Mechanistic Study of Plasmon Driven Surface Chemistry and Quantum Plasmonics

In this research project, significant experimental advances have been achieved at the frontiers of plasmon enhanced photochemistry, plasmon nanocavity resonances and plasmon enhanced light emission. The absorption bands of model organic molecules have been determined based on a novel approach that involves extracting electronic transition bands of adsorbates from moleculeplasmon excitation coupling. In contrast to past speculations that new electronic states are formed when organic dyes are adsorbed on metal surfaces, our experimental results show that the absorption bands of pi-conjugated organic dyes has the general shape of the corresponding absorption bands in solution apart from expected peak wavelength red-shifting and spectral broadening depending on the strength of the surface-molecule interaction. Establishing the absorption band has been foundational for our wavelength dependent analysis of plasmon enhanced photochemistry. By comparing the photochemistry of a model molecule (methylene blue) on gold nanoparticles at five excitation wavelengths (561, 633, 671,785 and 808 nm) at different adsorption conditions, we have demonstrated that the reaction mechanism can be switched from charge transfer to adsorbate electronic excitation using surface ligands as metal-molecule spacer and dipole orienting scaffold. This has led to a new understanding that hot electrons may simply play a role of preparing anion intermediates that have accessible electronic transitions in the visible spectral region as the PI Habteyes has highlighted in a recent invited perspective article in the Journal of Physical Chemistry. This advance has built on our proposed mechanism that involves plasmon-pumped adsorbate intramolecular electronic excitation. New findings have also been reported in quantum plasmonics and plasmon enhanced light emission.