Agriculture Reference
In-Depth Information
9.3
UNIQUE FEATURES OF NANO-PARTICLES
Because of the small size, the physical, chemical, electronic properties of
nano-structures changes as a function of size and are very different from
that of their bulk counterparts. Due to the small size of nano-particles,
there are more atoms on the surface compared to the interior of the par-
ticles, which leads to a large surface to volume ratio which in turn leads
to higher reactivity of nano-particles. For example, if a cube of 1 mm size
is broken into cubes of 1 nm size, the total volume remains the same, but
the surface area is increased by 10
6
times. One of the principle ways in
which a nano-particle differs from a larger or bulk material is that a high
proportion of the atoms that are associated with a NP occur at the surface.
As surface area increases in comparison to the volume, the behavior of
the atoms on the surface of the particle becomes more potent as compared
to those atoms that are inside the particle. Once particles become small
enough they exhibit quantum mechanical behavior. Because their size is
smaller than the order of wavelength, nanoparticles do not obstruct light.
The large surface to volume ratio also results in more interaction be-
tween atoms in intermixed materials in nanoparticles, which may lead to
increased strength, increased heat resistance, etc. Melting points of na-
no-material decreases for clusters smaller than a few hundred angstroms.
For example the melting temperature of gold decreases by approximately
a factor of two when the cluster size is reduced from 10 nm to 2 nm.
The melting point of gold in bulk is 1337°K whereas melting point of
gold nano-particle (~2 nm) is 650°K. Magnetic Properties of nano-clus-
ters are also very different from that of the corresponding bulk material.
For example nanoclusters of certain materials like Pd, Na, K and Rh are
ferromagnetic, where as in bulk form, these elements are paramagnetic.
Super-paramagnetism is a phenomenon that arises from the small size of
nanoclusters. Discretezation of energy level is an important property of
nano-particles. In bulk, the overlapping of the molecular orbitals of a large
number of atoms results in a continuum of energy levels or energy band.
But in nano-particles, due to fewer atoms the overlapping of their orbilt-
als is not much and thereby exhibit discrete energy levels. Thus discreti-
zation of the electronic energy levels takes place in nano-particles along
with an increase in the electronic band gape energy, which in turns, results
in interesting optical and electronic properties in nano-particles. Most of
these
changes are related to the appearance of quantum effects as
the size
