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"Cedric, Robert"
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When bright and dark bind together
Magneto-photoluminescence experiments reveal the role of dark excitons in the formation of biexcitons and charged biexcitons in tungsten diselenide monolayers.
Journal Article
Controlling interlayer excitons in MoS2 layers grown by chemical vapor deposition
Combining MoS
2
monolayers to form multilayers allows to access new functionalities. Deterministic assembly of large area van der Waals structures requires concrete indicators of successful interlayer coupling in bilayers grown by chemical vapor deposition. In this work, we examine the correlation between the stacking order and the interlayer coupling of valence states in both as-grown MoS
2
homobilayer samples and in artificially stacked bilayers from monolayers, all grown by chemical vapor deposition. We show that hole delocalization over the bilayer is only allowed in 2H stacking and results in strong interlayer exciton absorption and also in a larger A-B exciton separation as compared to 3R bilayers. Comparing 2H and 3R reflectivity spectra allows to extract an interlayer coupling energy of about
t
⊥
= 49 meV. Beyond DFT calculations including excitonic effects confirm signatures of efficient interlayer coupling for 2H stacking in agreement with our experiments.
The authors investigate the interplay between the stacking order and the interlayer coupling in MoS
2
homobilayers as well as artificially stacked bilayers grown by chemical vapour deposition, and identify the interlayer exciton absorption and A-B exciton separation as indicators for interlayer coupling.
Journal Article
Giant Stark splitting of an exciton in bilayer MoS2
Transition metal dichalcogenides (TMDs) constitute a versatile platform for atomically thin optoelectronics devices and spin–valley memory applications. In monolayer TMDs the optical absorption is strong, but the transition energy cannot be tuned as the neutral exciton has essentially no out-of-plane static electric dipole1,2. In contrast, interlayer exciton transitions in heterobilayers are widely tunable in applied electric fields, but their coupling to light is substantially reduced. In this work, we show tuning over 120 meV of interlayer excitons with a high oscillator strength in bilayer MoS2 due to the quantum-confined Stark effect3. We optically probed the interaction between intra- and interlayer excitons as they were energetically tuned into resonance. Interlayer excitons interact strongly with intralayer B excitons, as demonstrated by a clear avoided crossing, whereas the interaction with intralayer A excitons is substantially weaker. Our observations are supported by density functional theory (DFT) calculations, which include excitonic effects. In MoS2 trilayers, our experiments uncovered two types of interlayer excitons with and without in-built electric dipoles. Highly tunable excitonic transitions with large in-built dipoles and oscillator strengths will result in strong exciton–exciton interactions and therefore hold great promise for non-linear optics with polaritons.Interlayer excitons in bilayer MoS2 exhibit both a high oscillator strength and highly tunable energies in an applied electric field.
Journal Article
Interlayer exciton mediated second harmonic generation in bilayer MoS2
Second-harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. Efficient SHG occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here we show tuning of non-linear optical processes in an inversion symmetric crystal. This tunability is based on the unique properties of bilayer MoS
2
, that shows strong optical oscillator strength for the intra- but also interlayer exciton resonances. As we tune the SHG signal onto these resonances by varying the laser energy, the SHG amplitude is enhanced by several orders of magnitude. In the resonant case the bilayer SHG signal reaches amplitudes comparable to the off-resonant signal from a monolayer. In applied electric fields the interlayer exciton energies can be tuned due to their in-built electric dipole via the Stark effect. As a result the interlayer exciton degeneracy is lifted and the bilayer SHG response is further enhanced by an additional two orders of magnitude, well reproduced by our model calculations. Since interlayer exciton transitions are highly tunable also by choosing twist angle and material combination our results open up new approaches for designing the SHG response of layered materials.
Efficient second-harmonic generation (SHG) occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here the authors show SHG tuning in bilayer MoS
2
- an inversion-symmetric crystal - mediated by interlayer excitons.
Journal Article
Spin/valley pumping of resident electrons in WSe2 and WS2 monolayers
Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe
2
and WS
2
monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.
Optical excitation of transition metal dichalcogenide monolayers mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here, the authors demonstrate efficient spin/valley optical pumping of resident electrons in n-doped WSe
2
and WS
2
monolayers.
