Figure 2.

(A) Ionic radius and valence configuration of RE. From La³⁺ to Lu³⁺, the number of electrons on the 4f orbitals increases with increasing atomic number. The electron configurations of La³⁺, Gd³⁺, and Lu³⁺ show empty, half-filled, and completely filled 4f orbitals, respectively. Adapted with permission from ²¹, copyright 2022, American Chemical Society. (B) Simplified energy level diagrams of lanthanide ions for a basic upconversion process. The 4f ^N configuration of lanthanide ions splits into multiple energy sublevels due to the effects of the coulombic (H_c), spin-orbit (H_so) and crystal-field (H_cf) interactions. The energy levels are denoted as ^2S+1L_J, where S, L, and J stand for total spin, orbital, and total angular momentum quantum numbers. (C) Schematic illustrations for UCNPs and the tipical energy transfer process. (D) Schematic diagrams of six upconversion processes. Created with BioRender.com. λex: excitation spectrum; λem: emission spectrum; ESA: excited state absorption; ETU: energy transfer upconversion; CUC: cooperative upconversion; CR: cross-relaxation; PA: photon avalanche; EMU: energy migration upconversion.