Inversion of surface composition and evolution of nanostructure in Cu Co nanocrystals

Nanocrystals of Cu/Co with a nearly pure Cu core and a Co-rich shell have b een chemically synthesized. This structure results from reaction kinetics a nd represents an inversion of surface composition since the surface energy of Co is larger than that of Cu. Both the Cu core and the Co-rich shell ini tially have an fcc structure and a common lattice constant. Annealing at te mperatures in the range of 300 to 600 degrees C causes the material to phas e separate, forming small, increasingly pure Co nanocrystals which aggregat e on the surface of the pure Cu nanocrystals. (C) 1999 American Institute o f Physics. [S0003-6951(99)02621-2].