Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful technique for investigating the local physico-chemical properties of low-dimensional materials. Combined with scanning probe microscopy (SPM) junctions acting as precisely controlled plasmonic nanocavities, the simultaneous access to the electronic and vibrational channels can be achieved by coupling the SPM tip with external light sources using simple custom optimizations [1]. In analogy with surface enhanced Raman scattering (SERS), the intense Raman signals arise from two distinct mechanisms: (1) electromagnetic (EM) enhancement through spatially confined optical fields and (2) chemical enhancement (CHEM) arising from specific adsorbate-environment interactions. 

 

We systematically explore these enhancement mechanisms using precisely controlled SPM junctions and atomically well-defined systems spanning from single molecules to extended low-dimensional materials. Our experimental approach enables tracking the evolution of Raman spectra from the tunneling regime through to the formation of atomic or molecular point contacts [2]. We observe a dramatic signal enhancement upon point contact formation, which we primarily attribute to charge transfer processes underlying the CHEM mechanism. Notably, this chemical enhancement remains effective even on non-plasmonic substrates, as demonstrated for reconstructed silicon surfaces [3]. The exceptional spectral sensitivity allows us to investigate dynamic processes on materials traditionally considered unsuitable for TERS studies, such as monitoring local heating in individual C60 molecules adsorbed on catalytically active platinum surfaces [4]. In addition to the observed spectral enhancement, vanishing tip-sample distances of a few angstrom are responsible for the strong localization of the electromagnetic field sustained on atomic protrusions. We leverage this exceptional spatial resolution to perform vibrational mapping of single iron phthalocyanine (FePc) molecules adsorbed on non-equivalent Ag crystal surfaces [5]. These measurements reveal how subtle variations in adsorption geometry – particularly the precise registry with the underlying atomic lattice – induce significant modifications of the molecular vibrational landscape through symmetry-breaking effects (see Figure 1). Our findings establish TERS as a powerful tool to probe local properties of low-dimensional materials and pave the way for precisely tailoring adsorbate-substrate interactions at the atomic level.

 

Figure 1. Two adsorption configurations of iron phthalocyanine (FePc) on Ag(110) with the corresponding TERS map with the spatial distribution of the vibrational mode at ca. 385 cm-1.

References

[1]   Borja Cirera et al., MethodsX (2025) 14, 103156

[2]   Borja Cirera et al., Nano Letters (2022) 22, 6, 2170-2176

[3]   Borja Cirera* et al., PCCP (2024) 26, 32, 21325-21331

[4]   Borja Cirera et al., ACS Nano (2022) 16, 10, 16443-16451

[5]   Borja Cirera et al, In preparation

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Meeting ID: 508 144 0931

Seminar hosts: Neven Šantić i Matija Čulo