Three-dimensional characterization of 2D materials using few-tilt microsecond dwell time electron ptychography

From ripples to defects, edges and grain boundaries, the 3D atomic structure of 2D materials can drastically alter their properties. However, determining the 3D structure of 2D materials experimentally can be difficult. Conventional electron tomography requires images from a large number of tilt angles, necessitating many times the electron dose needed for a single micrograph. Such doses can alter structures before they can be determined. Here we provide an alternative route to 3D structure determination of 2D materials with a greatly reduced dose requirement. An event driven camera is used to enable rapid microsecond dwell time 4D scanning transmission electron microscopy (STEM), facilitating simultaneous annular dark field (ADF) and ptychographic imaging with minimal dose and drift. Crucially this combination of imaging modalities allows us to determine the projected positions of all the atoms in arbitrary 2D materials at each tilt angle. With this data, the 3D structures can be solved from very few tilts indeed. While as few as two tilts can be used, our low dose experimental setup and the efficiency of ptychography enables us to use several more tilt angles without exceeding our dose budget. This can be vital to providing picometer precision reconstructions of more complex structures. We demonstrate the technique with simulations for WS$_2$ and experimental data from a new 2D material, hexagonal CuI, both of which have complex layered structures containing both heavy and light elements.