Nanoconfinement: an Effective Way to Enhance PVDF Piezoelectric Properties Valentina Cauda,* ,†,§ Stefano Stassi, †,‡,§ Katarzyna Bejtka, † and Giancarlo Canavese †,§ † Center for Space Human Robotics@PoliTo, Istituto Italiano di Tecnologia, C.so Trento, 21, 10129 Torino, Italy ‡ Department of Applied Science and Technology, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy ABSTRACT: The dimensional confinement and oriented crystallization are both key factors in determining the piezoelectric properties of a polymeric nanostructured material. Here we prepare arrays of one-dimensional polymeric nanowires showing piezoelectric features by template-wetting two distinct polymers into anodic porous alumina (APA) membranes. In particular, poly(vinylidene fluoride), PVDF, and its copolymer poly(vinylidene fluoride-tri fluoroethylene), PVTF, are obtained in commercially available APA, showing a final diameter of about 200 nm and several micrometers in length, reflecting the templating matrix features. We show that the crystallization of both polymers into a ferroelectric phase is directed by the nanotemplate confinement. Interestingly, the PVDF nanowires mainly crystallize into the β-phase in the nanoporous matrix, whereas the reference thin film of PVDF crystallizes in the α nonpolar phase. In the case of the PVTF nanowires, needle-like crystals oriented perpendicularly to the APA channel walls are observed, giving insight on the molecular orientation of the polymer within the nanowire structure. A remarkable piezoelectric behavior of both 1-D polymeric nanowires is observed, upon recording ferroelectric polarization, hysteresis, and displacement loops. In particular, an outstanding piezoelectric effect is observed for the PVDF nanowires with respect to the polymeric thin film, considering that no poling was carried out. Current versus voltage (I-V) characteristics showed a consistent switching behavior of the ferroelectric polar domains, thus revealing the importance of the confined and oriented crystallization of the polymer in monodimensional nanoarchitectures. KEYWORDS: confined PVDF nanowires, nanostructured polymer, template-wetting, piezoelectric properties, anodic porous alumina ■ INTRODUCTION Scaling the size of materials down to the nanometer dimension is offering innovative opportunities not only to study their behavior at the nanoscale but also to discover new useful properties, which can widely extend the range of possible applications. One of the common strategies to generate monodimensional (1-D) nanostructures is a use of a template filled with the desired material. Several comprehensive reviews on this topic were already published, 1,2 showing that miniaturized, oriented, and anisotropic structures can be prepared by this approach starting from a material precursor or solution. In particular, to form polymeric nanowires, the template-wetting approach from a melt polymer or solution is commonly used. 3 In such a form, the polymer spreads on a porous templating substrate to form a thin surface film, thus covering the pore walls in the initial stages of wetting. Indeed, the polymer preferentially nucleates and grows on the surface of the pore walls, resulting in tubes at short infiltration times. Complete filling of the pores takes place later, and is normally hindered by thermal quenching in the case of melts or by solvent evaporation in the case of solutions; thus, controlled temperature and pressures or a higher amount of feeding material to fill the pores are required. 3,4 The final structure, either a nanotube or a nanowire, shows the same size distribution and shape of the templating matrix. Thus, by appropriately selecting a monodispersed size distribution, as well as aligned and long pores of the templating matrix, one can obtain ordered and monodispersed arrays of nanowires. Several authors have already studied the wetting template in anodic porous alumina (APA) membrane in order to produce arrays of 1-D ceramic and metallic nanowires, showing ferroelectric, dielectric, or conductive properties. 5-7 Porous alumina exhibits a high surface energy, completely wettable by liquids, i.e., polymer melts or solutions. 2 Among several polymers templated into APA, 8,9 the confinement of poly(vinylidene fluoride), PVDF, and its copolymer poly(vinylidene fluoride-trifluoroethylene), PVTF, was reported, obtaining a vertical array of oriented nanowires with a preferential crystalline direction. 10-12 How- ever, despite the nanowire preparation and the study on their crystallization and orientation, to our knowledge, the final piezoelectric properties of these monodimensional polymeric structures were not deeply investigated neither compared to the bulk nor to the thin film form. 13 Here we report on the preparation of both PVDF and PVTF nanowires into the pores of a templating commercial alumina membrane. This template-directed method is simple, high- throughput, cost-effective, and able to result in nanowires in a single-step procedure. The key point of the present study relies on the preferred crystallization of both polymers into a β Received: May 9, 2013 Accepted: June 18, 2013 Published: June 18, 2013 Research Article www.acsami.org © 2013 American Chemical Society 6430 dx.doi.org/10.1021/am4016878 | ACS Appl. Mater. Interfaces 2013, 5, 6430-6437