Agriculture Reference
In-Depth Information
decreases, and are the origin of phenomena such as
the super-paramagnet-
ism, Coulomb blockade, surface plasmon resonance,
etc.
Surface Plasmon
resonance (SPR) is a collective excitation of the electrons in the conduc-
tion band near the surface of the nano-particles. The surface of the nano-
particles is like plasma having free electrons in the conduction band with
positively charged nuclei. The position of the specific Plasmon absorbance
band indicates the presence of specific size of nano-particles. Specific
Plasmon absorbance at 412 nm (λ
max
) indicates the presence of silver nano-
particles. Similarly, a single absorbance band at 529 nm confirms the pres-
ence of gold nano-particles (7-20 nm). The wavelength of the absorption
peak depends on various factors like the particle size, dielectric constant
of the surrounding medium and inter particle distance. For example, with
the increase in diameter of gold nano-particles from 22 nm to 100 nm, the
λ
max
value increases from 526.5 nm to 540 nm. As mentioned earlier, the
origin of novel properties in metal nano-particle is attributed to an increase
in surface to bulk atoms and appearance of surface plasmon resonance.
The dependence of properties on the shape of the particle originated due to
different reasons: the surface energy, electronic structure and the stability
of different facets. The spherical particles of gold absorb electromagnetic
spectrum in the visible region whereas nanotriangles of gold absorb in the
near infrared region. On the contrary, gold nanorods show two plasmonic
structures: longitudinal plasmon (owing to electronic oscillation parallel
to the longitudinal axis) and transverse plasmon (owing to electronic oscil-
lation parallel to the transverse axis).
9.4
STABILITY OF NANO-PARTICLES
The stabilization of ultra fine or nano-particles in suspension is very im-
portant for both controlling the particle size and for developing process
based application of these suspensions to achieve a desired result. The
formation, stabilization and sedimentation of nano-particles depend upon
the discreet steps of nucleation, condensation and coagulation into larger
particles. Therefore, stabilization requires the optimization of these com-
peting factors. When the size of the particle is reduced to the nano level,
the ratio of surface to the bulk atoms increases thereby increasing energy
of the system as a whole, leading to a decrease in the system stability. A
number of laboratory studies indicate that nano-particles will aggregate to
