Highly Luminous Ba2SiO4-δN2/3δ:Eu2+ Phosphor for NUV-LEDs: Origin of PL-Enhancement by N3--Substitution

Abstract

Ba₂SiO_4-δN_2/3δ:Eu²⁺ (BSON:Eu²⁺) materials with different N^3- contents were successfully prepared and characterized. Rietveld refinements showed that N^3- ions were partially substituted for the O^2- ions in the SiO₄-tetrahedra because the bond lengths of Si‒(O,N) (average value = 1.689 Å) were slightly elongated compared with those of Si‒O (average value = 1.659 Å), which resulted in the minute compression of the Ba(2)‒O bond lengths from 2.832 to 2.810 Å. The average N^3- contents of BSON:Eu²⁺ phosphors were determined from 100 nm to 2000 nm depth of grain using a secondary ion mass spectrometry (SIMS): 0.064 (synthesized using 100% α-Si₃N₄), 0.035 (using 50% α-Si₃N₄ and 50% SiO₂), and 0.000 (using 100% SiO₂). Infrared (IR) and X-ray photoelectron spectroscopy (XPS) measurements corroborated the Rietveld refinements: the new IR mode at 850 cm^-1 (Si‒N stretching vibration) and the binding energy at 98.6 eV (Si-2p) due to the N^3- substitution. Furthermore, in UV-region, the absorbance of N^3--substituted BSON:Eu²⁺ (synthesized using 100% α-Si₃N₄) phosphor was about two times higher than that of BSO:Eu²⁺ (using 100% SiO₂). Owing to the N^3- substitution, surprisingly, the photoluminescence (PL) and LED-PL intensity of BSON:Eu²⁺ (synthesized using 100% α-Si₃N₄) was about 5.0 times as high as that of BSO:Eu²⁺ (using 100% SiO₂). The compressive strain estimated by the Williamson-Hall (W-H) method, was slightly increased with the higher N^3- content in the host-lattice of Ba₂SiO_4, which warranted that the N^3- ion plays an important role in the highly enhanced PL intensity of BSON:Eu²⁺ phosphor. These phosphor materials could be a bridgehead for developing new phosphors and application in white NUV-LEDs field.

Keywords: Ba2SiO4:Eu2+; IR; N3− substitution; PL; SIMS; XPS.