Enhancing Single Photon Emission Purity via Design of van der Waals Heterostructures.

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects. Here, we show that design of a van der Waals heterostructure effectively eliminates this spurious light, resulting in purities suitable for a variety of quantum technological applications. Single photon purity from emitters in monolayer WSe2 increases from 60% to 92% by incorporating this monolayer in a simple graphite/WSe2 heterostructure. Fast interlayer charge transfer quenches a broad photoluminescence background by preventing radiative recombination through long-lived defect bound exciton states. This approach is generally applicable to other 2D emitter materials, circumvents issues of material quality, and offers a path forward to achieve the ultrahigh single photon purities ultimately required for photon-based quantum technologies.

Low-Resistance 2D/2D Ohmic Contacts: A Universal Approach to High-Performance WSe2, MoS2, and MoSe2 Transistors.

We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ∼0.3 kΩ µm, high on/off ratios up to >10(9), and high drive currents exceeding 320 µA µm(-1). These favorable characteristics are combined with a two-terminal field-effect hole mobility µFE ≈ 2 × 10(2) cm(2) V(-1) s(-1) at room temperature, which increases to >2 × 10(3) cm(2) V(-1) s(-1) at cryogenic temperatures. We observe a similar performance also in MoS2 and MoSe2 FETs with 2D/2D drain and source contacts. The 2D/2D low-resistance ohmic contacts presented here represent a new device paradigm that overcomes a significant bottleneck in the performance of TMDs and a wide variety of other 2D materials as the channel materials in postsilicon electronics.

High mobility WSe2 p- and n-type field-effect transistors contacted by highly doped graphene for low-resistance contacts.

We report the fabrication of both n-type and p-type WSe2 field-effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal-insulator transition at a characteristic conductivity close to the quantum conductance e(2)/h, a high ON/OFF ratio of >10(7) at 170 K, and large electron and hole mobility of µ ≈ 200 cm(2) V(-1 )s(-1) at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ∼330 cm(2) V(-1) s(-1) and that of holes to ∼270 cm(2) V(-1) s(-1). We attribute our ability to observe the intrinsic, phonon-limited conduction in both the electron and hole channels to the drastic reduction of the Schottky barriers between the channel and the graphene contact electrodes using IL gating. We elucidate this process by studying a Schottky diode consisting of a single graphene/WSe2 Schottky junction. Our results indicate the possibility to utilize chemically or electrostatically highly doped graphene for versatile, flexible, and transparent low-resistance ohmic contacts to a wide range of quasi-2D semiconductors.

Highly Efficient Spin-Orbit Torque Switching in Bi2Se3/Fe3GeTe2 van der Waals Heterostructures.

Topological insulators (TIs) have shown promise as a spin-generating layer to switch the magnetization state of ferromagnets via spin-orbit torque (SOT) due to charge-to-spin conversion efficiency of the TI surface states that arises from spin-momentum locking. However, when TIs are interfaced with conventional bulk ferromagnetic metals, the combination of charge transfer and hybridization can potentially destroy the spin texture and hamper the possibility of accessing the TI surface states. Here, we fabricate an all van der Waals (vdW) heterostructure consisting of molecular beam epitaxy grown bulk-insulating Bi2Se3 and exfoliated 2D metallic ferromagnet Fe3GeTe2 (FGT) with perpendicular anisotropy. By detecting the magnetization state of the FGT via anomalous Hall effect and magneto-optical Kerr effect measurements, we determine the critical switching current density for magnetization switching to be Jc ≈ 1.2 × 106 A/cm2, the lowest reported for the switching of a perpendicular anisotropy ferromagnet using Bi2Se3. From second harmonic Hall measurements, we further determine the SOT efficiency (ξDL) to be in the range of 1.8 ± 0.3 and 1.4 ± 0.08 between 5 and 150 K, comparable to the highest values reported for Bi2Se3. Our density functional theory calculations find that the weak interlayer interactions at the Bi2Se3/FGT interface lead to a weakened dipole at the interface and suppress the proximity induced magnetic moment on Bi2Se3. This enables direct access to the TI surface states contributed by the first quintuple layer, where the spins are singly degenerate with significant net in-plane spin polarization. Our results highlight the clear advantage of all-vdW heterostructures with weak interlayer interactions that can enhance SOT efficiency and minimize critical current density, an important step toward realizing next generation low-power nonvolatile memory and spintronic devices.

