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

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 (S-4(3/2) + F-2(5/2) -> F-4(9/2) + F-2(7/2), F-4(9/2) + F-2(5/2) -> I-4(9/2) + F-2(7/2), I-4(9/2) + F-2(5/2) -> I-4(11/2) + F-2(7/2), I-4(11/2) + F-2(5/2) -> I-4(13/2) + F-2(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.