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Quantum dot

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File:Fluorescence in various sized CdSe quantum dots.png
Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots.

A quantum dot, also called a semiconductor nanocrystal or an artificial atom, is a semiconductor crystal whose size is on the order of just a few nanometers. They contain anywhere from 100 to 1,000 electrons and range from 2 to 10 nanometers, or 10 to 50 atoms, in diameter. At 10 nanometers in diameter, nearly 3 million quantum dots could be lined up end to end and fit within the width of your thumb.

Description

These quantum dots confine electrons, holes, or electron-hole pairs or so-called excitons to zero dimensions to a region on the order of the electrons' de Broglie wavelength. This can be contrasted to quantum wires, which are confined to a line and quantum wells, which are confined to a planar region. This confinement leads to discrete quantized energy levels and to the quantization of charge in units of the elementary electric charge e. Quantum dots are particularly significant for optical applications due to their theoretically high quantum yield. Quantum dots have also been suggested as implementations of a qubit for quantum information processing.

Because the quantum dot has discrete energy levels, much like an atom, they are sometimes called "artificial atoms". The energy levels can be controlled by changing the size and shape of the quantum dot, and the depth of the potential. Like in atoms, the energy levels of small quantum dots can be probed by optical spectroscopy techniques. In contrast to atoms it is relatively easy to connect quantum dots by tunnel barriers to conducting leads, which allows the application of the techniques of tunneling spectroscopy for their investigation.

One of the optical features of small excitonic quantum dots immediately noticeable to the unaided eye is coloration. While the material which makes up a quantum dot is significant, more significant in terms of coloration is the size. The larger the dot, the redder (the more towards the red end of the spectrum) the fluorescence. The smaller the dot, the bluer (the more towards the blue end) it is. The coloration is directly related to the energy levels of the quantum dot. Quantitatively speaking, the bandgap energy that determines the energy (and hence color) of the fluoresced light is inversely proportional to the square of the size of the quantum dot. Larger quantum dots have more energy levels which are more closely spaced. This allows the quantum dot to absorb photons containing less energy, i.e. those closer to the red end of the spectrum. Recent articles in Nanotechnology and other journals have begun to suggest that the shape of the quantum dot may well also be a factor in the colorization, but as yet not enough information has become available.

The ability to tune the size of quantum dots is advantageous, as the larger and more red-shifted the quantum dots, the less the quantum properties are. The small size of the quantum dot allows people to take advantage of these quantum properties.

Fabrication

In semiconductors, quantum dots are small regions of one material buried in another with a larger band gap. Quantum dots sometimes occur spontaneously in quantum well structures due to monolayer fluctuations in the well's thickness. Self-assembled quantum dots nucleate spontaneously under certain conditions during molecular beam epitaxy(MBE) and metallorganic vapor phase epitaxy (MOVPE), when a material is grown on a substrate to which it is not lattice matched. The resulting strain produces coherently strained islands on top of a two-dimensional "wetting-layer". This growth mode is known as Stranski-Krastanov growth. The islands can be subsequently buried to form the quantum dot. This fabrication method has the most potential for applications in quantum cryptography (i.e. single photon sources) and quantum computation. The main limitations of this method are the cost of fabrication and the lack of control over positioning of individual dots.

Individual quantum dots can be created by a technique called electron beam lithography, in which a pattern is etched onto a semiconductor chip, and conducting metal is then deposited onto the pattern.

Mass Production

In large numbers, quantum dots may also be synthesized by means of a colloidal synthesis. Epitaxy, lithography, and colloidal synthesis all have different positive and negative aspects. By far the cheapest, colloidal synthesis also has the advantage of being able to occur at benchtop conditions and is acknowledged to be the least toxic of all the different forms of synthesis.

Highly ordered arrays of quantum dots may also be self assembled by electrochemical techniques. A template is created by causing an ionic reaction at an electrolyte-metal interface which results in the spontaneous assembly of nanostructures, including quantum dots, on the metal which is then used as a mask for mesa-etching these nanostructures on a chosen substrate.

Yet another method is pyrolytic synthesis, which produces large numbers of quantum dots that self-assemble into preferential crystal sizes.

Applications

Researchers at Los Alamos National Laboratory have developed a wireless nanodevice that efficiently produces visible light, through energy transfer from nano-thin layers of quantum wells to nanocrystals above the nanolayers.

Being quasi-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.

Quantum dots have quickly found their way into homes in many electronics. The new PlayStation 3 and high-definition DVD players (notably Blu-ray and HD-DVD) to come out all use a blue laser for data reading. The blue laser up until only a few years ago was beginning to be seen as something of an impossibility, until the synthesis of a blue quantum dot laser.

Quantum dots are one of the most hopeful candidates for 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.

Another cutting edge application of quantum dots is also being researched as potential artificial fluorophore for intra-operative detection of tumors using fluorescence spectroscopy.

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 simply unable to meet the necessary standards at times. 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. Currently under research as well is tuning of the toxicity.

In a paper published in the May 2004 issue of Physical Review Letters a team from Los Alamos National Laboratory found that quantum dots produce as many as three electrons from one high energy photon of sunlight. When today's photovoltaic solar cells absorb a photon of sunlight, the energy gets converted to at most one electron, and the rest is lost as heat. This could boost the efficiency of panels produced in research labs from today's 20-30% to 42%.[1] This work was reproduced one year later by an NREL team.

Another paper, published in the October 18, 2005 issue of the Journal of the American Chemical Society, reports that Michael Bowers II at Vanderbilt University discovered that certain size crystals of cadmium and selenium emit white light when excited by an ultraviolet laser. This emission appears to be coming from the surface of the crystal, rather than the center. The crystals contain either 33 or 34 pairs of atoms. While they are being pyrolytically synthesized, they preferentially form into just this size; so Bowers can make a batch of such crystals in about an hour. Another student then mixed these quantum dots into ordinary varnish, applied it to a blue LED, and observed that the emission is yellowish-white, like a light bulb. The researchers believe that it will be possible to achieve this emission of white light via electrical stimulation as well as photonic, and hope to demonstrate it soon.

There are several inquiries into using quantum dots to make displays and light sources: "QD-LED" displays, and "QD-WLED" (White LED) [1]. In June, 2006, QD Vision announced technical success in making a proof of concept quantum dot display. [2] Quantum dots are valued for displays, because they are very small, they emit colored light in very specific frequencies, and because they require very little power, since they are entirely self-illuminating. [3]

See also

References

  1. ^ "Peter Weiss". "Quantum-Dot Leap". Science News Online. Retrieved 2005-06-17.
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