Electroluminescent vertical tunneling junctions based on WSe2 monolayer quantum emitter arrays: Exploring tunability with electric and magnetic fields.

We experimentally demonstrate the creation of defects in monolayer WSe2 via nanopillar imprinting and helium ion irradiation. Based on the first method, we realize atomically thin vertical tunneling light-emitting diodes based on WSe2 monolayers hosting quantum emitters at deterministically specified locations. We characterize these emitters by investigating the evolution of their emission spectra in external electric and magnetic fields, as well as by inducing electroluminescence at low temperatures. We identify qualitatively different types of quantum emitters and classify them according to the dominant electron-hole recombination paths, determined by the mechanisms of intervalley mixing occurring in fundamental conduction and/or valence subbands.

Transition metal dichalcogenide nanospheres for high-refractive-index nanophotonics and biomedical theranostics.

Recent developments in the area of resonant dielectric nanostructures have created attractive opportunities for concentrating and manipulating light at the nanoscale and the establishment of the new exciting field of all-dielectric nanophotonics. Transition metal dichalcogenides (TMDCs) with nanopatterned surfaces are especially promising for these tasks. Still, the fabrication of these structures requires sophisticated lithographic processes, drastically complicating application prospects. To bridge this gap and broaden the application scope of TMDC nanomaterials, we report here femtosecond laser-ablative fabrication of water-dispersed spherical TMDC (MoS2 and WS2) nanoparticles (NPs) of variable size (5 to 250 nm). Such NPs demonstrate exciting optical and electronic properties inherited from TMDC crystals, due to preserved crystalline structure, which offers a unique combination of pronounced excitonic response and high refractive index value, making possible a strong concentration of electromagnetic field in the NPs. Furthermore, such NPs offer additional tunability due to hybridization between the Mie and excitonic resonances. Such properties bring to life a number of nontrivial effects, including enhanced photoabsorption and photothermal conversion. As an illustration, we demonstrate that the NPs exhibit a very strong photothermal response, much exceeding that of conventional dielectric nanoresonators based on Si. Being in a mobile colloidal state and exhibiting superior optical properties compared to other dielectric resonant structures, the synthesized TMDC NPs offer opportunities for the development of next-generation nanophotonic and nanotheranostic platforms, including photothermal therapy and multimodal bioimaging.

Bi-MoSe2 Contacts in the Ultraclean Limit: Closing the Theory-Experiment Loop.

Achieving robust electrical contacts is crucial for realizing the promise of monolayer 2D semiconductors such as semiconducting transition metal dichalcogenides (s-TMDs) in electronics. Despite recent breakthroughs, a gap remains between the experimental and theoretical understanding of metal-s-TMDs contacts. This study explores bismuth semimetal contacts to monolayer MoSe2, using a platform that minimizes experimental sources of uncertainty; we combine contact-front and contact-end measurements to measure key parameters like specific resistivity (ρc) and transfer length (Lt). We find that the resistivity of MoSe2 under the contacts is enhanced due to charge transfer that can be modeled using a self-consistent approach. In contrast, ab initio calculations of the interlayer charge transfer rate are inconsistent with the measured value of ρc, highlighting the need for new theoretical approaches.

Temperature Induced, Reversible Switching of Ferro-Rotational Order Coupled to Superlattice Commensuralibity.

High-quality 1T-TaS2 crystals are investigated by angle-resolved photoelectron spectroscopy, Raman spectroscopy, and low-energy electron diffraction. The Ferro-Rotational Order (FRO) of the charge density wave switches configuration at the transition between the commensurate and the nearly commensurate phase. This process requires samples without built-in or externally induced strain. Moreover, temperature gradients generated by a focused laser beam can be employed in order to freeze the in-plane chirality. Based on such observations, we propose a protocol to obtain durable and nonvolatile state switching of the FRO configuration in bulk 1T-TaS2 crystals.

Electrostatic Control of Nonlinear Photonic-Crystal Polaritons in a Monolayer Semiconductor.

