Researchers have resolved the site-selective confinement of moiré excitons in a 2° twisted bilayer of molybdenum disulfide with nanometre resolution at room temperature, according to a study published in Nature Physics.
The team used photocurrent atomic force microscopy to observe the excitons, which are bound electron–hole pairs, inside individual moiré unit cells. The measurements revealed that direct and indirect excitons localize at different stacking registries across the moiré landscape. The spatial contrast in the measurements is governed by the alignment between site-selective exciton generation and confinement energy minima.
In the experiments, the researchers mapped indirect exciton channels at 1.46 eV and 1.63 eV alongside direct A1s excitons at 1.85 eV and B1s excitons at 2.03 eV. Fourier transform analysis revealed that the direct and indirect exciton channels display spatial anticorrelation across high-symmetry moiré sites. Earlier investigations had largely inferred moiré exciton behavior from spatially averaged far-field signals that could not resolve variations at the nanometre scale.
A Wannier-based effective moiré-exciton model captured the measured energy levels and the moiré-induced localization of the exciton wavefunction. Pantelis Bampoulis of the University of Twente conceived the project and supervised the experiments. Laurens J. M. Westenberg and Jort D. Verbakel prepared the twisted bilayer samples at the University of Twente, while Lumen Eek, Raphael Rossi Alves, and Cristiane Morais Smith carried out the theoretical calculations.
Hexagonal boron nitride crystals used in the research were synthesized by Takashi Taniguchi and evaluated by Kenji Watanabe at Japan's National Institute for Materials Science. The underlying experimental data and calculation code were published through the 4TU.ResearchData repository under DOI 10.4121/6f789d27-4204-48d4-bfaf-a330b7d7950c.
