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  1. Atomic force microscope
  2. Atomic nanoscope
  3. Atom probe
  4. Ballistic conduction
  5. Bingel reaction
  6. Biomimetic
  7. Bio-nano generator
  8. Bionanotechnology
  9. Break junction
  10. Brownian motor
  11. Bulk micromachining
  12. Cantilever
  13. Carbon nanotube
  14. Carbyne
  15. CeNTech
  16. Chemical Compound Microarray
  17. Cluster
  18. Colloid
  19. Comb drive
  20. Computronium
  21. Coulomb blockade
  22. Diamondoids
  23. Dielectrophoresis
  24. Dip Pen Nanolithography
  25. DNA machine
  26. Ecophagy
  27. Electrochemical scanning tunneling microscope
  28. Electron beam lithography
  29. Electrospinning
  30. Engines of Creation
  31. Exponential assembly
  32. Femtotechnology
  33. Fermi point
  34. Fluctuation dissipation theorem
  35. Fluorescence interference contrast microscopy
  36. Fullerene
  37. Fungimol
  38. Gas cluster ion beam
  39. Grey goo
  40. Hacking Matter
  41. History of nanotechnology
  42. Hydrogen microsensor
  43. Inorganic nanotube
  44. Ion-beam sculpting
  45. Kelvin probe force microscope
  46. Lab-on-a-chip
  47. Langmuir-Blodgett film
  48. LifeChips
  49. List of nanoengineering topics
  50. List of nanotechnology applications
  51. List of nanotechnology topics
  52. Lotus effect
  53. Magnetic force microscope
  54. Magnetic resonance force microscopy
  55. Mechanochemistry
  56. Mechanosynthesis
  57. MEMS thermal actuator
  58. Mesotechnology
  59. Micro Contact Printing
  60. Microelectromechanical systems
  61. Microfluidics
  62. Micromachinery
  63. Molecular assembler
  64. Molecular engineering
  65. Molecular logic gate
  66. Molecular manufacturing
  67. Molecular motors
  68. Molecular recognition
  69. Molecule
  70. Nano-abacus
  71. Nanoart
  72. Nanobiotechnology
  73. Nanocar
  74. Nanochemistry
  75. Nanocomputer
  76. Nanocrystal
  77. Nanocrystalline silicon
  78. Nanocrystal solar cell
  79. Nanoelectrochemistry
  80. Nanoelectrode
  81. Nanoelectromechanical systems
  82. Nanoelectronics
  83. Nano-emissive display
  84. Nanoengineering
  85. Nanoethics
  86. Nanofactory
  87. Nanoimprint lithography
  88. Nanoionics
  89. Nanolithography
  90. Nanomanufacturing
  91. Nanomaterial based catalyst
  92. Nanomedicine
  93. Nanomorph
  94. Nanomotor
  95. Nano-optics
  96. Nanoparticle
  97. Nanoparticle tracking analysis
  98. Nanophotonics
  99. Nanopore
  100. Nanoprobe
  101. Nanoring
  102. Nanorobot
  103. Nanorod
  104. Nanoscale
  105. Nano-Science Center
  106. Nanosensor
  107. Nanoshell
  108. Nanosight
  109. Nanosocialism
  110. Nanostructure
  111. Nanotechnology
  112. Nanotechnology education
  113. Nanotechnology in fiction
  114. Nanotoxicity
  115. Nanotube
  116. Nanovid microscopy
  117. Nanowire
  118. National Nanotechnology Initiative
  119. Neowater
  120. Niemeyer-Dolan technique
  121. Ormosil
  122. Photolithography
  123. Picotechnology
  124. Programmable matter
  125. Quantum dot
  126. Quantum heterostructure
  127. Quantum point contact
  128. Quantum solvent
  129. Quantum well
  130. Quantum wire
  131. Richard Feynman
  132. Royal Society's nanotech report
  133. Scanning gate microscopy
  134. Scanning probe lithography
  135. Scanning probe microscopy
  136. Scanning tunneling microscope
  137. Scanning voltage microscopy
  138. Self-assembled monolayer
  139. Self-assembly
  140. Self reconfigurable
  141. Self-Reconfiguring Modular Robotics
  142. Self-replication
  143. Smart dust
  144. Smart material
  145. Soft lithography
  146. Spent nuclear fuel
  147. Spin polarized scanning tunneling microscopy
  148. Stone Wales defect
  149. Supramolecular assembly
  150. Supramolecular chemistry
  151. Supramolecular electronics
  152. Surface micromachining
  153. Surface plasmon resonance
  154. Synthetic molecular motors
  155. Synthetic setae
  156. Tapping AFM
  157. There's Plenty of Room at the Bottom
  158. Transfersome
  159. Utility fog

