Low-Temperature Synthesis of WSe2 by the Selenization Process under Ultrahigh Vacuum for BEOL Compatible Reconfigurable Neurons

Low-temperature large-area growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) is critical for their integration with silicon chips. Especially, if the growth temperatures can be lowered below the back-end-of-line (BEOL) processing temperatures, the Si transistors can interface with 2D devices (in the back end) to enable high-density heterogeneous circuits. Such configurations are particularly useful for neuromorphic computing applications where a dense network of neurons interacts to compute the output. In this work, we present low-temperature synthesis (400 °C) of 2D tungsten diselenide (WSe2) via the selenization of the W film under ultrahigh vacuum (UHV) conditions. This simple yet effective process yields large-area, homogeneous films of 2D TMDs, as confirmed by several characterization techniques, including reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and different spectroscopy methods. Memristors fabricated using the grown WSe2 film are leveraged to realize a novel compact neuron circuit that can be reconfigured to enable homeostasis.

High-Performance Contact-Doped WSe2 Transistors Using TaSe2 Electrodes

Two-dimensional (2D) transitional metal dichalcogenides (TMDs) have garnered significant attention due to their potential for next-generation electronics, which require device scaling. However, the performance of TMD-based field-effect transistors (FETs) is greatly limited by the contact resistance. This study develops an effective strategy to optimize the contact resistance of WSe2 FETs by combining contact doping and 2D metallic electrode materials. The contact regions were doped using a laser, and the metallic TaSe2 flakes were stacked on doped WSe2 as electrodes. Doping the contact areas decreases the depletion width, while introducing the TaSe2 contact results in a lower Schottky barrier. This method significantly improves the electrical performance of the WSe2 FETs. The doped WSe2/TaSe2 contact exhibits an ultralow Schottky barrier height of 65 meV and a contact resistance of 11 kΩ·μm, which is a 50-fold reduction compared to the conventional Cr/Au contact. Our method offers a way on fabricating high-performance 2D FETs.

Exciton Dynamics of TiOPc/WSe2 Heterostructure

The van der Waals (vdW) heterostructures composed of two-dimensional (2D) transition metal dichalcogenides (TMDs) and organic semiconductors demonstrate numerous compelling optoelectronic properties. However, the influence of the vdW epitaxial effect and temperature on the optoelectronic properties and interface exciton dynamics of heterostructures remains unclear. This study systematically investigates the fluorescence properties of TiOPc/WSe2 heterostructure. Comprehensive spectral characterization elucidates that the emission behavior of the TiOPc/WSe2 heterostructure arises from charge/energy transfer at the heterostructure interfaces and the structural ordering of the organic layer on the 2D monolayer WSe2 induced by vdW epitaxy. The interface exciton dynamic features probed by ultrafast transient spectroscopy reveal that the face-to-face molecular stacking configuration of TiOPc exhibits ultrafast exciton dynamics. In particular, we observe picosecond-scale absorption of organic molecular dimer cations, providing direct evidence of interface charge transfer at room temperature. Moreover, energy transfer from the TiOPc to WSe2 may exist based on the tunability in the fluorescence emission of the TiOPc/WSe2 heterostructure as the temperature changes. This study unveils the critical role of vdW epitaxy and temperature in the exciton dynamics of organic/2D TMDs hybrid systems and provides guidance for studying interlayer charge and energy transfer in organic/inorganic heterostructures.

Multi-Layer Palladium Diselenide as a Contact Material for Two-Dimensional Tungsten Diselenide Field-Effect Transistors

Tungsten diselenide (WSe2) has emerged as a promising ambipolar semiconductor material for field-effect transistors (FETs) due to its unique electronic properties, including a sizeable band gap, high carrier mobility, and remarkable on-off ratio. However, engineering the contacts to WSe2 remains an issue, and high contact barriers prevent the utilization of the full performance in electronic applications. Furthermore, it could be possible to tune the contacts to WSe2 for effective electron or hole injection and consequently pin the threshold voltage to either conduction or valence band. This would be the way to achieve complementary metal-oxide-semiconductor devices without doping of the channel material.This study investigates the behaviour of two-dimensional WSe2 field-effect transistors with multi-layer palladium diselenide (PdSe2) as a contact material. We demonstrate that PdSe2 contacts favour hole injection while preserving the ambipolar nature of the channel material. This consequently yields high-performance p-type WSe2 devices with PdSe2 van der Waals contacts. Further, we explore the tunability of the contact interface by selective laser alteration of the WSe2 under the contacts, enabling pinning of the threshold voltage to the valence band of WSe2, yielding pure p-type operation of the devices.

Combined Experimental and Computational Insight into the Role of Substrate in the Synthesis of Two-Dimensional WSe2

Synthesis of large-area transition-metal dichalcogenides (TMDs) with controlled orientation is a significant challenge to their industrial applications. Substrate plays a vital role in determining the final quality of monolayer materials grown via the chemical vapor deposition process by controlling their orientation, crystal structure, and grain boundary. This study determined the binding energy and equilibrium distance for tungsten diselenide (WSe2) monolayers on crystalline and amorphous silicon dioxide and aluminum dioxide substrates. Differently oriented WSe2 monolayers are considered to investigate the role of the substrate in the orientation, binding strength, and equilibrium distance. This study can pave the way to synthesizing high-quality two-dimensional (2D) materials for electronic and chemical applications.

Intrinsic and Strain-Dependent Properties of Suspended WSe2 Crystallites toward Next-Generation Nanoelectronics and Quantum-Enabled Sensors

Two-dimensional (2D) layered materials exhibit great potential for high-performance electronics, where knowledge of their thermal and phononic properties is critical toward understanding heat dissipation mechanisms, considered to be a major bottleneck for current generation nanoelectronic, optoelectronic, and quantum-scale devices. In this work, noncontact Raman spectroscopy was used to analyze thermal properties of suspended 2D WSe2 membranes to access the intrinsic properties. Here, the influence of electron-phonon interactions within the parent crystalline WSe2 membranes was deciphered through a comparative analysis of extrinsic substrate-supported WSe2, where heat dissipation mechanisms are intimately tied to the underlying substrate. Moreover, the excitonic states in WSe2 were analyzed by using temperature-dependent photoluminescence spectroscopy, where an enhancement in intensity of the localized excitons in suspended WSe2 was evident. Finally, phononic and electronic properties in suspended WSe2 were examined through nanoscale local strain engineering, where a uniaxial force was induced on the membrane using a Au-coated cantilever within an atomic force microscope. Through the fundamental analysis provided here with temperature and strain-dependent phononic and optoelectronic properties in suspended WSe2 nanosheets, the findings will inform the design of next-generation energy-efficient, high-performance devices based on WSe2 and other 2D materials, including for quantum applications.

