Controlling the upconversion luminescence (UCL) intensity ratio, especially pumped at 808 nm, is of fundamental importance in biological applications due to the water molecules exhibiting low absorption at this excitation wavelength. In this work, a series of β-NaYbF 4 :Er microrods were synthesized by a simple one-pot hydrothermal method and their intense green (545 nm) and red (650 nm) UCL were experimentally investigated based on the single-particle level under the excitation of 808 nm continuous-wave (CW) laser. Interestingly, the competition between the green and red UCL can be observed in highly Yb 3+ -doped microcrystals as the excitation intensity gradually increases, which leads to the UCL color changing from green to orange. However, the microcrystals doped with low Yb 3+ concentration keep green color which is independent of the excitation power. Further investigations demonstrate that the cross-relaxation (CR) processes between Yb 3+ and Er 3+ ions result in the UCL competition. Graphical Abstract
Herein, user-friendly control of the synthesis of sub-10 nm hexagonal (β-) NaYbF4:Er nanocrystals (NCs) with extremely low-level Gd3+ doping (0%, 10 mol%) was achieved. We reveal for the first time that the effective sodium/fluoride levels during the formation of cubic (α-) nuclei are not only controlled by the sodium/fluoride to rare-earth precursor ratios used, but also sensitively restricted by the in situ NaF generation reaction in a sodium oleate-based solvothermal system. Excessive in situ NaF generation will lead to a respective sodium- and fluoride-deficient environment, delayed α-to-β transition and larger β-NCs. Based on these effects, sub-10 nm monodisperse low-level Gd3+-doped β-NaYbF4:Er was obtained with a user-friendly low fluoride dosage by finely balancing this NaF generation reaction and achieving an intrinsic optimized sodium-fluoride level for NC nucleation. Notably, our work represents the first example where the focus is on the competing in situ NaF generation reaction and its use for nucleation regulation, as well as for the user-friendly control of the solvothermal synthesis of sub-10 nm β-NaYbF4:Er.
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