Friday, January 8, 2010

NANO

NANOPARTICLES

A nanometer is Ǻ, so particles having a radius of ≤100 nm can be considered to be nanoparticles.

Nanostructured materials are having a characteristic length less than 100 nm. The nanoparticles have different electronic, optical, electrical, magnetic, chemical and mechanical properties which are different from the properties of bulk materials.

A number of metals, alloys, intermetallics, ceramics, polymers, semiconductors have been synthesized in nanoform. Apart from nanoparticles or nanocrystals made of metals, semiconductors or oxides, the special forms of nanomaterials are nanocomposites, nanotubes, quantum wires, quantum dots and quantum wells.

Nanocomosites are special type of materials originating from suitable combination of more nanoparticles by some appropriate techniques resulting in materials having unique physical properties.

Nanotubes are a one dimensional fullerence with a cylindrical tube. These cylindrical carbon molecules have novel properties and make them potentially useful in many applications in nanotechnology, electronics, optics and other fields.

Semiconductor structures in the nanometer size range exhibiting the characteristics of quantum confinement are commonly referred to as quantum dots when the confinement is in a three dimensions quantum wires and when the confinement is in two dimensions and quantum wells when the confinement is in one dimension. These can develop intense, long lasting colours due to excitation by UV and visible light, LEDs, lasers etc.

ORIGIN OF NANOTECHNOLOGY

While the topic nanotechnology seems to be relatively new, the existence of functional devices and structure of nanometer dimensions is not new and in fact nanostructures exists on earth as long as life exists.

In the fourth century, Roman glass makers fabricated glasses containing nanosized metals. They produced cups made up of soda lime glass containing silver and gold nanoparticles. The colour of the cup changes from green to deep red when a light source is placed in it.

In eighteenth century British scientists Thomas wedgewood and Sir Humprey davy were able to provide images using silver nitrate and chloride, but their images were not permanent.

Photography which has been developed in the nineteenth centuries depends on production of silver nanoparticles sensitive to light. Photographic film is an emulsion, a thin layer of gelatin containing silver halides such as silver bromide producing nanoparticles of silver which are pixels of the image.

In 1883, the American scientist Geroge Eastman produced a film containing a large paper strip coated with an emulsion containing silver halides. He later developed this into a flexible film that could be rolled which made photography very simple. Hence, colour films are named after him as Eastman colour film.

In 1960, Richard Feynman presented a visionary and prophetic lecture at a meeting of American physical society entitled “there are plenty of room at the bottom” where he speculated on the possibility and potent of nanosized particles. Richard Feynman

QUANTUM CONFINEMENT

Regarding the electronic, magnetic and optical properties, the internal electronic structure of the nanoparticles is of particular importance. Indeed the electronic properties of nanoparticles are special and in general summarized as quantum size effect. Metals in general are characterized by their unique electronic bonding system differing from non metals and semiconductors. However if the dimension decreases to a certain extent, this special behaviour is no longer possible. The electrons are trapped in a too small volume and they behave very differently and consequently change their chemical and physical properties.

In small nanocrystals the electronic energy levels are not continuous as in the bulk but are discrete (finite density of states), because of the confinement of the electronic wavefunction to the physical dimensions of the particles. This phenomenon is called quantum confinement and therefore nanocrystals are also referred to as quantum dots. In any material substantial variation of fundamental electrical and optical properties with reduced size will be observed when the energy spacing between the electronic levels exceeds the thermal energy.






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