Angle-resolved photoemission spectroscopy (ARPES) was developed for the determination of the electronic band structure of solids. In the last 10 years or so, ARPES energy/momentum resolution has improved to the point where it can illuminate more subtle electronic aspects, such as symmetry breaking and the many-body interactions (MBIs) that characterize ground states such as superconductivity. These MBIs involve exchange of momentum among electrons or with excitations such as phonons, and can therefore couple to nanoscale structures. By controlling the structure at the nanoscale, we can therefore hope to control or enhance the ground state properties of materials through nanoscale engineering. This dream has motivated the development of ARPES with nanoscale spatial resolution (nanoARPES), in order to probe these effects.
MAESTRO, the Microscopic and Electronic Structure Observatory, is a new synchrotron based user facility for the study of in situ prepared materials, including oxides, 2D van der Waals material, semiconductors, metals, and surfaces. With a combination of three ARPES microscopes with complementary spatial/energy/momentum resolutions, and in situ sample preparation (molecular beam epitaxy, pulsed laser deposition, and micro-mechanical sample transfer) we are able to examine the relationship between electron structure and topology with unprecedented spatial resolution, currently around 100nm, with 50 nm performance on the horizon.
As an example, I will show the spatially resolved electronic structure of two-dimensional metal dichalcogenide heterostructures of WS2, graphene, BN and TiO2 [1, 2]. Among the findings is a striking renormalization of the spin-orbit splitting of the WS2 valence band, which can be controlled by chemical doping or by choice of substrate. This is attributed to the impact of trion (charged exciton) formation on the self-energy of carriers in WS2. We have also observed modifications in the electronic structure of “twisted” layers of WS2 on graphene. Such twisted heterostructures are garnering a lot of attention due to the possibility of engineering novel electronic structures.
 Katoch, J. et al (2018). “Giant spin-splitting and gap renormalization driven by trions in single-layer WS2/h-BN heterostructures.” Nature Physics, 14, 355–359. https://doi.org/10.1038/s41567-017-0033-4
 S. Ulstrup et al (2019), “Imaging microscopic electronic contrasts at the interface of singlelayer WS2 with oxide and boron nitride substrates.” Applied Physics Letters, 114, 151601 (2019). https://aip.scitation.org/doi/10.1063/1.5088968
 S. Ulstrup et al (2019), “Direct observation of mini-bands in twisted heterobilayers.” arxiv.org/abs/1904.06681
Work done at the Advanced Light Source in Berkeley CA, in collaboration with Ohio State University and the Naval Research Laboratory