Veröffentlichungen 2023

  • 181. F. Tagarelli, E. Lopriore, D. Erkensten, R. Perea-Causin, S. Brem, J. Hagel, Z. Sun, G. Pasquale, K. Watanabe, T. Taniguchi, E. Malic, A. Kis, "Electrical control of hybrid exciton transport in a van der Waals heterostructure", Nature Photonics 17, 615 (2023)

    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 hitherto 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 dominating radiative decay mechanisms 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 the study of emerging states of matter, such as Bose-Einstein condensation, as well as for optoelectronic applications based on exciton propagation.

    Nature Photonics 17, 615 (2023)

  • 180. H. Fang, Q. Lin, Y. Zhang, J. Thompson, S. Xiao, Z. Sun, E. Malic, S. P. Dash and W. Wieczorek "Localization and interaction of interlayer excitons in MoSe2/WSe2 heterobilayers", Nature Communications 14, 6910 (2023)

    Transition metal dichalcogenide (TMD) heterobilayers provide a versatile platform to explore unique excitonic physics via the properties of the constituent TMDs and external stimuli. Interlayer excitons (IXs) can form in TMD heterobilayers as delocalized or localized states. However, the localization of IX in different types of potential traps, the emergence of biexcitons in the high-excitation regime, and the impact of potential traps on biexciton formation have remained elusive. In our work, we observe two types of potential traps in a MoSe2/WSe2 heterobilayer, which result in significantly different emission behavior of IXs at different temperatures. We identify the origin of these traps as localized defect states and the moiré potential of the TMD heterobilayer. Furthermore, with strong excitation intensity, a superlinear emission behavior indicates the emergence of interlayer biexcitons, whose formation peaks at a specific temperature. Our work elucidates the different excitation and temperature regimes required for the formation of both localized and delocalized IX and biexcitons and, thus, contributes to a better understanding and application of the rich exciton physics in TMD heterostructures.

    Nature Communications 14, 6910 (2023)

  • 179. R. Sebait, R. Rosati, S. Joon Yun, K. P. Dhakal, S. Brem, C. Biswas, A. Puretzky, E. Malic and Y. Hee Le "Sequential order dependent dark-exciton modulation in bi-layered TMD heterostructure", Nature Communications 14, 5548 (2023)

    We report the emergence of dark-excitons in transition-metal-dichalcogenide (TMD) heterostructures that strongly rely on the stacking sequence, i.e., momentum-dark K-Q exciton located exclusively at the top layer of the heterostructure. The feature stems from band renormalization and is distinct from those of typical neutral excitons or trions, regardless of materials, substrates, and even homogeneous bilayers, which is further confirmed by scanning tunneling spectroscopy. To understand the unusual stacking sequence, we introduce the excitonic Elliot formula by imposing strain exclusively on the top layer that could be a consequence of the stacking process. We further find that the intensity ratio of Q- to K-excitons in the same layer is inversely proportional to laser power, unlike for conventional K-K excitons. This can be a metric for engineering the intensity of dark K-Q excitons in TMD heterostructures, which could be useful for optical power switches in solar panels.

    Nature Communications 14, 5548 (2023)

  • 178. R. Rosati, I. Paradisanos, L. Huang, Z. Gan, A. George, K. Watanabe, T. Taniguchi, L. Lombez, P. Renucci, A. Turchanin, B. Urbaszek, E. Malic, "Interface engineering of charge-transfer excitons in 2D lateral heterostructures", Nature Communications 14, 2438 (2023)

    The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe2-WSe2 heterostructures. Based on a fully microscopic and material-specific theory, we reveal the many-particle processes behind the formation of CT excitons and how they can be tuned via interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius we predict the appearance of a low-energy CT exciton. The theoretical prediction is compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. Our joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.

    Nature Communications 14, 2438 (2023)

  • 177. E. Malic, R. Perea-Causin, R. Rosati, D. Erkensten and S. Brem,  "Exciton transport in atomically thin semiconductors", Nature Communications 14, 3430 (2023)

    Transition metal dichalcogenides (TMD) have emerged as an ideal material platform for exploring exciton transport phenomena. The reduced dimensionality and weak screening lead to a remarkably strong Coulomb interaction, giving rise to the formation of tightly bound excitons that are stable even at room temperature. TMDs host a rich exciton landscape, including bright, momentum-dark, and spin-dark excitons, which govern the spatial propagation. A key advantage of TMDs is that they can be laterally stitched and vertically stacked to form heterostructures with tailored properties. This gives rise to a new type of spatially separated excitons with large in-plane or out-of-plane dipole moments that can be tuned via electric fields, cf. Fig. 1. Furthermore, TMD monolayers in a heterostack can be twisted resulting in a long-range moiré pattern that can act as a trap for excitons strongly impeding their propagation. While there has been a significant advance in the understanding of linear and non-linear exciton transport phenomena in TMD monolayers, there is still relatively little knowledge about the propagation of interlayer, charge transfer, and moiré excitons. Here, we discuss the current status, the challenges, and potential technological impact of exciton transport in TMD monolayers, lateral and vertical heterostructures as well as moiré excitons in twisted TMD heterostacks.

