Quantum dots (QDs) are semiconductor nanoparticles that exhibit size and composition-dependent optical and electronic (optoelectronic) properties. QDs typically have a diameter in the 2-10 nanometer range (10-50 atoms). They display unique electronic properties, between those of bulk semiconductors and discrete molecules, in part due to their high surface-to-volume ratios. Their properties include superior photostability, size-dependent optical properties, high extinction coefficient and brightness, and large Stokes shift. These properties have led QD nanotechnology to be utilized by the electronic and biomedical industries, including applications in composites, solar cells, fluorescent biological labeling, displays, lighting, and medical imaging. The market for quantum dot applications in the United States reached $4 billion in 2021, and some 8% of the global TV market now relies on quantum dots to add brilliant colors.
Due to their small size, electrons in QDs are confined to a small space (quantum box). When the radii of a semiconductor nanocrystal is smaller than the exciton Bohr radius (average distance between the electron in the conduction band and the hole it leaves behind in the valence band), there is quantization of the energy levels according to Pauli’s exclusion principle. These discrete energy levels make QDs act closer to atoms than bulk materials. As the of the crystal decreases, the difference in energy between the highest valence band and the lowest conduction band increases. Therefore, more energy is needed to excite the dot, and more energy is released when the crystal returns to its ground state. Resulting in a color shift from red to blue in the light emitted. This phenomenon means nanomaterials can emit any color of light from the material by changing the dot size. During production, nanocrystals of different sizes can be accurately produced, tuned to emit a specific color of light.
In 1981, Alexei Ekimov first managed to make nanometer-size crystals of copper chloride embedded in glass while working at the Vavilov State Optical Institute in Russia. This work confirmed that dots of different sizes fluoresced in different colors. A few years later, Louis Brus, working at Bell Labs, was looking for catalysts to capture the energy of sunlight in a chemical reaction. Brus and his team crystallized particles of cadmium sulfide out of a solution, and they noticed that the larger ones reacted to light differently than the smaller ones, realizing the same quantum phenomenon independently. Brus's work was published in a 1983 paper. His work also removed a limitation of Ekimov's, with QDs suspended in a solution rather than embedded in glass.
The phrase quantum dot was coined in a 1986 paper titled "Spatial quantization in GaAs–AlGaAs multiple quantum dots" and authored by M. A. Reed, R. T. Bate, K. Bradshaw, W. M. Duncan, W. R. Frensley, J. W. Lee, and H. D. Shih.
Defects in these early quantum dots (in particular, their variable sizes) kept them from wide commercialization. In 1993, Moungi Bawendi and his team, also at Bell Labs, devised a way to make high-quality crystals of a well-defined size that would produce sharp, vivid light of one specific color.
On October 4, 2023, The Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry to Alexei Ekimov, Louis Brus, and Moungi Bawendi “for the discovery and synthesis of quantum dots.”
