Observation of the Interlayer Exciton Gases in WSe<sub>2</sub> -p:WSe<sub>2</sub> Heterostructures

Sun, Zheng; Beaumariage, Jonathan; Cao, Qingrui; Watanabe, Kenji; Taniguchi, Takashi; Hunt, Benjamin; Snoke, D. W.

doi:10.1021/acsphotonics.0c00476

Cited by 8 publications

(3 citation statements)

References 35 publications

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“…2H-stacked bilayer WSe 2 (Figure a,b) is a distinctive material to realize such a novel tunable electron–phonon system. First, it hosts two competing low-lying excitons, namely, the QK and QΓ intervalley excitons ( X QK , X QΓ ), which are associated with one Γ valence valley at the zone center, two Κ valence valleys (K, K′) at the zone corners, and six Q conduction valleys (Q, Q′) in the Brillouin zone (Figure c,d). − Although the electronic structure suggests the QK exciton as the lowest-lying exciton, recent theoretical research shows that the QΓ exciton may have similar energy due to the large effective mass of the Γ valley . Such competing intervalley excitons can hardly be found in other transition metal dichalcogenides (TMDs), such as MoS 2 , MoSe 2 , and WS 2 .…”

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confidence: 98%

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe₂

et al. 2022

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We report the observation of QΓ intervalley exciton in bilayer WSe2 devices encapsulated by boron nitride. The QΓ exciton resides at ∼18 meV below the QK exciton. The QΓ and QK excitons exhibit different Stark shifts under an out-of-plane electric field due to their different interlayer dipole moments. By controlling the electric field, we can switch their energy ordering and control which exciton dominates the luminescence of bilayer WSe2. Remarkably, both QΓ and QK excitons exhibit unusually strong two-phonon replicas, which are comparable to or even stronger than the one-phonon replicas. By detailed theoretical simulation, we reveal the existence of numerous (≥14) two-phonon scattering paths involving (nearly) resonant exciton–phonon scattering in bilayer WSe2. To our knowledge, such electric-field-switchable intervalley excitons with strong two-phonon replicas have not been found in any other two-dimensional semiconductors. These make bilayer WSe2 a distinctive valleytronic material with potential novel applications.

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confidence: 98%

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe₂

et al. 2022

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“…We note that this optimal N can vary with spacer index and thickness. Given that electronic interactions between two TMDC layers are non-negligible at spacer thicknesses of 1-1.5 nm 22,23 , there is little room for further improvement in light trapping while maintaining electronic isolation. The complex refractive index values used in the TMM simulations were obtained through spectroscopic ellipsometry measurements of the samples that were used for the fabrication of the superlattices.…”

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confidence: 99%

Light–matter coupling in large-area van der Waals superlattices

et al. 2021

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Two-dimensional (2D) crystals have renewed opportunities in design and assembly of arti cial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high delity. Recent availability of uniform, waferscale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. We present optical dispersion engineering in a superlattice structure comprised of alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate > 90 % narrowband absorption in < 4 nm active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in cm 2 samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tunable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically-thin layers.

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“…For example, CT between two graphene layers separated by an hBN barrier , has been utilized in tunneling field-effect transistors . Monolayer and few-layer hBN have been used to control the charge and energy transfer between TMD and graphene − and between two TMD monolayers. − Besides insulating hBN or Al 2 O 3 , which are often energy barriers for both the electrons and the holes, semiconducting TMDs offer more design flexibilities since they can act as energy barriers for one type of the carriers while allowing efficient transfer of the other type, as has been illustrated in recent experiments on TMD/TMD/graphene and TMD trilayer heterostructures. − …”

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confidence: 99%

Time-Resolved Observation of Hole Tunneling in van der Waals Multilayer Heterostructures

Zhang

Chang

et al. 2021

ACS Appl. Mater. Interfaces

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We reported a time-resolved study of quantum-mechanical tunneling of holes between two MoSe2 monolayers that are separated by a monolayer WS2 energy barrier. Four-layer heterostructures of MoSe2/WS2/MoSe2/graphene, as well as control samples, were fabricated by mechanical exfoliation and dry transfer techniques. To time-resolve the hole tunneling process, an ultrashort laser pulse was used to excite electrons and holes in both MoSe2 layers. By utilization of the graphene layer to eliminate carriers in the third MoSe2 layer, the first MoSe2 layer is selectively populated with the holes, which then tunnel to the third MoSe2 layer. By monitoring decay of the hole population with an ultrashort probe pulse, we measure a hole tunneling time of about 20 ps, which is found to slightly increase with the injected carrier density. Besides the fundamental interests of real-time observation of the quantum-mechanical tunneling effect across a nanometer barrier, these results provide quantitative understanding on tunneling mechanisms of charge transfer in van der Waals heterostructures, which is useful for designing sophisticated van der Waals multilayer heterostructures.

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Observation of the Interlayer Exciton Gases in WSe₂ -p:WSe₂ Heterostructures

Cited by 8 publications

References 35 publications

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe₂

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe₂

Light–matter coupling in large-area van der Waals superlattices

Time-Resolved Observation of Hole Tunneling in van der Waals Multilayer Heterostructures

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Observation of the Interlayer Exciton Gases in WSe2 -p:WSe2 Heterostructures

Cited by 8 publications

References 35 publications

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe2

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe2

Light–matter coupling in large-area van der Waals superlattices

Time-Resolved Observation of Hole Tunneling in van der Waals Multilayer Heterostructures

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Observation of the Interlayer Exciton Gases in WSe₂ -p:WSe₂ Heterostructures

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe₂

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe₂