Cross Relaxation Enables Spatiotemporal Color‐Switchable Upconversion in a Single Sandwich Nanoparticle for Information Security

Abstract Smart control of ionic interaction dynamics offers new possibilities for tuning and editing luminescence properties of lanthanide‐based materials. However, it remains a daunting challenge to achieve the dynamic control of cross relaxation mediated photon upconversion, and in particular the involved intrinsic photophysics is still unclear. Herein, this work reports a conceptual model to realize the color‐switchable upconversion of Tm³⁺ through spatiotemporal control of cross relaxation in the design of NaYF₄:Gd@NaYbF₄:Tm@NaYF₄ sandwich nanostructure. It shows that cross relaxation plays a key role in modulating upconversion dynamics and tuning emission colors of Tm³⁺. Interestingly, it is found that there is a short temporal delay for the occurrence of cross relaxation in contrast to the spontaneous emission as a result of the slight energy mismatch between relevant energy levels. This further enables a fine emission color tuning upon non‐steady state excitation. Moreover, a characteristic quenching time is proposed to describe the temporal evolution of cross relaxation quantitatively. These findings present a deep insight into the physics of ionic interactions in heavy doping systems, and also show great promise in frontier applications including information security, anti‐counterfeiting and nanophotonics.