Case Study: Quantum Dots
Electrons flow across conducting material not confined to any atom. By creating a barrier that repels electrons, one can limit the space where an electron can travel.
When limiting an electron to 2 dimensions you can create a quantum well (also called a quantum plane) where an electron can travel freely in 2 directions while a third is limited
If another barrier is put in you can limit the electron to 1 dimension that it can travel freely in. That is called a quantum wire or line.
A quantum dot constrains a metal in all three dimensions creating a particle-in-a-box like system
It was determined experimentally that each side of the cube needed to be around 10 nm in length or smaller in order that the system falls into the visible region and also displays quantum principles instead of classical ones.
How Quantum Dots are made
There are two major types of synthesis of quantum dots. The first and least expensive way is to synthesize the dots in colloid system using surfactants and moderately high temperatures.
The second way is to etch columns into electron repelling material or an energy barrier type material then fill that up with metal or semi conducting material. This method allows for a specific amount of electrons to be trapped in the quantum dots. This is done by then applying a a known amount of voltage across the dots. This way the energy level of the quantum dots can be changed based on the number of electrons that are put into each dot. This control over the dot is not seen in the colloidal system but this method is much more expensive.
Uses of Quantum Dots
Quantum dots are used for a lot of different things ranging from dyes for biological assays to advanced smaller resistors in circuits. The currant most common usage is in dyes they are coated in a small molecule to help make them soluble in water and then attached to the protein desired. The benefit of using quantum dots is that there are so many different colors available that more then 1 or 2 different things can be tagged at a time. Another benefit is that quantum dots typically have a much longer life time for florescence then organic dyes.
For resistors the benefit is the size. Being that quantum dots are anywhere from 1 nm to 10 nm and are either conducting material or semi-conducting material the resistors size can be greatly reduced. There are also other applications in microchips that can be taken advantage of.
It is a developing technology and so there are many applications that have yet to be fully discovered.
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