Broadband dielectric dispersion in ferroelectric P(VDF-TrFE) copolymer films

Ferroelectric polyvinylidenefluoride-trifluoroethylene copolymer films with different thicknesses are prepared by a solvent-cast technique, by spin coating, and by a horizontal Langmuir–Blodgett technique respectively. The frequency dependent dielectric permittivity of these films is investigated with varying sample thickness and varying temperature in the ferroelectric as well as in the paraelectric phase. A dielectric relaxation according to a Vogel–Tamman–Fulcher law of the relaxation times is found in all samples. However, the relaxation times extracted from the dielectric permittivity in the frequency range are not consistent with the relaxation times determined from the temperature range. An explanation for this behavior is given by a temperature dependent distribution of relaxation times. Additionally, in thin samples a second relaxation with a weak anomalous temperature dependence, i.e. an increasing relaxation time with increasing temperature, is observed at high frequencies. Detailed investigations show that this behavior can be attributed to an electrode effect.     

Exchange coupling in P(VDF-TrFE) copolymer based all-organic composites with giant electrostriction

In this Letter, we discuss the dramatically enhanced electrostriction and dielectric constant in an all-organic composite consisting of polyvinylidene fluoride trifluoroethylene [P(VDF-TrFE)] copolymer matrix and copper-phthalocyanine (CuPc) particles, which could not be explained by traditional composite theory. Using a Landau-type potential energy combined with energy minimization, we demonstrate that the dramatic property enhancement is due to the exchange coupling between the dielectrically hard P(VDF-TrFE) and dielectrically soft CuPc, which becomes dominant when the heterogeneity size of the composite is comparable to the exchange length. The exchange coupling is a very effective mechanism for the enhancement of functional properties in ferroelectric and dielectric systems, and its variation with various material parameters is demonstrated and discussed.     

Dielectric responses of composite films based on P(VDF-TrFE) copolymer with TGS inclusions

The dielectric characteristics of film samples of P(VDF-TrFE) + TGS composite in the frequency range of 10³–10⁷ Hz are studied. The values of the real and imaginary parts of the complex dielectric constant in the temperature range of–40 to 140°C, including the points of the polymer matrix’s transition to the glassy state when T_g ≈–25°C and the ferroelectric phase transition. An analysis of the ferroelectric crystal inclusion effect on the dielectric response of P(VDF-TrFE) copolymer matrix is carried out.     

Effect of dielectric layer on ferroelectric responses of P(VDF-TrFE) thin films

The dielectric layer in the sandwich structural device plays a very important role in determining the electrical properties of the ferroelectric film. In this paper, we investigate the effect of the dielectric layers with different thicknesses on switching performance of ferroelectric P(VDF-TrFE) thin films. The hysteresis loops become slanting with increasing thickness of the dielectric layer. A negative slope of the ‘real’ hysteresis loop is apparently observed which demonstrates negative capacitance effect caused by the dielectric layer. This behavior is simulated qualitatively by the Weiss mean field model considering an interfacial dielectric layer in series with a ferroelectric layer. The agreement between experiments and simulations supports that negative capacitance results from the positive feedback among electric dipoles. Furthermore, the switching time of the ferroelectric film increases with the increase of dielectric layer thickness. This study shows that the ferroelectric sandwich structure provides great potential towards low power negative capacitance devices.     

Effect of misfit strain on the electrocaloric effect of P(VDF-TrFE) copolymer thin films

Based on the phenomenological Landau-Devonshire theory, we investigate the effect of misfit strain on the electrocaloric effect of P(VDF-TrFE) copolymer thin films. Theoretical analysis indicates that the compressive misfit strain reduces the working temperature to a great extent where the electrocaloric effect is maximized, which is different from the result of the conventional ferroelectric thin films, such as BaTiO₃. Although the compressive or tensile misfit strain does not change the maximum of the electrocaloric coefficient, the compressive misfit strain decreases the maximum of the adiabatic temperature change and the tensile misfit strain results in the opposite effect. Consequently, control of the misfit strain provides potential means to vary the working temperature for use in cooling systems.     

Crystalline morphologies of P(VDF-TrFE) (70/30) copolymer films above melting point

To further understand the crystallization behavior from melts of P(VDF-TrFE) (70/30) copolymer, the morphologies of P(VDF-TrFE) copolymer thin and ultra-thin films are studied by AFM. It is found that P(VDF-TrFE) (70/30) copolymer films exhibit morphologic transition from big grains to fiber-like rods above melting point. And this may be on account of the molecule chains plastic flow and self-organization, corresponding to polymer homoepitaxy.     

