Thom Snoeren1,Kimo Pressler1,Kelly Walsh1,Daniel Gamelin1
University of Washington1
Thom Snoeren1,Kimo Pressler1,Kelly Walsh1,Daniel Gamelin1
University of Washington1
The layered 2D van der Waals ferromagnets CrX<sub>3</sub> (X = Cl, Br, I) are some of the most promising and well-studied materials for spintronics and related quantum systems. Despite potential applications in quantum sensing and as single-photon sources, investigations on the optical properties of CrX<sub>3</sub>, characterized by broad <i>d-d</i> photoluminescence (PL), are severely lacking. The incorporation of carefully designed atomic defects such as lanthanides opens up a pathway to modulate the overall electronic and magneto-optical properties. Here we report preparation, structural characterization, and spectroscopic studies of all three CrX<sub>3</sub> compounds doped with the optical impurity Yb<sup>3+</sup>. 4 K PL measurements show efficient sensitization of Yb<sup>3+</sup> luminescence upon photoexcitation into lattice absorption bands for all three compounds, converting their nondescript broadband <i>d-d</i> PL into sharp <i>f-f</i> emission. The PL of CrCl<sub>3</sub>:Yb<sup>3+</sup> and CrBr<sub>3</sub>:Yb<sup>3+</sup> occur at energies typical for [YbX<sub>6</sub>]<sup>3-</sup> with these halides, with PL decay times of 0.5 – 1.0 ms at 4 K, but CrI<sub>3</sub>:Yb<sup>3+</sup> displays anomalously low-energy Yb<sup>3+</sup> emission and an unusually short PL decay time at 4 K. Data analysis and angular overlap model (AOM) calculations show that Yb<sup>3+</sup> in CrI<sub>3</sub>:Yb<sup>3+</sup> has a lower spin-orbit splitting energy than reported for any other Yb<sup>3+</sup> in any other compound. We attribute these observations to exceptionally high covalency of the Yb<sup>3+</sup> <i>f</i> orbitals in CrI<sub>3</sub>:Yb<sup>3+</sup> stemming primarily from the shallow valence-shell ionization potentials of the iodide anions. These results provide rare fundamental insights into the electronic structure and luminescence of the remarkably underexplored [YbI<sub>6</sub>]<sup>3-</sup> motif.