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QUANTUM DOTS
A QUANTUM DOT is a semiconductor nano structure that confines the motion of conduction band electrons
valence band holes or excitons (bound pairs of conduction band electrons and valence band holes) in all three
spatial directions or these are fluorescent semiconductor nanocrystal that confines one or more electrons.
The confinement can be due to electrostatic potentials (generated by external electrodes, doping, strain,
impurities), the presence of an interface between different semiconductor materials (e.g. in core-shell nanocrystal
systems), the presence of the semiconductor surface (e.g. semiconductor nanocrystal), or a combination of these.
A quantum dot has a discrete quantized energy spectrum. The corresponding wave functions are spatially
localized within the quantum dot, but extend over many periods of the crystal lattice. A quantum dot contains a
small finite number (of the order of 1–100) of conduction band electrons, valence band holes, or excitons, i.e., a
finite number of elementary electric charges.
Researchers at Los Alamos National Laboratory have developed a wireless nano device that efficiently produces
visible light, through energy transfer from nano thin layers of quantum wells to nano crystals above the nano
layers
THE PHENOMENON OF QUANTUM CONFINMENT:
Quantum dots are made of nanocrystal semiconductors. Electrons in quantum dots do still have a range of
energies and the basic concepts of semiconductors apply to quantum dots as well. But here lies a major difference
in that excitons have an average physical separation between the electron and the hole, normally referred to as
‘exciton Bohr radius ‘.
Exciton Bohr radius is a physical distance that must be taken into consideration when the size of the dot is
compared with. This parameter is different for each material, whereas in bulk semiconductors crystals tend to be
larger than the exciton Bohr radius, allowing the exciton to extend to its natural limit.
If the size of the semiconductor crystal is so small that it’s comparable with the material’s exciton Bohr radius ,the
electron energy levels must be treated as discrete and not continuous , which means a finite separation between
the energy levels. Under these conditions, the semiconductor behaves differently from the bulk semiconductor
and is called a “quantum dot”. The phenomenon is known as “QUANTUM CONFINEMENT”.
The formation of multiple excitons per absorbed photon happens when the energy of the photon absorbed is far
greater than the semiconductor band gap. This phenomenon does not readily occur in bulk semiconductors
where the excess energy simply dissipates away as heat before it can cause other electron-hole pairs to form. But
in semi-conducting quantum dots, the rate of energy dissipation is significantly reduced, and the charge carriers
are confined within a minute volume, thereby increasing their interactions and enhancing the probability for
multiple excitons to form.
FABRICATION OF QUANTUM DOTS
There are different techniques through which the quantum dots can be fabricated
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. Pillars
are etched into the entire surface. Multiple layers are applied this way to build up the properties and size
wanted.
Disadvantages : Slow, contamination, low density, defect formation.
Colloidal synthesis : Three components precursors, organic surfactants, and solvents In this form of synthesis
precursor molecules are dissolved in solvent. Solution is then heated at large temperature to start creating
monomers. Once the monomers reach a high enough supersaturation level, the nanocrystal growth starts with a
nucleation process by rearranging and annealing of atoms. For this process the temperature control is necessary.
And is done via heat or laser. Due to strong quantum confinement, the nanocrystals show size-tunable
absorption and luminescence. By control of the surface chemistry, we produced photochemically stable
nanocrystals
Disadvantage: Density of quantum dots limited by mask pattern
Epitaxy Synthesis : 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. Using 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
APPLICATIONS OF QUANTUM DOTS
PRODUCTION OF SOLAR CELLS
Quantum dots may have the potential to increase the efficiency and reduce the cost of today's typical silicon
photovoltaic cells. According to experimental proof from 2006, quantum dots of lead selenide can produce as
many as seven excitons from one high energy photon of sunlight (7.8 times the band gap energy). This compares
favorably to today's photovoltaic cells which can only manage one exciton per high-energy photon, with high
kinetic energy carriers losing their energy as heat. This would not result in a 7-fold increase in final output
however, but could boost the maximum theoretical efficiency from 31% to 42%. Quantum dot photovoltaic would
theoretically be cheaper to manufacture, as they can be made "using simple chemical reactions".
