Vertically aligned ZnO nanoarray directly orientated on Cu paper by h-BN monolayer for flexible and transparent piezoelectric nanogenerator

Piezoelectric nanogenerators (PENGs) harvest various types of mechanical energy and exhibit remarkable potential for powering portable devices and wearable electronic systems. The insulating layer in the device can enhance the output voltage, however, people are still curious on the extreme effect when the dielectric thickness reaches the limit of atomic monolayer. Here we demonstrate vertically aligned growth of zinc oxide (ZnO) nanorods array directly on polycrystalline copper paper through the pre-orientation with two-dimensional (2D) hexagonal boron nitride (h-BN) monolayer. The h-BN layer effectively releases the misfit strain in ZnO nanoarray and leads to highly aligned and ultralong nanorods over 75 µm through chemical vapor deposition method. A flexible and transparent PENG based on h-BN/ZnO nanoarray/h-BN sandwiched structure was designed and fabricated. Ultrathin thickness and high barrier of h-BN strongly suppresses the movement of free-carriers in PENG device and improves the output voltage. It is found that the capacitance of PENG increases exponentially inversely proportional to the dielectric layer thickness and reaches 0.01 mF with 2D h-BN monolayer. The multilayer h-BN based PENG can generate open-voltage of 5 V and short-current of ∼ 18 μA, and achieve a maximum output power density up to 169 mW/cm2. A prototype transparent PENG has been achieved for harvesting walking energy and exhibits the practical potential for self-powered wearable devices in the future.