| 1. |
- Choi, Junho, et al.
(författare)
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Fermi Pressure and Coulomb Repulsion Driven Rapid Hot Plasma Expansion in a van der Waals Heterostructure
- 2023
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Ingår i: Nano Letters. - 1530-6992 .- 1530-6984. ; 23:10, s. 4399-4405
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal dichalcogenide heterostructures provide a versatile platform to explore electronic and excitonic phases. As the excitation density exceeds the critical Mott density, interlayer excitons are ionized into an electron-hole plasma phase. The transport of the highly non-equilibrium plasma is relevant for high-power optoelectronic devices but has not been carefully investigated previously. Here, we employ spatially resolved pump-probe microscopy to investigate the spatial-temporal dynamics of interlayer excitons and hot-plasma phase in a MoSe2/WSe2 twisted bilayer. At the excitation density of ∼1014 cm-2, well exceeding the Mott density, we find a surprisingly rapid initial expansion of hot plasma to a few microns away from the excitation source within ∼0.2 ps. Microscopic theory reveals that this rapid expansion is mainly driven by Fermi pressure and Coulomb repulsion, while the hot carrier effect has only a minor effect in the plasma phase.
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| 2. |
- Erkensten, Daniel, 1995, et al.
(författare)
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Dark exciton-exciton annihilation in monolayer WSe2
- 2021
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 104:24
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Tidskriftsartikel (refereegranskat)abstract
- The exceptionally strong Coulomb interaction in semiconducting transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape consisting of bright and dark exciton states. At elevated densities, excitons can interact through exciton-exciton annihilation (EEA), an Auger-like recombination process limiting the efficiency of optoelectronic applications. Although EEA is a well-known and particularly important process in atomically thin semiconductors determining exciton lifetimes and affecting transport at elevated densities, its microscopic origin has remained elusive. In this joint theory-experiment study combining microscopic and material-specific theory with time- and temperature-resolved photoluminescence measurements, we demonstrate the key role of dark intervalley states that are found to dominate the EEA rate in monolayer WSe2. We reveal an intriguing, characteristic temperature dependence of Auger scattering in this class of materials with an excellent agreement between theory and experiment. Our study provides microscopic insights into the efficiency of technologically relevant Auger scattering channels within the remarkable exciton landscape of atomically thin semiconductors.
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| 3. |
- Erkensten, Daniel, 1995, et al.
(författare)
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Electrically tunable dipolar interactions between layer-hybridized excitons
- 2023
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Ingår i: Nanoscale. - 2040-3372 .- 2040-3364. ; 15:26, s. 11064-11071
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Tidskriftsartikel (refereegranskat)abstract
- Transition-metal dichalcogenide bilayers exhibit a rich exciton landscape including layer-hybridized excitons, i.e. excitons which are of partly intra- and interlayer nature. In this work, we study hybrid exciton-exciton interactions in naturally stacked WSe2 homobilayers. In these materials, the exciton landscape is electrically tunable such that the low-energy states can be rendered more or less interlayer-like depending on the strength of the external electric field. Based on a microscopic and material-specific many-particle theory, we reveal two intriguing interaction regimes: a low-dipole regime at small electric fields and a high-dipole regime at larger fields, involving interactions between hybrid excitons with a substantially different intra- and interlayer composition in the two regimes. While the low-dipole regime is characterized by weak inter-excitonic interactions between intralayer-like excitons, the high-dipole regime involves mostly interlayer-like excitons which display a strong dipole-dipole repulsion and give rise to large spectral blue-shifts and a highly anomalous diffusion. Overall, our microscopic study sheds light on the remarkable electrical tunability of hybrid exciton-exciton interactions in atomically thin semiconductors and can guide future experimental studies in this growing field of research.
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| 4. |
- Erkensten, Daniel, 1995, et al.
(författare)
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Microscopic origin of anomalous interlayer exciton transport in van der Waals heterostructures
- 2022
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Ingår i: Physical Review Materials. - 2475-9953. ; 6:9
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Tidskriftsartikel (refereegranskat)abstract
- Van der Waals heterostructures constitute a platform for investigating intriguing many-body quantum phenomena. In particular, transition-metal dichalcogenide (TMD) heterobilayers host long-lived interlayer excitons which exhibit permanent out-of-plane dipole moments. Here, we develop a microscopic theory for interlayer exciton-exciton interactions including both the dipolar nature of interlayer excitons as well as their fermionic substructure, which gives rise to an attractive fermionic exchange. We find that these interactions contribute to a drift force resulting in highly nonlinear exciton propagation at elevated densities in the MoSe2 - WSe2 heterostructure. We show that the propagation can be tuned by changing the number of hBN spacers between the TMD layers or by adjusting the dielectric environment. In particular, although counterintuitive, we reveal that interlayer excitons in freestanding samples propagate slower than excitons in hBN-encapsulated TMDs-due to an enhancement of the net Coulomb drift with stronger environmental screening. Overall, our work contributes to a better microscopic understanding of the interlayer exciton transport in technologically promising atomically thin semiconductors.
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| 5. |
- Malic, Ermin, 1980, et al.
(författare)
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Exciton transport in atomically thin semiconductors
- 2023
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Ingår i: Nature Communications. - 2041-1723 .- 2041-1723. ; 14:1
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Atomically thin semiconductors have been in the center of one of the most active research fields. Here, we discuss the main challenges in exciton transport that is crucial for nanoelec- tronics. We focus on transport phenomena in monolayers, lateral heterostructures, and twis- ted heterostacks of transition metal dichalcogenides.
