AVS 46th International Symposium
    Applied Surface Science Division Monday Sessions
       Session AS1-MoA

Paper AS1-MoA7
Enriched Spectral-information from TOF-SIMS Spectra of Self Assembled Monolayers: More Than Just Molecular Ions

Monday, October 25, 1999, 4:00 pm, Room 610

Session: Molecular Mass Spectrometry including Interpretation
Presenter: B.D. Ratner, University of Washington
Authors: D.J. Graham, University of Washington
B.D. Ratner, University of Washington
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Engineered biomaterials that accurately trigger complex biological processes such as healing require complex surface modifications. This increase in complexity must be met by equally capable surface analysis tools such as TOF-SIMS. To extract information from complex organic surfaces, one should consider the spectrum as a whole, instead of selecting only a few key peaks. This study demonstrates that the information content of a TOF-SIMS spectrum does not reside solely in the molecular and cluster ions, but that each region of the spectrum contains enough information to clearly distinguish the samples. For this study dodecanethiol self-assembled monolayers (SAMs) were prepared from 0.01mMol solutions in ethanol for different times (2sec,1min,5min,15 min,30 min,1hr,24hr,6d). Principal component analysis (PCA) models from the TOF-SIMS negative and positive spectra were constructed using the entire data set and from partial data sets from mass ranges m/z=0-100,100-200,200-300,400-500,500-1000. Scores plots from the PCA models show that data from each set were able to distinguish the samples. Regions containing the molecular ions showed enhanced ability to distinguish the samples. In the overall model it was seen that there is a relative increase in the intensity of low mass hydrocarbon fragments (C to C3) with increasing time. Similar trends were seen in the positive spectra where a relative increase of C to C4 hydrocarbon fragments was seen with increasing time. This increase was accompanied by a decrease in the intensity of C5 and above hydrocarbon fragments. This data suggests that as the SAM surface becomes more ordered and crystalline, the emission of longer fragments from the thiol chains is reduced relative to the emission of short fragments. As there is also an enhancement of the emission of the thiol head group ion, it is believed that these short fragments arise by the clipping of the tops of the assembled layer by the primary ion or energetic secondary ions.