Ferroelectric Langmuir-Blodgett Research Articles

Large-area atomic-smooth polyvinylidene fluoride Langmuir-Blodgett film exhibiting significantly improved ferroelectric and piezoelectric responses

Large roughness and structure disorder in ferroelectric ultrathin Langmuir-Blodgett (LB) film results in severe space scatter in electrical, ferroelectric and piezoelectric characteristics, thus limiting the nanoscale research and reliability of nano-devices. However, no effective method aiming at large-area uniform organic ferroelectric LB film has ever been reported to date. Herein, we present a facile hot-pressing strategy to prepare relatively large-area poly(vinylidene fluoride) (PVDF) LB film with ultra-smooth surface root mean square (RMS) roughness is 0.3 nm in a 30 μm × 30 μm area comparable to that of metal substrate, which maximized the potential of LB technique to control thickness distribution. More importantly, compared with traditionally annealed LB film, the hot-pressed LB film manifests significantly improved structure uniformity, less fluctuation in ferroelectric characteristics and higher dielectric and piezoelectric responses, owing to the uniform dipole orientation and higher crystalline quality. Besides, different surface charge relaxation behaviors are investigated and the underlying mechanisms are explained in the light of the interplay of surface charge and polarization charge in the case of nanoscale non-uniform switching. We believe that our work not only presents a novel strategy to endow PVDF LB film with unprecedented reliability and improved performance as a competitive candidate for future ferroelectric tunnel junctions (FTJs) and nano electro mechanical systems (NEMS), but also reveals an attracting coupling effect between the surface potential distribution and nanoscale non-uniform switching behavior, which is crucial for the understanding of local transport characterization modulated by band structure, bit signal stability for data-storage application and the related surface charge research, such as charge gradient microscopy (CGM) based on the collection of surface charge on the biased ferroelectric domains.

Characterisation of ferroelectric poly(vinylidene fluoride–trifluoroethylene) film prepared by Langmuir‐Blodgett deposition

Ferroelectric polymer is a flexible memory material that is insensitive to environmental variations and lends itself to research in emerging optoelectronic applications. Using Langmuir-Blodgett (LB) deposition technology, the thickness of a ferroelectric film can be controlled in the nanometre range, offering a pathway to molecular electronics. In this reported work, ultrathin ferroelectric polymer films were fabricated through LB deposition technology and characterised by observing the surface morphology, crystallinity and polarisation–voltage relationships. Unlike previously reported ferroelectric LB films, this work has shown a maximum remanent polarisation (PR) of 6 μC/cm2 at 35 nm, which is compatible with the thick film prepared by the spin-coating method. In addition, the polarisation stability in terms of depolarisation was investigated, which showed that the engineering of the interface between the ferroelectric LB film and the substrate is an important, deterministic factor for reliable memory applications with high signal-to-noise ratios.

SWITCHING AND DIELECTRIC RELAXATION IN NANOSTRUCTURE FERROELECTRICS FROM LANGMUIR-BLODGETT COPOLYMER FILMS

ABSTRACT The switching kinetics and dynamic dielectric measurements were carried out for multilayered ferroelectric Langmuir-Blodgett (LB) films based on vinylidene fluoride-trifluoroethylene copolymers P(VDF-TrFE). The switching kinetics of ultrathin films with thickness of 5–15 nm is well described by Landau-Khalatnikov equation and shows the critical behavior. Debye type dielectric relaxation has been revealed and investigated in the ferroelectric phase of LB films. The temperature dependence of relaxation time τ has a critical slowing-down phenomenon near the phase transition. The values of τ and damping coefficient ξ obtained from dielectric relaxation are by many orders of value smaller then the values of switching time and ξ characterizing the ferroelectric switching of ultrathin films.

Computational Nanostructures and Physical Properties of the Ultra-Thin Ferroelectric Langmuir-Blodgett Films

Computational molecular nanostructures and transition physical properties of the ultra-thin ferroelectric (FE) Langmuir-Blodgett (LB) films, based on polymer poly(vinylidene fluoride) PVDF and its copolymers with trifluoroethylene TrFE (CH2-CF2:CHF-CF2) were explored theoretically using ab initio quantum-chemical calculations (by HyperChem and Gaussian-98 code). As result the computational models for different molecular conformations were constructed and switching phenomena of FE LB films were explored. These results correspond to known experimental data for switching phenomena of ultra-thin FE LB films and clarify the nature of the ferroelectricity on the molecular level for these new nano-materials.

