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Quantum dot

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File:QD mini rainbow.jpg
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] They were discovered by Louis E. Brus, who was then at Bell Labs. The term "Quantum Dot" was coined by Mark Reed.

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.

In layman's terms, quantum dots are semiconductors whose conducting characteristics are closely related to the size and shape of the individual crystal. Generally, the smaller the size of the crystal, the larger the band gap, the difference in energy between the highest valence band and the lowest conduction bandbecomes, therefore more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state. For example, in fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light emitted. The main advantages in using quantum dots is that because of the high level of control possible over the size of the crystals produced, it is possible to have very precise control over the conductive properties of the material.[4]

Quantum confinement in semiconductors

Main article: Potential well

In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length, which is called the exciton Bohr radius and is estimated by replacing the positively charged atomic core with the hole in the Bohr formula. If the electron and hole are constrained further, then properties of the semiconductor change. This effect is a form of quantum confinement, and it is a key feature in many emerging electronic structures.[5][6]

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.

Making quantum dots

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 nanocrystals produced by chemical methods or by ion implantation, or in nanodevices made by state-of-the-art lithographic techniques.[7]

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 Ostwald ripening.

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.

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] to be the least toxic of all the different forms of synthesis.

Fabrication

  • Self-assembled quantum dots are typically between 5 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.
  • 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 called lateral quantum dots. 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 atoms.[8]

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

Viral assembly

Lee et al. (2002) reported using genetically engineered M13 bacteriophage viruses to create quantum dot biocomposite structures.[9] 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 phage display.[10] Additionally, it is known that liquid crystalline structures of wild-type viruses (Fd, M13, and 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 nanocrystals, 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 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-manufacturing of quantum dots

Conventional, small-scale quantum dot manufacturing relies on a process called “high temperature dual injection” which is impractical for most commercial applications that require large quantities of quantum dots. A reproducible method for creating larger quantities of consistent, high-quality quantum dots involves producing nanoparticles from chemical precursors in the presence of a molecular cluster compound under conditions whereby the integrity of the molecular cluster is maintained and acts as a prefabricated seed template. Individual molecules of a cluster compound act as a seed or nucleation point upon which nanoparticle growth can be initiated. In this way, a high temperature nucleation step is not necessary to initiate nanoparticle growth because suitable nucleation sites are already provided in the system by the molecular clusters. A significant advantage of this method is that it is highly scalable.

Cadmium-free quantum dots - “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.

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.[11]

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 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 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 fluorescent light is inversely proportional to the square of 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 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 [12] 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 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 edge of the band gap.

Qdots can be synthesized with larger(thicker) shells (CdSe qdots with CdS shells). The shell thickness has shown direct correlation to the lifetime and emission intensity.

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 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.

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. Quantum dots may be excited within the locally enhanced electromagnetic field produced by the 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 entangled quantum dots, or qubits, plus a way of performing operations, quantum calculations and the computers that would perform them might be possible.

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.[13] 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 (allowing much less photobleaching). It has been estimated that quantum dots are 20 times brighter and 100 times more stable than traditional fluorescent reporters.[13] For single-particle tracking, the irregular 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[14]. 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 [15]. Researchers were able to observe quantum dots in lymph nodes of mice for more than 4 months [16].

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.[17]

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 utilizes 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 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. 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.

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 article: Nanocrystal 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 2006 (controversial results [18]), 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).[19] 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."[19] The generation of more than one exciton by a single photon is called multiple exciton generation (MEG) or carrier multiplication.

Light emitting devices

There are several inquiries into using quantum dots as light-emitting diodes 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 distributions. 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.[20] A color liquid crystal display (LCD), for example, is powered by a single fluorescent lamp that is color filtered to produce red, green, and blue pixels. Displays that intrinsically produce monochromatic light can be more efficient, since more of the light produced reaches the eye.[21]

