Direct molecular level near-field plasmon and temperature assessment in a single plasmonic hot spot
in: Light-Science & Applications (2020)
Tip-enhanced Raman spectroscopy (TERS) is now widely recognized as an inescapable but still emergent technique for exploring the nanoscale. However, our lack of comprehension of critical parameters still limits its potential as a user-friendly analytical tool. Surface plasmon resonance of the tip, heating due to near-field temperature rise, and spatial resolution are undoubtedly three challenging experimental parameters to unravel, but also the most fundamentally relevant to explore because they ultimately decide the signal to be probed and the state of the molecule investigated. Here we propose a straightforward and accessible methodology to access molecular level information of the optical near-field experienced solely by the molecules contributing to the TERS signal. The detailed near-field optical response, at the molecular level and as a function of time, is evaluated using standard TERS experimental equipment, by probing simultaneously the Stokes and anti-Stokes spectral intensities. Self-assembled 16-mercaptohexadodecanoic acid monolayers covalently bound to an ultra-flat gold surface were used as a demonstrator. Observation of blinking lines in the spectra also lead to critical information on the lateral resolution and on the atomic scale thermally induced morphological changes at the tip apex during the experiment. This study gives access to unprecedent molecular level information under TERS conditions and aims at reducing the elusiveness of TERS in day-to-day experiments by providing access to the information that most critically affect the experiments. This knowledge is of central importance for any experimental plasmonic investigations and for the application of TERS in the field of nanoscale thermometry.