Broadband Van-der-Waals Photodetector Driven by Ferroelectric Polarization

The potential for various future industrial applications has made broadband photodetectors beyond visible light an area of great interest. Although most 2D van-der-Waals (vdW) semiconductors have a relatively large energy bandgap (>1.2 eV), which limits their use in short-wave infrared detection, they have recently been considered as a replacement for ternary alloys in high-performance photodetectors due to their strong light-matter interaction. In this study, a ferroelectric gating ReS2 /WSe2 vdW heterojunction-channel photodetector is presented that successfully achieves broadband light detection (>1300 nm, expandable up to 2700 nm). The staggered type-II bandgap alignment creates an interlayer gap of 0.46 eV between the valence band maximum (VBMAX ) of WSe2 and the conduction band minimum (CBMIN ) of ReS2 . Especially, the control of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric dipole polarity for a specific wavelength allows a high photoresponsivity of up to 6.9 × 103 A W-1 and a low dark current below 0.26 nA under the laser illumination with a wavelength of 405 nm in P-up mode. The achieved high photoresponsivity, low dark current, and full-range near infrared (NIR) detection capability open the door for next-generation photodetectors beyond traditional ternary alloy photodetectors.

High-Performance WSe2 Top-Gate Devices with Strong Spacer Doping

Because of the lack of contact and spacer doping techniques for two-dimensional (2D) transistors, most high-performance 2D devices have been produced with nontypical structures that contain electrical gating in the contact regions. In the present study, we used chloroauric acid (HAuCl4) as a strong p-dopant for WSe2 monolayers used in transistors. The HAuCl4-doped devices exhibited a record-low contact resistance of 0.7 kΩ·μm under a doping concentration of 1.76 × 1013 cm-2. In addition, an extrinsic carrier diffusion phenomenon was discovered in the HAuCl4-WSe2 system. With a suitably designed spacer length for doping, a normally off, high-performance underlap top-gate device can be produced without the application of additional gating in the contact and spacer regions.

Valley Polarized Holes Induced Exciton Polaron Valley Splitting

Monolayer transition metal dichalcogenide semiconductors are promising valleytronic materials. Among various quasi-particle excitations hosted by the system, the valley polarized holes are particularly interesting due to their long valley lifetime preserved by the large spin-orbit splitting and spin-valley locking in the valence band. Here we report that in the absence of any magnetic field a surprising valley splitting of exciton polarons can be induced by such valley polarized holes in monolayer WSe2. The size of the splitting is comparable to that of the Zeeman effect in a magnetic field as high as 7 T and offers a quantitative approach to extract the hole density imbalance between the two valleys. We find that the density difference can easily achieve more than 1011 per cm2, and it is tunable by gate voltage as well as optical excitation power. Our study highlights the response of exciton polarons to optical pumping and advances understanding of valley dependent phenomena in monolayer transition metal dichalcogenide.

Se-Vacancy Healing with Substitutional Oxygen in WSe2 for High-Mobility p-Type Field-Effect Transistors

Transition-metal dichalcogenides possess high carrier mobility and can be scaled to sub-nanometer dimensions, making them viable alternative to Si electronics. WSe₂ is capable of hole and electron carrier transport, making it a key component in CMOS logic circuits. However, since the p-type electrical performance of the WSe₂-field effect transistor (FET) is still limited, various approaches are being investigated to circumvent this issue. Here, we formed a heterostructural multilayer WSe₂ channel and solution-processed aluminum-doped zinc oxide (AZO) for compositional modification of WSe₂ to obtain a device with excellent electrical properties. Supplying oxygen anions from AZO to the WSe₂ channel eliminated subgap states through Se-deficiency healing, resulting in improved transport capacity. Se vacancies are known to cause mobility degradation due to scattering, which is mitigated through ionic compensation. Consequently, the hole mobility can reach high values, with a maximum of approximately 100 cm²/V s. Further, the transport behavior of the oxygen-doped WSe₂-FET is systematically analyzed using density functional theory simulations and photoexcited charge collection spectroscopy measurements.

Self-Powered Bidirectional Photoresponse in High-Detectivity WSe2 Phototransistor with Asymmetrical van der Waals Stacking for Retinal Neurons Emulation

An artificial retina system shows a promising potential to achieve fast response, low power consumption, and high integration density for vision sensing systems. Optoelectronic sensors, which can emulate the neurobiological functionalities of retinal neurons, are crucial in the artificial retina systems. Here, we propose a WSe₂ phototransistor with asymmetrical van der Waals (vdWs) stacking that can be used as an optoelectronic sensor in artificial retina systems. Through the utilization of the gate-tunable self-powered bidirectional photoresponse of this phototransistor, the neurobiological functionalities of both bipolar cells and cone cells, as well as the hierarchical connectivity between these two types of retinal neurons, are successfully mimicked by a single device. This self-powered bidirectional photoresponse is attributed to the asymmetrical vdWs stacking structure, which enables the transition from an n-p to p⁺-p homojunction in the WSe₂ channel under different polarities of gate bias. Moreover, the detectivity and ON/OFF ratio of this phototransistor reach as high as 1.8 × 10¹³ Jones and 5.3 × 10⁴, respectively, and a rise/fall time <80 μs is achieved, as well, which reveals good photodetection performance. The proof of this device provides a pathway for the future development of neuromorphic vision devices and systems.

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.

Molybdenum Diselenide and Tungsten Diselenide Interfacing Cobalt-Porphyrin for Electrocatalytic Hydrogen Evolution in Alkaline and Acidic Media

Easy and effective modification approaches for transition metal dichalcogenides are highly desired in order to make them active toward electrocatalysis. In this manner, we report functionalized molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) via metal-ligand coordination with pyridine rings for the subsequent covalent grafting of a cobalt-porphyrin. The new hybrid materials were tested towards an electrocatalytic hydrogen evolution reaction in both acidic and alkaline media and showed enhanced activity compared to intact MoSe₂ and WSe₂. Hybrids exhibited lower overpotential, easier reaction kinetics, higher conductivity, and excellent stability after 10,000 ongoing cycles in acidic and alkaline electrolytes compared to MoSe₂ and WSe₂. Markedly, MoSe₂-based hybrid material showed the best performance and marked a significantly low onset potential of -0.17 V vs RHE for acidic hydrogen evolution reaction. All in all, the ease and fast modification route provides a versatile functionalization procedure, extendable to other transition metal dichalcogenides, and can open new pathways for the realization of functional nanomaterials suitable in electrocatalysis.

