  Ferroelectricity at room temperature has been demonstrated in nanometer-thin
quasi 2D croconic acid thin films, by the polarization hysteresis loop
measurements in macroscopic capacitor geometry, along with observation and
manipulation of the nanoscale domain structure by piezoresponse force
microscopy. The fabrication of continuous thin films of the hydrogen-bonded
croconic acid was achieved by the suppression of the thermal decomposition
using low evaporation temperatures in high vacuum, combined with growth
conditions far from thermal equilibrium. For nominal coverages >=20 nm, quasi
2D and polycrystalline films, with an average grain size of 50-100 nm and 3.5
nm roughness, can be obtained. Spontaneous ferroelectric domain structures of
the thin films have been observed and appear to correlate with the grain
patterns. The application of this solvent-free growth protocol may be a key to
the development of flexible organic ferroelectric thin films for electronic
applications.
