Detailed Atomic Structure of Defects in 2D Materials: From Graphene to Transition Metal Dichalcogenides

Understanding the exact details of the position of atoms within defective regions in materials helps model and predict their properties accurately. Two dimensional materials that are only one atom thick, such as graphene and boron nitride, make identifying the positions of single atoms substantially easier than from 3D material structures. Aberration-corrected transmission electron microscopy at low accelerating voltages of 80 kV and below, provides high atomic contrast in both phase contrast TEM imaging as well as in annular dark field scanning transmission electron microscopy. In recent work we have shown how monochromation of the electron source reduces the energy spread and lowers the detrimental effects of chromatic aberration on spatial resolution. The combination of monochromation and image correction in phase contrast HRTEM mode at an accelerating voltage of 80kV yields images of graphene with contrast spots associated with the position of individual atoms, as shown in figure This led to the ability to fully resolve the position of individual carbon atoms in graphene around defects and measure the changes in C-C bond lengths. Correlating the atomic structural models deduced from HRTEM with density functional theory provided insights into how changes in charge density around defect clusters causes bond elongation and compression In new work this has been extended to high temperature measurements by using an in-situ heating holder with suspended graphene samples. Totally different defect structures are produced at these high temperatures (>600 o C) and are likely caused by changes to the flexibility of graphene and limits to out-of-plane distortions that help accommodate strain at low temperatures.