In Atomic Force Microscopy Study of Piezoelectric Polymers, the 23rd chapter of the book Applied Scanning Probe Methods XIII, the writers discuss the techniques to analyze the microstructure and belongingss of piezoelectric polymers by utilizing Atomic Force Microscopy ( besides known as Scaning Probe Microscopy ) . This chapter aimed at the survey of topography and belongingss of piezoelectric polymer utilizing Atomic Force Microscopy ( AFM ) . The survey was conducted on polyvinylidene fluoride ( PVDF ) , which is a alone polymer that has high pyroelectric and piezoelectric belongingss. Its high permittivity and comparatively low dissipation factor makes it a possible campaigner to utile in many applications. Piezoelectric polymer have advantage over piezoelectric ceramics for certain applications wherein acoustic impedence similar to that of H2O or life tissue is required.2 Due to this ground PVDF is progressively used for medical and industrial applications. This motivated the writers who are actively involved in strategically analyzing progress stuffs to uncover rules in effects of electronic, chemical, mechanical, and tribological belongingss to analyze PVDF every bit good. The results of these researches are in the development of fresh nanofabrication procedures and nanostructured stuffs that have the possible to be used as unreal articulations, car constituents, micro detectors, and energy harvest home devices. They work in close association with industries to assist them to work out corrosion and tribological jobs.
The piezoelectric consequence was discovered in 1880 by Pierre and Jacques Curie. Piezoelectricity is an interaction between electrical and mechanical systems. The direct piezoelectric consequence is that electric polarisation is produced by mechanical emphasis. Closely related to it is the converse consequence, whereby a crystal becomes strained when an electric field is applied. Both effects are manifestations of the same cardinal belongings of the crystal. Dielectric, elastic, and piezoelastic invariables are the cardinal parametric quantities depicting piezoelectric phenomena.1 There are many widely used piezoelectric stuffs 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 signifiers are observed with different conformations that is, TG+TG- in the ? and ? stages, all-trans ( TTT ) planar zigzag in the ? stage, and T3G-T3G+ in the ? and ? stages. These crystalline signifiers can transform to each other under specific conditions, such as under the application of mechanical distortion and high electrical field. The ?-crystalline signifier doesnot demo a net lattice polarisation due to antiparallel TG+TG- concatenation agreement in its unit cell. Therefore, the concatenation dipoles oppose each other. The ? stage consists of parallel wadding of the polymer ironss in all-trans conformations, and therefore, it shows a big self-generated lattice polarisation. Hence, out of all the stages, the polar ? stage exhibits the strongest piezoelectric and pyroelectric belongingss and is the most desirable crystalline signifier for PVDF.5
The surface morphology, surface force measurings, nano-piezoelectricity, conduction and clip dependent stage transmutations due to emphasize on PVDF were studied utilizing close-contact or tapping mode AFM as it eliminates the jobs associated with clash, adhesion, and electrostatic forces caused in contact-mode AFM. However, PVDF crystals and molecular ironss were aligned by utilizing contact-mode AFM.4
A brief reappraisal of the Atomic Force Microscopy techniques is provided in the beginning along with the two types of scanning manners, contact and close contact ( or tapping/vibrating manner ) . Contact-mode is more utile for doing clear topographical images of difficult stuffs with low mean raggedness. In the contact manner, the abhorrent force experienced by the tip is measured by entering the cantilever warp and the warp is normally measured by optical method. A close-contact or tapping manner is utile for stage sensing and non-destructive imagination of soft stuffs without any harm to the stuff under survey. In the tapping manner, the cantilever is excited to vibrate near its resonating frequence near to the sample surface. Once the nearing tip is in contact with the surface, the alteration of cantilever oscillation is reflected by the surface belongingss of the sample. AFM is besides used for the measuring of sidelong and adhesion force, electrostatic force, magnetic force, piezoresponse force, to call a few. The SPM is widely used for the survey of ferroelectric stuffs. It is used to map polarisation in a local sphere. The self-generated polarisation, ferroelectric stage passage, and pyroelectric alteration were observed through SPM in thin movie ferroelectric polymers. The writers cited the work where the orientation of PVDF crystals and molecules were controlled by utilizing alining technique developed for polymer crystals and molecular ironss utilizing contact-mode AFM. Lamellar crystals of poly ( vinylidenefluoride-trifluoroethylene ) thin movies were aligned to the scan way by scanning the movie surface utilizing an AFM cantilever tip at the temperature of 70-100 & A ; deg ; C and the molecular ironss were aligned by scanning at a higher temperature ( 135 & A ; deg ; C ) .4
In this peculiar survey, PVDF thin movies were prepared by blending farinaceous PVDF with propanone ( 80ml ) and dimethylsulfoxide ( DMSO ) solutions ( 20ml ) . The concentration was set at 40, 60, 80, and 100 g/L each for assorted viscousnesss. The solution was so heated and stirred at 40 & A ; deg ; C for approximately 30 min till all solid atoms were dissolved wholly. The solution was so spin-coated at the velocity of 3000 revolutions per minute for 20 s. Samples were so annealed at 23, 40, 60 and 80 & A ; deg ; C with corona poling of 30kV for 2 min. Different viscousnesss of PVDF solutions were spin-coated on a gold-coated Si substrate at different velocities. Movies were so heated at assorted temperatures before, during and after 30min with unmoved aureole punting a 30kV for 2 min. Using fisheye X-ray diffraction ( WAXD ) and Fourier Transform Infrared Spectroscopy ( FTIR ) , microstructural analysis was done. The diffraction lines of the samples were obtained utilizing WAXD and the designation of the crystalline phases present in the samples was done by agencies of an FTIR spectrometer. FTIR accurately measured the fluctuation of the ? stage in the polymer movies and the fraction of ?-phase crystals in each sample was calculated harmonizing to the specific soaking up sets of the different phases.5 Surface image analysis, surface force measurings, piezoelectric effect, conduction measuring and clip dependent stage transmutation survey were conducted on the 4 samples prepared.
