Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement.

A quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result, they have properties that are between those of bulk semiconductors and those of discrete molecules.^[1][2][3] A semiconductor is a solid material that has electrical conductivity in between that of a conductor and that of an insulator; it can vary over that wide range either permanently or dynamically. ... An exciton is a bound state of an electron and a hole in an insulator (or semiconductor), or in other words, a Coulomb correlated electron/hole pair. ... A potential well is the region surrounding a local minimum of potential energy. ... In science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ...

Researchers have studied quantum dots in transistors, solar cells, LEDs, and diode lasers. They have also investigated quantum dots as agents for medical imaging and hope to use them as qubits. Some quantum dots are commercially available.^[4][5] Assorted discrete transistors A transistor is a semiconductor device, commonly used as an amplifier or an electrically controlled switch. ... A solar cell, made from a monocrystalline silicon wafer A solar cell or photovoltaic cell is a device that converts light energy into electrical energy. ... LED redirects here. ... A packaged laser diode with penny for scale. ... For other uses, see Stain (disambiguation). ... Medical imaging designates the ensemble of techniques and processes used to create images of the human body (or parts thereof) for clinical purposes (medical procedures seeking to reveal, diagnose or examine disease) or medical science (including the study of normal anatomy and function). ... Molecule of alanine used in NMR implementation of error correction. ...

Quantum confinement in semiconductors

Main article: Potential Well

In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length called the Bohr exciton radius. If the electron and hole are constrained further, then the semiconductor's properties change. This effect is a form of quantum confinement, and it is a key feature in many emerging electronic structures.^[6][7] A potential well is the region surrounding a local minimum of potential energy. ...

Other quantum confined semiconductors include:

1. quantum wires, which confine electrons or holes in two spatial dimensions and allow free propagation in the third.
2. quantum wells, which confine electrons or holes in one dimension and allow free propagation in two dimensions.

In condensed matter physics, a quantum wire is an electrically conducting wire, in which quantum effects are affecting transport properties. ... A quantum well is a potential well that confines particles in one dimension, forcing them to occupy a planar region. ...

Making quantum dots

There are several ways to confine excitons in semiconductors, resulting in different methods to produce quantum dots.

Colloidal synthesis

Colloidal semiconductor nanocrystals are synthesized from precursor compounds dissolved in solutions, much like traditional 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 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 Oswald ripening. A Colloid or colloidal dispersion is a type of homogeneous mixture. ... A semiconductor is a solid material that has electrical conductivity in between that of a conductor and that of an insulator; it can vary over that wide range either permanently or dynamically. ... A nanocrystal is a crystalline material with dimensions measured in nanometers; a nanoparticle with a structure that is mostly crystalline. ... In chemistry, chemical synthesis is purposeful execution of chemical reactions in order to get a product, or several products. ... A colloid or colloidal dispersion, is a form of matter intermediate between a true solution and a mixture (suspension). ... In chemistry, a monomer (from Greek mono one and meros part) is a small molecule that may become chemically bonded to other monomers to form a polymer. ... Enormous highly pure, single crystal substances can be grown from a solution at the metastable boundary between an unsaturated and supersaturated solution. ... Anneal may refer to: Annealing (metallurgy), a heat treatment wherein the microstructure of a material is altered, causing changes in its properties such as strength and hardness. ...

There are colloidal methods to produce many different semiconductors, including cadmium selenide, cadmium sulfide, indium arsenide, and indium phosphide. 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 nanometers, and at 10 nm in diameter, nearly 3 million quantum dots could be lined up end to end and fit within the width of a human thumb. Cadmium selenide (CdSe) is a solid, binary compound of cadmium and selenium. ... Cadmium sulfide (UK English sulphide), the mineral greenockite, is an hexagonal, yellowish crystal with specific gravity of 4. ... Indium arsenide, InAs, or indium monoarsenide, is a semiconductor material, a semiconductor composed of indium and arsenic. ... Indium phosphide (InP) is a semiconductor composed of indium and phosphorus. ... For other uses, see Atom (disambiguation). ... A nanometre (American spelling: nanometer) is 1. ...

Large quantities of quantum dots may be synthesized via colloidal synthesis. Colloidal synthesis is by far the cheapest^{[citation needed]} and has the advantage of being able to occur at benchtop conditions. It is acknowledged^{[citation needed]} to be the least toxic of all the different forms of synthesis. In general, a colloid or colloidal dispersion, is a two-phase system of matter; a type of mixture intermediate between homogeneous mixtures and heterogeneous mixtures. ... Benchtop Conditions are conditions at which a bench in a laboratory would normally be under. ...

Fabrication

• Self-assembled quantum dots are typically between 10 and 50 nm in size. Quantum dots defined by lithographically patterned gate electrodes, or by etching on two-dimensional electron gases in semiconductor heterostructures can have lateral dimensions exceeding 100 nm.

Fluorescence emission spectra of CdTe quantum dots of different sizes.

• 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 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. 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.
• Confinement in quantum dots can also arise from electrostatic potentials (generated by external electrodes, doping, strain, or impurities).

