Molecular dynamics simulations of sputtering of Langmuir-Blodgett multilayers by keV C(60) projectiles

Mass Spectrometry of Nanoparticles is Different

Secondary ion mass spectrometry, SIMS, is a method of choice for the characterization of nanoparticles, NPs. For NPs with large surface-to-volume ratios, heterogeneity is a concern. Assays should thus be on individual nano-objects rather than an ensemble of NPs; however, this may be difficult or impossible. This limitation can be side-stepped by probing a large number of dispersed NPs one-by-one and recording the emission from each NP separately. A large collection of NPs will likely contain subsets of like-NPs. The experimental approach is to disperse the NPs and hit an individual NP with a single massive cluster (e.g., C-60, Au-400). At impact energies of ~1 keV/atom, they generate notable secondary ion (SI) emission. Examination of small NPs (≤20 nm in diameter) shows that the SI emission is size-dependent and impacts are not all equivalent. Accurate identification of the type of impact is key for qualitative assays of core or outer shell composition. For quantitative assays, the concept of effective impacts is introduced. Selection of co-emitted ejecta combined with rejection (anticoincidence) of substrate ions allows refining chemical information within the projectile interaction volume. Last, to maximize the SI signal, small NPs (≤5 nm in diameter) can be examined in the transmission mode where the SI yields are enhanced ~10-fold over those in the (conventional) reflection direction. Future endeavors should focus on schemes acquiring SIs, electrons, and photons concurrently.

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.

An experimental and theoretical view of energetic C60 cluster bombardment onto molecular solids

Recent experimental measurements and calculations performed by molecular dynamics computer simulations indicate, for highly energetic C60 primary ions bombarding molecular solids, the emission of intact molecules is unique. An energy- and angle-resolved neutral mass spectrometer coupled with laser photoionization techniques was used to measure the polar angle distribution of neutral benzo[a]pyrene molecules desorbed by 20-keV [Formula: see text] primary ions and observed to peak at off-normal angles integrated over all possible emission energies. Similarly, computer simulations of 20-keV C60 projectiles bombarding a coarse-grained benzene system resulted in calculations of nearly identical polar angle distributions. Upon resolving the measured and calculated polar angle distributions, sputtered molecules with high kinetic energies are the primary contributors to the off-normal peak. Molecules with low kinetic energies were measured and calculated to desorb broadly peaked about the surface normal. The computer simulations suggest the fast deposition of energy from the C60 impact promotes the molecular emission by fluid-flow and effusive-type motions. The signature of off-normal emission angles is unique for molecules because fragmentation processes remove molecules that would otherwise eject near normal to the surface. Experimental measurements from a Ni {001} single crystal bombarded by 20-keV [Formula: see text] demonstrate the absence of this unique signature.

Fluid Flow and Effusive Desorption: Dominant Mechanisms of Energy Dissipation after Energetic Cluster Bombardment of Molecular Solids

The angular distribution of intact organic molecules desorbed by energetic C(60) primary ions was probed both experimentally and with molecular dynamics computer simulations. For benzo[a]pyrene, the angular distribution of intact molecules is observed to peak at off-normal angles. Molecular dynamics computer simulations on a similar system show the mechanism of desorption involves fast deposition of energy followed by fluid-flow and effusive-type emission of intact molecules. The off-normal peak in the angular distribution is shown to arise from emission of intact molecules from the rim of a crater formed during the cluster impact. This signature is unique for molecules because fragmentation processes remove molecules that would otherwise eject at directions near-normal to the surface.

Molecular depth profiling of buried lipid bilayers using C(60)-secondary ion mass spectrometry

An organic delta layer system made of alternating Langmuir-Blodgett multilayers of barium arachidate (AA) and barium dimyristoyl phosphatidate (DMPA) was constructed to elucidate the factors that control depth resolution in molecular depth profile experiments. More specifically, one or several bilayers of DMPA (4.4 nm) were embedded in relatively thick (51 to 105 nm) multilayer stacks of AA, resulting in a well-defined delta layer model system closely resembling a biological membrane. 3-D imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile analysis was performed on this system using a focused buckminsterfullerene (C(60)) cluster ion beam. The delta layer depth response function measured in these experiments exhibits similar features as those determined in inorganic depth profiling, namely an asymmetric shape with quasi-exponential leading and trailing edges and a central Gaussian peak. The effects of sample temperature, primary ion kinetic energy, and incident angle on the depth resolution were investigated. While the information depth of the acquired SIMS spectra was found to be temperature independent, the depth resolution was found to be significantly improved at low temperature. Ion induced mixing is proposed to be largely responsible for the broadening, rather than topography, as determined by atomic force microscopy (AFM); therefore, depth resolution can be optimized using lower kinetic energy, glancing angle, and liquid nitrogen temperature.

Biological cluster mass spectrometry

This article reviews the new physics and new applications of secondary ion mass spectrometry using cluster ion probes. These probes, particularly C(60), exhibit enhanced molecular desorption with improved sensitivity owing to the unique nature of the energy-deposition process. In addition, these projectiles are capable of eroding molecular solids while retaining the molecular specificity of mass spectrometry. When the beams are microfocused to a spot on the sample, bioimaging experiments in two and three dimensions are feasible. We describe emerging theoretical models that allow the energy-deposition process to be understood on an atomic and molecular basis. Moreover, experiments on model systems are described that allow protocols for imaging on biological materials to be implemented. Finally, we present recent applications of imaging to biological tissue and single cells to illustrate the future directions of this methodology.