  Nanogenerators based on piezoelectric materials convert ever-present
mechanical vibrations into electrical power for energetically autonomous
wireless and electronic devices. Nanowires of piezoelectric polymers are
particularly attractive for harvesting mechanical energy in this way, as they
are flexible, lightweight and sensitive to small vibrations. Previous studies
have focused exclusively on nanowires grown by electrospinning, but this
involves complex equipment, and high voltages of $\approx$ 10 kV that
electrically pole the nanowires and thus render them piezoelectric. Here we
demonstrate that nanowires of poly(vinylidene fluoride-trifluoroethylene)
(P(VDF-TrFE)) grown using a simple and cost-effective template-wetting
technique, can be successfully exploited in nanogenerators without poling. A
typical nanogenerator comprising $\approx$ 10$^{10}$ highly crystalline,
self-poled, aligned nanowires spanning $\approx$ 2 cm$^2$ is shown to produce a
peak output voltage of 3 V at 5.5 nA in response to low-level vibrations. The
mechanical-to-electrical conversion efficiency of 11% exhibited by our
template-grown nanowires is comparable with the best previously reported
values. Our work therefore offers a scalable means of achieving
high-performance nanogenerators for the next generation of self-powered
electronics.
