Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide

Recent progress in large-area synthesis of monolayer molybdenum disulphide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapour deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulphide up to 120 μm in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane electrical conductivity, whereas tilt boundaries slightly decrease the conductivity.

Edge nonlinear optics on a MoS₂ atomic monolayer

The translational symmetry breaking of a crystal at its surface may form two-dimensional (2D) electronic states. We observed one-dimensional nonlinear optical edge states of a single atomic membrane of molybdenum disulfide (MoS2), a transition metal dichalcogenide. The electronic structure changes at the edges of the 2D crystal result in strong resonant nonlinear optical susceptibilities, allowing direct optical imaging of the atomic edges and boundaries of a 2D material. Using the symmetry of the nonlinear optical responses, we developed a nonlinear optical imaging technique that allows rapid and all-optical determination of the crystal orientations of the 2D material at a large scale. Our technique provides a route toward understanding and making use of the emerging 2D materials and devices.

In-Plane Anisotropy in Mono- and Few-Layer ReS2 Probed by Raman Spectroscopy and Scanning Transmission Electron Microscopy

Rhenium disulfide (ReS2) is a semiconducting layered transition metal dichalcogenide that exhibits a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various material properties. Here, we demonstrate the strong anisotropy in the Raman scattering response for linearly polarized excitation. Polarized Raman scattering is shown to permit a determination of the crystallographic orientation of ReS2 through comparison with direct structural analysis by scanning transmission electron microscopy (STEM). Analysis of the frequency difference of appropriate Raman modes is also shown to provide a means of precisely determining layer thickness up to four layers.

Evidence of high-temperature exciton condensation in two-dimensional atomic double layers

A Bose-Einstein condensate is the ground state of a dilute gas of bosons, such as atoms cooled to temperatures close to absolute zero¹. With much smaller mass, excitons (bound electron-hole pairs) are expected to condense at considerably higher temperatures^2-7. Two-dimensional van der Waals semiconductors with very strong exciton binding are ideal systems for the study of high-temperature exciton condensation. Here we study electrically generated interlayer excitons in MoSe₂-WSe₂ atomic double layers with a density of up to 10¹² excitons per square centimetre. The interlayer tunnelling current depends only on the exciton density, which is indicative of correlated electron-hole pair tunnelling⁸. Strong electroluminescence arises when a hole tunnels from WSe₂ to recombine with an electron in MoSe₂. We observe a critical threshold dependence of the electroluminescence intensity on exciton density, accompanied by super-Poissonian photon statistics near the threshold, and a large electroluminescence enhancement with a narrow peak at equal electron and hole densities. The phenomenon persists above 100 kelvin, which is consistent with the predicted critical condensation temperature^9-12. Our study provides evidence for interlayer exciton condensation in two-dimensional atomic double layers and opens up opportunities for exploring condensate-based optoelectronics and exciton-mediated high-temperature superconductivity¹³.

Tailoring the electronic structure in bilayer molybdenum disulfide via interlayer twist

Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacking single-crystal monolayers. Varying interlayer twist angle results in strong tuning of the indirect optical transition energy and second-harmonic generation and weak tuning of direct optical transition energies and Raman mode frequencies. Electronic structure calculations show the interlayer separation changes with twist due to repulsion between sulfur atoms, resulting in shifts of the indirect optical transition energies. These results show that interlayer alignment is a crucial variable in tailoring the properties of two-dimensional heterostructures.

Magnetic brightening and control of dark excitons in monolayer WSe2

Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light-matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX₂ (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe₂ and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.

Low-Temperature Ohmic Contact to Monolayer MoS2 by van der Waals Bonded Co/h-BN Electrodes

Monolayer MoS₂, among many other transition metal dichalcogenides, holds great promise for future applications in nanoelectronics and optoelectronics due to its ultrathin nature, flexibility, sizable band gap, and unique spin-valley coupled physics. However, careful study of these properties at low temperature has been hindered by an inability to achieve low-temperature Ohmic contacts to monolayer MoS₂, particularly at low carrier densities. In this work, we report a new contact scheme that utilizes cobalt (Co) with a monolayer of hexagonal boron nitride (h-BN) that has the following two functions: modifies the work function of Co and acts as a tunneling barrier. We measure a flat-band Schottky barrier of 16 meV, which makes thin tunnel barriers upon doping the channels, and thus achieve low-T contact resistance of 3 kΩ.μm at a carrier density of 5.3 × 10¹²/cm². This further allows us to observe Shubnikov-de Haas oscillations in monolayer MoS₂ at much lower carrier densities compared to previous work.

