Matt Poulsen

Nonvolatile memory element based on a ferroelectric polymer Langmuir-Blodgett film

We report the operation of a potential nonvolatile bistable capacitor memory element consisting of a metal gate, a 170 nm thick ferroelectric Langmuir–Blodgett film of vinylidene fluoride (70%) with trifluoroethylene (30%) copolymer, and a 100 nm thick silicon-oxide insulating layer, all deposited on an n-type silicon semiconductor substrate. The device exhibited clear capacitance hysteresis as the gate voltage was cycled between ±25 V, with a capacitance dynamic range of 8:1 and threshold voltage shift of 2.8 V. The results are in good agreement with the model of Miller and McWhorter [J. Appl. Phys. 72, 5999 (1992)].

Determination of the optical dispersion in ferroelectric vinylidene fluoride (70%)/trifluoroethylene (30%) copolymer Langmuir–Blodgett films

We report measurements of the optical dispersion in ferroelectric Langmuir–Blodgett films of polyvinylidene fluoride (70%)-trifluoroethylene (30%) copolymer, using variable-angle spectroscopic ellipsometry over a wide spectral range from infrared to ultraviolet. Film thickness averaged 1.78±0.07 nm per deposition layer for films ranging from 5 to 125 deposition layers as determined from multi-sample analysis. This deposition rate was consistent with capacitance measurements, yielding a dielectric constant of 9.9±0.4 normal to the film, by quartz microbalance measurements, and by atomic force microscopy.

Effects of annealing conditions on ferroelectric nanomesa self-assembly

We report the results of studies of the effects of annealing conditions on the morphology of ferroelectric nanomesas. The nanomesa patterns were fabricated by self-assembly from continuous ultra-thin Langmuir–Blodgett fi lms of copolymers of vinylidene fl uoride and trifl uoroethylene. Annealing in the paraelectric phase induced surface reorganization into disc-shaped ferroelectric nanomesas approximately 9 nm thick and 100 nm in diameter. Several factors affect the nanomesa dimensions, such as polymer composition, substrate material, deposition conditions, and annealing temperature. The height and diameter of the nanomesas both increase with increasing annealing temperature. Annealing studies in the ferroelectric–paraelectric coexistence region show that only the paraelectric phase is mobile. From this we conclude that the paraelectric phase supports a kind of plastic crystalline fl ow connected with dynamic disorder of the polymer conformation.

Comparison of the electronic structure of two polymers with strong dipole ordering

Two different polymers, with large local electric dipoles, are compared: copolymers of polyvinylidene fluoride with trifluoroethylene [P(VDF-TrFE, 70%:30%)] and polymethylvinylidenecyanide (PMVC). While the different local point group symmetries play a key role, both crystalline polymers exhibit intra-molecular band structure, though the Brillouin zone critical points differ.

Effects of an external electric field on the ferroelectric-paraelectric phase transition in polyvinylidene fluoride-trifluoroethylene copolymer Langmuir-Blodgett films

X-ray diffraction and capacitance measurements have been employed to study the structural and dielectric behavior of the ferroelectric-paraelectric phase transition under the influence of a large external electric field. The samples under study are ultrathin (15–100nm) Langmuir–Blodgett films of a copolymer of vinylidene fluoride (70%) with trifluoroethylene (30%) deposited on aluminum-coated silicon. In situ θ-2θ x-ray diffraction was used to measure the change in interlayer spacing perpendicular to the film surface, corresponding to the (110) direction and indicating that the polymer chains along (001) lie predominantly in the plane, while capacitance measurements were used to monitor the behavior of the dielectric constant of the film. Application of a large electric field, up to 265MV∕m, raises the phase transition temperature and can convert the nonpolar trans-gauche paraelectric phase to the polar all-trans ferroelectric phase in a reversible manner.

Investigation of Ferroelectricity in Poly(methyl vinylidene cyanide)

The ferroelectric and piezoelectric properties of newly synthesized polymer systems have been studied. To date PVDF and its copolymers P(VDF-TrFE) have provided the bulk of the knowledge pertaining to ferroelectricity in polymers. Recently, ultrathin ferroelectric films of P(VDF-TrFE) 70:30 have been fabricated using the Langmuir-Blodgett technique [4]. In this study, various new polymers have been synthesized by chemically altering the PVDF structure. This alteration was performed in order to enhance the amphiphilic nature of the polymer and thus improve the LB film quality and control. Various chemical groups have been used to replace the electropositive hydrogen and electronegative fluorine found in the traditional PVDF chemical structure, including Nitrile, Ester, and Methyl groups. In all cases the resulting chemical structure provides for a net dipole moment directed from the electronegative side of the monomer to the electropositive side. However, to obtain ferroelectricity these microscopic dipoles must first pack in a manner such that a reversible macroscopic dipole is obtained. Both structural and dielectric studies have been performed on a number of newly synthesized systems. The structural properties of these new materials were probed using both temperature-dependent x-ray diffraction and differential scanning calorimetry, while dielectric properties were investigated using electric field and temperature-dependent capacitance and polarization measurements. Communicated by Dr. George W. Taylor (Originally presented at the 10th European Meeting on Ferroelectricity; Cambridge; UK; August 3-8, 2003)