Enhancing the Purity of Deterministically Placed Quantum Emitters in Monolayer WSe2.

We present a method utilizing an applied electrostatic potential for suppressing the broad defect bound excitonic emission in two-dimensional materials (2DMs) which otherwise inhibits the purity of strain induced single photon emitters (SPEs). Our heterostructure consists of a WSe2 monolayer on a polymer in which strain has been deterministically introduced via an atomic force microscope (AFM) tip. We show that by applying an electrostatic potential, the broad defect bound background is suppressed at cryogenic temperatures, resulting in a substantial improvement in single photon purity demonstrated by a 10-fold reduction of the correlation function g(2)(0) value from 0.73 to 0.07. In addition, we see a 2-fold increase in the intensity of the SPEs as well as the ability to activate/deactivate the emitters at certain wavelengths. Finally, we present an increase in the operating temperature of the SPE up to 110 K, a 50 K increase when compared with the results when no electrostatic potential is present.

Laser-Patterned Submicrometer Bi2Se3-WS2 Pixels with Tunable Circular Polarization at Room Temperature.

Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS2) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of ×161), peak position (38.4 meV range), circular polarization (39.4% range), and valley polarization of a Bi2Se3-WS2 2D heterostructure using a low-power laser (0.762 µW) in ambient conditions. Changes are spatially confined to the laser spot, enabling submicrometer (814 nm) features, and are long-term stable (>334 days). PL and valley polarization changes can be controllably reversed through laser exposure in a vacuum, allowing the material to be erased and reused. Atmospheric experiments and first-principles calculations indicate oxygen diffusion modulates the exciton radiative vs nonradiative recombination pathways, where oxygen absorption leads to brightening and desorption to darkening.

Emergent Moiré Phonons Due to Zone Folding in WSe2-WS2 Van der Waals Heterostructures.

Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moiré superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moiré Raman modes from nearly aligned WSe2-WS2 vdWHs in the range of 240-260 cm-1, which are absent in both monolayers and homobilayers of WSe2 and WS2 and in largely misaligned WSe2-WS2 vdWHs. Using first-principles calculations and geometric arguments, we show that these moiré Raman modes are a consequence of the large moiré length scale, which results in zone-folded phonon modes that are Raman active. These modes are sensitive to changes in twist angle, but notably, they occur at identical frequencies for a given small twist angle away from either the 0-degree or 60-degree aligned heterostructure. Our measurements also show a strong Raman intensity modulation in the frequency range of interest, with near 0 and near 60-degree vdWHs exhibiting a markedly different dependence on excitation energy. In near 0-degree aligned WSe2-WS2 vdWHs, a nearly complete suppression of both the moiré Raman modes and the WSe2 A1g Raman mode (∼250 cm-1) is observed when exciting with a 532 nm CW laser at room temperature. Temperature-dependent reflectance contrast measurements demonstrate the significant Raman intensity modulation arises from resonant Raman effects.

Room-Temperature Oxygen Transport in Nanothin BixOySez Enables Precision Modulation of 2D Materials.

Oxygen conductors and transporters are important to several consequential renewable energy technologies, including fuel cells and syngas production. Separately, monolayer transition-metal dichalcogenides (TMDs) have demonstrated significant promise for a range of applications, including quantum computing, advanced sensors, valleytronics, and next-generation optoelectronics. Here, we synthesize a few-nanometer-thick BixOySez compound that strongly resembles a rare R3m bismuth oxide (Bi2O3) phase and combine it with monolayer TMDs, which are highly sensitive to their environment. We use the resulting 2D heterostructure to study oxygen transport through BixOySez into the interlayer region, whereby the 2D material properties are modulated, finding extraordinarily fast diffusion near room temperature under laser exposure. The oxygen diffusion enables reversible and precise modification of the 2D material properties by controllably intercalating and deintercalating oxygen. Changes are spatially confined, enabling sub-micrometer features (e.g., pixels), and are long-term stable for more than 221 days. Our work suggests few-nanometer-thick BixOySez is a promising unexplored room-temperature oxygen transporter. Additionally, our findings suggest that the mechanism can be applied to other 2D materials as a generalized method to manipulate their properties with high precision and sub-micrometer spatial resolution.

Stacking-dependent optical properties in bilayer WSe2.