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| 6. |
- Perea Causin, Raul, 1995, et al.
(författare)
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Exciton optics, dynamics, and transport in atomically thin semiconductors
- 2022
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 10:10
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Tidskriftsartikel (refereegranskat)abstract
- Atomically thin semiconductors such as transition metal dichalcogenide (TMD) monolayers exhibit a very strong Coulomb interaction, giving rise to a rich exciton landscape. This makes these materials highly attractive for efficient and tunable optoelectronic devices. In this Research Update, we review the recent progress in the understanding of exciton optics, dynamics, and transport, which crucially govern the operation of TMD-based devices. We highlight the impact of hexagonal boron nitride-encapsulation, which reveals a plethora of many-particle states in optical spectra, and we outline the most novel breakthroughs in the field of exciton-polaritonics. Moreover, we underline the direct observation of exciton formation and thermalization in TMD monolayers and heterostructures in recent time-resolved, angle-resolved photoemission spectroscopy studies. We also show the impact of exciton density, strain, and dielectric environment on exciton diffusion and funneling. Finally, we put forward relevant research directions in the field of atomically thin semiconductors for the near future.
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| 7. |
- Tagarelli, Fedele, et al.
(författare)
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Electrical control of hybrid exciton transport in a van der Waals heterostructure
- 2023
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Ingår i: Nature Photonics. - 1749-4885 .- 1749-4893. ; 17:7, s. 615-621
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Tidskriftsartikel (refereegranskat)abstract
- Interactions between out-of-plane dipoles in bosonic gases enable the long-range propagation of excitons. The lack of direct control over collective dipolar properties has so far limited the degrees of tunability and the microscopic understanding of exciton transport. In this work we modulate the layer hybridization and interplay between many-body interactions of excitons in a van der Waals heterostructure with an applied vertical electric field. By performing spatiotemporally resolved measurements supported by microscopic theory, we uncover the dipole-dependent properties and transport of excitons with different degrees of hybridization. Moreover, we find constant emission quantum yields of the transporting species as a function of excitation power with radiative decay mechanisms dominating over nonradiative ones, a fundamental requirement for efficient excitonic devices. Our findings provide a complete picture of the many-body effects in the transport of dilute exciton gases, and have crucial implications for studying emerging states of matter such as Bose–Einstein condensation and optoelectronic applications based on exciton propagation.
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| 8. |
- Feldstein, David, et al.
(författare)
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Microscopic Picture of Electron-Phonon Interaction in Two-Dimensional Halide Perovskites
- 2020
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 11:23, s. 9975-9982
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Tidskriftsartikel (refereegranskat)abstract
- Perovskites have attracted much attention due to their remarkable optical properties. While it is well established that excitons dominate their optical response, the impact of higher excitonic states and formation of phonon sidebands in optical spectra still need to be better understood. Here, we perform a theoretical study of excitonic properties of monolayered hybrid organic perovskites - supported by temperature-dependent photoluminescence measurements. Solving the Wannier equation, we obtain microscopic access to the Rydberg-like series of excitonic states including their wave functions and binding energies. Exploiting the generalized Elliot formula, we calculate the photoluminescence spectra demonstrating a pronounced contribution of a phonon sideband for temperatures up to 50 K, in agreement with experimental measurements. Finally, we predict temperature-dependent line widths of the three energetically lowest excitonic transitions and identify the underlying phonon-driven scattering processes.
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| 9. |
- Hofmann, Niklas, et al.
(författare)
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Link between interlayer hybridization and ultrafast charge transfer in WS 2 -graphene heterostructures
- 2023
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Ingår i: 2D Materials. - 2053-1583. ; 10:3
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast charge separation after photoexcitation is a common phenomenon in various van-der-Waals (vdW) heterostructures with great relevance for future applications in light harvesting and detection. Theoretical understanding of this phenomenon converges towards a coherent mechanism through charge transfer states accompanied by energy dissipation into strongly coupled phonons. The detailed microscopic pathways are material specific as they sensitively depend on the band structures of the individual layers, the relative band alignment in the heterostructure, the twist angle between the layers, and interlayer interactions resulting in hybridization. We used time- and angle-resolved photoemission spectroscopy combined with tight binding and density functional theory electronic structure calculations to investigate ultrafast charge separation and recombination in WS2-graphene vdW heterostructures. We identify several avoided crossings in the band structure and discuss their relevance for ultrafast charge transfer. We relate our own observations to existing theoretical models and propose a unified picture for ultrafast charge transfer in vdW heterostructures where band alignment and twist angle emerge as the most important control parameters.
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| 10. |
- Jago, Roland, 1990, et al.
(författare)
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Spatio-temporal dynamics in graphene
- 2019
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 11:20, s. 10017-10022
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Tidskriftsartikel (refereegranskat)abstract
- Temporally and spectrally resolved dynamics of optically excited carriers in graphene has been intensively studied theoretically and experimentally, whereas carrier diffusion in space has attracted much less attention. Understanding the spatio-temporal carrier dynamics is of key importance for optoelectronic applications, where carrier transport phenomena play an important role. In this work, we provide a microscopic access to the time-, momentum-, and space-resolved dynamics of carriers in graphene. We determine the diffusion coefficient to be D ≈ 360 cm 2 s -1 and reveal the impact of carrier-phonon and carrier-carrier scattering on the diffusion process. In particular, we show that phonon-induced scattering across the Dirac cone gives rise to back-diffusion counteracting the spatial broadening of the carrier distribution.
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