Two-dimensional ferroelectrics

Abstract

Blinov, L. M.; Fridkin, V. M.; Palto, S.P.; Bune, A. V.; Dowben, Peter A.; and Ducharme, Stephen, "Two-Dimensional Ferroelectrics" (2000). Abstract. The investigation of the finite-size effect in ferroelectric crystals and films has been limited by the experimental conditions. The smallest demonstrated ferroelectric crystals had a diameter of $ 200 A and the thinnest ferroelectric films were $ 200 A thick, macroscopic sizes on an atomic scale. Langmuir ± Blodgett deposition of films one monolayer at a time has produced high quality ferroelectric films as thin as 10 A, made from polyvinylidene fluoride and its copolymers. These ultrathin films permitted the ultimate investigation of finite-size effects on the atomic thickness scale. 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 a 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 in 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 phase transition associated with the twodimensional layers.

Surface structure of ultrathin copolymer films of ferroelectric vinylidene fluoride (70%) with trifluoroethylene (30%) on graphite

“…Furthermore, this first-order ferroelectric phase transition is almost independent of the film thickness in crystalline copolymer films, indicating the existence of two-dimensional ferroelectricity. 24,26,[35][36][37] A first-order surface ferroelectric phase transition was also observed at T C S =20°C, 25,29,31,36,37 attributed to the dimensionality effect associated with the lower coordination at the surface. 25,29 The spatial dimension of polarization control in thin films of P(VDF-TrFE) has decreased substantially in the past decade.…”

“…Local external electronic fields can affect the orientation of the monomer dipole moments of P(VDF-TrFE) films, 11,22,30,[35][36][37][38][39][40][41] thus resulting in local polarization and local switching (dipole reversal). 30 In previous experiments, the polarization alignment and conductance switching were achieved by changing the value and the polarity of the tipbias voltage.…”

“…The smallest region they could polarize was about 1 m. Similar results were obtained by poling and the imaging with an AFM. 22,35,[37][38][39] Kimura et al 40 obtained a size of 0.3 m ϫ 0.3 m polarized area. In 1998, Matsushige et al 41 reported that the ferroelectric domains of P(VDF-TrFE) could be formed locally with dimensions of about 30 nm in diameter by AFM.…”

Simulation of oxygen vacancy induced phenomena in ferroelectric thin films

“…Moreover, experiment has shown that ferroelectricity can still exist in the film of a few nanometers thick. 8 Thus, the assumption of a nonferroelectric interfacial is problematic. In our previous work, 9 instead of assuming a nonferroelectric layer, we proposed that this layer is still ferroelectric with parameters different from those of the interior region.…”