Experimental Evidence for Dark Excitons in Monolayer ${\mathrm{WSe}}_{2}$

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Experimental Evidence for Dark Excitons in Monolayer ${WSe}_{2}$

Xiao-Xiao Zhang, Yumeng You, Shu Yang Frank Zhao, and Tony F. Heinz

Phys. Rev. Lett. 115, 257403 – Published 15 December 2015

Abstract

Transition metal dichalcogenides in the class $M X_{2}$ ( $M = Mo$ , W; $X = S$ , Se) have been identified as direct-gap semiconductors in the monolayer limit. Here, we examine light emission of monolayer ${WSe}_{2}$ using temperature-dependent photoluminescence and time-resolved photoluminescence spectroscopy. We present experimental evidence for the existence of an optically forbidden dark state of the band-gap exciton that lies tens of meV below the optically bright state. The presence of the dark state is manifest in the strong quenching of light emission observed at reduced temperatures. The experimental findings are consistent with theoretical predictions of spin-polarized conduction and valence bands at the $K$ point of the Brillouin zone, with the minimum gap occurring between bands of opposite electron spin.

Received 7 July 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.257403

Physics Subject Headings (PhySH)

Excitons

Transition metal dichalcogenides

Photoluminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Xiao-Xiao Zhang¹, Yumeng You^1,2, Shu Yang Frank Zhao³, and Tony F. Heinz^4,5,*

¹Departments of Physics and Electrical Engineering, Columbia University, 538 West 120th Street, New York, New York 10027, USA
²Ordered Matter Science Research Center, Southeast University, Nanjing 211189, China
³Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
⁴Department of Applied Physics, Stanford University, Stanford, California 94305, USA
⁵SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA