Showing papers on "Exciton published in 2021"

Recent progress in hot exciton materials for organic light-emitting diodes

Abstract: According to Kasha's rule, high-lying excited states usually have little effect on fluorescence. However, in some molecular systems, the high-lying excited states partly or even mainly contribute to the photophysical properties, especially in the process of harvesting triplet excitons in organic electroluminescent devices. In the current review, we focus on a type of organic light-emitting diode (OLED) materials called “hot exciton” materials, which can effectively harness the non-radiative triplet excitons via reverse intersystem crossing (RISC) from high-lying triplet states to singlet states (Tn → Sm; n ≥ 2, m ≥ 1). Since Ma and Yang proposed the hot exciton mechanism for OLED material design in 2012, there have been many reports aiming at the design and synthesis of novel hot exciton luminogens. Herein, we present a comprehensive review of the recent progress in hot exciton materials. The developments of the hot exciton mechanism are reviewed, the fundamental principles regarding molecular design are discussed, and representative reported hot exciton luminogens are summarized and analyzed, along with their structure–property relationships and OLED applications.

Evidence of high-temperature exciton condensation in 2D atomic double layers

Abstract: A Bose-Einstein condensate is the ground state of a dilute gas of bosons, such as atoms cooled to temperatures close to absolute zero. With much smaller mass, excitons (bound electron-hole pairs) are expected to condense at significantly higher temperatures. Here we study electrically generated interlayer excitons in MoSe2/WSe2 atomic double layers with density up to 10^12 cm-2. The interlayer tunneling current depends only on exciton density, indicative of correlated electron-hole pair tunneling. Strong electroluminescence (EL) arises when a hole tunnels from WSe2 to recombine with electron in MoSe2. We observe a critical threshold dependence of the EL intensity on exciton density, accompanied by a super-Poissonian photon statistics near threshold, and a large EL enhancement peaked narrowly at equal electron-hole densities. The phenomenon persists above 100 K, which is consistent with the predicted critical condensation temperature. Our study provides compelling evidence for interlayer exciton condensation in two-dimensional atomic double layers and opens up exciting opportunities for exploring condensate-based optoelectronics and exciton-mediated high-temperature superconductivity.

Signatures of Wigner crystal of electrons in a monolayer semiconductor.

Abstract: When the Coulomb repulsion between electrons dominates over their kinetic energy, electrons in two-dimensional systems are predicted to spontaneously break continuous-translation symmetry and form a quantum crystal1. Efforts to observe2–12 this elusive state of matter, termed a Wigner crystal, in two-dimensional extended systems have primarily focused on conductivity measurements on electrons confined to a single Landau level at high magnetic fields. Here we use optical spectroscopy to demonstrate that electrons in a monolayer semiconductor with density lower than 3 × 1011 per centimetre squared form a Wigner crystal. The combination of a high electron effective mass and reduced dielectric screening enables us to observe electronic charge order even in the absence of a moire potential or an external magnetic field. The interactions between a resonantly injected exciton and electrons arranged in a periodic lattice modify the exciton bandstructure so that an umklapp resonance arises in the optical reflection spectrum, heralding the presence of charge order13. Our findings demonstrate that charge-tunable transition metal dichalcogenide monolayers14 enable the investigation of previously uncharted territory for many-body physics where interaction energy dominates over kinetic energy. The signature of a Wigner crystal—the analogue of a solid phase for electrons—is observed via the optical reflection spectrum in a monolayer transition metal dichalcogenide.

Pseudo-bilayer architecture enables high-performance organic solar cells with enhanced exciton diffusion length.

Abstract: Solution-processed organic solar cells (OSCs) are a promising candidate for next-generation photovoltaic technologies. However, the short exciton diffusion length of the bulk heterojunction active layer in OSCs strongly hampers the full potential to be realized in these bulk heterojunction OSCs. Herein, we report high-performance OSCs with a pseudo-bilayer architecture, which possesses longer exciton diffusion length benefited from higher film crystallinity. This feature ensures the synergistic advantages of efficient exciton dissociation and charge transport in OSCs with pseudo-bilayer architecture, enabling a higher power conversion efficiency (17.42%) to be achieved compared to those with bulk heterojunction architecture (16.44%) due to higher short-circuit current density and fill factor. A certified efficiency of 16.31% is also achieved for the ternary OSC with a pseudo-bilayer active layer. Our results demonstrate the excellent potential for pseudo-bilayer architecture to be used for future OSC applications.

