Tuning the NIR downconversion luminescence and photothermal conversion efficiencies of MNd x Y 1−x F ₄ (M = Na and Li) nanocrystals for use in anti-counterfeiting labels with opposite displays

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 NaNdxY1-xF4 nanocrystals are formed from the initially created cubic NaNdxY1-xF4 nanocrystals, whereas for LiOH precursor, tetragonal LiNd0.03Y0.97F4 nanocrystals are obtained. Due to the large size mismatch between Li+ and Nd3+, unstable LiNdxY1-xF4 undergoes phase separation to form either orthorhombic or hexagonal NdxY1-xF3 nanocrystals upon increasing the Nd3+ concentration. The latter dominates when Nd3+ is the majority rare earth element in the host matrix. NaNdxY1-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.