What Are "quantum Dots"?

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 (e.g. Ultraviolet) on nano-crystals (e.g. 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 colour, 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 raises 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 i.e. 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 nearly 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 might create a molecular quantum-effect device that would function in a fashion similar to solid-state resonance tunnelling 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 adatomsurface interactions. Many researchers in nano-electronics are talking 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 neighbourhood of the dot, for example by applying a voltage to a nearby 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 confined in a quantum dot. In nano-structures, the electrical properties can be markedly different from their macroscopic equivalents thereby 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 few examples: 1. 2. 3. 4. 5.

Quantum computers. Domestic and office lighting applications. Medical Applications. Television screens and monitors. Silicon Photovoltaic cells.

Conclusion Generally speaking, atoms are quantum dots, however, adding a number of molecules together in small space, produce 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 be still 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 organisations to invest heavily in this field.

Najib Altawell

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