Boosting triplet self-trapped exciton emission in Te(IV)-doped Cs 2 SnCl 6 perovskite variants

Abstract: Perovskite variants have attracted wide interest because of the lead-free nature and strong self-trapped exciton (STE) emission. Divalent Sn(II) in CsSnX3 perovskites is easily oxidized to tetravalent Sn(IV), and the resulted Cs2SnCl6 vacancy-ordered perovskite variant exhibits poor photoluminescence property although it has a direct band gap. Controllable doping is an effective strategy to regulate the optical properties of Cs2SnX6. Herein, combining the first principles calculation and spectral analysis, we attempted to understand the luminescence mechanism of Te4+-doped Cs2SnCl6 lead-free perovskite variants. The chemical potential and defect formation energy are calculated to confirm theoretically the feasible substitutability of tetravalent Te4+ ions in Cs2SnCl6 lattices for the Sn-site. Through analysis of the absorption, emission/excitation, and time-resolved photoluminescence (PL) spectroscopy, the intense green-yellow emission in Te4+:Cs2SnCl6 was considered to originate from the triplet Te(IV) ion 3P1→1S0 STE recombination. Temperature-dependent PL spectra demonstrated the strong electron-phonon coupling that inducing an evident lattice distortion to produce STEs. We further calculated the electronic band structure and molecular orbital levels to reveal the underlying photophysical process. These results will shed light on the doping modulated luminescence properties in stable lead-free Cs2MX6 vacancy-ordered perovskite variants and be helpful to understand the optical properties and physical processes of doped perovskite variants.

Excitons in a reconstructed moiré potential in twisted WSe2/WSe2 homobilayers

Abstract: Moire superlattices in twisted van der Waals materials have recently emerged as a promising platform for engineering electronic and optical properties. A major obstacle to fully understanding these systems and harnessing their potential is the limited ability to correlate direct imaging of the moire structure with optical and electronic properties. Here we develop a secondary electron microscope technique to directly image stacking domains in fully functional van der Waals heterostructure devices. After demonstrating the imaging of AB/BA and ABA/ABC domains in multilayer graphene, we employ this technique to investigate reconstructed moire patterns in twisted WSe2/WSe2 bilayers and directly correlate the increasing moire periodicity with the emergence of two distinct exciton species in photoluminescence measurements. These states can be tuned individually through electrostatic gating and feature different valley coherence properties. We attribute our observations to the formation of an array of two intralayer exciton species that reside in alternating locations in the superlattice, and open up new avenues to realize tunable exciton arrays in twisted van der Waals heterostructures, with applications in quantum optoelectronics and explorations of novel many-body systems. Scanning electron microscopy is used to image stacking domains in few-layer graphene, as well as moire patterns in twisted van der Waals heterostructures, allowing for the correlation of the local structure with their excitonic properties.

Twist Angle-Dependent Interlayer Exciton Lifetimes in van der Waals Heterostructures.

Abstract: In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in MoSe_{2}/WSe_{2} twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1° to 3.5°. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moire potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of "twistronics".

Perovskite semiconductors for room-temperature exciton-polaritonics.

Abstract: Lead-halide perovskites are generally excellent light emitters and can have larger exciton binding energies than thermal energy at room temperature, exhibiting great promise for room-temperature exciton-polaritonics. Rapid progress has been made recently, although challenges and mysteries remain in lead-halide perovskite semiconductors to push polaritons to room-temperature operation. In this Perspective, we discuss fundamental aspects of perovskite semiconductors for exciton-polaritons and review the recent rapid experimental advances using lead-halide perovskites for room-temperature polaritonics, including the experimental realization of strong light–matter interaction using various types of microcavities as well as reaching the polariton condensation regime in planar microcavities and lattices. An outlook on the potential of lead-halide perovskites as a playground for exciton-polariton studies and for the development of polaritonic devices operating at room temperature is provided.

