Painting potential landscapes on an atomically thin canvas

Atomically thin van der Waals crystals like graphene and transition metal dichalcogenides allow for the creation of arbitrary, atomically precise interfaces simply by stacking disparate monolayers without the constraints of covalent bonding or epitaxy. By leveraging the environmental sensitivity of interactions at the ultra-thin, two-dimensional (2D) limit, we can “paint” potential energy landscapes to create and control the electronic structure and excitations in these systems. In this talk, I will discuss stories from our joint experimental and theoretical work on the prototypical 2D semiconductor interface of monolayer WS₂ and monolayer WSe₂. In part one, we use ultrafast electron diffraction to reveal the role of layer-hybridized electronic states for controlling energy and charge transport across atomically sharp junctions [1]. In part two, we align the registry of the two layers and use electron energy loss spectroscopy to directly visualize the real space localization of excitonic states within a single moiré unit cell [2], opening the possibility of engineering excitonic superlattices with nanometer precision. In the final part, I will discuss the transport of energy across such a superlattice potential using interlayer excitons [3].

[1] Sood, Haber, Raja, et al. “Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer,” Nature Nanotechnology 18 (2023) 29–35 (2023); [2] Susarla, Naik, Raja, et al. “Hyperspectral imaging of excitons within a moiré unit-cell with a sub-nanometer electron probe,” Science 378 (2022) 1235–1239; [3] Rossi, Zipfel, Maity, Raja et al. “Anomalous interlayer exciton diffusion in WS₂/WSe₂moiré heterostructure,” ACS Nano 18 (2024) 18202–18210.