A gold particle a few nanometers across looks like a tiny faceted diamond, but with five faces instead of the usual ones. Inside it hides a defect, like a crack in glass that can either heal or shatter the whole object. Scientists set out to discover what makes this defect move and disappear.
They built a computer model of such a particle and began altering its shape: making the surface concave like a bowl, then convex like a dome. It turned out that the concave shape helps surface atoms crawl and fill in the defect, restoring symmetry. The convex shape, on the other hand, drives the defect quickly inward, and the particle loses its fivefold structure within billionths of a second.
This discovery is reminiscent of a jeweler who knows where to make a fine cut so that a gem doesn't split but instead becomes stronger. Understanding how a nanoparticle's shape controls its internal defects will allow us to create materials with desired properties — for example, catalysts for chemical reactions or components for microelectronics.
Gold nanoparticles with fivefold symmetry are used in medicine for targeted drug delivery because their shape helps them penetrate cells.