Researchers at ETH Zurich have developed a novel technique for studying surfaces, a crucial but complex field in materials science. This method, based on the use of an ultrathin gold membrane, promises to revolutionize surface characterization in many technological fields.
An interdisciplinary team of materials scientists and electrical engineers, led by Professor Lukas Novotny of ETH Zurich, together with colleagues from Humboldt University Berlin, has developed a method that significantly simplifies surface characterizationThe results of their research, based on the use of an extremely thin gold membrane, were published in the scientific journal Nature Communications.
Roman Wyss, a former PhD student in materials science and first author of the paper, highlights the importance of surfaces: “Whether catalysts, solar cells or batteries, surfaces are always extremely relevant for their functionality.».
Indeed, important processes usually take place at interfaces. For catalysts, these are the accelerated chemical reactions on their surface. In batteries, the surface properties of the electrodes are crucial for their efficiency and degradation behavior.
For many years, the Raman spectroscopy is used to examine the properties of materials non-destructively. However, its application to surfaces has significant limitations. Sebastian Heeg, who contributed to the experiments as a postdoc in Lukas Novotny’s group, explains: “The laser light penetrates the material by several micrometers, so the frequency spectrum is mainly affected by the volume of the material and only to a very small extent by its surface, which consists of only a few atomic layers.».
In order to exploit Raman spectroscopy also for surfaces, the ETH researchers have developed a special gold membrane with a thickness of only 20 nanometers, containing elongated pores of around 100 nanometers. When this membrane is transferred to a surface to be studied, two phenomena occur: first, the membrane prevents the laser beam from penetrating into the volume of the material. Second, at the locations of the pores, the laser light is concentrated and re-emitted only a few nanometers into the surface.
Sebastian Heeg adds: «The pores act as plasmonic antennas, similar to the antenna on a cell phone“. The antenna amplifies the Raman signal from the surface up to a thousand times compared to the signal from conventional Raman spectroscopy without the membrane. This amplification has been demonstrated on several materials, including strained silicon and lanthanum nickel oxide (LaNiO3), a perovskite crystal.
Strained silicon, which is important for applications in quantum technologies, could not previously be probed by Raman spectroscopy due to the measurement background noise. After application of the gold membrane, the strain signal was selectively amplified to the point where it can be clearly distinguished from other Raman signals of the material.
Mads Weber, former postdoctoral fellow at ETH Zurich and now assistant professor at the University of Le Mans, emphasizes the importance of this method for the study of metal perovskites such as lanthanum nickel oxide. Thanks to the new gold membrane method, the researchers were able to access the surface structure of this material for the first time.
Sebastian Heeg highlights the sustainable aspect of this approach: “Our approach is also interesting from a sustainability perspective, as existing Raman equipment can acquire completely new capabilities without much effort.».
In the future, researchers hope further improve their method and adapt it to user demands. For example, by producing a gold membrane with pores of equal size and aligned in parallel, the method could be optimized for specific materials, which would further improve the Raman signal strength by a factor of a hundred.
Caption: The pores in the gold membrane developed by the ETH researchers amplify the laser beam in Raman spectroscopy, allowing it to penetrate only the surface (light grey) but not the bulk of the material (dark grey). (Illustration: Scixel)
Wyss RM, Kewes G, Marabotti P et al. Bulk-suppressed and surface-sensitive Raman scattering by transferable plasmonic membranes with irregular slot-shaped nanopores. Nature Communications 15, 5236 (2024). DOI: 10.1038/s41467-024-49130-2
