Shock-wave synthesis of nanoparticles during ion sputtering

We report electron microscopy studies of nanoparticles (500 ≤ n ≤ 10⁴, where n is the number of atoms in a given cluster) that are sputtered from the surface by high-energy ion impacts. Measurements of the sizes of these clusters yielded an inverse power-law distribution with an exponent of -2 that is independent of irradiating ion species and total sputtering yield. This inverse-square dependence indicates that these nanoclusters are produced when shock waves, generated by sub-surface displacement cascades, impact and ablate the surface. Such nanoparticles consist of simple fragments of the original surface, i.e., ones that have not undergone any large thermal excursion. As discussed below, this "ion ablation" technique should therefore be useful for synthesizing nanoparticles of a wide variety of alloy compositions and phases.