Fabrication Of Quantum Dots And Their Use In Photodiodes...

The semiconductor quantum dots like CdSe are small clusters of one material capped in another whose electron hole pairs are confined throughout the spatial arrangement of the particle. Due to the extreme quantum confinement effect they possess intermediate properties of bulk material as well as single molecule or atom. A unique optical property of colloidal quantum dots is their coloration. Every quantum dot has its own intrinsic energy signature and the quantum confined size of quantum dots is more significant at energies near the band gap. As a result, quantum dots of the same material, but with different sizes, can emit light of different colours and obviously the physical reason is the quantum confinement effect. The energy levels directly effect their coloration of the quantum dots. Apart from this, the band gap energy that determines the energy of the fluorescent light (as well as the colour) is inversely proportional to the size of the quantum dot. Larger quantum dots have more energy levels which are also more closely spaced and this allows the quantum dot to absorb photons containing less energy, i.e., those closer to the red end of the spectrum (www.wikipedia.org). Nowadays, the colloidal quantum dots are widely available can also be made in the laboratory by means of common laboratory techniques.

In the case of conventional bulk semiconductor material, the percentage of electrons those occupy the conduction band is extremely low where as the vast majority of electrons occupy the valence band, filling it almost completely. An electron can only move to conduction band (which occupies relatively high energy) by absorbing enough energy more than that of the corresponding band gap, and most electrons in the bulk simply do not have enough energy to do so and under external stimuli such as heat, voltage, or photon flux can eventually promote some electrons to jump the forbidden gap to the conduction band. The consequence of valence location they vacate is commonly referred to as a hole which is the theoretical and mathematical opposite of an electron (http://www.evidenttech.com). Hence the electron which already took residence in the conduction band still maintains an electrostatic force of attraction with the hole in the valence band. This force of attraction eventually gives rise to a bound state like exciton. The residence of the previously excited electron in the conduction band is transitory and will return to its original valence band position very shortly. In doing so, the electron crosses the band gap and eventually releases electromagnetic radiation of certain wavelength. The nature of the electromagnetic radiation that has been emitted by the process is dependent on the size of the quantum dots. As for an example, in case of the fluorescent dye, we find relatively higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a colour shift from red to blue in the light emitted. The main advantage of such tuning with quantum dots is that, because of the high level of control possible over the size of the crystals produced, it is possible to have very precise control over the conductive properties of the material as well as quantum dots of different sizes can be assembled into a gradient multi-layer nanofilm (http://en.wikipedia.org). Quantum dots are also widely known as nanocrystals which are made of those elements which particularly belong to the periodic groups of II-VI, III-V, or IV-VI materials. Semiconductor quantum dots are the key to the modern electronics industry because of their highly tunable band gap which makes possible applications such as the high performance Photodiodes and solar cells. Like conventional semiconductors devices their electrical conductivity can also be greatly altered using a secondary stimulus like voltage, photon flux, etc which make them highly useful in sophisticated electronic devices like high performance solar cells and photodiodes.