Monolayer transition metal dichalcogenides (TMDs) support robust excitons in the visible to near‐infrared spectral range. Their reduced dielectric screening results in large binding energies, and combined with a direct bandgap in monolayer form, these excitons dominate the optical response of TMDs. In this work, a comprehensive investigation of the temperature‐dependent optical and polaritonic properties of high‐quality WS 2 , MoS 2 , WSe 2 , and MoSe 2 monolayers, encapsulated by hBN, in the range of 5‐300 K, is presented. Using reflection spectroscopy measurements, the optical and polaritonic constituents of the TMD excitons in terms of oscillator strength, linewidth, and negative permittivity, are evaluated and compared. It is found that MoSe 2 exhibits the most pronounced optical and polaritonic response, stemming from its rapid linewidth narrowing at low temperatures, as compared to the other TMDs. In addition, It is found that all four TMDs exhibit a temperature‐dependent negative real part permittivity, thus supporting surface‐exciton‐polaritons. The dispersion relation, confinement factor, and losses are derived and similarly reveal that MoSe 2 exhibits enhanced polaritonic properties. These findings establish a comparative framework for understanding the optical and polaritonic properties of monolayer TMDs, with implications on their utilization in optoelectronic devices based on 2D semiconductors.

Atomically-thin moiré superlattices offer an optically accessible platform for interacting bosons, where strong onsite repulsion U_xx suppresses double occupancy and supports excitonic Mott states at unit filling. However, moiré confinement also enhances phonon- and disorder-assisted relaxation, challenging the robustness of these correlated states under dissipation. Here we show that strengthening the intersite exciton repulsion V_xx between neighboring moiré cells offers a distinct route to stabilizing unit-filling excitonic Mott states. In H-stacked WSe2/WS2, moiré confinement endows interlayer excitons with an out-of-plane dipole and a pronounced in-plane quadrupolar charge distribution. Helicity-resolved transient photoluminescence, supported by first-principles-informed modelling, reveals that this quadrupolar geometry increases V_xx at unit filling by at least a factor of two relative to the dipolar R-stacked excitons. Despite a slight reduction in U_xx, the enhanced V_xx yields a long-lived, valley-polarized excitonic Mott state at unit filling that persists for 12 ns - more than twice as long as in R-stacks - and remains robust up to 50 K. Beyond unit filling, the same geometry supports valley-polarized doublons with fourfold longer lifetimes than in R-stacks. These results establish moiré-geometric control of intersite interactions as a route to stabilizing excitonic Mott states and doublons against dissipation in solids.

We report robust room temperature (RT) interlayer excitons in transition metal dichalcogenide (TMD) heterostructures engineered via precise stacking orientation and twist-angle control. We integrate a 2H-stacked / / heterotrilayer (HTL) onto a chirped distributed Bragg reflector (cDBR) that acts as a backside mirror. This way, we fabricate a platform that hosts distinct HTL, heterobilayer (HBL), and homobilayer (HoBL) regions with enhanced excitonic features at elevated temperatures. Although the HBL supports temperature-tunable singlet and triplet interlayer excitons, it exhibits low emission yield at 4 K. In comparison, the HTL shows remarkable excitonic properties, including pronounced band modulation, intervalley interlayer exciton transitions, and a 10-fold photoluminescence (PL) enhancement along with a sevenfold increase in exciton decay time at cryogenic temperatures compared to the HBL system. Temperature-dependent studies reveal intriguing interlayer exciton dynamics in the HTL, including the emergence of valley-polarized interlayer excitons, and the ability to maintain optical stability up to RT. Our results establish a clear strategy for engineering excitonic states across multilayer van der Waals heterostructures (VdWHSs) from 4 K to RT, providing a versatile platform for excitonic optoelectronics, quantum photonics, and tunable long-lived interlayer exciton states in scalable TMD heterostructures.

