Quantum dots are very small semiconductor particles comparable in size to the bohr radius of an exciton. They confine carrier movement (electrons and holes) in all three dimensions, leading to discrete energy levels similar to artificial atoms. As the size of quantum dots decreases, the separation of energy levels increases. Quantum dots can be fabricated using top-down approaches like lithography or bottom-up approaches like colloidal synthesis. They have applications in solar cells, biosensors, LEDs, quantum computation, displays, and lasers due to their tunable optical and electronic properties.

1. Quantum nono dots
NAME UMA SHANKAR
ROLL NO PH19M009

2. Quantum dots
• In solid, movement of carrier (e and hole) can be confined in all three dimensions of space
by surrounding material with larger band gap.
• Confinement leads to discrete energy levels (artificial atoms)
• A quantum dot has a larger band gap
• The crystal structure of QD is same as that of bulk system and spherical in shape.
• Very small semiconductor particle with a size comparable to the bohr radius of exciton (
correlated distance between electron and hole)

3. Quantum dot confinement

4. Discrete energy level
• As decreasing size of quantum dot separation of
energy level increases
• The energy levels depend on the size and also
the shape of quantum dot.
• Smaller quantum dot leads to higher band gap
energy.

5. Fabrication of quantum nanodots
• There are three main ways to confine exciton in semiconductors :
• Lithography
• Colloidal synthesis
• Ball milling
• Epitaxy:
• Pattern growth
• Self-organized growth

6. Synthasis of nanodots
Top to down approach
• A top down approach comprises breaking down of the bulk material into pieces of nanoscale dimensions.
Disadvantage
• Undesired imperfections at the surface structure. e.g. lithography can cause significant crystallographic damage to processed
pattern.
Bottom up approach

7. Schematic diagram of top-down and bottom-
up approach
• Less defect homogeneous chemical
composition.
• Refer to the building up of a system from
several smaller subunits such as, atoms,
molecules or cluster
Disadvantage
• Produce internal stress. Surface defect and
contamination.

8. Applications
• Good efficiency photovoltaic devices: solar cell
• Biology: biosensors : imaging MRI, drug delivery to cancer cells,
• Light emitting diodes: LEDs e.g. CdS
• Quantum coputation, qubit
• Flat panel display, LCD with good colour emission
• Memory elements
• Photodetectors
• Lasers

9. References
• Leutwyler, W. K., et al. Semiconductor clusters,
nanocrystals, and quantum dots, Science 271(5251),
933--937, 1996.
• Brus, L. Electronic wave functions in semiconductor
clusters: experiment and theory, J. Phys. Chem. 90(12),
2555--2560, 1986.
• Brus, L. E. Electron-electron and electron-hole
interactions in small
• semiconductor crystallites: The size dependence of the
lowest excited electronic state, J. Chem. Phys. 80(9),
4403--4409, 1984
• P. Alivisatos, “The use of nanocrystals in biological
detection,” Nature Biotechnology, vol. 22, no. 1, pp.
47–52, 2004. [2]
X.H.Gao,L.L.Yang,J.A.Petros,F.F.Marshall,J.W.Simons,
and S. M. Nie, “In vivo molecular and cellular imaging
with quantum dots,” Current Opinion in
Biotechnology, vol. 16, no. 1, pp. 63–72, 2005.