Nitric oxide assisted C60 secondary ion mass spectrometry for molecular depth profiling of polyelectrolyte multilayers

Cluster-Induced Desorption/Ionization of Polystyrene: Desorption Mechanism and Effect of Polymer Chain Length on Desorption Probability

Soft cluster-induced desorption/ionization of polystyrene oligomers was investigated with respect to application in mass spectrometry. Clear peak progressions corresponding to intact polystyrene molecules were observed in the mass spectra, and no fragmentation was detected; efficient desorption was deduced from quartz crystal microbalance measurements. Molecular dynamics (MD) simulations of the process revealed that even in the case of the nonpolar polystyrene molecules cluster-induced desorption proceeds via dissolvation in the polar clusters. Experimentally, a significantly lower desorption efficiency was observed for polystyrene molecules with larger chain length. Taking into account MD simulations and further experiments with mixed samples consisting of long- and short-chain polystyrene oligomers, the reduced desorption efficiency for longer chain polystyrene molecules was attributed to a stronger entanglement of the larger polystyrene molecules.

Revealing Contamination and Sequence of Overlapping Fingerprints by Unsupervised Treatment of a Hyperspectral Secondary Ion Mass Spectrometry Dataset

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been successfully applied for chemical imaging of overlapping fingermarks. The resulting big dataset has been treated by means of an unsupervised machine learning approach based on uniform manifold approximation and projection. The hyperspectral matrix was composed of 49 million pixels associated with 518 peaks. However, the single-pixel spectrum results in a very poor signal intensity, mostly like a barcode. Contrary to what has been reported in the literature recently, we have not applied a crude approach based on binning but a sophisticated machine learning method capable of separating the chemical signals of the two fingerprints from each other and from the substrate in which they were impressed. Moreover, using ToF-SIMS, an extremely surface-sensitive technique, the sequence of deposition of the fingerprints has been determined.

ToF-SIMS Depth Profiling of PS-b-PMMA Block Copolymers Using Arn+, C60++, and Cs+ Sputtering Ions

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a high performance tool for molecular depth profiling of polymer films, in particular when they are structured in microphases. However, a major issue is the degradation of polymer materials under ion irradiation in reactions such as cross-linking, chain breaking, or reorganization processes of polymers which have been demonstrated for materials such as polystyrene (PS) and poly(methyl methacrylate) (PMMA). This work aims at comparing ToF-SIMS molecular depth profiling of structured polymers (polystyrene (PS)-b-polymethyl methacrylate (PMMA) block copolymers (BCP)) using either ultralow energy cesium or the more recently introduced C₆₀⁺⁺ (under NO dosing and with sample cooling) and argon cluster ion beams (using Ar₁₅₀₀⁺ ions at 5 keV). The latter improved the quality of the depth profiles, especially the argon cluster ion beam, as it is characterized by a greater homogeneity for the sputter yields of PS and PMMA. No significant artifacts were observed, and this was confirmed by the comparison of depth profiles obtained from films with variable thickness, annealing time, and morphology (cylindrical blocks vs spherical blocks). Comparison to a theoretical model (hexagonal centered pattern) ensured that the ToF-SIMS depth profiles described the real morphology and may thus be a relevant characterization tool to verify the morphology of the films as a function of the deposition parameters.

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.