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{{Use dmy dates|date=July 2013}}
{{technical|date=June 2013}}
{{Nanomat}}
[[File:QD S.jpg|thumb|165px|Colloidal quantum dots irradiated with an UV light. Different sized quantum dots emit different color light due to quantum confinement.]]

A '''quantum dot''' is a [[nanocrystal]] made of [[semiconductor]] materials that are small enough to display quantum mechanical properties.  Specifically, its [[exciton]]s are [[potential well|confined]] in all three [[spatial dimensions]].  The electronic properties of these materials are intermediate between those of bulk semiconductors and of discrete [[molecules]].<ref>{{cite news|accessdate=7 July 2009|url=http://www.columbia.edu/cu/chemistry/fac-bios/brus/group/pdf-files/semi_nano_website_2007.pdf|author=Brus, L.E. |title= Chemistry and Physics of Semiconductor Nanocrystals|year=2007}}</ref><ref>{{cite news|id={{hdl|1721.1/11129}}|author= Norris, D.J.|title=Measurement and Assignment of the Size-Dependent Optical Spectrum in Cadmium Selenide (CdSe) Quantum Dots, PhD thesis, MIT|year= 1995}}</ref><ref>{{cite journal |title=Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies |journal=Annual Review of Materials Research |volume=30 |issue=1 |pages=545–610 |year=2000 |doi=10.1146/annurev.matsci.30.1.545|bibcode = 2000AnRMS..30..545M |last1=Murray |first1=C. B. |last2=Kagan |first2=C. R. |last3=Bawendi |first3=M. G. }}</ref> Quantum dots were discovered in a glass matrix by [[Alexei Ekimov]] and in [[colloid]]al solutions by [[Louis E. Brus]]. The term "quantum dot" was coined by [[Mark Reed (physicist)|Mark Reed]].<ref>{{cite journal |author=Reed MA, Randall JN, Aggarwal RJ, Matyi RJ, Moore TM, Wetsel AE |title=Observation of discrete electronic states in a zero-dimensional semiconductor nanostructure |journal=Phys Rev Lett |volume=60 |issue=6 |pages=535–537 |year=1988 |pmid=10038575|doi=10.1103/PhysRevLett.60.535 |bibcode=1988PhRvL..60..535R|url=http://www.eng.yale.edu/reedlab/publications/26%20QDot%20PRL.pdf}}</ref>

Researchers have studied applications for quantum dots in [[transistor]]s, [[solar cell]]s, [[light-emitting diode|LEDs]], and [[laser diode|diode lasers]]. They have also investigated quantum dots as [[Stain|agents]] for [[medical imaging]] and as possible [[qubit]]s in [[quantum computing]].

Quantum dots are semiconductors whose electronic characteristics are closely related to the size and shape of the individual crystal. Size and [[band gap]] are inversely related in quantum dots. For example, in fluorescent dye applications, emission frequencies increase as the size of the quantum dot decreases, resulting in a color shift from red to blue in the light emitted.<ref name="americanelements">{{cite web|title=Nanotechnology Information Center: Properties, Applications, Research, and Safety Guidelines|url=http://www.americanelements.com/nanotech.htm|publisher=[[American Elements]]}}</ref>  Excitation and emission of the quantum dot are therefore highly tunable.  Because the size of the crystals can be controlled during synthesis, the conductive properties can be carefully controlled. Quantum dots of different sizes can be assembled into a [[Gradient Multi-Layer nanofilm|gradient multi-layer nanofilm]].
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==Quantum confinement in semiconductors==
[[File:Quantum dot.png|thumb|330px|right|3D confined electron wave functions in a quantum dot. Here, rectangular and triangular-shaped quantum dots are shown. Energy states in rectangular dots are more ''s-type'' and ''p-type''. However, in a triangular dot the wave functions are mixed due to confinement symmetry. (Click for animation)|link=File:QuantumDot_wf.gif]]
{{main|Potential well}}

In a semiconductor [[crystallite]] whose diameter is smaller than the size of its [[exciton]] [[Bohr radius]], the excitons are squeezed, leading to [[Potential well#Quantum confinement|quantum confinement]]. The energy levels can then be modeled using the [[particle in a box]] model in which the energy of different states is dependent on the length of the box. Quantum dots are said to be in the 'weak confinement regime' if their radii are on the order of the exciton Bohr radius; quantum dots are said to be in the 'strong confinement regime' if their radii are smaller than the exciton Bohr radius. If the size of the quantum dot is small enough that the quantum confinement effects dominate (typically less than 10&nbsp;nm), the electronic and optical properties are highly tunable.

[[File:Quantum confinement effect.jpg|thumb|right|Splitting of energy levels for small quantum dots due to the quantum confinement effect. The horizontal axis is the radius, or the size, of the quantum dots and a<sub>b</sub>* is the Exciton Bohr radius.]]

