The 2023 Nobel Prize in Chemistry has been awarded to three researchers who pioneered quantum dots – particles whose properties can be tuned through their size. How does that work and what applications stand to benefit?
We need to start at the atom. Electrons orbiting an atomic nucleus can’t just adopt any energy: only a number of discrete energy levels are allowed. When two atoms get close enough, their outer atomic orbitals interact, forming two molecular orbitals of slightly different energy. Add another atom and four orbitals are formed. If a sufficiently large number of atoms come together to form a solid, so many orbitals are created that they essentially form two continuous bands. Through the power of many, the discrete nature of atomic orbitals is lost.
The relative position of the bands determines the electrical properties of a material. In a metal, the bands overlap, allowing electrons to freely move through the crystal lattice – the material conducts electricity well. An insulator features a bandgap, confining electrons to the lower-energy band, which is so overcrowded that movement is restricted – the material doesn’t conduct electricity. A semiconductor also has a bandgap, but it’s small enough for electrons to move between bands – the material conducts electricity a little.
The bandgap also dictates optical properties. The energy difference of a bandgap is the threshold for photons to be absorbed: only photons with an energy at least equal to that difference are capable of ‘promoting’ electrons across the gap. Reversely, electrons returning to the ground state release photons with energy roughly corresponding to that of the bandgap. Large bandgaps emit high-energy and therefore low-wavelength photons (towards the blue and UV part of the spectrum) and smaller ones emit higher wavelengths (towards the red and infrared part of the spectrum).
Now imagine going backward, picking off atoms one by one from a bulk solid. When the number of atoms gets low enough, the discrete nature of orbitals returns. At the same time, these clusters of atoms are so small that their size roughly coincides with the wavelength of electrons (at this scale all matter acts like waves). This introduces another effect: it changes the bandgap. The smaller the tiny crystals get, the more electrons are ‘squeezed,’ the bigger the bandgap. So, the smallest dots emit blue light, bigger ones shine red. Many colors in between are accessible as well, simply by changing the size of the quantum dots.
One of the most prominent applications of quantum dots is found in computer and television screens marketed as “quantumdot LED” (QLED). In these screens, blue light emitted by an LED backlight is transformed by quantum dots into red and green. In this respect, QLED is a variation of standard LED-lit LCD technology. However, Samsung is researching emissive quantum dot screens, in which quantum dots themselves are the light sources. This would be comparable to OLED technology.
Quantum dots also show promise as light-emitting labels in biomedical imaging. For example, they can guide surgeons to visualize blood vessels feeding tumors during surgery. Attaching quantum dot labels to various biomolecules helps researchers uncover what happens on a molecular level inside living cells. Compared to organic dyes conventionally used for the same purpose, quantum dots are brighter (more easily detectable) and more stable. In some cases, a change in emissions coincides with a chemical reaction having occurred. A downside is that many quantum dots are made from toxic metals, prohibiting medical use.
Quantum dots are also attractive base materials for solar cells, being efficient absorbers of light and consisting of abundant and inexpensive materials. Some types of quantum dots produce more electrons (and holes) for each photon that strikes them than conventional semiconductors, potentially offering a boost in efficiency. So far, however, quantum dot-based solar cells have failed to impress.
The global quantum dot market size reached 6.5 billion dollars in 2022 and is poised to grow 23.4 percent annually (CAGR), according to research firm Markets and Markets.
Main picture credit: Kenny Chou/Boston University