Impact of the Surface Chemistry of 2D Nanoplatelets on Cation Exchange

Among nanocrystals, 2D zinc blende nanoplatelets (NPLs) provide the highest level of growth control, resulting in submonolayer roughness on their surface and hence on their thickness. This characteristic enables a uniquely narrow spectral line width. The synthesis of 2D particles is now well controlled with cadmium chalcogenides, but the growth process remains difficult to extend to other materials. Therefore, an alternative strategy to achieve a roughness-free 2D object is to perform cation exchange on the as-synthesized NPLs. Nevertheless, if not properly conducted, this strategy leads to dramatic shape reconstruction, resulting in a loss of beneficial properties. Here, we demonstrate that surface chemistry can significantly impact the kinetic and thermodynamic properties of the cation exchange procedure. When copper exchange is performed on CdSe NPLs, we find that thiols are the most suitable ligands for shape preservation. This can be attributed to the ligand-induced lattice contraction on CdSe, which minimizes lattice distortion during cation exchange. Conversely, we determine that the initial flatness of the particle, ligand length, and ligand binding strength are not critical for maintaining the shape, although they affect the reaction duration. Finally, the method is generalized to multiple CdSe thicknesses and CdS NPLs.