To begin with in layman’s language or maybe for a person who has limited or little knowledge about physics, quantum dots are materials that are small but are sufficient to exhibit quantum mechanical properties. Quantum dots were first discovered in 1980. They exhibit electronic properties which are between semiconductors and discrete molecules. That is the very reason for the unusually high surface to volume ratio. The most visible use of quantum dots is in fluorescence where the nanocrystal is capable of producing different distinctive colors determined by their particle size.
For the more experienced and the more technical personals, Quantum dots are semiconductor nanostructures confining ...view middle of the document...
Generally as the size of the crystal decreases or reduces, the difference in the energy between the highest valence band and the lowest conduction band goes on increasing. This causes more energy requirement to excite the quantum dot and simultaneously more amount of energy is given out when the crystal would return to its initial ground state when it is non-excited. This results in a shift in the colour from red to blue when the light is emitted. This property is used to make the quantum dots exhibit light of any colour simply by changing the dot size. Because of the high control level on the size of the nanocrystal, the quantum dots can be tuned as desired at the time of production so as to emit the desired color. (Yoffe, 2001).
Types Of Quantum Dots
The quantum dots are classified into different types based on the compositions that they possess.
1. Core-Shell Quantum Dots
2. Alloyed Quantum Dots
Core-Shell Quantum Dots:
The luminous property of the quantum dots are because of the combination and the recombination of the hole-electron pairs which in other terms is the exciton decay through the radiative pathways. The exciton decay can also occur because of nonradiative methods and by reduction of the fluorescence quantum yield. One of the methods which is used to improve the brightness and efficiency of the semiconductor crystals is by growing the shells of another semiconductor material with a high band gap around the smaller crystal. These dots with a smaller region embedded in another with a much wider energy band gap are termed as the Core-Shell Dots.
Alloyed Quantum Dots:
By changing the crystalline size of the dots to tune the optical and electronic properties has become a trend in the field of quantum dots. This tuning of properties by changing the size of the crystal can cause problems in many applications because of the size restrictions. The way around is multicomponent quantum dots that offer alternate methods to tune the properties but without changing the size of the crystal and help retaining the properties. Alloyed semiconductors are homogenous and the gradient internal structures allow tuning of the optical and electronic properties by combining the combination and internal structure but it does not change the size of the crystal. Alloyed semiconductors exhibit properties which are interesting and distinct not only from the properties of their plethora of counterparts but also from the parent semiconductors.
Production of Quantum Dots
There are numerous ways for the fabrication or manufacturing of the quantum dots. These are based on confining excitons in semiconductors thus they produce quantum dots. Generally the quantum wires, wells and dots are made by advanced epitaxial ways in crystals. These crystals are produced by chemical processes or by ion implantation. In nano devices they are done through lithographic techniques.
There are several ways for manufacturing...