Nonadiabatic Effects on Defect Diffusion in Silicon-Doped Nanographenes

Single atom impurities in graphene,
substitutional silicon defects
in particular, have been observed to diffuse under electron beam irradiation.
However, the relative importance of elastic and inelastic scattering
in facilitating their mobility remains unclear. Here, we employ excited-state
electronic structure calculations to explore potential inelastic effects,
and find an electronically nonadiabatic excited-state silicon diffusion
pathway involving “softened” Si–C bonding that
presents an ∼2 eV lower diffusion barrier than the ground-state
pathway. Beam-induced transition rates to this state indicate that
the excited-state pathway is accessible through irradiation of the
defect site. However, even in the limit of fully elastic scattering,
upward nonadiabatic transitions are also possible along the diffusion
coordinate, increasing the diffusion barrier and further demonstrating
the potential for electronic nonadiabaticity to influence beam-induced
atomic transformations in materials. We also propose some experimentally
testable signatures of such excited-state pathways.