Functional Interface Engineering Enables Selective Multicolor Upconversion in NaErF ₄ @NaYF ₄ Core–Shell Nanocrystals

Abstract Lanthanide‐doped upconversion nanoparticles (UCNPs) have attracted extensive interest in broad frontier fields such as nanophotonics and biomedicine. However, low luminescence intensity, severe nonradiative energy loss, and limited color‐tuning capability still urgently need to be addressed. Herein, a functional interface engineering regulation strategy is proposed to achieve a significant enhancement in upconversion luminescence intensity and multicolor output from the NaErF 4 @NaYF 4 core–shell nanocrystals. It is found that adjusting the amount of CF 3 COONa in the precursor during the shell growth is able to modulate lattice defect densities at the interface and thereby tune the energy migration paths and non‐radiative transition processes of Er 3+ in the core. This also results in a multicolor tuning from green to yellow, orange, and red under 980 nm excitation, and the overall upconversion luminescence intensity is significantly enhanced by 17.1 times. Moreover, the interface engineering strategy is also applicable to 808 and 1540 nm excitations and the UCNPs with other shells (e.g., NaLuF 4 and NaGdF 4 ). These results provide new insights into the photochromic tuning of highly doped UCNPs and exhibit great promise in multiple anti‐counterfeiting, information security, and biosensing and bioimaging.