Surface nanocratering by localized plasma expansion induced by slow highly charged ions

Ion beam technology provides a powerful means for surface modification and nanostructuring of materials. In this work, slow highly charged xenon ions were used to fabricate well-defined nanopits in thin polymethyl methacrylate (PMMA) films. The pit surface density scales linearly with the applied ion fluence, demonstrating that each structure originates from a single ion impact. The formation of these nanopits is interpreted within a plasma expansion framework, in which the potential energy of an individual highly charged ion creates a transient, localized plasma within a nanometric surface region. The subsequent plasma expansion ejects material and produces a shallow surface depression. Numerical solutions of the hydrodynamic equations for the plasma components yield expansion parameters that agree with the experimentally observed pit dimensions and aspect ratios, confirming the plausibility of the proposed mechanism.