SrLaAlO₄:Mn⁴⁺ phosphors exhibited an intense deep-red emission band in 610-790 nm wavelength range peaking at 730 nm, which was assigned to the ²E_g → ⁴A_2g transition of Mn⁴⁺ ions. The deep red emission showed excellent responsiveness to phytochrome P_FR, revealing that the SrLaAlO₄:0.4% Mn⁴⁺ phosphors possessed a possible application in deep-red light-emitting diodes (LEDs) for plant cultivation. The optimal doping concentration of Mn⁴⁺ ions was found to be 0.4 mol%. The critical distance R _c for energy transfer among Mn⁴⁺ ions was determined to be 5.86 Å and the concentration quenching mechanism was confirmed to be the electric dipole-dipole interaction. In addition, the Commission International de I'Eclairage (CIE) colour coordinates of the SrLaAlO₄:0.4% Mn⁴⁺ phosphors (0.734, 0.266) were located in the deep red region and the corresponding internal quantum efficiency was measured to be about 29%. The above results confirmed that the as-prepared SrLaAlO₄:0.4% Mn⁴⁺ deep red emitting phosphors might be a potential candidate for plant cultivation LEDs.

Fig. 1. XRD patterns of pure SLA and 0.4% Mn⁴⁺ doped SLA phosphors.

Fig. 2. (a) Rietveld refinement XRD patterns of SLA:0.4% Mn⁴⁺ phosphors by the FULLPROF program. The solid black rounds, red lines and blue lines express the observed and calculated XRD patterns of the sample as well as their differences. The short vertical pink lines display the positions of Bragg reflection. (b) Schematic crystal structure of SLA:0.4% Mn⁴⁺ phosphors.

Fig. 3. (a) FE-SEM image and (b–f) elemental mapping of SLA:0.4% Mn⁴⁺ phosphors.

Fig. 4. (a) Room-temperature PLE and PL spectra of SLA:0.4% Mn⁴⁺ phosphors. (b) PL spectrum of SLA:0.4% Mn⁴⁺ phosphors and the absorption spectra of phytochrome P_FR and P_R.

Fig. 5. (a) PL spectra of SLA:xMn⁴⁺ (x = 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, and 1%) phosphors as a function of Mn⁴⁺ doping concentrations. (b) PL emission intensities with Mn⁴⁺ doping concentration. (c) The relationship between log(I/x) versus log(x). (d) PL decay curves and calculated lifetime values of SLA:0.4% Mn⁴⁺ phosphors under the excitation of 364 nm and monitored at 730 nm.

Fig. 6. CIE chromaticity coordinates of SLA:0.4% Mn⁴⁺ phosphors (λ_ex = 364 nm). Inset shows the image of SLA:0.4% Mn⁴⁺ phosphors under 365 nm UV lamp.

Fig. 7. (a) Temperature dependent PL spectra of SLA:0.4% Mn⁴⁺ phosphors (λ_ex = 364 nm). (b) Normalized PL intensity of SLA:0.4% Mn⁴⁺ phosphors as a function of various temperatures. (c) The relationship of ln(I₀/I − 1) versus 1/kT of the SLA:0.4% Mn⁴⁺ phosphors.

Fig. 8. EL spectrum of the fabricated deep-red LED device using SLA:0.4% Mn⁴⁺ phosphors and a 365 nm near-UV LED chip under a current of 20 mA. Inset shows the fabricated LED device and corresponding luminescent image.