Ultra-efficient thermo-convective solution-growth of vertically aligned ZnO nanowires

The integration of ZnO nanostructures in practical devices almost always takes advantage of wet-chemistry procedures which are largely inefficient in terms of energy, chemicals, time, length, and cost. For instance, in many cases the predeposition and annealing of a seed layer is required, which can also result in process-compatibility and functional issues. Forced flow, thermal convection, and localized heating may help, but can also introduce other difficulties. Here we demonstrate that heating the substrate by an adjacent temperature-controlled surface can naturally promote the ultra-efficient, convection-assisted, seed-less synthesis of high-density, vertically aligned ZnO nanowires on large areas. In particular, the sub-mm confinement of the high-temperature required for solution-growth can reduce the nanowire energetic-cost by orders of magnitude. As examples, we effortlessly grow with a single procedure high-density and long (up to several tens of μm) ZnO nanowires on silicon, gold, aluminium, and both the copper and the plastic of flexible printed circuit boards (PCBs). As proofs of concept, we show the unprecedentedly low-energy, easy and seed-less cointegration on a flexible PCB of a ZnO-nanowire piezotronic strain sensor with gauge factor around 1000. The proposed approach can be generalized to the solution-growth of other materials and can result in ultra-efficient depositions on a wide variety of substrates.