Atomic-Resolution Visualization of Complex Structures in Intercalated Bilayer Graphene*

Intercalation in two-dimensional materials has been widely investigated using spectroscopic, diffraction, and electrical measurements. However, these techniques generate spatially averaged data and provide information about the local atomic structure only inferentially. In this work, we deploy aberration-corrected scanning transmission electron microscopy to directly visualize the local atomic structure of FeCl₃-intercalated bilayer (BLG) and few-layer graphene (FLG). The data exhibit a crystalline monolayer of FeCl₃ inside BLG, atomically sharp intercalation boundaries, a variety of orientations for FeCl₃ monolayers in FLG, and regions where the iron is reduced to form monolayer FeCl₂. Our density-functional-theory calculations predict a low energy barrier of 0.04 meV/nm² between different orientations of FeCl₃, supporting the observation of multiple orientations. Furthermore, resonant-Raman spectroscopy yields evidence of two distinct graphene doping levels of E_F=0.98eV and E_F=1.06 in intercalated bilayer graphene, which may be attributed to the coexistence of FeCl₃ and FeCl₂. These results highlight the critical need for atomic-resolution studies of FeCl₃ and similar intercalants to understand the dynamics of doping in multilayer graphene and graphene superlattices.