Quantum Dots

By Swathi Mahashetti
12 December 2021

Quantum dots, also known as artificial atoms, are an intriguing and versatile class of semiconductors – materials whose ability to conduct electricity increases with temperature. Quantum dots have applications in a wide variety of fields ranging from cancer detection to high-definition display technology.

Quantum dots are ionic salt crystals, only about a billionth of a metre in size. As with most nanoparticles, quantum dots luminesce when UV light is shone on them. However, it is their ability to display a spectrum of colours due to small variations in their size which makes them really exciting. This phenomenon was first discovered in the 1980s by Alexei Ekimov, Alexander Efros, Louis Brus and their research groups. Today it is an accessible and commonplace technology.

Two quantum dots which both comprise the same elements yet are different in size (one with a radius of 1.7 nm and the other 15 nm), appear as two different colours at opposite ends of the visible spectrum: red and blue respectively.

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To best understand how this is achieved, it would be useful to define the underpinning theory. Indeed, ‘Band theory’ determines the functionality of semiconductors such as quantum dots. Under this theory, electrons in a molecule tend to occupy molecular orbitals which are lower in energy.

As a semiconductor is made up of infinite molecules joined in a network, there is an overlap between the regions of space which the electrons are likely to inhabit,  thus producing a ‘band’. The band is split into two regions; the lower energy electron-rich region is called the ‘valence band’ and the higher energy empty region is called the ‘conduction band’. Valence bands and conduction bands are separated by a band gap. If light is shone on these crystals, an excited electron can gain energy and ‘jump’ over the band gap from the conduction band into the valence band. However, this excitation is short lived; electrons subsequently lose energy and return to their ground state.

When this happens, visible light is emitted. The colour of the light is dependent on its wavelength (which in turn is dependent on the energy gap between the two states). The larger the dot, the smaller the energy gap and the longer the wavelength of photon which is emitted, ensuring a red glow.

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Quantum dots are distinguishable from larger semiconductors due to the concept of ‘quantum confinement’. Essentially, in larger crystals, the valence bands are larger (as there are more contributing molecules) ensuring that the electrons in these bands are free to move over a larger region of space. However, as the size of the crystal decreases, the number of contributing  molecules decreases. This ensures that the size of the band becomes smaller and the electrons are more confined. Due to this confinement, the crystal begins to behave more like a molecule; the continuous energy bands split into discrete energy levels.

The properties of quantum dots are applicable in many fields, ranging from high-definition television, to solar cells and cancer detection. The optical properties of quantum dots make them more suitable to the labelling and imaging of cells over the typical use of organic dyes and fluorescent proteins. Furthermore, the emission spectrum of quantum dots is narrow compared to the broad spectra of dyes. This ensures that they can be used to image multiple biological molecules simultaneously without any overlap in the spectrum.  Quantum dots are highly sensitive and less toxic compared to some common imaging dyes; hence, they can be employed a probes during cancer detection.

Most recently, quantum dots have been used to produce coloured LEDs in the ‘QLED’ televisions in development by companies such as Samsung, Sony, and LG. QLED TVs are thought to have improved colour and display quality and are more environmentally sustainable compared to the LED TVs in the market today. Quantum dots can also be exploited for their catalytic properties and are used extensively in solar cells as very potent photocatalysts.

Quantum dots is an highly active area for research. Their applications in developing flexible and durable electronics is an area of great investment by tech companies – especially as electronics are set to become a fundamental future technology.

Swathi Mahashetti is in her second year at Imperial College London, studying Chemistry with Molecular Physics. She hopes to take up research in the future and is interested in sustainable energy technology. She is also a movie buff and enjoys doing embroidery in her spare time.

All images appearing within the article, including cover image, are courtesy of Celeste De kock, MSc Science Communication student at Imperial College London.