  Van der Waals atomic solids of noble gases on metals at cryogenic
temperatures were the first experimental examples of two-dimensional systems.
Recently such structures have also been created on surfaces under encapsulation
by graphene, allowing studies at elevated temperatures through scanning
tunneling microscopy. However, for this technique, the encapsulation layer
often obscures the actual arrangement of the noble gas atoms. Here, we create
Kr and Xe clusters in between two suspended graphene layers, and uncover their
atomic structure through direct imaging with transmission electron microscopy.
We show that small crystals (N<9) arrange as expected based on the simple
non-directional van der Waals interaction. Crystals larger than this show some
deviations for the outermost atoms, possibly enabled by deformations in the
encapsulating graphene lattice. We further discuss the dynamics of the clusters
within the graphene sandwich, and show that while all Xe clusters with up to
N~100 remain solid, Kr clusters with already N~16 turn occasionally fluid under
our experimental conditions with an estimated pressure of ca. 0.3 GPa. This
study opens a way for the so-far unexplored frontier of encapsulated
two-dimensional van der Waals solids with exciting possibilities for
fundamental condensed matter physics research and possible applications in
quantum information technology.
