Single molecule measurements of fully quantum redox reactions at metal-molecule interface

We report on the measurements of the electron transfer events in the redox reaction at single molecule–metallic electrode interface, which exhibit the quantized nuclear state transitions mediated by an electron-vibron coupling. We use a sensitive electric charge sensing technique using an atomic force microscopy (AFM) cantilever as a mechanical resonator to detect the electron transfer events [1]. Ferrocene molecules tethered to C16 alkanethiol are adsorbed on a gold surface. We measure the electron transfer that takes place as single-electron tunneling through the C16 alkanethiol at 4 K in vacuum. The electron transfer events appear as the peaks in the bias voltage dependences of the resonance frequency and damping of the AFM cantilever. With increasing oscillation amplitude, the intensity of the observed peak increases and then exhibits a series of peaks which are indicative of vibronic excitation. The evolution of the peak shape can be explained by the single-electron tunneling model which takes into account the overlap integral of the nuclear wave functions of two charge states (Franck-Condon factor). The experimental spectra show a very good agreement with the theoretical ones. [1] Y. Miyahara, A. Roy-Gobeil and P. Grutter, Nanotechnology 28, 064001 (2017).