Integration of 2D semiconductors with photonic crystal slabs provides an attractive approach to achieving strong light-matter coupling and exciton-polariton formation in a chip-compatible geometry. However, for the development of practical devices, it is crucial that polariton excitations are easily tunable and exhibit a strong nonlinear response. Here we study neutral and charged exciton-polaritons in an electrostatically gated photonic crystal slab with an embedded monolayer semiconductor MoSe2 and experimentally demonstrate a novel approach to optical control based on polariton nonlinearity. We show that spatial modulation of the dielectric environment within the photonic crystal unit cell results in the formation of two distinct excitonic species with significantly different nonlinear responses of the corresponding charged exciton-polaritons under optical pumping. This behavior enables optical switching with ultrashort laser pulses and can be sensitively controlled via an electrostatic gate voltage. Our results open new avenues toward the development of active polaritonic devices in a compact chip-compatible implementation.

Probing the Nature of Single-Photon Emitters in a WSe2 Monolayer by Magneto-Photoluminescence Spectroscopy.

Monolayer transition metal dichalcogenides (TMDs) have emerged as promising materials to generate single-photon emitters (SPEs). While there are several previous reports in the literature about TMD-based SPEs, the precise nature of the excitonic states involved in them is still under debate. Here, we use magneto-optical techniques under in-plane and out-of-plane magnetic fields to investigate the nature of SPEs in WSe2 monolayers on glass substrates under different strain profiles. Our results reveal important changes on the exciton localization and, consequently, on the optical properties of SPEs. Remarkably, we observe an anomalous PL energy redshift with no significant changes of photoluminescence (PL) intensity under an in-plane magnetic field. We present a model to explain this redshift based on intervalley defect excitons under a parallel magnetic field. Overall, our results offer important insights into the nature of SPEs in TMDs, which are valuable for future applications in quantum technologies.

Bandgap Engineering of 2D Materials toward High-Performing Straintronics.

Straintronics leverages mechanical strain to alter the electronic properties of materials, providing an energy-efficient alternative to traditional electronic controls while enhancing device performance. Key to the application of straintronics is bandgap engineering, which enables tuning of the energy difference between the valence and conduction bands of a material to optimize its optoelectronic properties. This mini-review highlights the fundamental principles of straintronics and the critical role of bandgap engineering within this context. It discusses the unique characteristics of various two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), and black phosphorus, which make them suitable for strain-engineered applications. Detailed examples of how mechanical deformation can modulate the bandgap to achieve desired electronic properties are provided, while recent experimental and theoretical studies demonstrating the mechanisms by which strain influences the bandgap in these materials are reviewed, emphasizing their implications for device fabrication. The review concludes with an assessment of the challenges and future directions in the development of high-performing straintronic devices, highlighting their potential applications in flexible electronics, sensors, and optoelectronics.

Light-Induced Polaronic Crystals in Single-Layer Transition Metal Dichalcogenides.

Light-induced ordered states can emerge in materials after irradiation with ultrafast laser pulses. However, their prediction is challenging because the inverted band occupation confounds our chemical intuition. Hence, we use a recently developed constrained density functional perturbation theory approach to systematically screen single-layer transition metal dichalcogenides (TMDs) for light-induced ordered states. We demonstrate that all examined single-layer TMDs reveal similar light-induced charge orderings. The corresponding reconstructions are periodic arrangements of polarons (polaronic crystals), characterized by triangular metal clusters and having no equivalent at equilibrium conditions. The polarons are accompanied by localized midgap states in the electronic band structure, detectable by experimental methods. We assess the selenides as the most promising candidates for potential photoexcitation experiments because they transition at low critical fluences, have low transition barriers, and maintain an open band gap under photoexcitation. Our work paves the way for innovative material design approaches targeting light-induced phases.

Control of Subband Energies via Interlayer Twisting in an Artificially Stacked WSe2 Bilayer.