Three-Dimensional Assemblies of Edge-Enriched WSe2 Nanoflowers for Selectively Detecting Ammonia or Nitrogen Dioxide

Herein, we present, for the first time, a chemoresistive-type gas sensor composed of two-dimensional WSe₂, fabricated by a simple selenization of tungsten trioxide (WO₃) nanowires at atmospheric pressure. The morphological, structural, and chemical composition investigation shows the growth of vertically oriented three-dimensional (3D) assemblies of edge-enriched WSe₂ nanoplatelets arrayed in a nanoflower shape. The gas sensing properties of flowered nanoplatelets (2H-WSe₂) are investigated thoroughly toward specific gases (NH₃ and NO₂) at different operating temperatures. The integration of 3D WSe₂ with unique structural arrangements resulted in exceptional gas sensing characteristics with dual selectivity toward NH₃ and NO₂ gases. Selectivity can be tuned by selecting its operating temperature (150 °C for NH₃ and 100 °C for NO₂). For instance, the sensor has shown stable and reproducible responses (24.5%) toward 40 ppm NH₃ vapor detection with an experimental LoD < 2 ppm at moderate temperatures. The gas detecting capabilities for CO, H₂, C₆H₆, and NO₂ were also investigated to better comprehend the selectivity of the nanoflower sensor. Sensors showed repeatable responses with high sensitivity to NO₂ molecules at a substantially lower operating temperature (100 °C) (even at room temperature) and LoD < 0.1 ppm. However, the gas sensing properties reveal high selectivity toward NH₃ gas at moderate operating temperatures. Moreover, the sensor demonstrated high resilience against ambient humidity (Rh = 50%), demonstrating its remarkable stability toward NH₃ gas detection. Considering the detection of NO₂ in a humid ambient atmosphere, there was a modest increase in the sensor response (5.5%). Furthermore, four-month long-term stability assessments were also taken toward NH₃ gas detection, and sensors showed excellent response stability. Therefore, this study highlights the practical application of the 2H variant of WSe₂ nanoflower gas sensors for NH₃ vapor detection.

Synthesis of 1T WSe2 on an Oxygen-Containing Substrate Using a Single Precursor

The metallic property of metastable 1T' WSe₂ and its promising catalytic performance have attracted considerable interest. A hot injection method has been used to synthesize 1T' WSe₂ with a three-dimensional morphology; however, this method requires two or more precursors and long-chain ligands, which inhibit the catalytic performance. Here, we demonstrate the synthesis of 1T' WSe₂ on a substrate by a simple heating-up method using a single precursor, tetraethylammonium tetraselenotungstate [(Et₄N)₂WSe₄]. The triethylamine produced after the reaction is an electron donor that yields negatively charged WSe₂, which is stabilized by triethylammonium cations as intercalants between layers and induces 1T' WSe₂. The purity of 1T' WSe₂ is higher on oxygen-containing crystalline substrates than amorphous substrates because the strong adhesion between WSe₂ and the substrate can produce sufficient triethylammonium (TEA) intercalation. Among the oxygen-containing crystal substrates, the substrate with a lower lattice mismatch with 1T' WSe₂ showed higher 1T' purity due to the uniform TEA intercalation. Furthermore, 1T' WSe₂ on carbon cloth exhibited a more enhanced catalytic performance in the hydrogen evolution reaction (197 mV at 10 mA/cm²) than has been reported previously.

High-Density, Localized Quantum Emitters in Strained 2D Semiconductors

Two-dimensional chalcogenide semiconductors have recently emerged as a host material for quantum emitters of single photons. While several reports on defect- and strain-induced single-photon emission from 2D chalcogenides exist, a bottom-up, lithography-free approach to producing a high density of emitters remains elusive. Further, the physical properties of quantum emission in the case of strained 2D semiconductors are far from being understood. Here, we demonstrate a bottom-up, scalable, and lithography-free approach for creating large areas of localized emitters with high density (∼150 emitters/um²) in a WSe₂ monolayer. We induce strain inside the WSe₂ monolayer with high spatial density by conformally placing the WSe₂ monolayer over a uniform array of Pt nanoparticles with a size of 10 nm. Cryogenic, time-resolved, and gate-tunable luminescence measurements combined with near-field luminescence spectroscopy suggest the formation of localized states in strained regions that emit single photons with a high spatial density. Our approach of using a metal nanoparticle array to generate a high density of strained quantum emitters will be applied to scalable, tunable, and versatile quantum light sources.

Photonic bandgap engineering in (VO2)n/(WSe2)nphotonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal-insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO₂) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO₂as a strongly correlated phase transition material and tungsten diselenide (WSe₂) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO₂with WSe₂in a photonic superlattices structure through the near and mid-infrared (NIR-MIR) thermochromic phase transition regions. By increasing the thickness of the VO₂layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO₂thickness. For the monoclinic case of VO₂, the number of the forbidden bands increase with the number of layers of WSe₂. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR-MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study's findings may help understand vanadium oxide's role in tunable photonic superlattice for infrared switchable devices and optical filters.

Recessed-Channel WSe2 Field-Effect Transistor via Self-Terminated Doping and Layer-by-Layer Etching

Effective channel control with low contact resistance can be accomplished through selective ion implantation in Si and III-V semiconductor technologies; however, this approach cannot be adopted for ultrathin van der Waals materials. Herein, we demonstrate a self-aligned fabrication process based on self-terminated p-doping and layer-by-layer chemical etching to achieve low contact resistance as well as a high on/off current ratio in ultrathin tungsten diselenide (WSe₂) field-effect transistors (FETs). Damage-free layer-by-layer thinning of the WSe₂ channel is repeated up to a thickness of approximately 1.4 nm, while maintaining the selectively p-doped source/drain regions. The device characteristics of the recessed-channel WSe₂ FET are systematically monitored during this layer-by-layer recess-channel process. The WSe₂ etching rate is estimated to be 2-3 layers per cycle of oxidation and subsequent chemical etching. The self-terminated tungsten oxide (WO_X) layer grown through ultraviolet-ozone treatment induces robust p-doping in the neighboring (or underlying) WSe₂ through the electron withdrawal mechanism, which remains in the source/drain regions after channel oxide removal. The adopted self-terminated and self-aligned recess-channel process for ultrathin WSe₂ FETs enables the realization of a high on/off output current ratio (>10⁸) and field-effect mobility (∼190 cm²/V·s), while maintaining low contact resistance (0.9-6.1 kΩ·μm) without a postannealing process. The proposed facile and reproducible doping and atomic-layer-etching method for the fabrication of a recessed-channel FET with an ultrathin body can be helpful for high-performance two-dimensional semiconductor devices and is applicable to post-Si complementary metal-oxide semiconductor devices.

A Waveguide-Integrated Two-Dimensional Light-Emitting Diode Based on p-Type WSe2/n-Type CdS Nanoribbon Heterojunction

Transition metal dichalcogenides (TMDs) have emerged as two-dimensional (2D) building blocks to construct nanoscale light sources. To date, a wide array of TMD-based light-emitting devices (LEDs) have been successfully demonstrated. Yet, their atomically thin and planar nature entails an additional waveguide/microcavity for effective optical routing/confinement. In this sense, integration of TMDs with electronically active photonic nanostructures to form a functional heterojunction is of crucial importance for 2D optoelectronic chips with reduced footprint and higher integration capacity. Here, we report a room-temperature waveguide-integrated light-emitting device based on a p-type monolayer (ML) tungsten diselenide (WSe₂) and n-type cadmium sulfide (CdS) nanoribbon (NR) heterojunction diode. The hybrid LED exhibited clear rectification under forward biasing, giving pronounced electroluminescence (EL) at 1.65 eV from exciton resonances in ML WSe₂. The integrated EL intensity against the driving current shows a superlinear profile at a high current level, implying a facilitated carrier injection via intervalley scattering. By leveraging CdS NR waveguides, the WSe₂ EL can be efficiently coupled and further routed for potential optical interconnect functionalities. Our results manifest the waveguided LEDs as a dual-role module for TMD-based optoelectronic circuitries.

