Spectroscopic Signatures for Interlayer Coupling in MoS<sub>2</sub>–WSe<sub>2</sub> van der Waals Stacking

Chiu, Ming Hui; Li, Ming Yang; Zhang, Wengjing; Hsu, Wei-Ting; Chang, Wen‐Hao; Terrones, Mauricio; Terrones, Humberto; Li, Lain‐Jong

doi:10.1021/nn504229z

Cited by 304 publications

(332 citation statements)

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“…The second and third terms describe respectively the light coupling mediated by bright intralayer exciton X τ in MoX 2 layer through hole interlayer hopping, and X τ in WX 2 through electron hopping. E M and E W are the energies of the intralayer excitons, a few hundred meV above E I as determined from optical and scanning tunneling measurements [19][20][21]30]. D τ τ,Q can be extracted from ab initio calculations at several commensurate stackings [40], from which we can determine the dipole strength, and the ellipticity and major axes of the emission polarization of the six-fold light cones at a general twist angle.…”

Section: M0mentioning

confidence: 99%

“…MoX 2 /WX 2 (X=Se, S) heterobilayers have been realized [19][20][21][22][23][24][25], which feature a type-II band alignment with the conduction (valence) band edges located in MoX 2 (WX 2 ) layer. Exciton then has the lowest energy in an interlayer configuration (i.e.…”

mentioning

confidence: 99%

“…Exciton then has the lowest energy in an interlayer configuration (i.e. electron and hole in different layers), from which luminescence is observed [19][20][21]. Due to the spatially indirect nature, interlayer excitons in MoSe 2 /WSe 2 heterobilayers have shown long lifetime exceeding nanosecond, repulsive interaction, and electrostatically tunable resonance [19], all of which are highly desirable for the realization of excitonic circuits and condensation [26][27][28][29].…”

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Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers

et al. 2015

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We show that, because of the inevitable twist and lattice mismatch in heterobilayers of transition metal dichalcogenides, interlayer excitons have six-fold degenerate light cones anomalously located at finite velocities on the parabolic energy dispersion. The photon emissions at each light cone are elliptically polarized, with major axis locked to the direction of exciton velocity, and helicity specified by the valley indices of the electron and the hole. These finite-velocity light cones allow unprecedented possibilities to optically inject valley polarization and valley current, and the observation of both direct and inverse valley Hall effects, by exciting interlayer excitons. Our findings suggest potential excitonic circuits with valley functionalities, and unique opportunities to study exciton dynamics and condensation phenomena in semiconducting 2D heterostructures.PACS numbers: 74.78. Fk, 78.67.Pt, 72.25.Fe Monolayers of group-VIB transition metal dichalcogenides (TMDs) have recently emerged as a new class of direct-gap semiconductors in the two-dimensional (2D) limit [1][2][3][4][5]. These hexagonal 2D crystals have exotic properties associated with the valley degeneracy of the band edges, including the valley Hall effect [6,7], the valley magnetic moment [8][9][10][11], and the valley optical selection rules [6,[12][13][14][15][16], leading to rich possibilities for valley-based device applications. The visible range bandgap further makes these 2D semiconductors ideal platforms for optoelectronics. Due to the strong Coulomb interaction, the optical response is dominated by excitons, the hydrogen-like bound state of an electron-hole pair. The demonstrated electrostatic tunability and optical controllability of valley configurations of excitons in monolayer TMDs have implied new optoelectronic device concepts not possible in other material systems [13][14][15][16][17].Stacking different TMDs monolayers to form van der Waals heterostructures opens up a new realm to extend their already extraordinary properties [18]. MoX 2 /WX 2 (X=Se, S) heterobilayers have been realized [19][20][21][22][23][24][25], which feature a type-II band alignment with the conduction (valence) band edges located in MoX 2 (WX 2 ) layer. Exciton then has the lowest energy in an interlayer configuration (i.e. electron and hole in different layers), from which luminescence is observed [19][20][21]. Due to the spatially indirect nature, interlayer excitons in MoSe 2 /WSe 2 heterobilayers have shown long lifetime exceeding nanosecond, repulsive interaction, and electrostatically tunable resonance [19], all of which are highly desirable for the realization of excitonic circuits and condensation [26][27][28][29]. An unprecedented aspect of this interlayer exciton system is that the heterobilayers in general have incommensurate structures due to lattice mismatch and twist in the stacking which, together with the valley physics inherited from the monolayers, bring in radically new properties.Here we discover anomalous light coupling prope...

