Excitonic Linewidth Approaching the Homogeneous Limit in ${\mathrm{MoS}}_{2}$-Based van der Waals Heterostructures

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Excitonic Linewidth Approaching the Homogeneous Limit in ${MoS}_{2}$ -Based van der Waals Heterostructures

F. Cadiz, E. Courtade, C. Robert, G. Wang, Y. Shen, H. Cai, T. Taniguchi, K. Watanabe, H. Carrere, D. Lagarde, M. Manca, T. Amand, P. Renucci, S. Tongay, X. Marie, and B. Urbaszek

Phys. Rev. X 7, 021026 – Published 18 May 2017

Abstract

The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) ${MoS}_{2}$ an exciting 2D material for fundamental physics and optoelectronics applications. But, so far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the better-characterized ML materials ${MoSe}_{2}$ and ${WSe}_{2}$ . In this work, we show that encapsulation of ML ${MoS}_{2}$ in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at $T = 4 K$ . Narrow optical transition linewidths are also observed in encapsulated ${WS}_{2}$ , ${WSe}_{2}$ , and ${MoSe}_{2}$ MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.

Received 1 February 2017

DOI:https://doi.org/10.1103/PhysRevX.7.021026

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Luminescence Optoelectronics Semiconductors Valleytronics

Transition metal dichalcogenides Van der Waals systems

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

F. Cadiz^1,*, E. Courtade¹, C. Robert¹, G. Wang¹, Y. Shen², H. Cai², T. Taniguchi³, K. Watanabe³, H. Carrere¹, D. Lagarde¹, M. Manca¹, T. Amand¹, P. Renucci¹, S. Tongay², X. Marie¹, and B. Urbaszek^1,†

¹Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 Avenue de Rangueil, 31077 Toulouse, France
²School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, USA
³National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan

^*cadiz@insa-toulouse.fr
^†urbaszek@insa-toulouse.fr

Popular Summary

Two-dimensional materials, which are made of layers just a few atoms thick, offer several unique and desirable properties. Graphene, for example, has high electrical conductivity and is incredibly strong yet flexible. But graphene, a thin sheet of carbon atoms, lacks a band gap, an energy threshold that electrons need to surpass to become mobile, which is essential for creating semiconductors. A new class of 2D materials does not have this limitation. Transition-metal dichalcogenides (TMDCs), such as ${MoS}_{2}$ and ${WSe}_{2}$ , are two-dimensional semiconductors with a direct band gap in the visible region of the electromagnetic spectrum, making these materials useful for optoelectronic devices. We have developed a method for creating TMDC monolayers that avoids the limitations of previous techniques, leading to higher quality crystals.

Monolayers are typically deposited directly onto silicon substrates. Any imperfection on the substrate’s surface, however, impacts the properties of the monolayer. In our experiments, TMDC monolayers were instead “sandwiched” between ultrathin insulating layers of hexagonal boron nitride (hBN). The hBN acts like a protective barrier. As a result, light emission from these TMDCs is governed by intrinsic properties of the monolayer material and not extrinsic effects coming from the substrate or interactions with the environment. These encapsulated monolayers emit light over a narrow wavelength range, revealing the high intrinsic optical quality of these 2D crystals and allowing access to the electronic states and optical properties of these materials with unprecedented detail.

Our work opens the door for a better understanding of the physics that governs the properties of atomically thin materials for future optoelectronic devices and potential applications in spin electronics, which relies on control of the spin of electrons rather than their charge.

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Vol. 7, Iss. 2 — April - June 2017

