In 1998, Penn and Banfield discovered an important crystal growth mechanism named “oriented attachment”.18,44 The mechanism involves spontaneous self-organization of adjacent particles, as grown by addition of solid particles to surface, so that they share a common crystallographic orientation, followed by joining of these particles at a planar interface. Crystal growth via the OA mechanism often leads to the formation of nanoparticles with irregular shapes by consuming primary nanocrystals as “building blocks”, and the growth rate is size-dependent.