The study of heterogeneous catalytic reactions remains a major challenge because it involves a complex network of reaction steps with various intermediates. If the vibrational spectra of individual molecules could be monitored in real time, one could characterize the structures of the intermediates and the time scales of reaction steps without ensemble averaging. Surface-enhanced Raman scattering (SERS) spectroscopy does provide vibrational spectra with single-molecule sensitivity, but typical single-molecule SERS signals exhibit spatial heterogeneities and temporal fluctuations, making them difficult to be used in single-molecule kinetics studies. Here we show that SERS can monitor the single-molecule catalytic reactions in real time. The surface-immobilized reactants placed at the junctions of well-defined nanoparticle-thin film structures produce time-resolved SERS spectra with discrete, step-transitions of photoproducts. We interpret that such SERS-steps correspond to the reaction events of individual molecules occurring at the SERS hotspot. The analyses of the yield, dynamics, and the magnitude of such SERS steps, along with the associated spectral characteristics, fully support our claim. In addition, a model that is based on plasmonic field enhancement and surface photochemistry reproduces the key features of experimental observation. Overall, the result demonstrates that it is possible, under well-controlled conditions, to differentiate the chemical and physical processes contributing to the single-molecule SERS signals, and thus shows the use of single-molecule SERS as a tool for studying the metal-catalyzed organic reactions.
We carried out the tip-enhanced Raman scattering (TERS) with a tip that is functionalized with a Aunanoparticle (AuNP, with a diameter of 250 nm). The AuNP tip is fabricated by a direct mechanical pickup of a AuNP from a flat substrate, and the TERS signal from the AuNP tip -organic monolayer -Au thin film (thickness of 10 nm) is recorded. We find that such a AuNP-tip interacting with a thin film routinely yields signal enhancement larger than ~10 4 , which is sufficient not only for local (with detection area of ~200 nm 2 ) Raman spectroscopy, but also the nanometric imaging of organic monolayers within a reasonable acquisition time (~20 minutes/image).
Thin film growth through nonclassical crystallization with charged nanoparticles as building blocks has been studied in chemical vapor deposition and physical vapor deposition. A similar mechanism might be applied to aerosol deposition (AD), with which the mechanism for the evolution of dense ceramic films at room temperature has not been clearly understood. Aerosol particles fracture into fragments of secondary nanoparticles during AD. In this work, secondary particles passing between the two parallel biased and grounded electrodes were captured by the transmission electron microscopy (TEM) grid membrane on each electrode, and it was revealed that they were mainly positively charged. Neutral secondary nanoparticles, not deflected by the electric field, produced a porous film on the silicon substrate, whereas charged secondary nanoparticles produced a dense film. The positive charge of the secondary nanoparticles appeared to enhance the plastic deformation required for the evolution of dense films during AD, which was supported by ab initio calculations.
The solar-powered electrochemical chlorine generation system for which design information is provided here is a simple and affordable way to produce chlorine with which to convert contaminated water into clean drinking water.
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