Langmuir-Blodgett films of polyethylene

The possibility to obtain surface layers on water and prepare solid multilayer Langmuir–Blodgett films of medium-density polyethylene is shown. The polymer film on water is stable, demonstrates a reversible surface pressure-area isotherm up to 15 mN/m, and can be deposited onto a substrate using the Langmuir–Blodgett technique in a wide range of surface pressures. The thickness of a single deposited layer is 5.1 nm on average. The dielectric and optical constants of multilayer films are near their bulk values. The films exhibit high dielectric strength of at least 200 MV/m.

Infrared spectroscopic ellipsometry study of vinylidene fluoride (70%)-trifluoroethylene (30%) copolymer Langmuir–Blodgett films

We report the studies of the molecular conformation and chain orientations through characterization of the vibrational modes in crystalline Langmuir–Blodgett films of the polyvinylidene fluoride/trifluoroethylene copolymer. The infrared spectra obtained by polarized reflectometry and ellipsometry showed that the ferroelectric phase has predominantly all-trans conformation and the paraelectric phase has predominantly alternating trans-gauche conformation, as in solvent-formed films of the same copolymer. The results showed that the polymer chains are predominantly parallel to the film plane with a random in-plane orientation and most of the ferroelectric phase vibrational mode behaviors are consistent with the published mode assignments. The ferroelectric phase optical dispersion curves in the infrared range were extracted from the data analysis based on a uniaxial model.

Mesoscopic structures in two-dimensional ferroelectric polymers

Abstract Unique insight into the nature of ferroelectricity is emerging from the study of the first two-dimensional ferroelectric films, which are made by Langmuir-Blodgett deposition of vinylidene fluoride copolymers. These films are the first truly two-dimensional ferroelectrics, with thickness-independent bulk ferroelectric properties and a separate surface phase transition. The films offer a unique system for the study the relationship between ferroelectric properties and mesoscopic structure. The structure at all length scales was probed using complementary techniques: X-ray and neutron diffraction, scanning tunneling microscopy, atomic force microscopy, electric field microscopy, low-energy electron diffraction, angle-resolved inverse photoemission spectroscopy, infrared ellipsometry, electron energy-loss spectroscopy, scanning electron microscopy, optical polarization microscopy. We also introduce new results from dynamic pyroelectric scanning microscopy.

Water Absorption and Subsequent Lattice Changes in the Crystalline Poly(Methyl Vinylidene Cyanide) and in the Ferroelectric Poly(Vinylidene Fluoride with Trifluoroethylene)

Water absorption on surfaces has been a major part of surface science for decades [1, 2], but there is not yet a corresponding level of understanding in how water (or indeed any adsorbate) interacts with polymer surfaces [3]. Unlike single-crystal metal surfaces, polymers are typically not very good electrical or thermal conductors and are notorious for having very heterogeneous surfaces [3], adding considerable impediments to the undertaking of otherwise standard surface science techniques. For example, segregation of one component [4–10] complicates attempts to prepare a reproducible surface. Yet, water absorption by polymers is not only a subject of considerable research, but also an issue with considerable industrial applications, such as gels, in vivo implants, and water-resistant coatings. Despite the complexities of polymer surface characterization, water has been identified as a cause of reorientation at polymer surfaces [11–14], including the surface structure of fluorinated polymers [15, 16]. Due to water’s strong dipole, it is not surprising that water absorption is also known to change the dielectric properties of polymers, including the ferroelectric copolymer poly(vinylidene fluoride with trifluoroethylene) [17–19]. Water adsorption and absorption on crystalline poly(methyl vinylidene cyanide) and polyvinylidene fluoride with 30% trifluoroethylene, P(VDFTrFE, 70:30), was examined by thermal desorption spectroscopy [20–22]. Two distinctly different water adsorption sites are identified: one adsorbed species that resembles ice and another species that interacts more strongly with the polymer thin film. The existence of the latter species is consistent with Xray diffraction studies of water absorbed into the bulk of copolymers of polyvinylidene fluoride with trifluoroethylene crystalline thin films poly(vinylidene fluoride-trifluoroethylene) (70:30), P(VDF-TrFE), have been previously investigated as water adsorption systems