Tuning the electronic structure of artificially stacked bilayer crystals using their twist angle has attracted a significant amount of interest. In this study, resonant tunneling spectroscopy was performed on trilayer WSe2/h-BN/twisted bilayer (tBL) WSe2 devices with a wide range of twist angles (θBL) of tBL WSe2, from 0° to 34°. We observed two resonant tunneling peaks, identified as the first and second lowest hole subbands at the valence band Γ point of tBL WSe2. The subband separation, which directly measured the interlayer coupling strength, was tuned by ∼0.1 eV as θBL increased toward 6° and remained nearly constant for larger θBL values. The θBL dependence was attributed to the emergence of a stable W/Se (Se/W) stacking domain in the small θBL region, owing to the atomic reconstruction of the moiré lattice in tBL WSe2. Our findings demonstrate that the twist-controlled subband energies in tBL WSe2 are predominantly determined by local atomic reconstruction.

Chirality-Dependent Dynamic Evolution for Trions in Monolayer WS2.

Monolayer transition metal dichalcogenides exhibit valley-dependent excitonic characters with a large binding energy, acting as the building block for future optoelectronic functionalities. Herein, combined with pump-probe ultrafast transient transmission spectroscopy and theoretical simulations, we reveal the chirality-dependent trion dynamics in h-BN encapsulated monolayer tungsten disulfide. By resonantly pumping trions in a single valley and monitoring their temporal evolution, we identify the temperature-dependent competition between two relaxation channels driven by chirality-dependent scattering processes. At room temperature, the phonon-assisted upconversion process predominates, converting excited trions to excitons within the same valley on a sub-picosecond (ps) time scale. As temperature decreases, this process becomes less efficient, while alternative channels, notably valley depolarization process for trions, assume importance, leading to an increase of trion density in the unpumped valley within a ps time scale. Our time-resolved valley-contrast results provide a comprehensive insight into trion dynamics in 2D materials, thereby advancing the development of novel valleytronic devices.

Ultrafast Coherent Exciton Couplings and Many-Body Interactions in Monolayer WS2.

Transition metal dichalcogenides (TMDs) are quantum confined systems with interesting optoelectronic properties, governed by Coulomb interactions in the monolayer (1L) limit, where strongly bound excitons provide a sensitive probe for many-body interactions. Here, we use two-dimensional electronic spectroscopy (2DES) to investigate many-body interactions and their dynamics in 1L-WS2 at room temperature and with sub-10 fs time resolution. Our data reveal coherent interactions between the strongly detuned A and B exciton states in 1L-WS2. Pronounced ultrafast oscillations of the transient optical response of the B exciton are the signature of a coherent 50 meV coupling and coherent population oscillations between the two exciton states. Supported by microscopic semiconductor Bloch equation simulations, these coherent dynamics are rationalized in terms of Dexter-like interactions. Our work sheds light on the role of coherent exciton couplings and many-body interactions in the ultrafast temporal evolution of spin and valley states in TMDs.

Boosting Monolayer Transition Metal Dichalcogenides Growth by Hydrogen-Free Ramping during Chemical Vapor Deposition.

The controlled vapor-phase synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) is essential for functional applications. While chemical vapor deposition (CVD) techniques have been successful for transition metal sulfides, extending these methods to selenides and tellurides often faces challenges due to uncertain roles of hydrogen (H2) in their synthesis. Using CVD growth of MoSe2 as an example, this study illustrates the role of a H2-free environment during temperature ramping in suppressing the reduction of MoO3, which promotes effective vaporization and selenization of the Mo precursor to form MoSe2 monolayers with excellent crystal quality. As-synthesized MoSe2 monolayer-based field-effect transistors show excellent carrier mobility of up to 20.9 cm2/(V·s) with an on-off ratio of 7 × 107. This approach can be extended to other TMDs, such as WSe2, MoTe2, and MoSe2/WSe2 in-plane heterostructures. Our work provides a rational and facile approach to reproducibly synthesize high-quality TMD monolayers, facilitating their translation from laboratory to manufacturing.

Thermomechanical Properties of Transition Metal Dichalcogenides Predicted by a Machine Learning Parameterized Force Field.

