Atomically thin transition metal dichalcogenides (TMDs), such a MoS₂ and WSe₂, strongly interact with light. Their optical properties are governed by excitons, electrons and holes bound by Coulomb attraction, that remain stable at room temperature. The ability to additionally tune their transition energies is essential for various interesting opto-electronic applications. This can in principle be achieved via the quantum-confined Stark effect by applying an electric field perpendicular to the sample plane.

In a single-layer MoS₂, absorption can reach up to 100 %, but the optical transitions cannot be tuned in energy in devices with electrodes. Here, the carriers are confined to one single layer – these excitons have essentially no out-of-plane electric dipole. This demands to introduce a certain vertical distance between the electron and hole, forming electric dipole along the applied electric field direction.

 Experiments at the University of Basel and at LPCNO Toulouse, supported by beyond DFT calculations, have now uncovered optical transitions with strong absorption and wide tunability in the visible range of the optical spectrum in MoS₂ bilayers. The main idea is to use transitions based on electrons and holes that do not reside in the same but in different layers, so called interlayer excitons. The separation between the electrons and the holes gives rise to a static electric dipole (Fig. a), and allows therefore tuning these transitions in applied electric fields. For practical device integration the researchers in Basel and Toulouse use atomically thin electrodes based on few-layer graphene (Fig. b). To find out which transition is tunable and which transition is not, careful comparison between the theoretically predicted localization of the carriers (in one of the layers, L₁ or L₂, or delocalized over both layers) and the experiments in applied magnetic and electric fields was crucial.

The existence of an in-built dipole makes interlayer excitons not only tunable, but also introduces interactions between them (strong dipole-dipole interactions) (Fig. c). In the future it will be interesting to couple these excitons to optical microcavity modes (as recently demonstrated for III-V semiconductors in Nat. Mater. 18, 213–218 (2019)., Nat. Mater. 18, 219–222 (2019).), where strong interactions between the particles are needed to work towards single polariton switches.

This work was recently published in Nature Nanotechnology:

Giant Stark splitting of an exciton in bilayer MoS₂

Nadine Leisgang, Shivangi Shree, Ioannis Paradisanos, Lukas Sponfeldner, Cedric Robert, Delphine Lagarde, Andrea Balocchi, Kenji Watanabe, Takashi Taniguchi, Xavier Marie, Richard J. Warburton, Iann C. Gerber, and Bernhard Urbaszek

Nature Nanotechnology (2020), https://www.nature.com/articles/s41565-020-0750-1