Direct, in-situ spatial imaging of stacking order in van der Waals heterostructures*

The optical and electronic properties of van der Waals (vdW) heterostructures depend strongly on the atomic stacking order of the constituent layers. This is exemplified by Mott insulator states in ABC-stacked graphene, and topologically protected states along AB/BA boundaries in bilayer graphene. Moreover, periodic variation of local atomic registry, known as moiré patterns, has given rise to superconductivity in twisted bilayer graphene and exotic exciton states in TMD heterobilayers. However, the nm-scale moiré structure is typically indirectly deduced, because the conventional imaging technique, transmission electron microscopy (TEM), requires sample preparation that is incompatible with most optical and transport measurements. We present a method to directly image the local stacking order in complete vdW devices, including hBN-encapsulation, top- and bottom- graphite gates, and standard Si-substrates. Using this method, we demonstrate imaging of reconstructed moiré patterns in stacked TMDs, ABC/ABA stacking order in graphene multilayers, and AB/BA boundaries in bilayer graphene. Furthermore, we show that optical properties of the TMD are conserved after imaging, enabling correlation of local stacking order with optical and electronic properties.