Quantum dot
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 100,000
atoms 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.
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'
compton 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.
In semiconductors, quantum dots are small regions of one material buried in another with a larger
band gap. These can be e.g. CdSe in the core and ZnS in the shell. 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.
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.
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 (much less
photodestruction). For single particle tracking, the irregular blinking of quantum dots is a minor drawback. 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%.
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]
*
Quantum wire*
Quantum well*
Quantum point contact* Murray, C. B., Norris, D. J., & Bawendi, M. G.
Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites J. Am. Chem. Soc. 115, 8706-8715, 1993.
* Peng, Z. A., Peng, X.;
Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor (123), J. Am. Chem. Soc., 2001, 183-184.
* Wang, C., Shim, M. & Guyot-Sionnest, P.
Electrochromic nanocrystal quantum dots.,
Science 291 2390-2392 (2001).
* Michalet, X. & Pinaud, F. F. & Bentolila, L. A. & Tsay, J. M. & Doose, S. & Li, J. J. & Sundaresan, G. & Wu, A. M. & Gambhir, S. S. & Weiss, S. (2005, January 28).
Quantum dots for live cells, in vivo imaging, and diagnostics. In
Science, 307, 538 – 544.
* Shim, M. & Guyot-Sionnest, P.
N-type colloidal semiconductor nanocrystals., NATURE 407 (6807): 981-983 OCT 26 2000
* W. E. Buhro and V. L. Colvin,
Semiconductor nanocrystals: Shape matters, Nat. Mater., 2003, 2, 138 139.
* S. Bandyopadhyay and A. E. Miller (2001). "Electrochemically self-assembled ordered nanostructure arrays: Quantum dots, dashes, and wires",
Handbook of Advanced Electronic and Photonic Materials and Devices,
6.
*
High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion R. D. Schaller and V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004)
* Michael J. Bowers II, James R. McBride, and Sandra J. Rosenthal (2005).
White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals, Journal of the American Chemical Society, October 18, 2005.
*
How quantum dots work - flash animations*
Sizing Curve for CdSe Nanocrystals*
Sizing Curve for CdS Nanocrystals*
Quantum dots that produce white light could be the light bulb's successor*
Nanomaterial Database*
Quantum dots device counts single electrons - New Scientist
*
Cheaper Dots : New process slashes the cost of quantum dots Scientific American Magazine (December 2005)
*
Quantum dot on arxiv.org
