Synthesis and Characterization of FePS3 Crystals

We synthesized our FePS₃ crystals using a chemical vapor transport method (for details see ref. 34 (link)). All the powdered elements (purchased from Sigma-Aldrich): iron (99.99% purity), phosphorus (99.99%), and sulfur (99.998%) were prepared inside an argon-filled glove box. After weighing the starting materials in the correct stoichiometric ratio, we added an additional 5 wt of sulfur to compensate for its high vapor pressure. After the synthesis, we carried out the chemical analysis of the single-crystal samples using a COXI EM-30 scanning electron microscope equipped with a Bruker QUANTAX 70 energy-dispersive X-ray system to confirm the correct stoichiometry. We also checked the XRD using a commercial diffractometer (Rigaku Miniflex II). Prior to optical measurements, we determined the crystal axes of the samples using an X-ray diffractometer. We cleaved samples before placing them into high vacuum (∼10⁻⁷ torr) to expose a fresh surface without contamination and oxidation.