Unraveling the Influence of the Anchored Headgroup and Ligand Tail Group Length on the Self-Assembly of ZnO NCs at the Air-Water Interface.

Understanding the self-assembly of nanocrystals into ordered superlattices is fundamental for tailoring nanoscale assembly and developing functional materials. However, the rational-by-design hierarchical assembly is complex, as it needs to orchestrate several physical and chemical processes. In this work, we investigate how the character of a ligand shell, particularly the type of anchored headgroup and the length of the ligand tail group, affects the self-assembly properties of nanoscale building blocks and the properties of corresponding thin films. Thus, a series of colloidal zinc oxide nanocrystals (ZnO NCs) coated with different organic shells are prepared by a self-supporting organometallic approach (OSSOM) and then self-assembled at the air-water interface. We demonstrate that ligands bearing fewer and longer alkyl chains attached to the headgroup promote a higher level of ligand interdigitation. This, in turn, leads to stronger interparticle interactions and enhanced viscoelastic properties of the resultant films. Shorter ligands and a more compact capping layer (more aliphatic chains per ligand) prevent interdigitation, allowing ZnO NCs to retain their individual character and form more elastic films. Our findings reveal the importance of organic shell design in the self-assembly of nanocrystals due to the steric control, and allow an extra level of tailorability to obtain the desired properties for the material called "superlattices" or "NC solids".