Formation of Single-Crystalline ZnO Nanotubes without Catalysts or Templates
Samuel L. Mensah, Vijaya K. Kayastha, and Yoke Khin
Single-crystalline nanotubes of ZnO were directly grown on planar substrates by conventional thermal chemical vapor deposition (CVD) without the use of catalysts, multiple processing steps, or templates. This achievement resulted from careful control of the growth conditions and fundamental understanding of the vapor-solid crystal growth modes for ZnO at the nanoscale. Under typical growth conditions the surface migration of adatoms across a hexagonally-faceted ZnO protrusion will lead to growth of a solid nanorod. However at sufficiently lower temperatures, surface migration is suppressed and nucleation at the protrusion edges is preferred. Under these conditions, faceted, hollow nanotubes of ZnO grow only from the energetically-preferred edge sites along the  direction. By carefully separating the temperatures for nucleation and growth, single-crystalline nanotubes can grow. Multichanneled nanotubes grow if one or more nucleation sites occur along the surface of the original nanorod face.
Nanotubes of wide band gap materials offer alternative functionalities to carbon nanotubes in electronic, optical, photochemical, nanofluidic, biological, and chemical applications. Like carbon nanotubes, the synthesis of sufficient quantities of nanotubes with high crystalline quality has historically hindered the fundamental understanding of their properties and their technological development toward functional applications. The ZnO nanotubes demonstrated in this work maintain the band structure and crystal structure of bulk wurtzite ZnO crystals according to high-resolution transmission electron microscopy and x-ray, Raman, and photoluminescence spectra. Through further understanding and control of the vapor-solid growth kinetics of ZnO, the nucleation and growth of crystalline nanotubes with even smaller diameters and higher aspect ratios should be achievable. Understanding how the fundamentals of crystal growth can be guided to shape new nanostructures with high crystalline quality is significant for the general rational design of functional nanoscale materials.
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This user research was conducted at Michigan Technological University with support from the U.S. Department of Army, National Science Foundation CAREER award, and the Center for Nanophase Materials Sciences which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy.
Scanning electron micrographs reveal the appearance of single and multichannel ZnO nanotubes grown by catalyst-free chemical vapor deposition. The scale bars are 300 nm. Figure 3 (and cover art) is from Ref. 1.