By Dr Caroline Shenton-Taylor 

What is a quantum dot? What do quantum dots look like, and how are they changing entertainment, healthcare and security? Applied nuclear physics lecturer Dr Caroline Shenton-Taylor tells us how they work and why she loves them. 

What are quantum dots?

Quantum dots are tiny specks of material because it is so small, some people say they have no dimensions. They exist as points of material, typically 1/10,000 the size of a human hair.

Materials this small are called nanoparticles.

Quantum dots are nanoparticles made from semiconducting materials. The dots show quantum effects due to the fact that they are so little. This means that electrons inside the dot are trapped and can only occupy defined energy levels.

With only confined, discrete energy levels available, quantum dots can have different optical and electrical properties to a large quantity of the same material. This makes quantum dots useful in developing nanotechnology.

Quantum dots confine the motion of electrons in all three spatial directions. This restriction leads us into the quantum world and quantum dots’ electrical and optical properties.

What does a quantum dot look like?

A quantum dot is a small cluster of atoms.

For our research in the Surrey University radiation physics team, researchers including me work with quantum dots made from different combinations of elements. If you were to zoom in, you might see cadmium, selenium, indium or zinc atoms, to name just a few.

Quantum dots for research, often come with a protective shell made from semiconducting materials. The shell helps keep the quantum dot core confined, and provides some stability and protection.

During manufacturing, scientists can grow a quantum dot and control the diameter of the core and thickness of the shell.

Where do you get the quantum dots that you use for research? 

We get them from laboratories. Some laboratories are very skilled at making quantum dots, and there are different ways to manufacture them.

Scientists can grow them in a nucleation site or make them on a metal plate using an electrochemical approach. You can use a flowing gas or a drop in temperature to stop them from growing, which means you can control the size of the core and shell.

We buy them as pre-made powder for our research, with the core and the shell attached. 

The powders, core and shell arrive in glass vials, ready to use. If you excite them by shining a laser on them, they could start to glow. The colour they glow will depend on the sizes and the elements used to make the dot, some will give out visible light.

We may mix the powdered quantum dots with a liquid called toluene, or mix them into a resin, which then can be set into a solid. This gives us a dispersion of quantum dots we can play with within the lab.

You can also buy quantum dots that do not have shells with them.

How does a quantum dot affect the way I watch a movie?

Quantum dots improve the picture quality in televisions.

When shining a light on a quantum dot, it will give out light of different wave-length. and depending on its size and composition, the dot will appear to glow at a specific colour.

This is because of photoluminescence.

When an excited quantum dot relaxes, it releases energy in the form of light. In televisions this can be used to improve the quality of the television backlight, allowing much cleaner basic colours.

Researchers have also combined quantum dots with light-emitting diodes, generating light emissions with very narrow wavelength ranges. This makes the colours appear very bright.

Are quantum dots a part of my life in any other ways?

Quantum dots can capture light and convert it into electricity. They can do this efficiently and require less space than larger, conventional materials.

Changing the size of the quantum dot allows us to tune its ability to absorb and emit specific light frequencies.

Within the chemical growth process, we can use flowing gases and cold temperatures to control the size of the quantum dot. A bigger dot will emit light at a longer wavelength. In the visible range, larger dots glow red. Smaller dots shine blue.

What happens when you put a quantum dot near radiation?

We know that shining light on a quantum dot can make it glow, but did you know that a quantum dot placed near radiation can emit light?

At Surrey University, some of our PhD students are researching the light produced by different types of quantum dots when they are irradiated with X-rays. We are working with visible wavelengths of light, making the glowing dots beautiful to study.

How could quantum dots affect security?

Quantum dots emit light when they are near radiation, and this could be a way to show nearby radiological materials. Our dots could be useful radiation detectors for security and screening applications.

Laboratories around the world are researching new detector designs. 

Scientists would need to assess the elements which make up the dots to ensure they are biologically safe and to know whether they dissipate or remain within the patient. 

What is the link between quantum dots and the environment?

Sometimes cadmium atoms are used in the core of a quantum dot. Cadmium is a heavy metal, and inhalation can lead to respiratory problems. The injection can trigger cardiovascular and renal issues. Treatment for cadmium poisoning is limited.

Although the amount of cadmium is minuscule in quantum dots, in our laboratory we are researching other element combinations in quantum dots. 

Cadmium can be quite expensive, and that is another good motivator to avoid that element in our quantum dots.

Could a quantum dot save my life?

Maybe. Quantum dots might be tiny, but they are showing exciting and versatile applications within nanotechnology, especially with their medical applications. Who knows what they might help us achieve?

Without them, the world of nanotechnology would lack a useful group of nanoparticles.

Article Origin: British Council