Highlights

TERS is an extension of conventional Raman scattering, a valuable tool for determining the vibrational modes of molecules and materials. The spatial resolution of traditional Raman spectroscopy is limited by the diffraction limit to hundreds of nanometres. This resolution is insufficient to spatially resolve atomic motion in molecules, which are typically less than 1 nanometer in size. TERS achieves sub-nanometre resolution by combining two powerful techniques, namely surface-enhanced Raman and scanning probe microscopy.  A sharp metal (or metal-coated) tip, within the centre of a laser focus, is positioned close to the sample. This setup excites free electrons at the tip, leading to the so-called plasmonic effect, and the resulting strongly localized electromagnetic field significantly amplifies the sample’s Raman signal. Since its first experimental demonstration about 20 years ago, TERS has been successfully used to visualize the vibrational modes of single molecules and monitor the catalytic process at the nanoscale, among various other applications.

However, simulating TERS spectra is complex. Previous attempts relied on either expensive computational methods limited to the study of simple systems or empirical parameters lacking predictive power. The recent study led by the group of Dr. Mariana Rossi at MPSD was published online in the Journal of Physical Chemistry Letters in late 2023 and overcomes these challenges with a novel approach that profits from several features of the FHI-aims code. It enables a predictive comparison with experiments without empirical inputs. In addition, the novel method is computationally less demanding than previous first-principles simulations, facilitating the study of complex systems. The development also relied on a collaboration with the theory department in MPSD. The method is implemented in FHI-aims, and an accompanying tutorial showcases its usage.

These first-principles simulations enable a deeper understanding of the mechanisms that enhance the Raman signal. “Our new method establishes a direct link between experimental observations and atomistic simulations, crucial for characterizing the chemical interactions between molecules and solid substrates,“ said Yair Litman, first author and Walter Benjamin fellow in the Yusuf Hamied Department of Chemistry at the University of Cambridge. “Our results demonstrate the decisive role charge transfer plays in the TERS spectra.“ For instance, the authors show that the TERS spectrum for TCNE, a flat organic molecule in the gas phase, is strongly affected by a silver surface when adsorbed. The spatially-resolved spectra for different normal modes at the same adsorbed molecular geometry, but including or excluding the surface, showed distinctive patterns that were not related in a simple manner.  These findings highlight the chemical sensitivity of the method and the need for first-principles simulations to properly interpret the increasing amount of experimental TERS results.