The new class of 2D colloidal nanomaterials involves both nanocrystals (NCs) of 3D materials such as CdSe, which are synthesized as atomically flat nanoplatelets (NPLs), and nanosheets of van der Waals crystals such as MoS2 that are intrinsically two dimensional. Such materials differ from the widely studied 0D crystallites. Opposite from 0D CdSe NCs, 2D CdSe NPLs are exclusively terminated by atomically flat (100) facets, whereas the surfaces of MoS2 nanosheets offer no dangling bonds for ligands to interact with. It thus remains unclear whether insights in surface chemistry obtained from the study of quasi-spherical 0D NCs are transferable to colloidal 2D materials. Here, we use CdSe NPLs and MoS2 nanosheets as model systems to discuss the surface chemistry of colloidal 2D materials. By means of NMR spectroscopy, we demonstrate that CdSe NPLs are stabilized by tightly bound cadmium carboxylates. Using ligand displacement isotherms, we demonstrate that the (100) surfaces offer a heterogeneous set of binding sites with different adsorption energies. This finding concurs with DFT calculations on a model NPL, which features weaker binding sites at the platelet edges and stronger sites on the facets. In addition, we show that the PL efficiency of NPLs is highly sensitive to the displacement of Z-type ligands, where the removal of a minute fraction of Z-type ligands from edge sites induces localized states in the band gap and PL quenching. In the case of MoS2, NMR analysis indicates that ligands such as amines or carboxylic acids exhibit a weak, dynamic interaction with the nanosheet surface. We demonstrate that the resulting issues with colloidal stability can be overcome by choosing a solvent that minimizes the effective inter-sheet interaction energy. Alternatively, we demonstrate that Z-type ligands, such as metal carboxylates, can bind to the top and bottom facets and stabilize MoS2 nanocolloids in a wider range of solvents. We conclude that in the case of CdSe NPLs, surface termination fits within the framework that was derived from the study of 0D CdSe nanocrystals, where the well-defined 2D NPL morphology makes such NPLs ideal model systems to relate structural, chemical and electronic insights. MoS2 nanosheets, on the other hand, behave differently. In line with their being 2D van der Waals solids, ligands show little interaction with the nanosheet surface; a behavior that calls for a new surface termination approach with these materials.

Surface chemistry of colloidal nanocrystals, from 0D to 2D

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.