    Nature Communications 14, 3430 (2023)

  • 176. S. Brem and E. Malic "Bosonic Delocalization of Dipolar Moiré Excitons", Nano Letters 23, 4627 (2023)

    In superlattices of twisted semiconductor monolayers, tunable moiré potentials emerge, trapping excitons into periodic arrays. In particular, spatially separated interlayer excitons are subject to a deep potential landscape and they exhibit a permanent dipole providing a unique opportunity to study interacting bosonic lattices. Recent experiments have demonstrated density-dependent transport properties of moiré excitons, which could play a key role for technological applications. However, the intriguing interplay between exciton-exciton interactions and moiré trapping has not been well understood yet. In this work, we develop a microscopic theory of interacting excitons in external potentials allowing us to tackle this highly challenging problem. We find that interactions between moiré excitons lead to a delocalization at intermediate densities and we show how this transition can be tuned via twist angle and temperature. The delocalization is accompanied by a modification of optical moiré resonances, which gradually merge into a single free exciton peak. The predicted density-tunability of the supercell hopping can be utilized to control the energy transport in moiré materials.

    Nano Letters 23, 4627 (2023)

  • 175. J. Choi, J. Embley, D. D. Blach, R. Perea-Causín, D. Erkensten, D. Seob Kim, L. Yuan, W. Young Yoon, T. Taniguchi, K. Watanabe, K. Ueno, E. Tutuc, S. Brem, E. Malic, X. Li, and L. Huang, “Fermi Pressure and Coulomb Repulsion Driven Rapid Hot Plasma Expansion in a van der Waals Heterostructure”, Nano Letters 23, 4399 (2023)

    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.

    Nano Letters 23, 4399 (2023)

  • 174. J. J. P. Thompson, M. Gerhard, G. Witte, E. Malic "Optical Signatures of Förster-induced energy transfer in organic/TMD heterostructures", njp 2D Materials and Applications 7, 69 (2023)

    Hybrid van der Waals heterostructures of organic semiconductors and transition metal dichalcogenides (TMDs) are promising candidates for various optoelectronic devices, such as solar cells and biosensors. Energy-transfer processes in these materials are crucial for the efficiency of such devices, yet they are poorly understood. In this work, we develop a fully microscopic theory describing the effect of the Förster interaction on exciton dynamics and optics in a WSe2/tetracene heterostack. We demonstrate that the differential absorption and time-resolved photoluminescence can be used to track the real-time evolution of excitons. We predict a strongly unidirectional energy transfer from the organic to the TMD layer. Furthermore, we explore the role temperature has in activating the Förster transfer and find a good agreement to previous experiments. Our results provide a blueprint to tune the light-harvesting efficiency through temperature, molecular orientation and interlayer separation in TMD/organic heterostructures.

    njp 2D Materials and Applications 7, 69 (2023)

  • 173. G. Meneghini, M. Reutzel, S. Mathias, S. Brem, and E. Malic, "Hybrid Exciton Signatures in ARPES Spectra of van der Waals Materials", ACS Photonics, 10, 3570 (2023)

    Van der Waals heterostructures show fascinating physics including trapped moire exciton states, anomalous moire exciton transport, generalized Wigner crystals, etc. Bilayers of transition metal dichalcogenides (TMDs) are characterized by long-lived spatially separated interlayer excitons. Provided a strong interlayer tunneling, hybrid exciton states consisting of interlayer and intralayer excitons can be formed. Here, electrons and/or holes are in a superposition of both layers. Although crucial for optics, dynamics, and transport, hybrid excitons are usually optically inactive and have therefore not been directly observed yet. Based on a microscopic and material-specific theory, we show that time- and angle-resolved photoemission spectroscopy (tr-ARPES) is the ideal technique to directly visualize these hybrid excitons. Concretely, we predict a characteristic double-peak ARPES signal arising from the hybridized hole in the MoS2 homobilayer. The relative intensity is proportional to the quantum mixture of the two hybrid valence bands at the Γ point. Due to the strong hybridization, the peak separation of more than 0.5 eV can be resolved in ARPES experiments. Our study provides a concrete recipe of how to directly visualize hybrid excitons and how to distinguish them from the usually observed regular excitonic signatures.