The mechanical and thermal properties of transition metal dichalcogenides (TMDs) are directly relevant to their applications in electronics, thermoelectric devices, and heat management systems. In this study, we use a machine learning (ML) approach to parametrize molecular dynamics (MD) force fields to predict the mechanical and thermal transport properties of a library of monolayered TMDs (MoS2, MoTe2, WSe2, WS2, and ReS2). The ML-trained force fields were then employed in equilibrium MD simulations to calculate the lattice thermal conductivities of the foregoing TMDs and to investigate how they are affected by small and large mechanical strains. Furthermore, using nonequilibrium MD, we studied thermal transport across grain boundaries. The presented approach provides a fast albeit accurate methodology to compute both mechanical and thermal properties of TMDs, especially for relatively large systems and spatially complex structures, where density functional theory computational cost is prohibitive.

Enhanced Electromechanical Response Due to Inhomogeneous Strain in Monolayer MoS2.

2D transition metal dichalcogenides (TMDs) exhibit exceptional resilience to mechanical deformation. Applied strain can have pronounced effects on properties such as the bandgaps and exciton dynamics of TMDs, via deformation potentials and electromechanical coupling. In this work, we use piezoresponse force microscopy to show that the inhomogeneous strain from nanobubbles produces dramatic, localized enhancements of the electromechanical response of monolayer MoS2. Nanobubbles with diameters under 100 nm consistently produce an increased piezoresponse that follows the features' topography, while larger bubbles exhibit a halo-like profile, with maximum piezoresponse near the periphery. We show that spatial filtering enables these effects to be eliminated in the quantitative determination of effective piezoelectric or flexoelectric coefficients. Numerical strain modeling reveals a correlation between the hydrostatic strain gradient and the effective piezoelectric coefficient in large MoS2 nanobubbles, suggesting a localized variation in electromechanical coupling due to symmetry reduction induced by inhomogeneous strain.

Growth of Highly Oriented (VNbMoTaW)S2 Layers.

Compositional tunability, an indispensable parameter for modifying the properties of materials, can open up new applications for van der Waals (vdW) layered materials such as transition-metal dichalcogenides (TMDCs). To date, multielement alloy TMDC layers are obtained via exfoliation from bulk polycrystalline powders. Here, we demonstrate direct deposition of high-entropy alloy disulfide, (VNbMoTaW)S2, layers with controllable thicknesses on free-standing graphene membranes and on bare and hBN-covered Al2O3(0001) substrates via ultra-high-vacuum reactive dc magnetron sputtering of the VNbMoTaW target in Kr and H2S gas mixtures. Using a combination of density functional theory calculations, Raman spectroscopy, X-ray diffraction, scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, we determine that the as-deposited layers are single-phase, 2H-structured, and 0001-oriented (V0.10Nb0.16Mo0.19Ta0.28W0.27)S2.44. Our synthesis route is general and applicable for heteroepitaxial growth of a wide variety of TMDC alloys and potentially other multielement alloy vdW compounds with the desired compositions.

Identification and Structural Characterization of Twisted Atomically Thin Bilayer Materials by Deep Learning.

Two-dimensional materials are expected to play an important role in next-generation electronics and optoelectronic devices. Recently, twisted bilayer graphene and transition metal dichalcogenides have attracted significant attention due to their unique physical properties and potential applications. In this study, we describe the use of optical microscopy to collect the color space of chemical vapor deposition (CVD) of molybdenum disulfide (MoS2) and the application of a semantic segmentation convolutional neural network (CNN) to accurately and rapidly identify thicknesses of MoS2 flakes. A second CNN model is trained to provide precise predictions on the twist angle of CVD-grown bilayer flakes. This model harnessed a data set comprising over 10,000 synthetic images, encompassing geometries spanning from hexagonal to triangular shapes. Subsequent validation of the deep learning predictions on twist angles was executed through the second harmonic generation and Raman spectroscopy. Our results introduce a scalable methodology for automated inspection of twisted atomically thin CVD-grown bilayers.

Direct Hot-Electron Transfer at the Au Nanoparticle/Monolayer Transition-Metal Dichalcogenide Interface Observed with Ultrahigh Spatiotemporal Resolution.

