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41847-AC5
Surface Damage of Organic Films by Polyatomic Projectiles in Secondary Ion Mass Spectrometry
Secondary ion mass spectrometry (SIMS) is the most widely used mass spectrometric technique for the surface analysis of molecular solids composed of organics, polymers and/or biological material. C60+, SF5+, and other polyatomic projectiles have been demonstrated to improve secondary ion yields in SIMS when compared to atomic ions of similar or lower mass. Polyatomic projectiles also show promise for depth profiling of molecular solids. These properties have made polyatomic (or cluster) SIMS a central method in the burgeoning field of imaging mass spectrometry.
Poly(methyl methacrylate), Teflon AF1600, and poly(3-hexylthiophene) films were studied after exposure to different fluences of C60 ions with kinetic energies of 1 - 20 keV by a quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS). C 1s XP spectra showed little or no change in the film chemistry upon ion bombardment at fluences up to 1013 - 1014 ion/cm2. This result supported prior observations from secondary ion yields that at least some organic films have no apparent static limit in SIMS using C60 ion projectiles until the film is sputtered away. Changes in C 1s XP spectra observed at the highest ion energies and fluences >1015 ion/cm2 were explained predominantly by differential charging effects. Some changes in film composition were also observed, but their extent varied with the polymer studied. Measurements for the total sputter yield of polymer films acquired using a QCM showed that each C60 cluster lead to an efficient emission of ~105 amu per C60 ion at 20 keV impact. The energy dependence for the total sputtering yield changed from a power law (with n~1.4) to linear above 5 keV. The sputtering yield for polymer films above 10 keV showed an enhancement of approximately five times when compared with gold targets, but there was little difference between the three polymers.
Comparisons were also made for sputtering yields of poly(3-hexylthiophene) by C60+ and SF5+: C60+ was found to be approximately seven times more efficient than SF5+ at 8 keV ion energy. This result is consistent with prior studies on polymer films from other laboratories.
The above work examined polymer films, but much of the interest in SIMS depth profiling is focused on biological samples. A model of a biological film was therefore prepared based upon a polyelectrolyte multilayer of chitosan and alginate, with the first chitosan layer covalently bound to a gold-coated QCM crystal via a glutaraldehyde-alkanethiolate linkage. The QCM was used to measure sputtering yields by 8 keV C60+ ions for this polyelectrolyte multilayer and found similar values to those reported above for the three polymer films. These experiments are continuing despite the end of PRF funding, with future publication of these results under consideration.
The sputtering yield data and the polyelectrolyte multilayer were included in a grant application to the National Institutes of Health titled “Novel Chemical Analyses of the Biofilm-Biomaterial Interface”. This NIH grant was funded in July 2007 and it is the principal investigator's first NIH grant as principal investigator. PRF support was crucial in the success of this NIH proposal. The NIH funded project will use the aforementioned polyelectrolyte layer to evaluate depth profiling with non-mass-selected keV C60 ion beams. The techniques developed for the polyelectrolyte layer depth profiling will then be applied to the analysis of bacterial biofilms using laser desorption vacuum ultraviolet postionization mass spectrometry. Comparisons will be made to polyatomic SIMS and other techniques in imaging mass spectrometry.
One graduate student has been supported on this project for its duration and she has been a coauthor on the papers published under this grant. Thus, PRF support has been crucial to her graduate study. She plans to submit her M.S. thesis in the Fall of 2007 on the data she collected for this project. Two other graduate students joined the principal investigator's group in early 2007. They worked on these experiments during the summer of 2007 and learned the methods of preparing, analyzing, and sputtering the aforementioned polyelectrolyte multilayers. This was an excellent introduction to research for these students, who have begun to apply their experience to other projects. Finally, PRF support permitted the hiring of a research assistant professor, the principal investigator's first postdoctoral researcher, who remains in the group now supported entirely by a different project.
