New methods for imaging 2D materials:
Imaging strain, moirés, and phase transformations from micrometers to picometers

Because they are atomically thin, 2D materials offer a uniquely powerful platform for electron microscopy to extract the structure and properties of materials with single-atom or even picometer precision. In addition, understanding the relationships between the atomic structure and properties of two-dimensional materials is crucial for the development of next-generation electronics including deformable electronics, biosensors, and nanoelectromechanical systems. In this talk, I will discuss how we use utilize 2D mono- and multilayer stacks to create nanoscale laboratories for studying 2D materials inside atomic resolution electron microscopes. We then use aberration-corrected scanning transmission electron microscopy (STEM) and ptychography to measure phase transformations, strain, and bending of materials just a few atoms thick. For example, we utilize graphene encapsulation in combination with a MEMS-based heating holder to conduct in-situ studies of solid-solid phase transformations and interfacial restructuring in 2D transition metal dichalcogenides. The graphene encapsulation protects the 2D materials from the vacuum environment of the STEM, enabling high temperature studies up to 1000°C. We use these structures to directly visualize the layer-by-layer phase transformation of MoTe2 and the lattice reconstruction of 2D moirés.