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Quantum dots, or QDs, are part of nanocrystal semiconductor liquid crystal display (LCD) technology. They are electroactive thin layer sheets deposited over large surface areas on non-planar flexible substrates.
Conventional LED organic molecules are sensitive to oxidation and humidity and have a tendency to degrade. Organic and inorganic QD LEDs have many advantages and represent future-generation display technology that will be found in flat panel TV screens, tablets, mobile phones, and digital cameras.
Nanocrystal performance and properties are determined by the composition and size of the quantum dots, which are photoluminescent as well as electroluminescent.
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Optical Properties of Quantum Dots
QDs from a given material have unusual properties: their size depends on their energy levels.
For example, the light emission from CdSe quantum dots can be slowly tuned from the red zone of the spectrum for a 5mm diameter dot to the violet zone for a 1.5mm dot. The quantum confinement effect depends on QD coloration and is directly related to the energy levels of the QD.
Quantum dots are photoluminescent (or photoactive) and electroluminescent (or electroactive). They have unique physical properties which can easily be integrated into OLED displays. QDs have better color purity, and they also have flexibility due to the incorporation of soluble aqueous and non-aqueous solvents. This provides printable flexible displays of all sizes. Large areas can be covered as on large TV displays, and quantum dots have improved life expectancy.
The saturated colors of QD-based LED displays are of narrow bandwidth. The wavelength can be tuned by changing the QD size, and the displays can cover the entire visible wavelength range from 460 nm blue color to 650 nm red color.
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Fabrication Methods for Quantum Dot Sheets
The fabrication of layers of quantum dots involves phase separation. A layer of single QDs is first created by spin casting a mixed solution of QDs, which simultaneously produces QD mono layers. The configuration of the assembly consists of close-packed hexagonal arrays. These are placed atop co-deposited contacts, and phase separation causes the quantum dots to separate from the lower layer material and rise to the film surface.
The QD structure is affected by many parameters. These parameters are solvent separation, concentration of the solution, distribution of the QD sizes, QD aspect ratio, and the organic solvent. Phase separation is a simple process, but it is not suitable for display applications.
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Solvent-free Contact Printing of Quantum Thin Film Dot Displays
The contact printing process is a solvent-free, simple, and cost effective method of forming QD thin films. The device structure does not come into contact with the solvents during the printing process. The contact printing process has major advantages as it can produce RGB-patterned electroactive structures of 1,000 pixels per inch.
A silicon master is used for molding polydimethlysiloxane (PDMS). Its top side is coated with thin film of Parylene-c, which is a chemical vapor deposited aromatic organic polymer. The ink coated parylene-stamp is produced via spin casting of colloidal quantum dots suspended in organic solvent. When the solvent evaporates, the QD mono layer gets transformed on the substrate by contact printing. QD Vision's printing method can provide uniformity over the large areas necessary for printing displays
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Practical Uses of Quantum Dots
Scientists have ultimately succeeded in making silicon emit light through the quantum dot concept. Basically, the making of quantum dots involves the breaking of a semiconductor material like silicon into atomic levels.
Astonishingly, this attributes to the particles a property in which they get excited and start emitting light throughout the visible spectrum. Depending upon the size, quantum dots may emit lights of different colors, with relatively larger particles emiting light over the red spectrum and the color changing toward blue with a reduction in their dimensions.
Thus, quantum dots are attributed with properties of emitting multicolored lights which specifically becomes applicable in the field of biosensors. The conventional uses for flouroscent dyes have limited spectral range and thus are not fully effective in detecting harmful bio-agents during possible biological warfare. Quantum dots, with their wide spectral range, can be used for detecting hazmat/bio-agent exposure to the pinpoint extent and are being current researched for these types of applications.
The property of quantum dots of emitting lights of different colors over the visible range is also being exploited in the field of light emitting diodes or LEDs.
The potential is also there for QD to be used for producing better and more efficient flat panel TVs and for illuminating cell phone displays with minimal consumption.
QD technology may ultimately result in an efficient lighting solution for the world in the coming years, replacing all the less efficient conventional lighting systems like CFLs and mercury vapor lamps.