a nanocrystal

Take an ordinary crystal — atoms stacked in a neat repeating pattern, like oranges in a grocer's pyramid — and shrink the whole pyramid down until it is only a few dozen atoms across. That tiny grain, typically between about 1 and 100 nanometres (a nanometre is a millionth of a millimetre, roughly the width of ten atoms side by side), is a nanocrystal. It is still a real crystal inside, with the same neat stacking, but it is so small that a large share of its atoms are sitting right on the outer skin rather than buried safely in the middle.

That last point is the whole story of the nanoscale. In a marble-sized crystal, the surface atoms are a vanishingly small fraction of the total — practically everything is interior. But shrink the crystal and the surface starts to dominate. For a rough estimate, the fraction of atoms within one atomic layer of the surface of a sphere is about 3 times (surface layer thickness) divided by radius; for a 5 nm crystal (radius 2.5 nm, surface layer about 0.3 nm) that is roughly 3 times 0.3 over 2.5, near one third of every atom living on the surface. Surface atoms have missing neighbours, so they are less tightly held and higher in energy, and this simple fact reshapes the whole particle.

Because so many atoms feel the surface, a nanocrystal behaves unlike the same material in bulk: it can melt hundreds of degrees below the normal melting point, its outer atoms can pull inward so the lattice slightly contracts, its surfaces reconstruct into faceted shapes to lower surface energy, and — in a semiconductor — squeezing the electrons into so small a box shifts its colour (quantum confinement). Nanocrystals show up as catalyst particles, as the coloured dots in quantum-dot displays, as pigments, and as the building blocks of nanostructured solids. The single idea to carry away: shrink a crystal far enough and the surface stops being a detail and becomes the main character.