Main Content
Publications 2018
103. J. Lindlau, M. Selig, A. Neumann, L. Colombier, J. Kim, G. Berghäuser, F. Wang, E. Malic, A. Högele, "The role of momentum-dark excitons in the elementary optical response of bilayer WSe", Nature Communications 9, 2586 (2018)
Monolayer transition metal dichalcogenides (TMDs) undergo substantial changes in the single-particle band structure and excitonic optical response upon the addition of just one layer. As opposed to the single-layer limit, the bandgap of bilayer (BL) TMD semiconductors is indirect which results in reduced photoluminescence with richly structured spectra that have eluded a detailed understanding to date. Here, we provide a closed interpretation of cryogenic emission from BL WSe2 as a representative material for the wider class of TMD semiconductors. By combining theoretical calculations with comprehensive spectroscopy experiments, we identify the crucial role of momentum-indirect excitons for the understanding of BL TMD emission. Our results shed light on the origin of quantum dot formation in BL crystals and will facilitate further advances directed at opto-electronic applications of layered TMD semiconductors in van der Waals heterostructures and devices.
102. G. Berghaeuser, I. Bernal-Villamil, R. Schmidt, R. Schneider, I. Niehues, P. Erhart, S. Michaelis de Vasconcellos, R. Bratschitsch, A. Knorr and E. Malic, "Inverted valley polarization in optically excited transition metal dichalcogenides", Nature Communications 9, 971 (2018)
Large spin–orbit coupling in combination with circular dichroism allows access to spin-polarized and valley-polarized states in a controlled way in transition metal dichalcogenides. The promising application in spin-valleytronics devices requires a thorough understanding of intervalley coupling mechanisms, which determine the lifetime of spin and valley polarizations. Here we present a joint theory–experiment study shedding light on the Dexter-like intervalley coupling. We reveal that this mechanism couples A and B excitonic states in different valleys, giving rise to an efficient intervalley transfer of coherent exciton populations. We demonstrate that the valley polarization vanishes and is even inverted for A excitons, when the B exciton is resonantly excited and vice versa. Our theoretical findings are supported by energy-resolved and valley-resolved pump-probe experiments and also provide an explanation for the recently measured up-conversion in photoluminescence. The gained insights might help to develop strategies to overcome the intrinsic limit for spin and valley polarizations.
101. A. Raja, M. Selig, G. Berghäuser, J. Yu, H. Hill, A. Rigosi, L. Brus, A. Knorr, T. Heinz, E. Malic and A. Chernikov,"Enhanced exciton-phonon scattering from monolayer to bilayer WS2", Nano Letters 18, 6135 (2018)
Layered transition metal dichalcogenides exhibit the emergence of a direct bandgap at the monolayer limit along with pronounced excitonic effects. In these materials, interaction with phonons is the dominant mechanism that limits the exciton coherence lifetime. Exciton-phonon interaction also facilitates energy and momentum relaxation, and influences exciton diffusion under most experimental conditions. However, the fundamental changes in the exciton–phonon interaction are not well understood as the material undergoes the transition from a direct to an indirect bandgap semiconductor. Here, we address this question through optical spectroscopy and microscopic theory. In the experiment, we study room-temperature statistics of the exciton line width for a large number of mono- and bilayer WS2 samples. We observe a systematic increase in the room-temperature line width of the bilayer compared to the monolayer of 50 meV, corresponding to an additional scattering rate of ∼0.1 fs–1. We further address both phonon emission and absorption processes by examining the temperature dependence of the width of the exciton resonances. Using a theoretical approach based on many-body formalism, we are able to explain the experimental results and establish a microscopic framework for exciton–phonon interactions that can be applied to naturally occurring and artificially prepared multilayer structures.
Nano Letters 18, 6135 (2018)100. I. Niehues, R. Schmidt, M. Drueppel, P. Marauhn, D. Christiansen, M. Selig, G. Berghäuser, D. Wigger, R. Schneider, L. Braasch, R. Koch, A. Castellanos-Gomez, T. Kuhn, A. Knorr, and E. Malic, M. Rohlfing, Michaelis de Vasconcellos, R. Bratschitsch, "Strain control of exciton-phonon coupling in atomically thin semiconductors", Nano Letters 18, 1751 (2018)
Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton–phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe2, WSe2, WS2, and MoS2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS2. For MoS2 monolayers, the line width increases. These effects are due to a modified exciton–phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton–phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale.
