Yb3+ as a Sharp-Line Optical Defect in Magnetic 2D van der Waals Materials

Abstract

The sharp-line emitting lanthanides have been extensively used to transform the optical properties of materials, but as of yet their implementation in 2D layered van der Waals (vdW) materials is limited. Here, we investigate Yb3+ as an optical dopant in the magnetic 2D vdW materials CrX3 (X = Cl, Br, I), in which the broad and featureless Cr3+ photoluminescence (PL) is transformed into highly structured sharp-line Yb3+ PL. Notable shifts in PL energies and lifetimes indicate unusually high Yb3+ f-orbital covalency in CrI3:Yb3+. This is investigated further in MI3:Yb3+ (M = Y, Gd, Bi, Yb) and LuChX:Yb3+ (Ch = S, Se; X = Br, I) compounds, where clear trends are observed between the Yb3+ f-orbital covalency and the polarizability of the ligand. CrI3:Yb3+ remains an outlier for its high covalency, which is attributed to structural distortions driven by the size mismatch between Cr3+ and Yb3+. Due to its low ionization energy, this high degree of covalent bonding is unique to Yb3+ among lanthanides. Measurements in a magnetic field demonstrate that the sharp-line Yb3+ PL serves as an embedded optical sensor for the host-lattice magnetization and internal temperature. Furthermore, the magnetic structure of the host lattice acts as an effective field on Yb3+ dopant ions. This is manifested as exchange splitting of the Yb3+ spin sublevels, which can be individually resolved in PL measurements. The presence of exchange splittings allows for facile optical control of spin states at zero external field. High-pressure measurements show that the exchange splittings persist above the magnetic ordering temperature as a consequence of short-range spin alignment within the first coordination sphere. These results on Yb3+ f-orbital covalency and magnetic Cr3+-Yb3+ interactions serve as a guideline for the design of novel spin-photonic materials.