Growth mechanism and diameter control of well-aligned small-diameter ZnO nanowirearrays synthesized by a catalyst-free thermal evaporation method

Abstract

Well-aligned small-diameter ZnO nanowire arrays have been synthesized on Si wafers pre-coated withc-oriented ZnO thin films by a catalyst-free thermal evaporation method. The morphology ofthe products has been found to be greatly affected by the oxygen flow rate. Theself-catalyzed VLS mechanism is proposed to interpret the growth of the ZnO nanowiresand the change in the product morphology. Classical nucleation theory is employed toanalyze the growth process of the ZnO nanowires, and Zn vapor supersaturation isproposed to be a key factor to affect the diameter and the areal density of the ZnOnanowires. The average diameter of the ZnO nanowires can be finely controlled in the rangeof 12–31 nm by controlling the oxygen flow rate and hence the Zn vapor supersaturation.This result is consistent with our theoretical prediction. X-ray photoelectron spectroscopymeasurements were performed to characterize the stoichiometry of nanowires withvarious average diameters. It also revealed that there are many OH species on thenanowire surface. Photoluminescence measurements showed that the deep-levelemission of the nanowires with average diameter of 12 nm peaks in the yellowregion, which may be attributed to the presence of OH species and surface effects.