Van der Waals heterostructure polaritons with moiré-induced nonlinearity

Abstract: Controlling matter-light interactions with cavities is of fundamental importance in modern science and technology1. This is exemplified in the strong-coupling regime, where matter-light hybrid modes form, with properties that are controllable by optical-wavelength photons2,3. By contrast, matter excitations on the nanometre scale are harder to access. In two-dimensional van der Waals heterostructures, a tunable moire lattice potential for electronic excitations may form4, enabling the generation of correlated electron gases in the lattice potentials5-9. Excitons confined in moire lattices have also been reported10,11, but no cooperative effects have been observed and interactions with light have remained perturbative12-15. Here, by integrating MoSe2-WS2 heterobilayers in a microcavity, we establish cooperative coupling between moire-lattice excitons and microcavity photons up to the temperature of liquid nitrogen, thereby integrating versatile control of both matter and light into one platform. The density dependence of the moire polaritons reveals strong nonlinearity due to exciton blockade, suppressed exciton energy shift and suppressed excitation-induced dephasing, all of which are consistent with the quantum confined nature of the moire excitons. Such a moire polariton system combines strong nonlinearity and microscopic-scale tuning of matter excitations using cavity engineering and long-range light coherence, providing a platform with which to study collective phenomena from tunable arrays of quantum emitters.

Small Exciton Binding Energies Enabling Direct Charge Photogeneration Towards Low-Driving-Force Organic Solar Cells

Abstract: Organic solar cells (OSCs) with nonfullerene acceptors (NFAs) exhibit efficient charge generation under small interfacial energy offsets, leading to over 18% efficiency for the single-junction devices based on the state-of-the-art NFA of Y6. Herein, to reveal the underlying charge generation mechanisms, we have investigated the exciton binding energy ( E b ) in Y6 by a joint theoretical and experimental study. The results show that owing to strong charge polarization effects, Y6 has remarkable small E b of -0.11~0.15 eV, which is even lower than perovskites in many cases. Moreover, it is peculiar that the photoluminescence is enhanced with temperature, and the energy barrier for separating excitons into charges is evidently lower than the thermal energy according to the temperature dependence of photoluminescence, manifesting direct photogeneration of charge carriers enabled by weak E b in Y6. Thus, charge generation in NFA-based OSCs shows little dependence on interfacial driving forces.

Targeted regulation of exciton dissociation in graphitic carbon nitride by vacancy modification for efficient photocatalytic CO2 reduction

Abstract: The excitons are known to exist in graphitic carbon nitride (g-C3N4), but the contribution of excitons to the photocatalytic process has received only sporadic attention Targeted regulation of excitons dissociation into free charges is an effective measure to improve the utilization of carriers and enhance photocatalytic activity Here we designed the ultra-thin g-C3N4 nanosheets modified with rich N vacancies (Nv-rich-CN) through molecular self-assembly and molecular intercalation strategies N vacancies can effectively dissociate excitons into free charges The electron concentration of the Nv-rich-CN is 324 times that of bulk g-C3N4 (CN) Benefiting from the enhanced carrier utilization, Nv-rich-CN showed superior activity for CO2 photoreduction under visible light irradiation Femtosecond transient absorption spectroscopy revealed a photophysical model of exciton dissociation Theoretical calculation results explained that the essential reason for the vacancy to promote exciton dissociation may be that the lack of local order provided a driving force for exciton dissociation Besides, N vacancies acted as active sites in the process of CO2 reduction, promoting the adsorption and activation of CO2 by the photocatalyst In situ diffuse reflectance infrared Fourier transform spectroscopy revealed the change of intermediate products during the reduction of CO2 This work focuses on the contribution of excitons in the photocatalytic process and provides a novel idea for enhancing the carrier utilization of photocatalysts

Strongly correlated excitonic insulator in atomic double layers

Abstract: Excitonic insulators (EI) arise from the formation of bound electron-hole pairs (excitons) in semiconductors and provide a solid-state platform for quantum many-boson physics. Strong exciton-exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations. Although spectroscopic signatures of EIs have been reported, conclusive evidence for strongly correlated EI states has remained elusive. Here, we demonstrate a strongly correlated spatially indirect two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. An equilibrium interlayer exciton fluid is formed when the bias voltage applied between the two electrically isolated TMD layers, is tuned to a range that populates bound electron-hole pairs, but not free electrons or holes. Capacitance measurements show that the fluid is exciton-compressible but charge-incompressible - direct thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We further construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing the exotic quantum phases of excitons, as well as multi-terminal exciton circuitry for applications.

