  Proton-transfer type molecular ferroelectrics hold great application
potential due to their large spontaneous polarizations, high Curie
temperatures, and small switching fields. However, it is puzzling that
preparation of quasi-2D films with macroscopic ferroelectric behaviors has only
been reported in few molecular ferroelectrics. To resolve this puzzle, we
studied the effect of microstructures on macroscopic ferroelectric properties
of 5,6-Dichloro-2-methylbenzimidazole (DC-MBI) films grown using
low-temperature deposition followed by restrained crystallization (LDRC)
method. We revealed a competition between dense spherulites and porous
microstructures containing randomly oriented nanograins in as-grown films.
Post-growth annealing at moderate temperature promotes the formation of
spherulites which leads to macroscopic ferroelectric polarization switching.
These results highlight microstructure density as a critical factor for
macroscopic ferroelectric properties, potentially resolving the puzzle for
absence of macroscopic ferroelectric behavior in molecules ferroelectric films.
We expect the approach for enhancing microstructure density offered in this
work to greatly advance fabrication of quasi-2D molecular ferroelectrics films
and to unlock their potential in device applications.
