Size-encoded hierarchical self-assembly of nanoparticles into chains and tubules

Hierarchical structures with sophisticated patterns allow the emergence of challenging properties. However, the highly cooperative and specific interactions needed for assembly spanning different length scales are typically lacking in inorganic nanoparticles (NPs). Here we show that size can be a common structural driving force for controlling hierarchical assembly of inorganic NPs into anisotropic superstructures. It involves first the self-limiting assembly of small CdS NPs into large supraparticles and their subsequent spontaneous organization into chains and tubules hundreds of nanometers long. Our quantitative calculations based on DLVO theory reveals an intrinsic size effect relating to the dimension change of assembly units in accordance with a negative cooperativity. It is shown that the size increase in building blocks creates an effective kinetic barrier contrast at different attachment sites due to the increase of interparticle electrostatic repulsion, switching the assembly from thermodynamically preferred 3D to kinetically favored 1D pathway. The size-encoded hierarchical assembly is accompanied by the ligand-controlled Oswald ripening process, which is responsible for the variation of hierarchical patterns from chains to tubules. The general principle in governing multistage inorganic NP ordering represents an important guideline toward the complex mesoscale structures that may surpass traditional materials in both design and functionality.