Remarkably Enhanced Red Upconversion Emission in β-NaLuF₄:Er,Tm Microcrystals via Ion Exchange

Internal OH^- defects always cause significant depletion of excitation energy for low upconversion efficiency; however, preventing the incorporation of OH^- into the crystal lattice is very difficult during the synthesis of upconversion crystals. In this work, red upconversion emission is dramatically enhanced in Er³⁺-sensitized hexagonal NaLuF₄ (β-NaLuF₄) microcrystals through a collaborative effect of ion exchange and Tm³⁺ doping. The ion exchange can not only maintain the morphology very well but also strongly reduce the internal OH^- defects, which can enhance the upconversion luminescence due to improved excitation energy harvesting and red to green (R/G) ratio due to intense cross-relaxation. Doping of Tm³⁺ as an energy trapping center not only regulates the red emission output but also reduces the excitation energy loss. β-NaLuF₄:Er,Tm microcrystals after ion exchange exhibit stronger red upconversion emission compared with β-NaYF₄:Er,Tm and β-NaGdF₄:Er,Tm counterparts. Moreover, fluorescent labeling of the fingerprint pattern printed from β-NaLuF₄:Er,Tm microcrystals with stable and intense red fluorescence has been demonstrated under 980 nm excitation.