    ACS Photonics 10, 3570 (2023)

  • 172. J. J. P. Thompson, V. Lumsargis, M. Feierabend, Q. Zhao, K. Wang, L. Dou, L. Huang, E. Malic, "Interlayer exciton landscape in WS2/tetracene heterostructures", Nanoscale 15, 1730 (2023)

    The vertical stacking of two-dimensional materials into heterostructures gives rise to a plethora of intriguing optoelectronic properties and presents an unprecedented potential for technological concepts. While much progress has been made combining different monolayers of transition metal dichalgonenides (TMDs), little is known about TMD-based heterostructures including organic layers of molecules. Here, we present a joint theory-experiment study on a TMD/tetracene heterostructure demonstrating clear signatures of spatially separated interlayer excitons in low temperature photoluminescence spectra. Here, the Coulomb-bound electrons and holes are localized either in the TMD or in the molecule layer, respectively. In particular, we reveal both in theory and experiment that at cryogenic temperatures, signatures of momentum-dark interlayer excitons emerge. Our findings shed light on the microscopic nature of interlayer excitons in TMD/molecule heterostructures and could have important implications for technological applications of these materials.

    Nanoscale 15, 1730 (2023)

  • 171. D. Erkensten, S. Brem, R. Perea-Causin, J. Hagel, F. Tagarelli, E. Lopriore, A. Kis, E. Malic "Electrically tunable dipolar interactions between layer-hybridized excitons", Nanoscale 15, 11064 (2023)

    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.

    Nanoscale 15, 11064 (2023)

  • 170. F. Sousa, R. Causin, S. Hartman, L. Lafeta, B. Luiza Teixeira Rosa, S. Brem, C. Palekar, S. Reitzenstein, A. Hartschuh, E. Malic and L. Malard, "Ultrafast Hot Electron-Hole Plasma Photoluminescence in Two-Dimensional Semiconductors" Nanoscale 15, 7154 (2023)

    The transition metal dichalcogenide family of semiconducting two-dimensional materials has recently shown a prominent potential to be an ideal platform to study the exciton Mott transition into electron-hole plasma and liquid phases due to their strong Coulomb interactions. Here, we show that pulsed laser excitation at high pump fluences can induce this exciton Mott transition to an electron-hole plasma in mono and fewlayer transition metal dichalcogenides at room temperature. The formation of an electron-hole plasma leads to a broadband light emission spanning from the near infrared to the visible region. In agreement with our theoretical calculations, the photoluminescence emission at high energies displays an exponential decay that directly reflects the electronic temperature - a characteristic fingerprint of unbound electron-hole pairs recombination. Furthermore, two-pulse excitation correlation measurements are performed to study the dynamics of the electronic cooling, which shows two decay time components, one of less than 100 fs and a slower component of few ps associated with the electron-phonon and phonon-lattice bath thermalizations, respectively. Our work may shed light on further studies of exciton Mott transition in other two-dimensional materials and their heterostructures and its applications in nanolasers and other optoelectronic devices.

    Nanoscale 15, 7154 (2023)

  • 169. J. P. Bange, P. Werner, D. Schmitt, W. Bennecke, G. Meneghini, A. AlMutairi, M. Merboldt, K. Watanabe, T. Taniguchi, S. Steil, D. Steil, R. T. Weitz, S. Hofmann, G. S. M. Jansen, S. Brem, E. Malic, M. Reutzel and S. Mathias "Ultrafast dynamics of bright and dark excitons in monolayer WSe2 and heterobilayer WSe2/MoS2", 2D Materials 10, 035039 (2023)

    The energy landscape of optical excitations in mono- and few-layer transition metal dichalcogenides (TMDs) is dominated by optically bright and dark excitons. These excitons can be fully localized within a single TMD layer, or the electron- and the hole-component of the exciton can be charge-separated over multiple TMD layers. Such intra- or interlayer excitons have been characterized in detail using all-optical spectroscopies, and, more recently, photoemission spectroscopy. In addition, there are so-called hybrid excitons whose electron- and/or hole-component are delocalized over two or more TMD layers, and therefore provide a promising pathway to mediate charge-transfer processes across the TMD interface. Hence, an in-situ characterization of their energy landscape and dynamics is of vital interest. In this work, using femtosecond momentum microscopy combined with many-particle modeling, we quantitatively compare the dynamics of momentum-indirect intralayer excitons in monolayer WSe2 with the dynamics of momentum-indirect hybrid excitons in heterobilayer WSe2/MoS2, and draw three key conclusions: First, we find that the energy of hybrid excitons is reduced when compared to excitons with pure intralayer character. Second, we show that the momentum-indirect intralayer and hybrid excitons are formed via exciton-phonon scattering from optically excited bright excitons. And third, we demonstrate that the efficiency for phonon absorption and emission processes in the mono- and the heterobilayer is strongly dependent on the energy alignment of the intralayer and hybrid excitons with respect to the optically excited bright exciton. Overall, our work provides microscopic insights into exciton dynamics in TMD mono- and bilayers.

