An international research collaboration, including a group from Cornell Engineering, has applied a novel X-ray-based reconstruction technique to observe, for the first time, topological defects in a cubic lattice structure based on nanoscale self-assembly of a polymer-metal composite material imaged on a relatively large sample volume.
In the future, this technique and new materials knowledge could be applied to the study of other mesoscopic structures exhibiting this class of defects – which are known to be the origin of many known physical phenomena and can give rise to new or improved material properties – in self-assembled materials, both natural and synthetic.
“This is a new polymer, a new structure and a new technique that have made it possible to reconstruct unprecedented volumes of samples,” said Ulrich WiesnerSpencer T. Olin Professor in the Department of Materials Science and Engineering. “That’s really the key: If you have 70,000 unit cells of a material, instead of just a few dozen unit cells, you can really start to look carefully at the structure of the defects: what kind of defects and how often do those defects occur?”
Wiesner is co-author of “High-resolution three-dimensional imaging of topological textures in single-diamond nanoscale networkspublished July 23 in Nature Nanotechnology. The corresponding author is Justin Llandro, an assistant professor at the Electrical Communication Research Institute at Tohoku University in Sendai, Japan.
Wiesner, whose research group has been working on block copolymer self-assembly (BCP SA) since he arrived at Cornell 25 years ago, oversaw the synthesis of the triblock terpolymer material used in the study. The synthesis was carried out by Takeshi Yuasa and Hiroaki Sai, both former members of the Wiesner Group.
The question of how significant defects are in the materials generated by BCP SA has always been elusive, Wiesner said, in part because the technologies needed to measure sufficiently large sample volumes – with proportionally larger defect structures – have been slow to develop.
The new technology, hard X-ray ptychography, performed at the Swiss Light Source (SLS) at the Paul Scherrer Institute in Switzerland, is an advanced form of tomography that can penetrate deeper into a material than electron microscope beams can. The technique allowed the researchers to reconstruct a very large sample volume of a polymer-metal composite material derived from BCP SA.
“If you have a smaller defect, like a line or point defect, when you perturb the system you can often ‘fix’ the structure of the defect,” Wiesner said. “In contrast, topological defects are so large that they are very stable to external perturbations.”
Once the triblock terpolymer was synthesized, researchers in the group of Ulli Steiner at the Adolphe Merkle Institute in Fribourg, Switzerland, a long-time collaborator of Wiesner, generated thin films from it and replaced one of the terpolymer blocks with gold, so that the material could withstand repeated exposure to the intense coherent X-ray beams of the SLS.
SLS imaging and image reconstruction ultimately revealed a co-continuous network known as a single diamond structure, with topological defects that the researchers believe would have substantial effects on mechanical and other properties. Importantly, the defects closely resemble topological textures found in nematic liquid crystals and in the single-celled Hydra organisms, suggesting that self-assembly can be used as a model process to study the role of topology in nature.
Wiesner said this collaborative research could pave the way for future studies in an area his lab has already explored: block copolymer-driven superconductors.
“You would expect that the macroscopic, electronic, or transport properties of the superconductor would depend on the defects in your materials,” he said. “That’s what really excites me: We now have a technique that allows us to visualize larger volumes of these materials and generate correlations between the defect structure and the properties.”
Other collaborators came from the Paul Scherrer Institute, the Adolphe Merkle Institute of the University of Fribourg, both in Switzerland; the Max Planck Institute for Chemical Solid State Physics, Dresden, Germany; the University of Salzburg, Austria; and Hiroshima University and the Inamori Scientific Research Institute, Kyoto, both in Japan.
This study received financial support from the Swiss National Science Foundation, the National Competence Centre for Bioinspired Materials Research, the European Union’s Horizon 2020 research and innovation programme, the European Soft Matter Infrastructure, the Japan Society for the Promotion of Science, and the National Science Foundation.
The researchers used the Cornell Materials Research Center Shared Facilities.