Lanthanide-Doped KGd2F7 Nanocrystals: Controlled Synthesis, Optical Properties, and Spectroscopic Identification of the Optimum Core/Shell Architecture for Highly Enhanced Upconverting Luminescence

Trivalent Lanthanide (Ln3+)-doped fluoride nanocrystals (NCs) in the category of ALn2F7 (A = Li, Na, and K) have recently emerged as an attractive alternative to the well-developed ALnF4-type fluorides owing to their unique crystallographic structures. In this paper, we reported for the first time the controlled synthesis of monodisperse Ln3+-doped monoclinic-phase KGd2F7 NCs via a facile thermal decomposition method. Characteristic downshifting and upconverting luminescence (UCL) of Ln3+ ion ranging from the visible to near-infrared spectral regions were readily tuned in the KGd2F7:Ln3+ NCs. Moreover, we have also fabricated a series of KGd2F7:Yb3+/Er3+ core/shell NCs with different architectures of inert KGd2F7 or active KGd2F7:Yb3+ shells, which thereby allowed us to carry out a comparative spectroscopic investigation of the optimum core/shell architecture for highly enhanced UCL of Ln3+. Specifically, the KGd2F7:Yb3+/Er3+ core/shell NCs with optimized thickness of inert KGd2F7 shell were found to outperform their counterparts with active KGd2F7:Yb3+ shell in terms of UCL intensity and lifetime. These results would unambiguously provide new fundamental insights into the rational design of complicated core/shell nanostructures with highly enhanced UCL of Ln3+, and would further push forward the development of novel luminescent nanomaterials based on Ln3+-doped KGd2F7 NCs for versatile applications.