Cathodoluminescence of Y2O3:Ln3+ (Ln = Tb, Er and Tm) and Y2O3:Bi3+ nanocrystalline particles at 200 keV
The cathodoluminescence (CL) spectra of nanocrystalline Y2O3:Tb3+ (0.3%), Y2O3:Er3+ (1%), Y2O3:Tm3+ (2%) and Y2O3:Bi3+ (1%) were recorded in a transmission electron microscope at 200 keV, low current density and various temperatures. The quenching energy of the intrinsic luminescence of the various Y2O3:Ln3+ (Ln = Tb, Er and Tm) phosphors was found to be 0.25 eV. The intrinsic luminescence and the strongest spectral transitions of Ln3+ in these three phosphors exhibit similar temperature behaviour at temperatures > −50 °C, viz. a small increase of the spectral radiance upon increasing the temperature. Increasing the temperature beyond −50 °C led to complete quenching of the intrinsic luminescence at room temperature, whereas the radiance of the Ln3+ spectral transitions only decreased slightly. An extended Jablonski diagram for the energy transfer from the self-trapped exciton states in Y2O3 to the Ln3+ and Bi3+ ions is proposed. This diagram also indicates why Tb3+ is a better quencher of the intrinsic luminescence in Y2O3 than Er3+ and Tm3+. The intrinsic luminescence of Y2O3:Bi3+ largely overlapped with the blue Bi3+ emission band, which made an accurate analysis of its temperature behaviour impossible. Nevertheless, we concluded that upon increasing the temperature energy from Bi3+ ions at the C3i sites is transferred to Bi3+ ions at C2 sites. From the temperature behaviour of the 539 nm transition of the 2H11/2 → 4I15/2 manifold of Y2O3:Er3+ the activation energy for this transition could be determined: viz. 0.078 eV (623 cm−1).