Site-selective excitation and photoluminescence properties of a cyan-emitting NaGd9(SiO4)6O2: Bi3+ phosphor for potential application in white light-emitting diodes

Abstract

Bismuth ions, as excellent non-rare-earth activators for luminescent materials, possess characteristic excitation in the near ultraviolet (n-UV) region rather than in the visible region and could avoid reabsorption as compared to traditional rare-earth (e.g., Eu2+, Ce3+) phosphors. In this work, a series of apatite-type NaGd9(SiO4)6O2: xBi3+ (NGSO) phosphors with a hexagonal crystal system were synthesized through high-temperature solid-solid reactions. They showed intense n-UV absorption and cyan emission of Bi3+ ions. The optimal Bi3+ doping concentration was 0.02. Further, there were two types of Bi3+ ions that provided seven- and nine-coordination in NGSO: xBi3+ phosphors, resulting in two luminescence centers at 462 and 558 nm, respectively. Bi3+ (Ⅰ) ions were dominant for the emission at 558 nm excited at a wavelength of 340 nm, and Bi3+ (Ⅱ) ions governed the luminescence at 462 nm excitation at 370 nm; energy transfer occurred from Bi3+ (Ⅰ) to Bi3+ (Ⅱ) ions. Finally, good thermal stability with activation energy (Ea) of 0.296 eV was demonstrated. These findings exhibit the potential application of NGSO: 0.02Bi3+ phosphors as cyan phosphors in n-UV white light-emitting diodes.