The size-responsive phase transition mechanism and upconversion/downshifting luminescence properties of KLu2F7:Yb3+/Er3+ nanocrystals

Impurity doping has a significant influence on the nucleation and growth of nanocrystals. The influence of the ionic radius of the dopant and grain size on the crystallographic phase of KLu2F7: Yb3+/Er3+ nanocrystals has not been systematically investigated. Herein, we demonstrate an M ion (M = Mn2+, Gd3+, Nd3+) doping approach to dynamically fine-tune the grain size of KLu2F7: Yb3+/Er3+ nanocrystals, and study the influence of the ionic radius of the doping M ions and particle size reduction on the crystallographic phase. Through a comprehensive investigation of the impurity doping induced phase transition phenomenon and the growth mechanism of KLu2F7: Yb3+/Er3+ nanocrystals, we speculated that the impurity doping induced phase transition mechanism of KLu2F7: Yb3+/Er3+ mainly depends on the grain size of nanocrystals ignoring the ionic radius of the dopant. With the decrease of the grain size, high surface tension triggers phase transformation from anisotropic (orthorhombic KLu2F7: Yb3+/Er3+) to isotropic (cubic KLu3F10: Yb3+/Er3+). What is more, we have systematically investigated the visible (VIS)-upconversion and near infrared (NIR)-downshifting emission of various-sized KLu2F7: Yb3+/Er3+ UCNPs and Mn2+, Gd3+ or Nd3+ ion doped KLu2F7: Yb3+/Er3+ UCNPs. The findings provide keen insights into understanding the effect of size reduction on the phase transition mechanism and the photoluminescence properties of KLu2F7: Yb3+/Er3+ UCNPs, while offering the possibility for good academic studies and application prospects.