Two-dimensional ferroelectrics

Abstract

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 ∼ 200 Å and the thinnest ferroelectric films were ∼ 200 Å 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 Å, 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 nm) 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 field; 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.