SIMS2015 Session PM1-WeM: Polymers and Molecular Films

In the surface treatment of polymer-based materials by ambient “open air” plasma, the intervention of oxygen, nitrogen and water is inevitable. Often, the presence of water vapor in the plasma is considered as problematic because water is known to destabilize it. However, its presence induces the grafting of -OH groups onto the polymer surfaces, representing a promising functionalization route for industrial and medical applications. Hence, the aim of the present study is to trace the deuterated water reactivity with low density polyethylene (LDPE) films, modified by an atmospheric Ar-D₂O post-discharge, using static and dynamic time-of-flight secondary ion mass spectrometry (ToF-SIMS). Indeed, ToF-SIMS is an appropriate tool for the investigation of structural and chemical modifications of plasma-treated polymers both on the surface and along the depth, the latter being made possible by the recent application of argon gas cluster ion beams (GCIB) as sputter sources.[1]

In the present contribution, static-SIMS combined with PCA (principal component analysis) has been performed to investigate the chemical and structural modifications of the treated polyethylene, in terms of H-D exchange, oxygen/nitrogen-uptake, unsaturation, branching and cross-linking, as a function of two external plasma parameters: treatment time (30 seconds, 1 and 5 minutes) and sample-torch distance (3, 5, 7 and 10 mm). The SIMS characterization points out that the most important polymer modifications occur with decreasing treatment time going from 3 mm to 7 mm. The inversion of the time influence is observed for 10 mm. Furthermore, as a function of the sample-torch distance, the most pronounced -OD grafting on the polymer surface occurs at 5 mm. The surface functionalization might be explained on the basis of the lifetime of the active species and the etching process induced by the energetic particles present in the plasma medium. Finally, the plasma-induced modifications of the LDPE film have been studied by means of ultra-shallow depth-profiling,[2] demonstrating their dependence on the external plasma parameters along the third dimension. This SIMS approach for the plasma-treated polymer characterization provides important insights in order to tune the functionalization, simultaneously in the surface and along the depth, based on the desired applications of the investigated material.

[1] A. Delcorte, V. Cristaudo, M. Zarshenas, D. Merche, F. Reniers, P. Bertrand, Plasma Processes and Polymers2015, in press.

[2] V. Cristaudo, S. Collette, C. Poleunis, F. Reniers, A. Delcorte, Plasma Processes and Polymers2015, in press.

Organic thin films are anticipated to be widely used in applications such as gas sensors, insulating layers, optical devices or organic electronics. Depending on the application, these molecular materials present severe constraints in terms of composition. Time of Flight (ToF-SIMS) is one of the most powerful analytical techniques for the molecular characterization of these kinds of materials.

In this work poly(neopentyl methacrylate) p(npMa) thin films are studied as active adsorbing nano-layers for gas sensor applications where their abundance in methyl groups allows the absorption of a wide family of organic species. They are deposited by a promising new technique called initiated Chemical Vapor Deposition (iCVD) consisting in the injection of an initiator and monomers in the vacuum reaction chamber. The initiator, thermally or UV activated, reacts with the monomers starting the radical chain polymerization directly on the substrate.

Size-exclusion chromatography techniques are insufficient to characterize thin polymer films finely, notably to describe variations of molecular weight as a function of deposition time. The potential of ToF-SIMS to characterize and understand polymerization mechanisms of these new materials was demonstrated[1]. However, in order to obtain chemical information representative of the bulk of the polymeric chains, a pre-sputtering of the sample surface is crucial to avoid surface contamination. However, for very thin layers a too aggressive pre-sputtering can bring an undesired contribution from the substrate when the analysis is performed. Appropriate pre-sputtering conditions for the characterization of p(npMa) thin films were developed in the course of this study in order to minimize the artefacts in the spectra obtained.

Pre-sputtering with a gas cluster ion beam (GCIB) mounted on a ToF-SIMS 5 (ION-TOF) was used on 15 to 70 nm thick p(npMa) films. Clusters of Ar₂₅₀₀ at 5keV were tested at different current densities to determine the best conditions in terms of sample degradation and sputtered volume. Spectra were collected (Bi³⁺ primary ions at 15keV at increasing sputter ion doses and differences in chemical information collected are inspected, with the help of multivariate analysis.

