What you get when electrons are squeezed into a space only a few nanometres across: in one direction, a thin layer, for a quantum well; in all three, a tiny speck, for a quantum dot. Squeezing raises their energy in fixed steps, so the size decides the colour of light given off – which is how quantum-dot TV screens get their pure colours. A of a is a natural quantum well.
Going deeper
Squeeze electrons in one direction and their energies come in steps; squeeze them in all three and the result behaves like an artificial atom. The smaller the space, the bigger the steps – so the same material glows a different colour at a different size.
A particle in a box
An electron trapped in a small space behaves like a wave on a guitar string: only certain wavelengths fit, and the shorter the space, the higher the notes – for an electron, the higher its energy. A few nanometres is small enough for the steps between allowed energies to exceed the jiggling of room temperature, so they show up directly in how the material absorbs and emits light.
This is also why a 2D ’s depends on its thickness. As of InSe or black phosphorus are thinned layer by layer, the confinement grows and the gap widens step by step, turning the thickness itself into a way to choose the colour.
Wells, natural and built
Quantum wells are the workhorse of optoelectronics: the laser in a fibre-optic link or a Blu-ray player contains layers of one semiconductor a few nanometres thick, sandwiched between another with a wider gap. Building them takes growth one atomic layer at a time. Some layered crystals come with them ready-made. In a 2D perovskite, sheets of lead a few atoms thick alternate with layers of organic molecules that act as barriers, so the crystal is a stack of quantum wells straight from the flask. A quantum well of mercury telluride was also where the effect was first seen, in 2007.
Dots, made and found
Quantum dots are best known as nanocrystals a few nanometres across, made in solution, whose colour is set by their size; they turn blue light into pure red and green in some television screens, and their discovery and synthesis won the 2023 Nobel Prize in Chemistry. In 2D materials they turn up in two forms. Tiny flakes of graphene or MoS2 made in liquids behave as dots. And in single layers of WSe2 a local or defect can trap an in a spot small enough to act as a dot, giving out one at a time – among the first quantum light sources found in a 2D material, in 2015.
For specialists
Structures that confine carriers on the scale of their de Broglie wavelength in one (well) or all three (dot) dimensions, quantising their motion into subbands or discrete levels whose spacing scales roughly as 1/L2 for a deep well. Wells are made by sandwiching a narrow-gap layer between wider-gap barriers, as in HgTe/CdTe, or occur naturally in layered 2D perovskites and in monolayers; dots are colloidal nanocrystals, lithographically or electrostatically defined regions, or localised strain and defect sites in monolayers that act as .