Plasmon-induced hot-electron transfer at the metallic nanoparticle/semiconductor interface is the basis of plasmon-enhanced photocatalysis and energy harvesting. However, limited by the nanoscale size of hot spots and femtosecond time scale of hot-electron transfer, direct observation is still challenging. Herein, by using spatiotemporal-resolved photoemission electron microscopy with a two-color pump-probe beamline, we directly observed such a process with a concise system, the Au nanoparticle/monolayer transition-metal dichalcogenide (TMD) interface. The ultrafast hot-electron transfer from Au nanoparticles to monolayer TMDs and the plasmon-enhanced transfer process were directly measured and verified through an in situ comparison with the Au film/TMD interface and free TMDs. The lifetime at the Au nanoparticle/MoSe2 interface decreased from 410 to 42 fs, while the photoemission intensities exhibited a 27-fold increase compared to free MoSe2. We also measured the evolution of hot electrons in the energy distributions, indicating the hot-electron injection and decay happened in an ultrafast time scale of ∼50 fs without observable electron cooling.

Elucidating Piezoelectricity and Strain in Monolayer MoS2 at the Nanoscale Using Kelvin Probe Force Microscopy.

Strain engineering modifies the optical and electronic properties of atomically thin transition metal dichalcogenides. Highly inhomogeneous strain distributions in two-dimensional materials can be easily realized, enabling control of properties on the nanoscale; however, methods for probing strain on the nanoscale remain challenging. In this work, we characterize inhomogeneously strained monolayer MoS2 via Kelvin probe force microscopy and electrostatic gating, isolating the contributions of strain from other electrostatic effects and enabling the measurement of all components of the two-dimensional strain tensor on length scales less than 100 nm. The combination of these methods is used to calculate the spatial distribution of the electrostatic potential resulting from piezoelectricity, presenting a powerful way to characterize inhomogeneous strain and piezoelectricity that can be extended toward a variety of 2D materials.

Room Temperature, Twist Angle Independent, Momentum Direct Interlayer Excitons in van der Waals Heterostructures with Wide Spectral Tunability.

Interlayer excitons (IXs) in van der Waals heterostructures with static out of plane dipole moment and long lifetime show promise in the development of exciton based optoelectronic devices and the exploration of many body physics. However, these IXs are not always observed, as the emission is very sensitive to lattice mismatch and twist angle between the constituent materials. Moreover, their emission intensity is very weak compared to that of corresponding intralayer excitons at room temperature. Here we report the room-temperature realization of twist angle independent momentum direct IX in the heterostructures of bulk PbI2 and bilayer WS2. Momentum conserving transitions combined with the large band offsets between the constituent materials enable intense IX emission at room temperature. A long lifetime (∼100 ns), noticeable Stark shift, and tunability of IX emission from 1.70 to 1.45 eV by varying the number of WS2 layers make these heterostructures promising to develop room temperature exciton based optoelectronic devices.

Excitonic van der Waals Metasurfaces for Resonant Wavefront Shaping at Deep Subwavelength Thickness Scale.

Dielectric phase gradient metasurfaces have emerged as promising candidates to shrink bulky optical elements to subwavelength thickness scale based on dielectric meta-atoms. These meta-atoms strongly interact with light, thus offering excellent phase manipulation of incident light. However, to fulfill 2π phase control using meta-atoms, the metasurface thickness, to date, is limited to the order of 102 nm. Here, we present the thickness scaling down of phase gradient metasurfaces to <λ/20 by using excitonic van der Waals metasurfaces. High-refractive-index enabled by exciton resonances and symmetry-breaking nanostructures in the patterned layered tungsten disulfide (WS2) corporately enable quasibound states in the continuum in WS2 metasurfaces, which consequently yield complete phase regulation of 2π with the thickness down to 35 nm. To illustrate the concept, we have experimentally demonstrated beam steering, focusing, and holographic display using WS2 metasurfaces. We envision our results unveiling new venues for ultimate thin phase gradient metasurfaces.