Modifying plasmon resonances at nanocavities by molecular adsorption: A Scanning Tunnelling Microscopy Study

Scanning Tunnelling Microscopy (STM) is a promising tool to study the properties of plasmonic nano and pico-cavities^[1], as it allows atomic-scale control of the cavity size. STM electroluminescence spectra contains information about the optical properties of the cavity but also about the electronic structure of the system. Recently, our group has developed a normalization technique to separate these two factors by combining luminescence and I(V) measurements^[2]. In this work, we use this technique to investigate the optical modifications in nanocavities between an Au tip and noble metal surfaces, due to the adsorption of two different organic molecules (5,12-Bis(phenylethynyl)naphthacene (BPEN), 9,10-bis(phenylethynyl)anthracene (BPEA)). Our results show that most of the changes in the raw spectra can be attributed to electronic structure factors, but still true optical modifications in the gap plasmonic modes due to the presence of the molecules can be observed in the normalized spectra. As possible origins for such changes, Pauli repulsion from the molecule electron density or coupling of plasmonic and excitonic modes will be discussed.

[1]          Kuhnke K., et al. (2017). Chemical reviews, 117(7), 5174-5222.

[2]           Martín-Jiménez, A. et al. (2020). Nat. commun., 11(1), 1-8.