Exciton physics in two-dimensional semiconductors and heterostructures

The enhanced Coulomb interaction in two dimensions leads to not only tightly bound excitons but also many-particle excitonic complexes: excitons interacting with other quasiparticles, which results in improved and even new exciton properties with better controls. Here, we summarize studies of excitonic complexes in monolayer transition metal dichalcogenides and their moiré heterojunctions, envisioning how to utilize them for exploring quantum many-body physics.

In this Comment, the authors discuss the current status, the challenges, and potential technological impact of exciton transport in transition metal dichalcogenide (TMD) monolayers, lateral and vertical heterostructures as well as moiré excitons in twisted TMD heterostacks.

The formation dynamics of excitons in 2D transition metal dichalcogenides are challenging to probe directly because of their inherently fast timescales. Here, the authors use extremely short optical pulses to excite an electron-hole plasma, and show the formation of 2D excitons in MoS₂ on the timescale of 30 fs.

Excitons play an important role in the optical properties of 2D semiconductors, but their spatial characterization is usually constrained by the diffraction limit. Here, the authors report near-field optical spectroscopy of 2D transition metal dichalcogenides with 20 nm resolution, revealing their spatially dependent excitonic spectra and complex dielectric function.

Excitons in various spin and valley configurations control the optical properties of ultrathin transition metal dichalcogenides. Here, the authors develop theoretical and experimental methods to determine the exciton g-factors for all possible spin-valley configurations of excitons in monolayer and bilayer WSe₂, including valley-indirect excitons.

The rational design of optoelectronic devices based on 2D materials relies on quantitative knowledge of their excitonic properties. Here the authors perform circularly-polarized absorption spectroscopy on monolayer \({{\rm{MoS}}}_{2},{{\rm{MoSe}}}_{2},{{\rm{MoTe}}}_{2}\) and \({{\rm{WS}}}_{2}\) in magnetic fields up to 91 T, and derive the effective exciton masses, binding energies, radii, dielectric properties, and free-particle bandgaps of these monolayer semiconductors

Here, the authors report intrinsic donor bound dark exciton states with associated phonon replicas in monolayer WSe₂, and defect control crystal synthesis for the deterministic creation of these states.

Here, the authors report the observation of room temperature excitons in a single layer of bismuth atoms epitaxially grown on a SiC substrate - a material of non-trivial global topology - with excitonic and topological physics deriving from the very same electronic structure.

Room temperature photoluminescence of most 2D van der Waals semiconductors originate from in-plane dipoles. Here, the authors report a combination of far-field photoluminescence measurements and ab-initio calculations to demonstrate that 2D-InSe flakes sustain luminescent excitons with an intrinsic out-of-plane orientation.

The exciton–phonon coupling (EXPC) affects the opto-electronic properties of atomically thin semiconductors. Here, the authors develop two-dimensional micro-spectroscopy to determine the EXPC of monolayer MoSe₂.

Excitons control the optical properties of transition metal dichalcogenide monolayers. Here, the authors measure the exciton fine structure of MoS₂ and MoSe₂ monolayers encapsulated in hBN in magnetic fields up to 30 T, and observe a brightening of the spin-forbidden dark excitons in MoS₂.

Here, the authors determine the exciton polarizabilities for 3- to 11-layer black phosphorus via frequency-resolved photocurrent measurements on dual-gate devices, and unveil the exciton response for higher-index sub-bands under the gate electrical field, as well as a carrier screening effect in thicker samples.

The valley degree of freedom in monolayer transition metal dichalcogenides can be addressed by optical means. Here, the authors develop a waveguide-based method to detect emission from dark excitons in single-layer WSe2, access the valley degree of freedom through the Zeeman effect, and demonstrate long valley lifetime of charged dark excitons.

The long lifetime and spin properties of dark excitons in atomically thin transition metal dichalcogenides offer opportunities to explore light-matter interactions beyond electric dipole transitions. Here, the authors demonstrate that the coupling of the dark exciton and an optically silent chiral phonon enables the intrinsic photoluminescence of the dark-exciton replica in monolayer WSe₂

Here, the authors screen hundreds of 2D materials and identify the candidates where spontaneous excitonic condensation mediated by purely electronic interaction should occur in true equilibrium. Hetero-pairs Sb₂Te₂Se/BiTeCl, Hf₂N₂I₂/Zr₂N₂Cl₂, and LiAlTe₂/BiTeI emerge as promising.

Here, the authors report the emergence of dark-excitons in transition-metal-dichalcogenide heterostructures that strongly rely on the stacking sequence, i.e., momentum-dark K-Q excitons located exclusively at the top layer of the heterostructure.

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 MoS₂ - an inversion-symmetric crystal - mediated by interlayer excitons.

