Interactions of CdSe nanoplatelets
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Abstract
CdSe nanoplatelets show extraordinary (optical) properties. Therefore, they are candidates for applications in light-emitting devices, solar cells, field effect transistors, and lasers. They are usually synthesized with colloidal methods, and sterically stabilized in n-alkane solvents.
The properties of CdSe nanoplatelets have been investigated in many studies. However, little attention has been paid to the interactions between the nanoplatelets. The lack of knowledge has made the interpretation of experimental results difficult. For example, it has been unclear why CdSe nanoplatelets assemble into stacks in pure n-alkane solvents, which influences their steric stability. In this thesis, we consider the interplay of the effects that contribute to the interaction of CdSe nanoplatelets. Unlike metallic nanocrystals, the core-core van der Waals interaction between semiconducting CdSe nanoplatelets is weak. Also, the dipole-dipole interaction is weak. Thus, either of these two interactions cannot explain why the nanoplatelets form stacks. Here, we follow the hypothesis that solvation forces cause a strong attraction between the nanoplatelets, leading to the formation of stacks, and influencing their steric stability.
At the interface between a surface and a solvent, the solvent often restructures into layers. As two surfaces approach each other, the solvent layering intensifies, and strong solvation forces can appear. However, nanocrystals often have highly curved surfaces. Due to the curvature, the ligand packing density of these nanocrystals decreases with the distance to their surface. Therefore, the ligand shell and the solvent mix, no significant layer formation occurs, and solvation forces are usually negligible. Thus, the traditional continuum model that is often used to describe the effects of the ligand shell neglects solvation forces. However, the ligand shell of nanocrystals with extended flat facets can be very dense, so that solvent layer formation and solvation forces crucially influence their interaction.
We show that CdSe nanoplatelets are an ideal model system for the study of solvation forces. The ligand grafting density is very high. Furthermore, they can be synthesized with very large base facet areas. We show in molecular dynamics simulations that the solvent restructures into well-defined layers away from the ligand-solvent interface. Strong solvation forces occur between the base facets of the nanoplatelets. These forces cause an attraction that can lead to the stacking of the nanoplatelets. The strength of the attraction depends on the size and geometry of the nanoplatelets, the ligand shell, and the solvent type.
We anticipate that our research results are applicable to other systems. In general, we expect that solvation forces will play a crucial role in the interactions of nanocrystals with extended flat facets, large ligand densities, and well-defined, even ligand-solvent interfaces.