An international team of researchers, including from the Adolphe Merkle Institute Soft Matter Physics group, has used advanced imaging techniques to achieve a high-resolution, three-dimensional visualization of topological defects in nanoscale materials. This could pave the way for innovative applications in nanofabrication and engineering.

Nature produces structures of striking complexity from the molecular to the microscopic scale. The concept that nanoscale architectures can emerge from their building blocks holds great promise for materials science applications. This idea is particularly compelling given the energy and resource efficiency inherent in biological systems. To extract the advantages of this nanofabrication approach, a better understanding of the underlying forces that lead to the formation of both the structures and the defects in synthetic self-assembly networks is needed.

Defects are typically associated with irregularities in atomically crystalline materials. Materials that self-assemble, though, often lack a clear structure at the atomic scale but are periodically ordered in units measuring tens of nanometers. In such mesoscale materials, even larger defects exist that manifest themselves as long-range collective distortions of tens of unit cells. Studying these defects requires techniques capable of observing distortions of individual unit cells at high spatial resolution. This is important as such collective behavior can lead to profound changes in material properties.

Topological defects are fundamental to understanding the properties of materials and play a critical role in defining the properties of both hard and soft materials, impacting everything from the stability of crystalline structures to the behavior of liquid crystals. Despite their importance, these defects have been difficult to study in three dimensions at the nanoscale — until now.

The international team, including scientists from AMI, Switzerland’s Paul Scherrer Institute (PSI), the Max Planck Institute for Chemical Physics of Solids in Germany, Cornell University in the US, the University of Salzburg in Austria, and Hiroshima University and Tohoku University in Japan, has overcome this challenge. First, they fabricated topological defects within a single-diamond network composed of individual diamond crystals and templated from a block copolymer. They then used synchrotron-based X-ray nanotomography at PSI to image the network with unprecedented clarity. The rays are generated from a high-energy source, and have very short wavelengths, allowing for nanoscale-resolution imaging. As the sample rotates, projections are taken from multiple angles. The X-rays penetrate the sample, and the varying absorption or scattering of X-rays within the sample provides information about its internal structure.

The scientists were able to achieve a 3D spatial resolution of 11.2 nanometers, allowing them to visualize nearly 70,000 individual unit cells in a self-assembled polymer network. This resolution enabled them to identify and analyze two distinct types of topological defects, resembling 'comet' and 'trefoil' patterns. The high-resolution imaging revealed intricate details of these defects, which emerged at the boundaries between grains of different orientations within the network. The defects, though like patterns found in liquid crystals, showed strain behaviors more characteristic of hard matter, highlighting the unique properties of these nanoscale structures.

The use of non-destructive X-ray nanotomography marks a significant advancement over traditional methods like transmission electron (TEM) and focused-ion beam (FIB) microscopy, which either limit the sample volume or are destructive. It provides high spatial resolution without damaging the sample, allowing for detailed 3D analysis.

The study's findings, published in the leading journal Nature Nanotechnology, have far-reaching implications. By confirming the topological nature of the defects through number analysis and detailed strain mapping, the researchers suggest that controlling substrate geometry could influence the formation of topological defects in copolymer networks. The ability to manipulate these defects could lead to the engineering of new properties in nanoscale materials, with potential applications across various fields of technology and materials science.

Reference: Karpov, D.; Djeghdi, K.; Holler, M.; Abdollahi, S. N.; Godlewska, K.; Donnelly, C.; Yuasa, T.; Sai, H.; Wiesner, U. B.; Wilts, B. D.; Steiner, U.; Musya, M.; Fukami, S.; Ohno, H.; Gunkel, I.; Diaz, A.; Llandro, J. High-Resolution Three-Dimensional Imaging of Topological Textures in Nanoscale Single-Diamond Networks. Nat. Nanotechnol. 2024, 1–8. https://doi.org/10.1038/s41565-024-01735-w.

Original text: Paul Scherrer Institute