Piezo-to-piezo (P2P) conversion: simultaneous β-phase crystallization and poling of ultrathin, transparent and freestanding homopolymer PVDF films <i>via</i> MHz-order nanoelectromechanical vibration

An unconventional yet facile low-energy method for uniquely synthesizing neat poly(vinylidene fluoride) (PVDF) films for energy harvesting applications by utilizing nanoelectromechanical vibration through a 'piezo-to-piezo' (P2P) mechanism is reported. In this concept, the nanoelectromechanical energy from a piezoelectric substrate is directly coupled into another polarizable material (<i>i.e.</i>, PVDF) during its crystallization to produce an optically transparent micron-thick film that not only exhibits strong piezoelectricity, but is also freestanding-properties ideal for its use for energy harvesting, but which are difficult to achieve through conventional synthesis routes. We show, particularly through <i>in situ</i> characterization, that the unprecedented acceleration associated with the nanoelectromechanical vibration in the form of surface reflected bulk waves (SRBWs) facilitates preferentially-oriented nucleation of the ferroelectric PVDF β-phase, while simultaneously aligning its dipoles to pole the material through the SRBW's intense native evanescent electric field . The resultant neat (additive-free) homopolymer film synthesized through this low voltage method, which requires only -orders-of-magnitude lower than energy-intensive conventional poling methods utilizing high kV electric potentials, is shown to possess a 76% higher macroscale piezoelectric charge coefficient <i>d</i>₃₃, together with a similar improvement in its power generation output, when compared to gold-standard commercially-poled PVDF films of similar thicknesses.