#SCIENCE | Rainbows at the nanoscale

By Kat Dela Rama

LED screens, solar panels, and dyes—these widely used inventions have all been improved by tiny, colorful nanoparticles called quantum dots. The discovery and the synthesis of these nanoparticles set Alexei Ekimov, Louis Brus, and Moungu Bawendi on the path of being awarded the Nobel Prize in Chemistry 2023.

Nanoscience’s chameleon

Quantum dots are artificial droplets of charge or semiconductor particles that are only a few nanometers in size. Because of this, their electrons become so tightly packed that their properties change depending on their size—this is called the quantum effect.

The most visible quantum effect is the color of the quantum dot. Like a chameleon, the color of a quantum dot can change. As light is absorbed by a quantum dot, it causes electrons to move from their original positions and emit light. The wavelength, and thus the color, of the light depends on the “distance” moved by the electron.

Since larger quantum dots have more space between electrons, they emit red light. And as quantum dots get progressively smaller, the light emitted becomes orange, yellow, green, and finally, blue.

While scientists have theorized this since 1937, the small size of these nanoparticles made it difficult to prove. However, during the 1980s, Alexei Ekimov and Louis Brus were able to successfully synthesize quantum dots and prove their size-dependent properties.

Colorful discoveries

In 1981, Ekimov and his team at S.I. Vavilov State Optical Institute, Russia had been studying why the color of glass changed depending on how long it was heated, even if the substances used to dye them stayed the same.

When Ekimov created his own colored glass samples with copper chloride and heated them to various temperatures for different lengths of time, he X-rayed the samples to examine their crystal structure. He noticed that the copper chloride crystals varied in size and that this variation in size resulted in different wavelengths of light being absorbed.

On the other side of the world at Bell Laboratories in the US, Brus discovered the same phenomenon as he was researching how light affected chemical reactions.

He had created solutions filled with tiny particles of

Cadmium Sulfide (CdS) in hopes of absorbing more light, but after leaving them alone for a while, he noticed that the color of the solution had changed. Upon investigating, he found that the particles had grown in size, which changed the light that it had absorbed.

Based on their observations, they quickly determined that this was due to the quantum effects of nanoparticles.

Yet, despite their discovery, quantum dots were still difficult to produce in the scale and quality that commercialization demands. Ekinov’s method produced quantum dots trapped in glass, while Brus’ method produced quantum dots of inconsistent quality.

However, in 1993, a breakthrough was made in efficiently creating these quantum dots. Moungi Bawendi injected selected substances into a heated solvent and successfully grew a large number of highquality nanocrystals, or crystals with dimensions that are less than 100 nanometers. The size of these nanocrystals can easily be controlled by changing how long the solvent is heated. With this, scientists were finally able to efficiently study the properties of quantum dots further and develop new technologies utilizing these particles.

Lighting up the world

Today, quantum dots are being applied in a wide variety of fields. The most familiar application of quantum dots are Quantum Dot LED (QLED) screens. By using quantum dots, a QLED screen is able to show clearer and brighter images than a regular LED screen, allowing us to enjoy a more highquality viewing experience.

Another use of quantum dots is in bioimaging. Fluorescent dyes are often needed to mark proteins, antibodies, and other biomolecules in order to observe their roles in various biological processes. Dyes created using quantum dots are brighter and more stable than traditional organic dyes.

Quantum dots are also being utilized in improving the efficiency of solar cells by increasing the amount of solar energy they can convert to electrical energy. More efficient solar cells reduce the dependence on fossil fuels, which release harmful emissions when consumed.

Thanks to the contributions of Ekimov, Brus, and Bawendi, the field of nanotechnology flourished in the following decades. Even so, Heiner Linke of the Nobel Committee on Chemistry believes that there are more innovations for quantum dots just waiting to be explored.