Molecular Dopant-Dependent Charge Transport in Surface-Charge-Transfer-Doped Tungsten Diselenide Field Effect Transistors

The controllability of carrier density and major carrier type of transition metal dichalcogenides(TMDCs) is critical for electronic and optoelectronic device applications. To utilize doping in TMDC devices, it is important to understand the role of dopants in charge transport properties of TMDCs. Here, the effects of molecular doping on the charge transport properties of tungsten diselenide (WSe₂ ) are investigated using three p-type molecular dopants, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄ -TCNQ), tris(4-bromophenyl)ammoniumyl hexachloroantimonate (magic blue), and molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Mo(tfd-COCF₃ )₃ ). The temperature-dependent transport measurements show that the dopant counterions on WSe₂ surface can induce Coulomb scattering in WSe₂ channel and the degree of scattering is significantly dependent on the dopant. Furthermore, the quantitative analysis revealed that the amount of charge transfer between WSe₂ and dopants is related to not only doping density, but also the contribution of each dopant ion toward Coulomb scattering. The first-principles density functional theory calculations show that the amount of charge transfer is mainly determined by intrinsic properties of the dopant molecules such as relative frontier orbital positions and their spin configurations. The authors' systematic investigation of the charge transport of doped TMDCs will be directly relevant for pursuing molecular routes for efficient and controllable doping in TMDC nanoelectronic devices.

Piezoelectric Modulation of Excitonic Properties in Monolayer WSe2 under Strong Dielectric Screening

We investigate the interaction of excitons in monolayer WSe₂ with the piezoelectric field of surface acoustic wave (SAW) at room temperature using photoluminescence (PL) spectroscopy and report a large in-plane exciton polarizability of 8.43 ± 0.18 × 10^-6 Dm/V. Such large polarizability arises due to the strong dielectric screening from the piezoelectric substrate. In addition, we show that the exciton-piezoelectric field interaction and population distribution between neutral excitons and trions can be optically manipulated by controlling the field screening using photogenerated free carriers. Finally, we model the broadening of the exciton PL line width and report that the interaction is dominated by type-II band edge modulation, because of the in-plane electric field in the system. The results help understand the interaction of excitons in monolayer transition-metal dichalcogenides that will aid in controlled manipulation of excitonic properties for applications in sensing, detection, and on-chip communication.

Tightly-bound trion and bandgap engineering viaγ-ray irradiation in the monolayer transition metal dichalcogenide WSe2

Afterγ-ray irradiation treatment, a monolayer tungsten diselenide could be transitioned into an n-doped semiconductor due to the anion vacancies created by the radiation. Transmission electron microscope studies showed clear chemical modulation with atomically sharp interface. Change in the lattice vibrational modes induced by passivation of oxygen is captured by Raman spectroscopy. The frequency shifts in both in-plane and out-of-plane modes are dependent linearly on the oxidation content. We observe a negative trion, which is a neutral exciton bound with an electron, in the photoluminescence spectra. The binding energy of this trion is estimated to be ∼90 meV, making it a tightly bound exciton. The first-principles calculation suggests that an increase in the anion vacancy population is generally accompanied by a transition from a direct gap material to an indirect one. This opens up a new venue to engineer the electronic properties of transition metal dichalcogenides by using irradiation.

Material and Device Structure Designs for 2D Memory Devices Based on the Floating Gate Voltage Trajectory

Two-dimensional heterostructures have been extensively investigated as next-generation nonvolatile memory (NVM) devices. In the past decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate (FG) voltage measurements. Systematic comparisons of MoTe₂, WSe₂, and MoS₂ channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and p-n junction in the channel. Furthermore, the control experiment indicated that the access region in the device structure is required to achieve 2D channel/FG tunneling by preventing electrode/FG tunneling. The present understanding suggests that the ambipolar 2D-based FG-type NVM device with the access region is suitable for further realizing potentially high electrical reliability.

Band-Gap Landscape Engineering in Large-Scale 2D Semiconductor van der Waals Heterostructures

We present a growth process relying on pulsed laser deposition for the elaboration of complex van der Waals heterostructures on large scales, at a 400 °C CMOS-compatible temperature. Illustratively, we define a multilayer quantum well geometry through successive in situ growths, leading to WSe₂ being encapsulated into WS₂ layers. The structural constitution of the quantum well geometry is confirmed by Raman spectroscopy combined with transmission electron microscopy. The large-scale high homogeneity of the resulting 2D van der Waals heterostructure is also validated by macro- and microscale Raman mappings. We illustrate the benefit of this integrative in situ approach by showing the structural preservation of even the most fragile 2D layers once encapsulated in a van der Waals heterostructure. Finally, we fabricate a vertical tunneling device based on these large-scale layers and discuss the clear signature of electronic transport controlled by the quantum well configuration with ab initio calculations in support. The flexibility of this direct growth approach, with multilayer stacks being built in a single run, allows for the definition of complex 2D heterostructures barely accessible with usual exfoliation or transfer techniques of 2D materials. Reminiscent of the III-V semiconductors' successful exploitation, our approach unlocks virtually infinite combinations of large 2D material families in any complex van der Waals heterostructure design.

Role of Ferromagnetic Monolayer WSe2 Flakes in the Pt/Y3Fe5O12 Bilayer Structure in the Longitudinal Spin Seebeck Effect

The spin Seebeck effect (SSE) has attracted renewed interest as a promising phenomenon for energy harvesting systems. A noteworthy effort has been devoted to improving the SSE voltage by inserting ultrathin magnetic layers including Fe₇₀Cu₃₀ interlayers in Pt/Y₃Fe₅O₁₂ (Pt/YIG) systems with increased spin-mixing conductance at the interfaces. Nevertheless, the responsible underlying physics associated with the role of the interlayer in Pt/YIG systems in the SSE is still unknown. In this paper, we demonstrate that with a monolayer tungsten diselenide (ML WSe₂) interlayer in the Pt/YIG bilayer system, the longitudinal SSE (LSSE) voltage is significantly increased by the increased spin accumulation in the Pt layer; the spin fluctuation in ML WSe₂ amplifies the spin current transmission because the in-plane-aligned WSe₂ spins are coupled to thermally pumped spins under nonequilibrium magnetization conditions in the LSSE configuration at room temperature. The thermopower (V_LSSE/ΔT) improves by 323% with respect to the value of the reference Pt/YIG bilayer sample in the LSSE at room temperature. In addition, the induced ferromagnetic properties of the ML WSe₂ flakes on YIG increase the LSSE voltage (V_LSSE) of the sample; the ferromagnetic properties are a result of the improved magnetic moment density in the ML WSe₂ flakes and their two-dimensional (2D) ML nature in the LSSE under nonequilibrium magnetization conditions. The results can extend the application range of the materials in energy harvesting and provide important information on the physics of the LSSE with a transition metal dichalcogenide intermediate layer in spin transport.

