Enhanced green upconversion luminescence in tetrahedral LiYF4:Yb/Er nanoparticles by manganese(ii)-doping: the key role of the host lattice

Abstract

We report the enhancement of green upconversion luminescence in tetrahedral LiYF₄:Yb/Er nanoparticles by Mn²⁺ ion doping, which is different from the enhanced single-band red emission dominated by Mn²⁺ ions in cubic NaLnF₄:Yb/Er (Ln = Y, Gd, Lu) nanoparticles. The energy levels of the first excited state ⁴T₁ of Mn²⁺ in tetrahedral LiYF₄:Gd and cubic NaGd(Y)F₄ are compared by detection of emissions from Mn²⁺via the energy transition from Gd³⁺ to Mn²⁺ with excitation at 275 nm. The coordination environments of Mn²⁺ in these two host lattices have been investigated by X-ray absorption fine structure measurements. The results demonstrate that the formation of tetrahedral MnF₄ in tetragonal LiYF₄ arising from the replacement of Ln³⁺ ions with Mn²⁺ ions leads to a higher energy level of the Mn^{2+ 4}T₁ state than that in octahedral MnF₆ in cubic NaYF₄. The high-lying excited state of tetrahedral MnF₄ is close to the green emitting ⁴S_3/2 state of Er³⁺ and thus enhances green upconversion emission in tetragonal LiYF₄:Yb/Er, while the low-lying excited state of octahedral MnF₆ dominates red emission in cubic NaYF₄:Yb/Er. These findings provide direct evidence for the key roles of the host lattices and more possibilities in modulating the upconversion behaviour of lanthanide-based nanoparticles by transition-metal ion doping to achieve the desired goals of specific applications.