Twisted MoS2 Reveals Exciton Localization

Scientists imaged moiré-confined excitons in a 2° twisted bilayer MoS2 with nanometre resolution using room-temperature photocurrent atomic force microscopy. The study shows that direct and indirect excitons localise at different stacking registries within the moiré lattice.

The imaging technique, called photocurrent AFM, scans the surface of the bilayer while measuring the photocurrent generated by locally excited electron-hole pairs. By operating at ambient temperature, the method preserves the intrinsic excitonic properties of the material and avoids the need for cryogenic setups that can obscure subtle spatial variations.

The researchers focused on a bilayer twisted by 2°, a small angle that produces a large moiré supercell. Within this supercell, the atomic registry alternates between high-symmetry sites such as AA, AB, and BA. The photocurrent maps reveal that excitons generated at a given site are trapped at the nearest potential minimum, leading to a pattern that mirrors the underlying moiré geometry.

Direct excitons, which involve electron and hole in the same layer, are found to concentrate at AA-type registries, whereas indirect excitons, involving carriers in opposite layers, prefer AB-type sites. The contrast between the two species is governed by the alignment between the excitation spot and the confinement minima, a relationship that the authors confirm by varying the laser spot position.

To interpret the experimental data, the team employed a Wannier-based effective moiré-exciton model. The model reproduces the measured energies of the excitonic transitions and captures the spatial localisation of the wavefunction within the moiré unit cell. This agreement provides a quantitative benchmark for theoretical descriptions of moiré excitons.

The authors note that the ability to resolve exciton localisation at the unit-cell level imposes stringent constraints on microscopic models. It also offers a practical route to engineering excitonic lattices that can be integrated into optoelectronic devices. The study’s code and source data are publicly available on 4TU.ResearchData, and can be cross-referenced with the stats database for further analysis. The data can also be cross-referenced with the fixtures database for additional context.

The findings have implications for a range of applications, from efficient light sources to quantum information processing. By demonstrating that excitons can be trapped at specific lattice sites, the work opens the door to programmable excitonic arrays that can be tuned by adjusting the twist angle or external fields.

The study also highlights the importance of real-space imaging techniques in revealing the microscopic behaviour of moiré systems. While previous reports inferred exciton confinement from spatially averaged far-field signals, this work provides direct, nanometre-scale evidence of site-selective trapping.

The authors acknowledge that further work is needed to explore the dynamics of exciton trapping and to extend the technique to other two-dimensional materials. They also point out that the source data underlying the figures are available at 4TU.ResearchData, and that the code used for analysis can be accessed through the same repository.

The study demonstrates that a 2° twisted bilayer MoS2 can host moiré-confined excitons that are localised at distinct stacking registries, a result that is captured in the photocurrent maps and supported by the Wannier-based model.