Oxygen-induced downshifting and lanthanide upconversion luminescence in Sr2YbF7 nanoparticles for dual-mode security applications

Abstract

Lanthanide-doped fluorides are an important class of luminescent materials in anti-counterfeiting, optical information storage, and biological imaging applications due to their remarkable upconversion capabilities. Herein, Sr₂YbF₇ and Sr₂YbF₇:Ln³⁺ (Ho³⁺, Tm³⁺) nanoparticles are synthesized through a hydrothermal procedure and their luminescence properties are studied. Both undoped and Ln³⁺-doped Sr₂YbF₇ nanomaterials crystalize in a fluorite structure. Lattice expansion of Ln³⁺-doped Sr₂YbF₇ compared to undoped Sr₂YbF₇ confirms that Ln³⁺ ions replace the Yb³⁺ in the lattice. Undoped Sr₂YbF₇ exhibits a sky blue colour upon excitation with 365 nm light due to the presence of oxygen, which creates vacancies in the lattice. The Sr₂YbF₇:x% Ho³⁺ nanoparticles underwent excitation at 980 nm, showing intense green emission due to ⁵F₄ → ⁵I₈ transition of Ho³⁺ ions, and a prominent purple-emission because of ³F₃ → ³H₆ transition of Tm³⁺ in Sr₂YbF₇:y% Tm³⁺ nanoparticles. Finally, security ink is generated with the undoped Sr₂YbF₇, Sr₂YbF₇:5% Ho³⁺ and Sr₂YbF₇:7% Tm³⁺ nanoparticles to create multimode anti-counterfeiting patterns. These patterns are stable against practical conditions such as temperature, humidity, and light. This study demonstrates the development of multimode luminescent nanomaterials for high-level security and anti-counterfeiting applications.