Comparative Transmission Electron Microscopy Study of ZnO Nanowire Growth by a Vapor–Solid Method and by Thermal Oxidation during Joule Heating

Abstract

The microstructure, morphology, and structural quality of ZnO nanowires grown by a vapor–solid (VS) mechanism and by oxidation during Joule heating of Zn wires by a current are compared using high-resolution transmission electron microscopy (TEM) and scanning TEM (STEM). Growth of the nanowires by Joule heating is a rapid process, with times of treatment in the range of seconds or minutes, whereas times for more conventional and widespread VS growth are normally in the range of hours at an elevated temperature. Nanowires grown by the two techniques are shown to have similar morphologies and to be single crystalline. The incorporation of Tb dopants into both kinds of nanowires was investigated by using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. In nanowires grown by the VS method, Tb was found to aggregate and form Tb oxide nanoparticles on parts of the nanowire surfaces. In nanowires grown by Joule heating, no segregation of the dopants was observed. These results contribute to the understanding and development of inexpensive Joule synthesis methods for nanowires of ZnO and other oxides.

Advanced (S)TEM measurements revealed that ZnO nanowires grown by conventional vapor−solid mechanism for several hours have single crystalline hexagonal structures with an [0001] growth direction. ZnO nanowires grown by Joule heating by an intense current flow within a few seconds or minutes also have comparable high-quality crystalline structures but with different growth directions and much lower aspect ratio.