In situ via Contact to hBN-Encapsulated Air-Sensitive Atomically Thin Semiconductors.

Establishing reliable electrical contacts to atomically thin materials is a prerequisite for both fundamental studies and applications yet remains a challenge. In particular, the development of contact techniques for air-sensitive monolayers has lagged behind, despite their unique properties and significant potential for applications. Here, we present a robust method to create contacts to device layers encapsulated within hexagonal boron nitride (hBN). This method uses plasma etching and metal deposition to create 'vias' in the hBN with graphene forming an atomically thin etch-stop. The resulting partially fluorinated graphene (PFG) protects the underlying device layer from air-induced degradation and damage during metal deposition. PFG is resistive in-plane but maintains high out-of-plane conductivity. The work function of the PFG/metal contact is tunable through the degree of fluorination, offering opportunities for contact engineering. Using the in situ via technique, we achieve ambipolar contact to air-sensitive monolayer 2H-molybdenum ditelluride (MoTe2) with more than 1 order of magnitude improvement in on-current density compared to previous literature. The complete encapsulation provides high reproducibility and long-term stability. The technique can be extended to other air-sensitive materials as well as air-stable materials, offering highly competitive device performance.

Reproducible graphene synthesis by oxygen-free chemical vapour deposition.

Chemical vapour deposition (CVD) synthesis of graphene on copper has been broadly adopted since the first demonstration of this process1. However, widespread use of CVD-grown graphene for basic science and applications has been hindered by challenges with reproducibility2 and quality3. Here we identify trace oxygen as a key factor determining the growth trajectory and quality for graphene grown by low-pressure CVD. Oxygen-free chemical vapour deposition (OF-CVD) synthesis is fast and highly reproducible, with kinetics that can be described by a compact model, whereas adding trace oxygen leads to suppressed nucleation and slower/incomplete growth. Oxygen affects graphene quality as assessed by surface contamination, emergence of the Raman D peak and decrease in electrical conductivity. Epitaxial graphene grown in oxygen-free conditions is contamination-free and shows no detectable D peak. After dry transfer and boron nitride encapsulation, it shows room-temperature electrical-transport behaviour close to that of exfoliated graphene. A graphite-gated device shows well-developed integer and fractional quantum Hall effects. By highlighting the importance of eliminating trace oxygen, this work provides guidance for future CVD system design and operation. The increased reproducibility and quality afforded by OF-CVD synthesis will broadly influence basic research and applications of graphene.

Wafer-scale development, characterization, and high temperature stabilization of epitaxial Cr2O3 films grown on Ru(0001).

This work outlines conditions suitable for the heteroepitaxial growth of Cr2O3(0001) films (1.5-20 nm thick) on a Ru(0001)-terminated substrate. Optimized growth is achieved by sputter deposition of Cr within a 4 mTorr Ar/O2 20% ambient at Ru temperatures ranging from 450 to 600 °C. The Cr2O3 film adopts a 30° rotated honeycomb configuration with respect to the underlying Ru(0001) substrate and exhibits a hexagonal lattice parameter consistent with that for bulk Cr2O3(0001). Heating to 700 °C within the same environment during film preparation leads to Ru oxidation. Exposure to temperatures at or above 400 °C in a vacuum, Ar, or Ar/H2 3% leads to chromia film degradation characterized by increased Ru 3d XPS intensity coupled with concomitant Cr 2p and O 1s peak attenuations when compared to data collected from unannealed films. An ill-defined but hexagonally well-ordered RuxCryOz surface structure is noted after heating the film in this manner. Heating within a wet Ar/H2 3% environment preserves the Cr2O3(0001)/Ru(0001) heterolayer structure to temperatures of at least 950 °C. Heating an Ru-Cr2O3-Ru heterostacked film to 950 °C within this environment is shown by cross-sectional scanning/transmission electron microscopy (S/TEM) to provide clear evidence of retained epitaxial bicrystalline oxide interlayer structure, interlayer immiscibility, and epitaxial registry between the top and bottom Ru layers. Subtle effects marked by O enrichment and O 1s and Cr 2p shifts to increased binding energies are noted by XPS in the near-Ru regions of Cr2O3(0001)/Ru(0001) and Ru(0001)/Cr2O3(0001)/Ru(0001) films after annealing to different temperatures in different sets of environmental conditions.

Atomic Defect Quantification by Lateral Force Microscopy.

Atomic defects in two-dimensional (2D) materials impact electronic and optoelectronic properties, such as doping and single photon emission. An understanding of defect-property relationships is essential for optimizing material performance. However, progress in understanding these critical relationships is hindered by a lack of straightforward approaches for accurate, precise, and reliable defect quantification on the nanoscale, especially for insulating materials. Here, we demonstrate that lateral force microscopy (LFM), a mechanical technique, can observe atomic defects in semiconducting and insulating 2D materials under ambient conditions. We first improve the sensitivity of LFM through consideration of cantilever mechanics. With the improved sensitivity, we use LFM to locate atomic-scale point defects on the surface of bulk MoSe2. By directly comparing LFM and conductive atomic force microscopy (CAFM) measurements on bulk MoSe2, we demonstrate that point defects observed with LFM are atomic defects in the crystal. As a mechanical technique, LFM does not require a conductive pathway, which allows defect characterization on insulating materials, such as hexagonal boron nitride (hBN). We demonstrate the ability to observe intrinsic defects in hBN and defects introduced by annealing. Our demonstration of LFM as a mechanical defect characterization technique applicable to both conductive and insulating 2D materials will enable routine defect-property determination and accelerate materials research.

Spontaneous exciton dissociation in transition metal dichalcogenide monolayers.

Since the seminal work on MoS2, photoexcitation in atomically thin transition metal dichalcogenides (TMDCs) has been assumed to result in excitons, with binding energies order of magnitude larger than thermal energy at room temperature. Here, we reexamine this foundational assumption and show that photoexcitation of TMDC monolayers can result in a substantial population of free charges. Performing ultrafast terahertz spectroscopy on large-area, single-crystal TMDC monolayers, we find that up to ~10% of excitons spontaneously dissociate into charge carriers with lifetimes exceeding 0.2 ns. Scanning tunneling microscopy reveals that photocarrier generation is intimately related to mid-gap defects, likely via trap-mediated Auger scattering. Only in state-of-the-art quality monolayers, with mid-gap trap densities as low as 109 cm-2, does intrinsic exciton physics start to dominate the terahertz response. Our findings reveal the necessity of knowing the defect density in understanding photophysics of TMDCs.

Validating the Use of Conductive Atomic Force Microscopy for Defect Quantification in 2D Materials.

Defects significantly affect the electronic, chemical, mechanical, and optical properties of two-dimensional (2D) materials. Thus, it is critical to develop a method for convenient and reliable defect quantification. Scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) possess the required atomic resolution but have practical disadvantages. Here, we benchmark conductive atomic force microscopy (CAFM) by a direct comparison with STM in the characterization of transition metal dichalcogenides (TMDs). The results conclusively demonstrate that CAFM and STM image identical defects, giving results that are equivalent both qualitatively (defect appearance) and quantitatively (defect density). Further, we confirm that CAFM can achieve single-atom resolution, similar to that of STM, on both bulk and monolayer samples. The validation of CAFM as a facile and accurate tool for defect quantification provides a routine and reliable measurement that can complement other standard characterization techniques.

Automated Grain Boundary Detection for Bright-Field Transmission Electron Microscopy Images via U-Net.

Quantification of microstructures is crucial for understanding processing-structure and structure-property relationships in polycrystalline materials. Delineating grain boundaries in bright-field transmission electron micrographs, however, is challenging due to complex diffraction contrast in images. Conventional edge detection algorithms are inadequate; instead, manual tracing is usually required. This study demonstrates the first successful machine learning approach for grain boundary detection in bright-field transmission electron micrographs. The proposed methodology uses a U-Net convolutional neural network trained on carefully constructed data from bright-field images and hand tracings available from prior studies, combined with targeted postprocessing algorithms to preserve fine features of interest. The image processing pipeline accurately estimates grain boundary positions, avoiding segmentation in regions with intragrain contrast and identifying low-contrast boundaries. Our approach is validated by directly comparing microstructural markers (i.e., grain centroids) identified in U-Net predictions with those identified in hand tracings; furthermore, the grain size distributions obtained from the two techniques show notable overlap when compared using t-test, Kolmogorov-Smirnov test, and Cramér-von Mises test. The technique is then successfully applied to interpret new microstructures having different image characteristics from the training data, with preliminary results from platinum and palladium microstructures presented, highlighting the versatility of our approach for grain boundary identification in bright-field micrographs.

