2023 Nobel Prize in Chemistry: The periodic table now has a third dimension

The 2023 Nobel Prize in Chemistry was awarded for the "discovery and synthesis of quantum dots". This achievement is a model of the combination of nanotechnology and quantum mechanics, and its application is closely related to production and life. This article is a popular science introduction to the award-winning content released by the Nobel Prize official, which uses an easy-to-understand way to understand the scale dependence and manufacturing methods of quantum dots.

Translation | Dong Weiyuan

Proofreading | Li Dudu

They add color to nanotechnology

Moungi G. Bawendi, Louis E. Brus and Alexei I. Ekimov have been awarded the 2023 Nobel Prize in Chemistry for the discovery and development of quantum dots. These tiny particles have unique properties and are now shining in TV screens and LED lights. They can catalyze chemical reactions, and their bright light can also illuminate tumor tissue for surgeons.

"Toto, I have a feeling we're not in Kansas anymore," is a classic line from the movie The Wizard of Oz. A powerful tornado swept Dorothy's house away, and the twelve-year-old fainted in her bed. But when the house landed again, Dorothy walked out the door with her dog Toto, and everything changed. Suddenly, she was in a magical and colorful world.

If a magical tornado swept through our lives and shrunk everything to nanometer size, we would surely be as shocked as Dorothy in Kansas. Everything around us would become colorful and change. Our gold earrings would suddenly shimmer blue, and the gold rings on our fingers would glow ruby ​​red. If we tried to fry something on a gas stove, the frying pan might melt. And our white walls (which have titanium dioxide in their paint) would begin to produce large amounts of reactive oxygen species.

Figure 1 Quantum dots give us new opportunities to create colored light. Image source: Johan Jarnestad/The Royal Swedish Academy of Sciences

Size matters at the nanoscale

Things really do behave differently in the nanoworld. As soon as the size of matter starts to be measured in millionths of a millimeter, strange phenomena occur - quantum effects, which challenge our intuition. The winners of the 2023 Nobel Prize in Chemistry are all pioneers in the exploration of the nanoworld. In the early 1980s, Louis Brus and Alexei Ekimov independently succeeded in creating quantum dots, tiny nanoparticles whose properties depend on quantum effects. In 1993, Moungi Bawendi revolutionized the method of manufacturing quantum dots, making them of extremely high quality, an important prerequisite for their use in today's nanotechnology.

Thanks to the work of these laureates, humanity is now able to exploit some of the strange properties of the nanoworld. Quantum dots now appear in commercial products and are used in many scientific fields, from physics to chemistry to medicine - but it's too early to discuss these now, so let's first uncover the story behind the 2023 Nobel Prize in Chemistry.

Figure 2 Quantum dots are often made up of only a few thousand atoms. In terms of size, a quantum dot is about the same size as a soccer ball is to the Earth. Image source: Johan Jarnestad/The Royal Swedish Academy of Sciences

For decades, quantum phenomena in the nanoworld were only a prediction.

When Alexei Ekimov and Louis Brus created the first quantum dots, scientists already knew that they could theoretically have unusual properties. In 1937, physicist Herbert Fröhlich had already predicted that nanoparticles would not behave like ordinary particles. He derived the theoretical result from the famous Schrödinger equation, which states that when particles become very small, the space for electrons in a material becomes smaller. Or, one could say, the electrons, which are both waves and particles, are squeezed together. Fröhlich realized that this would lead to drastic changes in the properties of the material.

The researchers were fascinated by this insight and used mathematical tools to successfully predict many size-dependent quantum effects. They also worked hard to demonstrate these effects in reality, but this was easier said than done because they needed to carve nanostructures that were a million times smaller than a pinhead.

Few people thought quantum effects could be exploited

Nevertheless, in the 1970s, researchers succeeded in making such nanostructures. Using a molecular beam technique, they created a nanometer-thick coating of material on top of a bulk material. Once assembled, they were able to show how the optical properties of the coating varied with its thickness, an observation that matched the predictions of quantum mechanics.

This was a major breakthrough, but the experiment required very advanced technology. The researchers needed ultra-high vacuum and temperatures close to absolute zero, so few expected that quantum mechanical phenomena would be applied in practice. However, science sometimes brings unexpected things, and this time the turning point was brought by the study of an ancient invention - stained glass.

A single substance can give glass a variety of colors

The oldest archaeological discoveries of colored glass date back thousands of years. Glassmakers gradually understood how to create colorful glass through experimentation. They added substances such as silver, gold, and cadmium, and tried different temperatures to create beautiful glass with different shades of color.

