Quantum Dots and Nanocrystals

Quantum dots are semiconductor nanocrystals small enough that confinement of their electrons makes their optical and electronic properties depend on size, so that absorption and emission colours can be tuned by controlling how the crystals are grown.

Definition

A quantum dot is a semiconductor nanocrystal, typically a few nanometres across, in which charge carriers are confined in all three dimensions so that its electronic energy levels are discrete and size-dependent, giving optical properties intermediate between molecule and bulk solid.

Scope

This topic covers zero-dimensional semiconductor nanocrystals: the physics of quantum confinement that widens the effective band gap as size shrinks; the colloidal hot-injection and related syntheses that produce nearly monodisperse crystals of controlled size and shape; core-shell structures that improve emission; surface ligand chemistry; and the optical properties — size-tunable photoluminescence and sharp excitonic absorption — that make them useful.

Core questions

• How does quantum confinement make nanocrystal properties size-dependent?
• How are monodisperse nanocrystals synthesised in solution?
• Why do core-shell structures improve quantum-dot emission?
• How does surface ligand chemistry affect nanocrystal stability and function?

Key concepts

• Quantum confinement
• Exciton Bohr radius
• Hot-injection synthesis
• Core-shell nanocrystals
• Surface ligands
• Size-tunable photoluminescence

Key theories

Quantum confinement and size-tunable gap

When a semiconductor crystal is smaller than the natural exciton size, the carriers are confined and the allowed energies become discrete; the effective band gap rises as the crystal shrinks, so emission and absorption shift continuously with particle size.

Colloidal synthesis and shape control

Rapid nucleation followed by controlled growth in hot coordinating solvents yields nearly monodisperse nanocrystals; varying surfactants and conditions controls shape and exposed facets, tuning optical and surface properties.

Mechanisms

Photoexcitation creates a confined electron-hole pair (exciton) whose recombination emits a photon at an energy set by the confined band gap; surface traps and dangling bonds open non-radiative pathways, which a wider-gap shell passivates to raise the emission efficiency.

Clinical relevance

Size-tunable, bright, photostable emission makes quantum dots valuable in display backlights and electroluminescent screens, in fluorescent labelling and bio-imaging, and as light absorbers and emitters in photovoltaics and light-emitting devices.

History

Brus in the early 1980s explained the size dependence of nanocrystal optical spectra in terms of quantum confinement. The development of hot-injection colloidal synthesis in the 1990s by Bawendi and others gave nearly monodisperse, high-quality nanocrystals, and Alivisatos's 1996 review consolidated the field, which led to commercial quantum-dot displays and bio-imaging probes.

Key figures

• A. Paul Alivisatos
• Louis Brus
• Moungi Bawendi

Seminal works

• alivisatos1996
• elsayed2005

Frequently asked questions

What does the 'quantum' in quantum dot refer to?

It refers to quantum confinement: the dot is small enough that the wave-like electrons and holes are squeezed into a space comparable to their natural size, which quantises their energy into discrete, size-dependent levels rather than the continuous bands of a bulk crystal.

Why are core-shell quantum dots brighter than bare cores?

Surface atoms of a bare nanocrystal have unsatisfied bonds that trap carriers and quench emission. Growing a thin shell of a wider-band-gap semiconductor confines the carriers inside the core and passivates the surface, sharply increasing the fraction of excitations that emit light.