Opposite luminescence thermal behavior of upconversion core/shell nanocrystals for anticounterfeiting

Inorganic luminescent materials generally suffer from thermal quenching due to accelerated nonradiative transitions at high temperature, whereas Yb³⁺ sensitized core nanocrystals with small sizes (<30 nm) exhibit a temperature-dependent upconversion luminescence (UCL) enhancement. The related mechanism of the anomalous UCL thermal behavior is still under debate. In this work, we find that the UCL of NaGdF₄:Yb/Tm@NaGdF₄ inert-shell nanocrystals declines at elevated temperature, while that of NaGdF₄:Yb/Ho@NaGdF₄:Yb active-shell ones is enhanced. The thermally-induced UCL enhancement of active-shell nanocrystals is attributed to a gradually attenuated surface quenching effect. The initiators of the surface quenching are H₂O molecules, which mainly attenuate Yb³⁺ excited states through an overtone energy transfer. The energy transfer is a coupling effect between ion dipoles of Yb³⁺ and atomic dipoles of H₂O. Utilizing the opposite UCL temperature-dependence of active- and inert-shell nanocrystals, we designed their hybrids, which exhibit temperature-responsive multicolor emissions. The color-tunable hybrids are demonstrated to be excellent candidates for producing anticounterfeiting patterns with high security but simple recognition methods.