Size characterisation of noble-metal nano-crystals formed in sapphire by ion irradiation and subsequent thermal annealing

► Systematic study on the formation of Ag and Au nano-particles in Al2O3. ► Annealing in a reducing atmosphere, below the metal melting point is more suitable. ► Au nano-particles grow up to 15 nm and Ag nano-particles up to 45 nm in radius. ► Ostwald ripening is the mechanism responsible for the formation of large nanoparticles. ► Optical properties of metallic nano-particles in Al2O3 can be related to their size.