Shock-wave production of nanoparticles during high-energy ion sputtering

Several previous studies have shown that the size distributions of smaller nanoparticles (n≤40 where n is the number of atoms in a given cluster) generated by ion sputtering obey an inverse power law, with an exponent varying between -8 and -4, dependent upon the total sputtering yield. Such large negative exponents have not been explained by any simple physical mechanism. We reported electron microscopy studies of the size distributions of the larger nanoparticles (n > 500) that are sputtered from the surface by high-energy ion impacts. These measurements also yielded an inverse power law, but one with an exponent of -2, and one that is independent of total sputtering yield. This inverse-square dependence indicates that the clusters are produced when shock waves, generated by sub-surface displacement cascades, impact and ablate the surface. Many smaller clusters can result from fragmentation of these larger ones, which helps explain the large negative exponents that have been reported previously. In this paper, we briefly review the previous results. In addition, we present new results demonstrating that the same inverse-square size distribution is generated in both transmission and reflection sputtering geometries. An important corollary from these results is that the sputtered nanoparticles consist of simple fragments of the original surface, that is particles which have not experienced any large thermal excursions. Hence high-energy ion sputtering should provide a convenient method for synthesizing a broad distribution of nanoparticles of a wide variety of alloy phases.