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A quantum dot (QD) is a crystal of semiconductor material whose diameter is on the order of several nanometers - a size which results in its free charge carriers experiencing "quantum confinement" in all three spatial dimensions. The electronic properties of quantum dots are intermediate between those of bulk semiconductors and of discrete molecules and closely related to their size and shape. This allows properties such as the band gap, emission color, and absorption spectrum to be highly tuneable, as the size distribution of quantum dots can be controlled during fabrication. For example, the band gap in a quantum dot, which determines the frequency range of emitted light, is inversely related to its size. In fluorescent dye applications, the frequency of emitted light increases as the size of the quantum dot decreases, shifting the color of emitted light from red to violet.〔(【引用サイトリンク】url=http://www.americanelements.com/nanotech.htm )〕 Researchers have studied applications for quantum dots in transistors, solar cells, LEDs, and diode lasers. They have also investigated quantum dots as agents for medical imaging and as possible qubits in quantum computing. The small size of quantum dots allows them to be suspended in various solvents and thus compatible with solution processing techniques such as spin coating and inkjet printing. These are inexpensive compared to conventional semiconductor device fabrication involving small areas and ultra-high vacuum. The first commercial release of a product utilizing quantum dots was the Sony XBR X900A series of flat panel televisions released in 2013.〔Bullis, Kevin. (2013-01-11) (Quantum Dots Produce More Colorful Sony TVs | MIT Technology Review ). Technologyreview.com. Retrieved on 2015-07-19.〕 Quantum dots were first discovered by Alexey Ekimov in 1981 in a glass matrix〔E. V. Kolobkova, N. V. Nikonorov, V. A. Aseev, ("Optical Technologies Silver Nanoclusters Influence on Formation of Quantum Dots in Fluorine Phosphate Glasses" ), Scientific and Technical Journal of Information Technologies, Mechanics and Optics, Volume 5, Number 12, 2012〕 and then in colloidal solutions by Louis E. Brus in 1985.〔(【引用サイトリンク】work=National Nanotechnology Initiative )〕 The term "quantum dot" was coined by Mark Reed. ==Quantum confinement in semiconductors== (詳細はcrystallite whose size is smaller than twice the size of its exciton Bohr radius, the excitons are squeezed, leading to 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, much like the pitch of a string in a musical instrument is dependent on its length. Comparing the quantum dots size to the Bohr radius of the electron and hole wave functions, 3 regimes can be defined. A 'strong confinement regime' is defined as the quantum dots radius being smaller than both electron and hole Bohr radius, 'weak confinement' is given when the quantum dot is larger than both. For semiconductors in which electron and hole masses are markedly different, an 'intermediate confinement regime' exists, where the quantum dots radius is larger than the Bohr radius of one (typically the hole), but not the other charge carrier. ;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 ab * as the energy levels split up. :: :where ab is the Bohr radius=0.053 nm, m is the mass, μ is the reduced mass, and εr is the size-dependent dielectric constant (Relative permittivity). :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 = 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 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: : where ''μ'' is the reduced mass, ''a'' is the radius, ''me'' is the free electron mass, ''mh'' is the hole mass, and ''εr'' 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 is 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 has been experimentally verified and is a key feature of many emerging electronic structures. Besides confinement in all three dimensions (i.e., a quantum dot), other quantum confined semiconductors include: * Quantum wires, which confine electrons or holes in two spatial dimensions and allow free propagation in the third. * Quantum wells, which confine electrons or holes in one dimension and allow free propagation in two dimensions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「quantum dot」の詳細全文を読む スポンサード リンク
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