Hauptinhalt

2023

  • 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, 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.

    DOI: 10.1088/2053-1583/ace067

  • G. Meneghini, M. Reutzel, S. Mathias, S. Brem, and E. Malic, Direct visualization of hybrid excitons in a van der Waals heterostructure, 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 strong interlayer tunneling, hybrid exciton states consisting of interlayer and intralayer excitons can be formed. Here, electrons and 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 yet been directly observed yet. Based on microscopic and material-specific theory, we show that time- and angle-resolved photoemission spectroscopy (tr-ARPES) is a direct technique to 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, a peak separation of more than 0.5 eV can be resolved in ARPES experiments. Our study provides a concrete recipe for how to directly visualize hybrid excitons and how to distinguish them from the usually observed regular excitonic signatures.

    DOI: 10.1021/acsphotonics.3c00599

  • T. Potocnik, O. Burton, M. Reutzel, D. Schmitt, J. P. Bange, S. Mathias, F. R. Geisenhof, R. Thomas Weitz, L. Xin, H. J. Joyce, S. Hofmann, J. A. Alexander-Webber, Fast twist angle mapping of bilayer graphene using spectroscopic ellipsometric contrast microscopy, Nano Letters 23, 5506 (2023).

    Twisted bilayer graphene provides an ideal solid-state model to explore correlated material properties and opportunities for a variety of optoelectronic applications, but reliable, fast characterization of the twist angle remains a challenge. Here we introduce spectroscopic ellipsometric contrast microscopy (SECM) as a tool for mapping twist angle disorder in optically resonant twisted bilayer graphene. We optimize the ellipsometric angles to enhance the image contrast based on measured and calculated reflection coefficients of incident light. The optical resonances associated with van Hove singularities correlate well to Raman and angle-resolved photoelectron emission spectroscopy, confirming the accuracy of SECM. The results highlight the advantages of SECM, which proves to be a fast, nondestructive method for characterization of twisted bilayer graphene over large areas, unlocking process, material, and device screening and cross-correlative measurement potential for bilayer and multilayer materials.

    DOI: 10.1021/acs.nanolett.3c00619

  • H. Petek, A. Li, X. Li, S. Tan, M. Reutzel, Plasmonic decay into hot electrons in silver, Progress in Surface Science 100707 (2023).

    Light at optical frequencies interacting with a metal surface can excite interband quantum transitions, or intraband currents at frequencies approaching the PHz range. Momentum conservation enables the interband excitation to occur in first order as a dipole transition, while intraband excitations involve second-order momentum scattering processes. The free electron response to optical fields can also be collective, causing the optical field to be screened by the multipole plasmon response. We describe the exitation of single crystal silver surfaces in the region where the dielectric response transits from negative to positive passing through the epsilon near zero (ENZ) condition. There, electrons can no longer screen the optical field, so that it penetrates as a collective charge density wave of the free electron plasma, in other words, as a bulk transverse or longitudinal plasmon field. We examine two-photon photoemission (2PP) signals from Ag(1 1 1), (1 0 0) and (1 1 0) surfaces through the ENZ region under conditions where intraband, and interband single particle, and bulk plasmon collective responses dominate. We are specifically interested in the bulk plasmon decay into plasmonic photoemission. Plasmonic decay into excitation of electrons from the Fermi level, which we observe as a nonlinear 2PP process, has been established for the free electron and noble metals, but its significance to transduction of optical-to-electronic energy has not penetrated the plasmonics community. 2PP spectra show evidence for intraband hot electron generation, interband surface and bulk band excitation, and nonlinear bulk plasmon driven plasmonic single particle excitation. Because the intraband and plasmonic decay into hot electron distributions have been extensively considered in the literature, without reference to explicit experimental measurements, we discuss such processes in light of the directional anisotropy of the electronic structure of single crystalline silver. We note that projected band gaps in silver exclude large regions of the unoccupied state density from hot electron generation, such that it predominantly occurs in the (1 1 0) direction. Moreover, the excited hot electron distributions do not follow expectations from the joint density of the occupied and unoccupied states of a free electron metal, as assumed in majority of research on hot electron processes. We describe the strongly anisotropic hot electron distributions recorded by 2PP spectroscopy of Ag surfaces, and the plasmonic photoemission process that occurs on all surfaces irrespective of the momentum-dependent single particle band structure of silver. Plasmonic photoemission, or its linear analog that excites hot electrons at energies below the work function of Ag, is an important process for harvesting hot electron energy in photocatalytic and electronic device applications because the plasmon energy is not distributed between an electron and hole. This plasmonic decay channel is robust, but many aspects raise further questions. The accompanying publication by Gumhalter and Novko discusses the plasmonic photoemission from a theoretical point of view and its extension to Floquet engineering, as an exploration of novel plasmonic excitation processes in metals.

    DOI: 10.1016/j.progsurf.2023.100707

  • A. Adamkiewicz, T. Bohamud, M. Reutzel, U. Höfer, M. Dürr, Tip-Induced Dissociation of Diethyl Ether on Si (001)─ Influence of Molecular Structure on Final Reaction Products, The Journal of Physical Chemistry C 127, 15819 (2023).

    Scanning tunneling microscopy gives access to well-controlled manipulation of surface adsorbates by means of activating reaction pathways beyond thermal excitation schemes. For the conversion of diethyl ether on Si(001) from a datively bound intermediate into the covalently bound final state, we show that tip-induced electronic excitation leads to new products when compared to thermal excitation. We emphasize the molecular structure as a key parameter in creating such specific reaction products via electronic excitation by comparing the results for diethyl ether and tetrahydrofuran, the cyclic analogue of diethyl ether. The final configurations of the two systems differ in terms of the reacted surface atoms and their electronic structure; moreover, a new reaction product which indicates the desorption of one of the two molecular fragments induced by tip-induced ether cleavage is dominant in the case of diethyl ether.

    DOI: 10.1021/acs.jpcc.3c03505