The elevated colour rendering of white-LEDs by microwave-synthesized red-emitting (Li, Mg)3RbGe8O18:Mn4+ nanophosphors

Abstract

The bright red emissive nature of low-cost Mn⁴⁺ ions can replace the commercially available Eu²⁺-doped nitrides/oxynitrides for application in white light-emitting diodes (W-LED). Herein, the Mn⁴⁺-doped Li₃RbGe₈O₁₈ (LRGO) phosphor was synthesized via the solid-state reaction (SSR), microwave-assisted diffusion (MWD), and microwave-assisted sol–gel (MWS) techniques. The MWS-derived crystalline nanoparticles having sizes less than 200 nm exhibited higher red emission intensity at around 668 nm as compared to that of the micron-sized particles obtained with other approaches, owing to the improved compositional homogeneity provided by the MWS technique. The effect of microwaves was studied to gain the optimized morphology with enhanced red emission brightness. Obtained samples showed narrow red emission maxima at 668 nm under UV (300 nm) and blue (455 nm) excitations owing to ²E_g → ⁴A_2g: Mn⁴⁺ transitions with the possibility of degeneracy. The existence of doubly degenerate forms and the splitting of ²E_2g and ⁴A_2g levels were further confirmed via low-temperature photoluminescence (PL) analysis. The emission intensity was also enhanced by the Mg²⁺ co-doping of MWS-derived LRGO:Mn⁴⁺ nanophosphors. Comparative photoluminescence analysis indicated that the optimized MWS route and the Mg²⁺ co-doping enhanced the red emission intensity by 182% as compared to the solid-state-derived LRGO:Mn⁴⁺. The optimized Mg²⁺ co-doped nanophosphor showed ∼99% red colour purity under UV and blue excitations. Finally, several W-LEDs were fabricated by combining the mixture of yellow-emitting YAG:Ce³⁺ phosphor and the optimized red-emitting LRGO:Mn⁴⁺,Mg²⁺ nanophosphor on a 460 nm blue-LED chip. The chromaticity of W-LEDs was tuned from bluish-white with the correlated color temperature of 6952 K, to pure white with the CCT of 5025 K. The color rendering index was also improved from 71 to 92, which could be suitable for indoor lighting applications.