Strongly correlated excitonic insulator in atomic double layers

Abstract: Excitonic insulators (EIs) arise from the formation of bound electron–hole pairs (excitons)1,2 in semiconductors and provide a solid-state platform for quantum many-boson physics3–8. Strong exciton–exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations8–11. Although spectroscopic signatures of EIs have been reported6,12–14, conclusive evidence for strongly correlated EI states has remained elusive. Here we demonstrate a strongly correlated two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. A quasi-equilibrium spatially indirect exciton fluid is created when the bias voltage applied between the two electrically isolated TMD layers is tuned to a range that populates bound electron–hole pairs, but not free electrons or holes15–17. Capacitance measurements show that the fluid is exciton-compressible but charge-incompressible—direct thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing exotic quantum phases of excitons8, as well as multi-terminal exciton circuitry for applications18–20. So far only signatures of excitonic insulators have been reported, but here direct thermodynamic evidence is provided for a strongly correlated excitonic insulating state in transition metal dichalcogenide semiconductor double layers.

Self-Trapped Exciton Emission in a Zero-Dimensional (TMA)2SbCl5·DMF Single Crystal and Molecular Dynamics Simulation of Structural Stability.

Abstract: Lead-free lower-dimensional organic-inorganic metal halide materials have recently triggered intense research because of their excellent photophysical properties and chemical stability. Herein, we report a novel zero-dimensional (0D) organic-inorganic hybrid single crystal (TMA)2SbCl5·DMF (TMA = N(CH3)3, DMF= HCON(CH3)2), which exhibits typical self-trapped exciton (STE) emission with an efficient yellow emission at 630 nm and high photoluminescence quantum yield (PLQY) of 67.2%. The dual STE emission is attributed to the singlet and triplet STEs in inorganic [SbCl5]2-, respectively. Further, an ab initio molecular dynamics simulation was performed to estimate the stability of crystal structure at room temperature. The calculated excited-state structure indicates that the deformation parameter (Δd) of the excited-state structure is larger than that of the ground state, illustrating the origin of a large Stokes shift. These results indicate that these new 0D lead-free organic-inorganic hybrid metal halides are promising luminescent materials for optoelectronic applications.

Tuning layer-hybridized moiré excitons by the quantum-confined Stark effect.

Abstract: Moire superlattices offer an unprecedented opportunity for tailoring interactions between quantum particles1-11 and their coupling to electromagnetic fields12-18. Strong superlattice potentials generate moire minibands of excitons16-18-bound pairs of electrons and holes that reside either in a single layer (intralayer excitons) or in two separate layers (interlayer excitons). Twist-angle-controlled interlayer electronic hybridization can also mix these two types of exciton to combine their strengths13,19,20. Here we report the direct observation of layer-hybridized moire excitons in angle-aligned WSe2/WS2 and MoSe2/WS2 superlattices by optical reflectance spectroscopy. These excitons manifest a hallmark signature of strong coupling in WSe2/WS2, that is, energy-level anticrossing and oscillator strength redistribution under a vertical electric field. They also exhibit doping-dependent renormalization and hybridization that are sensitive to the electronic correlation effects. Our findings have important implications for emerging many-body states in two-dimensional semiconductors, such as exciton condensates21 and Bose-Hubbard models22, and optoelectronic applications of these materials.

Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping

Abstract: The outstanding optoelectronic performance of lead halide perovskites lies in their exceptional carrier diffusion properties. As the perovskite material dimensionality is reduced to exploit the quantum confinement effects, the disruption to the perovskite lattice, often with insulating organic ligands, raises new questions on the charge diffusion properties. Herein, we report direct imaging of >1 μm exciton diffusion lengths in CH3NH3PbBr3 perovskite nanocrystal (PNC) films. Surprisingly, the resulting exciton mobilities in these PNC films can reach 10 ± 2 cm2 V−1 s−1, which is counterintuitively several times higher than the carrier mobility in 3D perovskite films. We show that this ultralong exciton diffusion originates from both efficient inter-NC exciton hopping (via Forster energy transfer) and the photon recycling process with a smaller yet significant contribution. Importantly, our study not only sheds new light on the highly debated origins of the excellent exciton diffusion in PNC films but also highlights the potential of PNCs for optoelectronic applications. Excitons quasi-particles in methylammonium lead bromide perovskite nanocrystal films can travel over a surprisingly long distance. Tze Chien Sum and colleagues at Singapore’s Nanyang Technological University uncovered that the long diffusion lengths of more than one micrometer are largely due to excitons hopping from one nanocrystal to another. They are also caused, albeit to a lesser degree, by photon recycling, in which absorbed photons are re-emitted within the material, generating more charge carriers. The team found that perovskite nanocrystals connected by octylamine ligands showed the longest exciton hopping range compared to those connected by hexylamine or oleylamine ligands. The findings demonstrate the potential of lead halide perovskite nanocrystal films for fabricating smaller, faster, and more energy-efficient electronic devices. They also improve the basic understanding of the mechanisms behind long-range energy transport in these films.

