Reconstructing the Surface Structure of NaREF₄Upconversion Nanocrystals with a Novel K⁺Treatment

Property of the nanocrystals' surface structure plays a key role in developing novel nanomaterials with high performance and new functionalities. Conventional methods of nanocrystal surface engineering are commonly based on tuning the synthesis reaction parameters or growing core-shell structures, which usually results in increasing the size of the nanoparticles. Here, we report an approach to tailoring the surface crystalline structure of β-NaYF4 nanocrystals by reheating the nanocrystals in a K+-rich environment of the oleic acid-1-octadecene (OA-ODE) system. We found that the crystal surface stability of nanocrystals was decreased in the K+-rich solution, which reconstructs the nanocrystals' surface into a porous surface structure. With a systematic design of experiments, the roles of the cations, such as K+, K+-Gd3+, and Na+-Y3+, are individually identified, which leads to a reformation of the surface structure of the hexagonal NaYF4 nanocrystal into different forms, e.g., a mesostructured, spherical, and diamond surface. The technique of tailoring the surface crystalline structures will provide new insight for the shape and surface-dependent property studies and luminescence enhancement without a size increase.