34 results on '"Gunnar Berghäuser"'
Search Results
2. The role of momentum-dark excitons in the elementary optical response of bilayer WSe2
- Author
-
Jessica Lindlau, Malte Selig, Andre Neumann, Léo Colombier, Jonathan Förste, Victor Funk, Michael Förg, Jonghwan Kim, Gunnar Berghäuser, Takashi Taniguchi, Kenji Watanabe, Feng Wang, Ermin Malic, and Alexander Högele more...
- Subjects
Science - Abstract
The electronic band structure of atomically thin transition metal dichalcogenides is strongly sensitive to the number of layers, resulting in modified light emission. Here, the authors investigate the cryogenic emission from bilayer WSe2 to identify the role of momentum-indirect excitons for its optical response. more...
- Published
- 2018
- Full Text
- View/download PDF
Catalog
3. Inverted valley polarization in optically excited transition metal dichalcogenides
- Author
-
Gunnar Berghäuser, Ivan Bernal-Villamil, Robert Schmidt, Robert Schneider, Iris Niehues, Paul Erhart, Steffen Michaelis de Vasconcellos, Rudolf Bratschitsch, Andreas Knorr, and Ermin Malic
- Subjects
Science - Abstract
In atomically thin transition metal dichalcogenides, spin- and valley-polarised states can be addressed thanks to large spin–orbit coupling and circular dichroism. Here, the authors investigate theoretically and experimentally the decay dynamics of spin and valley polarisation in transition metal dichalcogenide monolayers. more...
- Published
- 2018
- Full Text
- View/download PDF
4. Proposal for dark exciton based chemical sensors
- Author
-
Maja Feierabend, Gunnar Berghäuser, Andreas Knorr, and Ermin Malic
- Subjects
Science - Abstract
Two-dimensional materials have shown great promise as efficient chemical sensors. Here, the authors present a sensing mechanism to allow the detection of molecules based on dark excitons in atomically thin transition metal dichalcogenides. more...
- Published
- 2017
- Full Text
- View/download PDF
5. Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides
- Author
-
Malte Selig, Gunnar Berghäuser, Archana Raja, Philipp Nagler, Christian Schüller, Tony F. Heinz, Tobias Korn, Alexey Chernikov, Ermin Malic, and Andreas Knorr
- Subjects
Science - Abstract
The interplay between dark and bright excitons has a significant impact on the optical properties of semiconducting transition metal dichalcogenides. Here, the authors perform computational and experimental studies which unveil the microscopic origin of the excitonic coherence lifetime in WS2 and MoSe2. more...
- Published
- 2016
- Full Text
- View/download PDF
6. Proximity control of interlayer exciton-phonon hybridization in van der Waals heterostructures
- Author
-
Ermin Malic, Gunnar Berghäuser, Philipp Merkl, Chaw-Keong Yong, Marlene Liebich, Rupert Huber, and Isabella Hofmeister
- Subjects
Phonon ,Science ,Exciton ,General Physics and Astronomy ,02 engineering and technology ,Orbital overlap ,Two-dimensional materials ,Polaron ,01 natural sciences ,Article ,General Biochemistry, Genetics and Molecular Biology ,Condensed Matter::Materials Science ,symbols.namesake ,chemistry.chemical_compound ,0103 physical sciences ,Tungsten diselenide ,010306 general physics ,Infrared spectroscopy ,Infrared spectroscopy, Two-dimensional materials ,Condensed Matter::Quantum Gases ,Physics ,Multidisciplinary ,Condensed matter physics ,ddc:530 ,General Chemistry ,530 Physik ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,chemistry ,Quantum dot ,symbols ,van der Waals force ,0210 nano-technology ,Ground state - Abstract
Van der Waals stacking has provided unprecedented flexibility in shaping many-body interactions by controlling electronic quantum confinement and orbital overlap. Theory has predicted that also electron-phonon coupling critically influences the quantum ground state of low-dimensional systems. Here we introduce proximity-controlled strong-coupling between Coulomb correlations and lattice dynamics in neighbouring van der Waals materials, creating new electrically neutral hybrid eigenmodes. Specifically, we explore how the internal orbital 1s-2p transition of Coulomb-bound electron-hole pairs in monolayer tungsten diselenide resonantly hybridizes with lattice vibrations of a polar capping layer of gypsum, giving rise to exciton-phonon mixed eigenmodes, called excitonic Lyman polarons. Tuning orbital exciton resonances across the vibrational resonances, we observe distinct anticrossing and polarons with adjustable exciton and phonon compositions. Such proximity-induced hybridization can be further controlled by quantum designing the spatial wavefunction overlap of excitons and phonons, providing a promising new strategy to engineer novel ground states of two-dimensional systems., Here, the authors demonstrate proximity-controlled strong-coupling between Coulomb correlations and lattice dynamics in neighbouring van der Waals materials (WSe2 and a gypsum layer), creating electrically neutral hybrid exciton-phonon eigenmodes called excitonic Lyman polarons. more...
