Piezoelectricity in twisted few-layer graphene composites enabling real-time FFT-resolved rotational motion sensing and biomechanical thumb-grip assessment

Abstract

Convergence of energy harvesting and multimodal sensing functionalities in a self-powered, flexible composite represents a major step towards self-powered and intelligent systems. Piezoelectricity in twisted few-layer graphene (FLG), an intermediate structure between monolayer graphene and bulk graphite, and their composites with PVDF-HFP, is being investigated in the present study. We demonstrate that twisting-induced structural modification of FLG acts as an effective non-functionalized strategy to enhance interfacial polarization and promote beta-phase stabilization in PVDF-HFP composites, resulting in improved piezoelectric performance. Twisting preserves the weak interaction between the layers while inducing distortion and the negative surface charge promotes the Maxwell-Wagner interaction between the filler and polymer phases, facilitating electroactive beta-phase nucleation. Investigation of key piezoelectric performance metrics through Ferroelectric loops and Piezoresponse Force Microscopy confirms that the optimized composite, PG-10, delivered a saturation polarization of 15.64 nC/cm(2), achieved a remarkable d(33)* value of 0.35 nm/V and a high recoverable energy density of 15.54 nCJ/cm(3) at 220 V/cm, attributed to diluted intragranular interaction and induced dielectric in-homogeneities. The PG-10 piezoelectric nanogenerator (PENG) produced an output of 108.1 V, and finger-tap actuation rapidly charged a 4.7 pF capacitor to similar to 2.5 V within 10 s, showcasing its fast response under sustained mechanical excitation. Demonstrating practical applicability beyond energy harvesting, the PG-10 PENG accurately tracked the rotational frequency of a spin coater through FFT-based frequency mapping and resolved nuanced thumb-grip dynamics, establishing FLG-PVDF-HFP composites as promising candidates for nondestructive structural monitoring and biomechanical sensing platforms, accelerating the integration of flexible piezoelectric modules into sustainable MEMS devices.