Engineering optical properties, such as luminescence purity and charge transfer, is crucial for harnessing the application potential of atomically thin transition metal dichalcogenides (TMDCs). While electrostatic gating is widely applied to gain charge control in TMDC monolayers, charge transfer can also be engineered via coupling of TMDC monolayers at semiconductor III/V, organic, or van der Waals interfaces. This confers great advantages, such as ease in implementation and compatibility in device integration. Here, we shed light on the optical properties of many-particle complexes emerging at the GaInP/MoSe2 interface as a highly relevant material combination to manipulate the optical properties of TMDCs in integrated photonic devices. Our study verifies its nature as a type II hetero-interface, which bears the feasibility to display disorder-free photoluminescence. Through optical absorption measurements, we verify that the charged complexes acquire substantial oscillator strength. Furthermore, temperature-dependent photoluminescence, supported by a microscopic theory framework, evidences the suppression of the characteristic carrier recoil effect that was previously observed in the photoluminescence of trions in TMDCs. These phenomena allow us to identify the optical signatures at the TMDC-GaInP interface as Fermi polaron quasiparticle resonances, which are of high importance in researching Bose-Fermi mixtures in condensed matter systems.

Graphene photodetectors offer ultrafast response and broadband operation, but their responsivity is typically limited by rapid hot-carrier cooling, leading to a trade-off between sensitivity and speed. Here, we demonstrate that modifying the dielectric environment provides an effective route to control hot-carrier cooling and enhance device performance. By employing a WSe2 encapsulation architecture, we suppress out-of-plane energy dissipation, leading to an increased cooling length ( 2.68 um) and a reduced heat-exchange coefficient. As a result, we obtain zero-bias graphene photodetectors with responsivities up to 0.12 A/W (potentially 0.4 A/W) while maintaining ultrafast operation beyond the setup-limited 110 GHz bandwidth. The devices enable direct detection at data rates of 120 Gb s-1 (NRZ) and 160 Gb s-1 (PAM-4), with performance achieved using minimal digital signal processing. These results establish dielectric engineering as a key design axis for controlling hot-carrier dynamics, enabling energy-efficient, high-speed optical receivers for next-generation interconnects and AI-driven data systems.

Electron Wigner solids (WSs)1-12 provide an ideal system for understanding the competing effects of electron-electron and electron-disorder interactions, a central unsolved problem in condensed matter physics. Progress in this topic has been limited by a lack of single-defect-resolved experimental measurements as well as accurate theoretical tools to enable realistic experiment/theory comparison. Here we overcome these limitations by combining atomically resolved scanning tunnelling microscopy (STM) with neural-quantum-state quantum Monte Carlo (NQS-QMC) simulation of disordered 2D electron WSs to discover new disorder-induced physical regimes of correlated electron behaviour. STM was used to image the electron density (ne)-dependent evolution of electron WSs in gate-tunable bilayer MoSe2 (BL-MoSe2) devices with varying long-range (nLR) and short-range (nSR) disorder densities. These images were compared with NQS-QMC simulations using realistic disorder maps extracted from experiment, thus allowing the roles of different disorder types to be disentangled. We identify two distinct physical regimes for disordered electron WSs that depend on nSR. For nSR ≲ ne, the WS behaviour is dominated by long-range disorder and features extensive mixed solid-liquid phases, a new type of local re-entrant melting/crystallization and prominent Friedel oscillations. By contrast, when nSR ≫ ne, these features are suppressed and a more robust amorphous WS phase emerges that persists to higher ne, highlighting the importance of short-range disorder in this regime. Our work establishes a powerful framework for studying disordered quantum solids through a combined experimental-theoretical approach.

Wireless data traffic has grown at an unprecedented rate, creating an urgent need for innovative solutions to overcome current technological limitations. Sub-terahertz (sub-THz) carrier frequencies offer increased capacity and low attenuation for short-range wireless applications. Here, we demonstrate sub-THz receivers based on graphene, which offer several advantages over state-of-the-art sub-THz receivers, such as a direct detection scheme, passive operation, and compactness. We exploit multiple concepts incorporated into a single device, including a high-quality sub-THz cavity placed in the vicinity of a high-mobility graphene channel to overcome its intrinsically low absorption. The graphene receivers achieve a multigigabit-per-second data rate with a maximum distance of  ~ 3 m from the transmitter. We demonstrate a trade-off between bandwidth and responsivity: a setup-limited 40 GHz bandwidth in low-responsivity devices, and a maximum responsivity of 0.16 A/W in devices with a 2 GHz bandwidth. Our findings enable applications such as chip-to-chip communication and close-proximity device-to-device communication.

