Direct molecular-level near-field plasmon and temperature assessment in a single plasmonic hotspot

Tip-enhanced Raman spectroscopy (TERS) is currently widely recognized as an essential but still emergent technique for exploring the nanoscale. However, our lack of comprehension of crucial parameters still limits its potential as a user-friendly analytical tool. The tip's surface plasmon resonance, heating due to near-field temperature rise, and spatial resolution are undoubtedly three challenging experimental parameters to unravel. However, they are also the most fundamentally relevant parameters to explore, because they ultimately influence the state of the investigated molecule and consequently the probed signal. Here we propose a straightforward and purely experimental method to access quantitative information of the plasmon resonance and near-field temperature experienced exclusively by the molecules directly contributing to the TERS signal. The detailed near-field optical response, both at the molecular level and as a function of time, is evaluated using standard TERS experimental equipment by simultaneously probing the Stokes and anti-Stokes spectral intensities. Self-assembled 16-mercaptohexadodecanoic acid monolayers covalently bond to an ultra-flat gold surface were used as a demonstrator. Observation of blinking lines in the spectra also provides crucial information on the lateral resolution and indication of atomic-scale thermally induced morphological changes of the tip during the experiment. This study provides access to unprecedented molecular-level information on physical parameters that crucially affect experiments under TERS conditions. The study thereby improves the usability of TERS in day-to-day operation. The obtained information is of central importance for any experimental plasmonic investigation and for the application of TERS in the field of nanoscale thermometry. Tip-enhanced Raman spectroscopy: convenient characterization Experiments involving tip-enhanced Raman spectroscopy (TERS) could benefit from a convenient method to quantitatively characterize the process and thus aid its optimization. TERS is a highly sensitive technique that combines the chemical specificity of Raman spectroscopy with the spatial resolution of tip-based atomic force microscopy. However, evaluating and quantifying the underlying plasmon mechanisms behind the technique is challenging. Now, Marie Richard-Lacroix and Volker Deckert from Jena, Germany, report that simultaneous collection and analysis of Stokes and anti-Stokes Raman signals provide critical information about the plasmon-based TERS hotspot, including the precise spectral position and width of the plasmon resonance and the near-field temperature. TERS experiments performed with a monolayer of mercaptododecanoic acid on a gold substrate confirm the viability of the approach.