Charge transfer-enhanced d-d emission in antiferromagnetic NiPS₃

Van der Waals (vdW) magnetic materials have gained tremendous attentions since the discovery of intrinsic ferromagnetic order in atomically thin chromium triiodide (CrI₃) and chromium germanium telluride (Cr₂Ge₂Te₆)_. Particularly, antiferromagnetic metal phosphorus trichalcogenides (MPX₃, M = Mn, Fe, Ni; X = S, Se) are becoming attractive for their promising potential for next-generation spintronics. Among this family of vdW magnetic materials, nickel phosphorus trisulfide (NiPS₃) has been extensively studied in recent years with many intriguing phenomena reported such as phonon-magnon coupling, charge-spin correlation, and spin-correlated photon emission. However, although the unpaired d electrons in transition metal atoms of magnetic materials play crucial roles in determining their properties, the d electron transitions especially d-d emission, have rarely been observed in magnetic materials due to the forbidden selection rules. Here, we report an observation of d-d emission in antiferromagnetic nickel phosphorus trisulfides (NiPS₃) and its strong enhancement by stacking it with monolayer tungsten disulfide (WS₂). We attribute the observation of the strong d-d emission enhancement to the charge transfer between NiPS₃ and WS₂ in the type-I heterostructure. The d-d emission peak splits into two peaks D₁ and D₂ at low temperature below 150 K, from where an energy splitting due to the trigonal crystal field is measured as 105 meV. Moreover, we find that the d-d emissions in NiPS₃ are nonpolarized lights, showing no dependence on the zig-zag antiferromagnetic configuration. These results reveal rich fundamental information on the electronic and optical properties of van der Waals antiferromagnetic materials.