Structure of the moiré exciton captured by imaging its electron and hole.

Interlayer excitons (ILXs) - electron-hole pairs bound across two atomically thin layered semiconductors - have emerged as attractive platforms to study exciton condensation1-4, single-photon emission and other quantum information applications5-7. Yet, despite extensive optical spectroscopic investigations8-12, critical information about their size, valley configuration and the influence of the moiré potential remains unknown. Here, in a WSe2/MoS2 heterostructure, we captured images of the time-resolved and momentum-resolved distribution of both of the particles that bind to form the ILX: the electron and the hole. We thereby obtain a direct measurement of both the ILX diameter of around 5.2 nm, comparable with the moiré-unit-cell length of 6.1 nm, and the localization of its centre of mass. Surprisingly, this large ILX is found pinned to a region of only 1.8 nm diameter within the moiré cell, smaller than the size of the exciton itself. This high degree of localization of the ILX is backed by Bethe-Salpeter equation calculations and demonstrates that the ILX can be localized within small moiré unit cells. Unlike large moiré cells, these are uniform over large regions, allowing the formation of extended arrays of localized excitations for quantum technology.

Quantum criticality in twisted transition metal dichalcogenides.

Near the boundary between ordered and disordered quantum phases, several experiments have demonstrated metallic behaviour that defies the Landau Fermi paradigm1-5. In moiré heterostructures, gate-tuneable insulating phases driven by electronic correlations have been recently discovered6-23. Here, we use transport measurements to characterize metal-insulator transitions (MITs) in twisted WSe2 near half filling of the first moiré subband. We find that the MIT as a function of both density and displacement field is continuous. At the metal-insulator boundary, the resistivity displays strange metal behaviour at low temperatures, with dissipation comparable to that at the Planckian limit. Further into the metallic phase, Fermi liquid behaviour is recovered at low temperature, and this evolves into a quantum critical fan at intermediate temperatures, before eventually reaching an anomalous saturated regime near room temperature. An analysis of the residual resistivity indicates the presence of strong quantum fluctuations in the insulating phase. These results establish twisted WSe2 as a new platform to study doping and bandwidth-controlled metal-insulator quantum phase transitions on the triangular lattice.

Transport Study of Charge-Carrier Scattering in Monolayer WSe_{2}.

Employing flux-grown single crystal WSe_{2}, we report charge-carrier scattering behaviors measured in h-BN encapsulated monolayer field effect transistors. We observe a nonmonotonic change of transport mobility as a function of hole density in the degenerately doped sample, which can be explained by energy dependent scattering amplitude of strong defects calculated using the T-matrix approximation. Utilizing long mean-free path (>500 nm), we also demonstrate the high quality of our electronic devices by showing quantized conductance steps from an electrostatically defined quantum point contact, showing the potential for creating ultrahigh quality quantum optoelectronic devices based on atomically thin semiconductors.

Enhanced Superconductivity in Monolayer Td-MoTe2.

Crystalline two-dimensional (2D) superconductors (SCs) with low carrier density are an exciting new class of materials in which electrostatic gating can tune superconductivity, electronic interactions play a prominent role, and electrical transport properties may directly reflect the topology of the Fermi surface. Here, we report the dramatic enhancement of superconductivity with decreasing thickness in semimetallic Td-MoTe2, with critical temperature (Tc) increasing up to 7.6 K for monolayers, a 60-fold increase with respect to the bulk Tc. We show that monolayers possess a similar electronic structure and density of states (DOS) as the bulk, implying that electronic interactions play a strong role in the enhanced superconductivity. Reflecting the low carrier density, the critical temperature, magnetic field, and current density are all tunable by an applied gate voltage. The response to high in-plane magnetic fields is distinct from that of other 2D SCs and reflects the canted spin texture of the electron pockets.

Correlated electronic phases in twisted bilayer transition metal dichalcogenides.

In narrow electron bands in which the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. Such flat bands in twisted graphene-based systems result in correlated insulator, superconducting and topological states. Here we report evidence of low-energy flat bands in twisted bilayer WSe2, with signatures of collective phases observed over twist angles that range from 4 to 5.1°. At half-band filling, a correlated insulator appeared that is tunable with both twist angle and displacement field. At a 5.1° twist, zero-resistance pockets were observed on doping away from half filling at temperatures below 3 K, which indicates a possible transition to a superconducting state. The observation of tunable collective phases in a simple band, which hosts only two holes per unit cell at full filling, establishes twisted bilayer transition metal dichalcogenides as an ideal platform to study correlated physics in two dimensions on a triangular lattice.

Exciton Dipole Orientation of Strain-Induced Quantum Emitters in WSe2.