- Published
- 2021
- Full Text
- View/download PDF
7. Enhancement of Exciton–Phonon Scattering from Monolayer to Bilayer WS2
- Author
-
Alexey Chernikov, Jaeeun Yu, Malte Selig, Tony F. Heinz, Ermin Malic, Archana Raja, Heather M. Hill, Andreas Knorr, Albert F. Rigosi, Louis E. Brus, and Gunnar Berghäuser
- Subjects
Materials science ,Phonon scattering ,Condensed matter physics ,Condensed Matter::Other ,Phonon ,Mechanical Engineering ,Exciton ,Bilayer ,Bioengineering ,02 engineering and technology ,General Chemistry ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Condensed Matter::Materials Science ,Scattering rate ,0103 physical sciences ,Monolayer ,General Materials Science ,Direct and indirect band gaps ,Microscopic theory ,010306 general physics ,0210 nano-technology - Abstract
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. more...
- Published
- 2018
- Full Text
- View/download PDF
8. Dielectric Engineering of Electronic Correlations in a van der Waals Heterostructure
- Author
-
Malte Selig, Kenji Watanabe, Rupert Huber, Alexey Chernikov, Philipp Merkl, Ermin Malic, Alexander Graf, Christian Schüller, Philipp Nagler, Jonas Zipfel, Philipp Steinleitner, Tobias Korn, Takashi Taniguchi, Samuel Brem, and Gunnar Berghäuser more...
- Subjects
Materials science ,Terahertz radiation ,Exciton ,FOS: Physical sciences ,Physics::Optics ,Bioengineering ,02 engineering and technology ,Dielectric ,01 natural sciences ,Condensed Matter::Materials Science ,symbols.namesake ,0103 physical sciences ,Monolayer ,General Materials Science ,Dichalcogenides, atomically thin 2D crystals, van der Waals heterostructures, dielectric engineering, dark excitons ,010306 general physics ,Spectroscopy ,Condensed Matter - Materials Science ,Condensed matter physics ,Mechanical Engineering ,ddc:530 ,Materials Science (cond-mat.mtrl-sci) ,Heterojunction ,General Chemistry ,530 Physik ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Quasiparticle ,symbols ,van der Waals force ,0210 nano-technology - Abstract
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 WSe$_2$ 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 linewidths of the atom-like $1s$-$2p$ transition of optically bright and dark excitons as well as the densities of these quasiparticles. The fundamental excitonic resonance red shifts and narrows in the WSe$_2$/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 non local screening on electron-hole correlations and open new possibilities of dielectric engineering of van der Waals heterostructures. more...
- Published
- 2018
- Full Text
- View/download PDF
9. Proposal for dark exciton based chemical sensors
- Author
-
Gunnar Berghäuser, Andreas Knorr, Maja Feierabend, and Ermin Malic
- Subjects
Range (particle radiation) ,Multidisciplinary ,Materials science ,business.industry ,Exciton ,Science ,General Physics and Astronomy ,Sensor materials ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Article ,General Biochemistry, Genetics and Molecular Biology ,Small peak ,Dipole ,Power consumption ,0103 physical sciences ,Moment (physics) ,Optoelectronics ,Sensitivity (control systems) ,010306 general physics ,0210 nano-technology ,business - Abstract
The rapidly increasing use of sensors throughout different research disciplines and the demand for more efficient devices with less power consumption depends critically on the emergence of new sensor materials and novel sensor concepts. Atomically thin transition metal dichalcogenides have a huge potential for sensor development within a wide range of applications. Their optimal surface-to-volume ratio combined with strong light–matter interaction results in a high sensitivity to changes in their surroundings. Here, we present a highly efficient sensing mechanism to detect molecules based on dark excitons in these materials. We show that the presence of molecules with a dipole moment transforms dark states into bright excitons, resulting in an additional pronounced peak in easy accessible optical spectra. This effect exhibits a huge potential for sensor applications, since it offers an unambiguous optical fingerprint for the detection of molecules—in contrast to common sensing schemes relying on small peak shifts and intensity changes., Two-dimensional materials have shown great promise as efficient chemical sensors. Here, the authors present a sensing mechanism to allow the detection of molecules based on dark excitons in atomically thin transition metal dichalcogenides. more...
- Published
- 2017
10. Dark exciton based strain sensing in tungsten-based transition metal dichalcogenides
- Author
-
Zahra Khatibi, Ermin Malic, Maja Feierabend, and Gunnar Berghäuser
- Subjects
Photoluminescence ,Materials science ,Strain (chemistry) ,Condensed Matter::Other ,business.industry ,Exciton ,chemistry.chemical_element ,Astrophysics::Cosmology and Extragalactic Astrophysics ,02 engineering and technology ,Gauge (firearms) ,Tungsten ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,01 natural sciences ,Condensed Matter::Materials Science ,Semiconductor ,Transition metal ,chemistry ,0103 physical sciences ,Monolayer ,Optoelectronics ,010306 general physics ,0210 nano-technology ,business - Abstract
The recent emergence of atomically thin two-dimensional (2D) materials has opened up possibilities for the design of ultrathin and flexible nanoelectronic devices. As truly 2D materials, they exhibit an optimal surface-to-volume ratio, which results in an extremely high sensitivity to external changes which can not be achieved by conventional semiconductors. This makes these materials optimal candidates for sensing applications. Here, we propose a dark exciton based concept for ultrasensitive strain sensors. By investigating both dark and bright excitons in tungsten-based monolayer transition metal dichalcogenides, we demonstrate that the dark-bright-exciton separation can be controlled by strain, which has a crucial impact on the activation of dark excitonic states. The predicted opposite strain-induced shifts for dark and bright excitons result in a pronounced change in photoluminescence stemming from dark excitons even at very small strain values. The predicted high optical gauge factors of up to 8000 are promising for the design of optical strain sensors. more...
