Self-assembly of ZnO nanocrystals in colloidal solutions

The self-organization in solution of ZnO nanocrystals into superlattices is monitored by dynamic light scattering. When long-alkyl-chain amines or carboxylic acids are used as stabilizing ligands, no organization is observed. In contrast, when binary mixtures of long-alkyl-chain amines and carboxylic acids are used, the presence of a thermodynamic equilibrium between free and organized ZnO nanoparticles is detected in THF or toluene. The superlattices of organized ZnO nanoparticles are independently observed by TEM and SEM. The coordination mode of the ligands at the surface of the ZnO nanoparticles is evidenced by NMR studies. The presence of ion-paired ammonium carboxylate surrounding the surface of ZnO nanoparticles appears to be a necessary requirement to govern this reversible organization. This is substantiated by the absence of organization of ZnO nanoparticles when either a solvent of high dielectric constant, such as acetone, or a strong hydrogen-bond acceptor is used.