Experimental and Computational Investigation of Layer-Dependent Thermal Conductivities and Interfacial Thermal Conductance of One- to Three-Layer WSe2

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs) have received extensive research interest and investigations in the past decade. In this research, we used a refined opto-thermal Raman technique to explore the thermal transport properties of one popular TMDC material WSe₂, in the single-layer (1L), bilayer (2L), and trilayer (3L) forms. This measurement technique is direct without additional processing to the material, and the absorption coefficient of WSe₂ is discovered during the measurement process to further increase this technique's precision. By comparing the sample's Raman spectroscopy spectra through two different laser spot sizes, we are able to obtain two parameters-lateral thermal conductivities of 1L-3L WSe₂ and the interfacial thermal conductance between 1L-3L WSe₂ and the substrate. We also implemented full-atom nonequilibrium molecular dynamics simulations (NEMD) to computationally investigate the thermal conductivities of 1L-3L WSe₂ to provide comprehensive evidence and confirm the experimental results. The trend of the layer-dependent lateral thermal conductivities and interfacial thermal conductance of 1L-3L WSe₂ is discovered. The room-temperature thermal conductivities for 1L-3L WSe₂ are 37 ± 12, 24 ± 12, and 20 ± 6 W/(m·K), respectively. The suspended 1L WSe₂ possesses a thermal conductivity of 49 ± 14 W/(m·K). Crucially, the interfacial thermal conductance values between 1L-3L WSe₂ and the substrate are found to be 2.95 ± 0.46, 3.45 ± 0.50, and 3.46 ± 0.45 MW/(m²·K), respectively, with a flattened trend starting the 2L, a finding that provides the key information for thermal management and thermoelectric designs.

An Ultrafast WSe2 Photodiode Based on a Lateral p-i-n Homojunction

High-quality homogeneous junctions are of great significance for developing transition metal dichalcogenides (TMDs) based electronic and optoelectronic devices. Here, we demonstrate a lateral p-type/intrinsic/n-type (p-i-n) homojunction based multilayer WSe₂ diode. The photodiode is formed through selective doping, more specifically by utilizing self-aligning surface plasma treatment at the contact regions, while keeping the WSe₂ channel intrinsic. Electrical measurements of such a diode reveal an ideal rectifying behavior with a current on/off ratio as high as 1.2 × 10⁶ and an ideality factor of 1.14. While operating in the photovoltaic mode, the diode presents an excellent photodetecting performance under 450 nm light illumination, including an open-circuit voltage of 340 mV, a responsivity of 0.1 A W^-1, and a specific detectivity of 2.2 × 10¹³ Jones. Furthermore, benefiting from the lateral p-i-n configuration, the slow photoresponse dynamics including the photocarrier diffusion in undepleted regions and photocarrier trapping/detrapping due to dopants or doping process induced defect states are significantly suppressed. Consequently, a record-breaking response time of 264 ns and a 3 dB bandwidth of 1.9 MHz are realized, compared with the previously reported TMDs based photodetectors. The above-mentioned desirable properties, together with CMOS compatible processes, make this WSe₂ p-i-n junction diode promising for future applications in self-powered high-frequency weak signal photodetection.

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 (n_s) dependence of the conductivity σ(T, n_s) 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.

Scalable Substitutional Re-Doping and its Impact on the Optical and Electronic Properties of Tungsten Diselenide

Reliable, controlled doping of 2D transition metal dichalcogenides will enable the realization of next-generation electronic, logic-memory, and magnetic devices based on these materials. However, to date, accurate control over dopant concentration and scalability of the process remains a challenge. Here, a systematic study of scalable in situ doping of fully coalesced 2D WSe₂ films with Re atoms via metal-organic chemical vapor deposition is reported. Dopant concentrations are uniformly distributed over the substrate surface, with precisely controlled concentrations down to <0.001% Re achieved by tuning the precursor partial pressure. Moreover, the impact of doping on morphological, chemical, optical, and electronic properties of WSe₂ is elucidated with detailed experimental and theoretical examinations, confirming that the substitutional doping of Re at the W site leads to n-type behavior of WSe₂ . Transport characteristics of fabricated back-gated field-effect-transistors are directly correlated to the dopant concentration, with degrading device performances for doping concentrations exceeding 1% of Re. The study demonstrates a viable approach to introducing true dopant-level impurities with high precision, which can be scaled up to batch production for applications beyond digital electronics.

Dynamic Exciton Funneling by Local Strain Control in a Monolayer Semiconductor

The ability to control excitons in semiconductors underlies numerous proposed applications, from excitonic circuits to energy transport. Two dimensional (2D) semiconductors are particularly promising for room-temperature applications due to their large exciton binding energy and enormous stretchability. Although the strain-induced static exciton flux has been observed in predetermined structures, dynamic control of exciton flux represents an outstanding challenge. Here, we introduce a method to tune the bandgap of suspended 2D semiconductors by applying a local strain gradient with a nanoscale tip. This strain allows us to locally and reversibly shift the exciton energy and to steer the exciton flux over micrometer-scale distances. We anticipate that our result not only marks an important experimental tool but will also open a broad range of new applications from information processing to energy conversion.

Up- and Down-Conversion between Intra- and Intervalley Excitons in Waveguide Coupled Monolayer WSe2

The presence of two spin-split valleys in monolayer (1L) transition metal dichalcogenide (TMD) semiconductors supports versatile exciton species classified by their spin and valley quantum numbers. While the spin-0 intravalley exciton, known as the "bright" exciton, is readily observable, other types of excitons, such as the spin-1 intravalley (spin-dark) and spin-0 intervalley (momentum-dark) excitons, are more difficult to access. Here we develop a waveguide coupled 1L tungsten diselenide (WSe₂) device to probe these exciton species. In particular, TM coupling to the atomic layer's out-of-plane dipole moments enabled us to not only efficiently collect but also resonantly populate the spin-1 dark excitons, promising for developing devices with long valley lifetimes. Our work reveals several upconversion processes that bring out an intricate coupling network linking spin-0 and spin-1 intra- and intervalley excitons, demonstrating that intervalley scattering and spin-flip are very common processes in the atomic layer. These experimental results deepen our understanding of tungsten diselenide exciton physics and illustrate that planar photonic devices are capable of harnessing versatile exciton species in TMD semiconductors.

