Metal catalysts are used to promote axial growth of nanowires via a vapor-liquid-solid mechanism.  For the growth of Si nanowires, gold particles are used to catalyze SiH₄ decomposition resulting in the formation of a Au-Si liquid alloy particle.  When the liquid droplet becomes supersaturated with Si, nanocrystalline Si precipitates and the liquid droplet continues to feed the growth of the Si nanowire in the axial direction.  The nanowire diameter is therefore controlled by the initial metal particle size.