Monolayer Tungsten Diselenide Research Articles

Sustained robust exciton emission in suspended monolayer WSe2 within the low carrier density regime for quantum emitter applications

The development of semiconductor optoelectronic devices is moving toward low power consumption and miniaturization, especially for high-efficiency quantum emitters. However, most of these quantum sources work at low carrier density regions, where the Shockley–Read–Hall (SRH) recombination may be dominant and seriously reduce the emission efficiency. In order to reduce the effect of carrier trapping and sustain a strong photoluminescence (PL) emission under low power pumping conditions, we investigated the influence of “suspending” a monolayer of tungsten diselenide (WSe2), a novel two-dimensional quantum material. Not only the PL intensity but also the fundamental photoluminescence quantum yield (PLQY) has exhibited a huge, order-scale enhancement through suspending; even surprisingly, we found the PLQY improvement to be far significant under small pumping powers and observed an exponential increase in tendency toward an even lower carrier density region. With its strong excitonic effect, suspended WSe2 offers a solution to reduce carrier trapping and participate in non-radiative processes. Moreover, in the low-power range, where SRH recombination dominates, suspended WSe2 exhibited a remarkably higher percentage of excitonic radiation compared to contacted WSe2. Herein, we quantitatively demonstrate the significance of the suspended WSe2 monolayer in a low carrier density region, highlighting its potential for developing compact, low-power quantum emitters in the future.

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.

Brightening and Directionality Control of Dark Excitons through Quasi-Bound States in the Continuum

Thanks to their long lifetime, spin-forbidden dark excitons in transition metal dichalcogenides are promising candidates for storage applications in opto-electronics and valleytronics. To date, their study has been hindered by inefficient generation mechanisms and the necessity for elaborate detection schemes. In this work, we propose a new hybrid platform that simultaneously addresses both challenges. We study an all-dielectric metasurface with two symmetrically protected quasi-bound states in the continuum to enhance both the excitation and emission of dark excitons in a tungsten diselenide monolayer under normal light incidence. Our simulations show a giant photoluminescence signal enhancement (∼520) along with directional emission, thus offering distinct advantages for opto-electronic and valleytronic devices.

Curvature-enhanced localised emission from dark states in wrinkled monolayer WSe2 at room temperature

ABSTRACT Localised emission from defect states in monolayer transition metal dichalcogenides is of great interest for optoelectronic and quantum device applications. Recent progress towards high temperature localised emission relies on the application of strain to induce highly confined excitonic states. Here we propose an alternative paradigm based on curvature, rather than in-plane stretching, achieved through free-standing wrinkles of monolayer tungsten diselenide (WSe2). We probe these nanostructures using tip-enhanced optical spectroscopy to reveal the spatial localisation of out-of-plane polarised emission from the WSe2 wrinkles. Based on the photoluminescence and Raman scattering signatures resolved with nanoscale spatial resolution, we propose the existence of a manifold of spin-forbidden excitonic states that are activated by the local curvature of the WSe2. We are able to access these dark states through the out-of-plane polarised surface plasmon polariton resulting in enhanced strongly localised emission at room temperature, which is of potential interest for quantum technologies and photonic devices.

Magnetic exchange field signature in ultrafast electron dynamics in monolayer WSe2 by ultrashort optical pulse

We theoretically investigate the ultrafast electron dynamics in monolayer tungsten diselenide interacting with an irradiated ultrashort strong optical pulse in the presence of a magnetic exchange field. The dynamics of Dirac fermions in monolayer WSe2 is affected by spin and valley splitting, of course, enhanced by applied magnetic exchange field. There exists remarkable interaction between the exchange field and valley index because of huge spin–orbit coupling at the valence band. Regarding the optical pulse duration is smaller than the electron scattering time, the electron dynamics remains coherent and is described by the time-dependent Schrodinger equation. The strong electric field of the pulse, interacted with monolayer WSe2 causes appearance of a dipole moment connecting two valence and conduction bands. Because of the K and K′ valleys behave differently by the magnetic exchange field, the perfect valley-spin polarization effect is achieved. We explore the irreversible conduction band electron transition affected by a magnetic exchange field. The signature of the exchange field on such transitions is quite obvious. The conduction band electron redistribution around the Dirac points is represented, when the pulse ends. Asymmetric hot spots pattern in two different K and K′ valleys depends more or less on the magnitude of the magnetic exchange field. By reversing the direction of the exchange field, the process of spin polarization is completely reversed and it results in the electron distribution pattern. The high spin-valley resolved splitting in monolayer WSe2 provides a unique platform to investigate the magnetic exchange field driven phenomena, leading to valley polarization effect.

