In Atomic Force Microscopy Study of Piezoelectric Polymers, the 23rd chapter of the book Applied Scanning Probe Methods XIII, the authors discuss the techniques to study the microstructure and properties of piezoelectric polymers by using Atomic Force Microscopy (also known as Scanning Probe Microscopy). This chapter aimed at the study of topography and properties of piezoelectric polymer using Atomic Force Microscopy (AFM). The study was conducted on polyvinylidene fluoride (PVDF), which is a unique polymer that has high pyroelectric and piezoelectric properties. Its high permittivity and relatively low dissipation factor makes it a potential candidate to useful in many applications. Piezoelectric polymer have advantage over piezoelectric ceramics for certain applications wherein acoustic impedence similar to that of water or living tissue is required.2 Due to this reason PVDF is increasingly used for medical and industrial applications. This motivated the authors who are actively involved in strategically studying advance materials to reveal principles in effects of electronic, chemical, mechanical, and tribological properties to study PVDF as well. The outcomes of these researches are in the development of novel nanofabrication processes and nanostructured materials that have the potential to be used as artificial joints, automobile components, micro sensors, and energy harvesting devices. They work in close association with industries to help them to solve corrosion and tribological problems.
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The piezoelectric effect was discovered in 1880 by Pierre and Jacques Curie. Piezoelectricity is an interaction between electrical and mechanical systems. The direct piezoelectric effect is that electric polarization is produced by mechanical stress. Closely related to it is the converse effect, whereby a crystal becomes strained when an electric field is applied. Both effects are manifestations of the same fundamental property of the crystal. Dielectric, elastic, and piezoelastic constants are the fundamental parameters describing piezoelectric phenomena.1 There are many widely used piezoelectric materials which includes crystals, ceramics and polymers. Polyvinylidene fluoride (PVDF) is a polymer, (-CH2-CF2-)n, that has crystallinity of 40-50%. PVDF is a semicrystalline polymer in which five crystallographic forms are observed with different conformations that is, TG+TG- in the α and δ phases, all-trans (TTT) planar zigzag in the β phase, and T3G-T3G+ in the γ and ε phases. These crystalline forms can transform to each other under specific conditions, such as under the application of mechanical deformation and high electrical field. The α-crystalline form doesnot show a net lattice polarization due to antiparallel TG+TG- chain arrangement in its unit cell. Therefore, the chain dipoles oppose each other. The β phase consists of parallel packing of the polymer chains in all-trans conformations, and thus, it shows a large spontaneous lattice polarization. Hence, out of all the phases, the polar β phase exhibits the strongest piezoelectric and pyroelectric properties and is the most desirable crystalline form for PVDF.5
The surface morphology, surface force measurements, nano-piezoelectricity, conductivity and time dependent phase transformations due to stress on PVDF were studied using close-contact or tapping mode AFM as it eliminates the problems associated with friction, adhesion, and electrostatic forces caused in contact-mode AFM. However, PVDF crystals and molecular chains were aligned by using contact-mode AFM.4
A brief review of the Atomic Force Microscopy techniques is provided in the beginning along with the two types of scanning modes, contact and close contact (or tapping/vibrating mode). Contact-mode is more useful for making clear topographical images of hard materials with low average roughness. In the contact mode, the repulsive force experienced by the tip is measured by recording the cantilever deflection and the deflection is usually measured by optical method. A close-contact or tapping mode is useful for phase detection and non-destructive imaging of soft materials without any damage to the material under study. In the tapping mode, the cantilever is excited to vibrate near its resonant frequency close to the sample surface. Once the approaching tip is in contact with the surface, the change of cantilever oscillation is reflected by the surface properties of the sample. AFM is also used for the measurement of lateral and adhesion force, electrostatic force, magnetic force, piezoresponse force, to name a few. The SPM is widely used for the study of ferroelectric materials. It is used to map polarization in a local domain. The spontaneous polarization, ferroelectric phase transition, and pyroelectric change were observed through SPM in thin film ferroelectric polymers. The authors cited the work where the orientation of PVDF crystals and molecules were controlled by using aligning technique developed for polymer crystals and molecular chains using contact-mode AFM. Lamellar crystals of poly(vinylidenefluoride-trifluoroethylene) thin films were aligned to the scan direction by scanning the film surface using an AFM cantilever tip at the temperature of 70-100°C and the molecular chains were aligned by scanning at a higher temperature(135°C).4
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In this particular study, PVDF thin films were prepared by mixing granular PVDF with acetone (80ml) and dimethylsulfoxide (DMSO) solutions (20ml). The concentration was set at 40, 60, 80, and 100 g/L each for various viscosities. The solution was then heated and stirred at 40°C for about 30 min till all solid particles were dissolved completely. The solution was then spin-coated at the speed of 3000 rpm for 20 s. Samples were then annealed at 23, 40, 60 and 80°C with corona poling of 30kV for 2 min. Different viscosities of PVDF solutions were spin-coated on a gold-coated silicon substrate at different speeds. Films were then heated at various temperatures before, during and after 30min with in-situ corona poling a 30kV for 2 min. Using wide-angle X-ray diffraction (WAXD) and Fourier Transform Infrared Spectroscopy (FTIR), microstructural analysis was done. The diffraction lines of the samples were obtained using WAXD and the identification of the crystalline phases present in the samples was done by means of an FTIR spectrometer. FTIR accurately measured the variation of the β phase in the polymer films and the fraction of β-phase crystals in each sample was calculated according to the specific absorption bands of the different phases.5 Surface image analysis, surface force measurements, piezoelectricity, conductivity measurement and time dependent phase transformation study were conducted on the 4 samples prepared.
