  Two-dimensional materials, such as transition metal dichalcogenides (TMDCs),
have the potential to revolutionize the field of electronics and photonics due
to their unique physical and structural properties. This research presents a
novel method for synthesizing crystalline TMDCs crystals with < 10 nm size
using ultra-fast migration of vacancies at elevated temperatures. Through
in-situ and ex-situ processing and using atomic-level characterization
techniques, we analyze the shape, size, crystallinity, composition, and strain
distribution of these nanocrystals. These nanocrystals exhibit electronic
structure signatures that differ from the 2D bulk i.e., uniform mono and
multilayers. Further, our in-situ, vacuum-based synthesis technique allows
observation and comparison of defect and phase evolution in these crystals
formed under van der Waals heterostructure confinement versus unconfined
conditions. Overall, this research demonstrates a solid-state route to
synthesizing uniform nanocrystals of TMDCs and lays the foundation for
materials science in confined 2D spaces under extreme conditions.
