Why do nanowires grow vertically-aligned in the absence of epitaxy

Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when epitaxial templating from the substrate is clearly absent. Here, we reveal one such hidden growth guidance mechanism through a specific example - the case of ZnO nanowires grown on silicon oxide and glass. We show how electric fields exerted by the insulating substrate may be manipulated through the surface charge to define the orientation of the nanowires. Our results suggest that the growth of wurtzite semiconductors may often be described as a process of electric-charge-induced self assembly.