Electron-exciton interactions in the exciton-polaron problem

Abstract: Recently, it has been demonstrated that the absorption of moderately doped two-dimensional semiconductors can be described in terms of exciton polarons. In this scenario, attractive and repulsive polaron branches are formed due to interactions between a photoexcited exciton and a Fermi sea of excess charge carriers. These interactions have previously been treated in a phenomenological manner. Here, we present a microscopic derivation of the electron-exciton interactions which utilizes a mixture of variational and perturbative approaches. We find that the interactions feature classical charge-dipole behavior in the long-range limit, and that they are only weakly modified for moderate doping. We apply our theory to the absorption properties and show that the dependence on doping is well captured by a model with a phenomenological contact potential.

Real-time GW-BSE investigations on spin-valley exciton dynamics in monolayer transition metal dichalcogenide.

Abstract: We develop an ab initio nonadiabatic molecular dynamics (NAMD) method based on GW plus real-time Bethe-Salpeter equation (GW + rtBSE-NAMD) for the spin-resolved exciton dynamics. From investigations on MoS2, we provide a comprehensive picture of spin-valley exciton dynamics where the electron-phonon (e-ph) scattering, spin-orbit interaction (SOI), and electron-hole (e-h) interactions come into play collectively. In particular, we provide a direct evidence that e-h exchange interaction plays a dominant role in the fast valley depolarization within a few picoseconds, which is in excellent agreement with experiments. Moreover, there are bright-to-dark exciton transitions induced by e-ph scattering and SOI. Our study proves that e-h many-body effects are essential to understand the spin-valley exciton dynamics in transition metal dichalcogenides and the newly developed GW + rtBSE-NAMD method provides a powerful tool for exciton dynamics in extended systems with time, space, momentum, energy, and spin resolution.

Interlayer electronic coupling on demand in a 2D magnetic semiconductor.

Abstract: When monolayers of two-dimensional (2D) materials are stacked into van der Waals structures, interlayer electronic coupling can introduce entirely new properties, as exemplified by recent discoveries of moire bands that host highly correlated electronic states and quantum dot-like interlayer exciton lattices. Here we show the magnetic control of interlayer electronic coupling, as manifested in tunable excitonic transitions, in an A-type antiferromagnetic 2D semiconductor CrSBr. Excitonic transitions in bilayers and above can be drastically changed when the magnetic order is switched from the layered antiferromagnetic ground state to a field-induced ferromagnetic state, an effect attributed to the spin-allowed interlayer hybridization of electron and hole orbitals in the latter, as revealed by Green’s function–Bethe–Salpeter equation (GW-BSE) calculations. Our work uncovers a magnetic approach to engineer electronic and excitonic effects in layered magnetic semiconductors. Interlayer hybridization in 2D van der Waals materials can change their properties. Here, it is shown that the coupling in CrSBr can be changed from switching the magnetic order from antiferromagnetic to ferromagnetic states.

Excitonic Dynamics in Janus MoSSe and WSSe Monolayers

Abstract: We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements determined the room-temperature exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron-phonon interaction by the built-in dipole moment. By combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial separation of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.

Direct measurement of key exciton properties: Energy, dynamics, and spatial distribution of the wave function

Abstract: 1 Fritz-Haber-Institut derMax-Planck-Gesellschaft, Berlin, Germany 2 Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), École polytechnique fédérale de Lausanne, ISIC, Lausanne, Switzerland 3 Institut für Theoretische Physik, Nichtlineare Optik undQuantenelektronik, Technische Universität Berlin, Berlin, Germany 4 Max Planck Institute for the Structure andDynamics ofMatter and Center for Free Electron Laser Science, Hamburg, Germany 5 Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, Fribourg, Switzerland 6 Department of Applied Physics, KTHRoyal Institute of Technology, Stockholm, Sweden 7 SwissFEL, Paul Scherrer Institute, Villigen, Switzerland 8 Department of Physics, Chalmers University of Technology, Gothenburg, Sweden