The spectrally narrow photoluminescence lines occurring in transition metal dichalcogenides (TMD) heterostructures at low temperature have been attributed to interlayer excitons (IXs) localized by the moiré potential between the TMD layers. Here, the authors show that these lines are present even when the moiré potential is suppressed by inserting an hBN spacer between the TMD layers.

The authors present electroluminescence measurements of light-emitting devices based on van der Waals heterostructures, and observe a lower than expected threshold voltage for intralayer electroluminescence, attributed to non-radiative Auger-type recombination of interlayer excitons and resulting energy transfer.

In monolayer semiconductors phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone couple strongly to carriers’ spin and valley degree of freedom. Here, the authors report the observation of multiple valley phonons and the resulting exciton complexes in the monolayer semiconductor WSe₂.

Here, the authors observe tightly bound, valley-polarized, UV-emissive trions in monolayer transition metal dichalcogenide transistors. These are quasiparticles composed of an electron from a high-lying conduction band with negative effective mass, a hole from the first valence band, and an additional charge from a band-edge state.

The unique valley and spin texture of atomically thin transition metal dichalcogenides (TMDs) allows the observation of the valley Zeeman effect for neutral and charged excitons. Here, the authors unveil the underlying physics of the magneto-optical response and valley Zeeman splitting of trions in tungsten-based TMDs.

The emission of light from correlated excitonic complexes has been recently observed in atomically thin transition metal dichalcogenides. Here, the authors report electroluminescence generated by a pulsed gate voltage from excitons, trions, and biexcitons in monolayer WSe₂ and WS₂ encapsulated with hBN.

Multi-exciton states may emerge in atomically thin transition metal dichalcogenides as a result of strong many-body interactions. Here, the authors report experimental evidence of four- and five-particle biexciton complexes in monolayer WSe₂ and their electrical control.

Biexciton complexes in atomically thin transition metal dichalcogenides have unusually large binding energies. Here, the authors explore biexciton formation dynamics in monolayer MoSe₂ in the presence of magnetic fields up to 25 T.

Owing to strong Coulomb interactions, atomically thin transition metal dichalcogenides host strongly bound excitonic complexes. Here, the authors report charge-neutral biexciton and negatively charged trion-exciton complexes in hBN encapsulated monolayer WSe₂ by employing low-temperature photoluminescence spectroscopy.

High-order correlated states in atomically thin transition metal dichalcogenides may be facilitated by long-lived optically dark excitons. Here, the authors report experimentally the emergence of neutral and charged biexciton species at low light intensities in encapsulated WSe₂ monolayers.

Valleytronic devices of atomically thin transition metal dichalcogenides may benefit from high-order exciton complexes. Here, the authors present experimental evidence for four-particle biexcitons and five-particle exciton-trions in high-quality monolayer WSe₂.

The authors observe the signatures of quadrupolar excitons in a WSe₂-WS₂-WSe₂ trilayer moiré superlattice, originating from the hybridization of the WSe₂ valence moiré flatbands. They further use electrostatic gating to reveal a hybridized interlayer Mott insulator state, with holes shared between the two WSe₂ layers but laterally confined in moiré superlattices.

2D electronic spectroscopy found experimental indications of coherently interacting excitons and trions in doped transition metal dichalcogenides (TMDCs). Here, the authors perform simulations of 2D spectra of monolayer TMDCs based on a many-body formalism, allowing to relate exciton-trion coherence to quantum beats based on microscopic principles.

Here, the authors perform a spectroscopic study of monolayer WS₂ coupled to a silver nanocavity and show the emergence of resonances in reflectance contrast measurements, which they attribute to exciton- (X), trion- (T) and charged biexciton- (XX-) polaritons, sustaining strong nonlinearity.

Here, the authors perform statistical measurements on hundreds of plasmonic nano-cavities embedding WSe₂ monolayers, and show the activation of anti-Stokes photoluminescence in WSe₂ through resonant excitation of a dark exciton at room temperature.

Here, the authors use tip-enhanced photoluminescence spectroscopy to show a discontinuity of the exciton density distribution on each side of the interface of a MoSe₂/WSe₂ lateral heterostructure. They introduce the concept of ‘exciton Kapitza resistance’ by analogy with the interfacial thermal resistance known as ‘Kapitza resistance’.

Here, the authors integrate a photonic crystal, supporting photonic bound states in the continuum (BICs), with monolayer WSe₂, and leverage the high energy confinement of the BIC modes to demonstrate coherent directional dark exciton emission.

The authors investigate the interplay between the stacking order and the interlayer coupling in MoS₂ 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.

The authors induce a nanoscale strain gradient in monolayer MoS₂ suspended on a waveguide and take advantage of propagating surface plasmon polaritons to localize hot electrons in the suspended area. They funnel excitons in the waveguide, facilitating all-optical control of exciton-to-trion conversion.

