Nano-photocurrent mapping of twisted bilayer graphene

We report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene. We show that the photocurrent as a function of carrier density changes sign at twist-induced insulating states. We use this property to identify domains of varying local twist angle in our sample. Consistent with the photocurrent study, infrared nano-imaging experiments reveal optical conductivity features dominated by twist-angle dependent interband transitions. Nano-photocurrent imaging can be broadly applied to moiré superlattices occurring in various other van der Waals heterostructures.