Switching time dispersion and retention of bistable states in Langmuir-Blodgett ferroelectric films

Polarization switching and retention of each of the two polarized states in Langmuir—Blodgett (LB) ferroelectric films are studied using nonlinear dielectric spectroscopy. It is found that polarized states can be preserved for a long time, but the polarization dynamics in 10–40-nm-thick LB films is characterized by a considerable switching-time dispersion. In addition, ferroelectric LB films exhibit clearly manifested asymmetry of switching to states with opposite directions of polarization. To explain the experimental results, a polarization-switching model is proposed that takes into account the energy of interaction of a ferroelectric polymer with boundary surfaces. The effect of inhomogeneity of the LB film structure on the ferroelectric switching dynamics is also discussed.

Ferroelectricity of weak-polar organic molecules in alternate Langmuir-Blodgett multilayer films

Ferroelectricity in Langmuir-Blodgett (LB) films (Z-type and alternated Y-type) of nitrogencrown (NC) and arachidic acid (AA) was investigated by electric polarization measurements and surface pressure-molecular area π(π-A) isotherm. For Z-type AA LB multilayer and the AA LB films incorporating barium and manganese ions, as well as for Z-type NC LB multilayer and the Y-type NC/AA alternating LB films, typical hysteresis loops of the samples were observed and confirmed existence of the remnant polarizations in the LB films. Their corresponding remnant polarizations were found to be 15, 180, 650, 450 and 9400 μC · m−2, respectively. Therefore, it was demonstrated that the enhancement of ferroelectricity in the LB films could be created by incorporation of barium and manganese ions into the LB film structures and strong interaction between alternating monolayers of the samples. These novel approaches can be employed to create strong ferroelectric LB films to make ferroelectric thin-film device for practical applications.

Surface ferroelectric phase transition in multilayer polymer Langmuir films

The ferroelectric phase transition on the free surface of a polymer ferroelectric Langmuir-Blodgett film was studied by the optical second harmonic generation (SHG) technique. A hysteresis in the temperature dependence of the SHG intensity observed for a multilayer film of a poly(vinylidene fluoride)-trifluoroethylene copolymer in the vicinity of T≈15°C is a manifestation of the first-order ferroelectric phase transition in the topmost surface monolayer of the film.

Two-dimensional ferroelectricity and phase transitions in PVDF Langmuir-Blodgett films probed by second harmonic generation

Ultrathin electrode-free ferroelectric Langmuir-Blodgett (LB) films of polyvinylidene fluoride (PVDF) with trifluoroethylene are experimentally studied by means of optical second harmonic generation (SHG) in a temperature range from 140 K to 380 K. The temperature behavior of spontaneous polarization induced SHG reveals distinct features in the vicinity of 355 K and 295 K which can be attributed to ferroelectric-paraelectric phase transitions in the bulk and at the surface of this two-dimensional structures. Lattice-stiffening transition recently observed in PVDF LB films at 160 K does not show remarkable peculiarities of temperature dependence of SHG that indicates nonpolar changes in LB film structure under this transition.

Two-dimensional ferroelectricity and second harmonic generation in PVDF Langmuir–Blodgett films

Two-dimensional (2D) ferroelectricity and 2D ferroelectric–paraelectric phase transitions are experimentally studied, for the first time, by means of optical second harmonic generation (SHG) in ultrathin Langmuir–Blodgett (LB) films of vinylidene fluoride with trifluoroethylene. The temperature dependence of the SHG intensity confirms the existence of two types of 2D ferroelectric–paraelectric phase transition: the first one in the topmost layer of LB films and the other, almost thickness independent, in the bulk of ferroelectric LB films.

Ferroelectric Langmuir-Blodgett films

Abstract The ferroelectric Langmuir-Blodgett films are prepared and investigated for the first time The films are prepared from the ferroelectric copolymer of vinylidene fluoride with trifluorethylene The films with the thickness off 150 Å show the pyroelectric effect being measured along the polar C-axis perpendicular to the substrate surface, partial remnant polarization switching and ferroelectric phase transition of the first order, characterized by the temperature hysteresis