Alexander Rubinstein

Ion bombardment is one of the main tools to modify the properties of materials by controllable introduction of impurities and defects. At the moment there are three wide-spread approaches to simulate the non-equilibrium processes of ion impacts onto the targets.
- The most common tools are the Monte Carlo TRIM/SRIM approach (and its high-fluence extension TRIDYN) based on the binary collision approximation (BCA), which are computationally very efficient to predict ion ranges and estimate the number of defects produced even on a desktop computer.
- The second approach to model the non-equilibrium processes is to use molecular dynamics (MD) simulations which include also detailed analysis of energy transfer and multi particle dynamics in a solid or fluid after an ion impact. Compared to BCA simulations, MD simulations are computationally rather expensive, but allow obtaining a detailed understanding of materials modification by ion bombardment on an atomic level. Several MD codes are available for ion impact simulations, e.g. LAMMPS, DL-POLY and PARCAS. However, simulations of irradiation effects in novel systems, including the modification of 2D materials with ultra-low energy ion implantation and large clusters require further modification of these codes.
- The third approach is an analytical approach to the slowing down of particles in a solid incorporating the spread of deposited energy into the material. This technique is the fastest and most efficient of all, but relies upon substantial simplification of the processes involved.

The Ion Beam Centre at the University of Surrey has several on-going research programmes, funded by the EU and UKRI (EPSRC) to understand these processes in detail to enable users of the Facilities to better predict the behaviour of their samples under various irradiation conditions.

In particular, it is proposed that the two-temperature molecular dynamics model aimed at describing the impacts of high-energy ions and clusters into solids to account for the reduced dimensionality of two-dimensional materials and correspondingly develop the dedicated software based on the LAMMPS code. Moreover, we aim to combine the three approaches above and develop an algorithm/simulation setup, which would make it possible to assess the amount of damage produced in 2D materials on Si/SiO2 and other substrates. For light ions, e.g., using a He ion microscope, the majority of the damage in the 2D material results from the backscattered ions and atoms sputtered from the substrate, not by direct impacts. The process cannot be simulated by MD alone, as it becomes computationally too demanding. Besides, interatomic potentials are not always available to describe the interactions between the atoms in the target and substrate. SRIM-type software can therefore be used to assess the number of backscattered ions and sputtered atoms, and the results will be fed into MD calculations with the substrate modelled as an external repulsive potential. In cases where the ion dose is extremely high (above 1018 ions/cm2) even the SRIM like models become complex and progression to analytic models is called for. This will be investigated as part of this proposal too.