The existence of sub-nanometer plasmonic hot-spots and their relevance in spectroscopy and microscopy applications remain elusive despite a few recent theoretical and experimental evidence supporting this possibility. In this Letter, we present new spectroscopic evidence suggesting that Angstrom-sized hot-spots exist on the surfaces of plasmon-excited nanostructures. Surface-enhanced Raman scattering (SERS) spectra of 4,4'-biphenyl dithiols placed in metallic junctions show simultaneously blinking Stokes and anti-Stokes spectra, some of which exhibit only one prominent vibrational peak. The activated vibrational modes were found to vary widely between junction sites. Such site-specific, single-peak spectra could be successfully modeled using single-molecule SERS induced by a hot-spot with a diameter no larger than 3.5 Å, located at the specific molecular sites. Furthermore, the model, which assumes the stochastic creation of hot-spots on locally flat metallic surfaces, consistently reproduces the intensity distributions and occurrence statistics of the blinking SERS peaks, further confirming that the sources of the hot-spots are located on the metallic surfaces. This result not only provides compelling evidence for the existence of Angstrom-sized hot-spots but also opens up the new possibilities for the vibrational and electronic control of single-molecule photochemistry and real-space visualization of molecular vibration modes.

Conspectus The initial observations of surface-enhanced Raman scattering (SERS) from individual molecules (single-molecule SERS, SMSERS) have triggered ever more detailed mechanistic studies on the SERS process. The studies not only reveal the existence of extremely enhanced and confined fields at the gaps of Ag or Au nanoparticles but also reveal that the spatial, spectral, and temporal behaviors of the SMSERS signal critically depend on many factors, including plasmon resonances of nanostructures, diffusion (lateral and orientational) of molecules, molecular electronic resonances, and metal–molecule charge transfers. SMSERS spectra, with their molecular vibrational fingerprints, should in principle provide molecule-specific information on individual molecules in a way that any other existing single-molecule detection method (such as the ones based on fluorescence, mechanical forces, or electrical currents) cannot. Therefore, by following the spectro-temporal evolution of SMSERS signals of reacting molecules, one should be able to follow chemical reaction events of individual molecules without any additional labels. Despite such potential, however, real applications of SMSERS for single-molecule chemistry and analytical chemistry are scarce. In this Account, we discuss whether and how we can use SMSERS to monitor single-molecule chemical kinetics. The central problem lies in the experimental challenges of separately characterizing and controlling various sources of fluctuations and spatial variations in such a way that we can extract only the chemically relevant information from time-varying SMSERS signals. This Account is organized as follows. First, we outline the standard theory of SMSERS, providing an essential guide for identifying sources of spatial heterogeneity and temporal fluctuations in SMSERS signals. Second, we show how single-molecule reaction events of surface-immobilized reactants manifest themselves in experimental SMSERS trajectories. Comparison of the reactive SMSERS data (magnitudes and frequencies of discrete transitions) and the predictions of SMSERS models also allow us to assess how faithfully the SMSERS models represent reality. Third, we show how SMSERS spectral features can be used to discover new reaction intermediates and to interrogate metal–molecule electronic interactions. Finally, we propose possible improvements in experimental design (including nanogap structures and molecular systems) to make SMSERS applicable to a broader range of chemical reactions occurring under ambient conditions. The specific examples discussed in this Account are centered around the single-molecule photochemistry of 4-nitrobenzenethiol on metals, but the conclusions drawn from each example are generally applicable to any reaction system involving small organic molecules.

The local temperatures of a metal nanostructure and its adsorbate carry essential information about the energy dissipation dynamics, calling for nanoscale thermometry techniques. Here we present a surface-enhanced Raman scattering (SERS) thermometry method providing an accurate local temperature of the adsorbates: we use the ratios of anti-Stokes (aS) and Stokes (S) SERS vibrational peaks at the limit of zero (0) probe laser intensity, extrapolated from the spectra acquired with varying laser intensities, as an internal reference of the spectrum− temperature correlation. This self-referencing removes most of the measurement bias and uncertainty created by the different electromagnetic enhancements in aS and S components of SERS spectra and enables reliable thermometry with an accuracy and a precision of <10 K. Using the method, we have quantified the photothermal heating of adsorbates on the surfaces of plasmon catalysts.

Recent studies on plasmon-assisted chemical reactions postulate that the hot electrons of plasmon-excited nanostructures may induce a non-thermal vibrational activation of metal-bound reactants. However, the postulate has not been fully validated at the level of molecular quantum states. We directly and quantitatively prove that such activation occurs on plasmon-excited nanostructures: The anti-Stokes Raman spectra of reactants undergoing a plasmon-assisted reaction reveal that a particular vibrational mode of the reactant is selectively excited, such that the reactants possess >10 times more energy in the mode than is expected from the fully thermalized molecules at the given local temperature. Furthermore, a significant portion (∼20%) of the excited reactant is in vibrational overtone states with energies exceeding 0.5 eV. Such mode-selective multi-quantum excitation could be fully modeled by the resonant electron-molecule scattering theory. Such observations suggest that the vibrationally hot reactants are created by non-thermal hot electrons, not by thermally heated electrons or phonons of metals. The result validates the mechanism of plasmon-assisted chemical reactions and further offers a new method to explore the vibrational reaction control on metal surfaces.

Interferometric scattering microscopy (iSCAT), the imaging of interference between scattered light from a nanoparticle and reflected light from a substrate, provides a sensitivity sufficient for single biomolecule detection. The imaged interference, however, does not provide a separate amplitude and phase of the scattered light, making quantitative or spectroscopic measurement difficult. We report a method to fully recover the scattering amplitude and phase from iSCAT: vertical (z) scanning of the objective lens across the focus adds a Gouy phase between scattered light and reflected light, modulating image contrasts. The zstack image analysis provides a highly accurate amplitude and phase of the scattered light. The method is validated by demonstrating that the recovered amplitude and phase spectra of a single gold nanoparticle reproduce well-known plasmon resonances. We also show that the phase spectra can be used to extract the damping parameter of the plasmon resonance and the coupling strength of the strongly coupled chromophore-nanoparticle assembly.