[[Fluorescence]] occurs when an excited electron relaxes to the ground state and combines with the [[Electron hole|hole]]. In a simplified model, the energy of the emitted photon can be understood as the sum of the band gap energy between the occupied level and the unoccupied energy level, the confinement energies of the hole and the excited electron, and the bound energy of the exciton (the electron-hole pair):

[[File:Exciton energy levels.jpg|the figure is a simplified representation showing the excited electron and the hole in an exciton entity and the corresponding energy levels. The total energy involved can be seen as the sum of the band gap energy, the energy involved in the Coulomb attraction in the exciton, and the confinement energies of the excited electron and the hole]]

;Band gap energy: The band gap can become larger in the strong confinement regime where the size of the quantum dot is smaller than the Exciton Bohr radius a<sub>b</sub>* as the energy levels split up.
::<math>a^*_b = \varepsilon_r\left(\frac{m}{\mu}\right) a_b</math>
:where a<sub>b</sub> is the Bohr radius=0.053&nbsp;nm, m is the mass, μ is the reduced mass, and ε<sub>r</sub> is the size-dependent dielectric constant
:This results in the increase in the total emission energy (the sum of the energy levels in the smaller band gaps in the strong confinement regime is larger than the energy levels in the band gaps of the original levels in the weak confinement regime) and the emission at various wavelengths; which is precisely what happens in the sun, where the quantum confinement effects are completely dominant and the energy levels split up to the degree that the energy spectrum is almost continuous, thus emitting white light.
;Confinement energy: The exciton entity can be modeled using the particle in the box. The electron and the hole can be seen as hydrogen in the [[Bohr model]] with the hydrogen nucleus replaced by the hole of positive charge and negative electron mass. Then the energy levels of the exciton can be represented as the solution to the particle in a box at the ground level (n&nbsp;=&nbsp;1) with the mass replaced by the [[reduced mass]]. Thus by varying the size of the quantum dot, the confinement energy of the exciton can be controlled.
;Bound exciton energy: There is Coulomb attraction between the negatively charged electron and the positively charged hole. The negative energy involved in the attraction is proportional to Rydberg's energy and inversely proportional to square of the size-dependent dielectric constant<ref>{{cite book |last1=Brandrup |first1=J. |last2= Immergut |first2=E.H. |title=Polymer Handbook |edition=2 |year=1966 |publisher=Wiley |pages=240–246 |location=New York}}</ref> of the semiconductor. When the size of the semiconductor crystal is smaller than the Exciton Bohr radius, the Coulomb interaction must be modified to fit the situation.

Therefore, the sum of these energies can be represented as:
:<math>\begin{align}
  E_\textrm{confinement} &= \frac{\hbar^2\pi^2}{2 a^2}\left(\frac{1}{m_e} + \frac{1}{m_h}\right) = \frac{\hbar^2\pi^2}{2\mu a^2}\\
  E_\textrm{exciton}    &= -\frac{1}{\epsilon_r^2}\frac{\mu}{m_e}R_y = -R_y^*\\
  E &= E_\textrm{band gap} + E_\textrm{confinement} + E_\textrm{exciton}\\
    &= E_\textrm{band gap} + \frac{\hbar^2\pi^2}{2\mu a^2} - R^*_y
\end{align}</math>

where ''μ'' is the reduced mass, ''a'' is the radius, ''m<sub>e</sub>'' is the free electron mass, ''m<sub>h</sub>'' is the hole mass, and ''ε<sub>r</sub>'' is the size-dependent dielectric constant.

Although the above equations were derived using simplifying assumptions, the implications are clear; the energy of the quantum dots are dependent on their size due to the quantum confinement effects, which dominate below the critical size leading to changes in the optical properties. This effect of quantum confinement on the quantum dots have been experimentally verified<ref>{{cite journal |author=Khare, Ankur, Wills, Andrew W., Ammerman, Lauren M., Noris, David J., and Aydil, Eray S. |title=Size control and quantum confinement in Cu2ZnSnS4 nanocrystals |journal=Chem. Commun. |year=2011 |page=47|doi=10.1039/C1CC14687D |volume=47 |issue=42}}</ref> and is a key feature of many emerging electronic structures.<ref>{{cite journal|url=http://www.sciam.com/article.cfm?id=metal-insulator-electronics-wireless |title=New Electronics Promise Wireless at Warp Speed|author=Greenemeier, L. |journal=Scientific American|date=5 February 2008}}</ref><ref>{{cite news|url=http://query.nytimes.com/gst/fullpage.html?res=9D0CE4DA1631F932A05751C1A967958260&scp=2&sq=%22quantum+well%22&st=nyt |title=SCIENCE WATCH; Tiny Lasers Break Speed Record| work=The New York Times | date=31 December 1991}}</ref>

Besides confinement in all three dimensions (i.e., a quantum dot), other quantum confined semiconductors include:
* [[Quantum wire]]s, which confine electrons or holes in two spatial dimensions and allow free propagation in the third.
* [[Quantum well]]s, which confine electrons or holes in one dimension and allow free propagation in two dimensions.

==Production==
[[File:Quantum Dots with emission maxima in a 10-nm step are being produced at PlasmaChem in a kg scale.jpg|thumbnail|Quantum Dots with gradually stepping emission from violet to deep red are being produced in a kg scale at PlasmaChem GmbH]]
There are several ways to confine excitons in semiconductors, resulting in different methods to produce quantum dots. In general, quantum wires, wells and dots are grown by advanced [[epitaxial]] techniques in [[nanocrystal]]s produced by chemical methods or by ion implantation, or in [[nanobot|nanodevices]] made by state-of-the-art [[Photolithography|lithographic]] techniques.<ref>{{cite book|author=C. Delerue, M. Lannoo|title=Nanostructures: Theory and Modelling|isbn=3-540-20694-9|publisher=Springer|year=2004|page= 47}}</ref>

