A quantum dot is a nanoscale structure, e.g. a semiconductor nanocrystal which can confine electrons or other carriers in all three dimensions. It is usually embedded in another semiconductor material or in a glass, but there are also colloidal quantum dots in a liquid (a colloidal suspension). A quantum dot must have a band gap energy which is smaller than that of the surrounding medium, because otherwise the excitation energy could easily be lost in the surroundings.
The carrier confinement is a quantum effect. It totally changes the density of states for the confined particles, compared with the density of states for a larger piece of the material. For an ideal isolated quantum dot, there are discrete energy levels, corresponding to a delta-shaped density of states with no states in between the delta peaks. Such a behavior is known from atoms; in this sense, quantum dots can be considered as a kind of artificial atoms where the energy levels can be adjusted by design, e.g. by controlling the quantum dot dimensions or the material composition. In reality, large ensembles of quantum dots are normally used, and their size distribution leads to a somewhat smeared-out density of states, i.e. to inhomogeneous broadening.
Quantum dots can be fabricated from semiconductors. This can even accidentally happen when thin quantum wells are fabricated with molecular beam epitaxy (MBE) or metal–organic chemical vapor deposition (MOCVD), where some fluctuations of the thickness can occur. Under certain conditions (Stranski–Krastanov growth of a lattice-mismatched material on some substrate, e.g. InAs on GaAs), quantum dots can be grown in a self-organized fashion, so that they acquire more consistent sizes and distribution. Subsequently, they are often overgrown with the substrate material. Typical dimensions of the often pyramidally shaped quantum dots are of the order of 5–20 nm; quantum dots are therefore considered to be a topic of nanotechnology. Their density on a surface can be of the order of 109 dots per square centimeter. In most cases, a large number of dots are simultaneously used in a device, although some quantum optics experiments require single quantum dots.
There are also techniques for solution processing of quantum dots. Here, some precursor compounds are dissolved in an organic solvent at high temperature. When the precursor is increasingly converted to monomers, the latter may become supersaturated in the solvent, so that nanocrystals start to nucleate and grow further. The resulting colloidal solution can then be used to fabricate a layer containing a substantial density of quantum dots. The main attraction of colloidally suspended quantum dots is the prospect of fabricating them cheaply in large amounts. This would be attractive e.g. for large light-emitting diodes, but other applications are also envisaged, e.g. in displays , where electrically excited quantum dots would provide the light emission of the pixels, removing the need for background illumination.
Position and size control of quantum dots is also possible by growth on patterned substrates.
Quantum dots are useful for a variety of applications:
- The perhaps oldest application of quantum dots is the formation of beautiful colors in stained glasses, e.g. in church windows.
- Quantum dots make possible the fabrication of laser diodes with very low threshold pump power and/or low temperature sensitivity.
- Quantum dots can be used in white light-emitting diodes (LEDs): they are excited with a blue or near-ultraviolet LED and emit e.g. red and green light (acting as a kind of phosphor), so that overall a white color tone is achieved.
- In semiconductor saturable absorber mirrors (SESAMs), quantum dots can serve as absorbers with a very low saturation fluence. Such quantum dot absorbers can also be contained in a glass matrix.
- Quantum dots can be parts of very sensitive photodetectors, and in the future they may function in efficient photovoltaic cells.
- In the context of quantum cryptography, quantum dots can serve as single-photon emitters.
- Quantum dots might also be suitable for performing quantum computations.
- The mentioned functions can also be realized in the context of quantum nanophotonics.
The so far technologically most important applications are probably those in LEDs and laser diodes. Quantum dot semiconductor lasers are expected to replace some more of the currently used quantum well laser diodes.
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