Observation of moire superlattices on twisted bilayer graphene by scanning microwave impedance microscopy

摘要

In the emerging field of twistronics, new electronic devices based on bilayer graphene have shown distinct electronic properties that depend on the rotational misalignment of one crystalline layer with respect to another. Given present methods of preparing these bilayers, there is always some uncertainty in the actual versus targeted twist angle of a specific bilayer that can only be resolved by measuring the moire patterns that are unique to a specific twist angle. Traditional methods enabling such a measurement, Transmission Electron Microscopy and Scanning Tunneling Microscopy, impose serious restrictions on the types of substrates supporting the bilayers, which, in turn, constrains the subsequent fabrication of any devices. We report here a new, non-destructive method to measure moire patterns of bilayer graphene deposited on any smooth substrate, using the scanning probe technique known as scanning microwave impedance microscopy (sMIM) which enables the simultaneous generation of localized topography, capacitance and conductance images with nanometer scale resolution. Moire patterns were observed in samples prepared on various substrates with twist angles ranging from 0.02 to 6.7 degrees, beyond which the moire patterns are too small to be resolved by the sMIM probes. We present some possible reasons for the various contrast mechanisms. Addressing the problem of variations across a bilayer surface due to localized moire distortions that result from the tensile and shear forces involved in transferring a twisted bilayer to a substrate, we demonstrate how sMIM can precisely map the twist angle distribution across the film, and enable direct device and circuit routing.