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Thursday, 15 January 2015

BASIC OF NANOTECHNOLOGY

Quantum wire

Quantum wire is also known as one dimensional structure.

Suppose we have a material or structure and we have reduced its two dimension up to Nano range and third dimension remains same then that material or structure are known as quantum wire.

Quantum wire fabrication

1.    Quantum wire can be made by process of etching
2.    Quantum wire can be made with the help of carbon nano tubes
3.    Novel non lithographic technique can be used to fabricate the quantum wire

There are several methods to fabricate Quantum wire
1.    Colloidal synthesis
2.    Lithography
3.    Epitaxy

Colloidal Synthesis

Colloidal Synthesis method could be used to produce a numbers of quantum dots at a time. One more advantage of this process is that it is the cheapest method and also it is able to occur at non-extreme conditions.

First step is to merge of semiconductor micro-crystals with glass dielectric matrices.
Take a silicate glass with 1% semiconducting phase and heat for few hours at higher temperature. Now we may see formation of micro-crystals of approximately equal size.

Lithography

In this method of fabrication Quantum wells are covered by a polymer mask & exposed to an electron beam or ion beam. Now we may see that the surface is covered by a thin layer of metal, now we have to clean the exposed surface, hence we can build up the properties and sizes that we require by applying multiple layers in this way.

Epitaxy- Patterned growth

Semiconducting compounds having a smaller band gap, such as GaAs, are grown on the surface of a compound having a larger band gap such as AlGaAs.
One point should be noted that Growth is limited by coating it with a masking compound such as SiO2 & etching that mask by shape of the required crystal cell wall shape.

Epitaxy- Self-Organized Growth

Large difference in the lattice constants of the substrate and the crystallizing material is used in this fabrication process. When the crystallized layer is thicker than the critical thickness; there is a strong strain on the layers. The breakdown results in randomly distributed islets of regular shape and size.

Quantum wire application 

  1. Quantum wire is used in the field of medical in terms Nano bar codes. Nano bar codes are made with the help of quantum wires of various type of metal with different type of reflectivity. 
  2. Quantum wire is used in the field of optoelectronic applications.
  3. Quantum wire is used for di-electrophoretic manipulation because of their high aspect ratio.

Quantum dots

  1. Quantum dots are also known as ‘0’ dimensional structures.
  2. Quantum dots are also known as semiconductor Nano crystals.
  3. Quantum dots are made with almost same materials as normal semiconductors made with.
  4. Normal semiconductors are known as macroscopic objects but Quantum dots are treated as very - very small and known as zero dimensional structure.

Quantum dots characteristics 

  1. Quantum dots made with similar material but with different sizes have ability to emit light of different color.
  2. As the size of quantum decreases the energy difference between energy levels increases.
  3. Smaller dots emit high energy and small wavelength, blue light.
  4. Larger dots emit lower energy light.
  5. In case of quantum dots, there will be very precise control over the conductive properties of material.

Applications of Quantum dots 

Quantum dots are normally used for optical applications. We can see application of quantum dot as in following areas.
  1. Computing.
  2. Biology.       
  3. Flat panel displays.
  4. Photovoltaic device.
  5. Light emitting device.
  6. Can be used in active LEDs and White LEDs.
  7. Can also be used in thermoelectric.
  8. Can also be used in telecom industries.
  9. Can also be used in pigments/night vision.
  10. Can also be used in solar energy.

Explanations of applications of quantum dots

QDs Application in Biological areas

These days in biological analysis, different type of dyes are used and as time passes more flexibility are required of these dyes. Traditional dyes are normally not able to achieve the expectation. Quantum dots are found more suitable as compared to traditional dyes. 

It has been observed that quantum dots are 20 times brighter and 100 more stable as compared to traditional fluorescent reporters. Semiconductors are also used in Vitro-imaging of pre-labelled cells and this ability of imaging single cell migration in real time is very important in various areas of research for example Cancer Metastasis, embryogenesis and stem-cell therapeutics.

Application of QDs in Light emitting device


Displays of television and computer based on Light emitting quantum dots provide more quality image and consume half power as compared to organic light emitting diodes. Quantum dots are also known as inorganic semiconducting nanoparticle that emits light at specific wavelength depending on their size. 

As light will fall in a very-very small range of wavelengths, it is concluded in a research that these inorganic semiconducting nanoparticle quantum dots provide ability to colored displays that produce light at the wavelengths to which our eyes will be more sensitive. But there is one drawback of quantum dot light emitting diodes or QLEDs. 

As these QLEDs have less efficiency of converting electrical charge, used to power them, in to light and it is in value 2 to 3 percentage that reaches to the eyes of viewers. But in case of organic LEDs ,they convert about 6 percentage.

Application of QDs in Computing


As we know that Quantum dots are nanoparticle with size in nanometer and the motion of charge carriers such as electron and holes is restricted in three dimensions. Electronic properties of these QD could be amended and controlled with the help of electronic device. Therefore QDs are nanostructure that will play a very important role in future computing devices. 

Layer plus island growth can be considered as Heteroepitaxy growth as in this type of growth a crystalline film will grow on a film of a different material.

Factor affects the heteroepitaxial growth

1.    Dislocation spacing
2.    Misfit
3.    Total system energy
4.    Film thickness

Some Important points

Volume plasmons- Free electron materials

Volume plasmons are defined as aggregated or collective one dimensional oscillation of free electrons in a metallic volume.

Van Hove singularity

A Van Hove singularity can be defined as discontinuity in the density of states of a crystalline solid. Important point is this.
What is density of states?
Density of states is defined as number of states per energy interval

Inverted pyramid

Inverted pyramid is used in order to write the most important point on top and then followed by other important point and less important point on bottom.

Frank–van der Merwe

Frank–van der Merwe growth is type of growth where adatom (an absorbed atom) preferred to surface sites appearing in fully formed layers.
Frank–van der Merwe layer-by-layer growth is two dimensional that indicates that complete films form before growth of subsequent layers.

Volmer–Weber growth

Volmer–Weber growth shows that adatom (an absorbed atom) -adatom intercourses or interactions are stronger as compared to those of the adatom with the surface, and Tends to the buildup of three-dimensional islands Growth. This type of formation will cause rough multi-layer films to grow on the substrate surface.

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