Journal Article
Spin-orbit engineering in transition metal dichalcogenide alloy monolayers
Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light–matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo
(1−
x
)
W
x
Se
2
alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe
2
monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4–300 K), whereas for WSe
2
we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe
2
.
Single atomic layers of transition metal dichalcogenides are semiconductors with possible applications in spintronics. Here, the authors demonstrate tuning of the spin-orbit splitting in molybdenum tungsten diselenide by altering the alloy’s composition, impacting valley polarization and light emission yield.
Journal Article
Guide to optical spectroscopy of layered semiconductors
Potential applications in photonics and optoelectronics are based on our understanding of the light–matter interaction on an atomic monolayer scale. Atomically thin 2D transition metal dichalcogenides, such as MoS2 and WSe2, are model systems for layered semiconductors with a bandgap in the visible region of the optical spectrum. They can be assembled to form heterostructures and combine the unique properties of the constituent monolayers. In this Technical Review, we provide an introduction to optical spectroscopy for layered materials as a powerful, non-invasive tool to access details of the electronic band structure and crystal quality. We discuss the physical origin of the main absorption and emission features in the optical spectra and how they can be tuned. We explain key aspects of practical set-ups for performing experiments in different conditions and the important influence of the direct sample environment, such as substrates and encapsulation layers, on the emission and absorption mechanisms. A survey of optical techniques that probe the coupling between layers and analyse carrier polarization dynamics for spin- and valleytronics is provided.Understanding light–matter interactions in layered materials is crucial for applications in photonics and optoelectronics. This Technical Review discusses the optical spectroscopy techniques to access details of the electronic band structure, crystal quality, crystal orientation and spin–valley polarization, including key aspects of practical set-ups to perform experiments for a broad range of applications.
Journal Article
Quantum interference between dark-excitons and zone-edged acoustic phonons in few-layer WS2
Fano resonance which describes a quantum interference between continuum and discrete states, provides a unique method for studying strongly interacting physics. Here, we report a Fano resonance between dark excitons and zone-edged acoustic phonons in few-layer WS
2
by using the resonant Raman technique. The discrete phonons with large momentum at the M-point of the Brillouin zone and the continuum dark exciton states related to the optically forbidden transition at K and Q valleys are coupled by the exciton-phonon interactions. We observe rich Fano resonance behaviors across layers and modes defined by an asymmetry-parameter
q
: including constructive interference with two mirrored asymmetry Fano peaks (weak coupling,
q
> 1 and
q
< − 1), and destructive interference with Fano dip (strong coupling, ∣
q
∣ < < 1). Our results provide new insight into the exciton-phonon quantum interference in two-dimensional semiconductors, where such interferences play a key role in their transport, optical, and thermodynamic properties.
Here, the authors investigate the Raman spectra of few-layered WS
2
when the excitation energy is in resonance with the dark exciton, and observe a Fano resonance between dark excitonsand zone-edge acoustic phonons.
Journal Article
Filtering the photoluminescence spectra of atomically thin semiconductors with graphene
Atomically thin semiconductors made from transition metal dichalcogenides (TMDs) are model systems for investigations of strong light–matter interactions and applications in nanophotonics, optoelectronics and valleytronics. However, the photoluminescence spectra of TMD monolayers display a large number of features that are particularly challenging to decipher. On a practical level, monochromatic TMD-based emitters would be beneficial for low-dimensional devices, but this challenge is yet to be resolved. Here, we show that graphene, directly stacked onto TMD monolayers, enables single and narrow-line photoluminescence arising solely from TMD neutral excitons. This filtering effect stems from complete neutralization of the TMD by graphene, combined with selective non-radiative transfer of long-lived excitonic species to graphene. Our approach is applied to four tungsten- and molybdenum-based TMDs and establishes TMD/graphene heterostructures as a unique set of optoelectronic building blocks that are suitable for electroluminescent systems emitting visible and near-infrared photons at near THz rate with linewidths approaching the homogeneous limit.Interfacing TMD monolayers with graphene enables the demonstration of bright, single and narrow-line photoluminescence arising solely from TMD neutral excitons.
Journal Article