Monodisperse β-NaYF4:Yb3+,Tm3+ hexagonal microplates with efficient NIR-to-NIR up-conversion emission developed via ion exchange

Abstract

Monodisperse β-NaYF₄:Yb³⁺,Tm³⁺ (NYF) hexagonal microplates with efficient near-infrared (NIR)-to-NIR up-conversion (UC) developed via a new ion-exchange modified hydrothermal method are reported. Ion exchange modification (IEM) not only significantly increases the UC intensity by up to 1000 times and prolongs the emission lifetimes of Tm³⁺ and Yb³⁺, but also enables the monodisperse morphology and size of hexagonal microplates to be maintained. A high UC quantum efficiency (QE) of 3.1% is obtained for IEM β-NaYF₄:20%Yb³⁺,1%Tm³⁺ when excited with 980 nm light at a power density of 10 W cm⁻². The UC emission properties can be finely tailored by changing the NaF/NYF molar ratio (NYF represents lanthanide doped β-NaYF₄) and the doping concentration of Tm³⁺. The two photon NIR UC emission centered at ∼803 nm (arising from Tm³⁺: ³H₄ → ³H₆) dominates the UC emission spectrum and high concentrations of Tm³⁺ favor NIR UC further. A proof-of-concept optical image of printed patterns is demonstrated to verify their applications in security. These results suggest the promising applications of the newly developed monodisperse β-NaYF₄:Yb³⁺,Tm³⁺ hexagonal microplates in security, luminescent labels, solid-state lasers, amplifiers, and biomedicine.