Controlled growth of semiconducting nanowire, nanowall, and hybrid nanostructures on graphene for piezoelectric nanogenerators.

Precise control of morphologies of one- or two-dimensional nanostructures during growth has not been easy, usually degrading device performance and therefore limiting applications to various advanced nanoscale electronics and optoelectronics. Graphene could be a platform to serve as a substrate for both morphology control and direct use of electrodes due to its ideal monolayer flatness with π electrons. Here, we report that, by using graphene directly as a substrate, vertically well-aligned zinc oxide (ZnO) nanowires and nanowalls were obtained systematically by controlling gold (Au) catalyst thickness and growth time without inflicting significant thermal damage on the graphene layer during thermal chemical vapor deposition of ZnO at high temperature of about 900 °C. We clarify Au nanoparticle positions at graphene-ZnO heterojunctions that are very important in realizing advanced nanoscale electronic and optoelectronic applications of such nanostructures. Further, we demonstrate a piezoelectric nanogenerator that was fabricated from the vertically aligned nanowire-nanowall ZnO hybrid/graphene structure generates a new type of direct current through the specific electron dynamics in the nanowire-nanowall hybrid.