Quantum Dot
A speck of crystal so small that picking its diameter is the same as picking its color.
Absorbs ultraviolet or blue light and re-emits it as a precise, narrow color set by its diameter — letting one semiconductor (CdSe, InP, perovskite) cover the entire visible spectrum from a single chemistry. With a passivating shell, photoluminescence quantum yield approaches 100%. The same combination of size-tunable color, high efficiency, and chemical processability also makes individual dots into on-demand single-photon sources.
In bulk, a semiconductor's electrons live in continuous valence and conduction bands separated by a fixed bandgap. When a crystal shrinks below its exciton Bohr radius (~6 nm for CdSe), the electron and hole are forced into discrete states like a particle in a box, with energies that scale as ~1/r². Smaller dots have a wider bandgap and emit bluer light. An incoming photon above the bandgap creates an exciton; rapid relaxation to the band edge is followed by radiative recombination at exactly the bandgap energy. A wider-gap shell (ZnS on CdSe) eliminates surface trap states and brings the quantum yield close to unity.
In plain terms
Imagine a guitar string. A short string plays a high note, a long string plays a low note. A quantum dot is like a tiny box that traps electrons the way a string traps vibrations. A very small box forces electrons into tightly spaced, high-energy levels, so they emit blue light when they relax. A larger box gives more room, lower energies, and red light. By choosing the right crystal size — to within a few atoms — chemists tune the color across the rainbow from a single recipe.
Also called: semiconductor nanocrystal, colloidal quantum dot, Q-dot.
Why it matters
- QLED televisions use quantum-dot films to convert blue LED light into pure red and green, achieving close to 100% Rec. 2020 color gamut.
- Bioconjugated dots are non-photobleaching fluorescent labels for single-molecule tracking, tumor imaging, and multiplexed diagnostics.
- Solar cells using dots can be tuned to match the solar spectrum and may eventually exploit multiple-exciton generation.
- A single epitaxial dot in a photonic cavity can emit one photon on demand — a building block of photonic quantum computing.
Where it lives
- Samsung Neo QLED and QD-OLED televisions
- ThermoFisher Qdot bioimaging reagents (used in thousands of labs)
- Nanosys quantum-dot films for displays
- Bawendi Lab (MIT), Brus Lab (Columbia), Vavilov Institute (Ekimov) — original discovery sites
Key numbers
- Diameter range: 2–10 nm (Tunes CdSe emission across roughly 450–700 nm)
- Photoluminescence quantum yield (InP core/shell): approaching 95–100% (ACS Chem. Mater. (2025))
- Emission linewidth (FWHM): ~20–30 nm (Narrow size-distribution CdSe/ZnS)
- Exciton Bohr radius (CdSe): ~6 nm (Below this size, strong confinement)
- QLED color gamut: ~100% of Rec. 2020 (Versus ~70% for standard white-LED LCD)
Surprising facts
- The exact same compound, CdSe, emits every color from deep blue to deep red purely by changing crystal size from about 2 nm to 7 nm — no dyes, no impurities required.
- A teaspoon of the CdSe quantum dots first synthesized cleanly by Bawendi's group in 1993 contained roughly 10²² individual nanocrystals, made in under an hour on a lab bench at ~150 °C.
- A single dot can be coaxed to emit exactly one photon per excitation pulse, a capability now used as a provably secure light source for quantum cryptography.
- The 2023 Nobel Prize in Chemistry was awarded for this whole picture — Ekimov, Brus, and Bawendi — for showing that nanoscale size alone is enough to control optical properties.
Open questions
- Fluorescence intermittency ("blinking"): Individual quantum dots randomly switch between bright and dark emission states on millisecond-to-second timescales, even under steady illumination. After three decades of study, no single mechanism — Auger ionization, surface trap dynamics, charge tunneling — fully explains the statistics across all systems.
A short history
In 1981, Alexei Ekimov reported size-dependent optical properties of CuCl nanocrystals embedded in glass. Louis Brus at Bell Labs (1983–1985) provided the theoretical link between particle size, quantum confinement, and bandgap. In 1993, Moungi Bawendi at MIT introduced the hot-injection synthesis that produced monodisperse CdSe quantum dots with under 5% size dispersion, making them practical. The 2023 Nobel Prize in Chemistry was awarded to Ekimov, Brus, and Bawendi for the discovery and synthesis of quantum dots.
Sources
- Nobel Prize 2023 — advanced scientific background (https://www.nobelprize.org/uploads/2023/10/advanced-chemistryprize2023.pdf)
- ACS Central Science (2023) — quantum dot editorial (https://pubs.acs.org/doi/10.1021/acscentsci.3c01296)
- ACS Chem. Mater. (2025) — InP PLQY measurement (https://pubs.acs.org/doi/10.1021/acs.chemmater.4c02490)
- Nano Letters (2025) — blue QLED progress (https://pubs.acs.org/doi/10.1021/acs.nanolett.4c02968)
- Chemistry World (RSC) — quantum dot Nobel explainer (https://www.chemistryworld.com/news/explainer-why-have-quantum-dots-won-the-2023-nobel-prize/4018168.article)