 



NANOTECHNOLOGY
This article is from:
http://en.wikipedia.org/wiki/Quantum_dot

All text is available under the terms of the GNU Free Documentation License: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License 

Quantum dot

From Wikipedia, the free encyclopedia

 

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons (pairs of conduction band electrons and valence band holes) in all three spatial directions. The confinement can be due to electrostatic potentials (generated by external electrodes, doping, strain, impurities), due to the presence of an interface between different semiconductor materials (e.g. in the case of self-assembled quantum dots), due to the presence of the semiconductor surface (e.g. in the case of a semiconductor nanocrystal), or due to a combination of these. A quantum dot has a discrete quantized energy spectrum. The corresponding wave functions are spatially localized within the quantum dot, but extend over many periods of the crystal lattice. A quantum dot contains a small finite number (of the order of 1-100) of conduction band electrons, valence band holes, or excitons, i.e., a finite number of elementary electric charges.

Description

Small quantum dots, such as colloidal semiconductor nanocrystals, can be as small as 2 to 10 nanometers, corresponding to 10 to 50 atoms in diameter and a total of 100 to 100,000 atoms within the quantum dot volume. Self-assembled quantum dots are typically between 10 and 50 nanometers 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 nanometers. At 10 nanometers in diameter, nearly 3 million quantum dots could be lined up end to end and fit within the width of a human thumb.

Quantum dots can be contrasted to other semiconductor nanostructures: 1) quantum wires, which confine the motion of electrons or holes in two spatial directions and allow free propagation in the third. 2) quantum wells, which confine the motion of electrons or holes in one direction and allow free propagation in two directions.

Quantum dots containing electrons can also be compared to atoms: both have a discrete energy spectrum and bind a small number of electrons. In contrast to atoms, the confinement potential in quantum dots does not necessarily show spherical symmetry. In addition, the confined electrons do not move in free space, but in the semiconductor host crystal. The quantum dot host material, in particular its band structure, does therefore play an important role for all quantum dot properties. Typical energy scales, for example, are of the order of ten electron volts in atoms, but only 1 millielectron volt in quantum dots. Quantum dots with a nearly spherical symmetry, or flat quantum dots with nearly cylindrical symmetry can show shell filling according to the equivalent of Hund's rules for atoms. Such dots are sometimes called "artificial atoms". In contrast to atoms, 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.

Like in atoms, the energy levels of small quantum dots can be probed by optical spectroscopy techniques. In quantum dots that confine electrons and holes, the interband absorption edge is blue shifted due to the confinement compared to the bulk material of the host semiconductor material. As a consequence, quantum dots of the same material, but with different sizes, can emit light of different colors.

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.

Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots.
Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots.

One of the optical features of small excitonic quantum dots immediately noticeable to the unaided eye is coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, more significant in terms of coloration is the size. The larger the dot, the redder (the more towards the red end of the spectrum) the fluorescence. The smaller the dot, the bluer (the more towards the blue end) it is. 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 colorization, but as yet not enough information has become available.

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.

Fabrication

  1. 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.
  2. Quantum dots sometimes occur spontaneously in quantum well structures due to monolayer fluctuations in the well's thickness.
  3. 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.
  4. 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.

Mass production

In large numbers, quantum dots may be synthesized by means of a colloidal synthesis. Colloidal synthesis is by far the cheapest and has the advantage of being able to occur at benchtop conditions. It is acknowledged to be the least toxic of all the different forms of synthesis.

