High thermal stability phosphors with a rigid structure similar to the benzene ring and application in plant growth

Abstract

For inorganic luminescent materials, it is a challenge that the emission intensity decreases gradually with increasing temperature due to the temperature quenching effect. To reduce this effect, an effective way is to improve structural stiffness. Herein, a new series of Cr³⁺ activated LiABO₄ (A = Al, Ga; B = Ge, Si) with high thermal stability has been synthesized, which has a structure similar to the benzene ring. Such a rigid net structure endows this phosphor with outstanding thermal stability. The crystal structure, luminescence lifetime, photoluminescence (PL) properties, and temperature-dependent luminescence of LiGaGeO₄, Li(Al,Ga)GeO₄ and LiAlSiO₄ were investigated in detail. When replacing part of Ga with Al in the LiGaGeO₄, it can be observed that the intensity of photoluminescence and thermostability improved a lot. Since LiGaGeO₄ has two different environments for the Cr³⁺ ions, the PL shows a combination of the broadband and the narrow band which correspond to the ⁴T₂ → ⁴A₂ and ²E₂ → ⁴A₂ transitions, respectively. The PL intensities of LiAl_0.1Ga_0.9GeO₄:0.002Cr³⁺ and LiAlSiO₄:0.02Cr³⁺ at 150 °C still keep 82% and 94% of their PL intensities at room temperature, indicating outstanding thermostability. Owing to its rigid tetrahedral network of LiAlSiO₄, a small Stokes shift and ultra-narrow band with fwhm ≈3 nm center at 719 nm were observed. The emission bands of mentioned phosphors matched well with the absorption band of phytochromes. Moreover, the packaged white LED device that was made up of a 395 nm LED chip, commercial phosphors, and samples of LiAlSiO₄:0.02Cr³⁺ shows a high color rendering index (R_a = 94.5) and low color temperature (CCT = 3926 K), indicating that the phosphors mentioned above have a promising application in pc-LED for plant growth.