Modeling Scanning Electrochemical Cell Microscopy (SECCM) in Twisted Bilayer Graphene

Abstract

Twisted 2D-flat band materials host exotic quantum phenomena and novel moirépatterns, showing immense promise for advanced spintronic and quantum applications. Here, we evaluate the nanostructure−activity relationship in twisted bilayer graphene by modeling it under the scanning electrochemical cell microscopy setup to resolve its spatial moirédomains. We solve the steady state ion transport inside a 3D nanopipette to isolate the current response at AA and AB domains. Interfacial reaction rates are obtained from a modified Marcus−Hush−Chidsey theory combining input from a tight binding model that describes the electronic structure of bilayer graphene. High rates of redox exchange are observed at the AA domains, an effect that reduces with diminished flat bands or a larger cross-sectional area of the nanopipette. Using voltammograms, we identify an optimal voltage that maximizes the current difference between the domains. Our study lays down the framework to electrochemically capture prominent features of the band structure that arise from spatial domains and deformations in 2D flat-band materials. I nterfacial electrode−electrolyte reactions are of key importance in electrochemical devices, where they regulate the limits of operation and can enable better overall performance 1−5 by aiding faster and more efficient charge transfer. 2D flat-band systems are a promising class of materials for this purpose due to their tunable superconductivity and correlated electronic phases. 6−10 Significantly enhanced electron-transfer rates have been observed near the "magic" angles of twisted bilayer and trilayer graphene 11,12 (tBLG, tTLG), almost reaching to those of bulk graphite. In these systems, flat bands at the magic angle can coincide energetically with redox couple states and thus drastically increase electron-transfer rates. Such topological twist-angle defects can help tune properties of 2D materials for robust nanoelectrochemical devices and energy storage. 11,13,14 Twisted 2D materials also form spatial domains, e.g., AA, AB, and domain walls in tBLG, 15 that enclose unique band structure features. To determine the electron-transfer rates, recent studies utilize nanopipettes of ∼100 nm radius over the 2D surface, 11,12 effectively averaging the current signal from domains with much smaller length scales (∼5 nm). Zooming into these domains can provide clear evidence of novel electronic features like the flat bands inside the AA domains of tBLG. 15 One can observe real-space signatures of density of states (DOS) artifacts like the Van Hove Singularity (VHS) peaks and separation with the twist angle. Hence, a direct correlation of the electronic structure with the moirépattern, structural deformation, and strain fields 16 can be established. This correlation can be utilized to study the nanostructure− activity relationship in twisted graphene and to discern its spatial features through its electrochemical response. The scanning electrochemical cell microscopy (SECCM) is based on this working principle and has bee...