Utilizing 2D materials in liquid crystal–based electro-optic devices: A perspective

The alignment agent plays a crucial role in the electro-optical characteristics of an electro-optic liquid crystal (LC) device. Conventionally, rubbed polyimide (PI) layers have been utilized as alignment agents in LC devices. However, these organic PI layers are sensitive to UV rays and high temperatures. The rubbing procedure involved in preparing PI layers also introduces drawbacks, including non-uniform brightness due to uneven rubbing mechanisms and the presence of residual fiber dust. In our recent research, we demonstrated that various hexagonal 2D (inorganic) materials, such as monolayer graphene, monolayer tungsten diselenide (WSe2), and monolayer hexagonal boron nitride (h-BN) nanosheets, can effectively serve as planar-alignment agents in electro-optic LC devices. In this Perspective, we review the rubbing-free LC alignment process, which relies on the coherent overlay of the benzene rings of LC molecules on the hexagonal lattice structure of 2D materials. We review our recent results on LC devices utilizing 2D materials as alignment agents, highlighting their potential advantages. Furthermore, we discuss the challenges associated with this research field. Finally, we provide our perspectives on the future directions and opportunities for advancing this area of study, aiming to overcome challenges and extend the benefits of employing 2D materials as planar-alignment agents in electro-optic LC devices.

Electron density control in WSe2 monolayers via photochlorination

Modulation of the Fermi level using an ultraviolet (UV)-assisted photochemical method is demonstrated in tungsten diselenide monolayers. Systematic shifts and relative intensities between charged and neutral exciton species indicate a progressive and controllable decrease of the electron density and switch tungsten diselenide from n-type to a p-type semiconductor. The presence of chlorine in the 2D crystal shifts the Fermi level closer to the valence band while the effect can be only partially reversible via continuous wave laser rastering process. Chlorine species in the lattice are validated by x-ray photoelectron spectroscopy, while density functional theory calculations predict that adsorption of chlorine on the selenium vacancy sites leads to p-type doping. The results of our study indicate that photochemical techniques have the potential to enhance the performance of various 2D materials, making them suitable for integrated optoelectronics such as lateral nanopatterned p–n junctions.

Lead halide perovskite sensitized WSe2 photodiodes with ultrahigh open circuit voltages

Two-dimensional semiconductors (2DSCs) have attracted considerable interests for optoelectronic devices, but are often plagued by the difficulties in tailoring the charge doping type and poor optical absorption due to their atomically thin geometry. Herein, we report a methylammonium lead iodide perovskite (CH3NH3PbI3)/2DSC heterojunction device, in which the electric-field controllable ion migration in the perovskite layer is exploited to induce reversible electron- and hole-doping effects in the underlying monolayer tungsten diselenide (WSe2) to form a programmable p–n photodiode. At the same time, the CH3NH3PbI3 layer functions as a highly efficient sensitization layer to greatly boost the optical absorption and external quantum efficiency (EQE) of the resulting photodiode. By asymmetrically poling the perovskite layer, gold-contacted CH3NH3PbI3/WSe2 devices show a switchable open circuit voltage up to 0.78 V, along with a high EQE of 84.3%. The integration of tunable graphene-contacts further improves the photodiode performance to achieve a highest open circuit voltage of 1.08 V and a maximum EQE of 91.3%, greatly exceeding those achieved previously in 2DSC lateral diodes. Our studies establish a non-invasive approach to switch optoelectronic functions and open up a new avenue toward high-performance reconfigurable optoelectronic devices from 2DSCs.

Enhanced superconductivity in spin-orbit proximitized bilayer graphene.