    2D Materials 10, 035039 (2023)

  • 168. N. Hofmann, L. Weigl, J. Gradl, N. Mishra, G. Orlandini, S. Forti, C. Coletti, S. Latini, L. Xian, A. Rubio, D. Perez Paredes, R. Perea Causin, S. Brem, E. Malic, and I. Gierz, “Link between interlayer hybridization and ultrafast charge transfer in WS2-graphene heterostructures”, 2D Materials 10, 035025 (2023)

    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 eterostructures where band alignment and twist angle emerge as the most important control parameters.

    2D Materials 10, 035025 (2023)

  • 167. N. Sokołowski, S. Palai, M. Dyksik, K. Posmyk, M. Baranowskki, A. Surrente, D. K. Maude, F. Carrascoso, O. Cakiroglu, E. Sánchez-Viso, A. Schubert, C. Munuera, T. Taniguchi, K. Watanabe, J. Hagel, S. Brem, A. Castellanos-Gomez, E. Malic and P. Plochocka "Twist-angle dependent dehybridization of momentum-indirect excitons in MoSe2/MoS2 heterostructures", 2D Materials 10, 034003 (2023)

    The moire superlattice has emerged as a powerful way to tune excitonic properties in two-dimensional van der Waals structures. However, the current understanding of the influence of the twist angle for interlayer excitons in heterostructures is mainly limited to momentum-direct K-K transitions. In this work, we use a judicious combination of spectroscopy and many-particle theory to investigate the influence of the twist angle on momentum-indirect interlayer excitons of a MoSe2/MoS2 heterostructure. Here, the energetically lowest state is a dark and strongly hybridized ΓK exciton. We show that increasing the twist angle from an aligned structure (0◦ or 60◦) gives rise to a large blue shift of the interlayer exciton, which is a manifestation of the strong dehybridization of this state. Moreover, for small twist angle heterostructures, our photoluminescence measurements reveal contributions from two interlayer exciton states, which our modelling attributes to transitions from different moire minibands. Our finding contributes to a better fundamental understanding of the influence of the moire pattern on the hybridization of momentum-dark interlayer exciton states, which may be important for applications in moire-tronics including novel quantum technologies.

    2D Materials 10, 034003 (2023)

  • 166. J. König, J. Fitzgerald, J. Hagel, D. Erkensten, E.Malic "Interlayer exciton polaritons in homobilayers of transition metal dichalcogenides", 2D Materials 10, 025019 (2023)

    Transition metal dichalcogenides integrated within a high-quality microcavity support well-defined exciton polaritons. While the role of intralayer excitons in 2D polaritonics is well studied, interlayer excitons have been largely ignored due to their weak oscillator strength. Using a microscopic and material-realistic Wannier-Hopfield model, we demonstrate that MoS2 homobilayers in a Fabry-Perot cavity support polaritons that exhibit a large interlayer exciton contribution, while remaining visible in linear optical spectra. Interestingly, with suitable tuning of the cavity length, the hybridization between intra- and interlayer excitons can be 'unmixed' due to the interaction with photons. We predict formation of polaritons where > 90% of the total excitonic contribution is stemming from the interlayer exciton. Furthermore, we explore the conditions on the tunneling strength and exciton energy landscape to push this to even 100%. Despite the extremely weak oscillator strength of the underlying interlayer exciton, optical energy can be effectively fed into the polaritons once the critical coupling condition of balanced radiative and scattering decay channels is met. These findings have a wide relevance for fields ranging from nonlinear optoelectronic devices to Bose-Einstein condensation.

    2D Materials 10, 025019 (2023)

  • 165. J. Hagel, S. Brem, E. Malic, "Electrical tuning of moiré excitons in MoSe2 bilayers", 2D Materials 10, 014013 (2023)

    Recent advances in the field of vertically stacked 2D materials have revealed a rich exciton landscape. In particular, it has been demonstrated that out-of-plane electrical fields can be used to tune the spectral position of spatially separated interlayer excitons. Other studies have shown that there is a strong hybridization of exciton states, resulting from the mixing of electronic states in both layers. However, the connection between the twist-angle dependent hybridization and field-induced energy shifts has remained in the dark. Here, we investigate on a microscopic footing the interplay of electrical and twist-angle tuning of moiré excitons in MoSe2 homobilayers. We reveal distinct energy regions in PL spectra that are clearly dominated by either intralayer or interlayer excitons, or even dark excitons. Consequently, we predict twist-angle-dependent critical electrical fields at which the material is being transformed from a direct into an indirect semiconductor. Our work provides new microscopic insights into experimentally accessible knobs to significantly tune the moiré exciton physics in atomically thin nanomaterials.

    2D Materials 10, 014013 (2023)