Intense hypersensitive luminescence of Eu3+-doped YSiO2N oxynitride with near-UV excitation

Abstract

Nowadays highly efficient red phosphors with blue or near-UV excitation are strongly required to improve performance of existing white LEDs. With this aim in view, the Eu3+-doped YSiO2N ceramics with the pseudo-wollastonite structure was fabricated and thoroughly studied spectroscopically. The presence of two anions – O2− and N3− − around Eu3+ ions causes a shift of the host valence band (VB) to lower energy and a certain asymmetry of the impurity sites. After illumination by near-UV excitation light (280–360 nm), the YSiO2N:Eu3+ sample exhibited strong red luminescence at around 620 nm assigned to the hypersensitive 5D0→7F2 transition of the Eu3+ ions. Compared with three Eu3+-doped oxides (α-CaSiO3:Eu3+ which has the same pseudo-wollastonite structure, and α- and y-Y2Si2O7:Eu3+ which have the same cation composition), the charge transfer (CT) band was red-shifted by more than 9.5 × 103 cm−1 because the VB top increases. From the Judd-Ofelt analysis, it was found out that the YSiO2N:Eu3+ sample has the large Judd-Ofelt intensity parameter, Ω2, which is correlated with the dopant's site asymmetry. As expected, the mixed-anion coordination which consists of oxide and nitride anions resulted in the strong hypersensitive transition at 620 nm, which is characteristic Eu3+ red luminescence at the lower symmetry site. In addition, the fluorescence lifetime of the Eu3+ luminescence at 0 K (τ0) was estimated experimentally to be 0.664 ms, which was shorter than the radiative lifetime (τR), 1.32 ms, based on the Judd-Ofelt analysis. The experimental quantum yield measured with an integrating sphere, 32.3%, was lower than the internal quantum efficiency, 50.3%, evaluated from the ratio of τ0/τR. The proposed material can be suitable for applications as a red phosphor.