QUANTUMDOTS

1. QUANTUM DOTS
Presentation By:
D.Anusha
Guide: Mr.Vishnu 151FA05227
ECE
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2.  CONTENTS
1. Introduction
2. Quantum dot
3. How to make QDss
4. Lithography
5. Colloidal synthesis
6. Epitaxy
7. Applications
8. Advantages
9. Disadvantages
10.Conclusion 2

3.  INTRODUCTION:
 Quantum dots are semiconductor Nanocrystals.
 They are made of many of the same materials as
ordinary semiconductors (mainly combinations of
transition metals and/or metalloids).
 Unlike ordinary bulk semiconductors, which are
generally macroscopic objects, quantum dots are
extremely small, on the order of a few
nanometers. They are very nearly zero-
dimensional.
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 QUANTUM DOT
 Very small semiconductor particles with a size comparable
to the Bohr radius of the excitons
(separation of electron and hole).
 Electrons and holes are confined in all three dimensions
of space by a surrounding material with a larger bandgap.
 Discrete energy levels (artificial atom).
 A quantum dot has a larger bandgap.
 Like bulk semiconductor, electrons tend to make transitions
near the edges of the bandgap in quantum dots.

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HOW TO MAKE QUANTUM DOTS
 There are three main ways to confine excitons in semiconductors:
1. Lithography
2. Colloidal synthesis
3. Epitaxy:
I. Patterned Growth
II. Self-Organized Growth

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 LITHOGRAPHY
 Quantum wells are covered with a polymer mask and exposed
to an electron or ion beam.
 The surface is covered with a thin layer of metal, then cleaned
and only the exposed areas keep the metal layer.
 Multiple layers are applied this way to build up the properties
and size wanted.
 Disadvantages: slow, contamination, low density,
defect formation.

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 COLLOIDAL SYNTHESIS
 Immersion of semiconductor microcrystals in glass dielectric
matrices.
 Taking a silicate glass with 1% semiconducting phase (CdS,
CuCl, CdSe, or CuBr).
 Heating for several hours at high temperature.
 Formation of microcrystals of nearly equal size.
 Typically group II-VI materials (e.g. CdS, CdSe).

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 EPITAXY: PATTERNED GROWTH
 Semiconducting compounds with a smaller bandgap (GaAs)
are grown on the surface of a compound with a larger
bandgap (AlGaAs).
 Growth is restricted by coating it with a masking compound
(SiO2) and etching that mask with the shape of the required
crystal cell wall shape.
 Disadvantage: density of quantum dots limited by
mask pattern.

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 EPITAXY: SELF-ORGANIZED GROWTH
 Uses a large difference in the lattice constants of the substrate and
the crystallizing material.
 When the crystallized layer is thicker than the critical thickness,
there is a strong strain on the layers.
 The breakdown results in randomly distributed islets of regular
shape and size.
 Disadvantages: size and shape fluctuations, ordering.

10.  APPLICATIONS OF QUANTUM DOTS
The ability to tune the size of quantum dots is advantageous for
many applications.
Computing
Biology
Photovoltaic
devices
Light emitting
devices
Photodetector
devices
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11.  ADVANTAGES :
 It requires small amount of energy in order to be excited.
 It is suitable for high sensitivity applications like fluorescent
tagging and live-cell imaging.
 There are multiple methods to develop them easily and cost
effectively.
 Quantum dot formations absorb photons of light and then re-emit
longer wavelength photons for a period of time.
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12.  DISADVANTAGES
 In biological applications they cannot diffuse across cellular
membranes due to large physical size.
 Quantum dots may blink and become invisible.
 It may lead to quantum yield deterioration,
meaning:the ratio of the emitted to the absorbed energy is low
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13.  CONCLUSION
 QUANTUM DOTS is a Nano crystal made of semi-
conductor materials that are small enough to exhibit quantum
mechanical properties.
 There are several ways to confine excitons in semiconductors,
resulting in different methods to produce quantum dots.
 Research effort around the world is being applied to expanding
the accuracy and capabilities of this Nano Particles for its usage
in the industry of Hardware Components and Electronics as it is
one of the most promising candidates for use in solid-
state quantum computation.
 Quantum dots have also been suggested as implementations
of qubits for Quantum Information Processing. 13

14. THANK YOU
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