===Colloidal synthesis===
[[Colloid]]al [[semiconductor]] [[nanocrystal]]s are [http://www.youtube.com/results?search_query=quantum+dot&search_type= synthesized] from precursor compounds dissolved in solutions, much like traditional [[chemical synthesis|chemical processes]]. The synthesis of [[colloidal]] quantum dots is based on a three-component system composed of precursors, organic surfactants, and solvents. When heating a reaction medium to a sufficiently high temperature, the precursors chemically transform into [[monomers]]. Once the monomers reach a high enough [[supersaturation]] level, the nanocrystal growth starts with a nucleation process. The temperature during the growth process is one of the critical factors in determining optimal conditions for the nanocrystal growth. It must be high enough to allow for rearrangement and [[Annealing (metallurgy)|annealing]] of atoms during the synthesis process while being low enough to promote crystal growth. Another critical factor that has to be stringently controlled during nanocrystal growth is the monomer concentration. The growth process of nanocrystals can occur in two different regimes, "focusing" and "defocusing". At high monomer concentrations, the critical size (the size where nanocrystals neither grow nor shrink) is relatively small, resulting in growth of nearly all particles. In this regime, smaller particles grow faster than large ones (since larger crystals need more atoms to grow than small crystals) resulting in "focusing" of the size distribution to yield nearly [[monodisperse]] particles. The size focusing is optimal when the monomer concentration is kept such that the average nanocrystal size present is always slightly larger than the critical size. When the monomer concentration is depleted during growth, the critical size becomes larger than the average size present, and the distribution "defocuses" as a result of [[Ostwald ripening]].

There are colloidal methods to produce many different semiconductors. Typical dots are made of binary alloys such as [[cadmium selenide]], [[cadmium sulfide]], [[indium arsenide]], and [[indium phosphide]]. Dots may also be made from ternary alloys such as cadmium selenide sulfide.
These quantum dots can contain as few as 100 to 100,000 atoms within the quantum dot volume, with a diameter of 10 to 50 atoms. This corresponds to about 2 to 10 [[nanometer]]s, and at 10&nbsp;nm in diameter, nearly 3 million quantum dots could be lined up end to end and fit within the width of a human thumb.

Large batches of quantum dots may be synthesized via [[colloidal synthesis]]. Due to this scalability and the convenience of [[benchtop conditions]], colloidal synthetic methods are promising for commercial applications. It is acknowledged{{Citation needed|date=July 2007}} to be the least toxic of all the different forms of synthesis.

===Fabrication===
*Self-assembled quantum dots are typically between 5 and 50&nbsp;nm in size. Quantum dots defined by [[Photolithography|lithographically]] patterned [[Logic gate|gate]] electrodes, or by etching on two-dimensional electron gases in semiconductor heterostructures can have lateral dimensions exceeding 100&nbsp;nm.
*Some quantum dots are small regions of one material buried in another with a larger [[band gap]]. These can be so-called core–shell structures, e.g., with CdSe in the core and ZnS in the shell or from special forms of [[silica]] called [[ormosil]].
*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 (chemistry)|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 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 from two-dimensional electron or hole gases present in remotely doped quantum wells or semiconductor heterostructures called [[lateral quantum dot]]s. The sample surface is coated with a thin layer of resist. A lateral pattern is then defined in the resist by [[electron beam lithography]]. This pattern can then be transferred to the electron or hole gas by etching, or by depositing metal electrodes (lift-off process) that allow the application of external voltages between the electron gas and the electrodes. Such quantum dots are mainly of interest for experiments and applications involving electron or hole transport, i.e., an electrical current.
*The energy spectrum of a quantum dot can be engineered by controlling the geometrical size, shape, and the strength of the confinement potential. Also, 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.
The quantum dot absorption features correspond to transitions between discrete,three-dimensional [[particle in a box]] states of the [[electron]] and the hole, both confined to the same [[nanometer]]-size box.These discrete transitions are reminiscent of atomic spectra and have resulted in quantum dots also being called ''[[artificial atom]]s''.<ref>{{cite book|author=Silbey, Robert J.; Alberty, Robert A.; Bawendi, Moungi G.|title=Physical Chemistry, 4th ed.|publisher=John Wiley &Sons|year=2005|page=835}}</ref>

*Confinement in quantum dots can also arise from [[electrostatic potential]]s (generated by external electrodes, doping, strain, or impurities).
*CMOS technology can be employed to fabricate silicon quantum dots. Ultra small (L=20&nbsp;nm, W=20&nbsp;nm) CMOS transistors behave as single electron quantum dots when operated at cryogenic temperature over a range of −269&nbsp;°C (4&nbsp;[[kelvin|K]]) to about −258&nbsp;°C (15&nbsp;[[kelvin|K]]). The transistor displays Coulomb blockade due to progressive charging of electrons one by one. The number of electrons confined in the channel is driven by the gate voltage, starting from an occupation of zero electrons, and it can be set to 1 or many.<ref>{{cite journal|doi=10.1088/0957-4484/23/21/215204|title=Few electron limit of n-type metal oxide semiconductor single electron transistors|year=2012|last1=Prati|first1=Enrico|last2=De Michielis|first2=Marco|last3=Belli|first3=Matteo|last4=Cocco|first4=Simone|last5=Fanciulli|first5=Marco|last6=Kotekar-Patil|first6=Dharmraj|last7=Ruoff|first7=Matthias|last8=Kern|first8=Dieter P|last9=Wharam|first9=David A|displayauthors=8|journal=Nanotechnology|volume=23|issue=21|pages=215204|pmid=22552118|arxiv = 1203.4811 |bibcode = 2012Nanot..23u5204P }}</ref>

