(Nanowerk Information) “Surfaces were invented by the devil” – this quote is attributed to the theoretical physicist Wolfgang Pauli, who taught at ETH Zurich for a few years and in 1945 obtained the Nobel Prize in physics for his contributions to quantum mechanics. Researchers do, certainly, wrestle with surfaces. On the one hand they’re extraordinarily vital each in animate and inanimate nature, however then again it may be devilishly troublesome to review them with standard strategies.
An interdisciplinary staff of supplies scientists and electrical engineers led by Lukas Novotny, Professor of Photonics at ETH Zurich, along with colleagues at Humboldt-Universität zu Berlin has now developed a technique that may make the characterization of surfaces significantly simpler sooner or later.
They just lately revealed the outcomes of their analysis, which relies on an especially skinny gold membrane, within the scientific journal Nature Communications (“Bulk-suppressed and surface-sensitive Raman scattering by transferable plasmonic membranes with irregular slot-shaped nanopores”).
The pores within the gold membrane amplify the laser beam in Raman spectroscopy, permitting it to penetrate solely into the floor (mild gray) however not into the majority of the fabric (darkish gray). (Illustration: Scixel)
Surfaces are vital for performance
“Whether we are dealing with catalysts, solar cells or batteries – surfaces are always extremely relevant for their functionality”, says Roman Wyss, a former PhD pupil in supplies science and first creator of the paper, who now works as a researcher on the ETH spin-off firm Enantios. The explanation for this relevance is that vital processes often occur at interfaces. For catalysts, these processes are the chemical reactions which can be accelerated on its floor. In batteries, the floor properties of the electrodes are essential for his or her effectivity and degradation habits.
For a few years, researchers have used Raman spectroscopy for analyzing materials properties non-destructively – that’s, with out destroying the fabric within the course of. In Raman spectroscopy, a laser beam is shipped onto the fabric, and the mirrored mild is analysed. From the properties of the mirrored mild, whose frequency spectrum was modified by the vibrations of the molecules within the materials, one can draw conclusions each on the chemical composition of the thing into account – also called its chemical fingerprint – in addition to on mechanical results akin to pressure.
Gold membrane with tiny pores
“This is a very powerful method, but it can only be applied to surfaces with strong limitations”, says Sebastian Heeg, who contributed to the experiments as a postdoc in Lukas Novotny’s group and who now leads a junior analysis group at Humboldt-Universität. Since in Raman spectroscopy the laser mild penetrates the fabric by a number of micrometres, the frequency spectrum is affected primarily by the majority of the fabric and solely to a really small diploma by its floor, which solely includes just a few atomic layers.
To harness Raman spectroscopy additionally for surfaces, the ETH researchers developed a particular gold membrane that’s solely 20 nanometres thick and accommodates elongated pores round 100 nanometres in measurement. When such a membrane is transferred onto a floor to be investigated, two issues occur: first, the membrane prevents the laser beam from penetrating into the amount of the fabric. Second, on the places of the pores the laser mild is concentrated and re-radiated just a few nanometres into the floor.
Thousand-fold sign amplification
“The pores act as so-called plasmonic antennas – just like the antenna in a mobile phone”, says Heeg. The antenna amplifies the Raman sign from the floor by as much as a thousand instances in comparison with the sign of standard Raman spectroscopy with out the membrane. Heeg and his colleagues had been in a position to show this on numerous supplies, together with strained silicon and the perovskite crystal lanthanum nickel oxide (LaNiO3).
Left: The gold membrane (left half) amplifies the Raman sign of the floor in comparison with the sign from the majority of the fabric (proper half). Proper: Gold membrane with pores 100 nanometres in measurement that act as antennas. (Graphic: S. Heeg, R. Wyss)
Strained silicon is vital for functions in quantum applied sciences, however thus far it has not been potential to probe the pressure utilizing Raman spectroscopy as a result of the sign produced by the surfaced was coated by the background noise of the measurement. After the gold membrane had been utilized, the pressure sign was selectively amplified to the purpose that it may very well be clearly distinguished from the opposite Raman alerts of the fabric.
The metallic perovskite lanthanum nickel oxide, then again, is a vital materials for producing electrodes.
“The strong coupling between its crystal structure and electrical conductivity make it possible to control the conductivity by changing the thickness of the electrode on the nanometre scale. The surface structure, one presumes, plays an essential role here”, says Mads Weber, a former postdoc at ETH Zurich and now assistant professor on the College of Le Mans, who investigates this class of supplies and was additionally concerned within the research.
Due to the brand new gold membrane methodology, the researchers had been now in a position, for the primary time, to realize entry to the floor construction of lanthanum nickel oxide.
“Our approach is also interesting from the point of view of sustainability, as existing Raman equipment can gain completely new capabilities without much effort”, says Heeg.
Sooner or later, the researchers need to additional enhance their methodology and adapt it to person calls for. For example, at present the pores within the gold membrane have completely different sizes and are randomly oriented. By producing a gold membrane with pores of equal measurement which can be aligned in parallel, the strategy may very well be optimised for particular supplies, which might enhance the power of the Raman sign by one other issue of 100.