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.

Yet another method is pyrolytic synthesis, which produces large numbers of quantum dots that self-assemble into preferential crystal sizes.

Applications

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

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, amplifiers, and biological sensors.

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, 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.

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 simply unable to meet the necessary standards at times. 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. Currently under research as well is tuning of the toxicity.

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 of panels produced in research labs from today's 20-30% to 42%.[1] This work was reproduced one year later by an NREL team.

Another paper, published in the October 18, 2005 issue of the Journal of the American Chemical Society, reports that Michael Bowers II at Vanderbilt University discovered that certain size crystals of cadmium and selenium emit white light when excited by an ultraviolet laser. This emission appears to be coming from the surface of the crystal, rather than the center. The crystals contain either 33 or 34 pairs of atoms. While they are being pyrolytically synthesized, they preferentially form into just this size; so Bowers can make a batch of such crystals in about an hour. Another student then mixed these quantum dots into ordinary varnish, applied it to a blue LED, and observed that the emission is yellowish-white, like a light bulb. The researchers believe that it will be possible to achieve this emission of white light via electrical stimulation as well as photonic, and hope to demonstrate it soon.

There are several inquiries into using quantum dots to make displays and light sources: "QD-LED" displays, and "QD-WLED" (White LED) [1]. In June, 2006, QD Vision announced technical success in making a proof of concept quantum dot display. [2] 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 reflect the colors that the human eye can perceive. Quantum dots also require very little power since they are not color filtered. A LCD display, for example, is powered by a single fluorescent lamp that is color filtered to produce red, green, and blue pixels. Thus, when a LCD display shows a fully white screen, two-thirds of the light is absorbed by the filters. Displays that intrinsically produce monochromatic light can for this reason be more efficient, since more produced light reaches the eye. [3]

See also

  • Quantum wire
  • Quantum well
  • Quantum point contact
  • Nanocrystal solar cell

References

  • M. A. Reed, J. N. Randall, R. J. Aggarwal, R. J. Matyi, T. M. Moore, and A. E. Wetsel, Observation of discrete electronic states in a zero-dimensional semiconductor nanostructure, Phys. Rev. Lett. 60, 535 (1988).[4]
  • M. A. Reed, Quantum Dots, Scientific American 268, Number 1, 118, 1993.[5]
  • Murray, C. B., Norris, D. J., & Bawendi, M. G. Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites J. Am. Chem. Soc. 115, 8706-8715, 1993.
  • Peng, Z. A., Peng, X.; Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor (123), J. Am. Chem. Soc., 2001, 183-184.
  • Wang, C., Shim, M. & Guyot-Sionnest, P. Electrochromic nanocrystal quantum dots., Science 291 2390-2392 (2001).
  • 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. In Science, 307, 538 – 544.
  • Shim, M. & Guyot-Sionnest, P. N-type colloidal semiconductor nanocrystals., NATURE 407 (6807): 981-983 OCT 26 2000
  • W. E. Buhro and V. L. Colvin, Semiconductor nanocrystals: Shape matters, Nat. Mater., 2003, 2, 138 139.
  • S. Bandyopadhyay and A. E. Miller (2001). "Electrochemically self-assembled ordered nanostructure arrays: Quantum dots, dashes, and wires", Handbook of Advanced Electronic and Photonic Materials and Devices,6.
  • High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion R. D. Schaller and V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004)
  • Michael J. Bowers II, James R. McBride, and Sandra J. Rosenthal (2005). White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals, Journal of the American Chemical Society, October 18, 2005.
  1. ^ "Peter Weiss". Quantum-Dot Leap. Science News Online. Retrieved on 2005-06-17.

External links

  • How quantum dots work - flash animations
  • Sizing Curve for CdSe Nanocrystals
  • Sizing Curve for CdS Nanocrystals
  • Quantum dots that produce white light could be the light bulb’s successor
  • Nanomaterial Database
  • Quantum dots device counts single electrons - New Scientist
  • Cheaper Dots : New process slashes the cost of quantum dots Scientific American Magazine (December 2005)
  • Quantum dot on arxiv.org
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