Imaging double- moiré pattern in twisted bilayer graphene aligned on <i>h</i>-BN substrate

Moiré superlattices in van der Waals heterostructures have been a promising platform to explore emergent electronic phenomena. Here, we report the imaging experiment of a double moiré pattern in twisted bilayer-graphene that is aligned on h-BN substrate, using microwave impedance microscopy. Two graphene layers are directly grown on h-BN flakes using CVD method. The first layer grows directly on the h-BN substrate with a 0-degree alignment, while the second layer grows on top of the first one at a small twist angle. We resolve two sets of moiré patterns formed in this tri-layer heterostructure, a larger one (~15 nm) between the first graphene and h-BN and a smaller one between the two graphene layers.The smaller moiré pattern exhibits abrupt discontinuities at the boundaries of the larger moiré pattern. We analyze the parameters of the two moiré superlattices based on both a simple trilayer stacking model and FFT calculation, which agrees well with our experimental observations.