Alleviating luminescence concentration quenching in lanthanide doped CaF2 based nanoparticles through Na+ ion doping

Abstract

Luminescence concentration quenching, mainly due to a cross relaxation (CR) process between lanthanide ions (Ln³⁺), widely occurs in Ln³⁺ doped luminescent materials, setting a limit in the dopant content of Ln³⁺ emitters to withhold the brightness. Here, we introduced Na⁺ ions into the CaF₂ host lattice codoped with Nd³⁺ emitters that alleviates concentration quenching greatly. And we show that the optimal dopant concentration of Nd³⁺ in colloidal CaF₂:Nd nanoparticles increased from 10 to 30 mol%, resulting in an ∼32 times near-infrared (NIR) (1052 nm) brightness under 800 nm laser irradiation. Our mechanistic investigation suggests that the enhancement of NIR photo-luminescence (PL) could be attributed to not only the increasing crystallinity of nanoparticles but also the reducing concentration quenching of Nd³⁺ by improving the dopant distribution of Nd³⁺ ions in the CaF₂ lattice, as evidenced by the high angle annular dark field images. These result in the optimal concentration increase to produce brightness enhancement greatly. This strategy can be utilized for other Ln³⁺ doped CaF₂ based nanomaterials for bio-imaging.