Adsorption-Driven Symmetry Lowering in Single Molecules Revealed by {\AA}ngstrom-scale Tip-Enhanced Raman Imaging

TL;DR

This study uses advanced tip-enhanced Raman spectroscopy to map vibrational modes of single molecules adsorbed on different silver surfaces, revealing how substrate symmetry influences molecular vibrations at the atomic scale.

Contribution

First demonstration of sub-nanometer TERS mapping across different symmetry configurations, linking vibrational properties to local atomic environments.

Findings

Subtle adsorption geometry variations affect vibrational mode degeneracy.

Site-specific vibrational properties depend on substrate symmetry.

Sub-nanometer TERS reveals atomic-scale surface-molecule interactions.

Abstract

The vibrational landscape of adsorbed molecules is central to understanding surface interactions at the atomic scale, influencing phenomena from catalysis to molecular electronics. Recent advances in atomic-scale tip-enhanced Raman spectroscopy (TERS) have enabled vibrational mapping of single molecules with sub-nanometer spatial resolution, providing unprecedented insights into molecule-surface interactions by confining light in plasmonic picocavities. Here, we exploit TERS in a cryogenic scanning tunneling microscope junction to perform Raman hyperspectral mapping of single iron phthalocyanine (FePc) molecules in three non-equivalent adsorption configurations on Ag surfaces. We explore the changes in the vibrational modes of FePc molecules adsorbed on two distinct silver crystal terminations with differing symmetry, Ag(111) and Ag(110), revealing how subtle variations in the adsorption geometry due to substrate anisotropy can strongly influence molecular vibrations, lifting the degeneracy of individual normal modes. Our findings not only demonstrate the first use of sub-nanometer TERS mapping across different symmetry configurations but also provide a deeper understanding of how site-specific vibrational properties are intimately linked to local atomic environments. This capability paves the way for precisely tailoring surface interactions and controlling chemical reactions at the atomic scale.