Sub-nanometric materials: Electron transfer, delocalization, and beyond

In sub-nanometric materials, interactions between building blocks exert an even larger impact on the structures and properties compared with their nanosized counterparts. Electrons may be shared equally or unequally between directly connected building blocks, resulting in electron transfer in heterojunctions or electron delocalization among symmetric structures. Due to the unique electronic properties of clusters, which are close to those of atoms, large-scale electron delocalization may occur in cluster-based constructions and thus bring about extraordinary properties. In this perspective, sub-nanometric materials with well-defined structures have been summarized, including supported catalysts and cluster-based structures, and the relationship between the electronic structures and properties are emphatically discussed. This perspective aims to provide a novel insight into the understanding of sub-nanometric materials and cluster-based constructions. Challenges and opportunities • Electron transfer in heterojunctions usually occurs near the interfaces. Due to the unique electronic structure of clusters that close to an atom, the junction effect in cluster-based materials involves all atoms from both the surface and interior, resulting in enriched active sites and promoted catalytic activities. • Electrons can be shared equally between directly connected identical clusters, for which large-scale electron delocalization may be realized in cluster-based structures with high structural ordering. The delocalized electrons may induce significant changes in cluster orbitals and overall electronic structures, and thus exceptional properties may emerge. • Interactions, as well as molecular orbital interactions between clusters and inorganic entities at the molecular level, should be the focus in future investigations, with the use of XAFS, in situ characterizations, and theoretical methods. Electron transfer and delocalization in sub-nanometric materials exert an even larger impact on their structures and properties, as compared to their nanosized counterparts. In this perspective, sub-nanometric materials with well-defined structures have been summarized, including supported catalysts and cluster-based structures, and the relationship between the electronic structures and properties are emphatically discussed.