Multiscale Hierarchical Structures from a Nanocluster Mesophase

The spontaneous self-organization of nanoscale materials into complex macroscopic architectures has intrigued scientists for decades, because it offers insight into how complex structures can emerge from primitive building blocks. Hierarchical self-assembled systems offer structural advantages that are absent in isolated constituent units. Nature is replete with fascinating examples of hierarchical assembly, such as proteins or DNA, and natural photonic structures as seen in butterfly wings or fish. Although synthetic materials that afford the same level of accuracy and complexity as biomolecules are currently beyond reach, research into the self-organization of artificial units has increased dramatically. Organic molecules and copolymers are among the most commonly used elementary units to form large-area patterning. More recently, advances in nanomaterial synthesis have provided access to a versatile library of nanoparticle building blocks with tunable composition, morphology and atomic structure that give rise to physiochemical properties unattainable in naturally occurring materials.<br/>In this study, highly aligned structures are obtained from an organic–inorganic mesophase composed of 1.5 nm monodisperse Cd37S18 magic-size cluster building blocks. Impressively, structural alignment spans over seven orders of magnitude in length scale: nanoscale magic-size clusters arrange into a hexagonal geometry organized inside micrometer-sized filaments; self-assembly of these filaments leads to fibers that then organize into uniform arrays of centimeter-scale bands with well-defined surface periodicity. Enhanced patterning can be achieved by controlling processing conditions, resulting in bullseye and ‘zigzag’ stacking patterns with periodicity in two directions. The multiscale arrangement of MSC building blocks in filaments and striped films presents intriguing analogies to hierarchical assemblies in biological systems as well as the pervasive hierarchical structural organization throughout the natural world. Additionally, our thin films show strong circular dichroism response and reversible, chemically induced isomerization which triggers more intriguing scientific questions on the properties. Overall, our work demonstrates the ability of semiconductor nanomaterials to assemble into complex macroscopic structures, which provides a potential platform to realize advanced function.