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Quantum dots in brief

Quantum dots (QDs), also known as semi conducting nanoparticles, are promising zero‐dimensional advanced materials because of their nanoscale size and because they can be engineered to suit particular applications. Quantum dots are nanosized semi conductors that generate electron-hole pairs confined in all three dimensions (quantum confinement) and hence behave like giant molecules rather than bulk semiconductors. Often referred to as artificial atoms, quantum dots range in size from 2-10 nanometers in diameter. While typically composed of several thousand atoms, all the atoms are shared and coordinated as if there is only one atomic nucleus at the centre. That property enables numerous revolutionary schemes for electronic devices. The single atom quantum dots have also demonstrated significant control over individual electrons by using very little energy which is the key to quantum dot application in entirely new forms of silicon-based electronic devices, such as ultra low power computers.
QD applications
Quantum dots find applications in light emitting diodes, transistors, solar cells, drug delivery, cancer therapy, cell imaging, made to fluoresce in different colors depending on their size, superb carrier and last much longer than conventional dyes when used to tag molecules, and usually stop emitting light in seconds. Researchers are using QDs in medicine, cell and molecular biology and working to develop nanocrystal quantum-dot-based lasers, amplifiers and biological sensors capable of detecting cancer. QD is used as nonlinear optical devices (NLO), electro‐optical devices, in computing applications and as single-photon emitters. QDs can be joined to polymers in order to produce nanocomposites which can be considered a scientific revolution of the 21st century. semiconductor QDs can be conjugated with biomolecules to produce a new class of markers and probes that have affinities for binding with selected biological structures.
QDs do not scatter light at visible or longer wavelengths and hence minimize optical losses in practical applications, quantum confinement and surface effects become very important and therefore manipulation of the dot diameter or modification of its surface allows the properties of the dot to be controlled. Quantum confinement affects the absorption and emission of photons from the dot.
Studies of semiconductor devices utilizing quantum dots are vigorous, such as quantum dot memory devices utilizing the hole burning effects. Quantum dots can be incorporated into solar-cell devices in many different ways.
Techniques are known which artificially form quantum dots by using fine patterning technologies. When a quantum dot is positioned in close vicinity with a plasmonic nanostructure, the electric field enhancement will strongly enhance the QD optical cross section and modify its radiative lifetime.
Quantum dots interact with a solid-state environment, necessitating cryogenic operation temperatures, and yet environment-induced decoherence, but this can be offset by being fixed in place with large dipole moments and integrated into monolithic optical micro cavity structures.
Most quantum dots contain highly toxic metals such as cadmium, which tends to be released when the quantum dots enter the cells or organisms. In one study, CdTe quantum dots coated with hydrophilic sodium thioglycolate caused disruption in a cultured monolayer of Caco-2 human intestinal cells and cell-death at 0.1 ppm, which was thought to be caused by the quantum dots, rather than cadmium.
In another study, CdSe/ZnS quantum dots injected intravenously into mice caused marked vascular thrombosis in the lungs at 0.7 to 3.6 nanomol per mouse, especially when the quantum dots had carboxylate surface groups.
Researchers claim that quantum dots activated the coagulation cascade through contact. In fact, many kinds of nanoparticles enhance the formation of insoluble fibrous protein aggregates (amyloids), which are associated with human diseases including Alzheimer’s, Parkinson’s and Creutzfeld-Jacob disease.
Quantum dot structures have drawn attention as the ultimate structure based upon quantum mechanics. A quantum dot is an ultra fine structure having an energy level lower than a potential of a nearby region and being able to three dimensionally confine carriers in an ultra fine region. Only two electrons can exist in one quantum dot at the ground level on the conduction band side. If a quantum dot is used as an active region of a laser device, interaction between electrons and holes can be made efficient. A laser device using quantum dots is expected to be a device which exceeds the limit of laser devices using a two dimensionally extending quantum well layer, from the viewpoint of an oscillation threshold value, the temperature characteristics of the oscillation threshold value and the like.