Measurement of Quantum Yields of Monolayer TMDs Using Dye-Dispersed PMMA Thin Films

In general, the quantum yields (QYs) of monolayer transition metal dichalcogenides (1L-TMDs) are low, typically less than 1% in their pristine state, significantly limiting their photonic applications. Many methods have been reported to increase the QYs of 1L-TMDs; however, the technical difficulties involved in the reliable estimation of these QYs have prevented the general assessment of these methods. Herein, we demonstrate the estimation of the QYs of 1L-TMDs using a poly methyl methacrylate (PMMA) thin film embedded with rhodamine 6G (R6G) as a reference specimen for measuring the QYs of 1L-TMDs. The PMMA/R6G composite films with thicknesses of 80 and 300 nm demonstrated spatially homogeneous emissions with the incorporation of well-dispersed R6G molecules, and may, therefore, be used as ideal reference specimens for the QY measurement of 1L-TMDs. Using our reference specimens, for which the QY ranged from 5.4% to 22.2% depending on the film thickness and R6G concentrations, we measured the QYs of the exfoliated or chemical vapor deposition (CVD)-grown 1L-WS₂, -MoSe₂, -MoS₂, and -WSe₂ TMDs. The convenient procedure proposed in this study for preparing the thin reference films and the simple protocol for the QY estimation of 1L-TMDs may enable accurate comparisons of the absolute QYs between the 1L-TMD samples, thereby enabling the development of a method to improve the QY of 1L-TMDs.

Two-Dimensional to Three-Dimensional Growth of Transition Metal Diselenides by Chemical Vapor Deposition: Interplay between Fractal, Dendritic, and Compact Morphologies

We investigate the role of growth temperature and metal/chalcogen flux in atmospheric pressure chemical vapor deposition growth of MoSe₂ and WSe₂ on Si/SiO₂ substrates. Using scanning electron microscopy and atomic force microscopy, we observe that the growth temperature and transition metal flux strongly influence the domain morphology, and the compact triangular or hexagonal domains ramify into branched structures as the growth temperature (metal flux) is decreased (increased). The competition between adatom attachment to the domain edges and diffusion of adatoms along the domain boundary determines the evolution of the observed growth morphology. Depending on the growth temperature and flux, two different branched structures-fractals and dendrites-grow. The fractals (with a dimension of ∼1.67) obey a diffusion-limited aggregation mechanism, whereas the dendrites with a higher fractal dimension of ∼1.80 exhibit preferential growth along the symmetry-governed directions. The effect of chalcogen environment is studied, where a Se-rich condition helps restrict Mo-rich nucleus formation, promoting lateral growth. For a Se-deficient environment, several multilayer islands cluster on two-dimensional domains, suggesting a transition from lateral to vertical growth because of insufficient Se passivation. X-ray photoelectron spectroscopy analysis shows a near perfect stoichiometry (Mo/Se = 1:1.98) of MoSe₂ grown in a Se-rich environment, whereas in the Se-deficient condition, a ratio of Mo/Se = 1:1.68 is observed. This also supports the formation of metal-rich nuclei (Mo_1+xSe_2-x) under Se-deficient conditions, leading to three-dimensional clustering. Tuning the growth temperature and metal/chalcogen flux, we propose an optimized CVD growth window for synthesizing large-area Mo(W) selenide.

Momentum-Dark Intervalley Exciton in Monolayer Tungsten Diselenide Brightened via Chiral Phonon

Inversion symmetry breaking and 3-fold rotation symmetry grant the valley degree of freedom to the robust exciton in monolayer transition-metal dichalcogenides, which can be exploited for valleytronics applications. However, the short lifetime of the exciton significantly constrains the possible applications. In contrast, the dark exciton could be long-lived but does not necessarily possess the valley degree of freedom. In this work, we report the identification of the momentum-dark, intervalley exciton in monolayer WSe₂ through low-temperature magneto-photoluminescence spectra. Interestingly, the intervalley exciton is brightened through the emission of a chiral phonon at the corners of the Brillouin zone (K point), and the pseudoangular momentum of the phonon is transferred to the emitted photon to preserve the valley information. The chiral phonon energy is determined to be ∼23 meV, based on the experimentally extracted exchange interaction (∼7 meV), in excellent agreement with the theoretical expectation of 24.6 meV. The long-lived intervalley exciton with valley degree of freedom adds an exciting quasiparticle for valleytronics, and the coupling between the chiral phonon and intervalley exciton furnishes a venue for valley spin manipulation.

High-Performance WSe2 Photodetector Based on a Laser-Induced p-n Junction

Two-dimensional heterojunctions exhibit many unique features in nanoelectronic and optoelectronic devices. However, heterojunction engineering requires a complicated alignment process and some defects are inevitably introduced during material preparation. In this work, a laser scanning technique is used to construct a lateral WSe₂ p-n junction. The laser-scanned region shows p-type behavior, and the adjacent region is electrically n-doped with a proper gate voltage. The laser-oxidized product WO_x is found to be responsible for this p-type doping. After laser scanning, WSe₂ displays a change from ambipolar to unipolar p-type property. A significant photocurrent emerges at the p-n junction. Therefore, a self-powered WSe₂ photodetector can be fabricated based on this junction, which presents a large photoswitching ratio of 10⁶, a high photoresponsivity of 800 mA W^-1, and a short photoresponse time with long-term stability and reproducibility. Therefore, this selective laser-doping method is prospective in future electronic applications.

Chemically Tuned p- and n-Type WSe2 Monolayers with High Carrier Mobility for Advanced Electronics

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post-silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom-thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p- and n-type semiconductors is essential for various device applications, such as complementary metal-oxide-semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe₂ is demonstrated by chemical doping. Two different molecules, 4-nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe₂ field-effect transistors (FETs) to p- and n-type, respectively. Moreover, the chemically doped WSe₂ show increased effective carrier mobilities of 82 and 25 cm² V^-1 s^-1 for holes and electrons, respectively, which are much higher than those of the pristine WSe₂ . The doping effects are studied by photoluminescence, Raman, X-ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe₂ FETs are integrated into CMOS inverters, exhibiting extremely low power consumption (≈0.17 nW). Furthermore, a p-n junction within single WSe₂ grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe₂ will contribute to the development of TMDC-based advanced electronics.

Versatile Electronic Devices Based on WSe2/SnSe2 Vertical van der Waals Heterostructures

Van der Waals heterostructures formed by stacking of various two-dimensional materials are promising in electronic applications. However, the performances of most reported electronic devices based on van der Waals heterostructures are far away from those of existing (Si, Ge, and III-V bulk material based) technologies. Here, we report high-performance heterostructure devices based on vertically stacked tungsten diselenide and tin diselenide. Due to the unique band alignment and the atomic thickness of the material, both charge carrier transport and energy barrier can be effectively modulated by the applied electrical field. As a result, the heterostructure devices show superb characteristics, with a high current on/off ratio of ∼3 × 10⁸, an average subthreshold slope of 126 mV/dec over 5 dec of current change due to band-to-band tunneling, an ultrahigh rectification ratio of ∼3 × 10⁸, and a current density of more than 10⁴ A/cm². Furthermore, a small signal half-wave rectifier circuit based on a majority-carrier-transport-dominated diode is successfully demonstrated, showing great potential in future high-speed electronic applications.

