We obtained the two‐ and three‐component nanostructures by means of ion implantation of low‐energy Co and Ba ions in the surface layers of Si and GaAs in combination with the post‐implantation annealing. We show that flat shaped nanocrystals (nanoislands) of two‐ and three‐component composition, Co‐Si and Ga‐Ba‐As, start forming at ion fluences of Φ = (6¸8)×1014 cm‐2. After the laser annealing and short time electron heating we observed the formation of CoSi2 and Ga0.4Ba0.6As nanocrystals. Our results show that the size effects are clearly seen in electronic properties (opening of the band gap) when the lateral size of nanocrystals is less than 15‐30 nm, or in case of solid nanofilms, similar effect is observed at the thickness less than 3‐4 nm. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)