2, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
3, Michigan Technological University, Houghton, Michigan, United States
4, University of Wisconsin-Stout, Menomonie, Wisconsin, United States
The ability to precisely engineer well-defined nanoparticles and nanostructures in ever more complex configurations will allow increased functionality for nanoscale devices. We have taken advantage of plasma characteristics and alloy chemistry to control the nucleation and growth behavior during inert gas condensation (IGC) to create complex and unique nanoparticles with a variety of shapes and atomic structures as well as controlled chemical distributions. For example, core/shell nanoparticles can be designed by proper selection of alloy constituents (that is, immiscible components) and processing parameters, which have been demonstrated in a number of systems such as Fe/Ag, Fe/W, Co/Mo, and Co/Zn. We have also discovered, using plasma diagnostics, that two distinct nucleation zones can exist during the formation of nanoparticles. The first nucleation event occurs inside the plasma and fosters a more thermodynamically-controlled growth process, while the second occurs outside the plasma region and the particles grow in a more kinetically-controlled environment. With this phenomena, we can then create a two-step nucleation process and engineer even more complex structures. During the first stage, we can condense one phase or atomic species, and even control its shape, without mixing or co-condensing the second, immiscible species. These nanoparticles then provide heterogeneous nucleation sites for the secondary nucleation stage. If the first nanoparticle is not a sphere, preferential site condensation occurs, leading to, for example, core/frame structures. As an example of this, we have grown Fe(Co)/Ag core/frame structures, where Fe(Co) cubes are first condensed from the Fe-Co-Ag vapor (plasma), and Ag condenses during the second stage, first at corners to form core/corner structures, then along edges to form core/frame structures, and then on faces to form core/shell structures. In this talk, we will more fully describe the processing of these unique nanoparticles, and detail some functional properties of these complex nanoparticles/nanostructures.