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confidence: 99%

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Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers

et al. 2015

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“…However, band gaps are different enough to generate band mismatch in heterostructures. We can combine TMDs in vertical heterostructures for quantum well growth [49,173,194], with the purpose of selective confinement of photogenerated excitons. Figure 22 shows the BSE spectra of single-layer MoSe 2 , WS 2 , and WSe 2 (solid lines) and the RPA spectra (dashed lines).…”

Section: Optical Spectra Of Other Transition Metal Dichalcogenidesmentioning

confidence: 99%

Vibrational and optical properties of MoS2: From monolayer to bulk

Molina-Sánchez

Hummer

Wirtz

2015

Surface Science Reports

216

167

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Molybdenum disulfide, MoS 2 , has recently gained considerable attention as a layered material where neighboring layers are only weakly interacting and can easily slide against each other. Therefore, mechanical exfoliation allows the fabrication of single and multi-layers and opens the possibility to generate atomically thin crystals with outstanding properties. In contrast to graphene, it has an optical gap of 1.9 eV. This makes it a prominent candidate for transistor and opto-electronic applications. Single-layer MoS 2 exhibits remarkably different physical properties compared to bulk MoS 2 due to the absence of interlayer hybridization. For instance, while the band gap of bulk and multi-layer MoS 2 is indirect, it becomes direct with decreasing number of layers. In this review, we analyze from a theoretical point of view the electronic, optical, and vibrational properties of single-layer, few-layer and bulk MoS 2 . In particular, we focus on the effects of spinorbit interaction, number of layers, and applied tensile strain on the vibrational and optical properties. We examine the results obtained by different methodologies, mainly ab initio approaches. We also discuss which approximations are suitable for MoS 2 and layered materials. The effect of external strain on the band gap of single-layer MoS 2 and the crossover from indirect to direct band gap is investigated. We analyze the excitonic effects on the absorption spectra. The main features, such as the double peak at the absorption threshold and the high-energy exciton are presented. Furthermore, we report on the phonon dispersion relations of single-layer, few-layer and bulk MoS 2 . Based on the latter, we explain the behavior of the Raman-active A 1g and E 1 2g modes as a function of the number of layers. Finally, we compare theoretical and experimental results of Raman, photoluminescence, and optical-absorption spectroscopy.

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“…In prior studInfluence of the substrate material on the optical properties of tungsten diselenide monolayers 2 S Lippert et al ies on WSe 2 , layers were deposited on SiO 2 /Si substrates [41,42], sapphire [43,44], graphene [45], and fused silica (quartz) [34,46] or sandwiched between layers of hBN [47]. Based on the literature on WSe 2 and its properties, it has been identified as an ideal/ suitable testbed for investigating the impact of substrate properties on its excitonic species.…”

Section: Introductionmentioning

confidence: 99%

Influence of the substrate material on the optical properties of tungsten diselenide monolayers

et al. 2017

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Monolayers of transition-metal dichalcogenides such as WSe 2 have become increasingly attractive due to their potential in electrical and optical applications. Because the properties of these 2D systems are known to be affected by their surroundings, we report how the choice of the substrate material affects the optical properties of monolayer WSe 2 . To accomplish this study, pump-density-dependent micro-photoluminescence measurements are performed with time-integrating and time-resolving acquisition techniques. Spectral information and power-dependent mode intensities are compared at 290K and 10K for exfoliated WSe 2 on SiO 2 /Si, sapphire (Al 2 O 3 ), hBN/Si 3 N 4 /Si, and MgF 2 , indicating substrate-dependent appearance and strength of exciton, trion, and biexciton modes. Additionally, one CVD-grown WSe 2 monolayer on sapphire is included in this study for direct comparison with its exfoliated counterpart. Time-resolved micro-photoluminescence shows how radiative decay times strongly differ for different substrate materials. Our data indicates exciton-exciton annihilation as a shortening mechanism at room temperature, and subtle trends in the decay rates in correlation to the dielectric environment at cryogenic temperatures. On the measureable time scales, trends are also related to the extent of the respective 2D-excitonic modes' appearance. This result highlights the importance of further detailed characterization of exciton features in 2D materials, particularly with respect to the choice of substrate.

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Spectroscopic Signatures for Interlayer Coupling in MoS₂–WSe₂ van der Waals Stacking

Cited by 304 publications

References 60 publications

Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers

Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers

Vibrational and optical properties of MoS2: From monolayer to bulk

Influence of the substrate material on the optical properties of tungsten diselenide monolayers

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Spectroscopic Signatures for Interlayer Coupling in MoS2–WSe2 van der Waals Stacking

Cited by 304 publications

References 60 publications

Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers

Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers

Vibrational and optical properties of MoS2: From monolayer to bulk

Influence of the substrate material on the optical properties of tungsten diselenide monolayers

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Spectroscopic Signatures for Interlayer Coupling in MoS₂–WSe₂ van der Waals Stacking