Photolithography is a process used in semiconductor device fabrication to transfer a pattern from a photomask (also called reticle) to the surface of a substrate. ... A logic gate performs a logical operation on one or more logic inputs and produces a single logic output. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ... Cadmium telluride (CdTe) is a crystalline compound formed from cadmium and tellurium with a zinc blende (cubic) crystal structure (space group F43m). ... This article or section does not adequately cite its references or sources. ... The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ... Ormosil is a nano-engineered, organically modified silica, which shows great promise in a wide range of applications such as: an alternative to viral vectors for gene delivery, with superior transfection efficiencies suspension media and substrates for next generation solarcells (quantum dots) This future world today technology has also already... A quantum well is a potential well that confines particles in one dimension, forcing them to occupy a planar region. ... Molecular beam epitaxy, abbreviated MBE, is the deposition of one or more pure materials onto a single crystal wafer, one layer of atoms at a time, under ultra-high vacuum, forming a perfect crystal. ... In chemistry a molecule experiences strain when in a chemical conformation there exist unfavorable bond angles or bond distances. ... Quantum cryptography, or quantum key distribution (QKD), uses quantum mechanics to guarantee secure communication. ... Molecule of alanine used in NMR implementation of error correction. ... // Conventional electron-beam lithography The practice of using a beam of electrons to generate patterns on a surface is known as Electron beam lithography. ... Electric potential is the potential energy per unit charge associated with a static (time-invariant) electric field, also called the electrostatic potential, typically measured in volts. ...

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, on the metal which is then used as a mask for mesa-etching these nanostructures on a chosen substrate. Electrochemistry is the study of the electronic and electrical aspects of chemical reactions. ...

Optical properties

An immediate optical feature of colloidal quantum dots is their coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, the quantum confined size of the nanocrystal 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. This article or section does not adequately cite its references or sources. ...

The larger the dot, the redder (lower energy) its fluorescence spectrum. Conversely, smaller dots emit 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 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 coloration, but as yet not enough information has become available. Furthermore it was shown recently^[8] that the lifetime of fluorescence is detemined by the size. 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 and thus these large dots show a larger lifetime. Color is an important part of the visual arts. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ... This article deals with the general meaning of spectrum and the history of its use. ... Color is an important part of the visual arts. ... In solid state physics and related applied fields, the band gap is the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. ... Nanotechnology refers broadly to a field of applied science and technology whose unifying theme is the control of matter on the atomic and molecular scale, generally 100 nanometers or smaller, and the fabrication of devices with critical dimensions that lie within that size range. ...

As with any crystalline semiconductor, a quantum dot's electronic wave functions extend over the crystal lattice. Similar to a molecule, a quantum dot has both a quantized energy spectrum and a quantized density of electronic states near the band edge. A wave function is a mathematical tool that quantum mechanics uses to describe any physical system. ... Enargite crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... Look up quantization in Wiktionary, the free dictionary. ... This article deals with the general meaning of spectrum and the history of its use. ... Density of states (DOS) is a property in statistical and condensed matter physics that quantifies how closely packed energy levels are in some physical system. ...

Applications

Quantum dots are particularly significant for optical applications due to their theoretically high quantum yield. 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 qubits for quantum information processing. The Quantum Yield of a radiation-induced process is the number of times that a defined event (usually a chemical reaction step) occurs per photon absorbed by the system. ... In physics, a Coulomb blockade, named after Charles-Augustin de Coulomb, is the increased resistance at small bias voltages of an electronic device comprising at least one low-capacitance tunnel junction. ... A qubit representation by a Bloch sphere. ... Quantum information processing is concerned with what we can and cannot do with quantum information. ...

The ability to tune the size of quantum dots is advantageous for many applications. For instance, larger quantum dots have spectra shifted towards the red compared to smaller dots, and exhibit less pronounced quantum properties. Conversely the smaller particles allow one to take advantage of quantum properties.

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

Being 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. Image File history File linksMetadata Download high resolution version (1500x2122, 266 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Quantum dot Portal:Science/Previous articles Metadata This file contains additional information, probably added from the... Image File history File linksMetadata Download high resolution version (1500x2122, 266 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Quantum dot Portal:Science/Previous articles Metadata This file contains additional information, probably added from the... Los Alamos National Laboratory, aerial view from 1995. ... The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ... A quantum well is a potential well that confines particles in one dimension, forcing them to occupy a planar region. ... Dimension (from Latin measured out) is, in essence, the number of degrees of freedom available for movement in a space. ... Density of states (DOS) is a property in statistical and condensed matter physics that quantifies how closely packed energy levels are in some physical system. ... A laser diode is a laser where the active medium is a semiconductor p-n junction similar to that found in a light-emitting diode. ...

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, 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. Molecule of alanine used in NMR implementation of error correction. ... It has been suggested that Quantum coherence be merged into this article or section. ... A qubit representation by a Bloch sphere. ... A fluorophore-labeled human cell. ... Fluorescence spectroscopy or fluorometry is a type of electromagnetic spectroscopy used for analyzing fluorescent spectra. ...

