Micron-scale Characterization of Exfoliated Graphene by Raman Spectroscopy*

Moiré quantum matter has emerged as a new tool for investigating the physics of strongly interacting systems. The most famous moiré system, twisted bilayer graphene (TBG), has been predicted and shown to exhibit both superconductivity and orbital ferromagnetism. The moiré patterns key to TBG devices are highly sensitive to both twist angle and strain, but the current fabrication processes are unable to control these parameters to the precision required, particularly when the 'magic angle' near 1.1 degrees is desired. Because the twist and strain parameters of most TBG devices cannot be tuned after fabrication, these devices are plagued by limited experimental repeatability.

TBG is commonly made by stacking exfoliated monolayer graphene (MLG). We describe a method to extract absolute values of uniaxial strain, biaxial strain, temperature, defect density, and charge carrier density with micron-level resolution over large areas of exfoliated MLG flakes using Raman spectroscopy. We characterize these parameters at successive stages of device fabrication, and in MLG exfoliated by differing techniques.