Nanowire Piezoelectric Nanogenerators on Plastic Substrates as Flexible Power Sources for Nanodevices

Research applications in biomedical science and technology usually require various portable, wearable, easy-to-use, and/or implantable devices that can interface with biological systems. Organic or hybrid organic–inorganic microelectronics and nanoelectronics have long been a possibility. However, these devices require a power source, such as electrochemical cells or piezoelectric, thermoelectric, and pyroelectric transducers, to generate or store the electrical energy created through chemical, mechanical, or thermal processes. Finding a suitable power source has remained a major challenge for many devices in bioengineering and medical fields. ZnO is a typical piezoelectric and pyroelectric inorganic semiconducting material used for electromechanical and thermoelectrical energy conversion. Nanostructures of ZnO, such as nanowires (NWs), nanobelts (NBs), nanotubes, nanorings, nanosprings, and nanohelices, have attracted extensive research interest because of their potential applications as nanoscale sensors and actuators. While most of the current applications focus on its semiconducting properties, only a few efforts have utilized the nanometerscale piezoelectric properties of ZnO. Using ZnO NW arrays grown on a single-crystal sapphire substrate, we have successfully converted mechanical energy into electrical energy at the nanoscale. A conductive atomic force microscopy (AFM) tip was used in contact mode to deflect the aligned NWs. The coupling of piezoelectric and semiconducting properties in ZnO creates a strain field and charge separation across the NWs as a result of their bending. The rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads to electrical current generation. This is the principle behind piezoelectric nanogenerators. The ceramic and semiconducting substrates used for growing ZnO NWs are hard and brittle and cannot be used in applications that require a foldable or flexible power source, such as implantable biosensors. In this Communication, by using ZnO NW arrays grown on a flexible plastic substrate, we demonstrate the first successful flexible power source built on conducting-polymer films. This approach has two specific advantages: it uses a cost-effective, large-scale, wet-chemistry strategy to grow ZnO NW arrays at temperatures lower than 80 °C, and the growth of aligned ZnO NW arrays can occur on a large assortment of flexible plastic substrates. The latter advantage could play an important role in the flexible and portable electronics industry. Various dimensions, shapes, and orientations of ZnO NWs and microwires on flexible plastic substrates have been shown to be capable of producing piezoelectric voltage output, giving a real advantage for energy harvesting using large-scale ZnO NW arrays. The voltage generated from a single NW can be as high as 50 mV, which is large enough to power many nanoscale devices. The ZnO NWs were grown in solution using a synthetic chemistry approach. Figure 1 shows a series of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of typical ZnO NW arrays grown on a conductive plastic substrate. Figure 1a shows a low-resolution, topdown view of the densely aligned NW arrays. The aligned NWs have a uniform diameter of 200–300 nm and a hexagoC O M M U N IC A IO N