Cooperativity in the Enhanced Piezoelectric Response of Polymer Nanowires

摘要

Multilayered, aligned arrays of organic nanowires show unique advantages in their piezoelectric response. Here, the cooperative, electromechanical mechanism at the base of the enhanced response of aligned arrays of piezoelectric nanostructures in mutual contact is unveiled. An enhancement of the piezoelectric voltage by two orders of magnitude compared with individual nanofibers is demonstrated in the arrays. Piezoelectricity, a Greek term for pressure-induced electricity, is the capability of a material to polarize by means of spatially separated electrical charges of opposite sign, in response to an external stress that produces a mechanical deformation. Generally, charges accumulate at two opposite side surfaces of the material body, and, in absence of short-circuited contacts, a voltage bias is generated. This effect can be observed in materials whose crystalline state has no center of symmetry (so-called non-centrosymmetric), including polymers and biological systems.1 To date, piezoelectricity represents one of the most valuable alternative source of energy with an associated fast-growing investment market and potential applications spanning across a wide range of fields, such as information and communications, industrial automation, healthcare and medical monitoring, defense industry, automation and robotics.2 Indeed, the capability of harvesting energy from small mechanical forces, through pressure, vibration, bending, elongation, and compression, is today subject of extensive research on both materials and device geometries, and the related development of self-powered wireless devices could be of great importance for the internet of things, that is for interconnecting individual uniquely identifiable objects and bodies.3, 4 In this respect, piezoelectric micro- and nanostructures have demonstrated improved properties that enable new functionalities not achievable with their bulk counterpart. Most of these are related to reduced dislocations and superior mechanical properties.5-7 For instance, in pion­eering work by the Wang group, aligned arrays and multilayer stacks of zinc oxide and lead zirconate titanate nanowires have been exploited to power light-emitting and wireless devices.8, 9 In this framework, piezoelectric polymers are very promising, since they can also provide structural flexibility and toughness, as well as low cost, improved biocompatibility, and ease of processing. In particular, the device-integration of polyvinylidene­fluoride (PVDF) and its copolymers is attracting increasing interest,10-12 because their micro and nanostructures such as films, belts and fibers have shown unique advantages in terms of material functionality and piezoelectric response, and self-poling during nanofabrication.13-17 Electrospinning is especially effective in this respect, producing self-poled piezoelectric nanofibers due to the very high stretching forces exerted on electrified solution jets.17 Consequently, polymer molecules mainly align parallel to the fiber longitudinal axis,18 and piezo­electric material phases are favored compared to films.16, 19 Furthermore, aligned arrays of PVDF-based fibers generally exhibit still superior piezoelectric performances.20-22 Most often, these fibers are aligned with low density and provide sub-monolayer coverage of solid supports, namely they are separated by distances of the order of microns from their nearest neighbors in deposited strands. This configuration results in open-circuit currents which correspond to the sum of currents generated by each single nanowire in the generator.23 Dense (107 fibers/mm2) arrays of electrospun aligned nanofibers of poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFe)] offer exceptional piezoelectric characteristics and output voltage significantly enhanced with respect to individual fibers.19 Such an arrangement is characterized by large sensitive areas (tens of cm2) and light weight, and it may be bent or twisted without fracture. Howe