Depth profiling by cluster projectiles as seen by computer simulations

Cluster TOF-SIMS imaging as a tool for micrometric histology of lipids in tissue

Recent developments in instrumentation, ion beams or analyzers, for cluster time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging are described here. The methods which are employed to increase the sensitivity or to perform three-dimensional analyses in the organic materials are also illustrated. This review shows the improvements made for lipid imaging by cluster TOF-SIMS in various types of material and applications, and gives reasons for the expansion of its utilization.

Mass spectrometric analysis of the dissociation of argon cluster ions in collision with several kinds of metal

RATIONALE

Collisions of clusters with solids have become important, especially in the fields of thin film growth or surface processing such as etching or topography smoothing. However, it is not clear how much of the theory or model used in macroscopic collisions is appropriate for the consideration of microscopic collisions.

METHODS

We considered a cluster ion consisting of thousands of argon atoms as a continuum and examined the possibility that classical mechanics could analyze its collision with metals. A mass spectrometric analysis of the dissociated ions of argon cluster ions (Ar(+)1500) in collision with five different metals was performed.

RESULTS

In the mass spectra at an incident kinetic energy per atom of less than 10 eV, no monatomic argon ions (Ar(+)) were observed regardless of the prominence of Ar2(+) or Ar3(+). The branching ratio for the ion yield Ar2(+)/∑Arn(+) (n  ≥ 2), representing the dissociation rate, was found to be significantly different for each metal. The relationship between the branching ratio and the impulsive stress caused by the collision of the cluster ion with metal was investigated. The impulsive stress was calculated based on the Young's modulus and density of the clusters and metal, under the assumption that the collision was initially elastic. As a result, the magnitude correlation in the branching ratio corresponded well with that in the impulsive stress.

CONCLUSIONS

This result is important in that it indicates that collision of nano-sized clusters with solids at low energies can be modeled using elastic theory. Furthermore, the result suggests a new method for evaluating a physical property of a material such as its Young's modulus.

Molecular Depth Profiling

Bombardment of molecular solids with polyatomic projectiles allows interrogation of the sample with reduced chemical damage accumulation. Hence, it is now common to perform depth profiling experiments using a variety of substrates in a fashion similar to that reported for inorganic materials ‐ in use for many decades. The possibility for chemical processes, however, creates a number of fundamental differences between organic and inorganic materials. To retain molecular information during beam‐induced erosion, any damage accumulation must be removed at least as rapidly as it is formed. Here we discuss a number of fundamental descriptions associated with molecular depth profiling. These descriptions, which include both analytical models valuable in parameterizing the acquired signals and a molecular dynamics approach important for visualizing the action on a molecular level, point towards experimental conditions that optimize the quality of a depth profile. For example, the size and kinetic energy of the polyatomic projectile, the angle of incidence and the temperature all have significant influence on whether the important molecular ion signals are retained. Atomic force microscopy (AFM) is shown to be an essential technique for quantitative characterization of any molecular profile. Copyright © 2012 John Wiley & Sons, Ltd.