Engineering deep-red Al20B4O36:Cr3+ phosphors for photomorphogenesis

Abstract

Cr3+ doped phosphors with deep-red luminescence hold great promise as artificial lighting sources for plants growth. Among them, α-Al2O3:Cr3+ is featured with an ultra-intense pure zero phonon line (ZPL) emission with negligible sidebands. Nonetheless, further enhancements in its luminescence thermal stability are required to fully unlock its potential for practical applications. To tackle this challenge, we propose that Al20B4O36 with a negative expansion coefficient could serve as a more favorable host materials for Cr3+ ion, thereby enhancing the luminescence thermal stability. Our first-principles calculations have unveiled similar electronic structures for α-Al2O3:Cr3+ and Al20B4O36:Cr3+, suggesting that the observed zero phonon R-line (2Eg) emission, arising from the 2Eg→4A2g transition in α-Al2O3:Cr3+, might also be present in Al20B4O36:Cr3+. Based on these findings, we prepared Al20B4O36:Cr3+ phosphors using a simple solid reaction method and confirmed that Al20B4O36:Cr3+ indeed exhibits a similar ultra-intense pure ZPL emission centered at 694 nm. Moreover, Al20B4O36:Cr3+ phosphors demonstrate remarkable properties including a high internal quantum efficiency (IQE) of 98.2 % and superior luminescence thermal stability compared to α-Al2O3:Cr3+. A phosphor converted light emitting diode (pc-LED) is developed by coating Al20B4O36:Cr3+ phosphor onto a near-ultraviolet LED chip. The lighting from this deep-red LED significantly impacts the photomorphogenesis of bean sprouts. Our study showcases a rational approach to design novel Cr3+ doped phosphors with enhanced luminescent properties, tailored for specific applications.