| Summary: | Highly charged ions are a well-known tool for the nanostructuring of surfaces. We report on the thickness dependence of nanostructures produced by single 260 keV Xe<sup>38+</sup> ions on ultrathin poly(methyl methacrylate) (PMMA) films (1 nm to 60 nm) deposited onto Si substrates. The nanostructures induced by slow highly charged ions are rimless craters with a diameter of around 15 nm, which are roughly independent of the thickness of the films down to layers of about 2 nm. The crater depth and thus the overall crater volume are, however, thickness-dependent, decreasing in size in films thinner than ~25 nm. Our findings indicate that although the potential energy of the highly charged ions is the predominant source of deposited energy, the depth of the excited material contributing to crater formation is much larger than the neutralization depth of the ions, which occurs in the first nanometer of the solid at the projectile velocity employed here. This suggests synergism between kinetic and potential-driven processes in nanostructure formation in PMMA.
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