In the 19th and 20th centuries, when physicists began to study the optical properties of light, the knowledge of glassmakers was put to use. Physicists used colored glass to filter specific wavelengths of light. To optimize their experiments, they began to make glass themselves, which led to important insights. One of the discoveries was that a single substance could produce glass of completely different colors. For example, a mixture of cadmium selenide and cadmium sulfide could make glass yellow or red, depending on how hot the glass was melted and how it was cooled. Eventually, they were able to show that the color came from particles formed inside the glass, and that the color depended on the size of the particles.

This was essentially the level of knowledge in the late 1970s, when one of this year's Nobel Prize winners, Alexei Ekimov, had just received his PhD and started working at the SI Vavilov State Optical Institute.

Alexei Ekimov reveals the secrets of colored glass

The fact that a single substance could produce different colors of glass intrigued Alexei Ekimov because it was actually illogical. If you paint a picture with cadmium red, it should always be cadmium red unless you mix something else in. So why would a single substance produce different colors of glass?

During his doctorate, Ekimo studied semiconductors - an essential component of microelectronics. In this field, optical methods are used as a diagnostic tool to assess the quality of semiconductor materials. Researchers shine light on the material and measure the absorbance. This can reveal what kind of substance the material is made of and how ordered the crystal structure is.

Ekimov was familiar with these methods, so he began to examine colored glass in this way. After some preliminary experiments, he decided to systematically create glass with added copper chloride. He heated molten glass to temperatures ranging from 500°C to 700°C for periods ranging from one hour to 96 hours. Once the glass had cooled and hardened, he examined it with X-rays. The scattered rays showed that tiny crystals of copper chloride had formed within the glass, and the manufacturing process affected the size of these particles. In some glass samples, they were only about 2 nanometers in size, and in others they could be up to 30 nanometers.

Interestingly, it turns out that the light absorption of glass is affected by the size of the particles. The largest particles absorb light in the same way as copper chloride normally does, while the smaller the particles, the bluer the light they absorb. As a physicist, Ekimov was very familiar with the laws of quantum mechanics, and he quickly realized that he was observing a size-dependent quantum effect (Figure 3).

Figure 3 Quantum effects appear as the size of the particles shrinks. When the particle diameter is only a few nanometers, the space available for electrons decreases. This affects the optical properties of the particle. The quantum dots absorb light and then emit it at another wavelength. Its color depends on the size of the particle. Image source: Johan Jarnestad/The Royal Swedish Academy of Sciences

This was the first time that humans succeeded in deliberately creating quantum dots, which are nanoparticles that can cause quantum size dependence. In 1981, Ekimov published his findings in a Soviet scientific journal, but it was difficult for researchers on the other side of the Iron Curtain to obtain them. Therefore, in 1983, when Louis Brus, this year's Nobel Prize winner in Chemistry, became the first scientist in the world to prove the quantum size dependence of free particles in a fluid, he was unaware of Alexei Ekimov's discovery.

Brus shows that the strange properties of particles are quantum effects

Louis Brus was working at Bell Labs in the United States at the time, and his long-term goal was to use solar energy to perform chemical reactions. To achieve this goal, he used cadmium sulfide particles, which can capture light and use its energy to drive reactions. These particles are suspended in a solution, and Brus made them very small because this increases the surface area for chemical reactions to occur; the more a substance is chopped up, the more surface area is exposed to the surrounding environment.

While studying these tiny particles, Brus noticed something strange - after he left the particles on his lab bench for a while, their optical properties changed. He suspected that the particles might have grown larger, and to confirm his suspicion, he made cadmium sulfide particles with a diameter of about 4.5 nanometers. Brus then compared the optical properties of these newly prepared particles with larger particles with a diameter of about 12.5 nanometers. The larger particles absorbed the same wavelength of light as cadmium sulfide, but the smaller particles shifted their absorption wavelength toward blue light (Figure 3).

Like Ekimov, Brus realized that he was observing a size-dependent quantum effect. He published his findings in 1983 and then began to investigate whether particles made of other substances had similar effects. The results were the same: The smaller the particles, the more blue the light they absorbed.

The periodic table has a third dimension

You might be wondering at this point: “Why would it matter if a substance’s absorbance shifted slightly toward blue light? Why is this so surprising?”

In fact, the optical change revealed that the properties of the material had completely changed. The optical properties of a substance are controlled by its electrons. These electrons also control other properties of the substance, such as its ability to catalyze chemical reactions or conduct electricity. So when researchers found a change in the absorption of photons, they thought in principle that they were looking at an entirely new material.