Biology

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 often unable to meet the expectations. 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. A dye can generally be described as a coloured substance that has an affinity to the substrate to which it is being applied. ... The Quantum Yield of a radiation-induced process is the number of times that a defined event (usually a chemical reaction step) occurs per photon absorbed by the system. ... Photobleaching is the photochemical destruction of a fluorophore. ...

The use 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. 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 ^[9]. Researchers were able to observe quantum dots in lymph nodes of mice for more than 4 months ^[10].

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. In vitro (Latin: within the glass) refers to the technique of performing a given experiment in a test tube, or, generally, in a controlled environment outside a living organism. ... Embryogenesis is the process by which the embryo is formed and develops. ... Cancer is a class of diseases or disorders characterized by uncontrolled division of cells and the ability of these to spread, either by direct growth into adjacent tissue through invasion, or by implantation into distant sites by metastasis (where cancer cells are transported through the bloodstream or lymphatic system). ... For the musical composition, see Metastasis (Xenakis composition). ... Mouse embryonic stem cells with fluorescent marker. ... A scanning electron microscope (SEM) image of a single human lymphocyte. ... Immunology is a broad branch of biomedical science that covers the study of all aspects of the immune system in all organisms. ...

First attempts have been made in using 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 specifically bind to tumor cells. Passive targeting utilizes 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. In vivo (Latin for (with)in the living). ...

One of the remaining issues with quantum dot probes is their in vivo toxicity. CdSe nanocrystals for example are highly toxic to cultured cells under UV illumination. The energy of UV irradiation is close to 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. Then again, only little is known about the excretion process of polymer-protected quantum dots from living organisms. These and other questions must be carefully examined before quantum dot applications in tumor or vascular imaging can be approved for human clinical use. Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons between atoms, in order to produce a mutual attraction, which holds the resultant molecule together. ... Photolysis refers to any chemical reaction in which a compound is broken down by light. ... Vascular is an adjective for the word vessel and refers to tube-like structures. ...

Photovoltaic devices

Main article: Nanocrystal solar cell

Quantum dots may have the potential to increase the efficiency and reduce the cost of today's typical silicon photovoltaic cells. According to experimental proof from 2006, quantum dots of lead selenide can produce as many as seven excitons from one high energy photon of sunlight (7.8 times the bandgap energy).^[11] This compares favourably 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. This would not result in a 7-fold increase in final output however, but could boost the maximum theoretical efficiency from 31% to 42%. Quantum dot photovoltaics would theoretically be cheaper to manufacture, as they can be made "using simple chemical reactions."^[11] This article or section does not cite its references or sources. ... A photovoltaic cell is a device that turns light into electric energy. ...

Light emitting devices

There are several inquiries into using quantum dots as light-emitting diodes to make displays and other light sources: "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. Quantum dots are valued for displays, because they emit light in very specific gaussian distributions. This can result in a display that can more accurately render the colors that the human eye can perceive. Quantum dots also require very little power since they are not color filtered. A liquid crystal display (LCD), for example, is powered by a single fluorescent lamp that is color filtered to produce red, green, and blue pixels. Thus, when an LCD display shows a fully white screen, two-thirds of the light is absorbed by the filters. Displays that intrinsically produce monochromatic light can be more efficient, since more of the light produced reaches the eye.^[12] LED redirects here. ... External links LEd Category: TeX ... Probability density function of Gaussian distribution (bell curve). ... Something which is monochromatic has a single color. ...

See also

• Quantum wire
• Quantum well
• Quantum point contact
• Nanocrystal solar cell
• Programmable matter
• Quantum dot laser
• Fluorescence

In condensed matter physics, a quantum wire is an electrically conducting wire, in which quantum effects are affecting transport properties. ... A quantum well is a potential well that confines particles in one dimension, forcing them to occupy a planar region. ... A Quantum Point Contact (QPC) is a small point-like connection between two electrically-conducting regions. ... This article or section does not cite its references or sources. ... Programmable matter or wellstone is a science fiction neologism meaning bulk matter of which the physical or chemical properties reversibly can be changed on demand. ... A quantum dot laser succeeds in minimizing temperature-sensitive output fluctuations, something not possible with previous semiconductor lasers. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ...

References

This article or section, while providing some complete reference citations, includes a list of references or external links, and its verifiability remains partly unclear because it has insufficient in-text citations. You can improve this article by introducing more precise citations.

Image File history File links This is a lossless scalable vector image. ...

Further reading

• Thomas Engel. Quantum Chemistry and Spectroscopy. ISBN 0-8053-3843-8. Pearson Education, 2006. Pages 75-76.

A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

External links

• Nanopatterning of Quantum Dots
• Sizing Curve for CdSe Nanocrystals
• Sizing Curve for CdS Nanocrystals
• Quantum dots that produce white light could be the light bulb’s successor
• Quantum dots device counts single electrons - New Scientist
• Quantum dot on arxiv.org
• Quantum Dots Research and Technical Data
• National Nanotechnologies Laboratories of CNR-INFM
• How quantum dots work - flash animations (registration required)
• Quantum dots on YouTube