Surface image analysis was done that includes morphology and stage image, which were obtained utilizing AFM. The Electrostatic force microscope ( EFM ) images were besides acquired and the electrical end products due to electrical force gradients were obtained. As adhesion and clash can readily destruct the electrical connexions during operation of MEMS ( Microelectromechanical systems ) and NEMS ( Nanoelectromechanical systems ) , hence surface force measurings are really of import. The adhesion forces for the four samples were measured by the force-displacement curve as shown in Figure 1 under the contact manner utilizing Lateral force microscope ( LFM ) . In the figure, the investigation is nearing from A to C with increasing attractive force and detaching from C to D due to the abhorrent force that caused a sudden rest area from D to E. The distance between B and D is caused by adhesion force. Adhesion force was calculated utilizing the equation:
Fadhesion= K * a?†x
where K is the spring invariable ( nN/nm ) of the AFM investigation.
Figure1. Force-displacement curve used for the measuring of adhesion force
The clash values of the PVDF samples were measured utilizing the left and right warp of the investigation during scanning with contact-mode LFM.
For mensurating nano-piezoelectricity, PVDF samples were metalized by sputtering a Ni/Cu movie on both sides. The samples were mounted on AFM holder with one loose terminal. The charge signals of the PVDF samples due to mechanical bending were measured straight through the AFM readings. The microstructures of PVDF samples and surface morphology under the influence of applied electromotive force were besides analyzed utilizing AFM. Electrical conductions of the PVDF samples were measured utilizing the AFM apparatus shown in figure 2. The apparatus contains an external power supply, a picoameter, a labview Personal computer system, and a shark box that maps as a splitter. The shark box distributes electrical potencies and base on ballss current from samples to the picoameter. PVDF samples were half-coated with Ag movie and electrical potency of 12V were applied. The positive terminal was connected to the Ag bed on the PVDF sample and the negative one to the AFM. The samples were scanned by reciprocating AFM investigation between the silver-coated country and PVDF parts. Current generated during scanning flows to the picoameter where amplitude is displayed and recorded.
Figure 2. Simple diagram of the experimental apparatus to mensurate the conduction of PVDF
Analysis of alteration of piezo-properties utilizing AFM is besides documented in this chapter. Using combined techniques of atomic force microscope and Fourier transform infrared spectroscope, observation of surface morphology and stage transmutation was made. The effects of emphasis on microstructure and its subsequent relaxation with clip were investigated. During electrical poling of the PVDF samples, uniaxial stretching was applied at temperatures right below the thaw point which disentangles concatenation wadding of the molecules along the tensile way. Bending distortion was induced in longitudinal and cross waies with regard to the initial stretching way. The initial AFM scan was performed on the samples every bit shortly as bending was applied, i.e. T=0 min. Keeping the same bending distortion, dynamic AFM measurings were continued at clip ( T ) equal to 6, 13, 25, 35, 43, 61, and 68 min. Surface morphology and stage images of each deformed PVDF samples were observed utilizing an AFM in close contact manner with Si3N4 tips. The scan rate was set at 0.5 Hz with 512 declarations. Fourier transform infrared spectroscopy measurings were conducted in optical density manner at room temperature to verify time-dependent alterations in stage transformation.6
During surface image analysis, the morphological images of the PVDF samples were acquired and it was found that there are two stretching waies, radial and digressive to the whirling gesture way. In the stage images obtained, stretched construction caused by the aureole procedure was clearly seen. In the Electrostatic force microscope ( EFM ) images acquired, it was found that the strengths are unvarying across the samples bespeaking a stable electrical charge. During surface force measurings, adhesion and clash 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 ? stage and a mixture of ? and ? are about the same. Sample D has the highest value of adhesion force where most stages are ? . It was observed that the sum of ? stage reduces adhesion force due to electrostatic force as the ? stage is less electrostatic than that of ? or ? stage. The non-polar ? stage sample has a high adhesion force due to the consequence of electrostatic forces.
( B )
Figure3. ( a ) Plot of adhesion force of PVDF samples. ( B ) Comparison of clash values through AFM
The clash force of the PVDF samples measured utilizing left-right warp of the investigation in contact-mode AFM were plotted in the signifier 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 sum of ? stage, provides a high value of clash. The clash of samples with assorted stages is high compared to samples with merely one stage.
Application ; POSFET3 is a device which integrates the piezoelectric polymer engineering with the integrated circuit engineering for usage in medical imagination. The transducer consists of a sheet of polarized PVDF bonded to the surface of a Si wafer on which an array of MOSFET amplifiers have been defined.