QUANTUM DOTS

 Dear students,

This post deals with Quantum dots, a new class of materials that is neither molecular nor bulk. They have the same structure and atomic composition as bulk materials, but their properties can be tuned using a single parameter, the particle’s size.

What is a quantum dot?

Quantum dots are tiny crystals of semiconducting material. The crystals are so small – just a few nanometers wide (diameter of 2–10 nm) – that their physical size actually confines the electrons in the material to the point that it changes their behaviour. This means that quantum phenomena determine the properties of these tiny crystals – hence the name quantum dots. 

Changing the size of the particle changes its properties in a predictable way. This effect can be used to tune the band gap of the semiconductor so that dots emit different colours of light when illuminated by a single-wavelength source – smaller dots (e.g., 2-3 nm) providing higher energy emissions   will emit blue light, while larger ones (e.g., 5-6 nm) providing lower energy emissions emit at the red end of the visible spectrum. This property is sometimes referred to as “quantum confinement. 

Like other semiconductors, quantum dots tend to be made of combinations of transition metal and metalloid elements. Cadmium selenide and cadmium telluride are among the most regularly used materials.

Quantum Dot Production

There are various methods of producing quantum dots. The most typical is via a colloidal synthesis, which is the process of heating a solution, causing the precursors to decompose to form monomers, which then produce nanocrystals.

Quantum dots produced using this method can consist of compounds including indium arsenide, lead sulfide, lead selenide, and cadmium sulfide. Colloidal synthesis is a popular method as quantum dots can be produced in batches large enough to be potentially used for commercial applications.

Plasma synthesis is another popular technique for the production of quantum dots. This process enables the control of the composition, surface, size, and shape of the quantum dot, and it also reduces the challenges associated with doping.

Not every nanoparticle is a quantum dot. Only some materials (such as semiconductors) will show quantum size effects in their electronic structure when they are made into nanoscale particles.

Working Principle of a Quantum Dot

Within a quantum dot, there are confined valence band holes, conduction band electrons, or excitons. These are the particles that carry the electricity, and because of this confinement, the quantum dot has a distinct energy level. The optical absorption and emission of CdSe quantum dots can be tuned across nearly the entire visible range of the optical spectrum. This is possible because the energy bandgap of CdSe quantum dots varies between 1.8 eV (its bulk value) to 3 eV (in the smallest quantum dots)

Applications:

One of their most widely known uses is in forming the basis QLED television screens recognized as the “Q” in QLED TVs. where dots of different sizes are excited by blue light and then emit pure red and green light to give the three-colour output of the pixels in a TV screen. But they also have uses in biotechnology, catalysis, sensors, solar cells and more. In particular, the Nobel committee highlighted the use of quantum dots in medical devices that are used to map biological tissue because the dots’ fluorescence is brighter and longer-lived than that of other fluorescent tags such as molecular fluorophores. This means they can help to guide surgeons when removing tumours, for example.