Tuning the NIR downconversion luminescence and photothermal conversion efficiencies of MNdxY1−xF4 (M = Na and Li) nanocrystals for use in anti-counterfeiting labels with opposite displays

Abstract

Rare earth (RE³⁺ = Y³⁺ and Nd³⁺) alkali (M⁺ = Na⁺ or Li⁺) tetrafluoride nanocrystals adopt various morphologies and crystal structures depending on the M⁺–RE³⁺ size compatibility and Nd³⁺ concentration. This in turn affects the downconversion NIR luminescence and photothermal properties of the nanocrystals. For NaOH precursor, hexagonal NaNd_xY_1−xF₄ nanocrystals are formed from the initially created cubic NaNd_xY_1−xF₄ nanocrystals, whereas for LiOH precursor, tetragonal LiNd_0.03Y_0.97F₄ nanocrystals are obtained. Due to the large size mismatch between Li⁺ and Nd³⁺, unstable LiNd_xY_1−xF₄ undergoes phase separation to form either orthorhombic or hexagonal Nd_xY_1−xF₃ nanocrystals upon increasing the Nd³⁺ concentration. The latter dominates when Nd³⁺ is the majority rare earth element in the host matrix. NaNd_xY_1−xF₄ nanocrystals display better luminescence and photothermal properties as compared to their Li⁺-based counterparts and the inverse relationship between emission and light-to-heat conversion efficiencies is exploited for anti-counterfeiting purposes. In this case, patterns deposited on different substrates (e.g., glass and Teflon) using Nd³⁺-concentrated nanocrystals, with efficient light-to-heat conversion and poor NIR luminescence properties, exhibit bright thermal and dim emission images when irradiated with 808 nm light. On the other hand, areas printed with Nd³⁺-diluted nanocrystals display dim thermal and bright emission images. Such anti-counterfeiting labels with opposite thermal and NIR emission displays provide enhanced security.