Experimental Evidence for Dark Excitons in Monolayer<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>WSe</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>

Zhang, Xiao-Xiao; You, Yu‐Meng; Zhao, Shu Yang Frank; Heinz, Tony F.

doi:10.1103/physrevlett.115.257403

Cited by 417 publications

(333 citation statements)

References 63 publications

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“…In addition, relaxation of the exciton population toward intervalley K−K′ 47 and K-Q/Σ states 30 is possible. However, due to the similar binding energies of the different states and the broadening of the measured 1s−2p resonance, the excitons in our experiment should be generally considered as a mixture of these configurations.…”

Section: Nano Lettersmentioning

confidence: 99%

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂

et al. 2017

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Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like states called excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron−hole pairs in single monolayers of WSe 2 on a diamond substrate following femtosecond nonresonant optical excitation. To this end, phaselocked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductivity of the nonequilibrium system and to discern the hallmark low-energy responses of bound and unbound pairs. While the spectral shape of the infrared response immediately after above-bandgap injection is dominated by free charge carriers, up to 60% of the electron−hole pairs are bound into excitons already on a subpicosecond time scale, evidencing extremely fast and efficient exciton formation. During the subsequent recombination phase, we still find a large density of free carriers in addition to excitons, indicating a nonequilibrium state of the photoexcited electron−hole system. KEYWORDS: Dichalcogenides, atomically thin 2D crystals, exciton formation, ultrafast dynamics A tomically thin transition metal dichalcogenides (TMDCs) have attracted tremendous attention due to their direct bandgaps in the visible spectral range, 1,2 strong interband optical absorption, 3,4 intriguing spin-valley physics, 5−7 and applications as optoelectronic devices. 8−11 The physics of twodimensional (2D) TMDCs are governed by strong Coulomb interactions owing to the strict quantum confinement in the out-of-plane direction and the weak dielectric screening of the environment. 12,13 Electrons and holes in these materials can form excitons with unusually large binding energies of many hundreds of millielectronvolts, 14−19 making these quasiparticles stable even at elevated temperatures and high carrier densities. 20,21 The properties of excitons in 2D TMDCs are a topic of intense research, investigating, for example, rapid exciton−exciton scattering, 22 interlayer excitons, 23 charged excitons and excitonic molecules, 24,25 ultrafast recombination dynamics, 19,26−28 or efficient coupling to light and lattice vibrations. 4,19,29,30 In many experiments, excitons are created indirectly through nonresonant optical excitation or electronic injection, which may prepare unbound charge carriers with energies far above the exciton resonance. 8,18 Subsequently, the electrons and holes are expected to relax toward their respective band minima and form excitons in the vicinity of the fundamental energy gap. In principle, strong Coulomb attraction in 2D TMDCs should foster rapid exciton formation. Recent optical pump−probe studies relying on interband transitions have reported characteristic formation times on subpicosecond time-scales. 31 The relaxation of large excess energies, however, requires many sca...

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Section: Nano Lettersmentioning

confidence: 99%

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂

et al. 2017

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“…In addition, WSe 2 possesses good luminescence at room temperature (RT) and reasonable emission at low temperatures (LT). Finally, prior studies have confirmed the existence of excitons, trions [34,38,48] and more recently even biexcitons [49] and dark excitons [50]. Ultimately, the role of the dielectric environment and surface properties on excitons shall be unravelled in detail soon.…”

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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“…The corresponding transitions are denoted as K À K 0 and K À L in the following. In a recent time-resolved and temperaturedependent photoluminescence study, the existence of such dark intervalley excitons was experimentally demonstrated 14 . In particular, it was shown that in tungsten-based TMDs, the intervalley dark excitonic state lies energetically below the optically accessible exciton, resulting in a strong quenching of photoluminescence at low temperatures 14,15 .…”

mentioning

confidence: 99%

Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

Selig

Berghäuser

Raja

et al. 2017

Ultrafast Phenomena and Nanophotonics XXI

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Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light-matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS 2 and MoSe 2 ) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS 2 , we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures.

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Experimental Evidence for Dark Excitons in MonolayerWSe2

Cited by 417 publications

References 63 publications

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂

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

Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

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Experimental Evidence for Dark Excitons in MonolayerWSe2

Cited by 417 publications

References 63 publications

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe2

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe2

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

Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

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Product

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Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe₂