Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices

摘要

By stacking various two-dimensional (2D) atomic crystals [1] on top of eachother, it is possible to create multilayer heterostructures and devices withdesigned electronic properties [2-5]. However, various adsorbates becometrapped between layers during their assembly, and this not only affects theresulting quality but also prevents the formation of a true artificial layeredcrystal upheld by van der Waals interaction, creating instead a laminate gluedtogether by contamination. Transmission electron microscopy (TEM) has shownthat graphene and boron nitride monolayers, the two best characterized 2Dcrystals, are densely covered with hydrocarbons (even after thermal annealingin high vacuum) and exhibit only small clean patches suitable for atomicresolution imaging [6-10]. This observation seems detrimental for any realisticprospect of creating van der Waals materials and heterostructures withatomically sharp interfaces. Here we employ cross sectional TEM to take a sideview of several graphene-boron nitride heterostructures. We find that thetrapped hydrocarbons segregate into isolated pockets, leaving the interfacesatomically clean. Moreover, we observe a clear correlation between interfaceroughness and the electronic quality of encapsulated graphene. This work provesthe concept of heterostructures assembled with atomic layer precision andprovides their first TEM images.