AFM Characterization of Electroactive Polymer Nanocomposites
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AFM Characterization of Electroactive Polymer Nanocomposites Ricardo Pérez, Zoubeida Ounaies Aerospace Engineering Department, TAMU, College Station TX Peter Lillehei and Joycelyn Harrison NASA LaRC, Langley VA ABSTRACT An AFM-based method is used to probe the electromechanical response of a polyimide-SWNT nanocomposite at the nanoscale level. Our previous investigations on bulk polyimide-SWNT nanocomposites using a fiber optic sensor-based method have shown that the nanocomposites are electroactive and exhibit a quadratic relationship between output displacement and input voltage. In the current study, the composites are probed at the nanoscale using the AFM in contact mode. One goal of the current study is to assess the AFM-based technique in measuring our nanocomposites and verifying their actuation. A second goal is to improve the methodology itself by characterizing a commercially available piezoelectric ceramic, PZT, and a commercially available piezoelectric polymer, PVDF. The experiments are conducted in contact mode under the application of a DC voltage. Values for the strain piezoelectric coefficients d33 are widely reported in the literature and consequently permit calibration of the methodology applied in the present work. These findings, coupled with bulk film characterization will be used to shed light on the polyimide-SWNT interaction and its effect on the electroactive mechanism displayed by the nanocomposites. INTRODUCTION Atomic force microscopy (AFM) is a demonstrated useful tool to quantify mechanical and electrical properties of materials at the nanoscale. In recent years, electroactive materials, more specifically ferroelectric ceramics, have been studied with this technique [1-3]. The capabilities of the AFM make it possible to characterize the electromechanical response of ferroelectric polycrystalline films as well as single crystals. The technique utilized for this purpose is called piezoresponse force microscopy (PFM) and it allows the quantification of the piezoelectric coefficients of a given sample using a nanoprobe. For more details on the challenges and progress of PFM as a technique, the reader is referred to a recent article by Kalinin et al. [4]. The PFM consists of an AFM conductive probe working in contact mode that functions as the top electrode on a sample with a single bottom electrode. Figure 1 shows the generic experimental setup of the PFM. Two configurations are possible: Tip-sample-electrode (TSE) (see Figure 1a) and tip-electrode-sample-electrode (TESE) (see Figure 1b and 1c). The measurement of the piezoelectric coefficient using this approach is not free of challenge due to the highly non uniform electric field in the sample and the measurement of displacements at nanoscale. The AFM probe may amplify the local electric field applied to the sample due to its size, or it could result in a potential drop if the tip quality is not carefully controlled. Zavala et al. [5] compared both configurations (TSE and TESE) for a PZT substrate, and found that TESE resu
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