Towards In-Plane Control of Neutral Exciton Flux in Semiconducting 2D Materials

For a wide range of next-generation applications in excitonic circuits, quantum optics and optoelectronics, neutral excitons in semiconducting 2D materials need to be controlled. Until now, and in particular in transition metal dichalcogenides, such control has been achieved through the local strain gradient engineering. However, the charge-neutral nature of the dominant exciton types in such materials limits their interaction with external electric or magnetic fields, still making their manipulation a challenging task. Recently, we demonstrated efficient unidirectional transport of excitons from high-gap to low-gap material at room-temperature in high-quality lateral heteromonolayer [1]. Tip enhanced Raman and Photoluminescence spectroscopies (∼30nm resolution) performed on high-quality heteromonolayers reveal that excitons generated inside the high-gap material are able to pass through the junction before recombining inside the low-gap material, while propagation in the opposite direction is forbidden. Going further, we have shown that the abrupt change in exciton energy and effective mass has resulted in a strong Kapitza resistance-like effect, allowing excitons to be accelerated locally at the junction as they move from high-gap to low-gap material while being blocked in the opposite direction [2]. This effect causes a strong discontinuity in the excitonic density profile at the junction. As a result, a quenching of high-gap material-related PL is observed while low-gap material-related PL is enhanced near the junction. Finally, by using µ-Photoluminescence(µ-PL) imaging combined with a simulation-based statistical approach of randomly moving excitons through specific geometries of a high-quality CVD-grown MoSe₂-WSe₂ lateral heteromonolayer, we demonstrated that the exciton Kapitza effect enhances the exciton diffusion length, and is able to focus and confine excitons down to sub-wavelength dimensions [3]. We believe that our work provides new tools for manipulating neutral excitons in flat semiconducting 2D materials and will enable the incorporation of lateral heteromonolayers in next-generation excitonic devices.<br/><br/><b>References</b><br/>[1] Dorian Beret, Ioannis Paradisanos, <u>Hassan Lamsaadi</u>, et al. Exciton spectroscopy and unidirectional transport in MoSe₂-WSe₂lateral heterostructures encapsulated in hexagonal boron nitride. <b>npj 2D Materials and Applications</b>, 6(1):84, 2022.<br/>[2] <u>Hassan Lamsaadi</u> et al. Kapitza-resistance-like in flat MoSe₂-WSe₂ lateral heterjunction. <b>Nature communications</b>, 5881(1):84, 2023.<br/>[3] <u>Hassan Lamsaadi</u> et al. Towards in-plane control of neutral exciton flux in transition metal dichalcogenide monolayers. in preparation.