If you want to understand the significance of this discovery, imagine that the periodic table suddenly gained a third dimension: the properties of an element are affected not only by the number of electron shells and the number of outer electrons, but also, at the nanoscale, by the size. A chemist who wants to develop new materials thus has another parameter to manipulate - and this certainly captures the imagination of researchers!

There was just one problem: The methods Brus used to make nanoparticles often resulted in particles of unpredictable quality. Quantum dots are tiny crystals (Figure 2), and the ones that could be made at the time often had defects and were of varying sizes. One could control how the crystals formed so that the particles had a specific average size, but if researchers wanted all the particles in a solution to be about the same size, they had to sort them after they were made. This was a difficult process that hampered development.

Moungi Bawendi revolutionizes the production of quantum dots

The third winner of this year's Nobel Prize in Chemistry decided to tackle this problem. Moungi Bawendi began working as a postdoc in Louis Brus' lab in 1988, and the lab had done a lot of work trying to improve methods for producing quantum dots. They used a variety of solvents, controlled temperatures, and techniques, trying different substances to form organized nanocrystals. The crystals they made did get better, but they were still not ideal.

Bawendi didn't give up. When he began working as a research leader at MIT, he continued to work on producing higher-quality nanoparticles. The big breakthrough came in 1993, when the team injected the substance that would form nanocrystals into a heated and carefully selected solvent. They injected just the right amount of substance to form a saturated solution, which caused tiny crystal embryos to begin forming all at once (Figure 4).

Then, by dynamically changing the temperature of the solution, Moungi Bawendi and his research team succeeded in growing nanocrystals of a specific size. In the process, the solvent helped give the crystals a smooth and uniform surface. The nanocrystals produced by Bawendi were almost perfect, producing obvious quantum effects. Because this production method is easy to use, it has a revolutionary impact - more and more chemists are beginning to engage in nanotechnology research and begin to explore the unique properties of quantum dots.

Figure 4 Bawendi's method for preparing quantum dots of uniform size. Bawendi injected a substance that can form cadmium selenide particles into a hot solvent. Tiny cadmium selenide crystals formed immediately, and adding a coolant stopped the crystal growth. When the solvent temperature was raised again, the crystals grew again; the longer the time, the larger the crystals. Source: Johan Jarnestad/The Royal Swedish Academy of Sciences

The light-emitting properties of quantum dots find commercial applications

Thirty years later, quantum dots have become an important part of the nanotechnology toolbox and are appearing in commercial products. Researchers mainly use quantum dots to generate colored light. If you shine blue light on the quantum dots, they absorb the light and emit a different color. By adjusting the size of the particles, people can guarantee that they will emit a certain color of light (Figure 3).

The light-emitting properties of quantum dots are used in computer and TV screens based on quantum dot light-emitting diode (QLED) technology, where the Q stands for quantum dot. In these screens, blue light is produced using high-efficiency diodes that were awarded the 2014 Nobel Prize in Physics. Quantum dots are used to change the color of some of the blue light, turning it into red or green. This allows the TV screen to produce the required three primary colors of light.

Similarly, quantum dots are used in some LED lights to adjust the color temperature of the LED. In this way, the light can become as vibrant as daylight or as calm as the warm glow of a dim bulb. Quantum dot light can also be used in biochemistry and medicine. Biochemists attach quantum dots to biomolecules to mark cells and organs; doctors have begun to study the use of quantum dots to track tumor tissue in the body; chemists use the catalytic properties of quantum dots to drive chemical reactions.

Quantum dots therefore offer numerous benefits to humanity, and we have only just begun to explore their potential. Researchers believe that in the future quantum dots could contribute to flexible electronics, tiny sensors, thinner solar cells and even encrypted quantum communications. One thing is certain – we still have a lot to learn about the amazing quantum phenomena. So if there is a 12-year-old Dorothy out there looking for adventure, the nanoworld offers many interesting opportunities.

This article is translated from:

https://www.nobelprize.org/prizes/chemistry/2023/popular-information/

Special Tips

1. Go to the "Featured Column" at the bottom of the menu of the "Fanpu" WeChat public account to read a series of popular science articles on different topics.

2. Fanpu provides a function to search articles by month. Follow the official account and reply with the four-digit year + month, such as "1903", to get the article index for March 2019, and so on.

Copyright statement: Personal forwarding is welcome. Any form of media or organization is not allowed to reprint or excerpt without authorization. For reprint authorization, please contact the backstage of the "Fanpu" WeChat public account.