Thermally induced multicolor emissions of upconversion hybrids with large color shifts for anticounterfeiting applications

Abstract

Upconversion materials with tunable emission colors are highly desired for novel anticounterfeiting strategies because they offer additional security features. Herein, we report a series of upconversion hybrids consisting of NaYF₄:Yb/Ln microrods (MRs) and NaGdF₄:Yb/Ln@NaYbF₄ (Ln = Er, Ce/Ho, Tm) core/active-shell nanocrystals (NCs). A normal thermal quenching of upconversion luminescence (UCL) is found for the MRs, but a thermal UCL enhancement occurs for the small-sized NCs. Consequently, the hybrids exhibit temperature-responsive multicolor emissions with large color shifts. The UCL enhancement mechanism is further revealed by temperature-dependent steady/transient spectral analysis in various atmospheres. Moreover, various color-shifting pathways are realized by adjusting the MR/NC combinations in the hybrids. Thanks to the effective photothermal conversion of core/active-shell NCs, color changes of the printed patterns by the hybrid ink are achieved simply by single-laser irradiation at a relatively low power density. These desirable properties make the designed hybrids promising for anticounterfeiting applications with advantages of high-level security and convenient authentication methods.