Measurement of high exciton binding energy in the monolayer transition-metal dichalcogenides WS2 and WSe2

Abstract

Monolayer transition-metal dichalcogenides are direct gap semiconductors with great promise for optoelectronic devices. Although spatial correlation of electrons and holes plays a key role, there is little experimental information on such fundamental properties as exciton binding energies and band gaps. We report here an experimental determination of exciton excited states and binding energies for monolayer WS2 and WSe2. We observe peaks in the optical reflectivity/absorption spectra corresponding to the ground- and excited-state excitons (1s and 2s states). From these features, we determine lower bounds free of any model assumptions for the exciton binding energies as E2sA - E1sA of 0.83 eV and 0.79 eV for WS2 and WSe2, respectively, and for the corresponding band gaps Eg >= E2sA of 2.90 and 2.53 eV at 4K. Because the binding energies are large, the true band gap is substantially higher than the dominant spectral feature commonly observed with photoluminescence. This information is critical for emerging applications, and provides new insight into these novel monolayer semiconductors.

15 page manuscript, 5 figures, 10 page supplemental information

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• Many-body effects in nonlinear optical responses of 2D layered semiconductors
• Exciton formation assisted by longitudinal optical phonons in monolayer transition metal dichalcogenides
• Valley-Polarized Exciton Dynamics in Exfoliated Monolayer WSe${}_2$
• Phonon-assisted exciton and trion conversion efficiency in transition metal Dichalcogenides
• Deterministic direct growth of WS2 on CVD graphene arrays
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• Optical signatures of exciton-polarons from diagrammatic Monte Carlo