  We show that atomically thin molybdenum disulfide (MoS2) crystals can grow
without any underlying substrates into free-standing atomically-thin layers,
maintaining their planar 2D form. Using this property, we present a new
mechanism for 2D crystal synthesis, i.e. reagent-limited nucleation near an
aperture edge followed by reactions that allow crystal growth into the
free-space of the aperture. Such an approach enables us, for the first time,
the direct and selective growth of freestanding membranes of atomically thin
MoS2 layers across micrometer-scale pre-fabricated solid-state apertures in
SiNx membranes. Under optimal conditions, MoS2 grows preferentially across
apertures, resulting in sealed membranes that are one to a few atomic layers
thick. Since our method involves free-space growth and is devoid of either
substrates or transfer, it is conceivably the most contamination-free method
for obtaining 2D crystals reported so far. The membrane quality was
investigated using atomic-resolution transmission electron microscopy, Raman
spectroscopy, photoluminescence spectroscopy, and low-noise ion-current
recordings through nanopores fabricated in such membranes.
