A simulation study of mega electron-volt helium ion channeling and shadow effect in titania nanotubes

Ion channeling refers to the guided motion of high energy ions through axial/inter-planar regions in single crystals and ordered nanostructures under the influence of the Coulomb scattering process. The process has the potential of focusing ion beams without the use of strong magnetic fields. While the channeling phenomenon in single crystal materials has been extensively explored, in recent years, there has been an increasing interest in using ordered nano-architectures, especially the nanotube structure, due to potential technological benefits. Here, we report a simulation study of He ion channeling in titanium dioxide (titania; TiO2) nanotubes with features similar to those fabricated using anodic oxidation. The anodic titania nanotubes stand out from the rest of the compound nanotubes owing to the ability to yield single nanotubes as well as vertically aligned nanotube arrays with dimensions tunable in a wide range to facilitate the desired effect from ion channeling. We explored mega electron-volt (MeV) He++ ion channeling in titania nanotubes through molecular dynamics simulations based on Lindhard's planar channeling potential. Our simulated trajectories of ions projected into titania nanotubes showed that ion channeling phenomenon could occur in straight nanotubes. The simulation results showed that ion channeling in titania nanotubes could be used to focus an ion beam to an area as small as 113 nm(2) with a nanotube of pore diameter 100 nm. The distance to the first focal point reduced with the use of higher atomic number particles for channeling. We studied the transmittance of He ion flux through tilted nanotubes and compared the variation of the transmittance of ion flux under the ion channeling effect to that of a hypothetical beam following geometric optics, as a function of the angle of incidence. We introduced the term "Shadow Effect" to describe the variation of the transmittance of the above-mentioned hypothetical beam. For nanotube aspect ratios >20, the transmittance under ion channeling effect surpassed that in the case of the hypothetical beam.