Probing atomic structure across higher dimensions in 2D materials using sub-Angstrom electron beams
Department of Materials Science & Engineering
University of Michigan
Modern materials are designed atomic layer by atomic layer with architectural complexity extending into the third dimension. For charge ordered materials, dramatic electronic changes are associated with periodic lattice distortions that require higher dimensions to describe the crystal. Here I discuss how scanning / transmission electron microscopy can probe atomic structure across sub-Angstrom to micron length scales in both two, three, and higher dimensions for the hierarchical engineering and design of 2D materials.
In 2D-materials—such as graphene, MoS2, and TaS2—reduced dimensionality leads to unique properties that could transform future of electronic devices. Local atomic structure of 2D materials dictates local topology which greatly influences electronic properties and implementation in actual devices. Using a modern electron microscope we can count the atoms across grain boundaries, identify stacking structure, and locate individual defects and dopants. Concurrently, diffraction techniques provide an understanding of structure across billions of atoms at larger length scales. In combination, we obtain a complete description of the structure of 2D materials and correlate them with macroscopic properties.
For quantum materials with charge ordering, the crystal is described in higher dimensions by the addition a periodic lattice distortions (PLD). We show the persistence and restructuring of PLDs down to the ultrathin limit and across a metal-insulator phase transition using cryogenic scanning transmission electron microscopy. Using picometer precision, we unearth inhomogeneity in the charge order at room temperature, and emergent phase coherence at 93 K. Such local phase variations govern the long-range correlations of the charge-ordered state and locally change the periodicity of the modulations, resulting in wave vector shifts in reciprocal space. These atomically resolved observations underscore the importance of lattice coupling and phase inhomogeneity, and provide a microscopic explanation for putative “incommensurate” order.
Dr. Hovden is an Assistant Professor in Materials Science at the University of Michigan. He completed his Ph.D. in Applied Physics at Cornell University in the small academic town of Ithaca, NY. Utilizing electron microscopy he unveils new understanding of how structure at the atomic and nanoscale determines material properties at the macroscale—spanning a wide class of systems including 2D materials, next-generation energy devices, and biominerals.