Magical polyhedral twist via chemical unit co-substitution in LaAlO3:Mn4+ to greatly enhance the zero phonon line for high-efficiency plant-growth LEDs

Abstract

Phytochrome and phytochrome-controlled physiological responses play an important role in plant growth. The emission of LaAlO₃:Mn⁴⁺ partly matches with the absorption spectrum of the P_FR region. However, the luminescence efficiency is not high due to the discontinuous emission spectrum, thus resulting in its limited application prospects in plant-growth LEDs. Here, new phosphors of LaAl_0.999−x(Mg/Ge)_xO₃:0.001Mn⁴⁺ (LAMG:Mn⁴⁺, x = 0–0.4) were successfully synthesized via a solid-state reaction. Through a promising strategy of chemical unit co-substitution, that is substituting Mg²⁺–Ge⁴⁺ for Al³⁺–Al³⁺, a significant octahedral tilting distortion and a decreased symmetry of MnO₆ took place. Interestingly, the zero phonon line (ZPL) of Mn⁴⁺, located in the middle between the Stokes and the anti-Stokes vibronic bands, appeared and became stronger with an increase of the x value from 0 to 0.4. This makes the emission peak become a continual broad band with a FWHM of ∼50 nm, instead of two original separate ones, thus inducing a high absorption efficiency of the phytochrome. The electroluminescence spectrum of the LED prepared using commercial red phosphors CaAlSiN₃:Eu²⁺ and LaAl_0.599(Mg/Ge)_0.4O₃:0.001Mn⁴⁺ matched well with P_R and P_FR, indicating that LaAl_0.599(Mg/Ge)_0.4O₃:0.001Mn⁴⁺ can be used as a near-infrared component in phosphor-converted LEDs for controlling plant growth.