===Viral assembly===
Lee et al. (2002) reported using [[Genetic engineering|genetically engineered]] [[M13 bacteriophage]] [[virus]]es to create quantum dot [[biocomposite]] structures.<ref name="pmid11988570">{{cite journal |author=Lee SW, Mao C, Flynn CE, Belcher AM |title=Ordering of quantum dots using genetically engineered viruses |journal=Science|volume=296 |year=2002 |pmid=11988570 |doi=10.1126/science.1068054 |issue=5569|bibcode = 2002Sci...296..892L |pages=892–5 }}</ref> As a background to this work, it has previously been shown that genetically engineered viruses can recognize specific [[semiconductor]] surfaces through the method of selection by [[Combinatorial biology|combinatorial phage display]].<ref name="pmid10864319">{{cite journal |author=Whaley SR, English DS, Hu EL, Barbara PF, Belcher AM |title=Selection of peptides with semiconductor binding specificity for directed nanocrystal assembly |journal=Nature |volume=405 |year=2000 |pmid=10864319 |doi=10.1038/35015043 |issue=6787 |pages=665–8}}</ref> Additionally, it is known that [[liquid crystal]]line structures of wild-type viruses (Fd, M13, and [[Tobacco mosaic virus|TMV]]) are adjustable by controlling the solution concentrations, solution [[ionic strength]], and the external [[magnetic field]] applied to the solutions. Consequently, the specific recognition properties of the virus can be used to organize inorganic [[nanocrystal]]s, forming ordered arrays over the length scale defined by liquid crystal formation. Using this information, Lee et al. (2000) were able to create self-assembled, highly oriented, self-supporting films from a phage and [[Zinc sulfide|ZnS]] precursor solution. This system allowed them to vary both the length of bacteriophage and the type of inorganic material through genetic modification and selection.

===Electrochemical assembly===
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, onto the metal which is then used as a mask for mesa-etching these nanostructures on a chosen substrate.

===Bulk-manufacture===
Quantum dot manufacturing relies on a process called "high temperature dual injection" which has been scaled by multiple companies for commercial applications that require large quantities (100's of kgs to tonnes) of quantum dots.  This is a reproducible production method that can be applied to a wide range of quantum dot sizes and compositions.

The bonding in certain cadmium-free quantum dots, such as III-V-based quantum dots, is more covalent than that in II-VI materials, therefore it is more difficult to separate nanoparticle nucleation and growth via a high temperature dual injection synthesis. An alternative method of quantum dot synthesis, the “molecular seeding” process, provides a reproducible route to the production of high quality quantum dots in large volumes. The process utilises identical molecules of a molecular cluster compound as the nucleation sites for nanoparticle growth, thus avoiding the need for a high temperature injection step. Particle growth is maintained by the periodic addition of precursors at moderate temperatures until the desired particle size is reached.<ref>A.M. Jawaid, S. Chattopadhyay, D.J. Wink, L.E. Page and P.T. Smee, ''ACS Nano'', 2013, '''7''', 3190</ref> The molecular seeding process is not limited to the production of cadmium-free quantum dots; for example, the process can be used to synthesise kilogram batches of high quality II-VI quantum dots in just a few hours.

Another approach for the mass production of colloidal quantum dots can be seen in the transfer of the well-known hot-injection methodology for the synthesis to a technical continuous flow system. The batch-to-batch variations arising from the needs during the mentioned methodology can be overcome by utilizing technical components for mixing and growth as well as transport and temperature adjustments. For the production of CdSe based semiconductor nanoparticles this method has been investigated and tuned to production amounts of kg per month. Since the use of technical components allows for easy interchange in regards of maximum through-put and size, it can be further enhanced to tens or even 100's of kgs.<ref>http://www.azonano.com/article.aspx?ArticleID=3473</ref>

Recently a consortium of U.S. and Dutch companies reported a "milestone" in high volume quantum dot manufacturing by applying the traditional high temperature dual injection method to a [[flow chemistry|flow system]].<ref>{{cite news|accessdate=7 July 2011|url=http://www.flowid.nl/news/index.html#20110627|author=Quantum Materials Corporation and the Access2Flow Consortium|title=Quantum materials corp achieves milestone in High Volume Production of Quantum Dots|year=2011}}</ref> However as of 2011, applications using bulk-manufactured quantum dots are scarcely available.<ref>{{cite news|accessdate=7 July 2011|url=http://www.economist.com/node/18833511|author=The Economist|title=Quantum-dot displays-Dotting the eyes|date=16 June 2011}}</ref>

===Cadmium-free quantum dots===
Cadmium-free quantum dots are also called "CFQD". In many regions of the world there is now a restriction or ban on the use of [[heavy metals]] in many household goods which means that most [[cadmium]] based quantum dots are unusable for consumer-goods applications.

For commercial viability, a range of restricted, heavy metal-free quantum dots has been developed showing bright emissions in the visible and near infra-red region of the spectrum and have similar optical properties to those of CdSe quantum dots. Among these systems are InP/ZnS and CuInS/ZnS, for example.

[[Cadmium]] and other restricted heavy metals used in conventional quantum dots is of a major concern in commercial applications. For Quantum Dots to be commercially viable in many applications they must not contain cadmium or other restricted metal elements.<ref>{{cite web|accessdate=7 July 2009|url=http://www.nanocotechnologies.com/content/AdvancedMaterials/CadmiumFreeQuantumDotsQFQDHeavyMetalFree.aspx|title=Cadmium-free quantum dots}}</ref>

==Environmental impact==
The environmental impact of bulk manufacturing and consumption of quantum dots is currently undergoing studies in both private and public labs.

