Energy transfer and luminescent properties of Tb3+ and (Tb3+, Yb3+) doped CNGG phosphors

ABSTRACT In this work, calcium niobium gallium garnet (Ca3Nb1.6875Ga3.1875O12 - CNGG) ceramic samples single-doped with Tb3+ and co-doped with Tb3+ and Yb3+ ions were sintered by the solid-state reaction method. The structural characterization of the samples was carried out by X-ray diffraction measurements. The optimal concentration of Tb3+ ions corresponding to the maximum luminescence in the green spectral range in CNGG: x at% Tb (x = 0.1, 0.5, 1, 2, 3, 4, and 5) was determined to be 4 at%. The time-resolved luminescence of the 5D4 level (Tb3+) in the CNGG:x at% Tb samples was analysed to explore the quenching mechanisms involved in the Tb3+ green emission. Co-doped CNGG:4 at% Tb,y at% Yb (y = 0.5, 2, 4, 6, 8, and 10) ceramics were prepared and investigated. It is shown that CNGG:4 at% Tb,y at% Yb phosphors exhibit intense green luminescence under ultra-violet (UV), visible (VIS), and near-infrared (NIR) excitation, thus demonstrating the presence of simultaneous down-conversion (DC) and up-conversion (UC) processes. The dependence of the UC luminescence intensity on the diode laser pumping power was measured and the results indicate a two-photon process based on cooperative energy transfer (CET). Under UV excitation, the lifetime of the 5D4 (Tb3+) level slowly increases with increasing of Yb3+ concentration suggesting the energy transfer from Yb3+ to Tb3+ ions, while under NIR excitation, the lifetime of the 5D4 (Tb3+) level decreases with increasing of Yb3+ ions concentration, indicating the presence of a strong energy transfer from Tb3+ to Yb3+ ions. The highest energy transfer efficiency of ηET ∼ 42 % was determined for the CNGG:4 at% Tb, 10 at% Yb sample. The obtained results indicate that CNGG:(Tb3+, Yb3+) could be efficient new yellowish-green-emitting phosphors.