In the presence of a large perpendicular electric field, Bernal-stacked bilayer graphene (BLG) features several broken-symmetry metallic phases1-3 as well as magnetic-field-induced superconductivity1. The superconducting state is quite fragile, however, appearing only in a narrow window of density and with a maximum critical temperature Tc ≈ 30 mK. Here we show that placing monolayer tungsten diselenide (WSe2) on BLG promotes Cooper pairing to an extraordinary degree: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in Tc and occurs over a density range that is wider by a factor of eight. By mapping quantum oscillations in BLG-WSe2 as a function of electric field and doping, we establish that superconductivity emerges throughout a region for which the normal state is polarized, with two out of four spin-valley flavours predominantly populated. In-plane magnetic field measurements further reveal that superconductivity in BLG-WSe2 can exhibit striking dependence of the critical field on doping, with the Chandrasekhar-Clogston (Pauli) limit roughly obeyed on one end of the superconducting dome, yet sharply violated on the other. Moreover, the superconductivity arises only for perpendicular electric fields that push BLG hole wavefunctions towards WSe2, indicating that proximity-induced (Ising) spin-orbit coupling plays a key role in stabilizing the pairing. Our results pave the way for engineering robust, highly tunable and ultra-clean graphene-based superconductors.

Coherent momentum control of forbidden excitons

A double-edged sword in two-dimensional material science and technology is optically forbidden dark exciton. On the one hand, it is fascinating for condensed matter physics, quantum information processing, and optoelectronics due to its long lifetime. On the other hand, it is notorious for being optically inaccessible from both excitation and detection standpoints. Here, we provide an efficient and low-loss solution to the dilemma by reintroducing photonics bound states in the continuum (BICs) to manipulate dark excitons in the momentum space. In a monolayer tungsten diselenide under normal incidence, we demonstrated a giant enhancement (~1400) for dark excitons enabled by transverse magnetic BICs with intrinsic out-of-plane electric fields. By further employing widely tunable Friedrich-Wintgen BICs, we demonstrated highly directional emission from the dark excitons with a divergence angle of merely 7°. We found that the directional emission is coherent at room temperature, unambiguously shown in polarization analyses and interference measurements. Therefore, the BICs reintroduced as a momentum-space photonic environment could be an intriguing platform to reshape and redefine light-matter interactions in nearby quantum materials, such as low-dimensional materials, otherwise challenging or even impossible to achieve.

P-Type Ohmic Contact to Monolayer WSe2 Field-Effect Transistors Using High-Electron Affinity Amorphous MoO3

Monolayer tungsten diselenide (1L-WSe2) has been widely used for studying emergent physics due to the unique properties of its valence bands. However, electrical transport studies have been impeded by the lack of a reliable method to realize Ohmic hole-conducting contacts to 1L-WSe2 especially at low carrier densities and low temperatures. Here, we report low-temperature p-type Ohmic contact to 1L-WSe2 field-effect transistors at carrier densities (n) below n = 1 × 1012 cm–2 with negligible temperature dependence down to the lowest measured temperature (10 K). The non-rectifying barrier is achieved between 1L-WSe2 and molybdenum trioxide (MoO3), where 1L-WSe2 underneath MoO3 is heavily hole-doped through surface transfer doping. Electrical transport measurements reveal linear current–voltage relations at a temperature of 10 K and carrier densities from n = 7.7 × 1011 cm–2 to below the threshold. The finding is also supported by nearly temperature-independent output curves up to room temperature and a negligible contact barrier down to the subthreshold regime. Furthermore, the contact resistivity of MoO3-contacted 1L-WSe2 FET is 30.2–64.8 kΩ μm at n = 1.5 × 1012 cm–2, which is the lowest reported for 1L-WSe2 FETs at such low carrier density. Realizing robust p-type Ohmic contact to a 2D transition metal dichalcogenide semiconductor will enable direct electronic measurements of quantum transport in correlated phases in the valence bands of monolayer semiconductors.

Promotion of superconductivity in magic-angle graphene multilayers.