==Optical properties==
[[File:CdTe PlasmaChem spectra.gif|thumb|Fluorescence spectra of CdTe quantum dots of various sizes. Different sized quantum dots emit different color light due to quantum confinement.]]
An immediate optical feature of colloidal quantum dots is their color. While the material which makes up a quantum dot defines its intrinsic energy signature, the nanocrystal's quantum confined size is more significant at energies near the [[band gap]]. Thus quantum dots of the same material, but with different sizes, can emit light of different colors. The physical reason is the [[quantum confinement]] effect.

The larger the dot, the [[Spectral color|redder]] (lower energy) its [[fluorescence]] [[spectrum]]. Conversely, smaller dots emit [[Spectral color|bluer]] (higher energy) light. 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 fluorescent light is inversely proportional to the size of the quantum dot. Larger quantum dots have more energy levels which are also 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 (journal)|Nanotechnology]]'' and in other journals have begun to suggest that the shape of the quantum dot may be a factor in the coloration as well, but as yet not enough information is available. Furthermore, it was shown <ref>{{cite journal|doi=10.1103/PhysRevLett.95.236804|url=http://cops.tnw.utwente.nl/pdf/05/PHYSICAL%20REVIEW%20LETTERS%2095%20236804%20(2005).pdf|title=Frequency-Dependent Spontaneous Emission Rate from CdSe and CdTe Nanocrystals: Influence of Dark States|year=2005|author=Van Driel, A. F.|journal=Physical Review Letters|volume=95|page=236804|pmid=16384329|issue=23|bibcode=2005PhRvL..95w6804V|arxiv = cond-mat/0509565 }}</ref> that the lifetime of fluorescence is determined by the size of the quantum dot. Larger dots have more closely spaced energy levels in which the electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longer causing larger dots to show a longer lifetime.

As with any crystalline semiconductor, a quantum dot's electronic [[wave function]]s extend over the [[crystal structure|crystal lattice]]. Similar to a molecule, a quantum dot has both a [[Quantization (physics)|quantized]] energy [[Spectroscopic observations|spectrum]] and a quantized [[density of states|density of electronic states]] near the edge of the band gap.

Quantum dots can be synthesized with larger (thicker) shells (CdSe quantum dots with CdS shells). The shell thickness has shown direct correlation to the spectroscopic properties of the particles like lifetime and emission intensity, but also to the stability.

==Applications==
Quantum dots are particularly significant for optical applications due to their high [[extinction coefficient]].<ref>{{cite doi|10.1021/jp025698c}}</ref> In electronic applications they have been proven to operate like a [[single electron transistor]] and show the [[Coulomb blockade]] effect. Quantum dots have also been suggested as implementations of [[qubit]]s for [[quantum information processing]].

The ability to tune the size of quantum dots is advantageous for many applications. For instance, larger quantum dots have a greater spectrum-shift towards red compared to smaller dots, and exhibit less pronounced quantum properties. Conversely, the smaller particles allow one to take advantage of more subtle quantum effects.

[[File:Achermann7RED.jpg|thumb|right|Researchers at [[Los Alamos National Laboratory]] have developed a device that efficiently produces [[visible light]], through energy transfer from thin layers of quantum wells to crystals above the layers.<ref>{{cite journal |title= Energy-transfer pumping of semiconductor nanocrystals using an epitaxial quantum well|journal=Nature |volume=429 |issue=6992 |pages=642–646|year=2004 |doi= 10.1038/nature02571|last1=Achermann |first1=M. |last2=Petruska |first2=M. A. |last3=Smith |first3=D. L. |last4=Koleske|first4=D. D.|last5=Klimov |first5=V. I. |bibcode = 2004Natur.429..642A }}</ref>]]
Being [[Zero-dimensional space|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 laser]]s, amplifiers, and biological sensors. Quantum dots may be excited within a locally enhanced electromagnetic field produced by gold nanoparticles, which can then be observed from the surface [[plasmon resonance]] in the photoluminescent excitation spectrum of (CdSe)ZnS nanocrystals. High-quality quantum dots are well suited for optical encoding and multiplexing applications due to their broad excitation profiles and narrow/symmetric emission spectra. The new generations of quantum dots have far-reaching potential for the study of intracellular processes at the single-molecule level, high-resolution cellular imaging, long-term in vivo observation of cell trafficking, tumor targeting, and diagnostics.

===Computing===
Quantum dot technology is one of the most promising candidates for use in solid-state [[quantum computation]]. By applying small voltages to the leads, the flow of electrons through the quantum dot can be controlled and thereby precise measurements of the spin and other properties therein can be made. With several [[quantum entanglement|entangled]] quantum dots, or [[qubit]]s, plus a way of performing operations, quantum calculations and the [[quantum computer|computers]] that would perform them might be possible.

===Biology===
In modern biological analysis, various kinds of [[dyes|organic dyes]] are used. However, with each passing year, more flexibility is being required of these dyes, and the traditional dyes are often unable to meet the expectations.<ref name="Walling2009">{{cite journal|last=Walling|first=M. A.|coauthors=Novak, Shepard|date=February 2009|title=Quantum Dots for Live Cell and In Vivo Imaging|journal=Int. J. Mol. Sci.|volume=10|issue=2|pages=441–491|url=http://www.mdpi.com/1422-0067/10/2/441/|doi=10.3390/ijms10020441|pmid=19333416|pmc=2660663}}</ref> 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 extinction co-efficient combined with a comparable quantum yield to fluorescent dyes<ref>{{cite journal |author=Michalet X, Pinaud FF, Bentolila LA, ''et al.'' |title=Quantum dots for live cells, in vivo imaging, and diagnostics |journal=Science |volume=307 |issue=5709 |pages=538–44 |year=2005 |pmid=15681376 |doi=10.1126/science.1104274 |pmc=1201471|bibcode = 2005Sci...307..538M }}
</ref>) as well as their stability (allowing much less [[photobleaching]]). It has been estimated that quantum dots are 20 times brighter and 100 times more stable than traditional fluorescent reporters.<ref name="Walling2009" /> For single-particle tracking, the irregular [[fluorescence intermittency|blinking of quantum dots]] is a minor drawback.

