Publication date: 8th June 2021
Transition metal dichalcogenide monolayers with the 1H structure (WS2, MoSe2, etc...) poses rich exciton and trion physics, stable up to room temperature due these excitations having large binding energies. The lower energy excitons are split due to spin-orbit coupling (up to 400 meV), which can be selectively excited by circularly polarized light as a result of the lack of inversion symmetry in these materials crystal structure. Excitons in 2D materials behave intrinsically differently than in 3D ones, as screening is significantly reduced (field lines lie mostly outside the material). For the same reason, local modifications of the dielectric environment can easily alter the energy of these transitions. Finally, from a technological perspective, these materials are intrinsically hard to produce (exfoliation and transfer processes) and keep in their pristine state.
Therefore, local probes of the optical response of these monolayers, coupled to structural and chemical information is a requirement to understand the physics behind these excitations and to produce devices aimed at controlling them. This ensemble of measurements can be performed using a fast (30 to 300 keV kinetic energy) and focused (below the nanometer scale) electron beam.
In this contribution, we will first discuss how electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) are nanoscale counterparts of optical absorption and photoluminescence. As shown in Figure 1, EELS permits to detect transitions due to the excitons and their excited states. In CL, emission related to the lowest energy exciton and associated trions are detected, along with lower energy lines linked to localized emitters.
Then, we will focus on a specific sample, an h-BN/WS2/h-BN heterostructure [1]. We will show how the local electromagnetic environment can modify the exciton/trion dynamics: i) at places with composition changes at the heterostructure interfaces, and ii) at regions close to the heterostructure carbon support film. Finally, we will show that these materials present localized light emitters, with lateral scales down to 30 nm (limited by the typical reported excitation diffusion length in these monolayers).
[1] N. Bonnet et al.,arXiv:2102.06140 (2021).
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