Measurement of Lateral and Interfacial Thermal Conductivity of Single- and Bilayer MoS2 and MoSe2 Using Refined Optothermal Raman Technique

Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides (TMDCs) have attracted extensive interest in recent years, motivating investigation into multiple properties. In this work, we demonstrate a refined version of the optothermal Raman technique to measure the thermal transport properties of two TMDC materials, MoS2 and MoSe2, in single-layer (1L) and bilayer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ∼40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different radial positions. For 1L MoS2 and MoSe2, the room-temperature thermal conductivities are 84 ± 17 and 59 ± 18 W/(m·K), respectively. For 2L MoS2 and MoSe2, we obtain values of 77 ± 25 W and 42 ± 13 W/(m·K). Crucially, the interfacial thermal conductance is found to be of order 0.1-1 MW/m(2) K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices.

Deterministic coupling of site-controlled quantum emitters in monolayer WSe2 to plasmonic nanocavities

Solid-state single-quantum emitters are crucial resources for on-chip photonic quantum technologies and require efficient cavity-emitter coupling to realize quantum networks beyond the single-node level^1,2. Monolayer WSe₂, a transition metal dichalcogenide semiconductor, can host randomly located quantum emitters^3-6, while nanobubbles⁷ as well as lithographically defined arrays of pillars in contact with the transition metal dichalcogenide act as spatially controlled stressors^8,9. The induced strain can then create excitons at defined locations. This ability to create zero-dimensional (0D) excitons anywhere within a 2D material is promising for the development of scalable quantum technologies, but so far lacks mature cavity integration and suffers from low emitter quantum yields. Here we demonstrate a deterministic approach to achieve Purcell enhancement at lithographically defined locations using the sharp corners of a metal nanocube for both electric field enhancement and to deform a 2D material. This nanoplasmonic platform allows the study of the same quantum emitter before and after coupling. For a 3 × 4 array of quantum emitters we show Purcell factors of up to 551 (average of 181), single-photon emission rates of up to 42 MHz and a narrow exciton linewidth as low as 55 μeV. Furthermore, the use of flux-grown WSe₂ increases the 0D exciton lifetimes to up to 14 ns and the cavity-enhanced quantum yields from an initial value of 1% to up to 65% (average 44%).

Force generated by actomyosin contraction builds bridges between adhesive contacts

Extracellular matrices in vivo are heterogeneous structures containing gaps that cells bridge with an actomyosin network. To understand the basis of bridging, we plated cells on surfaces patterned with fibronectin (FN)-coated stripes separated by non-adhesive regions. Bridges developed large tensions where concave cell edges were anchored to FN by adhesion sites. Actomyosin complexes assembled near those sites (both actin and myosin filaments) and moved towards the centre of the non-adhesive regions in a treadmilling network. Inhibition of myosin-II (MII) or Rho-kinase collapsed bridges, whereas extension continued over adhesive areas. Inhibition of actin polymerization (latrunculin-A, jasplakinolide) also collapsed the actomyosin network. We suggest that MII has distinct functions at different bridge regions: (1) at the concave edges of bridges, MIIA force stimulates actin filament assembly at adhesions and (2) in the body of bridges, myosin cross-links actin filaments and stimulates actomyosin network healing when breaks occur. Both activities ensure turnover of actin networks needed to maintain stable bridges from one adhesive region to another.

