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

Abstract

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⁺ ²F_5/2 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