Polymer ferroelectrics are flexible and lightweight electromechanical materials that are widely studied due to their potential application as sensors, actuators and energy harvesters. However, one of the biggest challenges is their low piezoelectric coefficient. Here, we report a mechanical annealing effect based on local pressure induced by a nanoscale tip that enhances the local piezoresponse. This process can control the nanoscale material properties over micro-scale area at room temperature. We attribute this improvement to the formation and growth of β-phase extended chain crystals via sliding diffusion and crystal alignment along the scan axis under high mechanical stress. In addition, we found that scraping the screening charges on the surface by applying mechanical pressure to the charge gradient microscopy (CGM) tip and scanning the surface would generate at least seventy times more charges than piezoelectric effect. Both the direction and amount of the collected current were controlled by changing the polarity and area of the poled domains. However, the endurance of charge collection by rubbing the CGM tip on the polymer film was limited to 20 scan cycles, after which the current reduced to almost zero. This degradation was attributed to the increase of the chemical bonding strength between the external screening charges and the polarization charges. Once this degradation mechanism is mitigated, the CGM technique can be applied to efficient energy harvesting devices using polymer ferroelectrics.

Mechanical annealing and scraping of ferroelectric polymers at the nanoscale

253rd National Meeting (2017)

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