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.…”

“…29 The global ordering of similar films has been characterized using low-energy electron diffraction (LEED). 26,[29][30][31] STM measurements were done with the base pressure lower than 2 ϫ 10 −10 Torr. A sharp W tip was used to obtain atomic resolution image, reorient the dipole moment(s) and measure I-V curves.…”

“…Such films show the maximum spontaneous polarization P ϳ 0.1 C m −2 and large thermal hysteresis in thick films. 26 After being transferred into the ultrahigh vacuum (UHV) chamber, the sample was cleaned by gently annealing at ϳ420 K in an ultrahigh vacuum (UHV) chamber. The vacuum annealing at ϳ420 K not only cleaned the surface of the sample, but also increased the crystallinity of P(VDFTrFE) films.…”

“…[24][25][26][27]34,42 While the P(VDF-TrFE) polymer chains in the bulk and in the thin films may adopt a number of different configurations, the ferroelectric and paraelectric phases are exemplified by two different monomer arrangement configurations below or above the bulk phase transition temperature ͑T C B ͒, respectively. In the ferroelectric ␤ phase, below T C B , P(VDF-TrFE) chains adopt an all-trans (TTTT) configuration [ Fig.…”

“…Polymer of Vinylidene fluoride [P(VDF)] and random copolymers of vinylidene fluoride with trifluoroethylene [P(VDF-TrFE)] consist of monomers with large dipole moment and exhibit strong ferroelectric properties, even in the ultrathin film limit, drawing much attention because of their application to electromechanical transducers and nonvolatile memories. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] In the ferroelectric state, below the first-order ferroelectric phase transition temperature T C B =80°C, 17,18,23,33 the copolymer P(VDF-TrFE; 70%:30%) exhibits the properties typical of ferroelectricity, such as spontaneous electrical polarization, polarization reversal (switching), polarization hysteresis, etc. 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.…”