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

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 Mn2+ 4T1 state than that in octahedral MnF6 in cubic NaYF4. The high-lying excited state of tetrahedral MnF4 is close to the green emitting 4S3/2 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.