Computed Molecular Depth Profile for C60 Bombardment of a Molecular solid

Depth profiling cross-linked poly(methyl methacrylate) films: a time-of-flight secondary ion mass spectrometry approach

RATIONALE

In order to determine the degree of cross-linking on the surface and its variations in a nanometer-scale depth of organic materials, we developed an approach based on time-of-flight secondary ion mass spectrometry (TOF-SIMS), which provides rich chemical information in the form of fragment ions. TOF-SIMS is extremely surface-sensitive and capable of depth profiling with the use of a sputter ion beam to remove controllable amounts of substance.

METHODS

Poly(methyl methacrylate) (PMMA) films spin-coated on a Si substrate were cross-linked using a recently developed, surface sensitive, hyperthermal hydrogen projectile bombardment technique. The ion intensity ratio between two ubiquitous hydrocarbon ions, C₆ H^- and C₄ H^- , detected in TOF-SIMS, denoted as ρ, was used to assess the degree of cross-linking of the PMMA films. The cross-linking depth of the PMMA films was revealed by depth profiling ρ into the polymer films using a C₆₀⁺ sputter beam.

RESULTS

The control PMMA film spin-coated on a Si substrate was characterized by ρ = 32% on its surface when using a 25 keV Bi₃⁺ primary ion beam. This parameter on the PMMA films subjected to HHIC treatment for 10, 100 and 500 s increased to 45%, 56% and 65%, respectively. The depth profiles of ρ obtained using a 10 keV C₆₀⁺ ion beam resembled an exponential decay, from which the cross-linking depth was estimated to be 3, 15 and 39 nm, respectively, for the three cross-linked PMMA films.

CONCLUSIONS

We demonstrated that the ion intensity ratio of C₆ H^- to C₄ H^- detected in TOF-SIMS provides a unique and simple means to assess the degree of cross-linking of the surface of PMMA films cross-linked by the surface sensitive hyperthermal hydrogen projectile bombardment technique. With a C₆₀⁺ sputter beam, we were able to depth profile the PMMA films and determine cross-linking depths of the cross-linked polymer films at nanometer resolutions. Copyright © 2017 John Wiley & Sons, Ltd.

Cluster secondary ion mass spectrometry imaging of interfacial reactions of TiO2 microspheres embedded in ionic liquids

RATIONALE

Our goal is to develop protocols for the elucidation of the identity and structure of reaction products embedded in a reaction medium. Results should find significance in a variety of disciplines ranging from the study of biological cells and tissues, to the steps associated with the functionalization of nanoparticles.

METHODS

We utilize cluster secondary ion mass spectrometry (cluster-SIMS) to acquire three-dimensional (3D) information about 5-30 µm TiO2 microspheres imbedded into an ionic liquid. The method allows molecular depth profiling with submicron spatial resolution and depth profiling with a resolution of several tens of nanometers. The ionic liquid matrix enshrouds the spheres, allowing them to be introduced into the vacuum environment of the mass spectrometer.

RESULTS

The results provide 3D chemical information about these microspheres as they are synthesized by interfacial sol-gel reactions. We show that with 40 keV C60 (+) , it is possible to erode through the reaction medium and map the distribution of those embedded TiO2 microspheres. Moreover, we demonstrate that it is possible to monitor surface modification of the particles and, via ion beam drilling, elucidate their internal structure.

CONCLUSIONS

Using cluster-SIMS imaging, we are able to elucidate the identity and structure of reaction products embedded in a reaction medium, a problem of long-standing interest for materials characterization. With this strategy, we have provided a new approach that may be especially useful for the characterization of biological tissue and cells within the vacuum confines of the mass spectrometer. Copyright © 2015 John Wiley & Sons, Ltd.

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

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 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.

On Universality in Sputtering Yields Due to Cluster Bombardment

Molecular dynamics simulations, in which atomic and molecular solids are bombarded by Arn (n = 60-2953) clusters, are used to explain the physics that underlie the "universal relation" of the sputtering yield Y per cluster atom versus incident energy E per cluster atom (Y/n vs E/n). We show that a better representation to unify the results is Y/(E/U0) versus (E/U0)/n, where U0 is the sample cohesive energy per atom or molecular equivalent, and the yield Y is given in the units of atoms or molecular equivalents for atomistic and molecular solids, respectively. In addition, we identified a synergistic cluster effect. Specifically, for a given (E/U0)/n value, larger clusters produce larger yields than the yields that are only proportional to the cluster size n or equivalently to the scaled energy E/U0. This synergistic effect can be described in the high (E/U0)/n regime as scaling of Y with (E/U0)(α), where α > 1.