Layer-dependent electromechanical response in twisted graphene moir\'e superlattices

TL;DR

This study investigates the layer-dependent electromechanical responses in twisted graphene moiré superlattices using LPFM, revealing different coupling mechanisms and potential for nanoscale engineering.

Contribution

It provides the first detailed comparison of electromechanical responses in tBLG and tMBG, identifying distinct mechanisms like flexoelectricity and piezoelectricity.

Findings

tBLG shows flexoelectric response near domain walls

tMBG exhibits piezoelectric behavior with higher coefficients

Distinct responses depend on stacking symmetry and layer number

Abstract

The coupling of mechanical deformation and electrical stimuli at the nanoscale has been a subject of intense investigation in the realm of materials science. Recently, twisted van der Waals (vdW) materials have emerged as a platform to explore exotic quantum states. These states are intimately tied to the formation of moir\'e superlattices, which can be visualized directly exploiting the electromechanical response. However, the origin of the response, even in twisted bilayer graphene (tBLG), remains unsettled. Here, employing lateral piezoresponse force microscopy (LPFM), we investigate the electromechanical responses of marginally twisted graphene moir\'e superlattices with different layer thicknesses. We observe distinct LPFM amplitudes and spatial profiles in tBLG and twisted monolayer-bilayer graphene (tMBG), exhibiting effective in-plane piezoelectric coefficients of 0.05 pm/V and 0.35 pm/V, respectively. Force tuning experiments further underscore a marked divergence in their responses. The contrasting behaviors suggest different electromechanical couplings in tBLG and tMBG. In tBLG, the response near the domain walls is attributed to the flexoelectric effect, while in tMBG, the behaviors can be comprehended within the context of piezoelectric effect. Our results not only provide insights to electromechanical and corporative effects in twisted vdW materials with different stacking symmetries, but may also show their potential for engineering them at the nanoscale.