By Hannah Johnson

Quantum dots (QDs) have been extensively studied over the last thirty years and found applicable in many ways, such as in medical imaging, solar cells, drug delivery, and even in QLED screens which have been in production since 2015 and allowed the transition to better color resolution in screens.^2,3 The 2023 Nobel Prize in Chemistry was awarded based on the work that contributed to the discovery and synthesis of Quantum dots. QDs are classified as semiconductor crystals in the range of 2-10 nanometers in diameter. Nanoparticles (1-100nm) have been extensively studied and found useful in various applications including medicine, catalysis, and agriculture where their small size can induce interesting chemical and physical properties. In the size range of QDs, an interesting optical property is revealed. When exposed to ultraviolet light (UV light), nanoparticles made of the same material will emit a different color of light dependent on the QD size. As seen in Figure 1, QDs with a larger diameter will emit a red light while QDs with smaller diameters will emit blue light.

In order to understand the size-dependent color effect demonstrated by QDs, one must first understand the “quantum confinement effect”. Semiconductors are materials with an electrical conductivity capability in-between that of conductors and insulators. When determining the conductivity of a material, the difference in energy between the valance band (highest electron energy) and the conduction band (lowest electron energy) is measured⁴. This difference of energy levels is known as the band gap. When given energy, (e.g. UV light) an electron will jump from the valence band to the conduction band. When the electron returns to the valence band, energy is released in the form of a photon emitting the color of light associated with the size of the band gap^4,5. As seen in Figure 2,  the loss of an electron from the valence gap results in a positively charged electron hole which forms an electron-hole pair with the electron in the conduction band (the distance between the pair is known as an exciton). For semiconductors of nanometer size, like QDs, the quantum confinement effect is induced, meaning the size of the nanoparticle is smaller than the exciton Bohr radius (the average distance between electron and the electron hole). As such, the quantum confinement effect causes the electron hole pairs to be squeezed together so the smaller the QD the larger the distance between the valance band and the conduction band. For example, in Figure 3, the QD emitting blue light requires more energy for an electron to shift between the valance band and the conduction band as compared to the QD emitting red light due to the larger band gap produced from the quantum confinement⁴. This explains why smaller quantum dots will emit shorter wavelengths which have higher energy (blue) and larger QDs emit longer wavelengths (red).

The Nobel Prize for the discovery and synthesis of quantum dots was awarded to the three laureates: Alexei Ekimov, Louis Brus, and Moungi Bawendi. The first discovery of QDs was attributed to Ekimov in the 1980s, when he was studying the optical effects of copper chloride (CuCl) in glass treated at different temperatures and for varying times. He was able to uncover that different heat treatments to the glass inserted with CuCl formed into different sized nanocrystals, with each absorbing different wavelengths. This suggested there were different quantum effects contained within these nanocrystals based on size⁵. Unfortunately, the nanocrystals embedded in glass were not in a medium that allowed for further study; however, around this time Louis Brus was working on cadmium sulfide (CdS) nanoparticles. Brus was able to discover CdS nanocrystals showcasing quantum effects based on particle size in solution, which allowed for better study. Though the QDs were now able to be processed for further use, the methods utilized for nanocrystal synthesis resulted in nanoparticles of various sizes⁵. Finally, Moungi Bawendi, who had previously been a student in Brus’ lab, was able to synthesize a consistently reproducible method for making size-specific quantum dots in 1993⁵.

Though the initial discovery and synthesis of QDs occurred over three decades ago, the vast impact of these contributions can be seen in the world today. Current research is investigating QDs in catalysis, biomedicine, and environmental appications⁶. From advancing technology with QLED screens to the colored staining of cells and tissues in research, the many functions for quantum dots are wide and still being discovered.

TL;DR

• Quantum dots are nanocrystals capable of emitting different colored light based on their diameter size-alone.
• The 2023 Nobel Prize in Chemistry was awarded for the discovery and synthesis of quantum dots.

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Reference

1. Boston University. “What Are Quantum Dots? | the Brink | Boston University.” Boston University, 3 Sept. 2019, http://www.bu.edu/articles/2017/quantum-dots-breast-cancer-tumors.
2. Haynes, Dave. “What Is Quantum Dot Display Technology?” Samsung Business Insights, 29 Dec. 2021, insights.samsung.com/2021/12/29/what-is-quantum-dot-display-technology.
3. Berger, Michael. What Are Quantum Dots? www.nanowerk.com/what_are_quantum_dots.php.
4. Reshma, V. G., and P. V. Mohanan. “Quantum Dots: Applications and Safety Consequences.” Journal of Luminescence, vol. 205, Jan. 2019, pp. 287–98. https://doi.org/10.1016/j.jlumin.2018.09.015.
5. https://www.nobelprize.org/uploads/2023/10/advanced-chemistryprize2023.pdf
6. Cotta, M. A. “Quantum Dots and Their Applications: What Lies Ahead?” ACS Applied Nano Materials, vol. 3, no. 6, June 2020, pp. 4920–24. https://doi.org/10.1021/acsanm.0c01386.
7. https://phys.org/news/2022-03-fast-moving-excitons-metal-potential-digital.html