Importance of Suppression of Yb³⁺ De-Excitation to Upconversion Enhancement in β‑NaYF₄: Yb³⁺/Er³⁺@β-NaYF₄ Sandwiched Structure Nanocrystals

Nanosized Yb³⁺ and Er³⁺ co-doped β-NaYF₄ cores coated with multiple β-NaYF₄ shell layers were synthesized by a solvothermal process. X-ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near-infrared spectra as well as the decay curves were also measured. A 40-fold intensity increase for the green upconversion and a 34-fold intensity increase for the red upconversion were observed as the cores are coated with three shell layers. The origin of the upconversion enhancement was studied on the basis of photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure mainly originates from the suppression of de-excitation of Yb³⁺ ions. We also explored the population of the Er³⁺⁴F_9/2 level. The results reveal that energy transfer from the lower intermediate Er³⁺⁴I_13/2 level is dominant for populating the Er³⁺⁴F_9/2 level when the nanocrystal size is relatively small; however, with increasing nanocrystal size, the contribution of the green emitting Er³⁺⁴S_3/2 level for populating the Er³⁺⁴F_9/2 level, which mainly comes from the cross relaxation energy transfer from Er³⁺ ions to Yb³⁺ ions followed by energy back transfer within the same Er³⁺–Yb³⁺ pair, becomes more and more important. Moreover, this mechanism takes place only in the nearest Er³⁺–Yb³⁺ pairs. This population route is in good agreement with that in nanomaterials and bulk materials

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⁺ 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

Enhancement of Eu³⁺ Red Upconversion in Lu₂O₃: Yb³⁺/Eu³⁺ Powders under the Assistance of Bridging Function Originated from Ho³⁺ Tridoping

The red upconversion (UC) emission of Eu³⁺ ions in Lu₂O₃: Yb³⁺/Eu³⁺ powders was successfully enhanced by tridoping Ho³⁺ ions in the matrix, which is due to the bridging function of Ho³⁺ ions. The experiment data manifest that, in Yb³⁺/Eu³⁺/Ho³⁺ tridoped system, the Ho³⁺ ions are first populated to the green emitting level ⁵F₄/⁵S₂ through the energy transfer (ET) processes from the excited Yb³⁺ ions. Subsequently, the Ho³⁺ ions at ⁵F₄/⁵S₂ level can transfer their energy to the Eu³⁺ ions at the ground state, resulting in the population of Eu³⁺ ⁵D₀ level. With the assistance of the bridging function of Ho³⁺ ion, this ET process is more efficient than the cooperative sensitization process between Yb³⁺ ion and Eu³⁺ ion. Compared with Lu₂O₃: 5 mol % Yb³⁺/1 mol % Eu³⁺, the UC intensity of Eu³⁺ ⁵D₀→⁷F₂ transition in Lu₂O₃: 5 mol % Yb³⁺/1 mol % Eu³⁺/0.5 mol % Ho³⁺ is increased by a factor of 8