- Published
- 2019
- Full Text
- View/download PDF
11. Enhancement of Exciton-Phonon Scattering from Monolayer to Bilayer WS
- Author
-
Archana, Raja, Malte, Selig, Gunnar, Berghäuser, Jaeeun, Yu, Heather M, Hill, Albert F, Rigosi, Louis E, Brus, Andreas, Knorr, Tony F, Heinz, Ermin, Malic, and Alexey, Chernikov
- Abstract
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 WS more...
- Published
- 2018
12. Strain Control of Exciton–Phonon Coupling in Atomically Thin Semiconductors
- Author
-
Tilmann Kuhn, Malte Selig, Lisa Braasch, Steffen Michaelis de Vasconcellos, Dominik Christiansen, Daniel Wigger, Philipp Marauhn, Robert Schneider, Andres Castellanos-Gomez, Andreas Knorr, Robert Schmidt, Rouven Koch, Iris Niehues, Matthias Drüppel, Gunnar Berghäuser, Rudolf Bratschitsch, Ermin Malic, Michael Rohlfing, German Research Foundation, School of Nanophotonics (Germany), European Commission, Swedish Research Council, Castellanos-Gómez, Andrés [0000-0002-3384-3405], and Castellanos-Gómez, Andrés more...
- Subjects
Photoluminescence ,Materials science ,Absorption spectroscopy ,Phonon ,Exciton ,Bioengineering ,Line width ,02 engineering and technology ,urologic and male genital diseases ,01 natural sciences ,Strain ,Condensed Matter::Materials Science ,Transition metal dichalcogenide ,0103 physical sciences ,Monolayer ,General Materials Science ,cardiovascular diseases ,010306 general physics ,Electronic band structure ,Line (formation) ,Condensed Matter::Quantum Gases ,Condensed matter physics ,Condensed Matter::Other ,business.industry ,urogenital system ,Mechanical Engineering ,fungi ,General Chemistry ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,female genital diseases and pregnancy complications ,Exciton−phonon coupling ,Semiconductor ,Excitons ,0210 nano-technology ,business - Abstract
Niehues, Iris et al., 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., A.K., M.S., and D.C. acknowledge support by the Deutsche Forschungsgemeinschaft (DFG) through SFB 951 (to A.K.) and SFB 910 (to D.C.) and the School of Nanophotonics SFB 787 (to M.S.). E.M. and G.B. were supported by funding from the European Unions Horizon 2020 research and innovation program under grant agreement No. 696656 (Graphene Flagship) and the Swedish Research Council (VR). more...
- Published
- 2018
- Full Text
- View/download PDF
13. Dark excitons in transition metal dichalcogenides
- Author
-
Gunnar Berghäuser, Maja Feierabend, Andreas Knorr, Dominik Christiansen, Ermin Malic, Samuel Brem, Florian Wendler, and Malte Selig
- Subjects
Condensed Matter - Materials Science ,Photoluminescence ,Materials science ,Physics and Astronomy (miscellaneous) ,Condensed matter physics ,Band gap ,Atom and Molecular Physics and Optics ,Exciton ,Momentum transfer ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Wannier equation ,02 engineering and technology ,Condensed Matter Physics ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,01 natural sciences ,Momentum ,Condensed Matter::Materials Science ,0103 physical sciences ,Coulomb ,General Materials Science ,010306 general physics ,0210 nano-technology ,Electronic band structure - Abstract
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 non-direct gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states. more...
- Published
- 2018
- Full Text
- View/download PDF
14. The role of momentum-dark excitons in the elementary optical response of bilayer WSe$_{2}$
- Author
-
Jessica Lindlau, Malte Selig, Andre Neumann, Léo Colombier, Jonathan Förste, Victor Funk, Michael Förg, Jonghwan Kim, Gunnar Berghäuser, Takashi Taniguchi, Kenji Watanabe, Feng Wang, Ermin Malic, and Alexander Högele more...
- Subjects
Photoluminescence ,Band gap ,Science ,Exciton ,FOS: Physical sciences ,General Physics and Astronomy ,02 engineering and technology ,01 natural sciences ,Article ,General Biochemistry, Genetics and Molecular Biology ,Condensed Matter::Materials Science ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,0103 physical sciences ,Monolayer ,lcsh:Science ,010306 general physics ,Electronic band structure ,Physics ,Multidisciplinary ,Condensed matter physics ,Condensed Matter - Mesoscale and Nanoscale Physics ,business.industry ,Bilayer ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Semiconductor ,Quantum dot ,lcsh:Q ,0210 nano-technology ,business - Abstract
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., The electronic band structure of atomically thin transition metal dichalcogenides is strongly sensitive to the number of layers, resulting in modified light emission. Here, the authors investigate the cryogenic emission from bilayer WSe2 to identify the role of momentum-indirect excitons for its optical response. more...
