Medha Dandu1,Jordan Hachtel2,Mit Naik3,1,Patrick Hays4,Sriram Sankar4,Elyse Barré1,Takashi Taniguchi5,Kenji Watanabe5,Steven Louie3,1,Felipe da Jornada6,7,Sefaattin Tongay4,Peter Ercius1,Archana Raja1,Sandhya Susarla4
Lawrence Berkeley National Laboratory1,Oak Ridge National Laboratory2,University of California, Berkeley3,Arizona State University4,National Institute for Materials Science5,Stanford University6,SLAC National Accelerator Laboratory7
Medha Dandu1,Jordan Hachtel2,Mit Naik3,1,Patrick Hays4,Sriram Sankar4,Elyse Barré1,Takashi Taniguchi5,Kenji Watanabe5,Steven Louie3,1,Felipe da Jornada6,7,Sefaattin Tongay4,Peter Ercius1,Archana Raja1,Sandhya Susarla4
Lawrence Berkeley National Laboratory1,Oak Ridge National Laboratory2,University of California, Berkeley3,Arizona State University4,National Institute for Materials Science5,Stanford University6,SLAC National Accelerator Laboratory7
Artificial superlattices [1] enable self-organized arrays of interacting quasiparticles on a long-range, offering routes for dynamic simulation of many-body physics and engineering quantum optoelectronic devices. Stacking of individual layered materials offers an unprecedented bottom-up approach to design artificial superlattices with tunable light-matter interactions. Such a stacking can yield nanoscale moiré superlattices [2] modulated by interlayer hybridization and atomic reconstruction, depending on the lattice mismatch and the twist angle across the constituent materials. So far, conventional optical spectroscopy techniques have revealed the moiré superlattice effects [2] only in an indirect manner due to their diffraction limits. Localized spectroscopy analysis is an absolute necessity to directly probe the nanoscale electronic and optical modulations across moiré superlattices in layered heterostructures.<br/>Monochromated-electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) is a versatile technique that can probe spectral information with sub-nanometer spatial resolution. In the context of atomically thin layered superlattices, EELS measurements can be directly correlated with the dielectric losses and attributed to excitonic resonances deduced from the optical absorption. In a recent work [3], in-plane structural reconstruction and the corresponding exciton localization were probed using the spatial and spectral resolution in cryogenic STEM-EELS, demonstrating the possibility of direct experimental correlation between the sub-nanometer moiré superlattice and the emergent excitonic transitions. However, the distinct spectral signatures of moiré excitons could not be directly probed due to limited energy resolution. In this work, we demonstrate both spatially and spectrally resolved features of individual intralayer moiré excitons from R-stacked WS<sub>2</sub>/WSe<sub>2</sub>. Using hyperspectral EELS from multiple regions across the sample, we deconvolute the contribution of oscillator strength of moiré excitons from different lattice sites. The unique advantages of high spatial and spectral resolution from the STEM-EELS technique could be extended to understand other layered superlattices where electrically tunable layer hybridization and moiré superlattice effects can be harnessed for tunable quantum emitters.