Plasma membrane area increases with spread area by exocytosis of a GPI-anchored protein compartment

The role of plasma membrane (PM) area as a critical factor during cell motility is poorly understood, mainly due to an inability to precisely follow PM area dynamics. To address this fundamental question, we developed static and dynamic assays to follow exocytosis, endocytosis, and PM area changes during fibroblast spreading. Because the PM area cannot increase by stretch, spreading proceeds by the flattening of membrane folds and/or by the addition of new membrane. Using laser tweezers, we found that PM tension progressively decreases during spreading, suggesting the addition of new membrane. Next, we found that exocytosis increases the PM area by 40-60% during spreading. Reducing PM area reduced spread area, and, in a reciprocal manner, reducing spreadable area reduced PM area, indicating the interconnection between these two parameters. We observed that Golgi, lysosomes, and glycosylphosphatidylinositol-anchored protein vesicles are exocytosed during spreading, but endoplasmic reticulum and transferrin receptor-containing vesicles are not. Microtubule depolymerization blocks lysosome and Golgi exocytosis but not the exocytosis of glycosylphosphatidylinositol-anchored protein vesicles or PM area increase. Therefore, we suggest that fibroblasts are able to regulate about half of their original PM area by the addition of membrane via a glycosylphosphatidylinositol-anchored protein compartment.

Imaging strain-localized excitons in nanoscale bubbles of monolayer WSe2 at room temperature

In monolayer transition-metal dichalcogenides, localized strain can be used to design nanoarrays of single photon sources. Despite strong empirical correlation, the nanoscale interplay between excitons and local crystalline structure that gives rise to these quantum emitters is poorly understood. Here, we combine room-temperature nano-optical imaging and spectroscopic analysis of excitons in nanobubbles of monolayer WSe₂ with atomistic models to study how strain induces nanoscale confinement potentials and localized exciton states. The imaging of nanobubbles in monolayers with low defect concentrations reveals localized excitons on length scales of around 10 nm at multiple sites around the periphery of individual nanobubbles, in stark contrast to predictions of continuum models of strain. These results agree with theoretical confinement potentials atomistically derived from the measured topographies of nanobubbles. Our results provide experimental and theoretical insights into strain-induced exciton localization on length scales commensurate with exciton size, realizing key nanoscale structure-property information on quantum emitters in monolayer WSe₂.

Modulation of Quantum Tunneling via a Vertical Two-Dimensional Black Phosphorus and Molybdenum Disulfide p-n Junction

Diverse diode characteristics were observed in two-dimensional (2D) black phosphorus (BP) and molybdenum disulfide (MoS₂) heterojunctions. The characteristics of a backward rectifying diode, a Zener diode, and a forward rectifying diode were obtained from the heterojunction through thickness modulation of the BP flake or back gate modulation. Moreover, a tunnel diode with a precursor to negative differential resistance can be realized by applying dual gating with a solid polymer electrolyte layer as a top gate dielectric material. Interestingly, a steep subthreshold swing of 55 mV/dec was achieved in a top-gated 2D BP-MoS₂ junction. Our simple device architecture and chemical doping-free processing guaranteed the device quality. This work helps us understand the fundamentals of tunneling in 2D semiconductor heterostructures and shows great potential in future applications in integrated low-power circuits.

Stripe phases in WSe2/WS2 moiré superlattices

Stripe phases, in which the rotational symmetry of charge density is spontaneously broken, occur in many strongly correlated systems with competing interactions^1-11. However, identifying and studying such stripe phases remains challenging. Here we uncover stripe phases in WSe₂/WS₂ moiré superlattices by combining optical anisotropy and electronic compressibility measurements. We find strong electronic anisotropy over a large doping range peaked at 1/2 filling of the moiré superlattice. The 1/2 state is incompressible and assigned to an insulating stripe crystal phase. Wide-field imaging reveals domain configurations with a preferential alignment along the high-symmetry axes of the moiré superlattice. Away from 1/2 filling, we observe additional stripe crystals at commensurate filling 1/4, 2/5 and 3/5, and compressible electronic liquid crystal states at incommensurate fillings. Our results demonstrate that two-dimensional semiconductor moiré superlattices are a highly tunable platform from which to study the stripe phases and their interplay with other symmetry breaking ground states.

Recoverable Slippage Mechanism in Multilayer Graphene Leads to Repeatable Energy Dissipation

Understanding the deformation mechanisms in multilayer graphene (MLG), an attractive material used in nanodevices as well as in the reinforcement of nanocomposites, is critical yet challenging due to difficulties in experimental characterization and the spatiotemporal limitations of atomistic modeling. In this study, we combine nanomechanical experiments with coarse-grained molecular dynamics (CG-MD) simulations to elucidate the mechanisms of deformation and failure of MLG sheets. Elastic properties of graphene sheets with one to three layers are measured using film deflection tests. A nonlinear behavior in the force vs deflection curves for MLGs is observed in both experiments and simulations: during loading/unloading cycles, MLGs dissipate energy through a "recoverable slippage" mechanism. The CG-MD simulations further reveal an atomic level interlayer slippage process and suggest that the dissipated energy scales with film perimeter. Moreover, our study demonstrates that the finite shear strength between individual layers could explain the experimentally measured size-dependent strength with thickness scaling in MLG sheets.

Monolayer Molybdenum Disulfide Transistors with Single-Atom-Thick Gates

Two-dimensional transition-metal dichalcogenides (TMDs) are unique candidates for the development of next-generation electronic devices. However, the large contact resistance between metal and the monolayer TMDs have significantly limited the devices' performance. Also, the integration of ultrathin high- k dielectric layers with TMDs remains difficult due to the lack of dangling bonds on the surface of TMDs. We present monolayer molybdenum disulfide field-effect transistors with bottom local gates consisting of monolayer graphene. The atomic-level thickness and surface roughness of graphene facilitate the growth of high-quality ultrathin HfO₂ and suppress gate leakage. Strong displacement fields above 8 V/nm can be applied using a single graphene gate to electrostatically dope the MoS₂, which reduces the contact resistances between Ni and monolayer MoS₂ to 2.3 kΩ·μm at low gate voltages. The devices exhibit excellent switching characteristics including a near-ideal subthreshold slope of 64 millivolts per decade, low threshold voltage (∼0.5 V), high channel conductance (>100 μS/μm), and low hysteresis. Scaled devices with 50 and 14 nm channels as well as ultrathin (5 nm) gate dielectrics show effective immunity to short-channel effects. The device fabricated on flexible polymeric substrate also exhibits high performance and has a fully transparent channel region that is desirable in optical-related studies and practical applications.

Moiré metrology of energy landscapes in van der Waals heterostructures

The emerging field of twistronics, which harnesses the twist angle between two-dimensional materials, represents a promising route for the design of quantum materials, as the twist-angle-induced superlattices offer means to control topology and strong correlations. At the small twist limit, and particularly under strain, as atomic relaxation prevails, the emergent moiré superlattice encodes elusive insights into the local interlayer interaction. Here we introduce moiré metrology as a combined experiment-theory framework to probe the stacking energy landscape of bilayer structures at the 0.1 meV/atom scale, outperforming the gold-standard of quantum chemistry. Through studying the shapes of moiré domains with numerous nano-imaging techniques, and correlating with multi-scale modelling, we assess and refine first-principle models for the interlayer interaction. We document the prowess of moiré metrology for three representative twisted systems: bilayer graphene, double bilayer graphene and H-stacked MoSe2/WSe2. Moiré metrology establishes sought after experimental benchmarks for interlayer interaction, thus enabling accurate modelling of twisted multilayers.

Enhanced nonlinear interaction of polaritons via excitonic Rydberg states in monolayer WSe2

Strong optical nonlinearities play a central role in realizing quantum photonic technologies. Exciton-polaritons, which result from the hybridization of material excitations and cavity photons, are an attractive candidate to realize such nonlinearities. While the interaction between ground state excitons generates a notable optical nonlinearity, the strength of such interactions is generally not sufficient to reach the regime of quantum nonlinear optics. Excited states, however, feature enhanced interactions and therefore hold promise for accessing the quantum domain of single-photon nonlinearities. Here we demonstrate the formation of exciton-polaritons using excited excitonic states in monolayer tungsten diselenide (WSe2) embedded in a microcavity. The realized excited-state polaritons exhibit an enhanced nonlinear response ∼[Formula: see text] which is ∼4.6 times that for the ground-state exciton. The demonstration of enhanced nonlinear response from excited exciton-polaritons presents the potential of generating strong exciton-polariton interactions, a necessary building block for solid-state quantum photonic technologies.

