Nanosized (Y_{1-: X}Gd_x)₂O₂S:Tb³⁺ particles: Synthesis, photoluminescence, cathodoluminescence studies and a model for energy transfer in establishing the roles of Tb³⁺ and Gd³⁺

Herein we describe the synthesis and spectral analysis of nanosized (Y_1-xGd_x)₂O₂S:Tb³⁺ phosphors between x = 0 and x = 1 with 0.1 and 2 mol% Tb³⁺. The concentration of Gd³⁺ was varied in steps of 0.1 (mole ratio Gd³⁺). X-ray diffraction analyses confirmed the purity and composition of the phosphor materials. The photoluminescence spectra of the series of phosphors with 0.1 mol% Tb³⁺ showed a colour change of the fluorescence light from blue to green when x was increased from 0 to 1, whereas the samples of the series with 2 mol% Tb³⁺ yielded essentially green light. These phenomena could be explained in terms of energy transfer from the Tb^{3+ 5}D₃-level at 26 316 cm^-1 to the CT-minimum of (Y_1-xGd_x)₂O₂S:Tb³⁺ for the series with 0.1 mol% Tb³⁺, while for the series with 2 mol% Tb³⁺ cross relaxation between the ⁵D₃, ⁷F_J and ⁵D₄ levels of Tb³⁺ caused additional depopulation of the ⁵D₃-level at low Gd³⁺ concentrations. Cathodoluminescence spectra recorded at temperatures between -170 °C and 20 °C confirmed the proposed energy flow and enabled the evaluation of the energy barrier for this energy flow. By modelling the energy flow we were able to show that the concentration of Gd³⁺ is critical in controlling the change in colour of the (Y_1-xGd_x)₂O₂S:Tb³⁺ for the series with 0.1 mol% and that the presence of just over x = 0.1 of Gd³⁺ is enough to switch to the energy transfer route present in the x = 1.0 parent phosphor. In addition the charge transfer band of (Y_1-xGd_x)₂O₂S:Tb³⁺ showed a red shift of about 6 nm upon increasing x from 0 to 1. This red shift has been described in terms of an electrostatic model that enabled a calculation of the rearrangement of the electrons during the charge transfer.