Rogue Atoms in Flatland: Probing atomic disorder in two-dimensional systems

Understanding and controlling how properties arise in disordered systems is one of the oldest and most challenging problems in physics. Meanwhile, disorder in materials strongly affects their properties and limits the performance of electronic devices from the mobility of electrons in transistors, to the quantum yield of energy harvesting devices. Two recent developments enable new insight into the physics of disordered systems: the syntheses of novel two-dimensional materials and advances in transmission electron microscopy (TEM). Together, these allow us to utilize the low-dimensionality of 2D materials to probe their structure, bonding, chemistry, and the dynamics of deformation and phase change at the single-atom scale. The applications of these studies span applied and fundamental physics. For example, we use TEM techniques to study defects in 2D crystals such as graphene and molybdenum disulfide and correlate the atomic structures of grain boundaries with their mechanical, electrical, and optical properties. These studies are critical to optimizing the synthesis of a growing array of 2D materials. Meanwhile, we explore structure and properties of complex systems by studying new 2D glasses. We show that it is possible to image atoms in disordered solids, track their motions in response to local strain, and directly visualize phase transitions. Overall, these examples illustrate the wide-ranging and fundamental materials physics that can now be studied at atomic-resolution via transmission electron microscopy of two-dimensional materials.