The usage of quantum dots for highly sensitive cellular imaging has seen major advances over the past decade. The improved photostability of quantum dots, for example, allows the acquisition of many consecutive focal-plane images that can be reconstructed into a high-resolution three-dimensional image.<ref>{{cite journal|pmid=15731014|date=Mar 2005|author=Tokumasu, F; Fairhurst, Rm; Ostera, Gr; Brittain, Nj; Hwang, J; Wellems, Te; Dvorak, Ja|title=Band 3 modifications in Plasmodium falciparum-infected AA and CC erythrocytes assayed by autocorrelation analysis using quantum dots|volume=118|issue=Pt 5|pages=1091–8|doi=10.1242/jcs.01662|journal=Journal of Cell Science|type=Free full text}}</ref> Another application that takes advantage of the extraordinary photostability of quantum dot probes is the real-time tracking of molecules and cells over extended periods of time.<ref>{{cite journal|doi=10.1126/science.1088525|date=Oct 2003|author=Dahan, M; Lévi, S; Luccardini, C; Rostaing, P; Riveau, B; Triller, A|title=Diffusion dynamics of glycine receptors revealed by single-quantum dot tracking|volume=302|issue=5644|pages=442–5|pmid=14564008|journal=Science|bibcode = 2003Sci...302..442D }}</ref> Antibodies, streptavidin,<ref>{{cite journal|title=Monovalent, reduced-size quantum dots for imaging receptors on living cells|journal=Nature methods|volume=5|issue=5|pages=397–9|author=Howarth M, Liu W, Puthenveetil S, Zheng Y, Marshall LF, Schmidt MM, Wittrup KD, Bawendi MG, Ting AY. Nat Methods. 2008 May;5(5):397-9|year=2008 |pmid=18425138|pmc=2637151|doi=10.1038/nmeth.1206}}</ref> peptides,<ref>{{cite journal|title=Nanocrystal targeting in vivo|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=99|issue=20|pages=12617–21|author=Akerman ME, Chan WC, Laakkonen P, Bhatia SN, Ruoslahti E. Proc Natl Acad Sci U S A. 2002 Oct 1;99(20):12617-21|year=2002 |pmid=12235356|pmc=130509|doi=10.1073/pnas.152463399|bibcode = 2002PNAS...9912617A }}</ref> DNA,<ref>{{cite journal|title=Formation of targeted monovalent quantum dots by steric exclusion|journal=Nature Methods |author=Farlow J, Seo D, Broaders, KE, Taylor, MJ, Gartner ZJ, Jun, YW. Nat. Methods. U S A. 2013 Oct |year=2013 |doi=10.1038/nmeth.2682 }}</ref> nucleic acid [[aptamer]]s,<ref>{{cite journal|title=Quantum dot-antibody and aptamer conjugates shift fluorescence upon binding bacteria|journal=Biochemical and Biophysical Research Communications|volume=325|issue=3|pages=739–43|year=2004|author=Dwarakanath S, Bruno JG, Shastry A, Phillips T, John AA, Kumar A, Stephenson LD. Biochem Biophys Res Commun. 2004 Dec 17;325(3):739-43|pmid=15541352|doi=10.1016/j.bbrc.2004.10.099}}</ref> or small-molecule ligands can be used to target quantum dots to specific proteins on cells. Researchers were able to observe quantum dots in lymph nodes of mice for more than 4 months.<ref name="Ballou2004">{{cite journal|doi=10.1021/bc034153y|year=2004|author=Ballou, B; Lagerholm, Bc; Ernst, La; Bruchez, Mp; Waggoner, As|title=Noninvasive imaging of quantum dots in mice|volume=15|issue=1|pages=79–86|pmid=14733586|journal=Bioconjugate chemistry|type=Free full text}}</ref>

Semiconductor quantum dots have also been employed for [[in vitro]] imaging of pre-labeled cells. The ability to image single-cell migration in real time is expected to be important to several research areas such as [[embryogenesis]], [[cancer]] [[metastasis]], [[stem cell]] therapeutics, and [[lymphocyte]] [[immunology]].

Scientists have proven that quantum dots are dramatically better than existing methods for delivering a gene-silencing tool, known as [[siRNA]], into cells.<ref>{{cite news|url=http://newswise.com/articles/view/542018/ |title=Gene Silencer and Quantum Dots Reduce Protein Production to a Whisper|publisher=Newswise|accessdate=24 June 2008}}</ref>

First attempts have been made to use quantum dots for tumor targeting under [[in vivo]] conditions. There exist two basic targeting schemes: active targeting and passive targeting. In the case of active targeting, quantum dots are functionalized with tumor-specific binding sites to selectively bind to tumor cells. Passive targeting uses the enhanced permeation and retention of tumor cells for the delivery of quantum dot probes. Fast-growing tumor cells typically have more permeable membranes than healthy cells, allowing the leakage of small nanoparticles into the cell body. Moreover, tumor cells lack an effective lymphatic drainage system, which leads to subsequent nanoparticle-accumulation.

