Diverse nature of excitonic states in transition metal dichalcogenide moiré superlattices

Moiré patterns of 2D van der Waals materials have proven to be an ideal platform to host unusual correlated electronic phases, emerging magnetism, and correlated exciton physics. The existence of novel moire exciton states in transition metal dichalcogenide heterostructures has been established through the observation of multiple emergent peaks in the optical absorption and luminescence spectra. However, despite all the experimental efforts, the origin and nature of these excitonic states remains a mystery. The atomistic details of an exciton are particularly difficult to probe in experiments due to the diffraction limit of optical measurements.

Using state-of-the-art first-principles GW-Bethe Salpeter equation calculations we discover a rich diversity of excitonic states in large-area transition metal dichalcogenide moiré superlattices. These studies, which involve thousands of atoms in the reconstructed moiré unit-cell, are made possible by a novel computational approach, the pristine unit-cell matrix projection (PUMP) method [1]. In rotationally aligned WSe2/WS2 moiré superlattice, we find some excitons of a modulated Wannier character and others of a previously unidentified intralayer charge-transfer character [1,2]. In near-60° twisted bilayer WS2, we discover layer-hybridized excitons with in-plane charge transfer character [3]. These characteristics originate from the strong modulation of electron wavefunctions due to atomic reconstructions of the moiré superlattice. Experimental reflection contrast [1], electron energy loss spectroscopy [2] and scanning tunneling spectroscopy [3] measurements confirm these predictions.

References:

[1] M. H. Naik*, E. C. Regan*, Z. Zhang*, …, F. H. da Jornada, F. Wang and S. G. Louie, “Intralayer charge-transfer moiré excitons in van der Waals superlattices”, Nature 609, 52–57 (2022)

[2] S. Susarla*, M. H. Naik*, …, F. H. da Jornada, S. G. Louie, P. Ercius and A. Raja, “Hyperspectral imaging of excitons within a moiré unit-cell with a sub-nanometer electron probe”, Science, 378, 6625, 1235-1239 (2022)

[3] H. Li*, Z. Xiang*, M. H. Naik*, .., S. G. Louie, M. F. Crommie and F. Wang, "Imaging Moiré Excited States with Photocurrent Tunneling Microscopy", Nature Materials (2024).