Tip-Enhanced Raman Chemical and Chemical Reaction Imaging in H₂O with Sub-3-nm Spatial Resolution

Reproducible chemical and chemical reaction nanoimaging at solid–liquid interfaces remains challenging, particularly when resolutions on the order of a few nanometers are sought. In this work, we demonstrate the latter through liquid-tip-enhanced Raman (TER) measurements that target gold nanoplates functionalized with 4-mercaptobenzonitrile (MBN). In addition to chemical imaging and local optical field nanovisualization with high spatial resolution, we observe the signatures of 4-mercaptobenzoic acid, which forms as a result of plasmon-induced hydrolysis of MBN. Evidently, the solvent leads to distinct plasmon-induced/enhanced chemical reaction pathways that have not been documented. This work shows that such reactions that take place at solid–liquid interfaces can be tracked with a record sub-3-nm spatial resolution via TER spectral nanoimaging in liquids

Vibronic Raman Scattering at the Quantum Limit of Plasmons

We record sequences of Raman spectra at a plasmonic junction formed by a gold AFM tip in contact with a silver surface coated with 4,4′-dimercaptostilbene (DMS). A 2D correlation analysis of the recorded trajectories reveals that the observable vibrational states can be divided into subsets, by virtue of the symmetry of DMS (C2h). The first set comprises the totally symmetric vibrations of DMS (ag) that are neither correlated with each other nor with the fluctuating background, assigned to the signature of charge-transfer plasmons mediated by DMS. The second set consists of bu modes, which are correlated both with each other and with the background. Our findings are rationalized on the basis of the charge-transfer theory of Raman scattering and illustrate how current carrying plasmons modulate the vibronic coupling terms from which the intensities of the bu states are derived. In effect, this study identifies gateway molecular modes for mediating charge shuttling across a plasmonic gap

Multimodal Tip-Enhanced Nonlinear Optical Nanoimaging of Plasmonic Silver Nanocubes

Optical field localization at plasmonic tip–sample nanojunctions has enabled high-spatial-resolution chemical analysis through tip-enhanced linear optical spectroscopies, including Raman scattering and photoluminescence. Here, we illustrate that nonlinear optical processes, including parametric four-wave mixing (4WM), second-harmonic/sum-frequency generation (SHG and SFG), and two-photon photoluminescence (TPPL), can be enhanced at plasmonic junctions and spatiospectrally resolved simultaneously with few-nm spatial resolution under ambient conditions. Through a detailed analysis of our spectral nanoimages, we find that the efficiencies of the local nonlinear signals are determined by sharp tip–sample junction resonances that vary over the few-nanometer length scale. Namely, plasmon resonances centered at or around the different nonlinear signals are tracked through TPPL, and they are found to selectively enhance nonlinear signals with closely matched optical resonances

A Closer Look at Tip-Enhanced Raman Chemical Reaction Nanoimages

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique that enables ultrahigh spatial resolution and ultrasensitive chemical imaging. This technique’s ability to track plasmon-induced/enhanced chemical reactions in real space has gained increasing popularity in recent years. In this study, we expose inherent difficulties associated with assigning TERS signatures that accompany chemical transformations. Namely, distinct selection rules as well as the possibility of multiple physical processes/chemical reaction pathways complicate spectral assignments and necessitate caution in assigning the experimental observables. We illustrate the latter using 4,4′-dimercaptostilbene-functionalized plasmonic silver nanocubes, wherein we identify the TERS signatures of product formation, molecular charging, multipolar Raman scattering, and preferred molecular orientations that all lead to distinct and assignable spectral patterns

Multimodal Tip-Enhanced Nonlinear Optical Nano-Imaging of Plasmonic Silver Nanocubes

