Atomically thin transition-metal dichalcogenides (TMDs) such as MoS2 and WSe2 have a direct bandgap at the K-points of the Brillouin zone in the visible region of the optical spectrum. These are widely investigated materials for optoelectronics and spintronics. Their light matter interaction is governed by Coulomb bound electron-hole pairs (excitons). First, I will discuss the impact of the local dielectric environment on the optical transitions and how we can largely suppress the dielectric disorder in chemical vapor deposition (CVD)-grown MoS2 monolayers by encapsulation [1, 2]. The considerably improved optical sample quality allows us to inves- tigate interlayer coupling in CVD-grown multilayers .
The light-matter interaction is strongly modified when going from mono- to multilayers. Heterobilayers host new type of excitons named ‘interlayer excitons’ (IEs) where electrons and holes are residing in different layers. The main features of heterobilayers such as MoSe2/WSe2 are: a periodic moir ́e potential for localized emitters , and IEs with an in-built static electric dipole but weak optical oscillator strength. In our work we aim to combine tuning of exciton transition energies in applied electric fields with strong absorption and 2H stacked multilayers of MoS2 are ideal systems for investing both. We show in homo-multilayer systems that IE transitions are visible in absorption upto room temperature . We determine a correlation between the stacking order and interlayer coupling in artificially stacked and as-grown CVD layers . We show that hole delocalization over the bilayer is only allowed in 2H stacking and results in strong interlayer exciton absorption and also in a modified valence band spin splitting as compared to 3R bilayers. In gated multilayer MoS2 we measure a giant IE Stark splitting and a widely tunable energy shift in applied electric fields due to the in-built static electric dipole . We show strong coupling between intra- and interlayer excitons when they are energetically tuned into resonance. In trilayer MoS2, we uncover two distinct types of IEs in magneto-optics with very different in-built static electric dipoles.
(1) S. Shree, A. George, T. Lehnert, C. Neumann et al. 2D Materials 7, 1 (2019).
(2) S. Shree, I. Paradisanos, X. Marie, C. Robert et al. arXiv:2006.16872 (2020).
(3) I. Paradisanos, S. Shree, A. George, N. Leisgang et al. Nature Comms 11, 2391 (2020).
(4) Kyle L. Seyler, Pasqual Rivera, Hongyi Yu et al. Nature 567, 66–70(2019).
(5) Iann C. Gerber, E. Courtade, S. Shree et al. Phys. Rev. B. 99, 035443 (2019).
(6) N. Leisgang, S. Shree, I. Paradisanos, L. Sponfeldner et al. arXiv:2002.02507 (2020) and (in press).
This work is done in fruitful collaboration with NIMS Tsukuba, Basel University, Ulm University and Friedrich- Schiller University Jena.