Development of a Fast in Situ Scanning Tunneling Microscope for Studies of Electrocatalyst Surfaces
Authors: Schroefel Fabian , Greve Matthias , Tarhouni Karsten , Magnussen Olaf M. 
Published: February 2024

The atomic-scale understanding of processes at the interface between solid electrodes and liquid electrolytes is of high importance for electrochemical energy storage and conversion. Electrochemical scanning tunneling microscopy (ECSTM) is a key technique for the investigation of these interfaces and as such, it has seen widespread use. However, the image acquisition in a conventional ECSTM is a rather slow process, requiring tens of seconds or minutes per image. To help understand the precise reaction mechanisms of atomic and molecular species at solid-liquid interfaces, their movement and interactions need to be resolved. For this, much higher imaging rates are necessary. High-speed STMs (video STMs) are capable of operating at rates >10 images per second, which is sufficiently fast to observe and quantitatively study a wide range of surface dynamic processes, e.g., surface diffusion and growth [1]. However, this technique has not been widely employed, mainly because of the instrumental requirements.
The development of an STM for fast in situ measurements poses a set of challenges, which require a carefully planned implementation. For operation in electrochemical environment, the potentials of both STM tip and sample need to be controlled and electrochemical currents at the tip need to be kept way below the tunneling current. Fast image acquisition requires a scanner with high mechanical stability to avoid the excitation of uncontrolled tip oscillations at its resonance frequencies. Additionally, high-bandwidth measurement and control electronics and a sophisticated control software with fast scan generation and data processing capabilities are essential. No commercial system that is capable of fulfilling these requirements exists up to now. For this reason, all existing video-rate STM studies have been performed with home-built setups that employ highly specialized hardware that is not easy to reproduce.
In this contribution, we present a new ECSTM developed and built in our group. The setup is based on a Nanonis SPM controller by SPECS and a custom scanner, bipotentiostat, and coarse approach control that were integrated into this system. We show example data and images to demonstrate the performance of the STM. Furthermore, we reveal the modifications employed to make it capable of video-rate imaging. These include a novel scanner design with two independent piezo stacks for slow and high-speed movements and a custom high-bandwidth preamplifier integrated into the scan head as close as possible to the tip. Fast data acquisition is realized by an FPGA-based control software, which features a user-friendly frontend and a backend with good performance even at high image acquisition rates. This software is designed to run alongside the commercial control software for the slow STM operation so that switching between the slow and fast imaging modes is as frictionless as possible. We will show preliminary test results of the early implementation of the fast imaging mode.
[1] O. M. Magnussen, Chem. Eur. J. 2019, 25, 12865.