Quantum dots can be formed by various methods such as lithography with electron beams; a method of disposing quantum dots on vertices of pyramid crystals stacked on a mask pattern, a method of disposing quantum dots on vertices of quadrilateral pyramids formed under a mask pattern; a method utilizing initial lateral growth of crystals on a slanted substrate; a method utilizing atom manipulation based upon STM (scanning tunneling microscopy); and the like. These methods have the common aspect that semiconductor materials are artificially processed. These methods are therefore advantageous in that the position of each quantum dot can be controlled freely.
Another method of forming quantum dots is by self-organization. Specifically, a semiconductor layer is formed through vapor phase epitaxial growth under the specific conditions of lattice mismatch. In this case, not a film which two-dimensionally and uniformly extends on an underlying surface but a three dimensional fine structure (quantum dot structure) is formed by itself. With this method, as compared to artificial fine patterning, a quantum dot structure can be formed in which quantum dots are distributed at a higher density and each quantum dot has a high quality.
SK mode
The best known one of self-organization of quantum dots is the Stranski-Krastanov mode (SK mode). During the growth in the SK mode, a two dimensionally extending thin film (wetting layer) is grown initially on an underlying surface, and as source material continues to be supplied, quantum dots are formed by themselves. The quantum dots formed in the SK mode are buried in a quantum well layer so that the wavelength of luminescence of quantum dots can be controlled. Quantum dots having a uniform size can be formed by the SK mode.
Deposition method
The quantum dot deposition method used by Hwang and his colleagues is typical but, ultimately, random. A layer of fairly uniform thickness can be obtained using spin-coating techniques, for example, but it is no way to get a finely arranged array of particles with submicron resolution between particles.
The ability to precisely lay down individual quantum dots could be the next step toward improved biosensors, but also to better LEDs, organic LEDs, solar panels and other optoelectronics.
But it’s not easy as one must create the particles, and then move them around individually to the desired location. Doing this with optical tweezers would be direct, but besides other challenges, would be dreadfully slow when spacing out an array of particles on a commercial scale.
Instead, placing arrangements of quantum dots has typically been done through photochemical means. One such method is to deposit quantum dots with molecular tags, or ligands, that adhere the particles onto a substrate, then, with lasers, chemicals or both, strip the bonds of select adherents, leaving only the quantum dots required.
Researchers at Texas A&M University have developed a method named litho synthesis that takes advantage of the photo-oxidation process. Litho synthesis is a process wherein CdSe quantum dots are capped with a photo-oxidizable molecule, then spread onto a positively charged glass, silicon or other substrate. When a laser scans the layered surface, a portion of the caps are broken, leaving behind patterned arrays of quantum dots that have various emission intensities and wavelengths.
Ion beam scanner
Another method is provided for forming quantum holes of nanometer levels. In an ion beam scanner, ions are projected from an ion gun onto a semiconductor substrate. During the projection, ions are focused into an ion beam whose focal point is controlled to determine the diameter of the ion beam, and the ion beam is accelerated. When being incident upon the semiconductor substrate, the ion beam is deflected so as to form a plurality of quantum holes. Also provided is a semiconductor for use in a light emitting device with quantum dots. Impurities are doped onto a semiconductor substrate to form a P-type semiconductor layer on which an undoped, intrinsic semiconductor is grown to a certain thickness. A plurality of quantum holes are provided for the intrinsic semiconductor layer followed by filling materials smaller in energy band gap than the intrinsic semiconductor in annealed quantum holes through recrystallization growth. Next, an N-type semiconductor layer is overlaid on the quantum hole layer. Composition of the materials filled in the quantum holes determines the color of the light emitted from the semiconductor for use in a light emitting device.
Thus, the semiconductor is fabricated to emit light of the three primary colors or one of them. By cutting the semiconductor, unit display panels or elements can be prepared which emit radiation at wavelengths corresponding to red, green and blue colors.

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