Quantum dot
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A quantum dot is a semiconductor crystal with a diameter of a few nanometers, also called a nanocrystal, that because of its small size behaves like a potential well that confines electrons in three dimensions to a region on the order of the electrons' de Broglie wavelength in size, a few nanometers in a semiconductor. Compare to quantum wires (confined in 2D) and quantum wells (confined in 1D).
Because of the confinement, electrons in the quantum dot have quantized, discrete energy levels, much like an atom. For this reason, quantum dots 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.
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Fabrication
In semiconductors, quantum dots are small regions of one material buried in another with a larger band gap. Quantum dots occur accidentally in quantum well structures due to monolayer fluctuations in the well's thickness. Densely-packed quantum dots form spontaneously under certain conditions during molecular beam epitaxy when a material is grown on a substrate to which it is not lattice matched. The resulting strain produces pyramid-shaped quantum dots in the interface between the materials.
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.
En Masse, 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 differant forms of synthesis.
Applications
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 and detectors.
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 (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 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 from today's 20-30% to 65%. This work was reproduced one year later by an NREL team.
References
- 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. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15681376) In Science, 307, 538 – 544.
- W. E. Buhro and V. L. Colvin, Semiconductor nanocrystals: Shape matters (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12612665), Nat. Mater., 2003, 2, 138 139.
External links
- How quantum dots works - flash animations (http://www.evidenttech.com:80/qdot-definition/quantum-dot-introduction.php)da:Kvanteø