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Scientists Develop A Method For Doping Super Tiny Quantum Dots

Question asked by kunal in #Coffee Room on Apr 11, 2011
kunal · Apr 11, 2011
Rank D1 - MASTER
Quantum dots are microscopic spherical crystal like particles with a diameter measuring a few nanometers. These particles are so miniscule that when a few atoms were tried to be inserted in these particles, the scientists found that the new atoms got pushed out due to the lack of space! But now a team of Israel based researchers have made it possible to add a few extra atoms to these nanocrystals and made them stay there as a stable structure. You will say what’s so great about adding a few particles of matter to a quantum dot? Well, this development assumes significance due to the fact that it has made the scientists realize that they can now dope the quantum dots to make them work in any particular way.

Doping is a process traditionally used for decades in intrinsic semiconductors to improve their electrical properties and to imbibe desirable electronic characteristics in them.  The Israeli team of scientists has created the first pairs of p type and n type miniature semiconductors using doped quantum particles. If these quantum dots can work satisfactorily then they can be used in futuristic applications like flexible electronic devices, flexible solar cells and LEDS.

[​IMG]Adding impurities can greatly vary the electrical properties of the crystals. Theoretically speaking, the doping process can help to alter and manipulate the electronic properties of the crystals. These tiny quantum dots consist of a little more than a few hundred atoms. Such small dimensions lead to a variety of strange quantum effects such as glowing when a particular wavelength light falls on the surface of these dots. Quantum dots have had a few biology related applications such as tagging of molecules and taking cell images. However, till now there was no successful method to change the conductivity and such other electrical characteristics of the dots. Someone might say that you can dope a silicon or germanium semiconductor wafer with impurities and then crush this wafer to the size of quantum particles. The idea sounds theoretically good but there are a few physical restrictions that do not allow such manufacturing procedure to complete. Also, you cannot force accelerated dopant atoms into these crystals because they will disturb the quantum structure.

Now, a team of scientists led by Uri Banin and Oded Millo from Hebrew University have collaborated with a research unit from Tel Aviv University led by Eran Rabani to device a reliable and controlled methodology for introducing doping agent atoms in these minute particles. The procedure followed by these scholars is very simple. They took a solution consisting of indium arsenide crystals with 3.3 nm diameter and copper or silver atoms. The atoms initially keep rebounding from the surfaces in a random motion as per the principle of Brownian motion. Finally, when their kinetic energy is lost as heat and potential energy conversions, they find a way into the quantum dots. When copper atoms were introduced into the solution, the copper atoms occupied the empty spots in the crystal lattice and donated their valence electrons to the crystals. Due to the increase in the number of negatively charged electrons, the nanoparticles became n type semiconductors. However, when silver atoms were used, they displaced the indium atoms in the lattice thus leading to a deficiency of electrons. So the quantum dots become p type semiconductors with holes.

The group of scientists confirmed the alteration in the conductivity and V-I characteristics of these modified particles with the help of a scanning tunneling microscope. The observed that the Fermi energy levels had increased and decreased for the n type and p type quantum particles respectively. These observations and the various conclusions drawn from them were published in the journal <em>Science.</em>

This research thus gives a proof that quantum dots can actually be used to make miniature transistors, diodes, LEDs, and all other semiconductor devices. The devices made by nanoparticles will be more efficient, cheaper and could be embedded in different substrates. The quantum dots can be of great use in solar energy devices because they can be integrated in flexible plastic bases and can be tunes for any particular wavelength of light which is to be absorbed. So now everything that is electronic and made from semiconductors is going to shrink in size with these quantum particles.

Image credit: IEEE Spectrum Posted in: #Coffee Room

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