TOF-SIMS molecular depth profiling of polymers and organic materials using low energy atomic primary ions for sputtering

  • Nicolas Mine

    Student thesis: Doc typesDoctor of Sciences

    Abstract

    In this work we demonstrate that erosion with low energy primary ions (150 eV to 1 keV) in ToF-SIMS allows to depth-profile polymers but also various organic materials with a real molecular information [1–4, 8]. The depth-profiles obtained with low energy cesium allow us to monitor specific negative molecular fragments. We demonstrate also that molecular information can be observed from the positive polarity mass spectra when cesium is used for erosion by following the 푀퐶푠+ 푛 clusters (where M is a specific molecular fragment) in the positive mass spectra. In our experiments obtained with a dual beam ToF-SIMS, we demonstrate the damaging effect of the analysis primary ions (for most of this work, gallium with 15 keV of kinetic energy). Thanks to sputtering yield measurements we differentiate between Type I and Type II polymers. The incorporation of cesium in the organic material increases the stopping power, which is beneficial to the molecular sputtering yield of the organic material. A 3 to 2 increase of the erosion rate is respectively achieved from 250 eV to 1000 eV using cesium compared to xenon depth-profiling. The sensitivity of the organic material to ion beam damage and specially to cross linking allowed us to explain our preferential use of different primary ions and bombardment energy for erosion. We demonstrated that xenon and oxygen used with low energy are not well suited for depth-profiling of polymers. Chemical damage occur leading to cross-linking or carbonization of the organic material. However oxygen can be partially usable on degrading polymers/type II like PMMA. The higher sputtering yield due to radical depolymerization compared to type I polymers allows us to limit the ion beam damage. A major result of the present work is the demonstration that cesium low energy (less than 300 eV) is successful on cross-linking polymers (PS, PC, PP,...), which are challenging materials to depth-profile. Even with a simple analysis primary ion beam like gallium, the useful ion yields are high enough to follow the specific fragments in depth, for high erosion beam fluences. On type I polymers and cross-linking biomolecules, 퐶푠+ primary ions can be used with the lowest energy possible that keeps a sufficient sputtering yield. On simple small biomolecules, depth-profiling with etching energy significantly higher than for cross linking materials is possible (500 eV to 1 keV on amino-acids compared to less than 250 eV on polystyrene or polycarbonate). The whole protonated or deprotonated molecule can be monitored during a depth-profile. For instance, the aromatic amino-acids are specially difficult to profile and suffer from higher damage under ion bombardment. We assume that cross-linking reactions also occur, which lower the useful yields in ToF-SIMS. But the coupling of cesium reactive primary ion and a low bombardment energy has a protecting effect on the specific molecular structure of Type I and aromatic biomolecules. The effects of low energy cesium, which leads to a successful depth-profile are described in details in the thesis: • Cesium gives higher molecular sputtering yield on polymers compared to ”non reactive” xenon primary ion. • The lower deposited energy induces lower damage and a smaller implantation depth. • The cesium free radical scavenging eect allows to heal or mostly to prevent the organic fragments to cross-link. XPS experiments on polymers confirmed the formation of a cesium carbide, which is assumed to come from radical scavenging effect. • A chemical and stabilizing eect of cesium on the aromatic structures, which are really sensitive to cross-linking, is assumed under cesium ion bombardment. • The cesium negative ionization enhancement, is a strength of cesium implantation during the erosion. For instance, cesium reacts with free radicals to form a salt or an anionic site. Then the energetic analysis beam releases the negative fragments. This assumption is summarized by the simple equation; 푀∙ + 퐶푠0 ⇒ 푀− + 퐶푠+ Where the 푀− is a molecular anion that can be detected during the analysis. OES analysis performed simultaneously to ToF-SIMS depth-profiling measurements confirms the neutral cesium presence thanks to electronic transitions, which correspond to neutral atomic cesium only in polymers depth-profiles (from the 6푃1/2 and 6푃3/2 to 6푆1/2 energy level). Finally, in this work we discuss the formation mechanism and ionization of specific aromatic amino-acids like tyrosine or phenylalanine. A high depth-resolution profile of three delta-layers of phenylalanine embedded in tyrosine is shown for the first time, using cesium low energy for erosion coupled with 퐵푖+ 3 for analysis.
    Date of Award17 Dec 2012
    Original languageFrench
    Awarding Institution
    • University of Namur
    SupervisorLaurent Houssiau (Supervisor), Benoit CHAMPAGNE (Jury), Jean-Jacques Pireaux (President), Patrick Bertrand (Jury) & T. Wirtz (Jury)

    Keywords

    • ToF-SIMS
    • Cesium
    • SIMS
    • mass spectrometry
    • amino-acids
    • Low energy
    • Depth-profile
    • Depth-profiling
    • Polymers
    • Polystyren
    • Polycarbonate
    • Polymethylmetacrylate
    • Ionization

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