An energy scavenging nanogenerator using a piezoelectric zinc oxide (ZnO) nanowire was developed in 2006 and, since, many studies have explored applications such as piezoelectric field-effect transistors, force/pressure sensors, and resonators.[1–4] The advantage of the piezoelectric nanodevice is that electrical energy can be generated by a variety of external stimuli, including body movement, vibrations, and hydraulic or air forces, resulting in a wireless self-powered system.[4–8] Our previous work has demonstrated that transparent flexible (TF) nanogenerators with piezoelectric ZnO nanorods can be driven directly by pushing or bending the TF nanogenerator itself.[8] Such TF nanogenerators have garnered a great deal of attention because of the potential they present for new types of energy harvesting technology, such as nanogenerator-equipped flags that are charged by wind, or nanogenerator-embedded touch screens, in which a touching action is used for operating the display as well as self-charging. Furthermore, these devices can lead to new applications such as deformable mobile electronics or tactile skin sensors that can simultaneously detect position and pressure. However, since the TF nanogenerators are designed using indium tin oxide (ITO) electrodes to create a transparent device, the ITO-based nanogenerators have limited flexibility due to the ceramic structure of the ITO, and defects can easily be introduced if the device is overflexed.[9, 10] Conventionally, metallic or metal oxide thin films have been used for transparent conductors in optical devices including light-emitting diodes and photovoltaics.[11] These transparent conducting films, however, have limited use in flexible optoelectronics due to their mechanical brittleness, chemical instability, and high cost (they often include noble or rare metals).[12, 13] As a workaround, carbon nanotube (CNT) films, which have good optical, mechanical, and electrical properties, have been developed for many flexible optical devices.[14–16] However, some unfavorable characteristics, such as the difficulty in separating metallic and semiconducting CNTs, diameterdependent electrical properties, and significant roughness, have greatly hindered the performance and development of further applications for CNT-based devices. Graphene is a 2D material with extraordinary electrical and mechanical properties.[17–20] Graphene sheets have extremely high mobility (as high as 26000cm2 VÀ1 sÀ1)[17] at room temperature and high mechanical elasticity (elastic modulus of about 1TPa)[20] based on the carbon–carbon covalent bonds, favorable for use in unique applications in the fields of nanoelectronics and spintronics.[21, 22] In particular, graphene, with a high optical transmittance and high chemical stability, provides an attractive building block as a window material for optoelectronic devices.[23, 24] Historically, graphene sheets have been prepared using mechanical exfoliation, ultrasonic cleavage of graphite, chemical oxidation of graphite, or epitaxial growth on silicon carbide.[25–31] These methods are, however, delicate and time-consuming, or have provided the graphene sheets with low optical and electrical quality (eg, about 2kV at 70% transmittance) due to the poor interlayer junction contact resistance and the structural defects formed during the vigorous exfoliation and reduction process.

Recently, our group developed a direct synthesis method of producing large-scale graphene sheets with high-quality optical and electrical properties using chemical vapor deposition (CVD).[32, 33] Such high-quality, large-area, and cost-effective graphene sheets are expected to provide an …