Probing Atomic Reconstruction at 2D Interfaces via Scanning Transmission Electron Microscopy

Scanning transmission electron microscopy (STEM) provides uniquely powerful tools to study the interfacial structure and interactions of 2D materials up to atomic resolution. In my talk, I will discuss how we create and utilize 2D multilayer stacks to create nanoscale laboratories for studying the atomic structure, structural transformations, and properties of 2D interfaces inside the STEM. 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 restructuring in 2D transition metal dichalcogenides (TMDCs). Here, graphene encapsulation protects the 2D materials of interest from the vacuum environment of the STEM, enabling in-situ high temperature studies up to 1000°C. We use these structures to directly visualize phenomena such as the layer-by-layer phase transformation of MoTe₂and the lattice reconstruction of 2D moirés.<br/>We also use custom-built 2D stacks to study the bending and bending-induced properties of 2D multilayers. We find that interlayer interactions play a major role in governing the bending of 2D stacks, where they lead to an unusually low bending stiffness in few-layer graphene and a curvature-dependence of the bending stiffness.[1] For example, we find that the bending stiffness of 4-layer heterostructures containing two layers each of graphene and MoS₂ can be tuned from 20 eV to 80 eV by simply changing the order and interface type of the layers present [2]. The key role of interfaces in the bending of 2D multilayers also indicates potential methods to tune the deformability of 2D systems though interfacial engineering. Our techniques should enable the rational design of interfacial properties of devices based on 2D materials, including illustrating how to incorporate slippable interfaces and create ultra-low bending stiffness structures for deformable, stress-resilient electronics.<br/>[1] E. Han et al., Nature Materials 19, 305-310 (2020).<br/>[2] J. Yu et al., Advanced Materials 33, 2007269 (2021).