Photon-to-electron conversion in a single molecule investigated by Photon-STM

抄録

Given its central role in various light energy conversion systems, PET from photoexcited molecules has been widely studied using optical spectroscopy and photocurrent measurement. Detailed insights into this process as an initial process of photocurrent generation have been obtained using microscopic techniques that combine scanning probe microscopes (SPMs) and optical systems. These techniques have allowed photocurrent generation efficiency to be related to local molecular morphology on the nanoscale, far below the diffraction limit. However, their spatial resolution remains insufficient to distinguish individual molecules, and observed PET signals from the photoexcited molecule are often obscured by ensemble-averaging over inhomogeneous local structures. Since electron transfer between two substances depends on the direct overlap of their electron wavefunctions, atomic-scale geometric changes can profoundly affect the efficiency of the process. Consequently, it would be highly desirable to develop a photocurrent measurement technique with atomic spatial resolution that could reveal the fundamental physics governing the PET process.

In this study, we report atomic-scale visualization of photocurrent channels through the molecular orbitals of a single free-base phthalocyanine (FBPc) molecule [1] using a scanning tunnelling microscope (STM) combined with a tuneable laser (Figure) [2-4]. PET from the single FBPc molecule in the first excited state was distinctly detected via the photoinduced tunnelling current through the STM tip. Depending on the applied bias voltage, not only the direction but also the spatial distribution of the photocurrent changes markedly. The atomically resolved photocurrent images allowed us to discover multiple counterflowing photocurrent channels even at a voltage where the averaged photocurrent is almost zero. Moreover, we found direct evidence of competition between PET and photoluminescence (PL), and demonstrated the controllability of their branching ratio during energy relaxation by positioning the STM tip with three-dimensional, atomic precision. Our findings present that specific photocurrent channels can be selectively promoted or suppressed by tuning the coupling between molecular orbitals and metal wavefunctions, which provides a new perspective for improving the energy conversion efficiency by atomic-scale electronic and geometric engineering of molecular interfaces.

[1] M. Imai-Imada et al., Nature 603, 829 (2022).

[2] H. Imada, M. Imai-Imada et al., J. Chem. Phys. 157, 104302 (2022).

[3] H. Imada, M. Imai-Imada et al., Science 373, 95 (2021).

[4] Rafael B. Jaculbia, et al., Nat. Nanotechnol. 15, 105 (2020).