Thanks to these results the evolution of molecular weight as a function of deposition time has been studied over a wider range of thicknesses.

References

[1] V. Cristaudo, C. Poleunis, B. Czerwinski, A. Delcorte, Surface and Interface Analysis 2014, 46, 79.

Acknowledgements

This work was supported by the French "Recherches Technologiques de Base" Program and was performed on the Nano Characterization Platform (PFNC) of the CEA Grenoble.

Negative hydrocarbon ion species of C_2nH¯ are ubiquitous in time-of-flight secondary ion mass spectrometry (ToF-SIMS) for any hydrocarbon-containing materials and hydrocarbon-contaminated substrates. Their abundance decreases with increased numbers of carbon atoms, with C₂H¯ being the most abundant. In an effort to assess the degree of crosslinking of thin polymeric films crosslinked using a surface-sensitive technique coined hyperthermal hydrogen projectile bombardment, we found that C₆H¯, as well as C₈H¯ and C₁₀H¯, serve to gauge the degree of crosslingking. Moreover, we found that the ratio of their intensities to that of C₄H¯ serves as a criterion for quantifying the degree of crosslinking of polymer films.¹

Inspired by this finding, we have investigated those ion species for common polymers of polyethylene (PE), polypropylene (PP), polymethylmethacryate (PMMA) and polystyrene (PS), whose monomers are C₂H₄, C₃H₆, C₅H₈O₂ and C₈H₈, respectively, as a function of their carbon "density" (i.e., how many carbon atoms are linked to a carbon atom). Among those four polymers, PE and PS have the lowest and highest carbon “density”, respectively. The intensity ratio ρ=C₆H¯/C₄H¯ for PE, PP, PMMA and PS were estimated to be 19.4±0.1%, 22.4±0.2%, 33.6±0.3% and 55.5±0.3%, respectively. From our experimental results we can draw two important conclusions about ρ : (1) each polymer has a specific ρ and (2) ρ increases with increased carbon “density”.

C₄H¯ and C₆H¯ detected in ToF-SIMS do not originate from a polymer film (because there are no such species existing in the polymer film), rather, they are formed from atoms/ions of carbon and hydrogen in vacuum above the surface, which are generated from the bombardment of an energetic Bi₃⁺ beam. Surprisingly, our results suggest that those species have intrinsic relationships with each other so that they ought to reflect the structural properties of hydrocarbons in terms of the carbon “density”. The findings shed insight to our understanding about ion fragmentation and our potential to develop ToF-SIMS analytical approaches. We emphasize that our demonstrated analytical approach to using ρ for differentiating polymers and quantifying crosslinking degree of polymer films will contribute significantly to expanding applications of ToF-SIMS in polymer science and engineering.

Reference

1. S. Naderi-Gohar, K.M.H. Huang, Y.L. Wu, W.M. Lau and H.-Y. Nie, submitted for publication.

We describe here the application of cluster-SIMS for analyzing photoresist polymeric materials with line-width resolution at the sub-30 nm scale. The objective was to test nanoscopic-size molecular brush architecture for surface homogeneity and vertical alignment.

Our SIMS technique uses individual Au₄₀₀⁴⁺ projectiles. At an impact energy of 1.2 keV/atom, they generate an average of ~13 secondary ions (SIs) per impact. These ejecta originate from molecules co-localized within a surface volume of 10-20 nm in diameter and up to 10 nm in depth. Their identification enables, in principle, to analyze surface composition at the nanoscale. Repeating the shot-by-shot bombardment-detection allows to probe an ensemble of nanodomains stochastically. One can then extract from the records of individual impacts, subsets of data that correspond to like-domains with sufficient statistics for molecular characterization. The method for data analysis is described elsewhere(Anal. Chem.2006,78. 7410).

The materials examined are diblock brush terpolymers (DBTs). A first observation is that their low-density brush architecture influences SI emissions. Projectile-related signals normally observed when Au₄₀₀ impacts solids, are virtually absent(Int. J. Mass Spectrom. 2007, 263. 298). The projectiles penetrate deeper into the low-density structure, concurrently the emission of large polymeric ions appears hindered by steric effects. Evidence for the latter is that yields for low mass ions are at expected levels.