See also

References

  1. ^ L.E. Brus (2007). "Chemistry and Physics of Semiconductor Nanocrystals" (PDF). Retrieved 2009-07-07.
  2. ^ D.J. Norris (1995). "Measurement and Assignment of the Size-Dependent Optical Spectrum in Cadmium Selenide (CdSe) Quantum Dots, PhD thesis, MIT". Retrieved 2009-07-07.
  3. ^ C.B. Murray, C.R. Kagan, M. G. Bawendi (2000). "Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies". Annual Review of Materials Research. 30 (1): 545–610. doi:10.1146/annurev.matsci.30.1.545.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ "www.evidenttech.com: How quantum dots work". 2009. Retrieved 2009-10-15.
  5. ^ "Greenemeier, L." Scientific American. 2008.
  6. ^ "New York Times Science Watch December 31, 1991".
  7. ^ C. Delerue, M. Lannoo (2004). Nanostructures: Theory and Modelling. Springer. p. 47. ISBN 3540206949.
  8. ^ Silbey,Robert J.; Alberty, Robert A.; Bawendi, Moungi G. (2005). Physical Chemistry, 4th ed. John Wiley &Sons. p. 835.{{cite book}}: CS1 maint: multiple names: authors list (link)
  9. ^ Lee SW, Mao C, Flynn CE, Belcher AM (2002). "Ordering of quantum dots using genetically engineered viruses". Science. 296: 892. doi:10.1126/science.1068054. PMID 11988570.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ Whaley SR, English DS, Hu EL, Barbara PF, Belcher AM (2000). "Selection of peptides with semiconductor binding specificity for directed nanocrystal assembly". Nature. 405: 665. doi:10.1038/35015043. PMID 10864319.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ "Cadmium-free quantum dots". Retrieved 2009-07-07.
  12. ^ Van Driel, A. F. (2005). "Frequency-Dependent Spontaneous Emission Rate from CdSe and CdTe Nanocrystals: Influence of Dark States" (PDF). Physical Review Letters. 95: 236804. doi:10.1103/PhysRevLett.95.236804.
  13. ^ a b Walling, M. A. (2009-02). "Quantum Dots for Live Cell and In Vivo Imaging". Int. J. Mol. Sci. 10 (2): 441–491. doi:10.3390/ijms10020441. ISSN 1422-0067. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: unflagged free DOI (link)
  14. ^ Tokumasu, F; Fairhurst, Rm; Ostera, Gr; Brittain, Nj; Hwang, J; Wellems, Te; Dvorak, Ja (2005). "Band 3 modifications in Plasmodium falciparum-infected AA and CC erythrocytes assayed by autocorrelation analysis using quantum dots" (Free full text). Journal of cell science. 118 (Pt 5): 1091–8. doi:10.1242/jcs.01662. ISSN 0021-9533. PMID 15731014. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  15. ^ Dahan, M; Lévi, S; Luccardini, C; Rostaing, P; Riveau, B; Triller, A (2003). "Diffusion dynamics of glycine receptors revealed by single-quantum dot tracking". Science (New York, N.Y.). 302 (5644): 442–5. doi:10.1126/science.1088525. ISSN 0036-8075. PMID 14564008. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  16. ^ Ballou, B; Lagerholm, Bc; Ernst, La; Bruchez, Mp; Waggoner, As (2004). "Noninvasive imaging of quantum dots in mice" (Free full text). Bioconjugate chemistry. 15 (1): 79–86. doi:10.1021/bc034153y. ISSN 1043-1802. PMID 14733586. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  17. ^ "Gene Silencer and Quantum Dots Reduce Protein Production to a Whisper". Newswise. Retrieved June 24, 2008.
  18. ^ Service, Rf (2008). "Materials Research Society fall meeting. Shortfalls in electron production dim hopes for MEG solar cells". Science (New York, N.Y.). 322 (5909): 1784. doi:10.1126/science.322.5909.1784a. ISSN 0036-8075. PMID 19095918. {{cite journal}}: Unknown parameter |month= ignored (help)
  19. ^ a b Nanocharging Solar
  20. ^ Exploration, Vanderbilt's Online Research Magazine - Shrinking quantum dots to produce white light
  21. ^ Nanocrystal Displays

Further reading

  • Reed MA, Randall JN, Aggarwal RJ, Matyi RJ, Moore TM, Wetsel AE (1988). "Observation of discrete electronic states in a zero-dimensional semiconductor nanostructure". Phys Rev Lett. 60 (6): 535–537. doi:10.1103/PhysRevLett.60.535. PMID 10038575.{{cite journal}}: CS1 maint: multiple names: authors list (link) (1988).[1]
  • Reed MA (1993). "Quantum Dots" (PDF). Scientific American. 268 (1): 118.
  • Murray CB, Norris DJ, Bawendi MG (1993). "Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites". J Am Chem Soc. 115: 8706–15. doi:10.1021/ja00072a025.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  • Peng ZA, Peng X (2001). "Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor". J Am Chem Soc. 123: 183–4. doi:10.1021/ja003633m.
  • Wang C, Shim M, Guyot-Sionnest P (2001). "Electrochromic nanocrystal quantum dots". Science. 291: 2390–2. doi:10.1126/science.291.5512.2390. PMID 11264530.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  • Michalet X, Pinaud FF, Bentolila LA; et al. (2005). "Quantum dots for live cells, in vivo imaging, and diagnostics". Science. 307 (5709): 538–44. doi:10.1126/science.1104274. PMID 15681376. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  • Shim M, Guyot-Sionnest P (2000). "n-type colloidal semiconductor nanocrystals". Nature. 407 (6807): 981–3. doi:10.1038/35039577. PMID 11069172.
  • Buhro WE, Colvin VL (2003). "Semiconductor nanocrystals: Shape matters". Nature materials. 2 (3): 138–9. doi:10.1038/nmat844. PMID 12612665.
  • Bandyopadhyay S, Miller AE (2001). "Electrochemically self-assembled ordered nanostructure arrays: Quantum dots, dashes, and wires". In Nalwa HS (ed.). Handbook of Advanced Electronic and Photonic Materials and Devices. Vol. 6. San Diego, Calif.: Academic. ISBN 0125137451.
  • Schaller RD, Klimov VI (2004). "High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion". Phys Rev Lett. 92 (18): 186601. doi:10.1103/PhysRevLett.92.186601.
  • Bowers MJ, McBride JR, Rosenthal SJ (2005). "White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals". J Am Chem Soc. 127 (44): 15378–9. doi:10.1021/ja055470d.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  • 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 3540206949. Springer, 2004.
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