Transition metal dichalcogenides are promising semiconductors to enable advances in photonics and electronics and have also been considered as a host for quantum emitters. Particularly, recent advances demonstrate site-controlled quantum emitters in WSe2 through strain deformation. Albeit essential for device integration, the dipole orientation of these strain-induced quantum emitters remains unknown. Here we employ angular-resolved spectroscopy to experimentally determine the dipole orientation of strain-induced quantum emitters. It is found that with increasing local strain the quantum emitters in WSe2 undergo a transition from in-plane to out-of-plane dipole orientation if their emission wavelength is longer than 750 nm. In addition, the exciton g-factor remains with average values of g = 8.52 ± 1.2 unchanged in the entire emission wavelength. These findings provide experimental support of the interlayer defect exciton model and highlight the importance of an underlying three-dimensional strain profile of deformed monolayer semiconductors, which is essential to optimize emitter-mode coupling in nanoplasmonics.

Approaching the Intrinsic Limit in Transition Metal Diselenides via Point Defect Control.

Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods, chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above 1013/cm2 to below 1011/cm2. Because these point defects act as centers for nonradiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

Infrared Interlayer Exciton Emission in MoS_{2}/WSe_{2} Heterostructures.

We report light emission around 1 eV (1240 nm) from heterostructures of MoS_{2} and WSe_{2} transition metal dichalcogenide monolayers. We identify its origin in an interlayer exciton (ILX) by its wide spectral tunability under an out-of-plane electric field. From the static dipole moment of the state, its temperature and twist-angle dependence, and comparison with electronic structure calculations, we assign this ILX to the fundamental interlayer transition between the K valleys in this system. Our findings gain access to the interlayer physics of the intrinsically incommensurate MoS_{2}/WSe_{2} heterostructure, including moiré and valley pseudospin effects, and its integration with silicon photonics and optical fiber communication systems operating at wavelengths longer than 1150 nm.

Nonvolatile Control of Valley Polarized Emission in 2D WSe2-AlScN Heterostructures.

Achieving robust and electrically controlled valley polarization in monolayer transition metal dichalcogenides (ML-TMDs) is a frontier challenge for realistic valleytronic applications. Theoretical investigations show that the integration of 2D materials with ferroelectrics is a promising strategy; however, an experimental demonstration has remained elusive. Here, we fabricate ferroelectric field-effect transistors using a ML-WSe2 channel and an Al0.68Sc0.32N (AlScN) ferroelectric dielectric and experimentally demonstrate efficient tuning as well as non-volatile control of valley polarization. We measure a large array of transistors and obtain a maximum valley polarization of ∼27% at 80 K with stable retention up to 5400 s. The enhancement in the valley polarization is ascribed to the efficient exciton-to-trion (X-T) conversion and its coupling with an out-of-plane electric field, viz., the quantum-confined Stark effect. This changes the valley depolarization pathway from strong exchange interactions to slow spin-flip intervalley scattering. Our research demonstrates a promising approach for achieving non-volatile control over valley polarization for practical valleytronic device applications.

Controlled interlayer exciton ionization in an electrostatic trap in atomically thin heterostructures.

Atomically thin semiconductor heterostructures provide a two-dimensional (2D) device platform for creating high densities of cold, controllable excitons. Interlayer excitons (IEs), bound electrons and holes localized to separate 2D quantum well layers, have permanent out-of-plane dipole moments and long lifetimes, allowing their spatial distribution to be tuned on demand. Here, we employ electrostatic gates to trap IEs and control their density. By electrically modulating the IE Stark shift, electron-hole pair concentrations above 2 × 1012 cm-2 can be achieved. At this high IE density, we observe an exponentially increasing linewidth broadening indicative of an IE ionization transition, independent of the trap depth. This runaway threshold remains constant at low temperatures, but increases above 20 K, consistent with the quantum dissociation of a degenerate IE gas. Our demonstration of the IE ionization in a tunable electrostatic trap represents an important step towards the realization of dipolar exciton condensates in solid-state optoelectronic devices.

Tuning Polarity in WSe2/AlScN FeFETs via Contact Engineering.

Recent advancements in ferroelectric field-effect transistors (FeFETs) using two-dimensional (2D) semiconductor channels and ferroelectric Al0.68Sc0.32N (AlScN) allow high-performance nonvolatile devices with exceptional ON-state currents, large ON/OFF current ratios, and large memory windows (MW). However, previous studies have solely focused on n-type FeFETs, leaving a crucial gap in the development of p-type and ambipolar FeFETs, which are essential for expanding their applicability to a wide range of circuit-level applications. Here, we present a comprehensive demonstration of n-type, p-type, and ambipolar FeFETs on an array scale using AlScN and multilayer/monolayer WSe2. The dominant injected carrier type is modulated through contact engineering at the metal-semiconductor junction, resulting in the realization of all three types of FeFETs. The effect of contact engineering on the carrier injection is further investigated through technology-computer-aided design simulations. Moreover, our 2D WSe2/AlScN FeFETs achieve high electron and hole current densities of ∼20 and ∼10 µA/µm, respectively, with a high ON/OFF ratio surpassing ∼107 and a large MW of >6 V (0.14 V/nm).