Graphene moiré superlattices show an abundance of correlated insulating, topological, and superconducting phases. Whereas the origins of strong correlations and nontrivial topology can be directly linked to flat bands, the nature of superconductivity remains enigmatic. We demonstrate that magic-angle devices made of twisted tri-, quadri-, and pentalayer graphene placed on monolayer tungsten diselenide exhibit flavor polarization and superconductivity. We also observe insulating states in the tril- and quadrilayer arising at finite electric displacement fields. As the number of layers increases, superconductivity emerges over an enhanced filling-factor range, and in the pentalayer it extends well beyond the filling of four electrons per moiré unit cell. Our results highlight the role of the interplay between flat and more dispersive bands in extending superconducting regions in graphene moiré superlattices.

Probing Nanoscale Schottky Barrier Characteristics at WSe2/Graphene Heterostructures via Electrostatic Doping (Adv. Electron. Mater. 9/2022)

Nanoscale Schottky Barrier Characteristics The local Schottky–Mott characteristics determine the quality of electronic interfaces in metal–semiconductor van der Waals heterostructures. In the article number 2200196, Sebastian Wood and co-workers demonstrate how advanced scanning probe microscopy can probe the interface characteristics of monolayer tungsten diselenide on graphene transistors, distinguishing contributions from interface dipoles and graphene work function, and providing evidence for unpinned Fermi levels through electrostatic doping.

Boost Lasing Performances of 2D Semiconductor in a Hybrid Tungsten Diselenide Monolayer/Cadmium Selenide Quantum Dots Microcavity Laser

AbstractMonolayer (ML) transition metal dichalcogenides (TMDCs) are suitable for use in low‐threshold, room‐temperature lasers. 2D semiconductor lasers are coherent, compact, and are suitable for multispectral light sources. 0D quantum dot technology has been applied in broadband‐emitting devices and high‐quality display systems. Herein, dual‐color continuous‐wave microcavity lasers are investigated by integrating a tungsten diselenide (WSe2) monolayer and cadmium selenide (CdSe) quantum dots (QDs) into a single microdisk cavity. The hybrid TMDC/QD microcavity device not only achieves lasing in two distinct wavelength regions but also boosts the lasing performances of the WSe2 monolayer due to the energy conversion between the two gain materials. Variations in the spectra obtained under power‐dependent lasing are examined, and the temporal coherence properties of the lasing signals are also characterized to verify the lasing actions. The results indicate that the lasing threshold of the 2D WSe2 monolayer cavity with the CdSe QDs is reduced by more than 2.5 times compared with the WSe2 cavity without the QDs. These findings both expand the wavelength range of TMDC‐based compact lasers at room temperature and facilitate their implementation in applications such as photonic integrated circuits, broadband light‐emitting diodes, and quantum display systems.

Femtomolar‐Level Molecular Sensing of Monolayer Tungsten Diselenide Induced by Heteroatom Doping with Long‐Term Stability

AbstractSurface‐enhanced Raman scattering (SERS) is a sensitive, fast, and nondestructive technology to detect trace amounts of molecules. The development of ultrasensitive and environmentally stable noble‐metal‐free SERS substrates is crucial for practical applications but still very challenging. In this contribution, an in situ substitutional doping strategy to synthesize Re‐doped WSe2 (Re‐WSe2) with different doping levels is reported. By increasing the Re content to ≈50 at%, the Re‐WSe2 alloy inherits the 1T″ phase of the ReSe2 lattice. Furthermore, Nb atoms are doped into the 1T″ Re‐WSe2 alloy to further modulate its electronic structure. The as synthesized 1T″ Nb, Re‐WSe2 demonstrates a femtomolar‐level molecular sensing capability with a detectable concentration of 5 × 10–15 m and the corresponding enhancement factor is 2.0 × 109, which is superior to that of most non‐noble‐metal SERS substrates and comparable or even superior to that of noble‐metal substrates to the best of the authors’ knowledge. More importantly, the as‐synthesized 1T″ Nb, Re‐WSe2 exhibits excellent air‐stability over a long term (≈6 months) and selective detection capability in the mixed molecular solution, which are essential for their practical applications. The work provides a new strategy for the rational design of noble‐metal‐free SERS substrates to achieve ultrasensitive molecular sensing.

