ZnO‐Coated Carbon Nanotubes: Flexible Piezoelectric Generators

In their study aligned oxide nanowires were defl ected by an ultrasonic-wave-induced mechanical stress and the number of active nanowires that produce electricity was estimated to be 1000 wires cm − 2 . However, the reported technique for generating piezoelectricity has disadvantages including: First, ceramics are brittle in nature and material fractures are inevitable with repeated defl ections, thus reducing piezoelectric performance and reproducibility. Second, the design is complex and an accurate spacing with tolerance less than 1 μ m is required between the electrode and the piezoelectric ZnO array. Third, the zigzag electrode induces various strain modes on the nanowires and thus the average power density is reduced. We present here an alternative approach for piezoelectric generation in which devices are constructed by coating carbon nanotubes (CNTs) with ZnO using atomic layer deposition (ALD). The study shows that the piezoelectric character does not fade away with repeated stress applications, and coating oxides remain intact because the fl exible CNTs act as underlying cushions. [ 4 ] The average open-circuit voltage ( V OC ) reaches a value as high as 1.6 V and the short-circuit current ( I SC ) is 75 μ A cm − 2 , approximately a factor of 3000 greater than what was previously reported. [ 1 ] A Schottky barrier induced rectifying effect at the oxide/tube interface is also identifi ed and accounts for the device polarity. Figure 1 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of ALD-treated CNT arrays and the oxide coating is evident. The nanotubes display different diameters in the upper and lower sections and SEM images from a microscope equipped with position marking software reveals 55–70 nm for the former and 20–30 nm for the latter (Figure 1 b,c). The CNTs are graphite sheets rolled into seamless cylinders and conjugated surfaces do not accumulate charges upon SEM beam irradiation. Accordingly, the