- Published
- 2017
15. Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides
- Author
-
Gunnar Berghäuser, Philipp Nagler, Ermin Malic, Alexey Chernikov, Tobias Korn, Malte Selig, Tony F. Heinz, Christian Schüller, Archana Raja, and Andreas Knorr
- Subjects
Materials science ,Phonon ,Science ,Atom and Molecular Physics and Optics ,Exciton ,Binding energy ,LAYER MOS2 ,QUANTUM KINETICS ,WSE2 ,PHOTOLUMINESCENCE ,SEMICONDUCTORS ,SPECTROSCOPY ,FOS: Physical sciences ,General Physics and Astronomy ,Astrophysics::Cosmology and Extragalactic Astrophysics ,02 engineering and technology ,Electron hole ,01 natural sciences ,General Biochemistry, Genetics and Molecular Biology ,Article ,Condensed Matter::Materials Science ,Laser linewidth ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,0103 physical sciences ,Monolayer ,010306 general physics ,Condensed Matter::Quantum Gases ,Condensed Matter - Materials Science ,Multidisciplinary ,Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed matter physics ,Phonon scattering ,business.industry ,Condensed Matter::Other ,Scattering ,ddc:530 ,Optical physics ,Materials Science (cond-mat.mtrl-sci) ,General Chemistry ,Condensed Matter Physics ,530 Physik ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,Semiconductor ,Microscopic theory ,business ,0210 nano-technology ,Coherence (physics) - Abstract
Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures., The interplay between dark and bright excitons has a significant impact on the optical properties of semiconducting transition metal dichalcogenides. Here, the authors perform computational and experimental studies which unveil the microscopic origin of the excitonic coherence lifetime in WS2 and MoSe2. more...
- Published
- 2017
- Full Text
- View/download PDF
16. Molecule signatures in photoluminescence spectra of transition metal dichalcogenides
- Author
-
Maja Feierabend, Samuel Brem, Timur Shegai, Malte Selig, Gunnar Berghäuser, Siegfried Eigler, and Ermin Malic
- Subjects
Condensed Matter - Materials Science ,Photoluminescence ,Materials science ,Physics and Astronomy (miscellaneous) ,Absorption spectroscopy ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Molecular physics ,Spectral line ,Dipole ,Transition metal ,0103 physical sciences ,Monolayer ,Molecule ,General Materials Science ,010306 general physics ,0210 nano-technology ,Excitation - Abstract
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. more...
- Published
- 2017
- Full Text
- View/download PDF
17. Phonon Sidebands in Monolayer Transition Metal Dichalcogenides
- Author
-
Andreas Knorr, Ashish Arora, Ermin Malic, Malte Selig, Steffen Michaelis de Vasconcellos, Iris Niehues, Rudolf Bratschitsch, Gunnar Berghäuser, Dominik Christiansen, Robert Schneider, and Robert Schmidt more...
- Subjects
Physics ,Condensed matter physics ,Phonon ,Scattering ,Exciton ,media_common.quotation_subject ,Physics::Optics ,General Physics and Astronomy ,02 engineering and technology ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,Polaron ,01 natural sciences ,Asymmetry ,Redshift ,Condensed Matter::Materials Science ,0103 physical sciences ,Monolayer ,010306 general physics ,0210 nano-technology ,Line (formation) ,media_common - Abstract
Excitons dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). Besides optically accessible bright exciton states, TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we show that efficient exciton-phonon scattering couples bright and dark states and gives rise to an asymmetric excitonic line shape. The observed asymmetry can be traced back to phonon-induced sidebands that are accompanied by a polaron redshift. We present a joint theory-experiment study investigating the microscopic origin of these sidebands in different TMD materials taking into account intra- and intervalley scattering channels opened by optical and acoustic phonons. The gained insights contribute to a better understanding of the optical fingerprint of these technologically promising nanomaterials. more...
- Published
- 2017
- Full Text
- View/download PDF
18. Dark and bright exciton formation, thermalization, and photoluminescence in monolayer transition metal dichalcogenides
- Author
-
Gunnar Berghäuser, Andreas Knorr, Marten Richter, Malte Selig, Ermin Malic, and Rudolf Bratschitsch
- Subjects
Photoluminescence ,Exciton ,Quantum yield ,chemistry.chemical_element ,FOS: Physical sciences ,02 engineering and technology ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Tungsten ,7. Clean energy ,01 natural sciences ,Condensed Matter::Materials Science ,Transition metal ,0103 physical sciences ,Monolayer ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,Coulomb ,General Materials Science ,010306 general physics ,Condensed Matter::Quantum Gases ,Condensed matter physics ,Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter::Other ,Mechanical Engineering ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Thermalisation ,chemistry ,Mechanics of Materials ,0210 nano-technology - Abstract
The remarkably strong Coulomb interaction in atomically thin transition metal dichalcogenides (TMDs) results in an extraordinarily rich many-particle physics including the formation of tightly bound excitons. Besides optically accessible bright excitonic states, these materials also exhibit a variety of dark excitons. Since they can even lie below the bright states, they have a strong influence on the exciton dynamics, lifetimes, and photoluminescence. 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 microscopic study shedding light on time- and energy-resolved formation and thermalization of bright and dark intra- and intervalley excitons as well as their impact on the photoluminescence in different TMD materials. We demonstrate that intervalley dark excitons, so far widely overlooked in current literature, play a crucial role in tungsten-based TMDs giving rise to an enhanced photoluminescence and reduced exciton lifetimes at elevated temperatures. more...
- Published
- 2017
- Full Text
- View/download PDF
19. Molecule–substrate interaction in functionalized graphene
- Author
-
Gunnar Berghäuser and Ermin Malic
- Subjects
Molecular switch ,Substrate Interaction ,Nanostructure ,Materials science ,Graphene ,Nanotechnology ,General Chemistry ,Carbon nanotube ,law.invention ,Chemical physics ,law ,Saddle point ,General Materials Science ,Physics::Chemical Physics ,Hybrid material ,Graphene nanoribbons - Abstract
The study of carbon-based hybrid nanostructures is an emerging field of current research. In particular, photo-active molecules have been shown to considerably influence optical properties of carbon nanotubes suggesting realization of molecular switches. Here, we focus on the qualitative nature of molecule–substrate coupling within carbon-based hybrid nanostructures including nanoribbons and graphene. Our theoretical approach is based on density-matrix formalism and predicts a molecule-induced splitting of the pristine spectral resonances combined with a considerable spectral shift. Both effects strongly depend on the electronic bandstructure of the substrate. Furthermore, we investigate the impact of the substrate dimension on the coupling by increasing the width of nanoribbons from the very narrow up to graphene. Our calculations reveal a clear increase of the optical absorption of graphene in the vicinity of the Dirac point and a peak broadening at the saddle point due to the appearance of a high-energy shoulder. Our results give new insights into the molecule–substrate coupling and can guide future experiments towards the realization of tailored hybrid materials with desired optical properties. more...
- Published
- 2014
- Full Text
- View/download PDF
20. Coherent quantum dynamics of excitons in monolayer transition metal dichalcogenides
- Author
-
Kha Tran, Ermin Malic, Xiaoqin Li, Lain-Jong Li, Akshay Singh, Gunnar Berghäuser, Xiaodong Xu, Galan Moody, Chang-Hsiao Chen, Ming-Yang Li, Genevieve Clark, Chandriker Kavir Dass, Kai Hao, Andreas Knorr, and Lixiang Xu more...
- Subjects
Materials science ,Condensed matter physics ,Condensed Matter::Other ,Quantum dynamics ,Exciton ,02 engineering and technology ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,01 natural sciences ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,chemistry ,Quantum dot ,Coherent control ,0103 physical sciences ,Valleytronics ,Tungsten diselenide ,010306 general physics ,0210 nano-technology ,Coherent spectroscopy ,Biexciton - Abstract
Transition metal dichalcogenides (TMDs) have garnered considerable interest in recent years owing to their layer thickness-dependent optoelectronic properties. In monolayer TMDs, the large carrier effective masses, strong quantum confinement, and reduced dielectric screening lead to pronounced exciton resonances with remarkably large binding energies and coupled spin and valley degrees of freedom (valley excitons). Coherent control of valley excitons for atomically thin optoelectronics and valleytronics requires understanding and quantifying sources of exciton decoherence. In this work, we reveal how exciton-exciton and exciton-phonon scattering influence the coherent quantum dynamics of valley excitons in monolayer TMDs, specifically tungsten diselenide (WSe2), using two-dimensional coherent spectroscopy. Excitation-density and temperature dependent measurements of the homogeneous linewidth (inversely proportional to the optical coherence time) reveal that exciton-exciton and exciton-phonon interactions are significantly stronger compared to quasi-2D quantum wells and 3D bulk materials. The residual homogeneous linewidth extrapolated to zero excitation density and temperature is 1:6 meV (equivalent to a coherence time of 0.4 ps), which is limited only by the population recombination lifetime in this sample. (c) (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only more...
- Published
- 2016
- Full Text
- View/download PDF
21. Trion formation dynamics in monolayer transition metal dichalcogenides
- Author
-
Xiaoqin Li, Xiaodong Xu, Kha Tran, Nathaniel M. Gabor, Vincent Overbeck, Jiaqiang Yan, David Mandrus, Gunnar Berghäuser, Marie Scott, John Schaibley, Edward Seifert, Marten Richter, Dennis Pleskot, Ermin Malic, Akshay Singh, and Galan Moody more...
- Subjects
Materials science ,Exciton ,FOS: Physical sciences ,02 engineering and technology ,01 natural sciences ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,Delocalized electron ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,0103 physical sciences ,Monolayer ,010306 general physics ,Biexciton ,Condensed Matter::Quantum Gases ,Condensed Matter - Materials Science ,Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed matter physics ,Condensed Matter::Other ,Materials Science (cond-mat.mtrl-sci) ,Resonance ,Condensed Matter Physics ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,3. Good health ,chemistry ,Picosecond ,Molybdenum diselenide ,Condensed Matter::Strongly Correlated Electrons ,00A82 ,Trion ,0210 nano-technology - Abstract
We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides (TMDs), specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in TMDs. The quasiparticle formation and conversion processes are important for interpreting photoluminescence and photoconductivity in TMDs., 6 pages, 4 figures; accepted to PRB rapid more...
