Why do nanowires grow with their c-axis vertically-aligned in the absence of epitaxy?

TL;DR

This paper uncovers how electric fields from insulating substrates influence the vertical growth and orientation of nanowires, specifically ZnO, through surface charge effects, without the need for epitaxial templating.

Contribution

It introduces the concept that substrate electric fields and surface charges guide nanowire growth and orientation, revealing a new self-assembly mechanism driven by electric charge.

Findings

Electric fields from substrates influence nanowire orientation.

Surface charge manipulation can control nanowire polarity.

Growth can be explained by electric-charge-induced self assembly.

Abstract

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 the physics underlying 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 and polarity of the nanowires. Surface charge is ubiquitous on the surfaces of semiconductors and insulators, and as a result, substrate electric fields need always be considered. Our results suggest a new concept, according to which the growth of wurtzite semiconductors may often be described as a process of electric-charge-induced self assembly, wherein the internal built-in field in the polar material tends to align in parallel to an external field exerted by the substrate to minimize the interfacial energy of the system.