CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

exothermic reaction between the metal nitrates and fuel can rapidly heat the system to high
temperatures without an external heat source. Nanomaterials synthesized by combustion
route are generally homogeneous, contain fewer impurities, and have higher surface areas
than powders prepared by conventional solid-state methods. The parameters that influence
the reaction are the nature of fuel, fuel to oxidizer ratio, ignition temperature and water
content of the precursor mixture. The major advantage of this method is that large-scale
production can be made at relatively low temperatures and disadvantage is that the particles
obtained are highly agglomerated and can not be dispersed in solvents.
1.4 Properties of nanomaterials: A number of physical phenomena become noticeably
pronounced as the size of the material decreases. The electronic properties of solids are
significantly altered with reduction in particle size. This effect becomes dominant when the
nanometer size range is reached. Additionally, a number of physical (mechanical, electrical,
optical, etc.) properties change when compared to macroscopic systems. In the following
section, electronic and optical properties of materials with nanosize dimensions are discussed.
1.4.1 Electronic properties: In atoms or molecules, the electronic energy levels are discrete
and the electronic wave functions of many such atoms/ molecules overlap and give band
structure in bulk materials. As the particle size decreases the energy level corresponding to
the charge carriers increases. Further, the energy gap between valance band and conduction
band also increases when bands become discrete energy levels with decrease in the crystal/
particle size. Reduction in particles size not only causes an increase of the energy gap (blue
shift of the absorption edge) but also changes the density of states (DOS). Density of states is
defined as the number of energy states available in a unit energy interval in unit volume to be
occupied. Electronic and optical properties observed for the bulk materials and their
corresponding low dimensional system can be explained based on their differences in the
density of states.
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