Optical Imaging of Ultrafast Phonon-Polariton Propagation through an Excitonic Sensor.

Hexagonal boron nitride (hBN) hosts phonon polaritons (PhP), hybrid light-matter states that facilitate electromagnetic field confinement and exhibit long-range ballistic transport. Extracting the spatiotemporal dynamics of PhPs usually requires "tour de force" experimental methods such as ultrafast near-field infrared microscopy. Here, we leverage the remarkable environmental sensitivity of excitons in two-dimensional transition metal dichalcogenides to image PhP propagation in adjacent hBN slabs. Using ultrafast optical microscopy on monolayer WSe2/hBN heterostructures, we image propagating PhPs from 3.5 K to room temperature with subpicosecond and few-nanometer precision. Excitons in WSe2 act as transducers between visible light pulses and infrared PhPs, enabling visible-light imaging of PhP transport with far-field microscopy. We also report evidence of excitons in WSe2 copropagating with hBN PhPs over several micrometers. Our results provide new avenues for imaging polar excitations over a large frequency range with extreme spatiotemporal precision and new mechanisms to realize ballistic exciton transport at room temperature.

Two-Step Flux Synthesis of Ultrapure Transition-Metal Dichalcogenides.

Two-dimensional transition-metal dichalcogenides (TMDs) have attracted tremendous interest due to the unusual electronic and optoelectronic properties of isolated monolayers and the ability to assemble diverse monolayers into complex heterostructures. To understand the intrinsic properties of TMDs and fully realize their potential in applications and fundamental studies, high-purity materials are required. Here, we describe the synthesis of TMD crystals using a two-step flux growth method that eliminates a major potential source of contamination. Detailed characterization of TMDs grown by this two-step method reveals charged and isovalent defects with densities an order of magnitude lower than those in TMDs grown by a single-step flux technique. For WSe2, we show that increasing the Se/W ratio during growth reduces point defect density, with crystals grown at 100:1 ratio achieving charged and isovalent defect densities below 1010 and 1011 cm-2, respectively. Initial temperature-dependent electrical transport measurements of monolayer WSe2 yield room-temperature hole mobility above 840 cm2/(V s) and low-temperature disorder-limited mobility above 44,000 cm2/(V s). Electrical transport measurements of graphene-WSe2 heterostructures fabricated from the two-step flux grown WSe2 also show superior performance: higher graphene mobility, lower charged impurity density, and well-resolved integer quantum Hall states. Finally, we demonstrate that the two-step flux technique can be used to synthesize other TMDs with similar defect densities, including semiconducting 2H-MoSe2 and 2H-MoTe2 and semimetallic Td-WTe2 and 1T'-MoTe2.

Experimental and Computational Study of the Orientation Dependence of Single-Crystal Ruthenium Nanowire Stability.

Single-crystal nanowires are of broad interest for applications in nanotechnology. However, such wires are subject to both the Rayleigh-Plateau instability and an ovulation process that are expected to lead to their break up into particle arrays. Single crystal Ru nanowires were fabricated with axes lying along different crystallographic orientations. Wires bound by equilibrium facets along their length did not break up through either a Rayleigh-Plateau or ovulation process, while wires with other orientations broke up through a combination of both. Mechanistic insight is provided using a level-set simulation that accounts for strongly anisotropic surface energies, providing a framework for design of morphologically stable nanostructures.

Electrically tunable quantum confinement of neutral excitons.