Creation of moiré bands in a monolayer semiconductor by spatially periodic dielectric screening

Moiré superlattices of two-dimensional van der Waals materials have emerged as a powerful platform for designing electronic band structures and discovering emergent physical phenomena. A key concept involves the creation of long-wavelength periodic potential and moiré bands in a crystal through interlayer electronic hybridization or atomic corrugation when two materials are overlaid. Here we demonstrate a new approach based on spatially periodic dielectric screening to create moiré bands in a monolayer semiconductor. This approach relies on reduced dielectric screening of the Coulomb interactions in monolayer semiconductors and their environmental dielectric-dependent electronic band structure. We observe optical transitions between moiré bands in monolayer WSe₂ when it is placed close to small-angle-misaligned graphene on hexagonal boron nitride. The moiré bands are a result of long-range Coulomb interactions, which are strongly gate tunable, and can have versatile superlattice symmetries independent of the crystal lattice of the host material. Our result also demonstrates that monolayer semiconductors are sensitive local dielectric sensors.

Charge-Transfer Plasmon Polaritons at Graphene/α-RuCl3 Interfaces

Nanoscale charge control is a key enabling technology in plasmonics, electronic band structure engineering, and the topology of two-dimensional materials. By exploiting the large electron affinity of α-RuCl₃, we are able to visualize and quantify massive charge transfer at graphene/α-RuCl₃ interfaces through generation of charge-transfer plasmon polaritons (CPPs). We performed nanoimaging experiments on graphene/α-RuCl₃ at both ambient and cryogenic temperatures and discovered robust plasmonic features in otherwise ungated and undoped structures. The CPP wavelength evaluated through several distinct imaging modalities offers a high-fidelity measure of the Fermi energy of the graphene layer: E_F = 0.6 eV (n = 2.7 × 10¹³ cm^-2). Our first-principles calculations link the plasmonic response to the work function difference between graphene and α-RuCl₃ giving rise to CPPs. Our results provide a novel general strategy for generating nanometer-scale plasmonic interfaces without resorting to external contacts or chemical doping.

High-Throughput Mechanobiology Screening Platform Using Micro- and Nanotopography

We herein demonstrate the first 96-well plate platform to screen effects of micro- and nanotopographies on cell growth and proliferation. Existing high-throughput platforms test a limited number of factors and are not fully compatible with multiple types of testing and assays. This platform is compatible with high-throughput liquid handling, high-resolution imaging, and all multiwell plate-based instrumentation. We use the platform to screen for topographies and drug-topography combinations that have short- and long-term effects on T cell activation and proliferation. We coated nanofabricated "trench-grid" surfaces with anti-CD3 and anti-CD28 antibodies to activate T cells and assayed for interleukin 2 (IL-2) cytokine production. IL-2 secretion was enhanced at 200 nm trench width and >2.3 μm grating pitch; however, the secretion was suppressed at 100 nm width and <0.5 μm pitch. The enhancement on 200 nm grid trench was further amplified with the addition of blebbistatin to reduce contractility. The 200 nm grid pattern was found to triple the number of T cells in long-term expansion, a result with direct clinical applicability in adoptive immunotherapy.

LIM-Nebulette Reinforces Podocyte Structural Integrity by Linking Actin and Vimentin Filaments

BACKGROUND

Maintenance of the intricate interdigitating morphology of podocytes is crucial for glomerular filtration. One of the key aspects of specialized podocyte morphology is the segregation and organization of distinct cytoskeletal filaments into different subcellular components, for which the exact mechanisms remain poorly understood.

METHODS

Cells from rats, mice, and humans were used to describe the cytoskeletal configuration underlying podocyte structure. Screening the time-dependent proteomic changes in the rat puromycin aminonucleoside-induced nephropathy model correlated the actin-binding protein LIM-nebulette strongly with glomerular function. Single-cell RNA sequencing and immunogold labeling were used to determine Nebl expression specificity in podocytes. Automated high-content imaging, super-resolution microscopy, atomic force microscopy (AFM), live-cell imaging of calcium, and measurement of motility and adhesion dynamics characterized the physiologic role of LIM-nebulette in podocytes.

