Two-dimensional ferroelectrics

The investigation of the finite-size effect in the ferroelectric crystals a nd films has been limited by the experimental conditions. The smallest demo nstrated ferroelectric crystals had diameter similar to 200 Angstrom and th e thinnest ferroelectric films were similar to 200 Angstrom thick, macrosco pic sizes on an atomic scale. Langmuir - Blodgett deposition of films one m onolayer at a time has resulted in high quality ferroelectric films as thin as 10 Angstrom, made from poly(vinylidene fluoride) and its copolymers. Th ese ultrathin films permitted the ultimate investigation of the finite-size effects on the atomic thickness scale. The Langmuir - Blodgett films also revealed the fundamental two-dimensional character of ferroelectricity in t hese materials by demonstrating that there is no so called critical thickne ss; films as thin as two monolayers (1 nn) are ferroelectric with transitio n temperature near that of the bulk material. The films exhibit all the mai n properties of ferroelectricity with a first-order ferroelectric-paraelect ric phase transition: polarization hysteresis (switching); the jump in spon taneous polarization at the phase transition temperature; thermal hysteresi s in the polarization; the increase of the transition temperature with appl ied held; double hysteresis above the phase transition temperature; the exi stence of the ferroelectric critical point. The films also exhibit a new ph ase transition associated with the two-dimensional layers.