Ultrafast Charge Transfer Dynamics in Van der Waals Heterostructures

Van der Waals heterostructures built by vertically stacked transition metal dichalcogenides (TMDs) exhibit a rich exciton energy landscape including spatially separated interlayer states, momentum-dark intervalley states, and hybrid exciton states. Recent experiments have demonstrated an ultrafast charge transfer in TMD heterostructures. However, the nature of the charge transfer process has remained elusive. Based on a microscopic and material-realistic exciton theory combined with time-resolved ARPES measurements, we reveal that phonon-mediated scattering via strongly hybridized dark intervalley excitons governs the charge transfer process [1,2]. We track the time-, momentum-, and energy-resolved relaxation dynamics of optically excited excitons and determine the temperature- and stacking-dependent charge transfer times for different TMD bilayers.<br/><br/>Furthermore, we demonstrate how the Coulomb interaction between the correlated electron- and hole-components of intra- and interlayer excitons facilitates the study of the ultrafast hole transfer mechanism in a twisted WSe₂/MoS₂ heterostructure. Intriguingly, we find an increase of the photoelectron energy in the ARPES spectrum upon the hole transfer process across the interface. This is surprising at first, because the electron remains rigid in the conduction band during the hole transfer process, and also because any relaxation mechanism is typically expected to cause an overall decrease of the measured electronic energies. However, we do not observe a free photoelectron, but the blue-shift is a direct consequence of the correlated nature of the Coulomb-bound electron-hole-pair.<br/><br/>Compared to the electron transfer occuring on a timescale of sub-100fs, we find both in experiment and theory that the hole transfer is considerably slower and occurs rather on a timescale of a few picoseconds. This can be traced back to different relaxation pathways: In the case of the electron transfer, optically excited intralayer excitons are subject to a relaxation cascade via layer-hybridized KΛ excitons to the lowest interlayer exciton states. In contrast, the hole transfer occurs via layer-hybridized ΓK excitons. Since the Λ valleys are three-fold degenerate, while there is only one Γ valley, the density of states for the electron transfer pathway is more efficient. Furthemore, the exciton energy difference between the initial optically excited intralayer exciton and the final interlayer exciton is roughly 200 meV larger in the case of the hole transfer, further contributing to a slower transfer dynamics of holes.<br/><br/>The provided insights present an important step forward in microscopic understanding of the technologically important charge transfer process in van der Waals heterostructures.<br/><br/>References:<br/>[1] D. Schmitt, J. Bange, W. Bennecke, A. AlMutairi, G. Meneghini, K. Watanabe, T. Taniguchi, D. Steil, D. Luke, R. Weitz, S. Steil, G. Jansen, S. Brem, E. Malic, S. Hofmann, M. Reutzel and S. Mathias, Formation of moiré interlayer excitons in space and time, Nature 608, 499 (2022)<br/>[2] Meneghini, S. Brem, E. Malic, Ultrafast phonon-driven charge transfer in van der Waals heterostructure, Natural Sciences, e20220014 (2022)<br/>[3] J. Bange, D. Schmitt, W. Bennecke, G. Meneghini, A. AlMutairi, K. Watanabe, T. Taniguchi, D. Steil, S. Steil, R. Weitz G. Jansen, S. Hofmann, S. Brem, E. Malic, M. Reutzel, S. Mathias, Probing electron-hole Coulomb correlations in the exciton landscape of a moire heterostructure, arXiv 2303.17886 (2023)