Negative effective excitonic diffusion in monolayer transition metal dichalcogenides

• Published in: Nanoscale Vol. 12; no. 1; pp. 356 - 363
• Main Authors: Rosati, Roberto, Perea-Causín, Raül, Brem, Samuel, Malic, Ermin
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 07.01.2020
• Subjects: Chalcogenides; Diffusion; Excitons; Halos; Low temperature; Monolayers; Optoelectronics; Quantum mechanics; Temperature dependence; Transition metal compounds
• ISSN: 2040-3364; 2040-3372; 2040-3372
• DOI: 10.1039/c9nr07056g

While exciton relaxation in monolayers of transition metal dichalcogenides (TMDs) has been intensively studied, spatial exciton diffusion has received only a little attention - in spite of being a key process for optoelectronics and having already shown interesting unconventional behaviours ( e.g. spatial halos). Here, we study the spatiotemporal dynamics in TMD monolayers and track optically excited excitons in time, momentum, and space. In particular, we investigate the temperature-dependent exciton diffusion including the remarkable exciton landscape constituted by bright and dark states. Based on a fully quantum mechanical approach, we show at low temperatures an unexpected negative effective diffusion characterized by a shrinking of the spatial exciton distributions. This phenomenon can be traced back to the existence of dark exciton states in TMD monolayers and is a result of an interplay between spatial exciton diffusion and intervalley exciton-phonon scattering. The bright exciton propagation in monolayers of transition metal dichalcogenides (TMDs) shows at low temperatures a shrinking of the spatial profile, i.e. negative effective diffusion due to intervalley scattering in the TMDs excitonic landscape.