Figure 1. Atomic structure and optical characterization of exfoliated NiPS₃.

(a) Schematic crystal structure. View perpendicular to layers, only top layer shown (upper schematic). The atoms contained in the section shaded grey are shown in view parallel to layers (lower schematic). Unit cell (dashed outlines), covalent bonds within (P₂S₆)⁴⁻ anions (grey lines). (b) Brightfield microscope image of cleaved bulk NiPS₃ samples. (c) Image of NiPS₃ flake exfoliated onto oxidized silicon substrate, comprising mainly thick sheets (>10 layers). 60° and 120° angles indicated. (d) Image of thin exfoliated NiPS₃ sheets, 2–7 layers indicated. (e) Optical intensity contrast of ultrathin NiPS₃ sheets (2–7 layers), evaluated for red, green, and blue color channels separately, with reference to the substrate. (Photos in panels (c) and (d) were acquired at different illumination conditions to maximize visibility of relevant features).