Luminescence and Covalency in Ytterbium-Doped CrX3 (X = Cl, Br, I) van der Waals Compounds

• Published in: Journal of the American Chemical Society 2023-08, Vol.145 (31), p.17427-17434
• Main Authors: Snoeren, Thom J., Pressler, Kimo, Kluherz, Kyle T., Walsh, Kelly M., De Yoreo, James J., Gamelin, Daniel R.
• Format: Article
• Language: English
• Subjects: absorption; energy; iodides; ionization; photochemical reactions; photoluminescence; spectroscopy; van der Waals forces
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.3c05989

The layered 2D van der Waals ferromagnets CrX3 (X = Cl, Br, I) show broad d–d photoluminescence (PL). Here we report preparation, structural characterization, and spectroscopic studies of all three CrX3 compounds doped with the optical impurity, Yb3+. EXAFS measurements show very similar Cr K-edge and Yb L-edge data for each doped compound, and good fits of the latter are obtained for structures having Yb3+ occupying substitutional octahedral sites. Yb–X bond lengths are systematically ∼0.25 Å larger than their Cr–X counterparts. 4 K PL measurements show efficient sensitization of Yb3+ luminescence upon photoexcitation into lattice absorption bands [Cr3+ d–d and ligand-to-metal charge-transfer (LMCT)] for all three compounds, converting their nondescript broadband d–d PL into sharp f–f emission. The PL of CrCl3:Yb3+ and CrBr3:Yb3+ occurs at energies typical for [YbX6]3– with these halides, with PL decay times of 0.5–1.0 ms at 4 K, but CrI3:Yb3+ displays anomalously low-energy Yb3+ emission and an unusually short PL decay time of only 8 μs at 4 K. Data analysis and angular overlap model (AOM) calculations show that Yb3+ in CrI3:Yb3+ has a lower spin–orbit splitting energy than reported for any other Yb3+ in any other compound. We attribute these observations to exceptionally high covalency of the Yb3+ f orbitals in CrI3:Yb3+ stemming primarily from the shallow valence-shell ionization potentials of the iodide anions.