Bulk chromium tri-iodide (CrI3) has long been known as a layered van der Waals ferromagnet1. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet2, providing a new platform for investigating light–matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI3 under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI3 bilayers2. Distinct from the Wannier–Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors3, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI3. We attribute the photoluminescence to a parity-forbidden d–d transition characteristic of Cr3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.
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The authors thank D. Gamelin for insightful discussions on the optical response of CrI3, and A. Majumdar for testing the measurement system. Work at the University of Washington was mainly supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division (DE-SC0018171), and University of Washington Innovation Award. Work at MIT has been supported by the Center for Integrated Quantum Materials under NSF grant DMR-1231319 as well as the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4541 to P.J.-H. Device fabrication has been partly supported by the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Award Number DESC0001088. The contribution of D.H.C. is supported by DE-SC0002197. Work at CMU is supported by DOE BES DE-SC0012509. W.Y. is supported by the Croucher Foundation (Croucher Innovation Award), the RGC of Hong Kong (HKU17305914P), and the HKU ORA. Work at ORNL (M.A.M.) was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. X.X. and D.X. acknowledge the support of a Cottrell Scholar Award. S.G. and L.Y. are supported by NSF grant no. DMR-1455346 and EFRI-2DARE-1542815. X.X. acknowledges the support from the State of Washington funded Clean Energy Institute and from the Boeing Distinguished Professorship in Physics.
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Seyler, K.L., Zhong, D., Klein, D.R. et al. Ligand-field helical luminescence in a 2D ferromagnetic insulator. Nature Phys 14, 277–281 (2018). https://doi.org/10.1038/s41567-017-0006-7
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