Seduction of Finding Universality in Sputtering Yields Due to Cluster Bombardment of Solids

Abstract

Universal descriptions are appealing because they simplify the description of different (but similar) physical systems, allow the determination of general properties, and have practical applications. Recently, the concept of universality has been applied to the dependence of the sputtering (ejection) yield due to energetic cluster bombardment versus the energy of the incident cluster. It was observed that the spread in data points can be reduced if the yield Y and initial projectile cluster kinetic energy E are expressed in quantities scaled by the number of cluster atoms n, that is, Y/n versus E/n. The convergence of the data points is, however, not perfect, especially when the results for molecular and atomic solids are compared. In addition, the physics underlying the apparent universal dependence in not fully understood. For the study presented in this Account, we performed molecular dynamics simulations of Arn cluster bombardment of molecular (benzene, octane, and β-carotene) and atomic (Ag) solids in order to address the physical basis of the apparent universal dependence. We have demonstrated that the convergence of the data points between molecular and atomic solids can be improved if the binding energy of the solid U0 is included and the dependence is presented as Y/(E/U0) versus (E/U0)/n. As a material property, the quantity U0 is defined per the basic unit of material, which is an atom for atomic solids and a molecule for molecular solids. Analogously, the quantity Y is given in atoms and molecules, respectively. The simulations show that, for almost 3 orders of magnitude variation of (E/U0)/n, there are obvious similarities in the ejection mechanisms between the molecular and atomic solids, thus supporting the concept of universality. For large (E/U0)/n values, the mechanism of ejection is the fluid flow from a cone-shaped volume. This regime of (E/U0)/n is generally accessed experimentally by clusters with hundreds of atoms and results in the largest yields. For molecular systems, a large fraction of the total energy E is consumed by internal excitation and molecular fragmentation, which are energy loss channels not present in atomic solids. For small (E/U0)/n values, the cluster deforms the surface and the ejection occurs from a ring-shaped ridge of the forming crater rim. This regime of (E/U0)/n is generally accessed experimentally by clusters with thousands of atoms and results in the smallest yields. For the molecular systems, there is little or no molecular fragmentation. The simulations indicate, however, that the representation which includes U0 as the only material property cannot be completely universal, because there are other material properties which influence the sputtering efficiency. Furthermore, neither the Y/n nor Y/(E/U0) representation includes the energy loss physics associated with molecular fragmentation in the high (E/U0)/n regime. The analysis of the universal concept implies for practical applications that if the objective of the experiment is large material removal, then the...

Nitric oxide assisted C60secondary ion mass spectrometry for molecular depth profiling of polyelectrolyte multilayers

“…Nowadays, gas cluster ion beams (GCIB) are applicable to molecular depth profiling of a large variety of organic and polymer materials but, under the experimental conditions suitable for such purpose (typically quite large cluster ions, with thousands atoms per cluster), they appear rather inefficient in sputtering inorganic materials . For example, considering a typical case of an Ar n cluster with 10 eV/atom (such as 20 keV Ar 2000 ) the sputtering yield of inorganics (Si, Au, SiO 2 ) is reported to be about 2 orders of magnitude lower than that for organic targets (PMMA, PS, Irganox 1010) . Actually, the maximum sputtering yield of inorganics with Ar n (energy of some 10 keV) is reported in a size range of 10 < n < 200 (i.e.…”

Physical basis of energy per cluster atom in the universal concept of sputtering

“…The second change was needed in order to take into account the fact that the binding energies U 0 , for atomic and molecular solids, can differ several times; thus, their effect on the sputtering efficiency cannot be neglected. 14,17 The results of MD simulations showed that there are obvious similarities in the sputtering mechanisms between the atomic and molecular solids for almost 3 orders of magnitude variation of (E/U 0 )/n, therefore supporting the statement of universality. 14 The conclusion from the analysis of the universal relation with regard to the physical meaning of the ordinate variable Y/n is that, for a given E/n value, the dependence of the sputtering yield Y on the cluster size n is equivalent to the dependence of Y on the incident cluster energy E. This interpretation makes more physical sense because the cluster energy E is the primary factor inducing ejection rather than the cluster size n.…”

“…Due to high energy per cluster atom, individual cluster atoms penetrate into the bulk of the solid. 14 The penetration depth exceeds 2 nm. At the lowest E/n, the picture does not change much.…”

“…The cluster atoms keep together within the basin of deformed surface 14 and the penetration depth remains approximately 1 nm. The energy per cluster atom is not sufficient for the cluster atoms to penetrate into the solid.…”

“…The model samples of Ag(111) and molecular solids (benzene, octane, and b-carotene) were sputtered by Ar n clusters. [12][13][14] We demonstrated that a better representation to improve the convergence between the data points for atomic and molecular solids is Y/(E/U 0 ) vs (E/U 0 )/n, where U 0 is the binding energy of the solid. U 0 is defined per atom and per molecule for atomic and molecular solids, respectively.…”