properties, synthesis and applications of quantum dots

1. H QUANTUM DOTS

2. What really Quantum Dots are?

3.  QUANTUM DOTS (QDs) are nanometer-
scale semiconductor crystals composed
of groups II-VI (e.g.- CdS, CdSe ) or III-V
elements.
 They are defined as particles with
physical dimensions smaller than the
exciton Bohr radius , whose excitons are
confined to all three dimensions of
space.
 Quantum dots were discovered during
an research in 1980s by Alex Ekimov in
glass matrix and Louis E Brus in colloidal
solutions.

4. Quantum Dot Description :-
 Size, energy level and emission colour can be precisely controlled.
 Requisite adsorption and resultant emission wavelengths depend
on the dot size.
 The bigger the quantum dots larger the wavelength and smaller is
the frequency.
 The energy band gap increases with the decrease in the size of the
quantum dot.
 Hence more energy is needed to excite the dot and therefore more
energy is released when the crystal returns to the resting state.

5. Energy Bands
Splitting of energy levels in quantum dots due to the quantum
confinement effect, semiconductor band gap increases with decrease
in size of the nanocrystal..

6. SYNTHESIS
Quantum dots are synthesized by 3
common methods:-
 Lithography
 Colloidal synthesis
 Epitaxy

7. LITHOGRAHY
 Quantum wells are covered with
polymer mask and are exposed to
electron or ion beam.
 The surface is covered with a thin
layer of metal and then cleaned
and only the exposed areas keep
the metal layer .Pillars are etched
into the entire surface.
 Multiple layers are applied this way
to build up the properties and size
wanted
 Disadvantages:- slow , possible
contamination, low density, defect
formation

8. COLLOIDAL SYNTHESIS
 Immersion of semiconductor microcrystals in glass
dielectric matrices.
 Taking a silicate glass with 1% semi conducting phase (
CdS, CdSe, CuCl or CuBr).
 Heating for several hours at high temperatures.
 Formation of microcrystals of nearly equal size.
 Typically group II-VI elements.

9. Epitaxy
Semiconducting compounds with smaller band gap are
grown on the surface of compound with larger band gap.
For e.g. GaAs in AlGaAs
Growth is restricted by coating it with a masking compound
( Si02) and etching that mask with the shape of required
crystal cell well wall shape.

10. APPLICATIONS