- Published
- 2016
- Full Text
- View/download PDF
22. Ultrafast Coulomb-Induced Intervalley Coupling in Atomically Thin WS 2
- Author
-
Robert Schmidt, Robert Schneider, Philipp Tonndorf, Andreas Knorr, Rudolf Bratschitsch, Ermin Malic, Malte Selig, Steffen Michaelis de Vasconcellos, and Gunnar Berghäuser
- Subjects
Materials science ,Photoluminescence ,Condensed matter physics ,Graphene ,Mechanical Engineering ,Bioengineering ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Polarization (waves) ,01 natural sciences ,law.invention ,law ,0103 physical sciences ,Monolayer ,Coulomb ,General Materials Science ,Microscopic theory ,010306 general physics ,0210 nano-technology ,Ultrashort pulse ,Circular polarization - Abstract
Monolayers of semiconducting transition metal dichalcogenides hold the promise for a new paradigm in electronics by exploiting the valley degree of freedom in addition to charge and spin. For MoS2, WS2, and WSe2, valley polarization can be conveniently initialized and read out by circularly polarized light. However, the underlying microscopic processes governing valley polarization in these atomically thin equivalents of graphene are still not fully understood. Here, we present a joint experiment-theory study on the ultrafast time-resolved intervalley dynamics in monolayer WS2. Based on a microscopic theory, we reveal the many-particle mechanisms behind the observed spectral features. We show that Coulomb-induced intervalley coupling explains the immediate and prominent pump-probe signal in the unpumped valley and the seemingly low valley polarization degrees typically observed in pump-probe measurements compared to photoluminescence studies. The gained insights are also applicable to other light-emitting monolayer transition metal dichalcogenides, such as MoS2 and WSe2, where the Coulomb-induced intervalley coupling also determines the initial carrier dynamics. more...
- Published
- 2016
- Full Text
- View/download PDF
23. Exciton broadening and band renormalization due to Dexter-like intervalley coupling
- Author
-
Malte Selig, Ermin Malic, Steffen Michaelis de Vasconcellos, Andreas Knorr, Paul Erhart, Iris Niehues, Robert Schmidt, Gunnar Berghäuser, Philipp Tonndorf, Rudolf Bratschitsch, Robert Schneider, and Iván Bernal-Villamil more...
- Subjects
Coupling ,Physics ,Condensed matter physics ,Spintronics ,Mechanical Engineering ,Exciton ,Context (language use) ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Renormalization ,Transition metal ,Mechanics of Materials ,0103 physical sciences ,Valleytronics ,General Materials Science ,010306 general physics ,0210 nano-technology ,Circular polarization - Abstract
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 B 1s exciton. The effect is much more pronounced in tungsten-based TMDs, where the coupling excitonic states are quasi-resonant. We calculate a ratio γB B 1s /γA B 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 A 1s and B 1s states. more...
- Published
- 2018
- Full Text
- View/download PDF
24. Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides
- Author
-
Genevieve Clark, Galan Moody, Xiaoqin Li, Kha Tran, Andreas Knorr, Kai Hao, Lain-Jong Li, Ermin Malic, Chandriker Kavir Dass, Chang-Hsiao Chen, Xiaodong Xu, Gunnar Berghäuser, and Akshay Singh
- Subjects
Materials science ,Exciton ,Population ,General Physics and Astronomy ,02 engineering and technology ,01 natural sciences ,General Biochemistry, Genetics and Molecular Biology ,Article ,Laser linewidth ,chemistry.chemical_compound ,Condensed Matter::Materials Science ,0103 physical sciences ,Monolayer ,Metallurgy and Metallic Materials ,Tungsten diselenide ,010306 general physics ,education ,Condensed Matter::Quantum Gases ,education.field_of_study ,Multidisciplinary ,Condensed matter physics ,Condensed Matter::Other ,Relaxation (NMR) ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Brillouin zone ,chemistry ,Physical Sciences ,Quasiparticle ,0210 nano-technology - Abstract
The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors., The band-edge optical response of transition metal dichalcogenides is dominated by tightly bound valley excitons. Here, the authors use optical two-dimensional Fourier transform spectroscopy to determine the exciton homogeneous linewidth in monolayer tungsten diselenide. more...
- Published
- 2015
25. Ultrafast Coulomb Intervalley Interaction in Monolayer WS2
- Author
-
Philipp Tonndorf, Robert Schneider, Robert Schmidt, Rudolf Bratschitsch, Ermin Malic, Andreas Knorr, Steffen Michaelis de Vasconcellos, and Gunnar Berghäuser
- Subjects
Photoluminescence ,Materials science ,Condensed matter physics ,business.industry ,Exciton ,Physics::Optics ,Polarization (waves) ,Condensed Matter::Soft Condensed Matter ,Condensed Matter::Materials Science ,Semiconductor ,Monolayer ,Physics::Atomic and Molecular Clusters ,Coulomb ,Physics::Chemical Physics ,Microscopic theory ,business ,Ultrashort pulse - Abstract
We reveal the ultrafast intervalley dynamics in monolayer WS 2 with a spectrally-resolved ultrafast pump-probe experiment and a microscopic theory. We find strong intervalley Coulomb coupling in the atomically thin semiconductor. more...
