  Molecules intercalating two-dimensional (2D) materials form complex
structures that have been mostly characterized by spatially averaged
techniques. Here we use aberration-corrected scanning transmission electron
microscopy and density-functional-theory (DFT) calculations to study the atomic
structure of bilayer graphene (BLG) and few-layer graphene (FLG) intercalated
with FeCl$_3$. In BLG we discover two distinct intercalated structures that we
identify as monolayer-FeCl$_3$ and monolayer-FeCl$_2$. The two structures are
separated by atomically sharp boundaries and induce large but different
free-carrier densities in the graphene layers, $7.1\times10^{13}$ cm$^{-2}$ and
$7.1\times10^{13}$ cm$^{-2}$ respectively. In FLG, we observe multiple FeCl$_3$
layers stacked in a variety of possible configurations with respect to one
another. Finally, we find that the microscope's electron beam can convert the
FeCl$_3$ monolayer into FeOCl monolayers in a rectangular lattice. These
results reveal the need for a combination of atomically-resolved microscopy,
spectroscopy, and DFT calculations to identify intercalated structures and
study their properties.
