Improvement of Green Upconversion Monochromaticity by Doping Eu in LuO:Yb/Ho Powders with Detailed Investigation of the Energy Transfer Mechanism.

The monochromaticity improvement of green upconversion (UC) in LuO:Yb/Ho powders has been successfully realized by tridoping Eu. The integral area ratio of green emission to red emission of Ho increases 4.3 times with increasing Eu doping concentration from 0 to 20 mol %. The energy transfer (ET) mechanism in the Yb/Ho/Eu 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 HoF/S level tothe EuD level and ET from the HoI level to the EuF level. In addition, the population routes of the red-emitting HoF level in the Yb/Ho codoped system under 980 nm wavelength excitation have also been explored. The ET process from the YbF level to the HoI level and the cross-relaxation process between two nearby Ho ions in the F/S level and I level, respectively, have been demonstrated to be the dominant approaches for populating the HoF level. The multiphonon relaxation process originating from the HoF/S level is useless to populate the HoF level. As the energy level gap between the HoI level and HoI level matches well with that between EuF level and EuF level, the energy of the HoI level can be easily transferred to the EuF 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 HoF/S level to the EuD level as well as the ET from the HoI level to the EuF level, the integral area ratio of green emission to red emission of Ho has been improved significantly.