Strain Control of Exciton–Phonon Coupling in Atomically Thin Semiconductors

• Published in: Nano letters Vol. 18; no. 3; pp. 1751 - 1757
• Main Authors: Niehues, Iris, Schmidt, Robert, Drüppel, Matthias, Marauhn, Philipp, Christiansen, Dominik, Selig, Malte, Berghäuser, Gunnar, Wigger, Daniel, Schneider, Robert, Braasch, Lisa, Koch, Rouven, Castellanos-Gomez, Andres, Kuhn, Tilmann, Knorr, Andreas, Malic, Ermin, Rohlfing, Michael, Michaelis de Vasconcellos, Steffen, Bratschitsch, Rudolf
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 14-03-2018
• Subjects: exciton-phonon coupling; excitons; line width; strain; Transition metal dichalcogenide; strain; excitons; exciton−phonon coupling; Transition metal dichalcogenide; line width
• ISSN: 1530-6984; 1530-6992; 1530-6992
• DOI: 10.1021/acs.nanolett.7b04868

Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton–phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe2, WSe2, WS2, and MoS2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS2. For MoS2 monolayers, the line width increases. These effects are due to a modified exciton–phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton–phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale.