Hybridized intervalley moiré excitons and flat bands in twisted WSe<sub>2</sub> bilayers

Stacking monolayers of transition metal dichalcogenides into a heterostructure with a finite twist-angle gives rise to artificial moiré superlattices with a tunable periodicity. As a consequence, excitons experience a periodic potential, which can be exploited to tailor optoelectronic properties of these materials. Whereas recent experimental studies have confirmed twist-angle-dependent optical spectra, the microscopic origin of moiré exciton resonances has not been fully clarified yet. Here, we combine first-principles calculations with the excitonic density matrix formalism to study transitions between different moiré exciton phases and their impact on optical properties of the twisted MoSe 2 /WSe 2 heterostructure. At angles smaller than 2°, we find flat, moiré-trapped states for inter- and intralayer excitons. This moiré exciton phase changes into completely delocalized states at 3°. We predict a linear and quadratic twist-angle dependence of excitonic resonances for the moiré-trapped and delocalized exciton phases, respectively.

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“…The dispersions are plotted along a high-symmetry path of the hexagonal mBZ with the center γ and the edges κ = Δ K . 18 , 21 …”

Section: Resultsmentioning

confidence: 99%

Tunable Phases of Moiré Excitons in van der Waals Heterostructures

et al. 2020

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“…Decreasing the twist angle to 30°increases the interlayer distance and shifts X to the blue 25,26 , i.e., towards the energy of the A-exciton PL of single-layer WSe 2 20 . For the present analysis, we ignore signatures of intervalley excitons 6,27,28 (marked by an asterisk in Fig. 1d).…”

Section: Resultsmentioning

confidence: 99%

“…When they arise in twodimensional monolayer semiconductors, transition and binding energies become particularly sensitive to the immediate surrounding 1 . Whereas dielectric effects are straightforward to rationalize, the impact of artificial van-der-Waals stacking 2 of two monolayer crystals is dominated by the underlying electronic orbitals: electronic bands hybridize [3][4][5][6] , and moiré bands [7][8][9][10] and moiré excitons [11][12][13][14][15] form. To first order, the optoelectronic properties of transition-metal dichalcogenide (TMDC) monolayers 16 are dominated by the electronic transition between the highest-energy valence bands and the lowest conduction bands (CBs) at the K-points of the Brillouin zone.…”

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confidence: 99%

Twist-angle engineering of excitonic quantum interference and optical nonlinearities in stacked 2D semiconductors

et al. 2021

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Twist-engineering of the electronic structure in van-der-Waals layered materials relies predominantly on band hybridization between layers. Band-edge states in transition-metal-dichalcogenide semiconductors are localized around the metal atoms at the center of the three-atom layer and are therefore not particularly susceptible to twisting. Here, we report that high-lying excitons in bilayer WSe2 can be tuned over 235 meV by twisting, with a twist-angle susceptibility of 8.1 meV/°, an order of magnitude larger than that of the band-edge A-exciton. This tunability arises because the electronic states associated with upper conduction bands delocalize into the chalcogenide atoms. The effect gives control over excitonic quantum interference, revealed in selective activation and deactivation of electromagnetically induced transparency (EIT) in second-harmonic generation. Such a degree of freedom does not exist in conventional dilute atomic-gas systems, where EIT was originally established, and allows us to shape the frequency dependence, i.e., the dispersion, of the optical nonlinearity.

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“…The spatially dependent, periodic moiré potential gives rise to arrays of trapping potentials and can lead to flat excitonic bands and even superconductivity, as recently shown for twisted bilayer graphene. [ 128 ] Exciton formation, thermalization, and decay dynamics, the hybridization effects of intra and interlayer excitons, [ 129 ] exciton diffusion, [ 211 ] and dissociation at interfaces in BPNS/TMD heterostructures are additional topics that need to be explored in future studies. In terms of electronic and optoelectronic applications, tunable bandgaps covering infrared to visible frequencies, high carrier mobilities, and large on/off ratios at room temperature lift BPNS‐based heterostructures out of the shadow of graphene and TMDs.…”

Section: Discussionmentioning

confidence: 99%

The Art of Constructing Black Phosphorus Nanosheet Based Heterostructures: From 2D to 3D

et al. 2020

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Assembling different kinds of 2D nanosheets into heterostructures presents a promising way of designing novel artificial materials with new and improved functionalities by combining the unique properties of each component. In the past few years, black phosphorus nanosheets (BPNSs) have been recognized as a highly feasible 2D material with outstanding electronic properties, a tunable bandgap, and strong in‐plane anisotropy, highlighting their suitability as a material for constructing heterostructures. In this study, recent progress in the construction of BPNS‐based heterostructures ranging from 2D hybrid structures to 3D networks is discussed, emphasizing the different types of interactions (covalent or noncovalent) between individual layers. The preparation methods, optical and electronic properties, and various applications of these heterostructures—including electronic and optoelectronic devices, energy storage devices, photocatalysis and electrocatalysis, and biological applications—are discussed. Finally, critical challenges and prospective research aspects in BPNS‐based heterostructures are also highlighted.

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Hybridized intervalley moiré excitons and flat bands in twisted WSe₂ bilayers

Abstract: Twisted WSe2 bilayers exhibit hybridized K–Λ excitons with flat moiré bands, which become visible through phonon-assisted photoluminescence.

Cited by 78 publications

References 50 publications

Tunable Phases of Moiré Excitons in van der Waals Heterostructures

Tunable Phases of Moiré Excitons in van der Waals Heterostructures

Twist-angle engineering of excitonic quantum interference and optical nonlinearities in stacked 2D semiconductors

The Art of Constructing Black Phosphorus Nanosheet Based Heterostructures: From 2D to 3D

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Hybridized intervalley moiré excitons and flat bands in twisted WSe2 bilayers

Abstract: Twisted WSe2 bilayers exhibit hybridized K–Λ excitons with flat moiré bands, which become visible through phonon-assisted photoluminescence.

Cited by 78 publications

References 50 publications

Tunable Phases of Moiré Excitons in van der Waals Heterostructures

Tunable Phases of Moiré Excitons in van der Waals Heterostructures

Twist-angle engineering of excitonic quantum interference and optical nonlinearities in stacked 2D semiconductors

The Art of Constructing Black Phosphorus Nanosheet Based Heterostructures: From 2D to 3D

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Hybridized intervalley moiré excitons and flat bands in twisted WSe₂ bilayers