The twist angle between the monolayers in van der Waals heterostructures provides a new degree of freedom in tuning material properties. We compare the optical properties of WSe2 homobilayers with 2H and 3R stacking using photoluminescence, Raman spectroscopy, and reflectance contrast measurements under ambient and cryogenic temperatures. Clear stacking-dependent differences are evident for all temperatures, with both photoluminescence and reflectance contrast spectra exhibiting a blue shift in spectral features in 2H compared to 3R bilayers. Density functional theory (DFT) calculations elucidate the source of the variations and the fundamental differences between 2H and 3R stackings. DFT finds larger energies for both A and B excitonic features in 2H than in 3R, consistent with experimental results. In both stacking geometries, the intensity of the dominant A1g Raman mode exhibits significant changes as a function of laser excitation wavelength. These variations in intensity are intimately linked to the stacking- and temperature-dependent optical absorption through resonant enhancement effects. The strongest enhancement is achieved when the laser excitation coincides with the C excitonic feature, leading to the largest Raman intensity under 514 nm excitation in 2H stacking and at 520 nm in 3R stacked WSe2 bilayers.

Low-Temperature 2D/2D Ohmic Contacts in WSe2 Field-Effect Transistors as a Platform for the 2D Metal-Insulator Transition.

We report the fabrication of hexagonal-boron-nitride (hBN) encapsulated multiterminal WSe2 Hall bars with 2D/2D low-temperature Ohmic contacts as a platform for investigating the two-dimensional (2D) metal-insulator transition. We demonstrate that the WSe2 devices exhibit Ohmic behavior down to 0.25 K and at low enough excitation voltages to avoid current-heating effects. Additionally, the high-quality hBN-encapsulated WSe2 devices in ideal Hall-bar geometry enable us to accurately determine the carrier density. Measurements of the temperature (T) and density (ns) dependence of the conductivity σ(T, ns) demonstrate scaling behavior consistent with a metal-insulator quantum phase transition driven by electron-electron interactions but where disorder-induced local magnetic moments are also present. Our findings pave the way for further studies of the fundamental quantum mechanical properties of 2D transition metal dichalcogenides using the same contact engineering.

Twist Angle-Dependent Atomic Reconstruction and Moiré Patterns in Transition Metal Dichalcogenide Heterostructures.

Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding, which enables one to create so-called van der Waals heterostructures (vdWH). One promising attribute of vdWH is the ability to rotate the layers at arbitrary azimuthal angles relative to one another. Recent work has shown that control of the twist angle between layers can have a dramatic effect on TMD vdWH properties, but the twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction, that is, any rearrangement of atoms within the individual layers, has been reported experimentally to date. Here, we demonstrate that vdWH of MoSe2/WSe2 and MoS2/WS2 at twist angles ≤1° undergo significant atomic level reconstruction leading to discrete commensurate domains divided by narrow domain walls, rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior experimental work. Using conductive atomic force microscopy (CAFM), we show that TMD vdWH at small twist angles exhibit large domains of constant conductivity. The domains in samples with R-type stacking are triangular, whereas the domains in samples with H-type stacking are hexagonal. Transmission electron microscopy provides additional evidence of atomic reconstruction in MoSe2/WSe2 structures and demonstrates the transition between a rigid-lattice moiré pattern for large angles and atomic reconstruction for small angles. We use density functional theory to calculate the band structures of the commensurate reconstructed domains and find that the modulation of the relative electronic band edges is consistent with the CAFM results and photoluminescence spectra. The presence of atomic reconstruction in TMD heterostructures and the observed impact on nanometer-scale electronic properties provide fundamental insight into the behavior of this important class of heterostructures.

Synthesis of High-Quality Monolayer MoS2 by Direct Liquid Injection.