Electrically Confined Electroluminescence of Neutral Excitons in WSe2 Light-Emitting Transistors.

Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential in optoelectronic applications due to their direct band gap and exceptional quantum yield. However, TMD-based light-emitting devices have shown low external quantum efficiencies as imbalanced free carrier injection often leads to the formation of non-radiative charged excitons, limiting practical applications. Here, electrically confined electroluminescence (EL) of neutral excitons in tungsten diselenide (WSe2) light-emitting transistors (LETs) based on the van der Waals heterostructure is demonstrated. The WSe2 channel is locally doped to simultaneously inject electrons and holes to the 1D region by a local graphene gate. At balanced concentrations of injected electrons and holes, the WSe2 LETs exhibit strong EL with a high external quantum efficiency (EQE) of ≈8.2 % at room temperature. These experimental and theoretical results consistently show that the enhanced EQE could be attributed to dominant exciton emission confined at the 1D region while expelling charged excitons from the active area by precise control of external electric fields. This work shows a promising approach to enhancing the EQE of 2D light-emitting transistors and modulating the recombination of exciton complexes for excitonic devices.

Reduction of graphene damages during the fabrication of InGaN/GaN light emitting diodes with graphene electrodes.

Although graphene looks attractive to replace indium tin oxide (ITO) in optoelectronic devices, the luminous efficiency of light emitting diodes (LEDs) with graphene transparent conducting electrodes has been limited by degradation in graphene taking place during device fabrication. In this study, it was found that the quality of graphene after the device fabrication was a critical factor affecting the performance of GaN-based LEDs. In this paper, the qualities of graphene after two different device fabrication processes were evaluated by Raman spectroscopy and atomic force microscopy. It was found that graphene was severely damaged and split into submicrometer-scale islands bounded by less conducting boundaries when graphene was transferred onto LED structures prior to the GaN etching process for p-contact formation. On the other hand, when graphene was transferred after the GaN etch and p-contact metallization, graphene remained intact and the resulting InGaN/GaN LEDs showed electrical and optical properties that were very close to those of LEDs with 200 nm thick ITO films. The forward-voltages and light output powers of LEDs were 3.03 V and 9.36 mW at an injection current of 20 mA, respectively.

Improving the Optical Quality of MoSe2 and WS2 Monolayers with Complete h-BN Encapsulation by High-Temperature Annealing.

We improved the optical quality and stability of an exfoliated monolayer (ML) MoSe2 and chemical vapor deposition (CVD)-grown WS2 MLs by encapsulating and sealing them with both top and bottom few-layer h-BN, as tested by subsequent high-temperature annealing up to 873 K and photoluminescence (PL) measurements. These transition-metal dichalcogenide (TMD) MLs remained stable up to this maximum temperature, as seen visually. After the heating/cooling cycle, the integrated photoluminescence (PL) intensity at 300 K in the MoSe2 ML was ∼4 times larger than that before heating and that from exciton and trion PL in the analogous WS2 ML sample was ∼14 times and ∼2.5 times larger at 77 K and the exciton peak was ∼9.5 times larger at 300 K. This is attributed to the reduction of impurities, the lateral expulsion of contamination leading to clean and atomically flat surfaces, and the sealing provided by the h-BN layers that prevents the diffusion of molecules such as trace O2 and H2O to the TMD ML. Stability and optical performance are much improved compared to that in earlier work using top h-BN only, in which the WS2 ML PL intensity decreased even for an optimal gas environment. This complete encapsulation is particularly promising for CVD-grown TMD MLs because they have relatively more charge and other impurities than do exfoliated MLs. These results open a new route for improving the optical properties of TMD MLs and their performance and applications both at room and higher temperatures.

Free Trions with Near-Unity Quantum Yield in Monolayer MoSe2.

Trions, quasiparticles composed of an electron-hole pair bound to a second electron and/or hole, are many-body states with potential applications in optoelectronics. Trions in monolayer transition metal dichalcogenide (TMD) semiconductors have attracted recent interest due to their valley/spin polarization, strong binding energy, and tunability through external gate control. However, low materials quality (i.e., high defect density) has hindered efforts to understand the intrinsic properties of trions. The low photoluminescence (PL) quantum yield (QY) and short lifetime of trions have prevented harnessing them in device applications. Here, we study the behavior of trions in a series of MoSe2 monolayers, with atomic defect density varying by over 2 orders of magnitude. The QY increases with decreasing defect density and approaches unity in the cleanest material. Simultaneous measurement of the PL lifetime yields both the intrinsic radiative lifetime and the defect-dependent nonradiative lifetime. The long lifetime of ∼230 ps of trions allows direct observation of their diffusion.

