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.
In this work, monodisperse β-NaLuF 4 :Yb,Er microcrystals with intense upconversion emission were synthesized via a modified hydrothermal method . With the increase of reactant concentration, their production yield is increased obviously, and the upconversion emission intensity in β-NaLuF 4 :Yb,Er microcrystals is also enhanced significantly. The luminescence enhancement should be attributed to minimal internal OH − defects, validated by a combination of analytical X-ray diffraction (XRD), energy dispersive spectrum (EDS), and Eu 3+ structural probe measurements. We also reveal that high Na + :RE 3+ ratio in the β-NaLuF 4 :Yb,Er microcrystals prepared under Na + -rich reaction will arouse the increased repulsive energy of β-NaLuF 4 microcrystals between F − and OH − anions and then facilitate the substitution of large OH − ions by small F − ions under F − -rich reaction conditions. Minimal OH − concentrations can limit nonradiative relaxation and promote excitation energy harvesting for high upconversion efficiency. The presented results not only offer a facile method for the simultaneous production yield and luminescence intensity increase of β-NaREF 4 microcrystals, but also uncover a better insight into the upconversion emission alterations, which is favorable to broaden their practical applications.
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