Thursday, 26 January 2012

What are Quantum Dots?

Quantum dots are tiny particles, or “nanoparticles”, of a semiconductor material, traditionally chalcogenides (selenides or sulfides) of metals like cadmium or zinc (CdSe or ZnS, for example), which range from 2 to 10 nanometers in diameter (about the width of 50 atoms).

Source: Physorg
Because of their small size, quantum dots display unique optical and electrical properties that are different in character to those of the corresponding bulk material. The most immediately apparent of these is the emission of photons under excitation, which are visible to the human eye as light. Moreover, the wavelength of these photon emissions depends not on the material from which the quantum dot is made, but its size.

The ability to precisely control the size of a quantum dot enables the manufacturer to determine the wavelength of the emission, which in turn determines the colour of light the human eye perceives. Quantum dots can therefore be “tuned” during production to emit any colour of light desired. The ability to control, or “tune” the emission from the quantum dot by changing its core size is called the “size quantisation effect”.

The smaller the dot, the closer it is to the blue end of the spectrum, and the larger the dot, the closer to the red end. Dots can even be tuned beyond visible light, into the infra-red or into the ultra-violet.
At the end of the production process, quantum dots appear physically either as a powder or in a solution. Because of their tiny size, the ability to produce even a relatively “small” volume of quantum dots (e.g. one kilo) will yield enough actual quantum dots for industrial scale applications. Nanoco Technologies has patented a molecular seeding process which enables such “large scale” production to occur.

Now, the ability to mass-produce consistently high quality quantum dots enables product designers to envisage their use in consumer products and a wide range of other applications for the first time, and then bring these superior, next-generation products to market.

Quantum dots are particularly significant for optical applications due to their high extinction co-efficient. 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.

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 a locally enhanced electromagnetic field produced by gold nanoparticles, which can then be observed from the surface Plasmon resonance in the photo luminescent 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, tumour targeting, and diagnostics.

1 comment:

  1. Thanks for great article.I have searched various blogs on net but no one has given information in such details.