Control of optical properties of single molecules by plasmonic nanostructures is an important issue in plasmonics and nanophotonics, particularly valuable for the development of molecular plasmonic devices and ultrasensitive high-resolution microscopic techniques. The nanocavity defined by the coinage-metal tip and substrate in a scanning tunneling microscope (STM) can provide highly localized and dramatically enhanced electrical fields upon appropriate plasmonic resonant tuning, which can modify the excitation and emission of a single molecule inside and produce intriguing new optoelectronic phenomena. In this talk, I shall demonstrate two STM-based phenomena related to single-molecule optical spectroscopy. The first phenomenon is single-molecule Raman scattering. The spatial resolution of tip enhanced Raman spectromicroscopy has been further driven down to a single chemical-bond scale. Such simultaneous energetically and spatially resolved capability for vibrational-mode mapping opens up a new route to determine molecular chemical structure by optical imaging at only a single molecule. The second phenomenon is single-molecule electroluminescence (SMEL). Following a brief discussion on the excitation mechanism of SMEL including upconversion electroluminescence, I shall demonstrate how sub-nanometer resolved electroluminescence imaging can be used to gain insights into the intramolecular vibronic coupling caused by the interaction between electronic and nuclear motions. The photon-map patterns for certain vibrational modes are dramatically different from that associated with the (0,0) pure electronic transition, indicating directly a significant contribution from the Herzberg-Teller coupling. By combining with theoretical analyses, such photon-map patterns can unveil in real space how molecular vibrations shape the molecular wavefunction and induce the electronic transition, giving rise to light via intensity borrowing from specific higher-lying states.

STM-based single-molecule optical spectroscopy

2019 Fall National Meeting

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