Nanostructure formation due to impact of highly charged ions on mica R. Ritter a , G. Kowarik a , W. Meissl a , A.S. El-Said a,1 , L. Maunoury b , H. Lebius b , C. Dufour b , M. Toulemonde b , F. Aumayr a, * , 2 a Institute of Applied Physics, TU Wien (Vienna University of Technology), Wiedner Hauptstr. 8-10/E134, A1040 Vienna, Austria, EU b CIMAP, ENSICAEN, CEA, CNRS, Univ.Caen, 14070 Caen, France, EU Keywords: Mica AFM Nanostructuring Hillocks Friction Feature erasure abstract Muscovite mica was irradiated with slow highly charged Ar qþ (charge state q ¼ 12, 16) and Xe qþ (q ¼ 23, 27) ions in a kinetic energy range of 150–216 keV and subsequently observed by contact mode atomic force microscopy. Surprisingly, on samples irradiated with Xe ions nano-sized hillock-like structures were found well below the charge state threshold reported in earlier experimental investigations. However, the structures found are not the result of a true topographic surface modification induced by the ion bombardment, because the absence of these nanostructures in tapping mode images and the dependence of the detected structures on scan conditions points towards a surface modification which manifests itself only in frictional forces and therefore in height measurement artifacts. Furthermore the generated defects are not stable but can be erased by continuous scanning. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Slow (eV to keV) highly charged ions (HCI) have been suggested as a novel tool for gentle nanostructuring [1–3] of surfaces and meanwhile a variety of materials have been investigated by scan- ning probe methods [4,5]. Systematic studies in which charge state and kinetic energy of the HCI have been varied, however, have so far only been carried out for CaF 2 , KBr, HOPG and Mica (see [4–6] and references therein). In the case of CaF 2 [7–9], for example, it has been shown by decelerating the HCI projectiles to low kinetic energies (150 q  eV) that the potential energy of a single ion alone can be sufficient to create a topographic, non-erasable nano-sized protrusion (hillock) on the surface. This holds true above a certain threshold of potential energy (around 12 keV), above which height and diameter of the hillocks increase with increasing potential energy. By simulations on the basis of an extended classical over-the-barrier model the observed threshold could be successfully linked to a solid-liquid phase transition (nano-melting) [4,7,10]. On KBr(001) surfaces bombarded with slow highly charged Xe ions, on the other hand, pit structures with lateral sizes of 10–25 nm and monoatomic depth are created above certain thresholds for both potential and kinetic energy [11]. The mean pit volume shows a linear dependence on the potential energy of the ions. Potential sputtering [12] by a defect-mediated desorption mechanisms was invoked to explain these results [11]. For HOPG nano-sized hillock-like structures were found by scanning tunneling microscopy (STM) for all projectile charge states and kinetic energies [13–19]. While the structure size increases with the potential energy of the ions [4,20], no pronounced dependence on the kinetic energy was found [18]. However in atomic force microscopy (AFM) measurements, the dependence of the detected structures on scan conditions points towards a surface modification which manifests itself only in fric- tional forces and therefore in height measurement artifacts [20]. Furthermore the generated defects are not stable but can be erased by continuous scanning in contact mode [20]. In this paper we present recent results for HCI-induced nano- structures on muscovite mica, a phyllosilicate with the sum formula KAl 2 (Si 3 Al)O 10 (OH) 2 , which consists of layers with a net negative charge bonded through interlayer cations. This material is known to be a very good insulator and simple sample preparation by cleaving with adhesive tape or razor blades gives atomically flat surfaces with occasional occurrence of atomic steps. Mica is commonly used as a substrate for AFM investigations, or, as a calibration material and is known to be stable under contact mode atomic force microscopy. Due to these beneficial properties, mica has been considered a favorable material to explore ion-induced modifications. * Corresponding author. Tel.: þ43 1 58801 13430; fax: þ43 1 58801 13499. E-mail addresses: kowarik@iap.tuwien.ac.at (G. Kowarik), aumayr@iap.tuwien. ac.at (F. Aumayr). 1 On leave from Physics Department, Faculty of Sciences, Mansoura University, 35516 Mansoura, Egypt. 2 http://www.iap.tuwien.ac.at/www/atomic/ Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.10.018 Vacuum 84 (2010) 1062–1065