Two-dimensional materials such as graphene often have different properties from the corresponding 3D bulk materials (i.e., graphite in the case of graphene). They can be useful, e.g., in high-performance materials, electronics, or catalysis. Often, 2D materials have a layered structure in their bulk form, which makes it comparatively easy to synthesize them, e.g., by splitting these layers.

Matthew C. Beard, National Renewable Energy Laboratory, Golden, CO, USA, and colleagues have used a colloidal synthesis based on a cation-exchange reaction to prepare atomically thin 2D ZnS, CdS, CoS₂, and PbS. These compounds do not have a layered structure in the bulk, which makes their 2D forms more difficult to access.

The team used mono- and few-layer Ag₂S as a precursor, stabilized by 3-mercaptopropionic acid (MPA) and 1-octanethiol (OT) ligands. A colloidal solution of this 2D Ag₂S was then reacted with solutions of either zinc nitrate, cadmium nitrate, cobalt nitrate, or lead oleate to give the desired products via cation exchange. The material keeps its size, shape, and either single-layered or a few-layered nature during the reaction.

The optical properties of the resulting atomically thin metal sulfides (ATMS) are different from both the bulk material and from quantum dots (i.e., tiny 3D particles) made from the same compounds. This could lead to new applications. The ATMS have under-coordinated metal centers at the surface, which could make them highly active catalysts. According to the researchers, other 2D metal sulfides could be accessible via the same approach by choosing appropriate precursors and reaction conditions.

• Atomically Thin Metal Sulfides,
  Lenore Kubie, Marissa S. Martinez, Elisa M. Miller, Lance M. Wheeler, Matthew C. Beard,
  J. Am. Chem. Soc. 2019.
  https://doi.org/10.1021/jacs.9b05807