99. P. Steinleitner, P. Merkl, A. Graf, P. Nagler, C. Schueller, T. Korn, R. Huber, S. Brem, M. Selig, G. Berghaeuser, E. Malic "Dielectric engineering of electronic correlations in a van der Waals heterostructure", Nano Letters 18, 1402 (2018)
Heterostructures of van der Waals bonded layered materials offer unique means to tailor dielectric screening with atomic-layer precision, opening a fertile field of fundamental research. The optical analyses used so far have relied on interband spectroscopy. Here we demonstrate how a capping layer of hexagonal boron nitride (hBN) renormalizes the internal structure of excitons in a WSe2 monolayer using intraband transitions. Ultrabroadband terahertz probes sensitively map out the full complex-valued mid-infrared conductivity of the heterostructure after optical injection of 1s A excitons. This approach allows us to trace the energies and line widths of the atom-like 1s–2p transition of optically bright and dark excitons as well as the densities of these quasiparticles. The excitonic resonance red shifts and narrows in the WSe2/hBN heterostructure compared to the bare monolayer. Furthermore, the ultrafast temporal evolution of the mid-infrared response function evidences the formation of optically dark excitons from an initial bright population. Our results provide key insight into the effect of nonlocal screening on electron–hole correlations and open new possibilities of dielectric engineering of van der Waals heterostructures.
98. Gunnar Berghaeuser, Philipp Steinleitner, Philipp Merkl, Rupert Huber, Andreas Knorr and Ermin Malic,"Mapping of the dark exciton landscape in transition metal dichalcogenides", Phys. Rev. B, Rapid Communications 98, 020301(R) (2018), Editor's Suggestion
Transition metal dichalcogenides (TMDs) exhibit a remarkable exciton physics including bright and optically forbidden dark excitonic states. Here, we show how dark excitons can be experimentally revealed by probing the intraexcitonic 1s−2p transition. Distinguishing the optical response shortly after the excitation and after the exciton thermalization allows us to reveal the relative position of bright and dark excitons. We find both in theory and experiment a clear blueshift in the optical response demonstrating the transition of bright exciton populations into lower-lying dark excitonic states.
97. S. Brem, F. Wendler, S. Winnerl and E. Malic, "Electrically pumped graphene-based Landau-level laser", Phys. Rev. Materials 2, 034002 (2018)
Graphene exhibits a nonequidistant Landau quantization with tunable Landau-level (LL) transitions in the technologically desired terahertz spectral range. Here, we present a strategy for an electrically driven terahertz laser based on Landau-quantized graphene as the gain medium. Performing microscopic modeling of the coupled electron, phonon, and photon dynamics in such a laser, we reveal that an inter-LL population inversion can be achieved resulting in the emission of coherent terahertz radiation. The presented paper provides a concrete recipe for the experimental realization of tunable graphene-based terahertz laser systems.
96. E. Malic, M. Selig, M. Feierabend, S. Brem, D. Christiansen, F. Wendler, A. Knorr, G. Berghaeuser, "Dark excitons in transition metal dichalcogenides", Phys. Rev. Materials 2, 014002 (2018)
Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a nondirect gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.
95. M. Feierabend, G. Berghaeuser, M. Selig, S. Brem, T. Shegai, S. Eigler, E. Malic, "Molecule signatures in photoluminescence spectra of transition metal dichalcogenides", Phys. Rev. Materials 2, 014004 (2018)
Monolayer transition metal dichalcogenides (TMDs) show an optimal surface-to-volume ratio and are thus promising candidates for novel molecule sensor devices. It was recently predicted that a certain class of molecules exhibiting a large dipole moment can be detected through the activation of optically inaccessible (dark) excitonic states in absorption spectra of tungsten-based TMDs. In this paper, we investigate the molecule signatures in photoluminescence spectra in dependence of a number of different experimentally accessible quantities, such as excitation density, temperature, as well as molecular characteristics including the dipole moment and its orientation, molecule-TMD distance, molecular coverage, and distribution. We show that under certain optimal conditions even room-temperature detection of molecules can be achieved.
