Properties of Nanomaterials:
Nanomaterials have the structural features in between of those of atoms and the
bulk materials.(12): Fluorescence emission of (CdSe) ZnS quantum dots of various sizes and
absorption spectra of various sizes and shapes of gold nanoparticles
The optical properties of nanomaterials depend on parameters such as feature size,
shape, surface characteristics, and other variables including doping and interaction
with the surrounding environment or other nanostructures.Due to their small dimensions, nanomaterials have extremely large surface area to
volume ratio, which makes a large to be the surface or interfacial atoms, resulting
in more "surface" dependent material properties. Especially when the sizes of
nanomaterials are comparable to length, the entire material will be affected by the
30
surface properties of nanomaterials. This in turn may enhance or modify the
properties of the bulk materials. For example, metallic nanoparticles can be used as
very active catalysts. Chemical sensors from nanoparticles and nanowires
enhanced the sensitivity and sensor selectivity. The nanometer feature sizes of
nanomaterials also have spatial confinement effect on the materials, which bring
the quantum effects. The energy band structure and charge carrier density in the materials can be
modified quite differently from their bulk and in turn will modify the electronic
and optical properties of the materials. For example, lasers and light emitting
diodes (LED) from both of the quantum dots and quantum wires are very
promising in the future optoelections. High density information storage using
quantum dot devices is also a fast developing area. Reduced imperfections are also
an important factor in determination of the properties of the nanomaterials. Nanosturctures and Nanomaterials favors of a self-purification process in that the
impurities and intrinsic material defects will move to near the surface upon thermal
annealing. This increased materials perfection affects the properties of
nanomaterials. For example, the chemical stability for certain nanomaterials may
be enhanced, the mechanical properties of nanomaterials will be better than the
bulk materials. The superior mechanical properties of carbon nanotubes are well
known. Due to their nanometer size, nanomaterials are already known to have
many novel properties. Many novel applications of the nanomaterials rose from
these novel properties have also been proposed. Optical properties:
One of the most fascinating and useful aspects of nanomaterials is their optical
properties. Applications based on optical properties of nanomaterials include
optical detector, laser, sensor, imaging, phosphor, display, solar cell,In this class of material, polymers filled with silicate platelets exhibit the best
mechanical properties and are of the greatest economic relevance. The larger the
particles of the filler or agglomerates, the poorer are the properties obtained. Although, potentially, the best composites are those filled with nanofibers or
nanotubes, experience teaches that sometimes such composites have the least
ductility. On the other hand, by using carbon nanotubes it is possible to produce
composite fibers with extremely high strength and strain at rupture. Among the
most exciting nanocomposites are the polymer- ceramic nanocomposites, where
the ceramic phase is platelet-shaped. This type of composite is preferred in nature,
and is found in the structure of bones, where it consists of crystallized mineral
platelets of a few nanometers thickness that are bound together with collagen as the
matrix. Composites consisting of a polymer matrix and defoliated phyllosilicates
exhibit excellent mechanical and thermal properties. Magnetic properties:
Bulk gold and Pt are non-magnetic, but at the nano size they are magnetic. Surface
atoms are not only different to bulk atoms, but they can also be modified by
interaction with other chemical species, that is, by capping the nanoparticles. This
phenomenon opens the possibility to modify the physical properties of the
nanoparticles by capping them with appropriate molecules. Actually, it should be
possible that non-ferromagnetic bulk materials exhibit ferromagnetic-like behavior
when prepared in nano range. One can obtain magnetic nanoparticles of Pd, Pt and
the surprising case of Au (that is diamagnetic in bulk) from non-magnetic bulk
materials. In the case of Pt and Pd, the ferromagnetism arises from the structural
changes associated with size effects.Disadvantages of Nanomaterials:
? Instability of the particles Retaining the active metal nanoparticles is
highly challenging, as the kinetics associated with nanomaterials is rapid. In order to retain
nanosize of particles, they are encapsulated in some other matrix. Nanomaterials are thermodynamically metastable and lie in the region of
high-energy local-minima. Hence they are prone to attack and undergo
transformation. These include poor corrosion resistance, high solubility, and
phase change of nanomaterials. This leads to deterioration in properties and
retaining the structure becomes challenging. Fine metal particles act as strong explosives owing to their high surface area
coming in direct contact with oxygen. Their exothermic combustion can
easily cause explosion. ? Impurity - Because nanoparticles are highly reactive, they inherently interact
with impurities as well. In addition, encapsulation of nanoparticles becomes
necessary when they are synthesized in a solution (chemical route). The
stabilization of nanoparticles occurs because of a non-reactive species
engulfing the reactive nano-entities. Thereby, these secondary impurities
become a part of the synthesized nanoparticles, and synthesis of pure Electrical Properties:
Electrical Properties of Nanoparticles" discuss about fundamentals of electrical
conductivity in nanotubes and nanorods, carbon nanotubes, photoconductivity of
nanorods, electrical conductivity of nanocomposites.As the nanotubes have different lengths, then with increasing protrusion
of the fiber bundle an increasing number of carbon nanotubes will touch the
surface of the mercury droplet and contribute to the electrical current transport.photocatalysis, photoelectrochemistry and biomedicine
CdSe Nanoparticles
2.3nm
5.5nm
1.0-
0.8-
Gold Nanoparticles
06-
15nm nanospheres
30nm nanospheres
04-
2.5AR nanorods
-4.5AR nanorods
7.5AR nanorods
02-
Photo Felice Frankel
0.0-
400
500
600 700 800 900 1000 1100 1200 1300
Wavelength (nm)
Fig.This is mainly due to the nanometer size of the materials which render them: (i)
large fraction of surface atoms; (ii) high surface energy; (iii) spatial confinement;
(iv) reduced imperfections, which do not exist in the corresponding bulk materials.As the lengths and orientations of
the carbon nanotubes are different, they touch the surface of the mercury at
different times, which provides two sets of information: (i) the influence of carbon
nanotube length on the resistance; and (ii) the resistances of the different
nanotubes.While most microstructured materials have similar properties to the
corresponding bulk materials, the properties of materials with nanometer
dimensions are significantly different from those of atoms and bulks materials.One interesting method which
can be used to demonstrate the steps in conductance is the mechanical thinning of a
nanowire and measurement of the electrical current at a constant applied voltage.The important point here is that, with decreasing diameter of the wire, the number
of electron wave modes contributing to the electrical conductivity is becoming
increasingly smaller by well-defined quantized steps.(13): Electrical behavior of naotubes
In electrically conducting carbon nanotubes, only one electron wave mode is
observed which transport the electrical current.However, when an anisotropy is added to the
nanoparticle, such as growth of nanorods, the optical properties of the nanoparticles
change dramatically.With the CdSe semiconductor nanoparticles, a simple change in size
alters the optical properties of the nanoparticles.When metal nanoparticles are
enlarged, their optical properties change only slightly as observed for the different
samples of gold nanospheres.(12)
Exemplifies the difference in the optical properties of metal and semiconductor
nanoparticles.Likewise, shape can
have dramatic influence on optical properties of metal nanostructures.Metal
Semiconductor
Graphite
Fig.Fig.