Atomic force microscopy of the supramolecular organization and strength properties of ultrathin polysiloxane block copolymer films

Ultrathin films of polysiloxane block copolymers and their composites with modifying additives of the C₆₀ fullerene have been studied using atomic force microscopy. It has been revealed that, independently of the concentration of the additives, the surface relief of the films has an ordered structure with a period of approximately 35 nm, which is associated with the presence of a spatial network of rigid block domains of ladder phenylsilasesquioxane in the block copolymer. The mechanical properties of the films have been determined from indentation tests of their surface layers. Reliable quantitative measurements have been performed with specially fabricated spherical indenters of the calibrated submicron radius of curvature. The obtained values of the strength parameters correlate with the data derived from standard physical and mechanical tests of thick films. It has been found that the addition of the C₆₀ fullerene at a level of 0.01% significantly improves the elasticity of the surface layers of the block copolymer.     

Atomic force microscopy study of thermal stability of silver selenide thin films grown on silicon

Silver selenide thin films were grown on silicon substrates by the solid-state reaction of sequentially deposited Se and Ag films of suitable thickness. Transmission electron microscopy and particle-induced X-ray emission studies of the as-deposited films showed the formation of single phase polycrystalline silver selenide from the reaction of Ag and Se films. Atomic force microscopy images of the as-deposited and films annealed at different temperatures in argon showed the film morphology to evolve into an agglomerated state with annealing temperature. The results indicate that when annealed above 473 K, silver selenide films on silicon become unstable and agglomerate through holes generated at grain boundaries.     

Imaging of ferroelectric vinylidene fluoride and trifluoroethylene copolymer films by scanning tunneling microscopy

In this paper, we reported the possibility to image non-conducting P(VDF-TrFE) copolymer films by STM. The films had the thickness of ∼25.0 nm and were spin-coated onto Au or graphite substrates. For films deposited on Au substrates, STM images showed grain structures of ∼100 nm, much larger than the grains of bare Au substrate. With increased scan rate, the film surface was damaged by STM tip and extreme protrusions and holes were observed. For films deposited on graphite substrates, we only obtained an image of very flat plane and could not observe the topography of the film surface. It seemed that the tip had pierced through the uppermost P(VDF-TrFE) layers and only imaged the layers nearest to the substrate. Asymmetrical current-voltage curves were observed from copolymer films deposited on HOPG. Experimental results were discussed.     

Abnormal polarization enhancement effects of P(VDF-TrFE) films during fatigue process

The effects of alternating electric field on the fatigue behaviors of poly(vinylidene fluoride-trifluoroethylene) copolymer films were investigated. The value of the remanent polarization (Pr) reached a maximum with the increase of cycle number of alternating electric filed, and then decreased as observed from the curve of polarization vs. switching cycles. It was found that the maximum point is associated with the frequency of alternating electric field. Dual effects, i.e., polarization enhancement and degradation, were supposed to coexist during the process of fatigue. A model considering the two effects was proposed to describe the fatigue behaviors, and the simulated data fit well with the experimental data.     

Crystallization kinetics in ferroelectric thin films: Viability of atomic force microscopy

The pyrochlore to perovskite phase transformation was studied in lead zirconate titanate (PZT) thin films. The films were fabricated on platinum electrodes and annealed by rapid thermal processing (RTP). The phases which formed and their location in the film were analyzed using glancing angle XRD and depth profiling was demonstrated. Grain size and structure, nucleation sites and surface morphology were determined with transmission electron microscop (TEM) and atomic force microscopy (AFM). The role of AFM in this type of transformation study was assessed.

The PZT films crystallized with a (100) orientation which was preferentially nucleated at the platinum/film interface. RTP at 650°C for 15 s was sufficient to complete the transformation. However, columnar grain growth and improvements in the ferroelectric properties were obtained with increased RTP time. A PZT film with RTP at 650°C for 1 min possessed a remanent polarization of 25 μC/cm² and a dielectric constant of ε = 650.     

Analysis of the structure of block copolymer films by atomic force microscopy

The structure of thin films of the polysterene-polymethylacrylate-polysterene triblock copolymer was studied. Universal algorithms to analyze atomic-force-microscopy images of thin block-copolymer films were developed.     

Atomic force microscopy enabled roughness analysis of nanostructured poly (diaminonaphthalene) doped poly (vinyl alcohol) conducting polymer thin films

The Atomic Force Microscopy (AFM) helps in evaluating parameters like amplitude or height parameters, functional or statistical parameters and spatial parameters which describe the surface topography or the roughness. In this paper, we have evaluated the roughness parameters for the native poly (vinyl alcohol) (PVA), monomer diaminonaphthalene (DAN) doped PVA, and poly (diaminonaphthalene) (PDAN) doped PVA films prepared in different solvents. In addition, distribution of heights, skewness and Kurtosis moments which describe surface asymmetry and flatness properties of a film were also determined. At the same time line profiles, 3D and 2D images of the surface structures at different scanning areas i.e. 5 × 5 μm² and 10 × 10 μm² were also investigated. From the roughness analysis and the surface skewness and coefficient of Kurtosis parameters, it was concluded that for PVA film the surface contains more peaks than valleys and the PDAN doped PVA film has more valleys than peaks. It was also found that the PDAN doped PVA film with acetonitrile solvent was used for substrate in electronics applications because the film gives less fractal morphology. Thus, the AFM analysis with different parameters suggested that the PDAN doped PVA films are smooth at the sub-nanometer scale.     

