Bui Thuy An, Leuthner Gregor T, Madsen Jacob, Monazam Mohammad R A, Chirita Alexandru I, Postl Andreas, Mangler Clemens, Kotakoski Jani, Susi Toma
University of Vienna, Faculty of Physics, Boltzmanngasse 5, Vienna, 1090, Austria.
Small. 2023 Sep;19(39):e2301926. doi: 10.1002/smll.202301926. Epub 2023 Jun 1.
Understanding electron irradiation effects is vital not only for reliable transmission electron microscopy characterization, but increasingly also for the controlled manipulation of 2D materials. The displacement cross sections of monolayer hexagonal boron nitride (hBN) are measured using aberration-corrected scanning transmission electron microscopy in near ultra-high vacuum at primary beam energies between 50 and 90 keV. Damage rates below 80 keV are up to three orders of magnitude lower than previously measured at edges under poorer residual vacuum conditions, where chemical etching appears to dominate. Notably, it is possible to create single vacancies in hBN using electron irradiation, with boron almost twice as likely as nitrogen to be ejected below 80 keV. Moreover, any damage at such low energies cannot be explained by elastic knock-on, even when accounting for the vibrations of the atoms. A theoretical description is developed to account for the lowering of the displacement threshold due to valence ionization resulting from inelastic scattering of probe electrons, modeled using charge-constrained density functional theory molecular dynamics. Although significant reductions are found depending on the constrained charge, quantitative predictions for realistic ionization states are currently not possible. Nonetheless, there is potential for defect-engineering of hBN at the level of single vacancies using electron irradiation.
理解电子辐照效应不仅对可靠的透射电子显微镜表征至关重要，而且对于二维材料的可控操纵也越来越重要。使用像差校正扫描透射电子显微镜在近超高真空下，在50至90 keV的一次束能量下测量单层六方氮化硼（hBN）的位移截面。低于80 keV的损伤率比之前在较差残余真空条件下在边缘处测量的结果低多达三个数量级，在较差残余真空条件下化学蚀刻似乎占主导。值得注意的是，使用电子辐照可以在hBN中产生单空位，在低于80 keV时硼被逐出的可能性几乎是氮的两倍。此外，即使考虑到原子的振动，在如此低能量下的任何损伤也无法用弹性碰撞来解释。开发了一种理论描述来解释由于探针电子非弹性散射导致的价电离而引起的位移阈值降低，使用电荷约束密度泛函理论分子动力学进行建模。尽管发现根据约束电荷有显著降低，但目前还无法对实际电离态进行定量预测。尽管如此，利用电子辐照在单空位水平上对hBN进行缺陷工程仍具有潜力。
