Single-molecule chemistry and spectroscopy by near-field light

Yousoo Kim^1*

¹Surface and Interface Science Laboratory, RIKEN, Wako, Japan

* Presenter:Yousoo Kim, email:ykim@riken.jp

Excitation of molecules triggers various important energy conversion processes, such as luminescence, photochemical reactions, and photovoltaics. Detailed understanding of the molecular excited states is crucial to develop organic energy conversion devices based on opto-electronic/opto-chemical processes. We developed a scanning tunneling microscope (STM) combined with optical systems both for photon detection and for optical illumination to investigate energy transfer, conversion and dissipation processes at the single molecule level. The localized surface plasmon (LSP) generated near the STM tip either by optical illumination or by applying bias voltage has recently been attracting great attention mainly as a novel light source to overcome diffraction limit of optical wavelength. In recent years, we have applied the LSP towards exploring novel chemical (catalytic) reaction and spectroscopy based on the interaction between the LSP and electronic/vibrational quantum states of a single molecule at the STM junction. In this talk, I will discuss two issues focusing on single-molecule chemistry and spectroscopy based on the molecular excitation by localized surface plasmon at the STM junction.
Exciting the electronic states of molecules leads to various important energy conversion processes, such as luminescence, photochemical reactions, and photovoltaic systems. Thus, detailed understanding of the electronically excited states of molecules is crucial to improve and develop organic energy conversion devices based on opto-electronic and/or chemical processes. Recently, we developed single-molecule absorption spectroscopy [1] with unprecedented sensitivity and spatial resolution, in which the LSP around the STM tip is utilized for molecular excitation with sub-atomic precision. Further, in combination with the already developed single-molecule luminescence spectroscopy, we successfully visualized energy transfers between two molecules [2].
We have also investigated single-molecule chemical dynamics on metal surfaces, which provides a deep insight into the surface chemical (catalytic) processes. The single-molecule investigation of surface chemical processes has been demonstrated and the behind mechanisms based on the excitation and deexcitation of molecular electronic states interacting with visible light [3] and LSP induced by optical illumination of the STM tip [4].

References:
[1]　 “Single molecule investigation of energy dynamics in a coupled plasmon-exciton system”, H. Imada, K. Miwa, M. Imai-Imada, S. Kawahara, K. Kimura, Y. Kim, Phys. Rev. Lett. 119 (2017) 013901, 1-6.
[2]　 “Real-space investigation of energy transfer in heterogeneous molecular dimers”, H. Imada, K. Miwa, M. Imai-Imada, S. Kawahara, K. Kimura, Y. Kim, Nature 538 (2016) 364-367.
[3]　 “Direct pathway to molecular photodissociation on metal surfaces using visible light”, E. Kazuma, J. Jung, H. Ueba, M. Trenary, Y. Kim, J. Am. Chem. Soc. 139 (2017) 3115-3121.
[4]　 “Real-space and real-time observation of a plasmon-induced chemical reaction of a single molecule”, E. Kazuma, J. Jung, H. Ueba, M. Trenary, Y. Kim, Science 360 (2018) 521-526.

Keywords: STM, Single-molecule spectroscopy, Single-molecule chemistry, surface, luminescence