Nanoscale localization of electromagnetic fields using metallic nanostructures can catalyze chemical reactions in their immediate vicinity. Local optical field confinement and enhancement is also exploited to attain single-molecule detection sensitivity in surface-and tip-enhanced Raman (TER) spectroscopy. In this work, we observe and investigate the sporadic formation of 4-nitrobenzenethiolate upon TER imaging of a 4-nitrobenzenethiol (4NBT) monolayer on Au(111). Density functional theory (DFT), finite-difference timedomain (FDTD), and finite element method (FEM) calculations together confirm that this chemical reaction does not occur as a result of thermal desorption of the molecule, which requires temperatures in excess of 2100 K at the tip−sample junction. Our combined experimental and theoretical analyses strongly suggest that the chemical transformations observed throughout the course of TERS mapping is not driven by plasmonic photothermal heating, but rather by plasmon-induced hot carriers.

Bimetallic nanostructures exhibit unique catalytic activity and selectivity that are not evident for their monometallic analogues. Such nanostructures contain plasmonic metals, such as gold or silver, which afford highly efficient harvesting of electromagnetic radiation and its conversion into hot carriers. These highly energetic species are transferred to the catalytic metal subcomponent of the bimetallic nanostructure, where a large spectrum of chemical reactions may be catalyzed. The strength of the electric field and the interplay between catalytic and plasmonic metals at the nanoscale are thus critically important for the catalytic activity of bimetallic nanostructures. In this study, we investigate the relationship between the catalytic activity and local electric fields sustained on the surface of gold–palladium (Au@PdNPs) and gold–platinum (Au@PtNPs) nanoplates using tip-enhanced Raman spectroscopy. We image the spatially varying magnitudes of rectified (DC) local electric fields on the surface of these nanostructures and compare them to the fields sustained on the surface of monometallic nanoplates. We find substantially larger electric field magnitudes on Au@PdNPs and Au@PtNPs as compared to their monometallic analogues. These findings suggest that catalytic efficiency of bimetallic nanostructures may be mediated and potentially tuned through precise control of electric fields sustained on their surfaces.

We report on a scanning tunneling microscopy-induced luminescence in MoSe2 monolayers supported by uniform and nanopatterned gold substrates. Luminescence intensity mappings, recorded with a nanometric spatial resolution, and spectroscopy measurements were performed and analyzed in terms of photon emission processes taking place within the tip–surface gap region, which supports strongly localized optical and electronic states. We found that, excited by tunneling electrons, the light emission is due to the radiative recombinations of free excitons confined within the MoSe2 monolayer. Light emission is observed at positive and negative bias voltages with very different emission rates. The results are interpreted in terms of charge carrier injection in the MoSe2 layer. Additionally, electrodynamic simulations also stress that resonance between the emitted radiation and the surface plasmons formed in the tip–surface gap region plays a critical role in the emission process. When the MoSe2 layer lays on a nanopatterned gold substrate, its luminescence intensity, induced by the tunneling electrons, is enhanced by nearly an order of magnitude. Such an effect is observed here for the first time in scanning tunneling microscopy experiments. The luminescence enhancement is attributed to the surface plasmon properties of the nanopatterned gold substrate, which spectrally match the excitonic transition of the MoSe2 layer. We show that this surface-enhanced scanning tunneling microscope-induced light emission (SESTM-LE) effect is very useful for investigating the photon emission from localized emitters (e.g., quantum wells, quantum dots, molecules) with a subnanometer spatial resolution.

Tip-enhanced Raman spectroscopy (TERS) is a spectroscopy technique that possesses single-molecule sensitivity and subnanometer spatial resolution. These unique properties are achieved thanks to the extremely high electromagnetic field confinement at the apex of the scanning probe. However, such strong field confinement can lead to photodecomposition and thermal decomposition of the analytes. Here, we demonstrate that the use of an aqueous solvent as tip−sample junction mediator drastically reduces possible molecule degradation. Using a combination of electrodynamic and heat transport simulations, we provide some theoretical insight into the plasmonic heating of the TERS system. The simulations of a realistic model system show that upon illumination of the tip−sample junction the temperature at the tip apex can increase by ∼180 K when TERS is performed in air with optical powers of ∼100 μW. On the other hand, in aqueous media, the temperature increase of the tip remains significantly lower (a few kelvins) thanks to the higher thermal conductivity of water.