Self-assembly of Ag∕SiOx spherules in triangular pattern on strained surface of primary particles

Abstract

Spontaneous emergence of nanoscaled structures provides a potential alternative to the conventional bottom-up and top-down fabrication techniques. By a two-step thermal evaporation of mixed Ag2O and SiO2 powder onto sapphire substrate, concentric Ag∕SiOx spherules were assembled into a triangular pattern upon cooling on the spherical surface of large particles, which, of generally a few microns in dimension, were primarily grown with the substrate held at 1270K. The occasional presence of pentagon or heptagon defect, as required by the spherical geometry of the primary particle, was also recognized. Strain field in the oxide shell arising from expansion mismatch with the silver core seeds the subsequent nucleation events. The triangular pattern of the secondary nanoparticles can be accounted for with a minimized strain energy in the spherical surface. The self-assembled structure of striking uniformity is promising for the generation of a large family of building blocks for device production.