Materials in the micrometer scale mostly exhibit
physical properties the same as that of bulk form;
however, materials in the nanometer scale may
exhibit physical properties distinctively different
from that of bulk. There are two major effects
that are responsible for the change in nanoparticle
properties due to the size variations. First,
the intrinsic properties of the nanoparticles
are transformed by quantum confinement. Below
a critical size, there is substantial variation
of fundamental optical and electrical properties
with size, which can be realized when the energy
level spacing exceeds the thermal energy. Second,
the number of surface atoms in a nanoparticle
is a large fraction of the total number of atoms
and as a result, melting temperature suppression,
solid-solid phase transition, etc. have been observed.
are being made to synthesize nanoparticles, nanowires
and carbon nanotubes by simple methods. Metal
oxide nanowires with very large aspect ratio have
already been achieved simply by heating the metal
in air atmosphere. We have also successfully synthesized
millimeter long carbon nanotubes by a pyrolysis
The properties of a material decide its application.
As the size of a material is reduced, the band
gap increases, metallic systems behave like semiconductors,
the particles melts at low temperature. Therefore,
it is essential to investigate different properties
of nanostructures to ascertain their applications
at different conditions. Electrical, optical and
thermodynamic properties of different nanostructures
are being investigated.
Field emission (FE) is a unique quantum-mechanical
effect where electrons tunnel from condensed matter
into a vacuum. FE is of great commercial interest
in flat panel displays, x-ray sources, and other
vacuum microelectronic devices. In past decades,
research in this area mainly focused on carbon-based
materials because of their high mechanical stability,
good conductivity and negative electron affinity.
One-dimensional (1D) nanostructured materials
such as carbon nanotubes (CNTs)-were particularly
thought to be good candidates for FE as they have
the added advantages of high aspect ratio, which
enhances the electric field on the sharp end of
their structures. Many researchers consider ZnO
1D nanostructures as good field emitters and has
been investigated intensively as a potential alternative
for producing field emission with low threshold
and high efficiency. Generally, ZnO 1D nanostructures
have shown better field emission characteristics
for needle-like structures because sharper tips
increase the effective electric field at the tips.
The other advantages of ZnO are that it is thermally
stable and intrinsically oxidation resistant.
Research on ZnO 1D nanostructures as field emitters
has only recently begun, so their field emission
characteristics have not been optimized sufficiently.
Various 1D ZnO nanostructures-such as nanowires,
nanoribbons, tetrapods-have been fabricated to
investigate the FE behavior and is under progress.
Gas sensing: The
use of sensors to monitor gas atmospheres represents
a growing market resulting from strategies for
intelligent process management, environmental
protection and medicinal diagnostics as well as
from the domestic, aerospace and automobile sector.
Hence, the development of fast responding, sensitive
and especially highly selective gas sensor materials
is of major interest. SnO2 is an n-type semiconductor
material and is one of the most investigated materials.
The investigation on the gas sensing behavior
of ZnO and CNTs is under progress.