Correlated interlayer exciton insulator in double layers of monolayer WSe2 and moiré WS2/WSe2

Abstract: Moire superlattices in van der Waals heterostructures have emerged as a powerful tool for engineering novel quantum phenomena. Here we report the observation of a correlated interlayer exciton insulator in a double-layer heterostructure composed of a WSe2 monolayer and a WS2/WSe2 moire bilayer that are separated by an ultrathin hexagonal boron nitride (hBN). The moire WS2/WSe2 bilayer features a Mott insulator state at hole density p/p0 = 1, where p0 corresponds to one hole per moire lattice site. When electrons are added to the Mott insulator in the WS2/WSe2 moire bilayer and an equal number of holes are injected into the WSe2 monolayer, a new interlayer exciton insulator emerges with the holes in the WSe2 monolayer and the electrons in the doped Mott insulator bound together through interlayer Coulomb interactions. The excitonic insulator is stable up to a critical hole density of ~ 0.5p0 in the WSe2 monolayer, beyond which the system becomes metallic. Our study highlights the opportunities for realizing novel quantum phases in double-layer moire systems due to the interplay between the moire flat band and strong interlayer electron interactions.

Phonon scattering and exciton localization: molding exciton flux in two dimensional disorder energy landscape

Abstract: Two dimensional excitonic devices are of great potential to overcome the dilemma of response time and integration in current generation of electron or/and photon based systems. The ultrashort diffusion length of exciton arising from ultrafast relaxation and low carrier mobility greatly discounts the performance of excitonic devices. Phonon scattering and exciton localization are crucial to understand the modulation of exciton flux in two dimensional disorder energy landscape, which still remain elusive. Here, we report an optimized scheme for exciton diffusion and relaxation dominated by phonon scattering and disorder potentials in WSe2 monolayers. The effective diffusion coefficient is enhanced by > 200% at 280 K. The excitons tend to be localized by disorder potentials accompanied by the steadily weakening of phonon scattering when temperature drops to 260 K, and the onset of exciton localization brings forward as decreasing temperature. These findings identify that phonon scattering and disorder potentials are of great importance for long-range exciton diffusion and thermal management in exciton based systems, and lay a firm foundation for the development of functional excitonic devices.

Ultralow Threshold Polariton Condensate in a Monolayer Semiconductor Microcavity at Room Temperature.

Abstract: Exciton-polaritons, hybrid light-matter bosonic quasiparticles, can condense into a single quantum state, i.e., forming a polariton Bose-Einstein condensate (BEC), which represents a crucial step for the development of nanophotonic technology. Recently, atomically thin transition-metal dichalcogenides (TMDs) emerged as promising candidates for novel polaritonic devices. Although the formation of robust valley-polaritons has been realized up to room temperature, the demonstration of polariton lasing remains elusive. Herein, we report for the first time the realization of this important milestone in a TMD microcavity at room temperature. Continuous wave pumped polariton lasing is evidenced by the macroscopic occupation of the ground state, which undergoes a nonlinear increase of the emission along with the emergence of temporal coherence, the presence of an exciton fraction-controlled threshold and the buildup of linear polarization. Our work presents a critically important step toward exploiting nonlinear polariton-polariton interactions, as well as offering a new platform for thresholdless lasing.

Exciton-exciton interaction in transition metal dichalcogenide monolayers and van der Waals heterostructures

Abstract: Due to a strong Coulomb interaction, excitons dominate the excitation kinetics in two-dimensional (2D) materials. While Coulomb scattering between electrons has been well studied, the interaction of excitons is more challenging and remains to be explored. As neutral composite bosons consisting of electrons and holes, excitons show nontrivial scattering dynamics. Here, we study exciton-exciton interaction in transition-metal dichalcogenides and related van der Waals heterostructures on microscopic footing. We demonstrate that the crucial criterion for efficient scattering is a large electron/hole mass asymmetry, giving rise to internal charge inhomogeneities of excitons and emphasizing their cobosonic substructure. Furthermore, both exchange and direct exciton-exciton interactions are boosted by enhanced exciton Bohr radii. We also predict an unexpected temperature dependence that is usually associated with phonon-driven scattering, and we reveal an orders of magnitude stronger interaction of interlayer excitons due to their permanent dipole moment. The developed approach can be generalized to arbitrary material systems and will help to study strongly correlated exciton systems, such as moire super lattices.