Researchers have a hard time with surfaces. On the one hand, they are very important in both animate and inanimate nature, but on the other hand, it is sometimes extremely difficult to study them using conventional detection methods.
"Whether it's catalysts, solar cells or batteries - surfaces are always extremely relevant for their functionality," says Roman Wyss, a former PhD student in materials science who is now conducting research at ETH start-up Enantios. This is because the important processes usually take place at interfaces. In the case of catalysts, it is about the chemical reactions that are accelerated on their surfaces. For batteries, on the other hand, the surface properties of the electrodes are crucial for their efficiency and long-term behavior.
Raman spectroscopy for surfaces not yet very precise
For the non-destructive investigation of material properties - i.e. without damaging the material - researchers have been using Raman spectroscopy for many years. This involves directing a laser beam at the material and analyzing the reflected light. The properties of the reflected light, whose frequency spectrum has been altered by the vibrations of the molecules in the material, can be used to draw conclusions about the chemical composition of the object under investigation - known as a chemical fingerprint - as well as to detect mechanical effects such as stresses.
However, even powerful methods such as Raman laser spectroscopy have their limits when it comes to obtaining precise information from surfaces, as the laser light penetrates a few micrometers deep into the material and the frequency spectrum is therefore mainly influenced by the material's interior and only to a very small extent by the surface, which is only a few atomic layers thick.
Selective signal amplification using a gold membrane
An interdisciplinary team of materials scientists and electrical engineers led by Lukas Novotny, Professor of Photonics at ETH Zurich, together with colleagues from Humboldt-Universität zu Berlin, has now developed a method using a wafer-thin, specially structured gold membrane. This is applied to the material to be examined - and amplifies the Raman signal of the surface up to a thousand times.
This membrane is only 20 nanometers thick and has elongated pores about a hundred nanometers in size. It absorbs or reflects parts of the laser light and reduces the intensity so that the laser beam cannot penetrate far into the material as it normally would. Only at the pores can the laser light penetrate a few nanometers deep into the surface. The results of this work were recently published in the scientific journal Nature Communications.
Thousand-fold signal amplification
"The pores also act as so-called plasmonic antennas - very similar to the antenna in a cell phone," explains Sebastian Heeg, who was involved in the experiments as a postdoc with Lukas Novotny and now heads a junior research group at Humboldt-Universität. This antenna effect amplifies the Raman signal of the material surface up to a thousand times. In the future, it will be possible to optimize this gold membrane for certain surfaces For example, the pores in the gold membrane are currently of different sizes and arranged irregularly. By producing membranes with special pore geometries and arrangements, the Raman signal strength for certain materials could be increased a hundredfold.
Measuring method successfully tested in practice
Heeg and his colleagues were able to demonstrate this impressively using the example of strained silicon and the perovskite crystal lanthanum nickel oxide (LaNiO3). Strained silicon is important for applications in quantum technologies, but until now the strain could not be investigated using Raman spectroscopy because the signal generated by the surface was drowned out by the background noise of the measurement. After the gold membrane was applied, the strain signal was selectively enhanced to such an extent that it could be clearly distinguished from the other Raman signals of the material. The metallic perovskite lanthanum nickel oxide, for example, is an important material for the production of electrodes. "The strong coupling between its crystal structure and electrical conductivity makes it possible to control the conductivity by changing the electrode thickness in the nanometer range. It is assumed that the surface structure plays an essential role in this," explains Mads Weber, former postdoc at ETH Zurich and now assistant professor at the University of Le Mans, who researches this class of materials and was also involved in the study. Thanks to the new gold membrane method, the researchers have now been able to gain an insight into the surface structure of lanthanum nickel oxide for the first time.
"Our approach is also interesting in terms of sustainability, as it gives existing Raman devices completely new capabilities without a great deal of effort," explains Heeg. In future, the researchers want to further improve their method and adapt it to the needs of users.
Further information can be found at https://www.physik.hu-berlin.de/en/pld).