Optical field localization at plasmonic tip-sample nanojunctions has enabled high spatial resolution chemical analysis through tip-enhanced linear optical spectroscopies, including Raman scattering and photoluminescence. Here, we illustrate that nonlinear optical processes, including parametric four-wave mixing (4WM), second harmonic/sum-frequency generation (SHG and SFG), and two-photon photoluminescence (TPPL), can be enhanced at plasmonic junctions and spatio-spectrally resolved simultaneously with few-nm spatial resolution under ambient conditions. More importantly, through a detailed analysis of our spectral nano-images, we find that the efficiencies of the local nonlinear signals are determined by sharp tip-sample junction resonances that vary over the few-nanometer length scale because of the corrugated nature of the probe. Namely, plasmon resonances centered at or around the different nonlinear signals are tracked through TPPL, and they are found to selectively enhance nonlinear signals with closely matched optical resonances

A Closer Look at Tip-Enhanced Raman Chemical Reaction Nanoimages

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique that enables ultrahigh spatial resolution and ultrasensitive chemical imaging. This technique’s ability to track plasmon-induced/enhanced chemical reactions in real space has gained increasing popularity in recent years. In this study, we expose inherent difficulties associated with assigning TERS signatures that accompany chemical transformations. Namely, distinct selection rules as well as the possibility of multiple physical processes/chemical reaction pathways complicate spectral assignments and necessitate caution in assigning the experimental observables. We illustrate the latter using 4,4′-dimercaptostilbene-functionalized plasmonic silver nanocubes, wherein we identify the TERS signatures of product formation, molecular charging, multipolar Raman scattering, and preferred molecular orientations that all lead to distinct and assignable spectral patterns

Tip-Enhanced Raman Nanographs of Plasmonic Silver Nanoparticles

Tip-enhanced Raman scattering (TERS) from plasmonic silver nanoparticles traces spatial variations in optical fields defined by the interaction of the plasmonic probe with nanoscale topographic features that are characteristic of crystalline particles. This is demonstrated through correlated atomic force microscopy (AFM)–TERS imaging of ∼100 nm silver nanoparticles coated with 4-mercaptobenzonitrile (MBN) molecules. In effect, the recorded spectral images are sensitive to the 3D topographic makeup of the particle and broadcast local optical fields that vary over a few nanometers of length scale

Imaging Charged Exciton Localization in van der Waals WSe₂/MoSe₂ Heterobilayers

Exciton localization in transition-metal dichalcogenide monolayers is behind a variety of interesting phenomena and applications, including broad-spectrum solar cells and single-photon emissions. Strain fields at the periphery of topographically distinct features such as nanoscopic bubbles were recently associated with localized charge-neutral excitons. Here, we use tip-enhanced photoluminescence (PL) to visualize excitons in WSe2/MoSe2 heterobilayers (HBL). We find strong optical emission from charged excitons, particularly positively charged trions, in HBL supported by interlayer charge transfer. Our results reveal strong trion confinement, with a localization length scale comparable to the trion size, at the apex region inside individual nanoscopic bubbles. Nano-PL mapping also shows sub-10-nm spatial variations in the localized trion emission spectra, which stem from atomic-scale potential energy fluctuations. These findings demonstrate the possibility of confining charged exciton complexes that are electrically tunable, opening up further opportunities to probe many-body exciton physics and to explore additional possible sites for strong exciton localization that can lead to quantum emission

Direct Visualization of Counter-Propagating Surface Plasmons in Real Space-Time

We deploy two-dimensional nanohole arrays as resonant surface plasmon polariton (SPP) couplers that enable counter-propagation and excitation field interference-free imaging of SPP wave packets. We monitor the spatiotemporal evolution of the resulting SPPs using two-color photoemission electron microscopy. The measurements track the electric field envelope of the SPP in real space and time and enable direct characterization of their spatiotemporal properties in a regime where the SPP wave packet is the principal observable. We provide an analysis of the observables for both the co- and counter-propagating directions via SPP trajectories that are recorded in tandem. Our results highlight the advantages of isolating SPPs through counter-propagation, where excitation field–SPP interactions are suppressed