EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

2D crystals like graphene and transition metal dichalcogenides are stable because of their fully saturated in-plane covalent bonds. Another way to stabilize crystals in a low-dimensional conformation is to restrict the inclusion of atoms at the crystal edges via kinetic barriers. Such systems that include for instance metal nanowires grown inside carbon nanotubes and molecular crystal in few-layer graphene encapsulation are intrinsically metastable but can continue to exist if the barriers remain unchanged.

Here we demonstrate two distinct crystals of the latter type that have been assembled within the van der Waals gap of two graphene monolayers that prevent the crystal expansion along the normal direction of their basal plane. Their structure is fully characterized via aberration corrected scanning transmission electron microscopy (STEM) and dynamics are observed at the atomic resolution. The first example is a hexagonally close packed monolayer of C$_{60}$ fullerene molecules grown by vacuum evaporation on free-suspended graphene and subsequent encapsulation in ambient conditions. The encapsulated C$_{60}$ molecules exhibit an anomalous, truncated intermolecular spacing of 9.6 Å but retain their rotational degree of freedom at room temperature. The second example is a more complex, trigonal 2D copper-iodine (CuI) crystal created via hydrogenation of Cu-intercalated graphene oxide by using hydrogen iodide. The experimental lattice parameters unexpectedly match well with the density functional theory model of a similar structure without graphene encapsulation. Finally, despite the occasional edge-reconstructions, rotational translations and the appearance of iodine vacancies, their basal plane retains integrity during the STEM experiments under a 60 keV electron beam.