Two-dimensional ferroelectrics

The minimum size for a ferroelectric crystal has not been demonstrated expe rimentally because of difficulties in making thin films or small particles of sufficient quality. Langmuir-Blodgett deposition of crystalline films of vinylidene fluoride copolymers has produced high quality ferroelectric fil ms as thin as 1 nm. These films permit the ultimate investigation of the fi nite-size effects on the atomic thickness scale and also reveal the fundame ntal two-dimensional character of ferroelectricity in these materials. They exhibit the main properties of ferroelectricity: a first-order ferroelectr ic-paraelectric phase transition; polarization hysteresis (switching); a ju mp in spontaneous polarization at the phase transition temperature; thermal hysteresis in the polarization; increase of the transition temperature wit h applied field; double hysteresis above the phase transition temperature; and the existence of the ferroelectric critical point. The films also exhib it a new surface-layer phase transition.