Controlled synthesis and upconversion luminescence properties of heterogeneous isomorphic Yb3+/Er3+ co-doped Na0.9Ca0.9Gd1.1F6 nanorods with multiple luminescence centers

Abstract

Rare-earth upconversion luminescent nanocrystals (UCLNCs) have limited application in biomedical fields due to their low luminescence efficiency. Here, heterogeneous isomorphic Na_0.9Ca_0.9Gd_1.1F₆:Yb³⁺/Er³⁺ nanorods are successfully obtained using a mild solvothermal method. Subsequently, the upconversion luminescence (UCL) properties of Na_0.9Ca_0.9Gd_1.1F₆:Yb³⁺/Er³⁺ are investigated by room temperature fluorescence spectroscopy, fluorescence lifetime, lifetime imaging and in situ variable temperature spectroscopy measurements. The results show that hexagonal-phase Na_0.9Ca_0.9Gd_1.1F₆:Yb³⁺/Er³⁺ exhibits excellent luminescence properties. In particular, the in situ variable temperature spectrum demonstrates that Na_0.9Ca_0.9Gd_1.1F₆:Yb³⁺/Er³⁺ can luminesce at temperatures up to 300 °C. Meanwhile, by comparing the UCL properties of orthorhombic-phase GdF₃:Yb³⁺/Er³⁺, hexagonal-phase NaGdF₄:Yb³⁺/Er³⁺ and hexagonal-phase Na_0.9Ca_0.9Gd_1.1F₆:Yb³⁺/Er³⁺ nanocrystals prepared under the same conditions, it is found that hexagonal-phase Na_0.9Ca_0.9Gd_1.1F₆:Yb³⁺/Er³⁺ displays the strongest UCL and the longest luminescence decay lifetime. We attribute this UCL enhancement to the fact that Na_0.9Ca_0.9Gd_1.1F₆:Yb³⁺/Er³⁺ has multiple luminescence centers, which is verified by the low-temperature selective excitation technique of Eu³⁺. Importantly, a transparent and flexible Yb³⁺/Er³⁺ co-doped Na_0.9Ca_0.9Gd_1.1F₆/TPU (thermoplastic polyurethane) luminescent film is obtained. The discovery of host materials with multiple luminescence centers not only presents new ideas for further improving the UCL efficiency of nanocrystals, but also lays a certain foundation for UC nanomaterials in the field of in vivo imaging.