Observation of in-plane exciton–polaritons in monolayer WSe2 driven by plasmonic nanofingers

Abstract The transition metal dichalcogenides (TMDs) have drawn great research attention, motivated by the derived remarkable optoelectronic properties and the potentials for high-efficient excitonic devices. The plasmonic nanocavity, integrating deep-sub wavelength confinement of optical mode with dramatic localized field enhancement, provides a practical platform to manipulate light–matter interaction. In order to obtain strong exciton–plasmon coupling effects, it’s crucial to match the vibration direction of exciton to the available strong localized in-plane electric field. Herein, we demonstrate the coupling effect of in-plane exciton in monolayer tungsten diselenide (WSe2) to deterministic gap-plasmon field which is produced by nanometrically gapped collapsed nanofingers. The gap-plasmon field which is completely parallel to the in-plane excitons in WSe2 will drive a strong exciton–plasmon coupling at room temperature. More interestingly, it is experimentally observed that the luminescence of exciton–polariton cannot be influenced by the temperature in the range from 77 K to 300 K due to the presence of nanofingers. According to the theoretical analysis results, we attribute this finding to the dielectric screening effect arising from the extremely strong localized electric field of plasmonic nanofingers. This work proposes a feasible way to harness and manipulate the exciton of low-dimensional semiconductor, which might be potential for quantum optoelectronics.

Utilizing Valley–Spin Hall Effect in Monolayer WSe2 for Designing Low Power Nonvolatile Spintronic Devices and Flip-Flops

In this article, we propose low power nonvolatile flip-flops (NVFFs) utilizing valley–spin Hall effect (VSHE) in monolayer tungsten diselenide (WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). The proposed designs are based on nonvolatile spintronic devices comprising of WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based charge-to-spin converter coupled with magnetic tunnel junctions. The proposed devices have an integrated back-gate to control the flow of charge and spin currents. VSHE leads to the flow of opposite spins in divergent directions perpendicular to the WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer with valley/spin Hall angle ~1. This enables encoding true and complementary bits in a single device utilizing magnets with perpendicular magnetic anisotropy (PMA). By introducing exchange coupling between PMA magnets mediated by Ta and FeCo-oxide layers, we electrically isolate but magnetically couple the PMA free layers, which enable the design of backup/restore module of NVFFs with minimal number of transistors. Using object-oriented micromagnetic framework (OOMMF) simulation, we establish the robustness of the exchange-coupled PMA systems and show that the misalignment and reduction in exchange coupling strength, induced by process variations, have a minimal impact on their functionality. Exploiting the unique features of the proposed devices, we design two flavors of VSHE-based NVFFs (VSHE-NVFFs) and carry out their energy–delay characterization, including an extensive variation analysis. The proposed VSHE-NVFFs achieve 74%–75% lower backup energy and 55%–59% lower restore energy than the existing NVFFs based on giant spin Hall effect (GSHE).

Probing Nanoscale Schottky Barrier Characteristics at WSe2/Graphene Heterostructures via Electrostatic Doping

AbstractThe adoption of 2D transition metal dichalcogenide (TMD) based optoelectronic devices is limited by Fermi level pinning effects and consequent large contact resistances upon contacting TMDs with bulk metal electrodes. A potential solution for near‐ideal Schottky–Mott behavior and concomitant Schottky barrier height control is proposed by contacting TMDs and (semi‐)metals in van der Waals heterostructures. However, measurement approaches to directly assess interface parameters relevant to the Schottky–Mott rule on a local scale are still lacking. In the present work, a heterostructure of monolayer tungsten diselenide (WSe2) with monolayer graphene (1LG) and bilayer graphene (2LG) is investigated on a bottom‐gate substrate. Kelvin probe force microscopy and tip‐enhanced photoluminescence measurements at different electrostatic doping induced Fermi levels in graphene enable decoupling and quantification of contributions from the interface dipole and electrode work function. These are used to locally probe Schottky barrier characteristics with below 32 nm lateral resolution, demonstrating that the WSe2/1LG junction operates at the Schottky–Mott limit (S ≈ 1). At the WSe2/2LG junction, a reduction of the interface dipole is directly related to changes in excitonic emission properties. These are attributed to charge transfer modulation across the interface, critical for obtaining high‐performance transfer characteristics in transistors and related devices.

Generating Bright Emissive States by Modulating the Bandgap of Monolayer Tungsten Diselenide

Generating Bright Emissive States by Modulating the Bandgap of Monolayer Tungsten Diselenide

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.