High-resolution electron microscopy studies of metal nanoparticles: shape and twin defects, and surface stress effects

Potential applications of metal nanoparticles, because of specific physical or chemical properties, are in close relationship to their structural peculiarities. Structural characterisation of nanoparticles in various configurations, using high resolution electron microscopy (HREM), may reveal detailed information on particle shape and planar lattice defects. The variations observed for most noble metals comprise single crystalline particles of cuboctahedral shape, particles containing single twin faults, multiply twinned particles containing parallel twin lamellae, and particles composed of cyclic twinned segments arranged around axes of fivefold symmetry. Nanoparticles with free surface exhibit, because of their large surface-tovolume ratio, a size-dependent lattice contraction due to the surface stress. For spherical particles, the lattice contraction is a function of their radius. Thus, it can be calculated from the lattice spacings determined by HREM. In the case of strong interaction with a surrounding matrix, underlying support, or oxide coating, additional misfit stresses occur which contribute to an effective interface stress causing a larger lattice contraction, or even lattice dilatation. In the latter case the interface stress may take on negative values, which limits the curvature of the interface and for very small particles favours a selflimitation of their oxidation.