Quantum dot

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.

Researchers have studied quantum dots in transistors, solar cells, LEDs, diode lasers, and hope to use them as qubits. Some quantum dots are commercially available.

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 called quantum confinement, and it is a key feature in many emerging electronic structures. Other quantum confined semiconductors include:

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

Colloidal Synthesis

Colloidal semiconductor nanocrystals are synthesized from precursor compounds dissolved in solutions, much like traditional chemical processes. The excitons in these quantum dots are confined by the semiconductor's spatial boundaries.

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 quantities of quantum dots may be synthesized via 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.

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)

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.

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.

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 coloration, but as yet not enough information has become available. Furthermore it was shown recently 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.

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.

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 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 nanodevice that efficiently produces visible light, through energy transfer from nano-thin layers of quantum wells to nanocrystals above the nanolayers.

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.

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.

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. Currently under research as well is tuning of the toxicity.

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). 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".

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 LCD display, 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.

See also

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

References

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. PMID 10038575.{{cite journal}}: CS1 maint: multiple names: authors list (link) (1988).[2]

• 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" (PDF). J Am Chem Soc. 115: 8706–15.{{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" (PDF). J Am Chem Soc. 123: 183–4.

• Wang C, Shim M, Guyot-Sionnest P (2001). "Electrochromic nanocrystal quantum dots" (PDF). Science. 291: 2390–2.{{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" (PDF). 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. 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. "White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals". J Am Chem Soc. 127 (44): 15378–9. doi:10.1021/ja055470d S0002-7863(05)05470-3. {{cite journal}}: Check |doi= value (help)CS1 maint: multiple names: authors list (link)

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

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