Mechanical strain is a powerful tuning knob for excitons in two-dimensional semiconductors. Here, the authors find that under the application of strain, dark and localized excitons in monolayer WSe₂ are brought into energetic resonance, forming a new hybrid state that inherits the properties of the constituent species.

Here, the authors investigate the Raman spectra of few-layered WS₂ when the excitation energy is in resonance with the dark exciton, and observe a Fano resonance between dark excitonsand zone-edge acoustic phonons.

Here, the authors show brightening of dark excitons by strong coupling between cavity photons and high energy, spin-allowed, bright excitons in monolayer WSe₂. In this regime, the commonly observed photoluminescence quenching stemming from the fast relaxation to the dark ground state is prevented.

Cavity-enhanced light-matter interaction in the weak-coupling regime is known to result in Purcell enhancement. Here the authors demonstrate Purcell enhancement in the photoluminescence of vertical MoSe₂-WSe₂ heterostructures coupled to a micro-cavity and determine the light-matter coupling strength for interlayer excitons.

Here, the authors show that the interaction between microcavity photons and excitons in an atomically thin WSe₂ results in a hybridized regime of strong light-matter coupling. Coherence build-up is accompanied by a threshold-like behaviour of the emitted light intensity, which is a fingerprint of a polariton laser effect.

Here, the authors devise a strategy for prolonging the valley polarization lifetime in monolayer MoTe₂ by converting excitons to trions through gate control, and by taking advantage of the longer valley polarization lifetime of trions.

The ultrafast carrier dynamics across the van der Waals interface of transition metal dichalcogenide heterostructures govern the formation and funnelling of excitons. Here, the authors demonstrate a reversible switch from exciton dissociation to exciton funnelling in a MoSe₂/WS₂ heterostructure, which manifests itself as a photoluminescence quenching-to-enhancement transition.

Quantum emitters in atomically thin materials host optically addressable single spins. Here, the authors demonstrate spin selectivity in the preparation of the spin-valley state of site-controlled localized single holes in CrI3/WSe2 heterostructures.

Room-temperature exciton polaritons in a monolayer WS₂ are shown to display strong motional narrowing of the linewidth and enhanced first-order coherence. They can propagate for tens of micrometers while maintaining partial coherence, and display signatures of ballistic (dissipationless) transport.

Strain engineering can manipulate the propagation of excitons in atomically thin transition metal dichalcogenides. Here, the authors observe an anti-funnelling behavior, i.e., the exciton photoluminescence moves away from high-strain regions, and attribute it to the dominating role of propagating dark excitons.

Excitons in atomically thin crystals couple strongly with light. Here, the authors observe lattice polaritons in a tunable open optical cavity at room temperature, with an imprinted photonic lattice strongly coupled to excitons in a WS₂ monolayer.

Here, the authors use a tapered optical fibre to create a dynamic, reversible strain in a suspended WSe₂ monolayer, and observe that dark excitons are funnelled to high-strain regions and are the principal participants in drift and diffusion at cryogenic temperatures.

Excitons in 2D semiconductors suffer from a weak response to in-plane electric fields, inhibiting their transport beyond the diffusion length. Here, the authors demonstrate the directional, long-range transport of interlayer excitons in bilayer WSe₂ driven by the propagating potential traps induced by surface acoustic waves.

The authors unveil the many-particle processes underpinning the formation of bound charge transfer excitons at the interface of hBN-encapsulated lateral MoSe₂-WSe₂ heterostructures. The excitons can be tuned via interface (i.e. high quality lateral junction) and dielectric (i.e. hBN encapsulation) engineering.

Here, the authors demonstrate that dark excitons in two-dimensional transition metal dichalcogenides can diffuse over several micrometers, and prove that this repulsion-driven propagation is robust across non-uniform samples.

Here, the authors investigate the interactions between Fermi polarons in monolayer WS₂ by multi-dimensional coherent spectroscopy, and find that, at low electron doping densities, the dominant interactions are between polaron states that are dressed by the same Fermi sea. They also observe a bipolaron bound state with large binding energy, involving excitons in different valleys cooperatively bound to the same electron.

The authors embed a multiple quantum-well WS₂ heterostructure in a planar microcavity and show the systematic control of the normal mode coupling-strength. They find a strong enhancement of the characteristic time scale, which they attribute to long-lived dark excitations emerging in the structure.

The authors investigate the optical response of marginally twisted MoSe₂/WSe₂ heterobilayers and identify two types of trapped excitons depending on laser excitation power and temperature: excitons trapped in shallow defects and excitons trapped in the moire potential. At strong excitation powers, interlayer biexcitons are identified.

Here, the authors observe that valley-polarized dark excitons in monolayer WSe₂ show a distinct doping dependence when the carriers reach a critical density. This is indicative of the onset of strongly modified Fermi sea interactions.

