Electron knock-on damage in hexagonal boron nitride monolayers

We combine first-principles molecular-dynamics simulations with high-resolution transmission electron microscopy experiments to draw a detailed microscopic picture of irradiation effects in hexagonal boron nitride ($h$-BN) monolayers. We determine the displacement threshold energies for boron and nitrogen atoms in $h$-BN, which differ significantly from the tight-binding estimates found in the literature and remove ambiguity from the interpretation of the experimental results. We further develop a kinetic Monte Carlo model which allows to extend the simulations to macroscopic time scales and make a direct comparison between theory and experiments. Our results provide a comprehensive picture of the response of $h$-BN nanostructures to electron irradiation.

Figure 1

Examples of the $h$-BN structures after an exposure to a TEM beam with different acceleration voltages: (a) 80 kV, (b) 120 kV, and (c) 200 kV.

Figure 2

(Color online) (a) Knock-on displacement thresholds for atoms in the pristine structure and for monovacancy edges as a function of the charge of the system $q$ (total charge of the system). Squares denote the results obtained for nitrogen and circles for boron. (b) Displacement rates for B and N atoms in pristine structure (B/N), at monovacancy edges $\left({\text{B}}^{\prime }/{\text{N}}^{\prime }\right)$, and at edges of larger vacancies $\left({\text{B}}^{\Delta }/{\text{N}}^{\Delta }\right)$. These values were calculated for an electron-beam current of $10.0\text{A}/{\text{cm}}^2$.

Figure 3

(Color online) Results of kinetic Monte Carlo simulations corresponding to the experimental images shown in Fig. 1. Simulated acceleration voltages were (a) 80 kV, (b) 120 kV, and (c) 200 kV. Examples of removed atoms in unstable areas are shown with small circles, and large circles highlight areas where merging of vacancies could be expected to occur in a real experiment. Nitrogen-terminated edges are marked with lines in panels (b) and (c).