When examining the micro or nanostructure of functional materials—both natural and synthetic—you encounter a fascinating landscape of thousands of coherent domains or grains. These distinct regions, where atoms and molecules are orderly arranged, are fundamental to understanding material performance.
The way these domains are sized, oriented, and distributed can dramatically influence properties. For instance, it can determine whether you’re holding a robust brick or a disintegrating stone; it impacts the ductility of metals, the electron transfer efficiency in semiconductors, and the thermal conductivity of ceramics. This phenomenon extends to biological materials, too; collagen fibers, for example, are produced from a network of fibrils, and their organization is vital for the effectiveness of connective tissue. These grains are often remarkably small—just tens of nanometers across.
However, it’s their three-dimensional arrangement over extensive volumes that truly dictates material characteristics. Historically, methods for probing nanoscale organization have been restricted to two-dimensional analysis or have been destructive to the materials involved.
Now, a new imaging technique developed by a collaborative effort involving the Paul Scherrer Institute (PSI), ETH Zurich, the University of Oxford, and the Max Planck Institute for Chemical Physics of Solids employs X-rays from the Swiss Light Source (SLS) to unveil this critical three-dimensional structural information. This innovation marks a significant advancement, providing unprecedented insights into the foundations of material properties.
Their method is referred to as X-ray linear dichroic orientation tomography, abbreviated as XL-DOT. XL-DOT utilizes polarized X-rays from the Swiss Light Source SLS to investigate how materials absorb X-rays differently based on the orientation of their internal structural domains.
By varying the polarization of the X-rays and rotating the sample to capture images from different perspectives, this technique generates a three-dimensional representation that reveals the internal arrangement of the material. The research team used their technique on a piece of vanadium pentoxide catalyst, approximately one micron in size, employed in sulfuric acid production.
In this case, they were able to identify fine details within the catalyst’s structure, such as crystalline grains, the boundaries where these grains intersect, and variations in crystal orientation. They also detected topological defects within the catalyst. Such characteristics directly influence the effectiveness and durability of catalysts, making an understanding of this structure essential for enhancing performance.
The technique provides high spatial resolution. Due to the short wavelength of X-rays, the method can discern structures that are merely tens of nanometers in size, which corresponds to the dimensions of features like crystalline grains.
“Linear dichroism has been used to measure anisotropies in materials for many years, but this is the first time it has been extended to 3D. We not only look inside but with nanoscale resolution,” says Valerio Scagnoli, Senior Scientist in the Mesoscopic Systems, a joint group between PSI and ETH Zurich. “This means that we now have access to information that was not previously visible, and we can achieve this in small but representative samples, several micrometers in size.”
The concept of XL-DOT was first envisioned by researchers in 2019, but its practical application required an additional five years of dedication and innovation.
A significant challenge was the extraction of detailed three-dimensional maps of crystal orientations from terabytes of raw data. This complex mathematical issue was expertly addressed by Andreas Apseros, the lead author of the study, who developed a specialized reconstruction algorithm during his doctoral research at PSI, with support from the Swiss National Science Foundation (SNSF).
The team attributes a large part of their success in creating XL-DOT to PSI’s unwavering commitment to mastering coherent X-ray techniques. This investment has resulted in unparalleled control and stability at the coherent Small Angle X-ray Scattering (cSAXS) beamline—essential for executing the intricate measurements needed. This is an area that is set to leap forwards after the SLS 2.0 upgrade.
“Coherence is where we’re really set to gain with the upgrade,” says Apseros. “We’re looking at very weak signals, so with more coherent photons, we’ll have more signal and can either go to more difficult materials or higher spatial resolution.”
Due to the non-destructive characteristics of XL-DOT, the researchers anticipate conducting operando studies on systems like batteries and catalysts.
“Catalyst bodies and cathode particles in batteries are typically between ten and fifty micrometers in size, so this is a reasonable next step,” says Johannes Ihli, formerly of cSAXS and currently at the University of Oxford, who led the study.
However, the power of this technique extends beyond just catalysts. Researchers say its applicability to a broad array of materials with ordered microstructures, including biological tissues and cutting-edge materials for information technology and energy storage.
At the core of the research team’s ambitions is the need to explore the three-dimensional magnetic organization of materials. Take, for instance, the orientation of magnetic moments in antiferromagnetic materials. In these materials, magnetic moments alternate in the direction from atom to atom, resulting in no detectable net magnetization at a distance yet possessing a distinct local order in their magnetic structure. This characteristic is incredibly promising for technological applications aimed at faster and more efficient data processing.
“Our method is one of the only ways to probe this orientation,” says Claire Donnelly, group leader at Max Planck Institute for Chemical Physics of Solids in Dresden, who, since carrying out her doctoral work in the Mesoscopic Systems group, has maintained a strong collaboration with the team at PSI.
During his doctoral studies, Donnelly and his team at PSI published a method in Nature for conducting magnetic tomography using circularly polarised X-rays, unlike XL-DOT, which employs linearly polarised X-rays. This innovative technique has already been adopted globally by synchrotrons.
With a solid foundation established for XL-DOT, the team is optimistic that it will achieve the same level of widespread application as its circularly polarized counterpart in synchrotrons. XL-DOT’s applicability to a broader spectrum of samples and its critical role in understanding the structural ordering essential for material performance positions it to make an even more significant impact.
“Now that we’ve overcome many of the challenges, other beamlines can implement the technique. And we can help them to do it,” adds Donnelly.
Journal reference:
- Andreas Apseros, Valerio Scagnoli, Mirko Holler, Manuel Guizar-Sicairos, Zirui Gao, Christian Appel, Laura J. Heyderman, Claire Donnelly & Johannes Ihli. X-ray linear dichroic tomography of crystallographic and topological defects. Nature, 2024; DOI: 10.1038/s41586-024-08233-y