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.

Dipolar excitons enable large nonlinear interaction but are usually hampered by their weak oscillator strength. Here, the authors demonstrate the strong light-matter coupling of interlayer dipolar excitons having unusually large oscillator strength in bilayer MoS₂ resulting in highly nonlinear dipolar polaritons.

Naturally occurring hyperbolic polaritons exist in a class of layered materials. Here, the authors show evidence, via optical spectroscopy, of hyperbolic exciton-polaritons in phosphorene, originating from its in-plane anisotropy and strong exciton resonances.

Microcavity exciton-polaritons in atomically thin semiconductors are a promising platform for valley manipulation. Here, the authors show valley-selective control of polariton energies in monolayer WS2 using the optical Stark effect, thereby extending coherent valley manipulation to a hybrid light-matter regime

Here, the authors show the formation of exciton-polaritons with enhanced nonlinear response using excited excitonic Rydberg states in monolayer WSe₂ embedded in a microcavity.

Here, the authors report the creation of trion-polaritons in monolayer MoSe₂ in an open microcavity exhibiting strong nonlinear interactions, one order of magnitude bigger than those observed for exciton polaritons in GaAs.

Owing to the presence of a valley degree of freedom, atomically thin transition metal dichalcogenides show promise for room temperature valleytronic applications. Here, the authors use polarization-resolved Raman spectroscopy to gain insight to the exciton-phonon coupling in charge tunable single layer MoS₂.

The authors investigate the optical properties of a heterostructure formed by a metallic substrate and a nanostructured transition metal dichalcogenide multilayer by measuring the reflectance spectrum at different multilayer thicknesses, filling factors and grating periods. The spectra show strong dispersion and avoided crossing of excitons, plasmons and cavity photons along with excitonic mode suppression at the anti-crossing point.

An out-of-plane magnetic field is expected to strongly modify exciton-phonon interactions in atomically thin transitional metal dichalcogenides. Here, the authors show that the phonon-exciton interaction in monolayer WSe₂ lifts the inter-Landau-level transition selection rules for dark trions.

The short exciton life time in atomically thin transition metal dichalcogenides poses limitations to efficient control of the valley pseudospin and coherence. Here, the authors manipulate the exciton coherence in a WSe₂ monolayer embedded in an optical microcavity in the strong light-matter coupling regime.

Correlated insulator states of moire excitons in transition metal dichalcogenide heterostructures have attracted significant attention recently. Here the authors use time-resolved pump-probe spectroscopy to demonstrate the effects of non-equilibrium correlations of moire excitons in WSe₂/WS₂ heterobilayers.

Here, the authors show the dynamic tuning of the moiré potential in a WS₂/WSe₂ heterobilayer by gate voltage and optical power, allowing for simultaneous observation of the first and second order Stark shift for the ground state and first excited state, respectively, of the moiré trapped exciton.

The moiré lattice of trilayer WSe₂ and monolayer WS₂ host interlayer excitons localized at different moiré sites. Using an electric field, the authors tune the superposition between two moiré sites via hybridization with intralayer excitons in WSe₂.

Twisted heterostructures of transition metal dichalcogenides host the so-called moiré excitons, or intralayer excitons modified by the moiré potential. Here the authors show tunability of the moiré excitons and the coexisting correlated electronic states in WSe₂/WS₂ superlattices with varying WSe₂ layer thickness

Here, the authors show that the photoluminescence of MoSe₂/WSe₂ heterobilayers is dominated by valley-direct excitons, whereas, in heterotrilayers, interlayer hybridization turns momentum-indirect interlayer excitons into energetically lowest states with phonon-assisted emission.

Here, the authors show that the properties of the moiré excitons in twisted van der Waals bilayers of transition metal dichalcogenides are determined by the moiré reciprocal lattice period, and can be controlled via twist-angle tuning.

A correlated insulator has recently been observed in twisted bilayer WSe₂ moiré superlattices. Here, Bi and Fu present a theory and predict the insulating state to be an excitonic density wave as well as a spin-valley superfluid, which can be tested by optical pump-probe experiments.

Heterobilayers of transition metal dichalcogenides host moiré superlattices that give rise to strong electron interactions. Here, the authors study the photoluminescence from interlayer excitons in a WS₂/WSe₂ heterobilayer to reveal the onset of various correlated insulating states.

Interlayer electronic states in twisted bilayer graphene are characterized by flat-band regions hosting many-body electronic effects. Here, the authors observe two-photon photoluminescence excitation and excited-state absorption spectra on graphene containing a variety of twist angles to access the dark exciton transitions and estimate the exciton binding energy.

The nature of localized interlayer excitons (LIXs) in moiré superlattices is still elusive Here, the authors propose a donor-acceptor pair mechanism for LIXs in MoSe₂/WSe₂ heterobilayers.