Core/Shell Upconversion Nanocrystal Hybrids with Temperature‐Dependent Emission Color Changes for Multilevel Anticounterfeiting Applications

This work presents a novel and highly secure anticounterfeiting strategy based on core/shell upconversion nanocrystal (UCNC) hybrids with temperature-responsive multicolor emissions. Opposite luminescent temperature-dependences are found for active-core@inert-shell (thermal quenching) and active-core@active-shell (thermally induced enhancement) UCNCs. Accordingly, their hybrids are designed to show obvious color changes with increasing temperature under 975 nm excitation. Various color-shifting pathways (from white to green, blue to green, etc.) are achieved by adjusting the core/shell NC combinations in the hybrids. Moreover, color changes of the printed patterns using the hybrid NC inks can be realized by the simple heating treatment, or by increasing the laser power and prolonging the irradiation time. The results indicate the great potential of these core/shell NC hybrids for anticounterfeiting applications with high-level security and convenient authentication methods.