Veröffentlichungen 2023

  • 165. 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)

  • 164. 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" accepted in Nanoscale

    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.

    accepted in Nanoscale

  • 163. J. König, J. Fitzgerald, J. Hagel, D. Erkensten, E.Malic "Interlayer exciton polaritons in homobilayers of transition metal dichalcogenides", accepted in 2D Materials

    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.

    accepted in 2D Materials

  • 162. 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