The counting of coincidentally emitted SIs allows to determine the surface coverage and analyze the degree of ordered alignment(Int. J. Mass Spectrom. 2011, 303. 97). Surface coverage results (~88%) are in agreement with data on reference specimens and information from XPS. Correlation coefficients between SIs from different functional groups are typically close to unity, suggesting that DBT nanodomains are aligned vertically in a homogeneous arrangement. The structural information is in agreement with that inferred from AFM surface morphology.

The features and limitation of the event-by-event bombardment mode and data analysis methodology will be illustrated with experiments on a series of thin films containing vertically aligned hole transport layers. This work supported by grants from NSF (CHE-1308312 & DMR-1105304), the Dow Chemical Company and the Welch Foundation (A-0001).

Polymer additives can provide a wide range of properties to bulk polymer materials. To provide specific molecular quantification of these trace additives, solvent extraction of the additives from the polymers followed by a MS/MS analysis technique is frequently used [1]. Although the additives may be present in the ppm by weight range in practical polymers, frequently these additives will segregate to the surface of a polymer changing the desired surface properties of the polymer. For many years, TOF-SIMS has been shown to be a useful tool for imaging the surface segregation of polymer additives [2].

Despite dramatic improvements in surface sensitivity and the resulting measurable spatial resolution, the unique identification of each additive within the mixture of additives and polymer compositions on the surface of real polymer samples has remained problematic. The combination of TOF-SIMS imaging with simultaneous MS/MS TOF-SIMS imaging should therefore significantly increase the analytical certainty of molecular identification of additives on polymer surfaces.

We employ a new and patented [3] TOF-SIMS imaging MS/MS (tandem imaging mass spectrometry), exploiting the unique characteristics of the TRIFT spectrometer of the nanoTOF II TOF-SIMS instrument. The tandem mass spectrometry is based on the selection of a high mass molecular ion (precursor ion) from the entire secondary ion mass spectrum in a the first-stage mass spectrometer (MS1), followed by high energy collision induced dissociation (CID) of the precursor ion and collection of the fragment ion (product ion) spectrum in a second-stage mass spectrometer (MS2). The unique ion optics of the TRIFT mass spectrometer allows the simultaneous collection of the imaging secondary ion mass spectra (MS1) and of the product ion (MS2) imaging MS data sets.

A series of pure polymer additive reference spectra were acquired in the MS and MS/MS modes of operation. Simultaneous MS and MS/MS images from the same analytical volume were then acquired for unambiguous identification of the additives on the polymer surfaces. The results indicate that this analytical mode using a simultaneous TOF-SIMS imaging MS/MS should provide a powerful tool for the analysis of polymers and biological samples.

[1] S. Beissmann, M. Reisinger, L. Toelgyesi, C. Klampfl and W. Buchberger, Anal. Bioanal. Chem. 2013, 405: 6879-6884.

[2] M. J. Walzak, N. S. McIntyre, T. Prater, S. Kaberline and B. A. Graham, Anal. Chem., 1999, 71 (7): 1428–1430.

[3] P.E. Larsen, J.S. Hammond, R.M.A. Heeren and G.L. Fisher, Method and Apparatus to Provide Parallel Acquisition of MS/MS Data, U.S. Patent 20150090874, 02 April 2015.

Many properties and applications of polymers are strongly dependent on their molecular conformations/orientations at surfaces and hidden interfaces. With its high molecular specificity and small sampling depth, TOF-SIMS has become one of the most important techniques for the analysis of polymer conformations at surfaces. On the other hand, the recent advent in the organic depth profiling using polyatomic ion sources has made it possible to analyze polymer conformations at the hidden interfaces. In this talk, we will present a few examples that were recently studied in our laboratory to illustrate the progress in this field.

