Quantum Dots

Summary:  

Quantum Dots are tiny nano crystals that are made up of semiconductor materials like Silicon (Si), Germanium (Ge), Cadmium Sulfide (CdS) and Cadmium Selenide (CdSe). These dots light up when stimulated by another external source like ultra violet (UV) rays giving different colors depending on their size.

 

Quantum dots are tiny nano crystals that glow when stimulated by an external source such as ultraviolet (UV) light. They are composed of hundreds to thousands of atoms. The number of atoms included inside the quantum dots determines their size which determines the colour of light emitted. These semiconductor materials can be made of an element such as silicon or germanium, or even compounds such as CdS or CdSe. The figure below is a collection of CdSe quantum dot nano particles that differ in size as a result of how long they were allowed to form in the synthesis reaction. Colour is well known to be influenced by particle size in both quantum dots and nano particles.

 

Figure(1)

Top: Long wave UV illumination. Bottom: Ambient illumination. Solutions are in order of increasing particle size (longer growth time).

 

Now, how do these dots light up?! If we looked firstly on how atoms emit light we’ll find that after absorbing some energy, an electron inside the atom is promoted to a higher energy level further from the nucleus, when it returns back to its lower and stable state, amount of energy equals to the difference of energies of the two levels¬† is produced as a photon of light, The colour of the light depends on the energy released from electrons in different energy levels and varies from one atom to another.

Quantum dots produce light in a similar manner because the electrons and holes are confined inside them, similarly it produces discrete quantized energy levels. However, the energy levels are governed by the size of the dot rather than the substance from which it’s made.

The biggest quantum dots produce the highest wavelengths (and shortest frequencies), while the smallest dots make shorter wavelengths (and higher frequencies), in practice that means big dots make red light and small dots make blue, intermediate-sized dots produce green light (and the familiar spectrum of other colours too). The explanation for this is that a small dot has a bigger band gap ( the minimum energy it takes to free electrons so they’ll carry electricity through a material) so it needs more energy to be excited as the frequency of emitted light is proportional to the energy, smaller dots with higher energy produce higher frequencies (and lower wavelengths) while larger dots have more (and more closely) spaced energy levels which means smaller band gaps so they give out lower frequencies (and higher wavelengths).

 

Figure (2)

The colour emitted by a quantum dot changes according to its size

 

One of the famous applications of quantum dots is the optical applications as quantum dots have interesting optical properties, they’re being used for all sorts of applications where precise control of colored light is important.

Quantum dots can be used instead of pigments and dyes. Embedded in other materials, they absorb incoming light of one color and give out light of an entirely different color, they’re brighter and more controllable than organic dyes (artificial dyes made from synthetic chemicals). They can be used in screens as they provide three important advantages. Firstly, in a typical LCD (liquid crystal display screen), the image you see is made by tiny combinations of red, blue, and green crystals (effectively color filters that switch on and off under electronic control) that are illuminated from behind by a very bright backlight. Quantum dots can be tuned to give off light of any color so the colors of a quantum dot display are likely to be much more realistic. Secondly, quantum dots produce light themselves so they need no backlight, making them much more energy efficient (an important consideration in portable devices such as cell phones where battery life is very important). And thirdly, quantum dots are much smaller than liquid crystals so they’d give a much higher-resolution image.

 

References:

1) http://www.explainthatstuff.com/quantum-dots.html

2) http://education.mrsec.wisc.edu/background/quantum_dots/

 

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