- Published
- 2015
- Full Text
- View/download PDF
26. Optical properties of functionalized graphene
- Author
-
Ermin Malic and Gunnar Berghäuser
- Subjects
Spiropyran ,Nanostructure ,Materials science ,Absorption spectroscopy ,Graphene ,Physics::Optics ,Nanotechnology ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,law.invention ,chemistry.chemical_compound ,Dipole ,chemistry ,law ,Chemical physics ,Surface modification ,Molecule ,Merocyanine ,Physics::Chemical Physics - Abstract
Hybrid nanostructures are an emerging field in current research. The goal is to tailor material systems with desired electronic or optical properties by functionalization with molecules. Based on the density matrix formalism, we present a theoretical study on the optical properties of graphene functionalized with photoactive spiropyran molecules. Our calculations reveal the impact of the functionalization conditions on the absorption spectrum of graphene-based hybrid nanostructures. In particular, we find that the molecular coverage has a significant effect on the optical transition energy and shape, while the molecular dipole orientation plays a minor role. The external molecular dipole field induces changes in the optical properties of graphene. Inset: The photoactive spiropyran molecules can be optically switched into the merocyanine configuration that is characterized by a pronounced dipole moment [after Guo et al., J. Am. Chem. Soc. 127(43), 15045 (2005)]. more...
- Published
- 2013
- Full Text
- View/download PDF
27. Optical Response From Functionalized Atomically Thin Nanomaterials
- Author
-
Gunnar Berghäuser, Maja Feierabend, Andreas Knorr, and Ermin Malic
- Subjects
Spiropyran ,Nanostructure ,Materials science ,Graphene ,Exciton ,General Physics and Astronomy ,Nanotechnology ,02 engineering and technology ,Carbon nanotube ,021001 nanoscience & nanotechnology ,01 natural sciences ,Nanomaterials ,law.invention ,chemistry.chemical_compound ,chemistry ,Transition metal ,law ,0103 physical sciences ,Molecule ,010306 general physics ,0210 nano-technology - Abstract
Chemical functionalization of atomically thin nanostructures presents a promising strategy to create new hybrid nanomaterials with remarkable and externally controllable properties. Here, we review our research in the field of theoretical modeling of carbon nanotubes, graphene, and transition metal dichalcogenides located in molecular dipole fields. In particular, we provide a microscopic view on the change of the optical response of these technologically promising nanomaterials due to the presence of photo-active spiropyran molecules. The feature article presents a review of recent theoretical work providing microscopic view on the optical response of chemically functionalized carbon nanotubes, graphene, and monolayered transition metal dichalcogenides. In particular, we propose a novel sensor mechanism based on the molecule-induced activation of dark excitons. This results in a pronounced additional peak presenting an unambiguous optical fingerprint for the attached molecules. more...
- Published
- 2017
- Full Text
- View/download PDF
28. Analytical approach to excitonic properties of MoS2
- Author
-
Gunnar Berghäuser and Ermin Malic
- Subjects
Physics ,Condensed Matter - Materials Science ,Absorption spectroscopy ,business.industry ,Oscillator strength ,Exciton ,Binding energy ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Polarization (waves) ,Electronic, Optical and Magnetic Materials ,Semiconductor ,Coulomb ,Trion ,Atomic physics ,business - Abstract
We present an analytical investigation of the optical absorption spectrum of monolayer molybdenum-disulfide. Based on the density matrix formalism, our approach gives insights into the microscopic origin of excitonic transitions, their relative oscillator strength, and binding energy. We show analytical expressions for the carrier-light coupling element, which contains the optical selection rules and describes well the valley-selective polarization in ${\mathrm{MoS}}_{2}$. In agreement with experimental results, we find the formation of strongly bound electron-hole pairs due to the efficient Coulomb interaction. The absorption spectrum of MoS${}_{2}$ features two pronounced peaks corresponding to the A and B exciton. For MoS${}_{2}$ on a SiO${}_{2}$ substrate, these are characterized by binding energies of 455 meV and 465 meV, respectively. Our calculations reveal their relative oscillator strength and predict the appearance of further low-intensity excitonic transitions at higher energies. The presented approach is applicable to other transition metal dichalcogenides and can be extended to investigations of trion and biexcitonic effects. more...
- Published
- 2013
29. Impact of strain on the optical fingerprint of monolayer transition-metal dichalcogenides
- Author
-
Alexandre Morlet, Ermin Malic, Maja Feierabend, and Gunnar Berghäuser
- Subjects
Materials science ,Condensed matter physics ,Condensed Matter - Mesoscale and Nanoscale Physics ,Oscillator strength ,Dephasing ,FOS: Physical sciences ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Condensed Matter::Materials Science ,Bloch equations ,Lattice (order) ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,0103 physical sciences ,Monolayer ,Radiative transfer ,Microscopic theory ,010306 general physics ,0210 nano-technology ,Electronic band structure - Abstract
Strain presents a straightforward tool to tune electronic properties of atomically thin nanomaterials that are highly sensitive to lattice deformations. While the influence of strain on the electronic band structure has been intensively studied, there are only a few works on its impact on optical properties of monolayer transition-metal dichalcogenides (TMDs). Combining microscopic theory based on Wannier and Bloch equations with nearest-neighbor tight-binding approximation, we present an analytical view on how uni- and biaxial strain influences the optical fingerprint of TMDs, including their excitonic binding energy, oscillator strength, optical selection rules, and the radiative broadening of excitonic resonances. We show that the impact of strain can be reduced to changes in the lattice structure (geometric effect) and in the orbital functions (overlap effect). In particular, we demonstrate that the valley-selective optical selection rule is softened in the case of uniaxial strain due to the introduced asymmetry in the lattice structure. Furthermore, we reveal a considerable increase of the radiative dephasing due to strain-induced changes in the optical matrix element and the excitonic wave functions. more...
