Ligand-field helical luminescence in a 2D ferromagnetic insulator

Bulk chromium tri-iodide (CrI ₃ ) has long been known as a layered van der Waals ferromagnet ¹ . However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet ² , 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 CrI ₃ under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI ₃ photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI ₃ bilayers ² . Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors ³ , our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI ₃ . We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr ³⁺ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.