ABSTRACT The ability to tune the degree of interaction among particles at the nanoscale is highly intriguing. The spectroscopic signature of such interaction is often subtle and requires special probes to observe. To this end, inter‐layer excitons trapped in the periodic potential wells of a moiré superlattice offer rich interaction physics, specifically due to the presence of both attractive and repulsive components in the interaction. Here we show that the Coulomb force between two inter‐layer excitons switches from repulsive to attractive when the length scale reduces from inter‐moiré‐pocket to intra‐moiré‐pocket in a WS 2 /WSe 2 hetero‐bilayer ‐ thanks to the complex competition between direct and exchange interaction. The finding is a departure from the usual notion of repelling inter‐layer excitons due to layer polarization. This manifests as the simultaneous observation of an anomalous superlinear power‐law of moiré exciton and a stabilization of moiré trapped biexciton. The experimental observation is facilitated by placing the hetero‐bilayer on a polymer‐nanopillar/gold‐film stack which significantly reduces the inhomogeneous spectral broadening by selectively probing a smaller ensemble of moiré pockets compared with a flat sample. This creates an interesting platform to explore interaction among moiré trapped excitons and higher order quasiparticles.

, the heterostructure exhibits a pronounced thermal dependence of the enhancement factor, implying that the exciton lifetime dominates over interfacial transfer processes. Our findings demonstrate reversible, electric-field-driven PL control at a TMDC/metal interface, providing a pathway to electrically tunable light emission and improved contact engineering in two-dimensional optoelectronic devices.

Exciton dissociation via the excitonic Mott transition (EMT) governs the high-density optical response of semiconductors and sets fundamental limits for optoelectronic devices. The EMT is conventionally linked to the onset of population inversion and the emergence of optical gain. Here, we demonstrate that this paradigm can break down under ultrafast non-equilibrium excitation. Using femtosecond pump-probe optical spectroscopy, we drive a monolayer transition metal dichalcogenide into a dense photoexcited state in which the excitonic resonance is completely quenched within 100 fs, while the optical gain is entirely absent across the explored fluence range. State-of-the-art real-time ab initio simulations reveal that the EMT is governed by an interplay of strongly nonthermal carrier populations and nonequilibrium dynamical screening of the Coulomb interaction. The quantitative agreement between theory and experiment identifies a distinct, ultrafast pathway to exciton ionization beyond quasi-equilibrium descriptions and demonstrates that population inversion is not a universal prerequisite for the EMT.

Monolayer transition metal dichalcogenides (TMDCs) are promising materials for next-generation optoelectronic devices owing to their strong excitonic responses and atomic thickness. Controlling their light emission electrically is a crucial step toward realizing practical nanoscale optoelectronic devices, such as light-emitting diodes and optical modulators. However, photoluminescence (PL) quenching in van der Waals TMDC/metal heterostructures, caused by ultrafast interlayer charge or energy transfer, impedes such electrical modulation. Here, we investigate monolayer MoSe2/bulk NbSe2 heterostructures and demonstrate that a vertical electric field tunes the PL intensity by nearly 3 orders of magnitude in bare MoSe2 and by about 1 order of magnitude in the MoSe2/NbSe2 heterostructure. First-principles calculations with spin-orbit coupling reveal stronger electronic coupling and band hybridization in the MoSe2/NbSe2 heterostructure than in conventional graphene-based counterparts. This enhances the sensitivity to a perpendicular electric field and enables a transition between direct and indirect bandgaps, strongly affecting the photoluminescence response. Unlike bare MoSe2, the heterostructure exhibits a pronounced thermal dependence of the enhancement factor, implying that the exciton lifetime dominates over interfacial transfer processes. Our findings demonstrate reversible, electric-field-driven PL control at a TMDC/metal interface, providing a pathway to electrically tunable light emission and improved contact engineering in two-dimensional optoelectronic devices.

We report the current conduction across multilayer and delaminated Ti3C2Tx two-dimensional transition metal carbides (MXene)-based Schottky interfaces formed on n-type GaAs. Two types of Ti3C2Tx MXene/n-type GaAs metal-semiconductor (MS) interfaces were created by a simple drop-casting technique using a multilayer MXene coating and a thin film of delaminated MXene. The Schottky barrier heights of the MS interfaces were analyzed from room-temperature as well as temperature-dependent current-voltage (I-V) characteristics across the junction. The Schottky barrier height calculated from the I-V data shows lower values for the delaminated MXene-based Schottky junctions compared to the multilayer-based device. We attribute this difference primarily to reduced Fermi-level pinning at the delaminated Ti3C2Tx/n-GaAs interface and to the presence of a large amount of trapped water in the multilayer Ti3C2Tx structure. The applicability of the rectifying Schottky interfaces as photodiodes was demonstrated as a photodiode at a wavelength of 785 nm. The delaminated MXene-based photodiode shows superior performance with a responsivity of 284 mAW(-1) compared to the multilayer MXene-based device.

