Atom-by-atom observations on defect formation and dynamics in 2D materials studied by HRTEM
Day of Ph.D. defence: 22.10.2013
In this work I perform high resolution transmission electron microscopy studies on 2D materials such as graphene, single layer boron nitride and MoS2 as well as on atomically thin SiO2 glass. This combination turns out to be extremely fruitful because for the first time it is possible to see single atoms instead of atomic columns in transmission mode. This offers experimental excess to fundamental phenomena in materials sciences that so far have been reserved to computer simulations or analytical theory as capturing the dynamics of individual atoms in a bulk material is beyond the spatial and temporal resolution limit of current characterization techniques. Examples are the atomic structure of amorphous materials or the migration of point defects, grain boundaries and dislocations. On the other hand, this setup is an excellent lab-bench to study electron-specimen interaction such as the effect of chemical bonding on the elastic scattering, changes in the crystal structure due to knock-on (bond rotations or sputtering) and surface modifications caused by beam induced chemical reactions (chemical etching). In addition, the well-defined and simple geometry (especially the precisely defined thickness) and high stability of these new 2D materials allows measurements at a level of accuracy that has not been possible before inspiring researchers all over the world to push the limit of traditional microscopy techniques. Some examples are the detection of chemical bonding in direct HRTEM images for N-doped graphene and boron nitride, the possibility to distinguish between different light elements (B, C, N, O) from HRTEM and ADF-STEM and the possibility to perform spectroscopic measurements on single atoms.