TEM-BASED INVESTIGATIONS ON CVD-ASSISTED GROWTH OF ZnO NANOWIRES INSIDE NANOCHANNELS OF ANODIZED ALUMINUM OXIDE TEMPLATE

Abstract

The role of Chemical Vapor Deposition's (CVD) temperature and time duration on the mass transport properties and repercussions of these properties on the crystalline growth of ZnO nanowires inside Anodic Aluminum Oxide (AAO) nanotubes (aspect ratio = 1500) have been investigated. CVD assisted growth of nanowires in AAO nanochannels was carried out for 27 min at deposition temperature of 813 K (A1) and for 180 min at 903 K (A2) under identical gas flow rates ( Ar : 23 sccm and O2 : 0.45 sccm) under a chamber pressure of 4.45 torr. Electron diffraction patterns (SAED) of the ZnO diffused inside A1 nanotube (exposed in Focused Ion Beam milled cross-sections) showed a ringed pattern, signifying polycrystalline morphology at the tip as well as the channel bottom. For A2, a single crystal formation at least in the tip region was confirmed by the SAED pattern while the corresponding High Resolution TEM image (HRTEM) revealed a lattice spacing of 0.24 nm corresponding to the (1 0 1) growth direction of zincite (JCPDS-36-1451). Chemical composition of the nanowires, reflected the inherent stoichiometry of preferentially nucleated ZnO in the grooved orifices of individual nanopores. Data points of normalised values of the Zn concentration (TEM-EDS line scan) versus penetration depth (~ 0.6 μm), were empirically fitted with solutions of modified Fickian diffusion equation in 1-D. The Diffusion coefficient values of the order of 0.06 nm2 s-1 and 0.22 nm2 s-1 were obtained for ZnO flow inside nanochannels of A1 and A2, respectively. Observed variations, in diffusivity and relative density, have been explained qualitatively by considering the transport of ZnO inside the nanochannels as arising from superposition of the temperature dependent contributions of the convectional viscous creep and surface diffusion components.