By Dr Caroline Shenton-Taylor

28 August 2019 - 07:34

Quantum physics
'When an excited quantum dot relaxes, it releases energy in the form of light.' Photo ©

Gerd Altmann used under licence and adapted from the original

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, so small that some people say they have no dimensions. They exist as points of materials, 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 because 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.

When we buy our quantum dots for research, they 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 manufacture, 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 growing, which means you can control the size of the core and shell.

We buy them as a 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 were to excite them, by example by shining a laser on them, they could start to glow. The colour they glow depends on their size and the elements used to make the dot, so some will give out visible light.

We mix the powdered quantum dots into a liquid called toluene, or we mix them into a resin which then sets as a solid. This give us a dispersion of quantum dots we can play with in the lab.

You can also buy quantum dots without the shell.

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

Quantum dots improve the picture quality in televisions.

When you shine light on a quantum dot, dependent on its size and composition, it will give out light at a different wavelength. 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 back light, 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 their 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 a useful radiation detector for security and screening applications.

Laboratories around the world are researching new detector designs. 

What are the medical applications of quantum dots?

In medical procedures that require radiation, it is important to know the patient's dose, and any radiation received by the medical team. Quantum dots might provide an alternative dosimeter.

We recently considered whether we could use quantum dots in neuroscience as a sensor, lighting up when a therapy x-ray beam was correctly located. Scientists are also testing quantum dots for targeted drug delivery and cell labelling.

If used inside a patient, quantum dots emitting across near-infrared wavelengths could allow light to pass out of the body. But the medical use of quantum dots raises challenges within physics, and safety and ethical questions.

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. 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, and 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.

Caroline Shenton-Taylor will present a talk at Hall of FameLab (link expired) on Friday 27 September at the Natural History Museum in London, UK.

Hall of FameLab is a free event and part of European Researchers' Night.

Follow @Dr_CST on Twitter or her website.

Editor's note 29 August 2019: This article has been amended to say that quantum dots are typically 1/10,000 the size of a human hair, rather than 1/100,000 the size of a human hair. 

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