CONTACT EPITAXY OBSERVED IN SUPPORTED NANOPARTICLES

We have observed the formation of heteroepitaxial interfacial layers b etween silver nanoparticles and a single crystal copper surface by a p henomenon we term ''contact epitaxy.'' Upon depositing Ag nanoparticle s (5-20 nm diameter) onto clean (001) Cu in an ultrahigh vacuum in sit u transmission electron microscope, a thin (111)-oriented layer of Ag was detected at the interface between the substrate and particles. Mol ecular dynamics simulations reveal that the epitaxial layers form with in picoseconds of impact, with rapid alignment arising from mechanical relaxation of the highly stressed interface formed upon initial conta ct. The simulations also show that multiple grains form in the nanopar ticle as a consequence of this relaxation process. The unique structur e of the nanoparticles, induced by contact epitaxy, is expected to sig nificantly influence physical properties such as interfacial bonding, diffusion, chemical activity, and electrical transport, as well as for ming a nucleus for grain growth and epitaxy which we also observe. Due to its simple origin, the phenomenon should also apply to materials s ystems beyond the field of nanoparticles with implications for cluster deposition, adhesion, rheology, and catalysis. (C) 1998 American Inst itute of Physics. [S0003-6951(98)01648-9].