Two-dimensional (2D) materials are emerging as promising semiconductors for ultrascaled FET devices in the post-Moore’s Law era. Achieving precise doping in these materials is critical for fine-tuning of their electronic properties. Similar to traditional semiconductors, such as Si or SiGe, doping concentration plays a key role in modulating performance. In this work, we demonstrate controlled synthesis of n-type rhenium (Re)-doped MoS2 monolayers via atmospheric pressure chemical vapor deposition, achieving Re concentrations as low as 0.17 at.

The emergence of charge-neutral collective modes is a hallmark of correlated quantum phases but is often challenging to probe. In two-dimensional flatband systems, charge responses have been intensively investigated, but neutral excitations have not been directly probed. In particular, the intervalley-coherent state has been proposed as a unifying theme across graphene- and semiconductor-based systems and features a neutral Goldstone mode due to spontaneously broken valley U(1) symmetry. However, this mode has not been observed. Here we demonstrate the transport of neutral modes in twisted WSe2 moir & eacute; superlattices using a space-and-time-resolved ultrafast imaging technique. We show two propagating collective modes with different velocities that emerge near the Van Hove singularity. The fast-propagating mode is consistent with a Goldstone mode for an intervalley-coherent state, whereas the slow-moving mode is probably a gapped amplitude mode. They can be understood as the spin-valley analogues of the collective modes of a superfluid. Our study provides a powerful approach for probing neutral modes in quantum materials and offers key insights into the interplay between charge and spin-valley physics in moir & eacute; superlattices.

A robust and CMOS-compatible approach is introduced for the synthesis of high-quality Bi2O2Se thin films using reactive pulsed laser deposition (RPLD). The process leverages an oxygen-assisted transformation of Bi2Se3 to enable the direct formation of Bi2O2Se with a strong out-of-plane (00l) orientation on amorphous SiO2 (a-SiO2), c-cut sapphire (c-Al2O3), and lattice-matched SrTiO3 (STO) substrates. Systematic optimization of oxygen background pressure, substrate temperature, and laser fluence reveals critical phase boundaries governing the transformation and allows suppression of vertically extended features such as (103)-oriented Bi2O2Se and over-oxidized (231)-oriented vertical Bi2SeO5. Comprehensive structural and spectroscopic analyses, including Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), confirm the formation of phase-pure, atomically smooth Bi2O2Se layers. Conductive AFM (C-AFM) measurements establish a correlation between local film morphology and electronic transport, showing enhanced conductivity in misoriented Bi2O2Se domains and insulating behavior in Bi2SeO5 regions. Preliminary field-effect transistors (FETs) fabricated using RPLD-grown Bi2O2Se as the semiconductor channel material on (100) STO exhibit a moderate on/off ratio, efficient gate control, and current driving capability. These results highlight RPLD as a powerful technique for integrating layered oxychalcogenides into next-generation electronic and optoelectronic platforms.

ABSTRACT A robust and CMOS‐compatible approach is introduced for the synthesis of high‐quality Bi 2 O 2 Se thin films using reactive pulsed laser deposition (RPLD). The process leverages an oxygen‐assisted transformation of Bi 2 Se 3 to enable the direct formation of Bi 2 O 2 Se with a strong out‐of‐plane (00 l ) orientation on amorphous SiO 2 ( a ‐SiO 2 ), c‐cut sapphire (c‐Al 2 O 3 ), and lattice‐matched SrTiO 3 (STO) substrates. Systematic optimization of oxygen background pressure, substrate temperature, and laser fluence reveals critical phase boundaries governing the transformation and allows suppression of vertically extended features such as (103)‐oriented Bi 2 O 2 Se and over‐oxidized (231)‐oriented vertical Bi 2 SeO 5 . Comprehensive structural and spectroscopic analyses, including Raman spectroscopy, X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), confirm the formation of phase‐pure, atomically smooth Bi 2 O 2 Se layers. Conductive AFM (C‐AFM) measurements establish a correlation between local film morphology and electronic transport, showing enhanced conductivity in misoriented Bi 2 O 2 Se domains and insulating behavior in Bi 2 SeO 5 regions. Preliminary field‐effect transistors (FETs) fabricated using RPLD‐grown Bi 2 O 2 Se as the semiconductor channel material on (100) STO exhibit a moderate on/off ratio, efficient gate control, and current driving capability. These results highlight RPLD as a powerful technique for integrating layered oxychalcogenides into next‐generation electronic and optoelectronic platforms.