Surface image analysis was done that includes morphology and phase image, which were obtained using AFM. The Electrostatic force microscope (EFM) images were also acquired and the electrical outputs due to electrical force gradients were obtained. As adhesion and friction can readily destroy the electrical connections during operation of MEMS (Microelectromechanical systems) and NEMS (Nanoelectromechanical systems), therefore surface force measurements are very important. The adhesion forces for the four samples were measured by the force-displacement curve as shown in Figure 1 under the contact mode using Lateral force microscope (LFM). In the figure, the probe is approaching from A to C with increasing attractive force and detaching from C to D due to the repulsive force that caused a sudden pull-off from D to E. The distance between B and D is caused by adhesion force. Adhesion force was calculated using the equation:
Fadhesion= k * âˆ†x
where k is the spring constant (nN/nm) of the AFM probe.
Figure1. Force-displacement curve used for the measurement of adhesion force
The friction values of the PVDF samples were measured using the left and right deflection of the probe during scanning with contact-mode LFM.
For measuring nano-piezoelectricity, PVDF samples were metalized by sputtering a Ni/Cu film on both sides. The samples were mounted on AFM holder with one loose end. The charge signals of the PVDF samples due to mechanical bending were measured directly through the AFM readings. The microstructures of PVDF samples and surface morphology under the influence of applied voltage were also analyzed using AFM. Electrical conductivities of the PVDF samples were measured using the AFM setup shown in figure 2. The setup contains an external power supply, a picoameter, a labview PC system, and a shark box that functions as a splitter. The shark box distributes electrical potentials and passes current from samples to the picoameter. PVDF samples were half-coated with silver film and electrical potential of 12V were applied. The positive end was connected to the silver layer on the PVDF sample and the negative one to the AFM. The samples were scanned by reciprocating AFM probe between the silver-coated area and PVDF regions. Current generated during scanning flows to the picoameter where amplitude is displayed and recorded.
Figure 2. Simple diagram of the experimental setup to measure the conductivity of PVDF
Analysis of change of piezo-properties using AFM is also documented in this chapter. Using combined techniques of atomic force microscope and Fourier transform infrared spectroscope, observation of surface morphology and phase transformation was made. The effects of stress on microstructure and its subsequent relaxation with time were investigated. During electrical poling of the PVDF samples, uniaxial stretching was applied at temperatures right below the melting point which disentangles chain packing of the molecules along the tensile direction. Bending deformation was induced in longitudinal and transversal directions with respect to the initial stretching direction. The initial AFM scan was performed on the samples as soon as bending was applied, i.e. T=0 min. Maintaining the same bending deformation, dynamic AFM measurements were continued at time (T) equal to 6, 13, 25, 35, 43, 61, and 68 min. Surface morphology and phase images of each deformed PVDF samples were observed using an AFM in close contact mode with Si3N4 tips. The scan rate was set at 0.5 Hz with 512 resolutions. Fourier transform infrared spectroscopy measurements were conducted in absorbance mode at room temperature to verify time-dependent changes in phase transformation.6
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During surface image analysis, the morphological images of the PVDF samples were acquired and it was found that there are two stretching directions, radial and tangential to the spinning motion direction. In the phase images obtained, stretched structure caused by the corona process was clearly seen. In the Electrostatic force microscope (EFM) images acquired, it was found that the intensities are uniform across the samples indicating a stable electrical charge. During surface force measurements, adhesion and friction forces were measured. The adhesion forces of the four PVDF samples were acquired and a graph was plotted as shown in figure 3(a). From the graph, it was found that the adhesion force of α phase and a mixture of β and γ are almost the same. Sample D has the highest value of adhesion force where most phases are γ. It was observed that the amount of β phase reduces adhesion force due to electrostatic force as the β phase is less electrostatic than that of α or γ phase. The non-polar α phase sample has a high adhesion force due to the effect of electrostatic forces.
Figure3. (a) Plot of adhesion force of PVDF samples. (b) Comparison of friction values through AFM
The friction force of the PVDF samples measured using left-right deflection of the probe in contact-mode AFM were plotted in the form of a graph and the values were compared as shown in figure 3(b). It was found that samples A and B, which contains high amount of β phase, provides a high value of friction. The friction of samples with mixed phases is high compared to samples with only one phase.
Application; POSFET3 is a device which integrates the piezoelectric polymer technology with the integrated circuit technology for use in medical imaging. The transducer consists of a sheet of polarized PVDF bonded to the surface of a silicon wafer on which an array of MOSFET amplifiers have been defined.