Strain localization of excitons in 2D semiconductors investigated by Raman & PL microscopies

Stanford AFM Seminar Series, organized by nano@stanford and in collaboration with HORIBA Scientific. Webinar recorded April 10, 2020. Please note, some material has been redacted by the request of the speaker.

Strain localization of excitons in 2D semiconductors investigated by nanoscale Raman and PL microscopies
Thomas Darlington, Columbia University

The monolayer transition metal dichalcogenides (TMDs) are an atomically thin family of semiconductors that have attracted immense scientific interest, particularly in regard to their strong light-matter interactions from tightly bound exciton states. Their maximally thin character further allows few layer TMDs to endure huge strains on the order of 10%, far above the fracture point of bulk semiconductors. Studies on the interplay of strain, both uniaxial and biaxial, with the TMD excitons have shown great tunability of the emission properties such as their emission energy and coupling to phonons. When the application of strain is inhomogeneous, excitons have been shown to localize and become single photon sources at cryogenic temperatures. Despite the strong interest in the strain and its effect on excitons, many of the details remain unclear, arising from the scale mismatch of conventional optical microscopes (300-1000 nm) and the exciton (1 – 50 nm) in TMDs.

In this talk I will present our efforts to bridge this gap using nanoscopic photoluminescence and Raman scattering studies of inhomogeneous strain in TMDs. By mapping the nano-PL and nano-Raman response of naturally occurring nanoscale bubbles (nanobubbles) in several of the canonical group semiconducting TMDs, I will show that the strain of the bubble can strongly localize excitons around the periphery of nanobubbles in monolayers. Combining our experimental results with advanced theoretical modeling we show the localization arises from bending and wrinkling occurring at the nanobubble edges. Following up this work on nanobubbles in heterostructures of WSe2 and MoSe2, we find strong localization extends to the interlayer exciton, localized in some cases to regions less than 3 nm, with unexpectedly strong emission intensities at room temperature. This points to a potentially important experimental challenge in the study of these states for future optoelectronic devices.