Site-dependent TERS study of a porphyrin molecule on Ag(100) at 7 K

Abstract

Single-molecule tip-enhanced Raman spectroscopy (TERS) has emerged as an important technique for structural analysis at sub-molecular scale. Here in this work, we report a TERS study of an isolated free-base porphyrin molecule adsorbed on the Ag(100) surface at cryogenic temperature (∼7 K). Site-dependent TERS spectra reveal distinct local vibrational information for the chemical constituents within a single molecule. Moreover, distinct spatial features among different Raman peaks can be resolved from the TERS mapping images. These images are found to associate with related vibrational modes, enabling to resolve the mode associated with N−H bonds at the sub-nanometer level. This study will provide deep insights into the symmetry of adsorption configurations and local vibrational information within a single molecule.

The Expanding Frontiers of Tip-Enhanced Raman Spectroscopy

“…This study opened the door for further work exploring the limits of intramolecular resolution for TERS of a single molecule. 203 Continuing with the trend of the previously mentioned work, in 2019 the Apkarian group investigated CoTPP on Cu(100) at a temperature of 6 K. 43 The authors stated that through scanning with the STM tip in a close approach, the CoTPP molecule could be flattened on the surface. Once the molecule was manually flattened, they were able to image the vibrational normal modes of the planar molecule by maintaining the tip within the atomistic contact limit, closer than previously published TERS studies.…”

“…This study opened the door for further work exploring the limits of intramolecular resolution for TERS of a single molecule. 203…”

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

“…261 The SRP technique can also be applied to more complex molecular systems, e.g., for the H 2 TBPP porphyrin molecule on Ag(100), the orientation of the N−H bonds around the porphyrin core can be directly identified through the spatial mapping. 323 For the [12] cycloparaphenylene nanohoop molecule, its adsorption configuration and structural deformation on different metal substrates can be directly identified through different features in the spatial mapping images, revealing the local structural deformation associated with different tilting of the benzene units in real space caused by inhomogeneous molecule−substrate interactions. 324 As a brief summary to the SRP methodology, the various experimental studies described above have demonstrated its strong structural characterization ability, regardless of whether a molecule is planar or three-dimensional.…”

“…Furthermore, by monitoring the spectral evolution, one can even track the breaking of one top N–H bond and the formation of a new N–H bond on the cyanuric ring, resulting in the transformation from an adsorbed melamine to its tautomer through photon-induced hydrogen transfer . The SRP technique can also be applied to more complex molecular systems, e.g., for the H 2 TBPP porphyrin molecule on Ag(100), the orientation of the N–H bonds around the porphyrin core can be directly identified through the spatial mapping . For the [12] cycloparaphenylene nanohoop molecule, its adsorption configuration and structural deformation on different metal substrates can be directly identified through different features in the spatial mapping images, revealing the local structural deformation associated with different tilting of the benzene units in real space caused by inhomogeneous molecule–substrate interactions …”

Bicomponent supramolecular self‐assemblies studied with tip‐enhanced Raman spectroscopy

“…On the other hand, tip-enhanced Raman spectroscopy (TERS) offers an attractive alternative to directly probe the molecular characteristic vibrational signal, which is sensitive to the molecular configurations and local molecular environment. [14][15][16][17][18] Great progresses have been made in the field since the introduction of TERS in 2000; single-molecule sensitivity and subnanometer spatial resolution have been successfully achieved on isolated molecules and molecular self-assemblies, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] thanks to the dramatic enhancement that resulted from the highly confined plasmonic field in the tip-substrate junction. The chemical sensitivity with high spatial resolution has been nicely demonstrated in identifying different adjacent molecules in a variety of bicomponent molecular samples.…”