Excitons in monolayer transition metal dichalcogenides (TMDs) dominate their optical response due to exceptionally large binding energies arising from their two-dimensional nature. Several theoretical models have been proposed to describe this excitonic behavior, however, it remains unclear which model most accurately captures the underlying physical properties of the response. In this work, we experimentally measure the optical response of high-quality monolayer TMD heterostructures and compare the results with the different theoretical models to address this uncertainty. We find that in high-quality heterostructures, quantum mechanical interactions in the form of pure dephasing plays a dominant role, which has been challenging to isolate experimentally in previous studies. Furthermore, accounting for an additional decay rate to the commonly used radiative and non-radiative rates is found to be important for the accurate description of the excitonic response. These findings establish a robust framework for understanding and predicting the optical properties of TMD-based heterostructures, crucial for both fundamental research and optoelectronic applications.

Van der Waals heterostructures (VdWHs) composed of 2D materials have attracted significant attention in recent years due to their intriguing optical properties, such as strong light-matter interactions and large intrinsic anisotropy. In particular, VdWHs support a variety of polaritons-hybrid quasiparticles arising from the coupling between electromagnetic waves and material excitations-enabling the confinement of electromagnetic radiation to atomic scales. The ability to predict and simulate the optical response of 2D materials heterostructures is thus of high importance, being commonly performed until now via methods such as the TMM, or Fresnel equations. While straight forward, these often yield long and complicated expressions, limiting intuitive and simple access to the underlying physical mechanisms that govern the optical response. In this work, we demonstrate the adaptation of the transmission line model for VdWHs, based on expressing its constituents by distributed electrical circuit elements described by their admittance. Since the admittance carries fundamental physical meaning of the material response to electromagnetic fields, the approach results in a system of propagating voltage and current waves, offering a compact and physically intuitive formulation that simplifies algebraic calculations, clarifies the conditions for existence of physical solutions, and provides valuable insight into the fundamental physical response. To demonstrate this, we derive the transmission line analogs of bulk to monolayer 2D materials and show it can be used to compute the reflection/transmission coefficients, polaritonic dispersion relations, and electromagnetic field distributions in a variety of VdWHs, and compare them to experimental measurements yielding very good agreement. This method provides a valuable tool for exploring and understanding the optical response of layered 2D systems.

The increasing use of two-dimensional (2D) materials in nanoelectronics demands robust metrology techniques for electrical characterization, especially for large-scale production. While atomic force microscopy (AFM) techniques like conductive AFM (C-AFM) offer high accuracy, they suffer from slow data acquisition speeds due to the raster scanning process. To address this, we introduce SparseC-AFM, a deep learning model that rapidly and accurately reconstructs conductivity maps of 2D materials like MoS_2 from sparse C-AFM scans. Our approach is robust across various scanning modes, substrates, and experimental conditions. We report a comparison between (a) classic flow implementation, where a high pixel density C-AFM image (e.g., 15 minutes to collect) is manually parsed to extract relevant material parameters, and (b) our SparseC-AFM method, which achieves the same operation using data that requires substantially less acquisition time (e.g., under 5 minutes). SparseC-AFM enables efficient extraction of critical material parameters in MoS_2, including film coverage, defect density, and identification of crystalline island boundaries, edges, and cracks. We achieve over 11x reduction in acquisition time compared to manual extraction from a full-resolution C-AFM image. Moreover, we demonstrate that our model-predicted samples exhibit remarkably similar electrical properties to full-resolution data gathered using classic-flow scanning. This work represents a significant step toward translating AI-assisted 2D material characterization from laboratory research to industrial fabrication. Code and model weights are available at github.com/UNITES-Lab/sparse-cafm.