Shock-Wave Synthesis of Nanoparticles During Ion Sputtering
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Shock-Wave Synthesis of Nanoparticles during Ion Sputtering L. E. Rehn, R. C. Birtcher, S. E. Donnelly1, P. M. Baldo and L. Funk, Materials Science Division, Argonne National Laboratory Argonne, IL 60439, U.S.A. 1 Joule Physics Laboratory University of Salford, Salford M5 4WT, U.K.
ABSTRACT 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.
INTRODUCTION The emission of intact clusters of atoms during ion sputtering was first reported more than 40 years ago [1]. This observation elicited considerable surprise [2 - 5] because the energies involved in the ion-target collisions that generate sputtered atoms are typically much larger, ranging up to many keV, than the 1-2 eV typical binding energies holding the clusters together. We recently reported [6] transmission electron microscopy (TEM) studies of the sizedistributions of the larger clusters ( >500) that are sputtered from the surface by high-energy ion impacts. These new measurements yielded a power-law size distribution with an exponent of – 2, independent of ion species and total sputtering yield. Such an inverse-square dependence revealed that the clusters are produced when shock waves, generated by sub-surface displacement cascades, impact and ablate the surface, as originally proposed in the model of Bitensky and Parilis [7]. Such nanoparticles consist of simple fragments of the original surface, i.e., ones that have not undergone any significant thermal excursion. As discussed in greater detail below, this ion-ablation technique should therefore be useful for synthesizing nanoparticles of a wide variety of alloy compositions and phases.
EXPERIMENTAL DETAILS The experiments were performed as follows. Au films, 50 ± 5nm thick with a surface normal, were deposited by e-beam evaporation of 99.999 at.% Au onto NaCl substrates held at a temperature of 350 oC. Pieces of the evaporated Au films were floated off the NaCl in a W7.6.1
water/alcohol mixture and sifted onto Cu TEM grids. The ion sputtering experiments were performed in the Hitachi H-9000 electron microscope operating at 300 kV, located in the IVEMTandem facility at Argonne National Laboratory [8]. The ion beam was oriented 30o from the microscope axis, and the specimen was tilted 15 o toward the ion beam so that both ions an
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