Threshold Ion Energies for Creating Defects in 2D Materials from First-Principles Calculations: Chemical Interactions Are Important.

The characteristics of two-dimensional (2D) materials can be tuned by low-energy ion irradiation provided that the ion energy is correctly chosen. The optimum ion energy is related to E th ion , the minimum kinetic energy the ion should have to displace an atom from the material. E th ion can be assessed using the binary collision approximation (BCA) when the displacement threshold of the atom is known. However, for some ions the experimental data contradict the BCA results. Using density functional theory molecular dynamics (DFT-MD), we study the collisions of low-energy ions with graphene and hexagonal boron nitride and demonstrate that the BCA can strongly overestimate E th ion because energy transfer takes a finite time, and therefore, chemical interactions of the ion with the target are important. Finally, for all projectiles from H up to Ar, we calculate the values of E th ion required to displace an atom from graphene and h-BN, the archetypal 2D materials.