Nanoimaging Reveals Topological Textures in Nanoscale Crystalline Networks

Self-assembly processes run deep in biological machinery, creating complex structures such as the lipid molecular assemblies in cell membranes and the dynamic multi-megadalton spliceosome complexes that precisely remove noncoding sequences from coding RNA in cell nuclei. The concept of spontaneous, bottom-up formation of nanoscale architectures, driven by weak interaction forces between molecular building blocks, holds significant promise for materials science applications. This approach contrasts with more conventional synthetic top-down methods and is particularly appealing due to the energy and resource efficiency inherent in biological systems. To extract the advantages of this nanofabrication approach, we must better understand the underlying forces that lead to the formation of both the structures and the defects in synthetic self-assembly networks.

Defects are typically associated with irregularities in atomically crystalline materials. Self-assembly materials are often amorphous at the atomic scale but periodically ordered at the mesoscale, i.e., with unit cells of tens of nanometers. In such mesoscale materials, even larger defects exist, however, that are manifested as long-range collective distortions of tens of unit cells. Studying these defects requires characterization techniques capable of observing distortions of individual unit cells at high spatial resolution over statistically significant length scales/volumes as well as methods to analyze the associated large and complex datasets. This is important as such collective behavior can lead to profound changes in material properties.

An international team of researchers has achieved a major milestone in nanotechnology by mapping the intricate topological textures within 3D nanoscale networks with cubic single-diamond crystal structure (space group Fd3̅m), synthetically produced by block copolymer self-assembly replicated in gold. This groundbreaking research utilized advanced non-destructive X-ray nanotomography at the cSAXS beamline of the Swiss Light Source (SLS), achieving a spatial resolution of 11 nm over a large volume of 90 μm3 thanks to state-of-the-art instrumentation for high-resolution ptychographic X-ray computed tomography. This allowed the detailed analysis of nearly 70,000 individual crystal unit cells with sizes of a few tens of nanometers.

With access to both large volume and high spatial resolution, the researchers were able to capture not just a single topological defect, i.e., a defect with high stability against perturbations, but a pair of oppositely charged entities: "comet" and "trefoil" patterns pinned at the domain boundary. These patterns resemble those observed in "soft" liquid crystals, yet exhibit typical "hard" crystal behavior. Inspired by the Moiré phenomenon of interference patterns, the researchers devised a method to analyze strain in the volume without requiring prior information on the nodes' positions in the network. The observed strain distribution suggests structural synchronization arising from the topological nature of defects. This means that topological defects attract a higher density of local "classical" defects to their location, helping to stabilize long-range ordering in the network.

This research marks a significant advancement in the reconstruction of large volumes of mesoscale structures as well as the understanding of topological textures that emerge in self-assembly networks, while simultaneously analyzing tens of thousands of unit cells with high resolution. While the current work deals with diamond structures grown as a soft condensed matter system and then templated with gold, the results of this study highlight the opportunities that will be presented to the soft-matter community with the upgrade of the SLS. The higher brilliance expected from the upgrade will likely allow for non-destructive structural studies of polymer systems at the deep nanoscale.

Teaser image © Paul Scherrer Institute PSI/Dmitry Karpov