Plasmonic metals are promising materials for investigating and initiating energetically unfavorable chemical reactions. Localized surface plasmons, which form after introducing an on-resonant light source to the metal, couple together to form enhanced electric fields near the metal’s surface, known as hot spots. In under a picosecond, the surface plasmon begins to decay and generates highly energetic hot carriers through various scattering processes. If an adsorbed molecule is near the surface, the carrier may transfer to the adsorbate and initiate a chemical reaction. While there have been several published demonstrations of plasmon-induced photochemical reactions, there is still a lack in the general understanding of the interactions between hot carriers and molecular adsorbates, such as the time-dependent reaction rates and the hot carrier transfer yield. Providing a quantifiable description of these hot carrier-molecular interactions will help increase the efficiencies of plasmon-driven multi-electron chemical reactions. Surface-enhanced Raman spectroscopy (SERS) is a useful technique to probe molecules adsorbed to a plasmonic metal’s surface due to the dramatic dependence of the signal magnitude on the electric field. Methyl viologen, a molecule with three readily-accessible redox states, is a promising molecular probe to quantify the rate and yield of plasmon-driven hot electron transfer. Here we observe the dynamics of single methyl viologen molecules when deposited at low concentration on the surface of metal film over nanosphere SERS substrates. Interestingly, the Raman signatures suggest that individual MV2+ molecules undergo both reduction and oxidation reactions between the MV●+ and MV0 states throughout prolonged laser irradiation, allowing for a determination of the lifetimes and quantum yields for each redox state at a single molecule level. Monitoring the molecular evolution during hot carrier transfer provides the necessary information to quantify the efficiency and reaction rates of plasmon-induced processes, which will help further advance the development and optimization of plasmonically-powered platforms for photocatalytic applications.

Monitoring plasmonic photocatalysis at the single molecule level with surface-enhanced Raman spectroscopy

255th National Meeting (2018)

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