From New Scientist:
Kevin Stamplecoskie and Juan Scaiano at the University of Ottawa in Ontario, Canada, can grow silver particles shaped as hexagons, rods, triangles, spheres or dodecahedrons by shining green, red, orange, violet and blue light on the liquid respectively.
The particles are called into being by a burst of ultraviolet light, which causes tiny silver “seeds”, each 3 nanometres across, to precipitate out of the solution. Switching to coloured LEDs of a specific frequency for around 24 hours makes the seeds grow into nanoparticles of a desired shape, each some 50 to 200 nanometres across.
The trick works because the coloured light induces an electromagnetic field around the silver seeds that makes them stick to their nearest neighbours. “The light causes the formation of particles that absorb at [the] wavelength [of the light], and the process continues until all the particles share that absorption,” Stamplecoskie says.