Molecular dynamics simulations of energetic Ar cluster bombardment of Ag(111)

Large-scale molecular dynamics computer simulations are used to investigate the dynamics of material ejection during high-energy Ar_{n} cluster bombardment of Ag(111) at normal incidence. The silver sample containing 7 million atoms is bombarded with Ar_{n} projectiles (n=45-30000) with kinetic energy spanning from a few keV up to 1 MeV. Such a wide range of projectile parameters allows probing processes taking place during low-density collision cascade as well as during high-density events characteristic of micrometeorite bombardment in space. The material modifications and total sputtering yield of ejected particles are investigated. While at low-energy impacts, ejection of individual silver atoms is the main emission channel, the ejection of large clusters from the corona of the created crater dominates for the high-energy impacts

Sputtering of benzene sample by large Ne, Ar and Kr clusters : molecular dynamics computer simulations

Molecular dynamics simulations are employed to probe the role of an impact angle on emission efficiency of organic molecules sputtered from benzene crystal bombarded by 15 keV $Ne_{2953}$, $Ar_{2953}$, and $Kr_{2953}$ clusters. It is found that both the cluster type and the angle of incidence have significant effect on the emission efficiency. The shape of the impact angle dependence does not resemble the dependence characteristic for medium size clusters ($C_{60}, Ar_{366}$), where sputtering yield only moderately increases with the impact angle, has a shallow maximum around 40° and then decreases. On the contrary, for the large projectiles ($Ne_{2953}, Ar_{2953}$, and $Kr_{2953}$) the emission efficiency steeply increases with the impact angle, has a pronounced maximum around 55° followed by rapid signal decay. It has been found that the sputtering yield is the most sensitive to the impact angle change for Kr cluster projectiles, while change of the impact angle of Ne projectile has the smallest effect on the efficiency of material ejection

Partnering analytic models and dynamic secondary ion mass spectrometry simulations to interpret depth profiles due to kiloelectronvolt cluster bombardment

The analytical steady-state statistical sputtering model (SS-SSM) is utilized to interpret molecular dynamics (MD) simulations of depth profiling of Ag solids with keV cluster beams of C60 and Au3 under different incident energy and angle conditions. Specifically, the results of the MD simulations provide the input to the SS-SSM and the result is a depth profile of a delta layer. It has been found that the rms roughness of each system correlates with the total displacement yield, a new quantity introduced in this study that follows naturally from the SS-SSM. The results indicate that the best depth profiles occur when the displacement yield is low and the sputtering yield is high. Moreover, it is determined that the expected value of the delta layer position as calculated from a depth profile rather than the peak position in the depth profile is the best indicator of the actual delta layer position

Computed molecular depth profile for C60C_{60} bombardment of a molecular solid

Molecular dynamics (MD) simulations have been performed for 10 keV $C_{60}$ bombardment of an octane molecular solid at normal incidence. The results are analyzed using the steady-state statistical sputtering model (SS-SSM) to understand the nature of molecular motions and to predict a depth profile of a $\delta$-layer. The octane system has sputtering yield of $\sim$ 150 $nm^{3}$ of which 85% is in intact molecules and 15% is fragmented species. The main displacement mechanism is along the crater edge. Displacements between layers beneath the impact point are difficult because the nonspherically shaped octane molecule needs a relatively large volume to move into and the molecule needs to be aligned properly for the displacement. Since interlayer mixing is difficult, the predicted depth profile is dominated by the rms roughness and the large information depth because of the large sputtering yield

Seduction of finding universality in sputtering yields due to cluster bombardment of solids

ConspectusUniversal 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 <i>Y</i> and initial projectile cluster kinetic energy <i>E</i> are expressed in quantities scaled by the number of cluster atoms <i>n</i>, that is, <i>Y</i>/<i>n</i> versus <i>E</i>/<i>n</i>. 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 Ar_<i>n</i> 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 <i>U</i>₀ is included and the dependence is presented as <i>Y</i>/(<i>E</i>/<i>U</i>₀) versus (<i>E</i>/<i>U</i>₀)/<i>n</i>. As a material property, the quantity <i>U</i>₀ is defined per the basic unit of material, which is an atom for atomic solids and a molecule for molecular solids. Analogously, the quantity <i>Y</i> is given in atoms and molecules, respectively. The simulations show that, for almost 3 orders of magnitude variation of (<i>E</i>/<i>U</i>₀)/<i>n</i>, there are obvious similarities in the ejection mechanisms between the molecular and atomic solids, thus supporting the concept of universality. For large (<i>E</i>/<i>U</i>₀)/<i>n</i> values, the mechanism of ejection is the fluid flow from a cone-shaped volume. This regime of (<i>E</i>/<i>U</i>₀)/<i>n</i> 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 <i>E</i> is consumed by internal excitation and molecular fragmentation, which are energy loss channels not present in atomic solids. For small (<i>E</i>/<i>U</i>₀)/<i>n</i> values, the cluster deforms the surface and the ejection occurs from a ring-shaped ridge of the forming crater rim. This regime of (<i>E</i>/<i>U</i>₀)/<i>n</i> 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 <i>U</i>₀ as the only material property cannot be completely universal, because there are other material properties which influence the sputtering efficiency. Furthermore, neither the <i>Y</i>/<i>n</i> nor <i>Y</i>/(<i>E</i>/<i>U</i>₀) representation includes the energy loss physics associated with molecular fragmentation in the high (<i>E</i>/<i>U</i>₀)/<i>n</i> regime. The analysis of the universal concept implies for practical applications that if the objective of the experiment is large material removal, then the high energy per cluster atom regime is applicable. If the objective is little or no molecular fragmentation in organic materials, then the low energy per atom regime is appropriate

Steady-state statistical sputtering model for extracting depth profiles from molecular dynamics simulations of dynamic SIMS

Recently a "divide and conquer" approach was developed to model by molecular dynamics (MD) simulations dynamic secondary ion mass spectrometry (SIMS) experiments in order to understand the important factors for depth profiling. Although root-mean-square (rms) roughness can be directly calculated from the simulations, calculating depth profiles is beyond the current capability of the MD simulations. The statistical sputtering model (SSM) of Krantzman and Wucher establishes the foundation for connecting information from the MD simulations to depth profiles. In this study, we revise the SSM to incorporate more extensive information from the MD simulations in the steady-state region, thus presenting the steady-state statistical sputtering model (SS-SSM). The revised model is utilized to interpret MD simulations of 20 keV C60 bombardment of Ag at normal incidence as well as the effect of sample rotation on depth profiling