Effects of excitation density and energy transfer on cathodoluminescence from powder phosphors with and without embedded nanoparticles.

摘要

The intensity of cathodoluminescence (CL) from combustion synthesized Ln2O2S:Eu3+ and nano-particulate oxide coated ZnS:Ag,Cl micron-sized powder phosphors was studied. In addition, the intensity of photoluminescence (PL) from nanometer-sized ZnO coated with MgO or embedded in SiO2S:Eu3+ was studied.; For combustion synthesized La2O2S:Eu3+ (0.5 mole %) powders, it was shown that CL intensities from the 5D 1 (5D1 => 7F3) and 5D0 transition (5D0 => 7F2) and their ratio (designated 5D1/5D 0) decayed over the first ∼5 sec of DC electron beam irradiation at a higher current density (e.g., 2 keV, 500 muA/cm2). Temporal CL quenching could not be completely explained by thermal quenching from electron beam heating or by surface voltage changes due to charging. Instead, it was postulated that the primary electron beam induced internal electric fields, which were significant at higher excitation densities and resulted in local redistribution of charge carriers. Carrier redistribution led to enhanced activator interactions with temporal quenching and color change.; In addition, the 5D1/5D 0 ratio increased and then decreased for both La2O2 S:Eu3+ (0.1 mole %) and Gd2O2S:Eu 3+ (0.4 mole %) as the current density was increased from 10 towards 1000 muA/cm2. These effects were shown to be consistent with the feeding from the higher 5D2 excited state to the lower 5D1, resulting in an increase of the 5D 1/5D0 ratio at low current densities. At higher current densities, energy was funneled from the 5D1 to 5D0 states, resulting in a decrease of the 5D1/5D0 ratio.; In contrast to non-coated ZnS:Ag,Cl powder phosphors, ZnS:Ag,Cl coated with SiO2 (22 or 130 nm), SnO2 or Al2O 3 showed a decrease of CL intensity over the first ∼15 sec of electron beam irradiation at higher current densities (e.g., 300∼800 muA/cm 2). This temporal CL quenching of coated ZnS:Ag,Cl was postulated to result from a large concentration of non-radiative surface centers generated during surface modification of the phosphor, and from localization of generated electrons at the surface due to induced internal electric fields. These effects resulted in increased non-radiative surface recombination between electrons and holes and CL quenching. (Abstract shortened by UMI.)