Fast-ion-induced surface tracks in bioorganic films

Surface tracks, induced by single grazing incidence 55 MeV I-127 ions, incident on various bioorganic films, were studied by intermittent contact (tapping-mode) scanning force microscopy (SFM). Targets composed of biomolecules with different masses (10(2)-10(5) u) were employed. Surface tracks appeared as craters with raised rims and tails. In SFM, probe-tip-induced deformation of surface features under study is a possible complication, since it can lead to false imaging. In tapping-mode-SFM, larger amounts of tip oscillation attenuation with respect to the free amplitude imply greater tip-surface contact force. We found that reproducible track size results were obtained with a low attenuation ( < 4%), and track dimensions on the various surfaces could be compared quantitatively. The crater lateral dimensions decreased with increased target molecular mass. For surfaces composed of large protein molecules the craters are smaller than the molecules, which implies an upper mass limit for the detection of intact proteins by PDMS. The measurements were compared with various sputtering model predictions. A pressure-pulse model calculation assuming extensive cluster ejection showed fair agreement with observed crater widths and lengths. (C) 1998 Elsevier Science B.V. All rights reserved.