One of the remaining issues with quantum dot probes is their potential in vivo toxicity. For example, CdSe nanocrystals are highly toxic to cultured cells under UV illumination. The energy of UV irradiation is close to that of the [[covalent]] chemical bond energy of CdSe nanocrystals. As a result, semiconductor particles can be dissolved, in a process known as [[photolysis]], to release toxic cadmium ions into the culture medium. In the absence of UV irradiation, however, quantum dots with a stable polymer coating have been found to be essentially nontoxic.<ref name="Ballou2004" /><ref>{{cite journal|title=State of academic knowledge on toxicity and biological fate of quantum dots|journal=Toxicological sciences : an official journal of the Society of Toxicology|volume=112|issue=2|pages=276–96|author=Pelley JL, Daar AS, Saner MA. Toxicol Sci. 2009 Dec;112(2):276-96|year=2009|pmid=19684286|pmc=2777075|doi=10.1093/toxsci/kfp188}}</ref> [[Hydrogel encapsulation of quantum dots]] allows for quantum dots to be introduced into a stable aqueous solution, reducing the possibility of cadmium leakage.Then again, only little is known about the excretion process of quantum dots from living organisms.<ref>{{cite journal|title=Renal clearance of quantum dots|journal=Nature Biotechnology|volume=25|issue=10|pages=1165–70|author=Choi HS, Liu W, Misra P, Tanaka E, Zimmer JP, Itty Ipe B, Bawendi MG, Frangioni JV. Nat Biotechnol. 2007 Oct;25(10):1165–70. Epub 2007 Sep 23|year=2007|pmid=17891134|pmc=2702539|doi=10.1038/nbt1340}}</ref> These and other questions must be carefully examined before quantum dot applications in tumor or [[vascular]] imaging can be approved for human clinical use.

Another potential cutting-edge application of quantum dots is being researched, with quantum dots acting as the inorganic [[fluorophore]] for intra-operative detection of tumors using [[fluorescence spectroscopy]].

===Photovoltaic devices===
{{main|Quantum dot solar cell}}
Quantum dots may be able to increase the efficiency and reduce the cost of today's typical silicon [[photovoltaic cells]]. According to an experimental proof from 2004,<ref>{{cite journal|last1=Schaller|first1=R.|last2=Klimov|first2=V.|title=High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion|journal=Physical Review Letters|volume=92|year=2004|doi=10.1103/PhysRevLett.92.186601|pmid=15169518|bibcode=2004PhRvL..92r6601S|arxiv = cond-mat/0404368 }}</ref> quantum dots of lead selenide can produce more than one exciton from one high energy photon via the process of carrier multiplication or [[multiple exciton generation]] (MEG). This compares favorably to today's photovoltaic cells which can only manage one exciton per high-energy photon, with high kinetic energy carriers losing their energy as heat. Quantum dot photovoltaics would theoretically be cheaper to manufacture, as they can be made "using simple chemical reactions."

===Light emitting devices===
There are several inquiries into using quantum dots as [[light-emitting diode]]s to make displays and other light sources, such as "QD-LED" displays, and "QD-WLED" (White LED). In June 2006, QD Vision announced technical success in making a proof-of-concept [[quantum dot display]] and show a bright emission in the visible and near infra-red region of the spectrum. Quantum dots are valued for displays, because they emit light in very specific [[gaussian distribution]]s. This can result in a display that more accurately renders the colors that the human eye can perceive. Quantum dots also require very little power since they are not color filtered. Additionally, since the discovery of "white-light emitting" QD, general solid-state lighting applications appear closer than ever.<ref>[http://www.vanderbilt.edu/exploration/stories/quantumdotled.html Shrinking quantum dots to produce white light]. Vanderbilt's Online Research Magazine. Vanderbilt.edu. Retrieved on 24 July 2013.</ref> A color liquid crystal display (LCD), for example, is usually [[Backlight|backlit]] by [[fluorescent lamp]]s (CCFLs) or [[LED backlight|conventional white LEDs]] that are color filtered to produce red, green, and blue pixels. A better solution is using a conventional blue-emitting LED as light source and converting part of the emitted light into ''pure'' green and red light by the appropriate quantum dots placed in front of the blue LED. This type of white light as backlight of an LCD panel allows for the best color gamut at lower cost than a RGB LED combination using three LEDs.

Quantum dot displays that intrinsically produce [[monochromatic]] light can be more efficient, since more of the light produced reaches the eye.QD-LEDs can be fabricated on a silicon substrate, which allows integration of light sources onto silicon-based
[[integrated circuits]] or [[microelectromechanical systems]].<ref name="Nano LEDs printed on silicon">{{cite web|date=3 July 2009|title=Nano LEDs printed on silicon|url=http://nanotechweb.org/cws/article/lab/39721}}</ref> A QD-LED integrated at a scanning microscopy tip was used to demonstrate fluorescence near-field scanning optical microscopy ([[NSOM]]) imaging.<ref>{{cite journal|title=Nanoscale fluorescence imaging with quantum dot near-field electroluminescence|journal=Applied Physics Letters|year=2012|volume=101|issue=4|doi=10.1063/1.4739235|bibcode = 2012ApPhL.101d3118H|last1=Hoshino|first1=Kazunori|last2=Gopal|first2=Ashwini|last3=Glaz|first3=Micah S.|last4=Vanden Bout|first4=David A.|last5=Zhang|first5=Xiaojing|pages=043118 }}</ref>

