a quantum dot
Imagine a box so small that an electron inside can only occupy a few sharply defined energy levels, exactly like the electron levels of a single atom — except this box is a whole tiny crystal you designed. That is a quantum dot: a nanocrystal small enough, in all three directions, that its electrons are quantum-confined. Because it hems electrons in from every side, it is called a zero-dimensional structure — there is no direction in which an electron is free to roam. People often call quantum dots artificial atoms, because their discrete, tunable energy levels mimic the crisp levels of real atoms.
Concretely, a quantum dot is typically a semiconductor nanocrystal (cadmium selenide, lead sulphide, indium arsenide, or perovskite) between about 2 and 10 nm across — a few thousand atoms. Confinement in all three dimensions means the energy gap, and hence the colour of light it absorbs or emits, is set by the dot's diameter: make it smaller and the gap widens toward the blue, make it larger and it narrows toward the red, following the same particle-in-a-box scaling of roughly 1 over size squared. Dots are grown wet-chemically in flasks (colloidal dots) or built by depositing one semiconductor as small islands on another (self-assembled epitaxial dots).
The structural point for this field is simple: a quantum dot is just a nanocrystal viewed through the lens of confinement. Its power comes from that combination — a real crystalline structure whose size you can dial, and whose optical and electronic behaviour follows the size. That is why dots colour the reds and greens of quantum-dot televisions, tag cells in biological imaging, and serve as single-photon sources; in every case the structure (a size-controlled 0D nanocrystal) is doing the work.
A modern quantum-dot television backlight contains films of tiny CdSe or InP dots. Blue LED light hits the film, the roughly 6 nm dots re-emit pure red and the roughly 3 nm dots re-emit pure green, giving a wider, more saturated colour range than older screens. The whole trick is choosing the dot diameters — a matter of nanostructure, not of new chemistry.
A 0D structure: confined in all three directions, a quantum dot's emission colour is set by its diameter.
Real dots are usually coated with a shell of a wider-gap semiconductor and organic ligands, because their enormous surface-to-volume ratio makes bare surface atoms trap charge and quench the light. The 'artificial atom' picture is a useful analogy, not a literal claim — a dot still contains thousands of real atoms in a real lattice.