Band Structure Engineering of Layered WSe2 via One-Step Chemical Functionalization

Chemical functionalization is demonstrated to enhance the p-type electrical performance of two-dimensional (2D) layered tungsten diselenide (WSe₂) field-effect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH₄)₂S(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe₂ surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the WSe₂ valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The mechanism of electronic transformation of WSe₂ by (NH₄)₂S(aq) chemical treatment is elucidated using density functional theory calculations which reveal that molecular "SH" adsorption on the WSe₂ surface introduces additional in-gap states near the VBE, thereby, inducing a Fermi level shift toward the VBE along with a reduction in the electronic band gap. As a result of the (NH₄)₂S(aq) chemical treatment, the p-branch ON-currents (I_ON) of back-gated few-layer ambipolar WSe₂ FETs are enhanced by about 2 orders of magnitude, and a ∼6× increase in the hole field-effect mobility is observed, the latter primarily resulting from the p-doping-induced narrowing of the Schottky barrier width leading to an enhanced hole injection at the WSe₂/contact metal interface. This (NH₄)₂S(aq) chemical functionalization technique can serve as a model method to control the electronic band structure and enhance the performance of devices based on 2D layered transition-metal dichalcogenides.

Tunable Negative Differential Resistance in van der Waals Heterostructures at Room Temperature by Tailoring the Interface

Vertically stacked two-dimensional van der Waals (vdW) heterostructures, used to obtain homogeneity and band steepness at interfaces, exhibit promising performance for band-to-band tunneling (BTBT) devices. Esaki tunnel diodes based on vdW heterostructures, however, yield poor current density and peak-to-valley ratio, inferior to those of three-dimensional materials. Here, we report the negative differential resistance (NDR) behavior in a WSe₂/SnSe₂ heterostructure system at room temperature and demonstrate that heterointerface control is one of the keys to achieving high device performance by constructing WSe₂/SnSe₂ heterostructures in inert gas environments. While devices fabricated in ambient conditions show poor device performance due to the observed oxidation layer at the interface, devices fabricated in inert gas exhibit extremely high peak current density up to 1460 mA/mm², 3-4 orders of magnitude higher than reported vdW heterostructure-based tunnel diodes, with a peak-to-valley ratio of more than 4 at room temperature. Besides, Pd/WSe₂ contact in our device possesses a much higher Schottky barrier than previously reported Cr/WSe₂ contact in the WSe₂/SnSe₂ device, which suppresses the thermionic emission current to less than the BTBT current level, enabling the observation of NDR at room temperature. Diode behavior can be further modulated by controlling the electrostatic doping and the tunneling barrier as well.

Encapsulation of a Monolayer WSe2 Phototransistor with Hydrothermally Grown ZnO Nanorods

Transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials for realizing next-generation electronics and optoelectronics with attractive physical properties. However, monolayer TMDCs (^1LTMDCs) have various serious issues, such as instability under ambient conditions and low optical quantum yield from their extremely thin thickness of ∼0.7 nm. To overcome these issues, we constructed a hybrid structure (HS) by growing zinc oxide nanorods (ZnO NRs) on a monolayer tungsten diselenide (^1LWSe₂) using the hydrothermal method. Consequently, we confirmed not only enhanced photoluminescence of ^1LWSe₂ but also improved optoelectronic properties by fabricating the HS phototransistor. Through various investigations, we found that these phenomena were due to the antenna and p-type doping effects attributed to the ZnO NRs. In addition, we verified that the optoelectronic properties of ^1LTMDCs are maintained for 2 weeks in ambient condition through the sustainable encapsulation effect induced by our HS. This encapsulation method with inorganic materials is expected to be applied to improve the stability and performance of various emerging 2D material-based devices.

Metal-Guided Selective Growth of 2D Materials: Demonstration of a Bottom-Up CMOS Inverter

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large-area electronics and circuits strongly relies on wafer-scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal-guided selective growth (MGSG), is reported. The success of control over the transition-metal-precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p- and n-type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom-up complementary metal-oxide-semiconductor inverter based on p-type WSe₂ and n-type MoSe₂ is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.

Luminescent Emission of Excited Rydberg Excitons from Monolayer WSe2

We report the experimental observation of radiative recombination from Rydberg excitons in a two-dimensional semiconductor, monolayer WSe₂, encapsulated in hexagonal boron nitride. Excitonic emission up to the 4 s excited state is directly observed in photoluminescence spectroscopy in an out-of-plane magnetic field up to 31 T. We confirm the progressively larger exciton size for higher energy excited states through diamagnetic shift measurements. This also enables us to estimate the 1 s exciton binding energy to be about 170 meV, which is significantly smaller than most previous reports. The Zeeman shift of the 1 s to 3 s states, from both luminescence and absorption measurements, exhibits a monotonic increase of the g-factor, reflecting nontrivial magnetic-dipole-moment differences between ground and excited exciton states. This systematic evolution of magnetic dipole moments is theoretically explained from the spreading of the Rydberg states in momentum space.

Room-Temperature Mesoscopic Fluctuations and Coulomb Drag in Multilayer WSe2

Mesoscopic fluctuations, manifesting the quantum interference (QI) of electrons, have been theoretically proposed in bilayer Coulomb drag systems. Unfortunately, these phenomena are usually observed at cryogenic temperatures, which severely limits their novel physics for pragmatic applications. In this paper, observation of room-temperature QI and Coulomb drag in a multilayer WSe₂ transistor is reported via graphene contacts separately at its top and bottom layers. The central layers of WSe₂ act as an insulating region with a width of few nanometers, which spatially separates the top and bottom conducting channels and provides a strong Coulomb interaction between them, leading to large conductance oscillations at room temperature. The gradual suppression of the oscillations with the increase in the applied magnetic field and/or injected current further confirms the QI phenomenon. With the decrease in temperature, the Coulomb drag effect is exhibited in the system owing to the increased thickness of the insulating region. This study reveals a novel approach for realization of advanced quantum electronics operating at high temperatures.

