Two-dimensional ferroelectrics

The minimum size for a ferroelectric crystal has not been demonstrated experimentally 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 films as thin as 1 nm. These films permit the ultimate investigation of the finite-size effects on the atomic thickness scale and also reveal the fundamental two-dimensional character of ferroelectricity in these materials. They exhibit the main properties of ferroelectricity: a first-order ferroelectric-paraelectric phase transition; polarization hysteresis (switching); a jump in spontaneous polarization at the phase transition temperature; thermal hysteresis in the polarization; increase of the transition temperature with applied field; double hysteresis above the phase transition temperature; and the existence of the ferroelectric critical point. The films also exhibit a new surface-layer phase transition.