Single ion induced surface nanostructures

Ion impact on surfaces can induce surface modifications on the nano-scale. Different types of topological modifications are possible, like hillocks and craters, but in most cases, hillocks have been observed. These modifications can be produced by different types of ions. Ions deposit energy mainly by their potential energy (charge state) or kinetic energy (velocity). Slow highly charged ions (HCI), due to capture processes in front of the surface, as well as swift heavy ions (SHI), due to ionization processes inside the target, disturb the electronic system of the target heavily. A first indicator for this interpretation is the fact that the surface damage process usually has a threshold, either in the charge state (HCI) or the kinetic energy (SHI). Understanding, comparing and adapting the effects of different ion species for targeted surface modifications is the goal of the proposed project. Besides their excitation of the electronic system, slow and fast ions induce their main damage in different locations: slow HCI produce predominantly surface damage, whereas SHI do their damage mainly in the bulk (ion track). In order to compare the two ion types more easily, we will concentrate on grazing incidence with SHI. This particular collision geometry forces the track to a region close to the surface, comparable with the shallow damage of slow HCI. So far studies were done with individual combinations of irradiation parameters, like ion type, charge state and velocity, as well as target type and structure. We plan to concentrate on a relatively limited number of targets, but to study them in detail with a large number of ion types. We will start with three materials, whose reaction to ion beams is partially known, i.e. SrTiO3, TiO2 and CaF2. Depending on the obtained results, we will expand this initial choice to other types of materials. Until now, the main tools for the study of surface modifications were near-field methods. In most cases, atomic force microscopy (AFM) was used, therefore studying the topology of the modifications. We propose to use other methods, like high-resolution transmission electron microscopy, to also study the structure of the damage, and especially its extent into the volume, and not only the surface shape. We will also perform chemical and structural investigations of the perturbed regions using surface characterization techniques such as XPS, Auger, LEED and RBS/channeling. These studies will be combined with tools with sufficiently high spatial resolutions, like AFM and MET, to examine the effect of individual ion impacts. This broad variety of techniques will allow us to explore the damaging processes, and can lead to a better understanding of the mechanisms involved, which may ultimately lead to the application of these specific ions in surface nano-engineering.

For decades singly ionized (charged) atoms have been used in semiconductor technology for etching and cleaning of wafers as well as for modifying the properties of semiconductor components by implantation of foreign atoms (doping). If the charge state of the ions is further increased by removing more and more electrons, a process which is only possible in special ion traps or ion sources, so-called highly charged ions are produced which, in addition to their kinetic energy, also carry considerable amounts of potential energy. The primary objective of this project was to investigate whether this type of ion is suitable as a new tool for surface nano-structuring and modification of material properties on the nanometer scale and to understand the role of the potential energy in these processes. For this purpose, the Austrian project participants have irradiated a variety of materials with highly charged and heavy ions available from German and French ion beam laboratories, and visualized the tiny traces and tracks of individual ions produced on the sample surfaces using an extremely high-resolution scanning force microscope. Depending on the type, charge state, angle of incidence and energy of the ions and the used material, nanometer (= one millionth of a millimeter) -sized craters, hillocks, but also volcanic cones, elongated grooves or chains of hillocks were observed on the microscopic (nanoscopic) landscape. The observed tracks often have a great similarity with meteorite impacts, however, on a completely different length scale. A particular discovery of the project team were nanometer-sized holes observed after bombarding highly charged ions (similar to a "nano" shot-gun) onto ultra-thin carbon nanomembranes. Such a perforated membrane could possibly serve as a nano-sieve, which allows molecules of a specific size to pass through while blocking others, and which therefore might be of considerable interest for biological applications. However, the project was less focused on possible applications of highly charged heavy ions but instead aimed at a deeper understanding of the microscopic mechanisms that lead to the formation of such nanostructures. Experiments were carried out with the thinnest of all possible membranes, namely graphene, which consists of only one atomic layer of carbon. These experiments have shown that a large number of electrons is removed from graphene by the high electric field of a bombarding highly charged ion around the impact site, but surprisingly graphene is able to resupply these electrons within a few femtoseconds only. This leads to extremely high currents in graphene, which under normal conditions would not be possible at all. This exceptionally high mobility of the electrons makes graphene ideally suited for applications in ultrafast nano- and optoelectronics.