Current Understanding on the Unique Relaxation Dynamics of Sub-nanometer Materials and Their Structure-Property Relationships

Sub-nanometer materials(SNMs) define the emergent functional material system with their characteristic dimensions at sub-1 nm scale and they can be generally constructed with the sub-1 nm molecular clusters as the basic structural units. Due to their extremely small sizes, the sub-nm scale particles possess diffusive dynamics in their bulk with an energy level close to typical thermal fluctuation. Meanwhile, the volume fraction of surface structures becomes dominant and the dynamics of surface structures can be distinguishable from their diffusive dynamics. The research on the dynamics of SNM is key to the understanding of their unique properties in comparison to small molecule and nano-material systems. This review paper summarizes recent progresses in the studies of relaxation dynamics of SNM upon the combinatory application of X-ray/neutron scattering, dielectric spectroscopy and rheometric technology. The functional materials inspired by the dynamics investigations with applications in mechanical strengthening, ion conduction, and gas separation are also reviewed. In the end, challenges and outlooks on the theories, characterizations and the prediction of possible new functionalities of SNMs are discussed.