Abstract: Atomically thin van der Waals crystals like graphene and transition metal dichalcogenides allow for the creation of arbitrary, atomically precise heterostructures simply by stacking disparate monolayers without the constraints of covalent bonding or epitaxy. While these are commonly described as nanoscale LEGO blocks, many intriguing phenomena have been discovered in the recent past that go beyond this simple analogy.
In this talk, I will discuss two stories from our joint experimental and theoretical work focusing on the prototypical 2D semiconductor interface of monolayer WS2 and WSe2. In part one, we use ultrafast electron diffraction to uncover the role of layer-hybridized electronic states as a powerful route to control ultrafast energy transport across atomic junctions [1]. In part two, we use electron energy loss spectroscopy to directly visualize the nanoscale real space localization of excitonic states within the moiré unit cell of WS2 and WSe2, opening the possibility for on-demand engineering of excitonic superlattices with nanometer precision [2]. The theoretical part of these works utilized BerkeleyGW and are a result of fruitful collaborations with colleagues at various institutions including UC Berkeley, SLAC National Laboratory, Stanford, Molecular Foundry, Purdue University and NIMS Tsukuba.
[1] A. Sood†, J. Haber, J. D. Carlström, E. A. Peterson, E. Barré, A. H. M. Reid, X. Shen, M. E. Zajac, E. C. Regan, J. Yang, T. Taniguchi, K. Watanabe, F. Wang, X. Wang, J. B. Neaton, T. F. Heinz, A. Lindenberg†, F. H. da Jornada†, A. Raja†. “Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer,” Nature Nanotechnology 18 (1), 29-35 (2023)
[2] S. Susarla*, M. H. Naik*, D. D. Blach, J. Zipfel, T. Taniguchi, K. Watanabe, L. Huang, R. Ramesh, F. H da Jornada, S. G Louie†, P. Ercius†, A. Raja†. “Hyperspectral imaging of excitons within a moiré unit-cell with a sub-nanometer electron probe,” Science 378 (6625), 1235-1239 (2022)
Bio: Archana Raja completed her PhD in Chemical Physics from Columbia University under the supervision of Profs. Tony Heinz and Louis Brus in 2016. After spending a year as a postdoc in the Applied Physics department at Stanford University, she joined the Kavli Energy and Nanoscience Institute at UC Berkeley as a Heising-Simons postdoctoral fellow in Prof. Paul Alivisatos' group. In July 2019, she became a Staff Scientist at the Imaging and Manipulation of Nanostructures Facility at the Molecular Foundry. She is the recipient of the LBNL Director’s Award for Exceptional Achievement in Early Scientific Career, Early Career Lab Directed Research Development Award, Blanche R. and David Kasindorf Fellowship in Physical Chemistry at Columbia University, the Institute Silver Medal at the Indian Institute of Technology in Bombay and was recognized as part of the 2019 Rising Stars in Physics cohort.