On the origin of fine structure in the photoluminescence spectra of the β-sialon:Eu2+ green phosphor

Abstract

The photoluminescence (PL) and PL excitation (PLE) spectra of Si_6?zAl_zO_zN_8?z (β-sialon):Eu²⁺ phosphors with small z values (z=0.025–0.24) were studied at room temperature and 6 K. The PL and PLE spectra exhibit fine structure with the PL lines being as sharp as 45–55 nm even at room temperature; this fine structure was enhanced by decreasing the z value. These results can be used for expanding the color gamut of liquid crystal displays, particularly in the blue–green region. From low-temperature measurements, the fine PLE structure was ascribed to discrete energy levels of ⁷F_J states. The 4f⁶5d excited states of Eu²⁺ are considered to be localized near the 4f orbital. This is because the bonding of Eu²⁺ with surrounding atoms is ionic rather than covalent. Lattice phonon absorptions were also observed in the PLE spectrum, revealing that the optically active Eu²⁺ ions are located in the β-sialon crystal. The PL spectrum of the sample with the smallest z value (0.025) consists of a sharp zero-phonon line and lattice phonon replicas, which results in a sharp and asymmetric spectral shape.