The growth rate of ZnO nanowires grown epitaxially on GaN/sapphire substrates was studied. An inverse proportional relation between diameter and length of the nanowires was observed, i.e., nanowires with smaller diameters grew faster than larger ones. This unexpected result was attributed to surface diffusion of ZnO admolecules along the sidewalls of the nanowires. In addition, the unique c-axis growth of ZnO nanowires, which did not require a catalytic particle at the tip of the growing nanowires was explained by taking into account polarity, surface free energy, and ionicity.

Surface-Diffusion Induced Growth of ZnO Nanowires. Kim, D.S., Gösele, U., Zacharias, M.: Journal of Crystal Growth, 2009, 311[11], 3216-9