94. T. Mueller and E. Malic, "2D transition metal dichalcogenide semicondutors: exciton physics and devices", npj 2D Materials and Applications 2, 29 (2018)
Two-dimensional group-VI transition metal dichalcogenide semiconductors, such as MoS2, WSe2, and others, exhibit strong light-matter coupling and possess direct band gaps in the infrared and visible spectral regimes, making them potentially interesting candidates for various applications in optics and optoelectronics. Here, we review their optical and optoelectronic properties with emphasis on exciton physics and devices. As excitons are tightly bound in these materials and dominate the optical response even at room-temperature, their properties are examined in depth in the first part of this article. We discuss the remarkably versatile excitonic landscape, including bright, dark, localized and interlayer excitons. In the second part, we provide an overview on the progress in optoelectronic device applications, such as electrically driven light emitters, photovoltaic solar cells, photodetectors, and opto-valleytronic devices, again bearing in mind the prominent role of excitonic effects. We conclude with a brief discussion on challenges that remain to be addressed to exploit the full potential of transition metal dichalcogenide semiconductors in possible exciton-based applications.
npj 2D Materials and Applications 2, 29 (2018)93. S. Brem, G. Berghaeuser, M. Selig, E. Malic, "Exciton relaxation cascade in two-dimensional transition-metal dichalcogenides", Sci. Rep 8, 8238 (2018)
Monolayers of transition metal dichalcogenides (TMDs) are characterized by an extraordinarily strong Coulomb interaction giving rise to tightly bound excitons with binding energies of hundreds of meV. Excitons dominate the optical response as well as the ultrafast dynamics in TMDs. As a result, a microscopic understanding of exciton dynamics is the key for a technological application of these materials. In spite of this immense importance, elementary processes guiding the formation and relaxation of excitons after optical excitation of an electron-hole plasma has remained unexplored to a large extent. Here, we provide a fully quantum mechanical description of momentum- and energy-resolved exciton dynamics in monolayer molybdenum diselenide (MoSe2) including optical excitation, formation of excitons, radiative recombination as well as phonon-induced cascade-like relaxation down to the excitonic ground state. Based on the gained insights, we reveal experimentally measurable features in pump-probe spectra providing evidence for the exciton relaxation cascade.
92. M. Selig, G. Berghaeuser, M. Richter, R. Bratschitsch, A. Knorr and E. Malic, "Dark and bright exciton formation, thermalization and photoluminescence in monolayer TMDs", 2D Mater. 5, 035017 (2018)
The remarkably strong Coulomb interaction in atomically thin transition metal dichalcogenides (TMDs) re-sults in an extraordinarily rich many-particle physics including the formation of tightly bound excitons. Besidesoptically accessible bright excitonic states, these materials also exhibit a variety of dark excitons. Since theycan even lie below the bright states, they have a strong influence on the exciton dynamics, lifetimes, and photo-luminescence. While very recently, the presence of dark excitonic states has been experimentally demonstrated,the origin of these states, their formation, and dynamics have not been revealed yet. Here, we present a mi-croscopic study shedding light on time- and energy-resolved formation and thermalization of bright and darkintra- and intervalley excitons as well as their impact on the photoluminescence in different TMD materials. Wedemonstrate that intervalley dark excitons, so far widely overlooked in current literature, play a crucial role intungsten-based TMDs giving rise to an enhanced photoluminescence and reduced exciton lifetimes at elevatedtemperatures.
91. I. Bernal-Villamil, G. Berghaeuser, M. Selig, I. Niehues, R. Schmidt, R. Schneider, P. Tonndorf, P. Erhart, S. Michaelis de Vasconcellos, R. Bratschitsch, A. Knorr and E. Malic, "Exciton broadening and band renormalization due to Dexter-like intervalley coupling", 2D Materials 5, 025011 (2018)
A remarkable property of atomically thin transition metal dichalcogenides (TMDs) is the possibility to selectively address single valleys by circularly polarized light. In the context of technological applications, it is very important to understand possible intervalley coupling mechanisms. Here, we show how the Dexter-like intervalley coupling mixes A and B states from opposite valleys leading to a significant broadening γB(1s) of the B1s exciton. The effect is much more pronounced in tungsten-based TMDs, where the coupling excitonic states are quasi-resonant. We calculate a ratio γB(1s)/γA(1s)≈4.0 , which is in good agreement with the experimentally measured value of 3.9±0.7. In addition to the broadening effect, the Dexter-like intervalley coupling also leads to a considerable energy renormalization resulting in an increased energetic distance between A1s and B1s states.
2D Materials 5, 025011 (2018)