In this study, we achieved the first atomic-scale visualization ofphotocurrent channels in a single molecule, and a description of the mechanismof photon-to-electron energy conversion with an ultimate precision based on
the molecular orbitals [1-3].
Photoinduced electron transfer (PET) plays an essential role in light energy
conversion processes, such as photocurrent generation, photocatalysis, andphotosynthesis. The quest for an atomic-scale comprehension of PET has beena longstanding pursuit. However, the spatial resolution of conventional
microscopic techniques for photocurrent measurement has previously been
insufficient to resolve individual molecules, leaving the detailed mechanism
obscured at the atomic scale.

Overcoming the limitations of the conventional methods imposed by the diffraction limitof light, we dedicated many years to developing Photon-STM, a scanning tunneling microscope
combined with laser irradiation and photon detection systems. Our instrumentation has opened
up a previously unexplored research frontier in exciton nanoscience [1-6].

Recently, we upgraded Photon-STM by installing a narrow-width tunable
laser, enabling single-molecule laser nanospectroscopy with micro-electron volt
energy resolution [5-6]. Applying this new technique to efficiently excite amolecule below the STM tip, we have recently succeeded to enhance the spatial
resolution in photocurrent measurement, which lead to achieve the longstanding goal of visualizing photocurrent channels in a single molecule withatomic resolution [1-3].
In ref. [1], Photocurrent imaging and theoretical analysis reveal that thegeneration of photocurrent is decisively governed by the individual frontier
molecular orbitals. Moreover, our technique has allowed us to discover thatphotocurrent flows locally through multiple counterflowing photocurrent
channels, even at a voltage where the averaged photocurrent is zero. We also
demonstrated the controllability of the quantum efficiencies of photocurrent
generation and competing photoluminescence by tuning the moleculeelectrode distance. These results clearly indicate that selective promotion andsuppression of a specific de-excitation process is possible by an atomic-scale
design of a molecular interface. Our findings provide a new design strategy for
improving the efficiency of light energy conversion with organic devices. In
addition, our method provides a technical basis for unprecedented atomic-scalevisualization of excited states, which has the potential to revolutionize ourfundamental understanding of functional energy conversion processes in the
excited states.
We believe that these results will captivate a wide range of scientists
across various disciplines, including organic devices, molecular biology,
molecular spectroscopy, nanoscience and technology, quantum optics, andsurface science.
[1] M. Imai-Imada, H. Imada*, K. Miwa, et al., Nature 603, 829 (2022).
[2] 今井みやび, 金 有洙*, 化学 77, 21 (2022).
[3] 今井みやび*, フロンティア 5, 183 (2023). (Cover Article)[4] M. Imai-Imada, H. Imada, K. Miwa, et al., Phys. Rev. B 98, 201403(R) (2018).
[5] H. Imada*, M. Imai-Imada, K. Miwa, et al., Science 373, 95 (2021).
[6] H. Imada*, M. Imai-Imada, X. Ouyang, et al., J. Chem. Phys. 157, 104302 (2022).