Large-scale synthesis and screen printing of upconversion hexagonal-phase NaYF4:Yb3+,Tm3+/Er3+/Eu3+ plates for security applications

Abstract

Up-conversion materials can be applied in anti-counterfeit fields because of their high concealment and high up-conversion fluorescence efficiency. Herein, different lanthanide ions-doped β-NaYF₄ up-conversion micro-particles (UCMPs) with uniform hexagonal morphology were synthesized on a large-scale (more than 0.5 g) via a facile hydrothermal method. It is very interesting to find out the fluorescence intensity and the eventual morphology of the UCMPs were highly dependent on the reaction conditions. Then, the optimal UCMPs with uniform morphology and strong fluorescence intensity were selected as promising candidates for preparing ink. Eventually, we successfully printed designable and multicolor fluorescent patterns on flexible substrates (common paper and polyethylene terephthalate, PET) using the as-prepared β-NaYF₄:Yb³⁺,Tm³⁺/Er³⁺/Eu³⁺ UCMPs inks. Under ambient conditions, the printed patterns on conventional paper are invisible, while such printed patterns on PET are white. However, all the patterns could display blue, yellow-green and green fluorescence patterns under the irradiation of a 980 nm laser when using the β-NaYF₄:Yb³⁺,Tm³⁺/Er³⁺/Eu³⁺ UCMPs inks. We envision that the UCMPs fluorescent inks-based flexible and rapid screen printing patterns have enormous potential for anti-counterfeit and security applications.