Plugging into III–Vs: Crystalline, sub-100 nm gold nanoparticles are formed via simple galvanic displacement reactions on InP and GaAs surfaces (see images). The reactions require only four ingredients (gold salt, semiconductor, water, and dilute acid) and result in firmly bound gold nanoparticle arrays on the surface. The chemistry can be patterned via AFM-tip-mediated scribing of the sub-5-nm-thick surface oxide to reveal the underlying semiconductor, leading to galvanic displacement only in the exposed areas.