Nanotechnology-Enabled Sensors

4.3 Formation of Thin Films 147
In most of the one-dimensional deposition experiments, a lower supersaturation
result in more anisotropic structures. At higher supersaturation
secondary or inhomogenous nucleation may occur, resulting in secondary
growth effects such as bulk deposition and branching.
The pioneering and encompassing work of Wang et al are the first examples
of the deposition of one-dimensional metal oxides and other semiconductors
using simple low vacuum deposition in horizontal ovens. 33 They
used commercially available nano-powders of metal oxides and evaporated
them at temperatures much lower than their actual bulk evaporation. The
materials were evaporated at a high temperature and condensed at lower
temperature sites along the horizontal tube. 34 The Transmission electron
microscopy (TEM) micrographs of ZnO nanobelts deposited by Wang et al
are shown in (Fig. 4.7).
Fig. 4.7 (a) Transmission electron microscopy (TEM) image of the as-synthesized
ZnO nanobelts. (b) High-resolution TEM image recorded with the incident electrons
arriving normal to the top surface of the nanobelt. Reprinted with permission
from the Annual Review of Physical Chemistry. 34
Vapour-Liquid Solid (VLS) growth:
In this method the vapour is condensed on the surface with the help of a
second phase which acts as a catalitic (or an impurity) nucleation site.
The catalyst directs and confines growth within a predetermined direction
(Fig. 4.8). The catalyst acts as a trap for the growth species as it can
either amalgamate within or make a liquid phase on the substrate surface
during the deposition process. The growth species, which adsorbs onto the
catalyst, can participate within it and encourage a gradual one directional