Bilayer WSe2 as a natural platform for interlayer exciton condensates in the strong coupling limit.

Exciton condensates (ECs) are macroscopic coherent states arising from condensation of electron-hole pairs1. Bilayer heterostructures, consisting of two-dimensional electron and hole layers separated by a tunnel barrier, provide a versatile platform to realize and study ECs2-4. The tunnel barrier suppresses recombination, yielding long-lived excitons5-10. However, this separation also reduces interlayer Coulomb interactions, limiting the exciton binding strength. Here, we report the observation of ECs in naturally occurring 2H-stacked bilayer WSe2. In this system, the intrinsic spin-valley structure suppresses interlayer tunnelling even when the separation is reduced to the atomic limit, providing access to a previously unattainable regime of strong interlayer coupling. Using capacitance spectroscopy, we investigate magneto-ECs, formed when partially filled Landau levels couple between the layers. We find that the strong-coupling ECs show dramatically different behaviour compared with previous reports, including an unanticipated variation of EC robustness with the orbital number, and find evidence for a transition between two types of low-energy charged excitations. Our results provide a demonstration of tuning EC properties by varying the constituent single-particle wavefunctions.

Optical absorption of interlayer excitons in transition-metal dichalcogenide heterostructures.

Interlayer excitons, electron-hole pairs bound across two monolayer van der Waals semiconductors, offer promising electrical tunability and localizability. Because such excitons display weak electron-hole overlap, most studies have examined only the lowest-energy excitons through photoluminescence. We directly measured the dielectric response of interlayer excitons, which we accessed using their static electric dipole moment. We thereby determined an intrinsic radiative lifetime of 0.40 nanoseconds for the lowest direct-gap interlayer exciton in a tungsten diselenide/molybdenum diselenide heterostructure. We found that differences in electric field and twist angle induced trends in exciton transition strengths and energies, which could be related to wave function overlap, moiré confinement, and atomic reconstruction. Through comparison with photoluminescence spectra, this study identifies a momentum-indirect emission mechanism. Characterization of the absorption is key for applications relying on light-matter interactions.

Antiferromagnetic proximity coupling between semiconductor quantum emitters in WSe2 and van der Waals ferromagnets.

van der Waals ferromagnets have gained significant interest due to their unique ability to provide magnetic response even at the level of a few monolayers. Particularly in combination with 2D semiconductors, such as the transition metal dichalcogenide WSe2, one can create heterostructures that feature unique magneto-optical response in the exciton emission through the magnetic proximity effect. Here we use 0D quantum emitters in WSe2 to probe for the ferromagnetic response in heterostructures with Fe3GT and Fe5GT ferromagnets through an all-optical read-out technique that does not require electrodes. The spectrally narrow spin-doublet of the WSe2 quantum emitters allowed to fully resolve the hysteretic magneto-response in the exciton emission, revealing the characteristic signature of both ferro- and antiferromagnetic proximity coupling that originates from the interplay among Fe3GT or Fe5GT, a thin surface oxide, and the spin doublets of the quantum emitters. Our work highlights the utility of 0D quantum emitters for probing interface magnetic dipoles in vdW heterostructures with high precision. The observed hysteretic magneto response in the exciton emission of quantum emitters adds further new degrees of freedom for spin and g-factor manipulation of quantum states.

Stabilization of Chemical-Vapor-Deposition-Grown WS2 Monolayers at Elevated Temperature with Hexagonal Boron Nitride Encapsulation.

Chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS2 can be stabilized at elevated temperatures by encapsulation with several layer hexagonal boron nitride (h-BN), but to different degrees in the presence of ambient air, flowing N2, and flowing forming gas (95% N2, 5% H2). The best passivation of WS2 at elevated temperature occurs for h-BN-covered samples with flowing N2 (after heating to 873 K), as judged by optical microscopy and photoluminescence (PL) intensity after a heating/cooling cycle. Stability is worse for uncovered samples, but best with flowing forming gas. PL from trions, in addition to that from excitons, is seen for covered WS2 only for forming gas, during cooling below ∼323 K; the trion has an estimated binding energy of ∼28 meV. It might occur because of doping level changes caused by charge defect generation by H2 molecules diffusing between the h-BN and the SiO2/Si substrate. The decomposition of uncovered WS2 flakes in air suggests a dissociation and chemisorption energy barrier of O2 on the WS2 surface of ∼1.6 eV. Fitting the high-temperature PL intensities in air gives a binding energy of a free exciton of ∼229 meV.

Intrinsic donor-bound excitons in ultraclean monolayer semiconductors.

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (>6 µs) and polarization lifetimes (>100 ns). Resonant excitation of the free inter- and intravalley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellites' photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.