Confining particles to distances below their de Broglie wavelength discretizes their motional state. This fundamental effect is observed in many physical systems, ranging from electrons confined in atoms or quantum dots1,2 to ultracold atoms trapped in optical tweezers3,4. In solid-state photonics, a long-standing goal has been to achieve fully tunable quantum confinement of optically active electron-hole pairs, known as excitons. To confine excitons, existing approaches mainly rely on material modulation5, which suffers from poor control over the energy and position of trapping potentials. This has severely impeded the engineering of large-scale quantum photonic systems. Here we demonstrate electrically controlled quantum confinement of neutral excitons in 2D semiconductors. By combining gate-defined in-plane electric fields with inherent interactions between excitons and free charges in a lateral p-i-n junction, we achieve exciton confinement below 10 nm. Quantization of excitonic motion manifests in the measured optical response as a ladder of discrete voltage-dependent states below the continuum. Furthermore, we observe that our confining potentials lead to a strong modification of the relative wave function of excitons. Our technique provides an experimental route towards creating scalable arrays of identical single-photon sources and has wide-ranging implications for realizing strongly correlated photonic phases6,7 and on-chip optical quantum information processors8,9.

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.

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.

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.

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.

Diffusivity Reveals Three Distinct Phases of Interlayer Excitons in MoSe_{2}/WSe_{2} Heterobilayers.

Charge separated interlayer excitons in transition metal dichalcogenide heterobilayers are being explored for moiré exciton lattices and exciton condensates. The presence of permanent dipole moments and the poorly screened Coulomb interaction make many-body interactions particularly strong for interlayer excitons. Here we reveal two distinct phase transitions for interlayer excitons in the MoSe_{2}/WSe_{2} heterobilayer using time and spatially resolved photoluminescence imaging: from trapped excitons in the moiré potential to the modestly mobile exciton gas as exciton density increases to n_{ex}∼10^{11} cm^{-2} and from the exciton gas to the highly mobile charge separated electron-hole plasma for n_{ex}>10^{12} cm^{-2}. The latter is the Mott transition and is confirmed in photoconductivity measurements. These findings set fundamental limits for achieving quantum states of interlayer excitons.

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.

Direct Measurement of the Radiative Pattern of Bright and Dark Excitons and Exciton Complexes in Encapsulated Tungsten Diselenide.

The optical properties of particularly the tungsten-based transition-metal dichalcogenides are strongly influenced by the presence of dark excitons. Recently, theoretical predictions as well as indirect experimental insights have shown that two different dark excitons exist within the light cone. While one is completely dark, the other one is only dipole forbidden out-of-plane, hence referred to as grey exciton. Here, we present angle-resolved spectroscopic data of a high-quality hexagonal-BN-encapsulated WSe2 monolayer with which we directly obtain the radiation pattern of this grey exciton that deviates from that of the bright exciton and other exciton complexes obtained at cryogenic temperatures.

Simulation of Hubbard model physics in WSe2/WS2 moiré superlattices.

The Hubbard model, formulated by physicist John Hubbard in the 1960s1, is a simple theoretical model of interacting quantum particles in a lattice. The model is thought to capture the essential physics of high-temperature superconductors, magnetic insulators and other complex quantum many-body ground states2,3. Although the Hubbard model provides a greatly simplified representation of most real materials, it is nevertheless difficult to solve accurately except in the one-dimensional case2,3. Therefore, the physical realization of the Hubbard model in two or three dimensions, which can act as an analogue quantum simulator (that is, it can mimic the model and simulate its phase diagram and dynamics4,5), has a vital role in solving the strong-correlation puzzle, namely, revealing the physics of a large number of strongly interacting quantum particles. Here we obtain the phase diagram of the two-dimensional triangular-lattice Hubbard model by studying angle-aligned WSe2/WS2 bilayers, which form moiré superlattices6 because of the difference between the lattice constants of the two materials. We probe the charge and magnetic properties of the system by measuring the dependence of its optical response on an out-of-plane magnetic field and on the gate-tuned carrier density. At half-filling of the first hole moiré superlattice band, we observe a Mott insulating state with antiferromagnetic Curie-Weiss behaviour, as expected for a Hubbard model in the strong-interaction regime2,3,7-9. Above half-filling, our experiment suggests a possible quantum phase transition from an antiferromagnetic to a weak ferromagnetic state at filling factors near 0.6. Our results establish a new solid-state platform based on moiré superlattices that can be used to simulate problems in strong-correlation physics that are described by triangular-lattice Hubbard models.