The first example concerned the exploration of TOF-SIMS in determining polymer surface glass transition temperature (T_g^s). This work was inspired from the fact that polymer chains usually have higher mobility at surface than in the bulk and as a result, a lower surface transition temperature compared to its bulk is normally expected. To verify this, TOF-SIMS spectra of polystyrene (PS) films on silicon wafer were obtained at various temperatures. The detailed principal component analysis (PCA) of the spectra revealed a transition temperature (T_g^s) at which the PS surface conformation changed abruptly. In particular, the surface concentration of phenyl groups greatly decreased above T_g^s. This suggested that TOF-SIMS combined with PCA can be used to determine the T_g^s of PS. This conclusion was further confirmed recently using two other polymers: poly(2, 3, 4, 5, 6-pentafluorostyrene) and poly(methyl methacrylate).

The second example concerned the determination of polymer chain conformations in the thin film using TOF-SIMS depth profiling. Thin films of bromine-terminated poly(bisphenol A octane ether) (BA-C10) were depth profiled using C60 source. The polymer chain conformations in the films and at the interfaces can be revealed from the depth distributions of the polymer fragments and the end groups.

Acknowledgements

The work described in this paper was fully supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (grant nos. 600513 and 16300314).

Polymers or polymer-coated systems become more and more important for the technical or medical industries, e.g. thin organic film systems such as OLEDs or polymer-coated drug eluting stents. For characterizing and improving such thin organic film systems, it is very important to determine the 3D chemical composition of such systems.

For simultaneous detection of the elemental and molecular lateral distribution in very small surface areas as well as measuring the molecular concentration as a function of depth with high depth resolution only few techniques are available. Two powerful methods are time-of-flight secondary ion mass spectrometry (ToF-SIMS) and laser postionization secondary neutral mass spectrometry (Laser-SNMS). Both techniques are based on the detection of sputtered particles from the surface produced by ion bombardment. In the case of ToF-SIMS the sputtered secondary ions can be directly detected while in the case of Laser-SNMS the sputtered neutrals have to be post-ionized with a pulsed laser beam prior detection. Particularly, the use of an Ar cluster ion beam for sputtering makes it possible to obtain 3D depth profiles also from organic materials.

^* Corresponding author e-mail address: kecmchan@ust.hk [mailto:kecmchan@ust.hk] (C.-M. Chan)

Acknowledgements

The work described in this paper was fully supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (grant nos. 600513 and 16300314).

Tuning the surface potential of different metals is of particular interest for molecular electronic devices since the metal work function can be adjusted according to the electronic structure of the organic part. An attractive way to achieve this goal is the deposition of self-assembled monolayers (SAMs) consisting of molecules with strong and inherent dipole moments, for instance carboranes [1]. For silver, an additional essential aspect of the interaction of the carboranes with the surface is related to the strong Ag-S bond that forms and the resulting surface protection ability obtained with these SAMs. The combination of these two properties is unique in the sense that a robust surface functionalizing allowing surface potential tuning is obtained. However, the exact conformations of these molecules adsorbed on flat metal surfaces, and a better understanding of their effect on surface potential changes or on the stability of substrate metals [2], are not yet fully investigated.

In this contribution, the formation of SAMs of two positional dicarba-closo-dodecaborane-dithiol isomers, 1,2-(HS)₂-1,2-C₂B₁₀H₁₀ and 9,12-(HS)₂-1,2-C₂B₁₀H₁₀, on a flat silver surface, was investigated using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Kelvin Probe Force Microscopy (KPFM) [3]. While KPFM shows the opposite dipole orientation with regard to the flat silver surface and demonstrates the effective tuning of the surface potential value by the co-adsorption of both isomers, ToF-SIMS reveals characteristic fragments of the two isomers. Interestingly, the two isomers' SAMs manifest large differences in their respective cationization pattern, possibly due to their dipolar properties.

[1] J.F. Lübben, T. Baše, et al., Tuning the surface potential of Ag surfaces by chemisorption of oppositely-oriented thiolated carborane dipoles, J. Colloid Interface Sci. 354 (2011) 168–174

[2] T. Baše, Z. Bastl, et al., Carborane–thiol–silver interactions. A comparative study of the molecular protection of silver surfaces, Surf. Coat. Technol. 204 (2010) 2639–2646.

[3] A. Vetushka, L. Bernard et al., Manipulation of Ag (111) surface work function by the adsorption of oriented carborane dipoles, submitted.