USAGE IN DIODE LASERS & BIOLOGICAL SENSORS
Being zero dimensional, quantum dots have a sharper density of states than higher-dimensional structures. As a
result, they have superior transport and optical properties, and are being researched for use in diode lasers,
amplifiers, and biological sensors.
Sony Blue-Ray DVD an Sony Blue-Ray DVD and HD-DVD are currently available technologies using quantum dot
lasers..
QUANTUM COMPUTING
Quantum dot technology is one of the most promising candidates for use in solid-state quantum computation. By
applying small voltages to the leads, one can control the flow of electrons through the quantum dot and thereby
make precise measurements of the spin and other properties therein. With several entangled quantum dots, or
qubits, plus a way of performing operations, quantum calculations might be possible.
In modern biological analysis, various kinds of organic dyes are used. However, with each passing year, more
flexibility is being required of these dyes, and the traditional dyes are often unable to meet the expectations. To
this end, quantum dots have quickly filled in the role, being found to be superior to traditional organic dyes on
several counts, one of the most immediately obvious being brightness (owing to the high quantum yield) as well
as their stability (much less photo destruction). For single-particle tracking, the irregular blinking of quantum
dots is a minor drawback. Currently under research as well is tuning of the toxicity.
LIGHT EMITTING DIODES
There are several inquiries into using quantum dots as light-emitting diodes to make displays and other light
sources: "QD-LED" displays, and "QD-WLED" (White LED). In June, 2006, QD Vision announced technical
success in making a proof-of-concept quantum dot display. Quantum dots are valued for displays, because they
emit light in very specific Gaussian distributions. This can result in a display that can more accurately render the
colors that the human eye can perceive. Quantum dots also require very little power since they are not color
filtered. A LCD display, for example, is powered by a single fluorescent lamp that is color filtered to produce red,
green, and blue pixels. Thus, when a LCD display shows a fully white screen, two-thirds of the light is absorbed by
the filters. Displays that intrinsically produce monochromatic light can be more efficient, since more of the light
produced reaches the eye.
OPTICAL BIOPSY
Quantum dots can be coated with materials that selectively bind to various biological molecules. The coating is
responsible for finding the desired biostructure, while the QDs inside allow us to track their movement.
"This gives the molecules something like a tail light, and you could follow them in the body by exciting their
luminescence with ultraviolet light." - T.J. Mountziaris, professor of chemical and biological engineering in the
School of Engineering and Applied Sciences of the University of Buffalo. After revealing the general area of the
tumor, the emitted light allows for detailed inspection of the tumor at a molecular level. Tracking of protein
movement inside individual cancer cells Some of the common coatings include antibodies, peptides, and nucleic
acids
CONCLUSION
Quantum dots due to their extraordinary properties find their application in various fields ranging from
engineering to medicine. This technology which is a branch of nano technology is still in its infancy and lots of
research is being done to open up the intricacies surrounding it. NASA has sanctioned a $ 11-million R&D project
to Rice University to develop an experimental power cable made from carbon nanotubes.The project is to produce
a 1m long prototype of quantum wire by 2010!. Nanotechnology without doubt is the technology of the future.
Quantum dots derive their trade mark from the technology of the future. Their miniature size and versatile
properties grant them the flexibility to be used for a variety of applications.
Quantum dots have not yet begun their journey, but once path is set the power of quantum dots will conquer the
GLOBE
References :
http://en.wikipedia.org/wiki/Quantum_dot
http://azhar-paperpresentation.blogspot.com/2010/04/quantum-dots.html
http://www.scribd.com/doc/23338281/Presentation-Quantum-dots