Defect-Controlled Nucleation and Orientation of WSe2 on hBN: A Route to Single-Crystal Epitaxial Monolayers

A defect-controlled approach for the nucleation and epitaxial growth of WSe₂ on hBN is demonstrated. The WSe₂ domains exhibit a preferred orientation of over 95%, leading to a reduced density of inversion domain boundaries (IDBs) upon coalescence. First-principles calculations and experimental studies as a function of growth conditions and substrate pretreatment confirm that WSe₂ nucleation density and orientation are controlled by the hBN surface defect density rather than thermodynamic factors. Detailed transmission electron microscopy analysis provides support for the role of single-atom vacancies on the hBN surface that trap W atoms and break surface symmetry leading to a reduced formation energy for one orientation of WSe₂ domains. Through careful control of nucleation and extended lateral growth time, fully coalesced WSe₂ monolayer films on hBN were achieved. Low-temperature photoluminescence (PL) measurements and transport measurements of back-gated field-effect transistor devices fabricated on WSe₂/hBN films show improved optical and electrical properties compared to films grown on sapphire under similar conditions. Our results reveal an important nucleation mechanism for the epitaxial growth of van der Waals heterostructures and demonstrate hBN as a superior substrate for single-crystal transition-metal dichalcogenide (TMD) films, resulting in a reduced density of IDBs and improved properties. The results motivate further efforts focused on the development of single crystal hBN substrates and epilayers for synthesis of wafer-scale single crystal TMD films.

Isotope Effect in Bilayer WSe2

Isotopes of an element have the same electron number but differ in neutron number and atomic mass. However, due to the thickness-dependent properties in MX₂ (M = Mo, W; X = S, Se, Te) transition metal dichalcogenides (TMDs), the isotopic effect in atomically thin TMDs still remains unclear especially for phonon-assisted indirect excitonic transitions. Here, we report the first observation of the isotope effect on the electronic and vibrational properties of a TMD material, using naturally abundant ^NAW^NASe₂ and isotopically pure ¹⁸⁶W⁸⁰Se₂ bilayer single crystals over a temperature range of 4.4-300 K. We demonstrate a higher optical band gap energy in ¹⁸⁶W⁸⁰Se₂ than in ^NAW^NASe₂ (3.9 ± 0.7 meV from 4.41 to 300 K), which is surprising as isotopes are neutral impurities. Phonon energies decrease in the isotopically pure crystal due to the atomic mass dependence of harmonic oscillations, with correspondingly longer E_2g and A²_1g phonon lifetimes than in the naturally abundant sample. The change in electronic band gap renormalization energy is postulated as being the dominant mechanism responsible for the change in optical emission spectra.

Gate-Tunable Thermal Metal-Insulator Transition in VO2 Monolithically Integrated into a WSe2 Field-Effect Transistor

Vanadium dioxide (VO₂) shows promise as a building block of switching and sensing devices because it undergoes an abrupt metal-insulator transition (MIT) near room temperature, where the electrical resistivity changes by orders of magnitude. A challenge for versatile applications of VO₂ is to control the MIT by gating in the field-effect device geometry. Here, we demonstrate a gate-tunable abrupt switching device based on a VO₂ microwire that is monolithically integrated with a two-dimensional (2D) tungsten diselenide (WSe₂) semiconductor by van der Waals stacking. We fabricated the WSe₂ transistor using the VO₂ wire as the drain contact, titanium as the source contact, and hexagonal boron nitride as the gate dielectric. The WSe₂ transistor was observed to show ambipolar transport, with higher conductivity in the electron branch. The electron current increases continuously with gate voltage below the critical temperature of the MIT of VO₂. Near the critical temperature, the current shows an abrupt and discontinuous jump at a given gate voltage, indicating that the MIT in the contacting VO₂ is thermally induced by gate-mediated self-heating. Our results have paved the way for the development of VO₂-based gate-tunable devices by the van der Waals stacking of 2D semiconductors, with great potential for electronic and photonic applications.

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.

Environmental Effects on the Electrical Characteristics of Back-Gated WSe₂ Field-Effect Transistors

We study the effect of polymer coating, pressure, temperature, and light on the electrical characteristics of monolayer WSe 2 back-gated transistors with Ni / Au contacts. Our investigation shows that the removal of a layer of poly(methyl methacrylate) (PMMA) or a decrease of the pressure change the device conductivity from p- to n-type. From the temperature behavior of the transistor transfer characteristics, a gate-tunable Schottky barrier at the contacts is demonstrated and a barrier height of ~ 70 meV in the flat-band condition is measured. We also report and discuss a temperature-driven change in the mobility and the subthreshold swing that is used to estimate the trap density at the WSe 2 / SiO 2 interface. Finally, from studying the spectral photoresponse of the WSe 2 , it is proven that the device can be used as a photodetector with a responsivity of ~ 0.5   AW - 1 at 700   nm and 0.37   mW / cm 2 optical power.

Recovery Improvement for Large-Area Tungsten Diselenide Gas Sensors

Semiconducting two-dimensional transition-metal dichalcogenides are considered promising gas-sensing materials because of their large surface-to-volume ratio, excellent electrical conductivity, and susceptible surfaces. However, enhancement of the recovery performance has not yet been intensively explored. In this study, a large-area uniform WSe₂ is synthesized for use in a high-performance semiconductor gas sensor. At room temperature, the WSe₂ gas sensor shows a significantly high response (4140%) to NO₂ compared to the use of NH₃, CO₂, and acetone. This paper demonstrates improved recovery of the WSe₂ gas sensor's NO₂-sensing performance by utilizing external thermal energy. In addition, a novel strategy for improving the recovery of the WSe₂ gas sensor is realized by reacting NH₃ and adsorbed NO₂ on the surface of WSe₂: the NO₂ molecules are spontaneously desorbed, and the recovery time is dramatically decreased (85 min → 43 s). It is expected that the fast recovery of the WSe₂ gas sensor achieved here will be used to develop an environmental monitoring system platform.

Giant Mechano-Optoelectronic Effect in an Atomically Thin Semiconductor

Transition metal dichalcogenides (TMDs) are particularly sensitive to mechanical strain because they are capable of experiencing high atomic displacements without nucleating defects to release excess energy. Being promising for photonic applications, it has been shown that as certain phases of layered TMDs MX₂ (M = Mo or W; X = S, Se, or Te) are scaled to a thickness of one monolayer, the photoluminescence response is dramatically enhanced due to the emergence of a direct electronic band gap compared with their multilayer or bulk counterparts, which typically exhibit indirect band gaps. Recently, mechanical strain has also been predicted to enable direct excitonic recombination in these materials, in which large changes in the photoluminescence response will occur during an indirect-to-direct band gap transition brought on by elastic tensile strain. Here, we demonstrate an enhancement of 2 orders of magnitude in the photoluminescence emission intensity in uniaxially strained single crystalline WSe₂ bilayers. Through a theoretical model that includes experimentally relevant system conditions, we determine this amplification to arise from a significant increase in direct excitonic recombination. Adding confidence to the high levels of elastic strain achieved in this report, we observe strain-independent, mode-dependent Grüneisen parameters over the entire range of tensile strain (1-3.59%), which were obtained as 1.149 ± 0.027, 0.307 ± 0.061, and 0.357 ± 0.103 for the E_2g, A_1g, and A²_1g optical phonon modes, respectively. These results can inform the predictive strain-engineered design of other atomically thin indirect semiconductors, in which a decrease in out-of-plane bonding strength may lead to an increase in the strength of strain-coupled optoelectronic effects.