Single Molecular Optoelectronics by STM

Zhenchao Dong

Hefei National Lab for Physical Sciences at Microscale

University of Science and Technology of China

Hefei, China



Control of optical properties of single molecules by plasmonic nanostructures is an important issue in nanoplasmonics and nanophotonics, particularly valuable for the development of molecular plasmonic devices and ultrasensitive high-resolution microscopic techniques. One fundamental issue here is to understand the interplay between molecular excitons and surface plasmons in nearby metallic nanostructures. While extensive research has been carried out to such end using photon-excited techniques, electron induced luminescence from the molecular junction of a scanning tunneling microscope (STM) can provide additional information on the excitation and decay of local electromagnetic modes and could thus offer insights into the nature of optical transitions and energy transfer between excitons and plasmons at the nanoscale.

In this talk, I shall demonstrate two STM-induced phenomena related to single molecular optoelectronics. The first is single molecular electroluminescence arising from the LUMO-HOMO transitions of a well-decoupled neutral porphyrin molecule by the excitation of highly localized tunneling electrons. The generation of molecule-specific fluorescence is found to depend on two crucial factors: (1) the strength of electronic decoupling to suppress fluorescence quenching and to align molecular energy levels at the interface, and (2) the spectral matching of the nanocavity plasmon resonance to the molecular vibronic transitions for enhancing and coupling the emission to the far field. These findings help to substantially deepen our understanding on the coupling and decay of electronic excitations in single molecular optoelectronics and may offer new strategies for the development of electrically driven organic point-light sources and nanoscale optoelectronic integration. The second phenomenon to be addressed in this talk is single-molecule Raman scattering. I shall demonstrate single-molecule Raman spectromicroscopy with unprecedented spatial resolution below 1 nm, resolving even the inner structure of a single molecule and its configuration on the surface. This is achieved through a nonlinear spectral-matching technique provided by the combination of plasmon enhanced Raman scattering with scanning tunneling microscopy. Our findings not only suggest that chemical identification and imaging can now go sub-molecular and sub-nanometer, but also offer a new avenue to study nonlinear optical processes and photochemistry at the single-molecule level.



[1] Z. C. Dong,* X. L. Zhang, H. Y. Gao, Y. Luo, C. Zhang, L. G. Chen, R. Zhang, X. Tao, Y. Zhang, J. L. Yang, J. G. Hou,* “Generation of molecular hot electroluminescence by resonant nanocavity plasmons” , Nature Photonics 4, 50-54 (2010).

[2] R. Zhang, Y. Zhang, Z. C. Dong*, S. Jiang, C. Zhang, L. G. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. Luo, J. L. Yang , and J. G. Hou*, "Chemical mapping of a single molecule by plasmon enhanced Raman scattering", Nature 498, 82 (2013).