The winners of the Nobel Prize in chemistry were leaked early to a Swedish newspaper this week, in a mistake that the Nobel committee “deeply regret”. But ultimately, the difference of a few hours won’t matter too much in the story of “quantum dots” – light-shifting particles that were discovered in the 1980s, predicted by quantum theorists in the 1930s, but were harnessed by Roman glassmakers and beauty stylists thousands of years ago.
Following the leak on Wednesday, the Swedish Royal Academy of Sciences announced that the 2023 Nobel Prize in chemistry was awarded to Moungi Bawendi, Louis Brus and Alexei Ekimov. The three scientists, two Americans and a Russian, are credited with the discovery and development of quantum dots (QDs), which has had a major impact on nanotechnology.
The physical properties of a material, such as its colour or chemical reactivity, are typically determined by its chemical makeup. The atoms that form the material, and how they are arranged, will usually determine the behaviour of the material. But when atoms are arranged into tiny crystals, just nanometres in size, then their behaviour starts to change and size becomes a critical factor. On the nanoscale, the properties of these particles, or quantum dots, is determined by the laws of quantum mechanics. “Quantum dots have many fascinating and unusual properties. Importantly, they have different colours depending on their size,” says Johan Åqvist, the Swedish chair of the Nobel committee.
The unique behaviour of particles on the nanoscale was predicted as early as 1937, when scientists realised that electrons confined within a very small crystal would display the same quantum effects that are usually observed in individual atoms. But it took many decades before the practical demonstration of these effects were seen.
In the late 1970s, the Soviet scientist Alexei Ekimov was studying the effects of small copper chloride crystals suspended in glass. It was known that different amounts of copper, or other additives, could dramatically change the colour of glass, and that the size of the particles played an important role. Ekimov showed that the temperature used to heat the glass, and the time it was heated for, created glass with different optical properties. By observing the scatting of X-rays through the glass, he could determine the size of the crystals, which ranged from about 2 to 30 nanometres. (A nanometre is a billionth of a metre, or just under half the width of a strand of DNA.) The largest particles absorbed light like copper chloride normally would, but the smaller particles absorbed light closer to blue.
Ekimov was well versed in microelectronics and semiconductor physics, and in 1981 was able to recognise the role of quantum effects in the blue-shifting of the smallest nanoparticles.
Working on the other side of the iron curtain, Louis Brus from the Bell Laboratories in the US was unaware of Ekimov’s work in 1983, when he published his own studies on the quantum effects of QDs. Brus was working with tiny particles of cadmium sulphide, which were known to absorb light and help to drive chemical reactions. Brus noticed that the colour of his particles would shift over time after they were made. He theorised that this could be caused by the growth of larger particles affecting the optical properties of the material. He compared the light absorbance of freshly made QDs with a diameter of 4.5 nanometres with an older sample with particles of 12.5 nanometres. Just like in Ekimov’s particles suspended in glass, these cadmium sulphide particles shifted their absorbance towards shorter wavelengths as the crystals became smaller.
The discoveries of Ekimov and Brus electrified the scientific community, as their work showed that quantum effects could be harnessed to control the properties of different materials. But making quantum dots of the desired size and purity remained a major hurdle. Scientists struggled to produce crystals that were of a consistent and uniform size, which hampered their practical application. For example, it was shown that QDs could absorb light of a particular wavelength and emit light at a different wavelength (a process called fluorescence) depending on the particle size. But the quantum efficiency of this process, the percentage of light that was emitted at the new wavelength, was only around 1%.
Moungi Bawendi formerly worked in Brus’ lab, and continued to work with quantum dots as a research leader at MIT. Bawendi and his team painstakingly studied the conditions used to synthesise quantum dots, made from nanocrystals of cadmium selenide. They found conditions, by careful control of temperature, solvent and mixing rates, that produced quantum dots of nearly perfect size and quality. The quantum efficiency of these new particles shot up from 1% to 40% using Bawendi’s new methods. The ability to make these particles, and precisely control their size on the nanoscale, opened up the study of quantum dots to the scientific community and the field has continued to grow.
Most scientists working with quantum dots are looking towards their future development, but some are also looking to the past. QDs have been found in artefacts produced by the ancient Romans, who harnessed their optical properties without understanding their nature. The Lycurgus cup is a mesmerising piece of Roman glasswork produced in the 4th century that appears green in low light, but glows red when light passes through it. Roman glassmakers used gold and silver additives to colour their glass, and under the right conditions, these metals can form nanoparticles that display quantum effects. The red glow of this glass is caused by the optical properties of gold nanoparticles, which are drastically different to that of typical gold.
Quantum dots have also been found in recreations of Greco-Roman hair dyes, produced from recipes dating back 2000 years. Modern experiments have shown that the black colour of these dyes is produced by lead sulphide QDs that form within the hair fibre.
The unique optical properties of quantum dots are used today in numerous applications.
The unique optical properties of quantum dots are used today in numerous applications. The ability of quantum dots to absorb light and re-emit it at a different wavelength is used in QD displays. These display screens use highly efficient blue LEDs to produce light, which can then be absorbed and re-emitted by quantum dots at specific wavelengths, controlled by the particle size. This produces displays with increased brightness and access to a wider range of the colour gamut.
Quantum dots are also having an impact on solar cells, with photovoltaic devices using this technology starting to become commercially available. Traditional solar cell materials can only absorb part of the light across the solar spectrum, with much of the energy left unused. Because of their precise tuneabilty, quantum dots can be made to absorb light into the infrared region, which is difficult to achieve with traditional solar cell materials. Given that half of the sun’s energy reaching earth is in the infrared region, this offers great promise for renewable energy.