What are "Quantum Dots"?


Introduction

Quantum dots are nano-meter-scale "boxes" for selectively holding or releasing electrons. Over the past 14 years they have been transformed from laboratory curiosities to the building blocks for a future computer industry. Quantum dots are small metal or semiconductor boxes that hold a well-defined number of electrons. The number of electrons in a dot may be adjusted by changing the dot's electrostatic environment. Dots have been made ranging from 30nm to 1 micron in size, and holding from zero to hundreds of electrons.

Brief History

During the1980's ideas concerning the Quantum Dot surfaced when researchers in the field of computing were trying to construct something close to "nano-scale" in the field of computing.

The Mechanism of Quantum Dot

By using an external light (eg Ultraviolet) on nano-crystals (eg made from semiconductor materials such as zinc sulphide, cadmium selenide, indium phosphide or lead sulphide), the nano-crystal will absorb the light and then, as a result of the crystal being stimulated by the absorbed light, it will re-emit the light, usually of a certain color, depending on the size of the quantum dot.

It has been observed in experiments and shown theoretically that reducing the dimensions of a quantum dot increases the effective operating temperature of the electron confinement device. Present day quantum dots are large enough (approximately 1-10 microns long and wide) that they require cooling with liquid helium or, at least, liquid nitrogen, to cryogenic temperatures. However, for a practical technology with widespread applications based upon such quantum-effect devices, it will be necessary to achieve room temperature operation. This requirement implies that it is necessary to invent and manufacture molecular-scale quantum dots that are only approximately 1 to 10 nanometers in linear dimension. Such a quantum dot would probably be constructed as a single molecule ie a molecular quantum dot . Molecular quantum dots are one example of the next-generation technology known as Molecular-scale electronics .

Professor James Tour of the University of South Carolina and Professor Mark Reed of Rale University are collaborating on the chemical synthesis and testing of molecular wires. These operate by allowing electrons to move near ballistically along the length of a chain of ring-like chemical structures with conjugated pi-orbitals.

It has been suggested by Tour and by others, that it may be possible to insert chemical groups of lower conductance into such a molecular wire, creating paired barriers to electron migration through the chain. Such barriers may create a molecular quantum-effect device that would function in a fashion similar to solid-state resonance tunneling devices that already have been fabricated, tested, and applied in prototype quantum-effect logic.

Work in the area of ​​quantum-based devices for nano-scale metrology will be directed to fabricating an ultra-small SQUID (Superconducting Quantum Interference Device) for applications in single-particle detection. The fabrication of such a device will be a significant achievement, and should prove important in areas such as future nano-scale frequency standards, emerging quantum computer and single-particle sensor technologies and in the study of adatom-surface interactions.

Many researchers in nano-electronics are speaking of a possible architecture for computer logic based on quantum dots. As mentioned previously, a quantum dot is a box that holds a discrete number of electrons. Adjusting electric fields in the vicinity of the dot, for example by applying a voltage to a near metal gate, can change this number. Of course, since quantum dots are fabricated in solids, not in vacuum, there are many electrons in them. However, almost all of these are tightly bound to atoms in the solid. The few electrons spoken of are extra ones beyond those that are tightly bound. These extra electrons could roam free in a solid were they are not limited in a quantum dot.

In nano-structures, the electrical properties can be markedly different from their macroscopic equivalents thatby revealing many novel effects. "Progress in the field has been hampered by two problems," said Arizona State University Chemistry Professor Devens Gust. "The first has been in making robust, reproducible electrical connections to both ends of molecules. After this has been achieved, the next problem is knowing how many molecules there actually are between the electrical contacts."

Applications

The uses or possible future uses of Quantum Dots can cover various applications with impressive futuristic results.

The following are just a few examples:

1. Quantum computers.

2. Domestic and office lighting applications.

3. Medical Applications.

4. Television screens and monitors.

5. Silicon Photovoltaic cells

Conclusion

Generally speaking, atoms are quantum dots, however, adding a number of molecules together in small space, producing the quantum dots effects.

Addition or removal of an electron changes the properties of a quantum dot, resulting in a "benefit" in one way or another.

Quantum Dots and their applications are the next step in the field of nanotechnology, which in the future will bring applications in commercial and non-commercial fields. Quantum Dots may still remain in the research stage at the present time, however, their applications and the benefits which they will bring along has already encouraged companies and governmental organizations to invest heavily in this field.

References

  1. AF van Driel, G. Allan, C. Delerue, P. Lodahl, WL Vos and D. Vanmaekelbergh, Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: Influence of dark states, Physical Review Letters, 95, 236804 (2005) .
  2. 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. (1988).
  3. Reed MA (1993). "Quantum Dots" (PDF). Scientific American 268 (1): 118.



Source by Najib Altawell


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