- Full Text
- View/download PDF
30. Dark and bright exciton formation, thermalization, and photoluminescence in monolayer transition metal dichalcogenides.
- Author
-
Malte Selig, Gunnar Berghäuser, Marten Richter, Rudolf Bratschitsch, Andreas Knorr, and Ermin Malic
- Published
- 2018
- Full Text
- View/download PDF
31. Exciton broadening and band renormalization due to Dexter-like intervalley coupling.
- Author
-
Ivan Bernal-Villamil, Gunnar Berghäuser, Malte Selig, Iris Niehues, Robert Schmidt, Robert Schneider, Philipp Tonndorf, Paul Erhart, Steffen Michaelis de Vasconcellos, Rudolf Bratschitsch, Andreas Knorr, and Ermin Malic more...
- Published
- 2018
- Full Text
- View/download PDF
32. Optical fingerprint of non-covalently functionalized transition metal dichalcogenides
- Author
-
Maja Feierabend, Gunnar Berghäuser, Andreas Knorr, and Ermin Malic
- Subjects
Condensed Matter - Materials Science ,Materials science ,Absorption spectroscopy ,Graphene ,Exciton ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,02 engineering and technology ,Spin–orbit interaction ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Molecular physics ,3. Good health ,law.invention ,Dipole ,law ,0103 physical sciences ,Monolayer ,General Materials Science ,Direct and indirect band gaps ,010306 general physics ,0210 nano-technology ,Spin (physics) - Abstract
Atomically thin transition metal dichalcogenides (TMDs) hold promising potential for applications in optoelectronics. Due to their direct band gap and the extraordinarily strong Coulomb interaction, TMDs exhibit efficient light-matter coupling and tightly bound excitons. Moreover, large spin orbit coupling in combination with circular dichroism allows for spin and valley selective optical excitation. As atomically thin materials, they are very sensitive to changes in the surrounding environment. This motivates a functionalization approach, where external molecules are adsorbed to the materials surface to tailor its optical properties. Here, we apply the density matrix theory to investigate the potential of non-covalently functionalized TMDs. Considering exemplary spiropyran molecules with a strong dipole moment, we predict spectral redshifts and the appearance of an additional side peak in the absorption spectrum of functionalized TMDs. We show that the molecular characteristics, e.g. coverage, orientation and dipole moment, crucially influence the optical properties of TMDs, leaving a unique optical fingerprint in the absorption spectrum. Furthermore, we find that the molecular dipole moments open a channel for coherent intervalley coupling between the high-symmetry K and K' points which may open new possibilities for spin-valleytronics application., 6 pages, 5 figures more...
- Full Text
- View/download PDF
33. Optical fingerprint of dark 2p-states in transition metal dichalcogenides
- Author
-
Andreas Knorr, Gunnar Berghäuser, and Ermin Malic
- Subjects
Photoluminescence ,Absorption spectroscopy ,Exciton ,FOS: Physical sciences ,Astrophysics::Cosmology and Extragalactic Astrophysics ,02 engineering and technology ,01 natural sciences ,law.invention ,law ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,0103 physical sciences ,Coulomb ,General Materials Science ,010306 general physics ,Coupling ,Physics ,Condensed Matter - Materials Science ,Condensed matter physics ,Condensed Matter - Mesoscale and Nanoscale Physics ,business.industry ,Graphene ,Mechanical Engineering ,Materials Science (cond-mat.mtrl-sci) ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Semiconductor ,Geometric phase ,Mechanics of Materials ,0210 nano-technology ,business - Abstract
Atomically thin transition metal dichalcogenides exhibit a remarkably strong Coulomb interaction. This results in a fascinating many-particle physics including a variety of bright and dark excitonic states that determine optical and electronic properties of these materials. So far, the impact of dark states has remained literally in the dark to a large extent, since a measurement of these optically forbidden states is very challenging. Here we demonstrate a strategy to measure a direct fingerprint of dark states even in standard linear absorption spectroscopy. We present a microscopic study on bright and dark higher excitonic states in the presence of disorder for the exemplary material of tungsten disulfide (WS2). We show that the geometric phase cancels the degeneration of 2s and 2p states and that a significant disorder-induced coupling of these bright and dark states offers a strategy to circumvent optical selection rules. As a proof, we show a clear fingerprint of dark 2p states in the absorption spectrum of WS2. The predicted softening of optical selection rules through exciton-disorder coupling is of general nature and therefore applicable to related two-dimensional semiconductors. more...
- Full Text
- View/download PDF
34. Optical fingerprint of non-covalently functionalized transition metal dichalcogenides.
- Author
-
Maja Feierabend, Ermin Malic, Andreas Knorr, and Gunnar Berghäuser
- Published
- 2017
- Full Text
- View/download PDF
Discovery Service for Jio Institute Digital Library
For full access to our library's resources, please sign in.