Infrared Interlayer Exciton Emission in ${\mathrm{MoS}}_{2}/{\mathrm{WSe}}_{2}$ Heterostructures

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Infrared Interlayer Exciton Emission in ${MoS}_{2} / {WSe}_{2}$ Heterostructures

Ouri Karni, Elyse Barré, Sze Cheung Lau, Roland Gillen, Eric Yue Ma, Bumho Kim, Kenji Watanabe, Takashi Taniguchi, Janina Maultzsch, Katayun Barmak, Ralph H. Page, and Tony F. Heinz

Phys. Rev. Lett. 123, 247402 – Published 13 December 2019

Abstract

We report light emission around 1 eV (1240 nm) from heterostructures of ${MoS}_{2}$ and ${WSe}_{2}$ transition metal dichalcogenide monolayers. We identify its origin in an interlayer exciton (ILX) by its wide spectral tunability under an out-of-plane electric field. From the static dipole moment of the state, its temperature and twist-angle dependence, and comparison with electronic structure calculations, we assign this ILX to the fundamental interlayer transition between the $K$ valleys in this system. Our findings gain access to the interlayer physics of the intrinsically incommensurate ${MoS}_{2} / {WSe}_{2}$ heterostructure, including moiré and valley pseudospin effects, and its integration with silicon photonics and optical fiber communication systems operating at wavelengths longer than 1150 nm.

Received 30 July 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.247402

Physics Subject Headings (PhySH)

Band gap Excitons Luminescence Optoelectronics

2-dimensional systems Dichalcogenides

Infrared spectroscopy Photoluminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ouri Karni^1,*, Elyse Barré², Sze Cheung Lau¹, Roland Gillen³, Eric Yue Ma^1,4, Bumho Kim⁵, Kenji Watanabe⁶, Takashi Taniguchi⁶, Janina Maultzsch³, Katayun Barmak⁷, Ralph H. Page ^1,4, and Tony F. Heinz^1,4,†

¹Department of Applied Physics, Stanford University, Stanford, California, 94305, USA
²Department of Electrical Engineering, Stanford University, Stanford, California, 94305, USA
³Department Physik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudstrasse 7, 91058 Erlangen, Germany
⁴SLAC National Accelerator Laboratory, Menlo Park, California, 94025, USA
⁵Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA
⁶National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
⁷Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA

^*Corresponding author. oulrik@gmail.com
^†Corresponding author. tony.heinz@stanford.edu

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Vol. 123, Iss. 24 — 13 December 2019

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Figure 1
Theoretical description of the electronic structure of the ${MoS}_{2} / {WSe}_{2}$ heterostructure. (a) The calculated band structure (at the DFT level of theory) unfolded onto the primitive BZ of ${MoS}_{2}$ . The color represents the degree of localization of states within the ${MoS}_{2}$ layer (light green, yellow, and red indicate stronger localization of the band states in the layer.) (b) The same for the ${WSe}_{2}$ layer. (c) Schematic representation of the band alignment in a ${MoS}_{2} / {WSe}_{2}$ heterostructure. The configuration for the momentum-direct fundamental exciton investigated in this work is shown by the dashed dark green oval. The momentum-indirect exciton involving the $Γ$ point, where the states are hybridized across the layers, is indicated by the teal oval. The arrows show charge transfer processes leading to the formation of the ILXs.
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Figure 2
Infrared emission of the ${MoS}_{2} / {WSe}_{2}$ heterostructure. (a) Room-temperature PL spectra for isolated ${MoS}_{2}$ and ${WSe}_{2}$ monolayers compared with the heterostructure in both the visible and IR ranges. Emission from ${WSe}_{2}$ (blue) is scaled down by a factor of 50 for comparison with the other spectra. The intensity scale applies for both visible and IR spectral ranges. (b) Microscope image of the corresponding heterostructure. The red, blue, and dashed black boundaries, respectively, delineate the ${MoS}_{2}$ ML, the ${WSe}_{2}$ ML, and the region of the spatial map of the 1200–1250 nm IR emission shown in the inset. (c) Temperature dependent PL spectra for unencapsulated sample U1. Temperatures range from 20 to 295 K, as indicated. (d) The same for encapsulated sample E2.
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Figure 3
(a) Schematic representation of the gated device used to apply an out-of-plane electric field to the ${MoS}_{2} / {WSe}_{2}$ heterostructure. The electron-hole pair of the ILX, split between the two different layers, gives rise to a static electric-dipole moment (green arrow) that is responsible for a linear Stark shift of the exciton energy under an applied electric field (purple arrows). (b) False-color map of the PL spectra measured as a function of voltage applied between the gates (top horizontal axis) and the corresponding electric field strength (bottom horizontal axis) within the heterostructure. The left and right vertical axes are the corresponding emission energy and wavelength. The dashed line shows a linear fit to the emission energy with applied electric field. The slope yields a dipole moment $p = 0.6 e nm$ .
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Physical Review Letters

Infrared Interlayer Exciton Emission in MoS2/WSe2 Heterostructures

Ouri Karni, Elyse Barré, Sze Cheung Lau, Roland Gillen, Eric Yue Ma, Bumho Kim, Kenji Watanabe, Takashi Taniguchi, Janina Maultzsch, Katayun Barmak, Ralph H. Page, and Tony F. Heinz

Phys. Rev. Lett. 123, 247402 – Published 13 December 2019

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Infrared Interlayer Exciton Emission in ${MoS}_{2} / {WSe}_{2}$ Heterostructures