Energy-transfer from Gd(iii) to Tb(iii) in (Gd,Yb,Tb)PO4 nanocrystals

Abstract

The photoluminescence properties of (Gd,Yb,Tb)PO₄ nanocrystals synthesized via a hydrothermal route at 150 °C are reported. Energy-transfer from Gd³⁺ to Tb³⁺ is witnessed by the detailed analyses of excited-state lifetimes, emission quantum yields, and emission and excitation spectra at room temperature, for Tb³⁺ concentrations ranging from 0.5 to 5.0 mol%. Absolute-emission quantum yields up to 42% are obtained by exciting within the ⁶I_7/2–17/2 (Gd³⁺) manifold at 272 nm. The room temperature emission spectrum is dominated by the ⁵D₄ → ⁷F₅ (Tb³⁺) transition at 543 nm, with a long decay-time (3.95–6.25 ms) and exhibiting a rise-time component. The ⁵D₃ → ⁷F₆ (Tb³⁺) rise-time (0.078 ms) and the ⁶P_7/2 → ⁸S_7/2 (Gd³⁺) decay-time (0.103 ms) are of the same order, supporting the Gd³⁺ to Tb³⁺ energy-transfer process. A remarkably longer lifetime of 2.29 ms was measured at 11 K for the ⁶P_7/2 → ⁸S_7/2 (Gd³⁺) emission upon excitation at 272 nm, while the emission spectrum at 11 K is dominated by the ⁶P_7/2 → ⁸S_7/2 transition line, showing that the Gd³⁺ to Tb³⁺ energy-transfer process is mainly phonon-assisted with an efficiency of ∼95% at room temperature. The Gd³⁺ to Tb³⁺ energy transfer is governed by the exchange mechanism with rates between 10² and 10³ s⁻¹, depending on the energy mismatch conditions between the ⁶I_7/2 and ⁶P_7/2 levels of Gd³⁺ and the Tb^{3+ 5}I₇, ⁵F_2,3 and ⁵H_5,6,7 manifolds and the radial overlap integral values.