Research presentation: Low-energy ion irradiation of surfaces and 2D materials

The complexity of surface phenomena, famously characterized by Wolfgang Pauli’s remark that “God created the bulk solids, but the devil the surfaces,” becomes even more relevant for two-dimensional (2D) materials. Low-energy ions can be used to modify and shape surfaces and 2D materials in a unique way to change their properties. To do this in a controlled way, we need to understand the multitude of processes initiated by the irradiation of ions at low energies from few tens eV to few hundreds of eV.

In the first part of my talk I will delve into the self-organizing mechanisms leading to a plethora of nanoscale patterns emerging on ion irradiated surfaces. Depending on the irradiation conditions, hexagonally ordered dot or pit patterns, checkerboard patterns, as well as periodic ripple patterns are formed spontaneously due to the non-equilibrium conditions induced by continuous ion irradiation. In-situ studies of the surface morphology can reveal the kinetics of the patterning process, yielding further insight into the dominant mechanisms and thus enabling to gain precise process control. For instance, by real-time in-situ Grazing Incidence Small Angle X-Ray Scattering (GISAXS) investigation the significant morphological parameters of the surface are deduced, thus tracking the development of the crystalline Ge(100) surface morphology during ion irradiation. Observing the kinetics of pattern formation in the non-linear regime, we find that the temporal evolutions of characteristic length and roughness conform to power laws, their exponents agreeing with scaling laws for conserved continuum equations with four-fold symmetry.

In the second part I will focus on doping and defect engineering of 2D materials with low energy single charged and highly charged ions. Low-energy ions can create defects in 2D materials, such as graphene and hexagonal boron nitride (h-BN) above a minimum kinetic ion energy required to displace an atom. Traditional models using the binary collision approximation (BCA) often overestimate this threshold due to neglecting the time-dependent nature of energy transfer and chemical interactions during collisions. By employing density functional theory molecular dynamics, it can be shown that for many ions the threshold energy is significantly lower than predicted by BCA. Highly charged ions can additionally create extended defects by the release of their potential energy, which can largely exceed their kinetic energy.

Finally, I will also present some recent development of focussed ion beams at the Ion Beam Center and their applications for modifying surfaces and 2D materials.