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Figure 1
(a) Top: Optical microscope image of van der Waals heterostructure hBN/ML ${MoS}_{2} / hBN$ . Bottom: Schematic of sample (b) PL spectrum (filled curve) at $T = 4 K$ for one capped ML when excited with a 2.33-eV cw laser with a power density of $50 μ W / μ m^{2}$ . The neutral exciton transition, which is also the main feature of the differential reflectivity spectrum (red curve), exhibits a very narrow linewidth of 4.5 meV for this particular sample. Also shown is the PL spectrum of an uncapped ${MoS}_{2}$ ML deposited directly onto the ${SiO}_{2}$ substrate (black curve) measured under the same conditions. (c) PL spectrum of a capped sample for selected sample temperatures. The inset shows a color map revealing the temperature evolution of the spectrum’s intensity. The white full line tracks the evolution of the peak position according to Eq. (1), whereas the dashed lines are a guide to the eye indicating the linewidth (FWHM). The excitation density is kept as low as $1 μ W / μ m^{2}$ . (d) Temperature evolution of the neutral exciton linewidth extracted from the PL spectra when excited with $1 μ W / μ m^{2}$ . Typical error bars are $\pm 1 meV$ ( $\pm 5 meV$ ) at 4 K (300 K). The solid line is a fit according to Eq. (2), and the dashed line represents the linear term which dominates at low temperatures. (e) Linewidth extracted from the first derivative of the differential reflectivity as a function of temperature. The linear fit is consistent with a broadening induced by scattering with low-energy acoustic phonons. (f) The first derivative of the differential reflectivity for selected temperatures, centered around the $X^{0}$ absorption.
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Figure 2
(a) Typical PL spectra for different TMDC MLs at $T = 300 K$ when deposited directly onto ${SiO}_{2}$ (top) and when capped with hBN (bottom). The excitation density is $1 μ W / μ m^{2}$ . (b) Same as (a) at $T = 4 K$ .
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Figure 3
(a) Chiral optical selection rules for interband optical transitions in nonequivalent valleys $K^{+}$ and $K^{-}$ [30, 76]. (b) hBN/ML ${MoS}_{2} / hBN$ . Degree of linear PL polarization $P_{lin} = (I^{X} - I^{Y}) / (I^{X} + I^{Y})$ and circular polarization $P_{c} = (I^{σ^{+}} - I^{σ^{-}}) / (I^{σ^{+}} + I^{σ^{-}})$ for linearly and circularly polarized excitation, respectively, as a function of laser power density. Laser energy 1.96 eV, $T = 4 K$ . (c) Integrated PL intensity (open squares) and linewidth (full circles) as a function of excitation power density. The full line represents a linear relationship between intensity and power density. Laser energy 1.96 eV, $T = 4 K$ .
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Figure 4
$T = 4 K$ . hBN/ML ${MoS}_{2} / hBN$ . (a) The left-hand panel shows typical PL spectra for a capped ${MoS}_{2}$ ML under $σ^{+}$ circularly polarized excitation at selected values of the applied magnetic field $B_{z}$ . Both circularly polarized components of the PL are shown. (b) Zeeman splitting between both circular components of the PL for $σ^{+}$ and $σ^{-}$ polarized excitation at $10 μ W / μ m^{2}$ . The measured splitting corresponds to an exciton $g$ factor of $g_{X} = - 1.7 \pm 0.1$ . (c) Degree of circular polarization $P_{c}$ as a function of the magnetic field and for different incident polarization of the excitation laser. (d) Linearly polarized components of the PL spectrum after linearly polarized excitation along the $X$ direction at 633 nm. The $X^{0}$ emission is highly colinearly polarized, revealing robust valley coherence. (e) Degree of linear polarization $P_{lin}$ following linear excitation as a function of the magnetic field. (f) Schematic of the experimental configuration used to control the valley coherence. (g) PL intensity under linear excitation as a function of the angle of the linear polarization analyzer, for selected values of the magnetic field. A rotation of $\pm 5 °$ is observed for the PL polarization at $B_{z} = \pm 9 T$ .
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Figure 5
$T = 4 K$ . ML ${MoS}_{2} / {Si O}_{2} / Si$ . (a) Typical PL spectra for an acid-treated, uncapped ${MoS}_{2}$ ML at selected values of the applied magnetic field. A multiple-Lorentzian fit is employed in order to extract the Zeeman splitting of both neutral and charged exciton (trion) lines, shown in (b). Valley Zeeman splitting as a function of the applied field $B_{z}$ . Landé $g$ factors of $g_{X} = - 1.9 \pm 0.2$ and $g_{T} = - 6.6 \pm 0.2$ are found.
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Physical Review X

Excitonic Linewidth Approaching the Homogeneous Limit in MoS2-Based van der Waals Heterostructures

F. Cadiz, E. Courtade, C. Robert, G. Wang, Y. Shen, H. Cai, T. Taniguchi, K. Watanabe, H. Carrere, D. Lagarde, M. Manca, T. Amand, P. Renucci, S. Tongay, X. Marie, and B. Urbaszek

Phys. Rev. X 7, 021026 – Published 18 May 2017

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Excitonic Linewidth Approaching the Homogeneous Limit in ${MoS}_{2}$ -Based van der Waals Heterostructures