Adsorption-Driven Symmetry Lowering in Single Molecules Revealed by Ångstrom-Scale Tip-Enhanced Raman Imaging

TL;DR

Researchers used a new imaging technique to study how molecules interact with surfaces at the atomic level, revealing how their structure changes based on the surface they're on.

Contribution

The first subnanometer Raman hyperspectral mapping of single molecules across different symmetry configurations using cryogenic TERS.

Findings

Subtle variations in adsorption geometry on Ag surfaces strongly influence molecular vibrations.

TERS reveals lifting of degeneracy in normal modes due to substrate anisotropy.

Site-specific vibrational properties are closely linked to local atomic environments.

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 subnanometer 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 nonequivalent 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 subnanometer 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 on the atomic scale.

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