===Photodetector devices===
Quantum dot photodetectors (QDPs) can be fabricated either via solution-processing,<ref>{{cite journal|last1=Konstantatos|first1=G.|last2=Sargent|first2=E. H.|title=Solution-Processed Quantum Dot Photodetectors|journal=Proceedings of the IEEE|volume=97|issue=10|pages=1666–1683|year=2009|doi=10.1109/JPROC.2009.2025612 }}</ref> or from conventional single-crystalline semiconductors.<ref>{{cite journal|last1=Vaillancourt|first1=J.|last2=Lu|first2=X.-J.|title=A High Operating Temperature (HOT) Middle Wave Infrared (MWIR) Quantum-Dot Photodetector|journal=Optics and Photonics Letters|volume=4|issue=2|pages=1–5|year=2011|doi=10.1142/S1793528811000196|last3=Lu|first3=Xuejun }}</ref> Conventional single-crystalline semiconductor QDPs are precluded from integration with flexible organic electronics due to the incompatibility of their growth conditions with the process windows required by organic semiconductors. On the other hand, solution-processed QDPs can be readily integrated with an almost infinite variety of substrates, and also postprocessed atop other integrated circuits. Such [[colloidal]] QDPs have potential applications in surveillance, machine vision, industrial inspection, [[spectroscopy]], and fluorescent biomedical imaging.

==See also==
{{colbegin|3}}
*[[Fluorescence]]
*[[Nanocrystal solar cell]]
*[[Programmable matter]]
*[[Quantum dot display]]
*[[Quantum dot laser]]
*[[Quantum point contact]]
*[[Quantum well]]
*[[Quantum wire]]
*[[Trojan wave packet]]
{{colend}}

==References==
{{reflist|35em}}

===General references===
{{More footnotes|date=February 2008}}
{{refbegin|2}}

*{{cite journal |author=Reed MA |title=Quantum Dots|journal=Scientific American |volume=268 |issue=1 |page=118 |year=1993|url = http://www.eng.yale.edu/reedlab/publications/58%20QDotsSciAm1993.pdf|format = PDF |doi=10.1038/scientificamerican0193-118 |bibcode = 1993SciAm.268..118R  }}
*{{cite journal |author=Murray CB, Norris DJ, Bawendi MG |title=Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites|journal=J Am Chem Soc |volume=115 |issue=19 |pages=8706–15 |year=1993|doi=10.1021/ja00072a025}}
*{{cite journal |author=Peng ZA, Peng X |title=Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor|journal=J Am Chem Soc |volume=123 |pages=183–4 |year=2001|doi=10.1021/ja003633m |pmid=11273619 |issue=1}}
*{{cite journal |author=Wang C, Shim M, Guyot-Sionnest P |title=Electrochromic nanocrystal quantum dots|journal=Science |volume=291 |pages=2390–2 |year=2001|doi=10.1126/science.291.5512.2390 |pmid=11264530 |issue=5512|bibcode = 2001Sci...291.2390W }}
*{{cite journal |author=Shim M, Guyot-Sionnest P |title=n-type colloidal semiconductor nanocrystals |journal=Nature |volume=407 |issue=6807 |pages=981–3 |year=2000 |pmid=11069172 |doi=10.1038/35039577}}
*{{cite journal |author=Buhro WE, Colvin VL |title=Semiconductor nanocrystals: Shape matters |journal=Nature materials |volume=2 |issue=3 |pages=138–9 |year=2003 |pmid=12612665 |doi=10.1038/nmat844|bibcode = 2003NatMa...2..138B }}
*{{cite book|author = Bandyopadhyay S, Miller AE|chapter = Electrochemically self-assembled ordered nanostructure arrays: Quantum dots, dashes, and wires|title = Handbook of Advanced Electronic and Photonic Materials and Devices|editor = Nalwa HS|volume = 6|year = 2001|isbn = 0-12-513745-1|publisher = Academic|location = San Diego, Calif.}}
*{{cite journal|author = Schaller RD, Klimov VI|title = High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion|journal = Phys Rev Lett|year = 2004|volume = 92 |issue = 18|page = 186601|doi = 10.1103/PhysRevLett.92.186601|pmid = 15169518|bibcode=2004PhRvL..92r6601S|arxiv = cond-mat/0404368 }}
*{{cite journal|author = Bowers MJ, McBride JR, Rosenthal SJ|title = White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals|journal=J Am Chem Soc |volume = 127|issue = 44 |pages = 15378–9|doi = 10.1021/ja055470d|year = 2005|pmid = 16262395}}
*Thomas Engel. ''Quantum Chemistry and Spectroscopy.'' ISBN 0-8053-3843-8. Pearson Education, 2006. Pages 75–76.
*C. Delerue, M. Lannoo. ''Nanostructures: Theory and Modelling.'' ISBN 3-540-20694-9. Springer, 2004.

{{refend}}

==External links==
*[http://www.bccresearch.com/report/NAN027B.html Quantum Dots: Technical Status and Market Prospects]
*[http://exploration.vanderbilt.edu/news/news_quantumdot_led.htm Quantum dots that produce white light could be the light bulb's successor]
*[http://www.nrl.navy.mil/nanoscience/files/QDPhysicsTodayArticle.pdf Single quantum dots optical properties]
*[http://xstructure.inr.ac.ru/x-bin/theme3.py?level=2&index1=161737 Quantum dot on arxiv.org]
*[http://www.americanelements.com/AEquantumdots.html Quantum Dots Research and Technical Data]

{{Emerging technologies}}

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