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

The photoluminescence properties of (Gd,Yb,Tb)PO4 nanocrystals synthesized via a hydrothermal route at 150 degrees 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 D-5(4) -> F-7(5) (Tb3+) transition at 543 nm, with a long decay-time (3.95-6.25 ms) and exhibiting a rise-time component. The D-5(3) -> F-7(6) (Tb3+) rise-time (0.078 ms) and the P-6(7/ 2) -> S-8(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 P-6(7/2) -> S-8(7/2) (Gd3+) emission upon excitation at 272 nm, while the emission spectrum at 11 K is dominated by the P-6(7/2) -> S-8(7/2) transition line, showing that the Gd3+ to Tb3+ energy-transfer process is mainly phonon-assisted with an efficiency of similar to 95% at room temperature. The Gd3+ to Tb3+ energy transfer is governed by the exchange mechanism with rates between 10(2) and 10(3) s(-1), depending on the energy mismatch conditions between the (6I)(7/2) and P-6(7/2) levels of Gd3+ and the Tb3+ I-5(7), F-5(2,3) and H-5(5,6,7) manifolds and the radial overlap integral values.