We report the synthesis of high-quality single monolayer MoS2 samples using a novel technique that utilizes direct liquid injection (DLI) for the delivery of precursors. The DLI system vaporizes a liquid consisting of a selected precursor dissolved in a solvent into small, micron-sized droplets in an expansion chamber maintained at a selected temperature and pressure, before delivery to the deposition chamber. We demonstrate the synthesis of monolayer MoS2 on SiO2/Si substrates using the DLI technique with film quality superior to exfoliated samples or those grown by traditional tube furnace chemical vapor deposition (CVD) methods. Photoluminescence measurements of DLI monolayers exhibit consistently brighter emission, narrower line width, and higher emission energy than their exfoliated and CVD counterparts. Conductive atomic force microscopy identifies a defect density of 8.3 × 1011/cm2 in DLI MoS2, lower than the measured density in CVD material and nearly an order of magnitude improvement over the exfoliated MoS2 investigated under the same conditions. The DLI method is directly applicable to many other van der Waals materials, which require the use of challenging low vapor pressure precursors, to the growth of alloys, and sequential growths of dissimilar materials leading to van der Waals heterostructures.

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

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

Chemical Identification of Interlayer Contaminants within van der Waals Heterostructures.

van der Waals heterostructures (vdWHs) leverage the characteristics of two-dimensional (2D) material building blocks to create a myriad of structures with unique and desirable properties. Several commonly employed fabrication strategies rely on polymeric stamps to assemble layers of 2D materials into vertical stacks. However, the properties of such heterostructures frequently are degraded by contaminants, typically of unknown composition, trapped between the constituent layers. Such contaminants, therefore, impede studies of the intrinsic properties of heterostructures and hinder their application. Here, we use the photothermal induced resonance (PTIR) technique to obtain infrared spectra and maps of the contaminants down to a few attomoles and with nanoscale resolution. Heterostructures comprised of WSe2, WS2, and hexagonal boron nitride layers were found to contain significant amounts of poly(dimethylsiloxane) (PDMS) and polycarbonate, corresponding to the stamp materials used in their construction. Additionally, we verify that an atomic force microscope-based "nanosqueegee" technique is an effective method for locally removing contaminants by comparing spectra within as-fabricated and cleaned regions. Having identified the source of the contaminants, we demonstrate that cleaning PDMS stamps with isopropyl alcohol or toluene prior to vdWH fabrication reduces PDMS contamination within the structures. The general applicability of the PTIR technique for identifying the sources corrupting vdWHs provides valuable guidance for devising mitigation strategies (e.g., stamp cleaning or pre-/post-treatments) and enhances capabilities for producing materials with precisely engineered properties.

Spatially Selective Enhancement of Photoluminescence in MoS2 by Exciton-Mediated Adsorption and Defect Passivation.

Monolayers of transition-metal dichalcogenides (TMDs) are promising components for flexible optoelectronic devices because of their direct band gap and atomically thin nature. The photoluminescence (PL) from these materials is often strongly suppressed by nonradiative recombination mediated by midgap defect states. Here, we demonstrate up to a 200-fold increase in PL intensity from monolayer MoS2 synthesized by chemical vapor deposition (CVD) by controlled exposure to laser light in the ambient. This spatially resolved passivation treatment is stable in air and vacuum. Regions unexposed to laser light remain dark in fluorescence despite continuous impingement of ambient gas molecules. A wavelength-dependent study confirms that PL brightening is concomitant with exciton generation in the MoS2; laser light below the optical band gap fails to produce any enhancement in the PL. We highlight the photosensitive nature of the process by successfully brightening with a low-power broadband white light source. We decouple changes in absorption from defect passivation by examining the degree of circularly polarized PL. This measurement, which is independent of exciton generation, confirms that laser brightening reduces the rate of nonradiative recombination in the MoS2. A series of gas exposure studies demonstrate a clear correlation between PL brightening and the presence of water. We propose that H2O molecules passivate sulfur vacancies in the CVD-grown MoS2 but require photogenerated excitons to overcome a large adsorption barrier. This work represents an important step in understanding the passivation of CVD-synthesized TMDs and demonstrates the interplay between adsorption and exciton generation.

Quantum Calligraphy: Writing Single-Photon Emitters in a Two-Dimensional Materials Platform.

We present a paradigm for encoding strain into two-dimensional materials (2DMs) to create and deterministically place single-photon emitters (SPEs) in arbitrary locations with nanometer-scale precision. Our material platform consists of a 2DM placed on top of a deformable polymer film. Upon application of sufficient mechanical stress using an atomic force microscope tip, the 2DM/polymer composite deforms, resulting in formation of highly localized strain fields with excellent control and repeatability. We show that SPEs are created and localized at these nanoindents and exhibit single-photon emission up to 60 K, the highest temperature reported in these materials. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities, and plasmonic structures. In addition to enabling versatile placement of SPEs, these results present a general methodology for imparting strain into 2DM with nanometer-scale precision, providing an invaluable tool for further investigations and future applications of strain engineering of 2DM and 2DM devices.

Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS2.

Transition-metal dichalcogenides (TMDs) are an exciting class of 2D materials that exhibit many promising electronic and optoelectronic properties with potential for future device applications. The properties of TMDs are expected to be strongly influenced by a variety of defects which result from growth procedures and/or fabrication. Despite the importance of understanding defect-related phenomena, there remains a need for quantitative nanometer-scale characterization of defects over large areas in order to understand the relationship between defects and observed properties, such as photoluminescence (PL) and electrical conductivity. In this work, we present conductive atomic force microscopy measurements which reveal nanometer-scale electronically active defects in chemical vapor deposition-grown WS2 monolayers with defect density varying from 2.3 × 1010 cm-2 to 4.5 × 1011 cm-2. Comparing these defect density measurements with PL measurements across large areas (>20 µm distances) reveals a strong inverse relationship between WS2 PL intensity and defect density. We propose a model in which the observed electronically active defects serve as nonradiative recombination centers and obtain good agreement between the experiments and model.

Nano-"Squeegee" for the Creation of Clean 2D Material Interfaces.

Two-dimensional (2D) materials exhibit many exciting phenomena that make them promising as materials for future electronic, optoelectronic, and mechanical devices. Because of their atomic thinness, interfaces play a dominant role in determining material behavior. In order to observe and exploit the unique properties of these materials, it is therefore vital to obtain clean and repeatable interfaces. However, the conventional mechanical stacking of atomically thin layers typically leads to trapped contaminants and spatially inhomogeneous interfaces, which obscure the true intrinsic behavior. This work presents a simple and generic approach to create clean 2D material interfaces in mechanically stacked structures. The operating principle is to use an AFM tip to controllably squeeze contaminants out from between 2D layers and their substrates, similar to a "squeegee". This approach leads to drastically improved homogeneity and consistency of 2D material interfaces, as demonstrated by AFM topography and significant reduction of photoluminescence line widths. Also, this approach enables emission from interlayer excitons, demonstrating that the technique enhances interlayer coupling in van der Waals heterostructures. The technique enables repeatable observation of intrinsic 2D material properties, which is crucial for the continued development of these promising materials.

Double Indirect Interlayer Exciton in a MoSe2/WSe2 van der Waals Heterostructure.

An emerging class of semiconductor heterostructures involves stacking discrete monolayers such as transition metal dichalcogenides (TMDs) to form van der Waals heterostructures. In these structures, it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron-hole pairs with the electron in one TMD layer and the hole in an adjacent layer. We are able to clearly resolve two distinct emission peaks separated by 24 meV from an ILE in a MoSe2/WSe2 heterostructure fabricated using state-of-the-art preparation techniques. These peaks have nearly equal intensity, indicating they are of common character, and have opposite circular polarizations when excited with circularly polarized light. Ab initio calculations successfully account for these observations: they show that both emission features originate from excitonic transitions that are indirect in momentum space and are split by spin-orbit coupling. Also, the electron is strongly hybridized between both the MoSe2 and WSe2 layers, with significant weight in both layers, contrary to the commonly assumed model. Thus, the transitions are not purely interlayer in character. This work represents a significant advance in our understanding of the static and dynamic properties of TMD heterostructures.

Optimizing Charge Injection across Transition Metal Dichalcogenide Heterojunctions: Theory and Experiment.

In search of an improved strategy to form low-resistance contacts to semiconducting transition metal dichalcogenides, we combine ab initio density functional electronic structure calculations for an NbSe2/WSe2 interface with quantum transport measurements of the corresponding heterojunction between a few-layer WSe2 semiconductor and a metallic NbSe2 layer. Our theoretical results suggest that, besides a rigid band shift associated with charge transfer, the presence of NbSe2 does not modify the electronic structure of WSe2. Since the two transition metal dichalcogenides are structurally similar and display only a small lattice mismatch, their heterojunction can efficiently transfer charge across the interface. These findings are supported by transport measurements for WSe2 field-effect transistors with NbSe2 contacts, which exhibit nearly ohmic behavior and phonon-limited mobility in the hole channel, indicating that the contacts to WSe2 are highly transparent.