Sunday, April 25, 2010

quantum dots

ABSTRACT:

Semiconductor’s are the milestones of the electronics industry .Traditional semiconductors such as silicon and germanium have their limitations .They are not versatile and the chips constructed using them cannot be shrunk beyond limits. Their optical and electronic quantities are difficult to control since their band gap cannot be easily adjusted .As their band gaps cannot be tuned to the requirements, emission frequencies are beyond adjustment.

Conventional semiconductors owing to their shortcomings are to be replaced by futuristic semiconductors – QUANTUM DOTS. Quantum dots are essentially nanocrystals with limitless applications .They have tunable band gap, and are versatile and extremely flexible to form. Their unique size and composition give them novel quantum properties .Quantum dots can be used to emit a wide range of wavelengths ,say, the seven colors of rainbow or even infrared.
Conventional solar cell made from a single material can theoretically achieve efficiency of about 30 percent. In practice, the best achievable is about 25 percent .In Quantum dots there is an optimum band gap that corresponds to the highest possible solar-electric energy conversion, and this can also be achieved by using a mixture of quantum dots of different sizes for harvesting the maximum proportion of the incident light.

INTRODUCTION:

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 QUANTUM DOTS 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



PROPERTIES:


NANOCRYSTALS are also referred to as quantum dots (QDs due to their incredibly small size (ranging from <1.5>8 nanometers).These usually non-fluorescing compounds develop intense, long-lasting colors excitable by UV and visible light LEDs, lasers, etc. The colors produced are a function of the particle size (blue to red and infrared, depending on the diameter of the nanocrystal) with the smallest NCs fluorescing in the blue and green. The fluorescence is due to a phenomenon called quantum confinement.

Size of Quantum Dot is the most influential determinant of the color emitted. Quantum Dots with the same material can emit different colors when their size is changed. Size manipulation is an easy way to fine tune the color of light emitted by Quantum Dots


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”.

Multiple excitons from one photon:
Researchers led by Arthur Nozik at the National Renewable Energy Laboratory Golden, Colorado in the United States really grabbed the headline when they demonstrated that the absorption of a single photon by their quantum dots yielded - not one exciton as usually the case - but three of them [7].
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.
The researchers report a quantum yield of 300 percent for 2.9nm diameter PbSe (lead selenide) quantum dots when the energy of the photon absorbed is four times that of the band gap. But multiple excitons start to form as soon as the photon energy reaches twice the band gap. Quantum dots made of lead sulphide (PbS) also showed the same phenomenon.
APPLICATIONS:
1.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".

2. 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..

3. 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.
5.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.
1. 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
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Conducting wires with minimal resistance, lighter weight and greater strength in nanoscopic form are known as “quantum wires”. Quantum wires basically consist of a bunch of carbon nanotubes.

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.

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