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.