LiErF4 upconversion nanoparticles for the first near-infrared emission via energy management

Abstract

In previous studies, LiErF₄ mostly served as a dipolar-coupled antiferromagnet and was less commonly used as a luminescent material to study the luminescence performance. Therefore, there are few research reports on the luminescent properties of LiErF₄ materials at this stage. In this work, the LiErF₄:Ce,Yb,Y@LiYbF₄:Tm@LiYF₄ core/shell/shell upconversion nanoparticles (UCNPs) and the relevant derivative samples were successfully prepared by coprecipitation method. The LiErF₄:Ce,Yb,Y@LiYbF₄:Tm@LiYF₄ UCNPs exhibit a prominent emission peak at 792 nm and several weak emission peaks in the visible light region under irradiation at 980 nm. Here, the cross relaxation between Ce³⁺ and Er³⁺ in the core can effectively reduce unwanted visible light emissions (⁴S_3/2 + ²F_5/2 → ⁴F_9/2 + ²F_7/2, ⁴F_9/2 + ²F_5/2 → ⁴I_9/2 + ²F_7/2, ⁴I_9/2 + ²F_5/2 → ⁴I_11/2 + ²F_7/2, ⁴I_11/2 + ²F_5/2 → ⁴I_13/2 + ²F_7/2). The Yb³⁺ ions not only work as an energy transfer medium promoting energy transfer, but also as an energy harvester effectively absorbing excitation energy, which can improve energy utilization and promote first near-infrared (NIR-I) emission. The intensity of NIR-I and the ratio of NIR-I to visible light are increased due to the cross relaxation between Ce³⁺ and Er³⁺ and the energy migration among Yb³⁺. The UCNPs emitting strong NIR-I light are advisable for biological imaging.