# Ion Beam Nanopatterning of Biomaterial Surfaces
## 2 Nanoscale Pattern Formation on Solid Surfaces during Ion Beam Irradiation  
In ion beam sputtering, a solid target surface is irradiated with heavy ions with a kinetic energy ranging from a few tens of eV up to a few hundred keV. This kinetic energy is mostly deposited in the near-surface region of the target material in the form of atomic collisions between the incoming ions and the nuclei of the target atoms. During these initial and subsequent collisions, surface atoms are ejected from the target, resulting in the removal of target material [28; 29]. Therefore, ion beam sputtering is routinely employed in the processing of numerous technological surfaces, for instance, in the cleaning, polishing, and etching of semiconductor and optical surfaces [30]. However, as was already observed in the 1960s, ion irradiation of solid surfaces may also lead to the formation of regular nanopatterns. In particular, [PERSON] et al. [26] and [PERSON] et al. [25] reported the development of anisotropic surface topographies on metal and glass surfaces, respectively, during keV ion irradiation under an oblique angle of incidence. In particular, the irradiated surfaces were found to exhibit shallow nanoscale ripple patterns whose orientation depended on the incident angle of the ion beam. Since then, similar patterns have been obtained on various amorphous and crystalline materials, including various metals [31; 32; 33; 34; 35] and semiconductors [36; 37; 38; 39] (see Figure 1a), but also oxides [40; 41; 42; 43] (see Figure 1b), ionic crystals [44; 45; 46; 47], and even polymers [48; 49; 50; 51] (see Figure 1c). Such ripple patterns typically have a quasi-sinusoidal shape with periodicities ranging from less than 20 nm [52] to almost 1 \(\upmu\)m [53], depending on the surface material and the irradiation conditions.  
In the last two decades, other pattern types were also found on ion-sputtered surfaces. In 1999, for instance, [PERSON] et al. reported the formation of isotropic, hexagonally ordered nanodots on GaSb surfaces [54] during normal incidence ion irradiation (see Figure 1d), while the negative of this pattern, i.e., hexagonally ordered nanoholes, was obtained more recently on Ge surfaces (see Figure 1e) [55]. In 2013, checkerboard-like patterns of inverted nanopyramids were also found on crystalline Ge(100) surfaces irradiated at high substrate temperature (see Figure 1f) [56]. Because of this great versatility, both with regard to surface material and pattern type and dimensions, ion beam nanopatterning has become a widely employed technique in the fabrication of functional surfaces for applications in plasmonics [57; 58; 59; 60], (bio)molecular sensing [61; 62; 63; 64; 65], photovoltaics [66; 67; 68], magnetism [69; 70; 71; 72], and nanoelectronics [73; 74]. In the remainder of this section, we will briefly summarize the mechanisms of nanoscale pattern formation during ion irradiation with a focus on aspects of high importance in biomaterials applications. For an in-depth discussion of all the processes involved in ion-induced pattern formation and their theoretical description, the reader is referred to a number of reviews that focus specifically on such aspects [75; 76; 77].