Nanoscopic measurements of the electrostriction responses in P(VDF/TrFE) ultra-thin-film copolymer using atomic force microscopy

Atomic force microscopy (AFM) has been used as a new method to perform nanoscale measurements of the electrostriction coefficients in the lamellae structure of the ferroelectric P(VDF/TrFE) 73/27 copolymers. The result found shows that the electrostriction coefficient inside (in the middle of) the lamella crystals is 6×10^-19 (m²V^-2), which is three times larger than that at the boundary, 2×10^-19 (m²V^-2). To explain the dependence of the electrostriction coefficients with those two regions, some suggestions are proposed. By heat treatment at 140 °C during 2 h, the sample changed its morphology as well as its crystallinity; the amorphous phase is much reduced and the degree of the crystallinity inside the lamellae is higher than that in the border. Also, it is suggested that in the lamellae’s boundary the macromolecular chains come to an end, or one monolayer folds over the other layer. In this case, the electrostriction was suppressed due to the loss of surface energy in the lamellae’s boundary. The achievements will supply a guideline to develop new and better devices for electromechanical and actuator applications. Received: 23 June 2000 / Accepted: 23 August 2000 / Published online: 5 October 2000     

原子力显微镜研究DPPC磷脂多层膜结构与力学性能

应用原子力显微镜(AFM)首先研究了在蔗糖溶液中二棕榈酰磷脂酰胆碱(DPPC)磷脂双层膜的结构,分析了其力学性能;其次,研究了在纯水中、CaCl2溶液中的DPPC磷脂多层膜的结构特性和杨氏模量.实验结果表明,在CaCl2溶液中DPPC多层膜的水层厚度大于在纯水中厚度,在CaCl2溶液中多层膜的杨氏模量变小.     

Atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) of nanoscale plate-shaped second phase particles

The feasibility of accurately measuring the size and the volume fraction of nanoscale plate-shaped precipitates by atomic force microscopy (AFM) has been explored. For quantitative evaluations their unhandy geometry is conveniently described by superellipsoids. The experimental alloy Ni₆₉Co₉Al₁₈Ti₄ served as a model system: plate-shaped disordered γ-precipitates form in the L1₂ long-range ordered γ′-matrix. The results obtained by AFM are compared with those derived from transmission (TEM) and from high-resolution scanning electron microscopy (SEM). The agreement between the AFM and the TEM results is good. In spite of the low number of SEM images taken, the same holds for the SEM results. In addition, magnetic force microscopy was applied; its results are acceptable. The main advantages of AFM are (i) the numerical output for all three dimensions, (ii) the simplicity of its operation and (iii) the lower cost of the microscope itself. The first point allows the numerical AFM output data to be directly subjected to automated computer-based evaluations. All present experimental and evaluation procedures are also applicable to cube-shaped particles with rounded edges and corners as found, for example, in γ′-strengthened nickel-based superalloys.     

Factors affecting phase and height contrast of diblock copolymer PS-b-PEO thin films in dynamic force mode atomic force microscopy

Asymmetric PS-b-PEO block copolymer exhibits well-ordered cylindrical morphology with nanoscale domain sizes due to microphase separation. Since the PS and PEO blocks have large stiffness difference, this polymer system represents an ideal candidate for studies of the phase contrast behavior in atomic force microscopy (AFM). In this paper, PS-b-PEO films are investigated under different scanning conditions using two different atomic force microscopes. It is found that the phase contrast of the film can be well described in terms of energy dissipation, though the exact phase image may also depend on the scanning parameters (e.g., the repulsive versus attractive regimes) as well as the settings of the microscope. Height variation on sample surface does not have significant effect on phase contrast. However, in order to obtain true topography of the polymer film, care has to be taken to avoid damage to the sample by AFM. Under certain conditions, true topography can be obtained during the first scan in spite of the surface-damaging forces are used.     

Synthesis of ZnS nanoparticles on a solid surface: Atomic force microscopy study

In this work, zinc sulfide (ZnS) nanoparticles had been synthesized on DNA network/mica and mica surface, respectively. The synthesis was carried out by first dropping a mixture of zinc acetate and DNA on a mica surface for the formation of the DNA networks or zinc acetate solution on a mica surface, and subsequently transferring the sample into a heated thiourea solution. The Zn²⁺ adsorbed on DNA network/mica or mica surface would react with S²⁻ produced from thiourea and form ZnS nanoparticles on these surfaces. X-ray diffraction and atomic force microscopy (AFM) were used to characterize the ZnS nanoparticles in detail. AFM results showed that ZnS nanoparticles distributed uniformly on the mica surface and deposited preferentially on DNA networks. It was also found that the size and density of ZnS nanoparticles could be effectively controlled by adjusting reaction temperature and the concentration of Zn²⁺ or DNA. The possible growth mechanisms have been discussed in detail.