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Probing the vibration of atoms provides detailed information on local structure and bonding that define material properties. Tip-enhanced Raman spectroscopy (TERS) offers extremely high resolution to probe such vibrations. Krystof Brezina and Mariana Rossi from the MPI for the Structure and Dynamics of Matter (MPSD), and Yair Litman from the MPI for Polymer Research (MPIP), have demonstrated that realistic, first-principles simulations are essential for interpreting TERS images of molecules and materials on surfaces. Their approach reveals how interactions with metallic substrates reshape vibrational imaging at the nanoscale. The work has now been published in ACS Nano.
At the nanoscale all atoms vibrate. These vibrations define heat dissipation, chemical reactions, and material properties. The different ways in which atoms can vibrate is determined by the local chemical bonding and its environment, thus providing an invaluable probe into the properties and composition of matter. In the laboratory, they can be studied indirectly using spectroscopies such as Raman scattering. Conventional measurements average over many atoms and are therefore limited in spatial resolution. Tip-enhanced Raman spectroscopy overcomes this limitation by combining laser light with a sharp metallic tip that concentrates the electromagnetic field into a tiny volume, allowing for resolution down to the Ångström (10-10 m) scale. This makes it possible to image vibrational motion even down to individual molecules or defects in metallic surfaces. However, interpreting such highly detailed images requires reliable theoretical models that can connect measured signals to atomic-scale motion.
Experimentalists struggle to disentangle different environmental factors that influence TERS signals, making it harder to understand the signatures of individual atomic motion. This is where simulations come to the rescue. This new study proposes a computational method that allows an efficient simulation of TERS signals of realistically sized systems containing hundreds of atoms, relying only on the most fundamental laws of quantum-mechanics. The study further shows that common simplifications previously made in theoretical modeling, such as treating molecules as isolated systems or approximating surfaces using small clusters, can be problematic.
The simulations unambiguously demonstrate that TERS is exquisitely sensitive to the symmetry of local environments and allows, for example, the identification of local defects in 2D materials. They also demonstrate that electronic screening of the metal surface dramatically alters images of molecular vibrations involving motion perpendicular to the supporting surface, while vibrations confined to the molecular plane are far less affected.
“TERS images are often interpreted as direct maps of atomic motion,” explains Mariana Rossi. “Our results show that the electronic response of the surface can dominate the signal and fundamentally change what these images mean.” Krystof Brezina adds, “A new physical insight gained from our work is that spatially non-local interactions between atoms can strongly influence TERS signals at a particular point in space, meaning that the brightest regions do not necessarily correspond to the largest atomic displacements”
By enabling realistic and predictive simulations, this advance enhances the quality of TERS images as nanoscale probe. Accurate modeling of TERS with such methods will be instrumental in various emergent areas of research in surface science, including genome sequencing, material characterization, design of molecular-scale devices, and operandomonitoring of surface-catalyzed reactions for green energy generation.
Krystof Brezina (MPSD) krystof.brezina@mpsd.mpg.de
Mariana Rossi (MPSD) mariana.rossi@mpsd.mpg.de
Yair Litman (MPIP) litmany@mpip-mainz.mpg.de
https://dx.doi.org/10.1021/acsnano.5c16052
First-Principle Calculations inform the creation of correct TERF images
Copyright: M. Rossi et al.
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