Angular-resolved optical radiation of adsorbates in the (sub)nanometer junctions

• Applicants: Professorin Dr. Stefanie Gräfe; Professor Dr. Alfred J. Meixner; Professorin Dai Zhang, Ph.D.
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Physical Chemistry of Solids and Surfaces, Material Characterisation ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 549456677

The goal of this proposal is to provide an experimental and theoretical description of light-matter interactions in peak-enhanced Raman spectroscopy (TERS). Recently published work using TERS demonstrated spatial resolutions on the sub-nanometer or Ångström scale and stimulated active research to explore the underlying super resolution mechanisms. An important contribution to the resolution is provided by the field enhancement due to the extremely narrow gap between tip and sample. Nevertheless, not all mechanistic contributions are fully understood by far, such as the contribution of charge transfer between molecule and tip. In this project, this will be systematically investigated in close synergy between experiment and theory. The central research hypothesis is that the Raman polarizability and optical transition dipole moments of molecules/adsorbates can be strongly modified in the (sub-)nanometer gaps between tip and surface, also for example by symmetry breaking. These modifications, in turn, affect the Raman spectra, and ultimately the observed sub-molecular resolution. Specifically, we will experimentally investigate Raman spectra, angle-resolved Raman scattering, and the role of charge transfer (CT) versus electromagnetic (EM) enhancement in different light-matter interaction regimes (weak vs. strong coupling). Theoretically, we will calculate and model the pure chemical and CT contributions, the pure (spatially inhomogeneous) EM contributions to the optical signal, Raman modes (symmetry, polarizability, and energetic position) of molecules in nanocavities and picocavities, and the role of CT and EM enhancement in nanocavities and picocavities. Direct comparison with experiment will then allow elucidation of the underlying mechanisms. The proposed research is at the frontiers of Raman nano- and picoscopy. The experimental findings and theoretical models obtained in this project will generate strong synergies and provide fundamental new impetus to the fields of surface and tip-enhanced Raman spectroscopy.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Heiko Peisert