RESULTS

Nebl knockout mice have increased susceptibility to adriamycin-induced nephropathy and display morphologic, cytoskeletal, and focal adhesion abnormalities with altered calcium dynamics, motility, and Rho GTPase activity. LIM-nebulette expression is decreased in diabetic nephropathy and FSGS patients at both the transcript and protein level. In mice, rats, and humans, LIM-nebulette expression is localized to primary, secondary, and tertiary processes of podocytes, where it colocalizes with focal adhesions as well as with vimentin fibers. LIM-nebulette shRNA knockdown in immortalized human podocytes leads to dysregulation of vimentin filament organization and reduced cellular elasticity as measured by AFM indentation.

CONCLUSIONS

LIM-nebulette is a multifunctional cytoskeletal protein that is critical in the maintenance of podocyte structural integrity through active reorganization of focal adhesions, the actin cytoskeleton, and intermediate filaments.

Long-Lived Phonon Polaritons in Hyperbolic Materials

Natural hyperbolic materials with dielectric permittivities of opposite signs along different principal axes can confine long-wavelength electromagnetic waves down to the nanoscale, well below the diffraction limit. Confined electromagnetic waves coupled to phonons in hyperbolic dielectrics including hexagonal boron nitride (hBN) and α-MoO₃ are referred to as hyperbolic phonon polaritons (HPPs). HPP dissipation at ambient conditions is substantial, and its fundamental limits remain unexplored. Here, we exploit cryogenic nanoinfrared imaging to investigate propagating HPPs in isotopically pure hBN and naturally abundant α-MoO₃ crystals. Close to liquid-nitrogen temperatures, losses for HPPs in isotopic hBN drop significantly, resulting in propagation lengths in excess of 8 μm, with lifetimes exceeding 5 ps, thereby surpassing prior reports on such highly confined polaritonic modes. Our nanoscale, temperature-dependent imaging reveals the relevance of acoustic phonons in HPP damping and will be instrumental in mitigating such losses for miniaturized mid-infrared technologies operating at liquid-nitrogen temperatures.

Continuous Wave Sum Frequency Generation and Imaging of Monolayer and Heterobilayer Two-Dimensional Semiconductors

We report continuous-wave second harmonic and sum frequency generation from two-dimensional transition metal dichalcogenide monolayers and their heterostructures with pump irradiances several orders of magnitude lower than those of conventional pulsed experiments. The high nonlinear efficiency originates from above-gap excitons in the band nesting regions, as revealed by wavelength-dependent second order optical susceptibilities quantified in four common monolayer transition metal dichalcogenides. Using sum frequency excitation spectroscopy and imaging, we identify and distinguish one- and two-photon resonances in both monolayers and heterobilayers. Data for heterostructures reveal responses from constituent layers accompanied by nonlinear signal correlated with interlayer transitions. We demonstrate spatial mapping of heterogeneous interlayer coupling by sum frequency and second harmonic confocal microscopy on heterobilayer MoSe₂/WSe₂.

Near-Unity Light Absorption in a Monolayer WS2 Van der Waals Heterostructure Cavity

Excitons in monolayer transition-metal-dichalcogenides (TMDs) dominate their optical response and exhibit strong light-matter interactions with lifetime-limited emission. While various approaches have been applied to enhance light-exciton interactions in TMDs, the achieved strength have been far below unity, and a complete picture of its underlying physical mechanisms and fundamental limits has not been provided. Here, we introduce a TMD-based van der Waals heterostructure cavity that provides near-unity excitonic absorption, and emission of excitonic complexes that are observed at ultralow excitation powers. Our results are in full agreement with a quantum theoretical framework introduced to describe the light-exciton-cavity interaction. We find that the subtle interplay between the radiative, nonradiative and dephasing decay rates plays a crucial role, and unveil a universal absorption law for excitons in 2D systems. This enhanced light-exciton interaction provides a platform for studying excitonic phase-transitions and quantum nonlinearities and enables new possibilities for 2D semiconductor-based optoelectronic devices.