Interconversion between KSc₂F₇:Yb/Er and K₂NaScF₆:Yb/Er nanocrystals: the role of chemistry

Scandium (Sc) sits at a unique position in the periodic table, i.e., the junction of the top of the rare earth column and the beginning of the transition metal row. Studies have shown that Sc-based nanomaterials are very sensitive to the surrounding chemical environment. A simple adjustment of the chemical reaction conditions such as temperature, surfactant molecules, and solvents (e.g., oleic acid (OA) or 1-octadecene (OD)) can easily lead to different products in terms of chemical composition and phase structure. Herein, under purposely adjusted reaction conditions, we have investigated the interconversion process between two representative Sc-based nanomaterials, that is, nanocrystals of orthorhombic KSc2F7:Yb/Er and cubic K2NaScF6:Yb/Er, both of which have characteristic red upconversion luminescence and high similarity in chemical composition and phase structure. Experimental results have indicated that conversion from KSc2F7:Yb/Er to K2NaScF6:Yb/Er may start from the edge of the nanocrystal where K+ in KSc2F7:Yb/Er was gradually substituted by the post-introduced Na+ in the solution and finally KSc2F7:Yb/Er nanorods were broken and K2NaScF6:Yb/Er nanocubes were formed. On the other hand, a simple variation of the OA : OD ratio facilitates the dissolution of K2NaScF6:Yb/Er and subsequent crystallization of KSc2F7:Yb/Er during the opposite conversion process. Possible chemical reaction mechanisms were further developed to elucidate the interconversion details. Meanwhile, the variation of the upconversion luminescence such as emission intensity, red to green ratio, and lifetime is interpreted to monitor the conversion progress at corresponding stages, which is highly consistent with the scenario discussed above.