size effect of nanomaterials

Size Effect Of Nanomaterials

(Solid state Phase Transformation and Bandgap Variation)

Presented By_ To_

Sanjeeb Limbu(14305019) Dr.A.Subramania Sir

M.Tech.-Nanoscience & Technology Associate Professor

(FIRST YEAR) (CNST)

PONDICHERRY UNIVERSITY

Solid state Phase Transformation:

In the Universe matter exists in three states; solid, liquid and gaseous

state.

The phase of a solid substance is stable when the thermodynamic

variables like volume, pressure, temperature and energy are minimum.

If any of the thermodynamic variables is varied, the Gibb's free energy

of the system also changes continuously. If the variation in free energy

leads to change in structural details of a phase, a “Phase transformation

or phase transition" is said to occur.


Phase diagram:

Phase diagrams are maps that give the relationship between

phases in equilibrium in a system as function of temperature

pressure and composition.

We can study the phase changes keeping one of the variable

as constant and varying the other two variables.


One component system:

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Two component system:


The Lead-Tin phase diagram:


Size dependence of phase transition:

There are many examples phase transition occurring when

the particle dimension is reduces below a critical value.

CERAMICS: Monoclinic-tetragonal transition of Zirconia.

normally zirconia crystallises in monoclinic form.

but when size is reduced to 10 nm range it crystallises in

tetragonal structure.


Size dependence of phase transition:

METALS: The bulk nickel crystallises in fcc structure.

when the grain size reduced beyond 4nm , the nickel

crystallises in hcp structure.

SEMICONDUCTORS: In materials such as CdS nano

particles shows wurtzite structure up to 5 nm, less than that it

shows zinc blende structure.


Band gap variation:

Bandgap: The energy gap between valence and conduction

band is of fundamental importance for the properties of a solid.

When the size of a solid is reduced to the nanometer length

scale any change of the gap may significantly alter the

material’s physics and chemistry.

Therefore, the science and the technology of nanomaterials

needs to take into account a bandgap, which is different from

that of the bulk.


Band gap variation:

The relation between particle size and bandgap is well

explained in quantum physics by the relation

E is energy Eigen value (bandgap)

A is particle size


The figure below represents the variation of band gap in

bulk and different quantum dot structures


Example: Calculated optical bandgap energies for silicon crystallites

dependent on their diameter

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The bandgap also differs for different nano morphologies

such as nanowires, dots and sheets.

The figure below represents the variation of bandgap of

different nano morphologies as a function of size.


References:

Material Science and Engineering- V Raghavan

Materials Science- MS Vijaya and G Rangarajan


QUERIES !!!!!!!!
