Two-dimensional ferroelectrics

The investigation of the finite-size effect in the ferroelectric crystals and films has been limited by the experimental conditions. The smallest demonstrated ferroelectric crystals had diameter similar to 200 Angstrom and the thinnest ferroelectric films were similar to 200 Angstrom thick, macroscopic sizes on an atomic scale. Langmuir - Blodgett deposition of films one monolayer at a time has resulted in high quality ferroelectric films as thin as 10 Angstrom, made from poly(vinylidene fluoride) and its copolymers. These 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 these materials by demonstrating that there is no so called critical thickness; films as thin as two monolayers (1 nn) are ferroelectric with transition temperature near that of the bulk material. The films exhibit all the main properties of ferroelectricity with a first-order ferroelectric-paraelectric phase transition: polarization hysteresis (switching); the jump in spontaneous polarization at the phase transition temperature; thermal hysteresis in the polarization; the increase of the transition temperature with applied held; double hysteresis above the phase transition temperature; the existence of the ferroelectric critical point. The films also exhibit a new phase transition associated with the two-dimensional layers.