Improvement of Green Upconversion Monochromaticity by Doping Eu3+ in Lu2O3:Yb3+/Ho3+ Powders with Detailed Investigation of the Energy Transfer Mechanism

The monochromaticity improvement of green upconversion (UC) in Lu2O3:Yb3+/Ho3+ powders has been successfully realized by tridoping Eu3+. The integral area ratio of green emission to red emission of Ho3+ increases 4.3 times with increasing Eu3+ doping concentration from 0 to 20 mol %. The energy transfer (ET) mechanism in the Yb3+/Ho3+/Eu3+ tridoping system has been investigated carefully by visible and near-infrared (NIR) emission spectra along with the decay curves, revealing the existence of ET from the Ho3+5F4/5S2 level tothe Eu3+5D0 level and ET from the Ho3+5I6 level to the Eu3+7F6 level. In addition, the population routes of the red-emitting Ho3+5F5 level in the Yb3+/Ho3+ codoped system under 980 nm wavelength excitation have also been explored. The ET process from the Yb3+2F5/2 level to the Ho3+5I7 level and the cross-relaxation process between two nearby Ho3+ ions in the 5F4/5S2 level and 5I7 level, respectively, have been demonstrated to be the dominant approaches for populating the Ho3+5F5 level. The multiphonon relaxation process originating from the Ho3+5F4/5S2 level is useless to populate the Ho3+5F5 level. As the energy level gap between the Ho3+5I7 level and Ho3+5I8 level matches well with that between Eu3+7F6 level and Eu3+7F0 level, the energy of the Ho3+5I7 level can be easily transferred to the Eu3+7F6 level by an approximate resonant ET process, resulting in a serious decrease in the red UC emission intensity. Since this ET process is more efficient than the ET from the Ho3+5F4/5S2 level to the Eu3+5D0 level as well as the ET from the Ho3+5I6 level to the Eu3+7F6 level, the integral area ratio of green emission to red emission of Ho3+ has been improved significantly.