Electron Transfer Kinetics Using GHz Scanning Tunnelling Electrochemical Microscope

The electron transfer kinetics are fundamental for various applications such as supercapacitors, molecular electronics and transistors and is typically studied by techniques like Cyclic Voltammetry CV and Electrochemical Impedance Spectroscopy EIS [1]. Although these classical techniques inherit important information about electron transfer kinetics of the full ensemble of molecules, many questions related to the charge dynamics at the solid-electrolyte interface at nanoscale are still difficult to answer. Therefore, Radio Frequency Electrochemical Scanning Tunneling Microscopy (RF-ECSTM) has been developed as a non-invasive heterodyne capacitive sensing method for electrochemical application with the ability to measure electron transfer dynamics and CV locally with reliable sensitivity to the current down to a few attoamperes (10^{&lt;span style="font-size:10.8333px"&gt;-18&lt;/span&gt;} A) [2]. The techniques is based on conventional ECSTM, where we additionally apply GHz and kHz frequencies to the conductive tip and substrate, respectively, to extract the local impedances of the sample. Here we report on RF-ECSTM as a tool for investigation of the rate constant of electrochemical reaction on ferrocene monolayers self-assembled on a gold Au (111) substrate. Local EIS spectra were recorded at the oxidation potential and outside the redox window to discard any parasitic impedance contribution from which we extracted the local rate constant of the faradic reaction to be K_ET ≈ 6 x 10^-5s^-1. To conclude, RF-ECSTM is a versatile technique with the ability to measure the local K_ET at nanoscale with a minimum parasitic contribution comparing to other methods, it also provides a large spectrum of frequencies that might play an essential role in many applications i.e., molecular biophysics, electrochemical catalysis, sensors, transistors and batteries.<br/>[1] Eckermann, A.L., Feld, D.J., Shaw, J.A. and Meade, T.J., 2010. Electrochemistry of redox-active self-assembled monolayers. Coordination chemistry reviews, 254(15-16), pp.1769-1802.<br/>[2] Grall, S., Alić, I., Pavoni, E., Awadein, M., Fujii, T., Müllegger, S., Farina, M., Clément, N. and Gramse, G., 2021. Attoampere Nanoelectrochemistry. Small, p.2101253.